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Novel insights into the genomic basis of citrus canker based on the genome
sequences of two strains of Xanthomonas fuscans subsp. aurantifolii
BMC Genomics 2010, 11:238 doi:10.1186/1471-2164-11-238
Leandro M Moreira (Immorei@gmail.com)
Nalvo F Almeida Jr (firstname.lastname@example.org)
Neha Potnis (email@example.com)
Luciano A Digiampietri (firstname.lastname@example.org)
Said S Adi (email@example.com)
Julio C Bortolossi (firstname.lastname@example.org)
Ana C da Silva (email@example.com)
Aline M da Silva (firstname.lastname@example.org)
Fabricio E de Moraes (email@example.com)
Julio C de Oliveira (firstname.lastname@example.org)
Robson F de Souza (email@example.com)
Agda P Fancincani (firstname.lastname@example.org)
Andre L Ferraz (email@example.com)
Maria I Ferro (firstname.lastname@example.org)
Luiz R Furlan (email@example.com)
Daniele F Gimenez (firstname.lastname@example.org)
Jeffrey B Jones (email@example.com)
Elliot W Kitajima (firstname.lastname@example.org)
Marcelo L Laia (email@example.com)
Rui P Leite Jr (firstname.lastname@example.org)
Milton Y Nishiyama (email@example.com)
Julio Rodrigues Neto (firstname.lastname@example.org)
Leticia A Nociti (email@example.com)
David J Norman (firstname.lastname@example.org)
Eric H Ostroski (email@example.com)
Haroldo A Pereira Jr (firstname.lastname@example.org)
Brian J Staskawicz (email@example.com)
Renata I Tezza (firstname.lastname@example.org)
Jesus A Ferro (email@example.com)
Boris A Vinatzer (firstname.lastname@example.org)
Joao C Setubal (email@example.com)
Article type Research article
2010 Moreira et al. licensee BioMed Central Ltd.
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Novel insights into the genomic basis of citrus canker based
on the genome sequences of two strains of Xanthomonas
fuscans subsp. aurantifolii
Leandro M Moreira 1,2, Nalvo F Almeida Jr 13, Neha Potnis 3, Luciano A Digiampietri 12,14, Said S
Adi 13, Julio C Bortolossi 5, Ana C da Silva 9, Aline M da Silva 2, Fabricio E de Moraes 5, Julio C
de Oliveira 5,6, Robson F de Souza 2, Agda P Facincani5, Andr6 L Ferraz 5, Maria I Ferro 5, Luiz
R Furlan 7, Daniele F Gimenez 5, Jeffrey B Jones 3, Elliot W Kitajima 10, Marcelo L Laia 5'8, Rui P
Leite Jr 17 Milton Y Nishiyama2, Julio Rodrigues Neto 1, Leticia A Nociti 5, David J Norman ,
Eric H Ostroski 14, Haroldo A Pereira Jr 5, Brian J Staskawicz 19, Renata I. Tezza 5, Jesus A
Ferro 5, Boris A Vinatzer 4, Joao C Setubal15, 16
1. Departamento de Ciencias Biol6gicas, Instituto de Ciencias Exatas e Biol6gicas,
Campus Morro do Cruzeiro, Universidade Federal de Ouro Preto, Ouro Preto, MG,
2. Departamento de Bioquimica, Instituto de Quimica, Universidade de Sao Paulo, Sao
Paulo, SP, Brazil
3. Department of Plant Pathology, University of Florida, Gainesville, FL, USA
4. Department of Plant Pathology, Physiology and Weed Sciences, Virginia Polytechnic
Institute and State University, Blacksburg, VA, USA
5. Departamento de Tecnologia, Faculdade de Ciencias Agrdrias e Veterindrias de
Jaboticabal, UNESP Univ. Estadual Paulista, Jaboticabal, SP, Brazil
6. Departamento de Ciencias Biol6gicas, Campus de Diadema, Universidade Federal de
Sao Paulo, Sao Paulo, SP, Brazil
7. Departamento de Melhoramento e Nutrig.o Animal, Faculdade de Medicina Veterinaria
e Zootecnia de Botucatu, UNESP Univ. Estadual Paulista, SP, Brazil
8. Departamento de Engenharia Florestal, Centro de Ciencias Agroveterindrias,
Universidade do Estado de Santa Catarina, Lages, SC, Brazil
9. Allelyx Applied Genomics, Campinas, SP, Brazil
10. Nucleo de apoio & pesquisa em microscopia eletr6nica aplicada & pesquisa
agropecudria, Escola Superior de Agricultura Luiz de Queiroz, Universidade de Sao
Paulo, Piracicaba, SP, Brazil
11. Laborat6rio de Bacteriologia Vegetal, Instituto Biol6gico Campinas, Campinas, SP,
12. Escola de Artes, Ciencias, e Humanidades, Universidade de Sao Paulo, Sao Paulo, SP,
13. Faculdade de Computag.o, Universidade Federal do Mato Grosso do Sul, Campo
Grande, MS, Brazil
14. Laborat6rio de Bioinform.tica, Instituto de Computag.o, Universidade Estadual de
Campinas, Campinas, SP, Brazil
15. Virginia Bioinformatics Institute, Virginia Polytechnic Institute and State University,
Blacksburg, VA, USA
16. Department of Computer Science, Virginia Polytechnic Institute and State University,
Blacksburg, VA, USA
17. Institute Agron6mico do Parand, Londrina, PR, Brazil
18. Institute of Food and Agricultural Sciences, Mid-Florida Research & Education Center,
University of Florida, Gainesville, FL, USA
19. Department of Plant & Microbial Biology, University of California, Berkeley, Berkeley,
APF: agdapfl @yahoo.com.br
ALJF: splinter firstname.lastname@example.org
Citrus canker is a disease that has severe economic impact on the citrus industry worldwide.
There are three types of canker, called A, B, and C. The three types have different phenotypes
and affect different citrus species. The causative agent for type A is Xanthomonas citri subsp.
citri, whose genome sequence was made available in 2002. Xanthomonas fuscans subsp.
aurantifolii strain B causes canker B and Xanthomonas fuscans subsp. aurantifolii strain C
causes canker C.
We have sequenced the genomes of strains B and C to draft status. We have compared their
genomic content to X. citri subsp. citri and to other Xanthomonas genomes, with special
emphasis on type III secreted effector repertoires. In addition to pthA, already known to be
present in all three citrus canker strains, two additional effectors, xopE3 and xopAl, are also
present in all three strains and are both located on the same putative genomic island. These two
effectors, along with other effector-like genes in the same region, are thus good candidates for
being pathogenicity factors on citrus. Numerous gene content differences also exist between the
three cankers strains, which can be correlated with their different virulence and host range.
Particular attention was placed on the analysis of genes involved in biofilm formation and
quorum sensing, type IV secretion, flagellum synthesis and motility, lipopolysacharide synthesis,
and on the gene xacPNP, which codes for a natriuretic protein.
We have uncovered numerous commonalities and differences in gene content between the
genomes of the pathogenic agents causing citrus canker A, B, and C and other Xanthomonas
genomes. Molecular genetics can now be employed to determine the role of these genes in
plant-microbe interactions. The gained knowledge will be instrumental for improving citrus
Citrus canker is a disease with worldwide distribution that has severe economic impact on the
citrus industry [1-2]. Disease symptoms consist of water soaked lesions that develop into
blisters, then pustules, and, finally, cankers. In severe cases, citrus canker can lead to
defoliation and premature fruit drop . Eradication of infected plants is the method of choice to
control the disease where it is not yet endemic. When the disease is endemic, control is
attempted by planting disease-free trees, limiting the spread between orchards, and using
preventive copper sprays [4-6]. However, none of these measures controls citrus canker
There are three types of citrus canker described in the literature: types A, B and C. Type A
originated in Asia, probably in Southern China, Indonesia or India, and it is the type that is most
widespread and causes the greatest economic damage [4, 7]. The other two types have only
been found in South America. Type B (or false canker) was originally identified in Argentina in
1923. This type is present only in Argentina, Paraguay, and Uruguay , whereas type C is
limited to the state of Sdo Paulo, Brazil .
The causal agent of canker A is Xanthomonas citri subsp. citri (which we abbreviate as XAC for
reasons explained below). XAC causes disease on many citrus species, with C. paradisi
(grapefruit) and C. aurantifolia (Mexican lime) being most susceptible in the field and C.
reticulata (mandarin/tangerine) and C. sinensis (sweet orange) being relatively tolerant [10-11].
Importantly, no citrus species is resistant to XAC after artificial inoculation, suggesting that there
is no true genetic resistance against XAC and that field tolerance is mainly due to variation in
growth habit .
The genome of XAC strain 306 from Brazil was completely sequenced in 2002  and
compared to the genomes of Xanthomonas species that are pathogenic in other plants [13-14].
This comparative genomics approach has greatly accelerated the study of the molecular basis
of pathogenicity and virulence of XAC. XAC has a hrp/hrc cluster coding for a type III secretion
system (T3SS) that is used by the pathogen to inject virulence proteins, called effectors, into
host cells. While several genes coding for putative effectors have been identified in the XAC
genome, the single most important effector is PthA [3, 15]. Even in the absence of the
pathogen, PthA induces canker-like symptoms when transiently expressed in plants .
Deletion of pthA abolishes the ability of XAC to cause cankers . Intriguingly, PthA induces
cankers in all citrus species while it triggers plant immunity in other plant species, thus being the
prime determinant of XAC specificity toward citrus [16, 18-19].
Two variant forms of canker A have been described. The first was found in Southeast Asia in
1998 infecting C. aurantifolia. The pathogen was classified as XAC variant A* . The second
variant was isolated in 2003 in Southern Florida in C. aurantifolia and C. macrophyla (alemow),
and was named Xanthomonas citri variant Aw . Aw strains have been shown to be a sub-
group within A* . These strains are primarily pathogenic on C. aurantifolia and do not cause
disease on C. paradisi, even after artificial inoculation [19-20]. A T3SS effector, called AvrGf1,
was found to contribute to the exclusion of C. paradisi from the A* host range . A recent
study  suggests that A* strains (including Aw) have a wider genetic diversity than the strains
that cause A-type canker.
Canker B is mostly restricted to C. limon (lemon), but has also been found in C. sinensis and in
C. paradisi . Its causal agent has been described as X. fuscans subsp. aurantifolii type B
(which we abbreviate as XauB for reasons explained below). Even though symptoms are similar
to canker A, they take longer to appear, perhaps reflecting the slower growth of XauB in culture
when compared to XAC. Canker C has the same symptoms as type A, but, similarly to XAC A*
and Aw, it is restricted to C. aurantifolia and does not occur in C. paradisi . The causal agent
has been described as X. fuscans subsp. aurantifolii type C (which we abbreviate as XauC for
reasons explained below). Fig. 1A summarizes observed phenotypes in three different citrus
species. Recently, what appears to be a new type of X. fuscans subsp. aurantifolii infecting
single citrumelo (C. paradisi Macf. x Poncirus trifoliata L. Raf.) has been reported in Brazil
XAC, XauB and XauC have been compared phenotypically and analyzed phylogenetically. All
three strains present polar flagella with perceptible motility when cultured in semi-solid media
. They all grow in the presence of lactose, manitol, and celobiose. However, only XAC is
able to grow in the presence of maltose and aspartic acid, and it is also capable of pectate and
gel hydrolysis . XauB and XauC have little or no affinity for polyclonal antisera prepared
against XAC, and XAC is susceptible to bacteriophages CP1 and CP2 while XauB and XauC
are not . It is notable that XauB has fastidious growth in culture media where both XAC and
XauC grow well, for example in Agar nutrient and tryptophan-sucrose-agar media. All three
grow well in media rich in glutamic acid . Multilocus sequence typing and other molecular
analyses [28-30] have shown that XauB and XauC are more closely related to each other than
Under the rationale that the availability of the genome sequences and annotations of the
causative agents of the B and C canker types can substantially improve our understanding of
the genomic basis of the disease, we have sequenced the genomes of XauB and XauC to draft
status. We have compared them with the genomes of XAC and other xanthomonads. Identified
commonalities among the three canker genomes represent candidate genes that may help
explain the differences between citrus canker and diseases caused by other xanthomonads. We
have also identified numerous gene differences between the three citrus canker genomes.
Some of these genes were previously shown to contribute to the virulence of XAC  and are
thus primary candidates for explaining the higher virulence of XAC compared to XauB and
XauC as well as the host range differences that exist between the three canker types.
A note on species abbreviations
The organisms studied in this work do not have names that are universally accepted. At the time
when its genome was sequenced, the accepted name for XAC was Xanthomonas axonopodis
subsp. citri. In the meantime, Xanthomonas citri subsp. citri has been validly published as a
name of this organism [32-33]. However, because the locus tag prefix of XAC genes is 'XAC' we
opted for this abbreviation to avoid confusion along the text. Similarly, the causative agents for
the B and C cankers are known by different names, but we use XauB and XauC as acronyms
so that they are in agreement with their respective locus tag prefixes. For the B species we use
the name X. fuscans subsp. aurantifolii type B and for the C species we use the name X.
fuscans subsp. aurantifolii type C . We refer to the three organisms collectively as the citrus
canker strains (abbreviated by CC strains/genomes).
Results and Discussion
Table 1 shows the genome features of the three canker strains. Even though we do not have
complete genome sequences for XauB and XauC, alignments of their scaffolds to the XAC
genome suggest that all three chromosomes are highly syntenic (Additional file 1: Fig. Sl).
Based on these alignments and on total length of contigs we estimate that we have obtained
94% of the XauB genome and 96% of the XauC genome. We estimate that the vast majority of
remaining gaps in the two incomplete genomes is under 2 kbp. Fig. 1 B shows that XAC shares
74% of its protein-coding genes with the other two strains. The fractions for XauB and XauC are
87% and 84%, respectively. The number of XAC-specific genes is much larger than the
analogous number for XauB and XauC. Although this difference could be attributed to the
incompleteness of the Xau genomes, we have hybridization results (see below) that suggest
that many of these XAC-specific genes are indeed absent from the other two genomes under
XAC strain 306 has two plasmids, pXAC33 (34 kbp) and pXAC64 (65 kbp). Based on similarities
between contig sequences and XAC plasmid sequences it appears that both XauB and XauC
have plasmids. At least 46% of plasmid pXAC33 sequence is found in XauB contigs (46% of
plasmid pXAC33 sequence is found in XauC contigs); at least 61% of plasmid pXAC64
sequence is found in XauB contigs (55% for XauC). We do not have enough sequence data to
ascertain the exact number of plasmids in each Xau genome.
The phylogenetic relationship of the three strains with respect to each other and to fully
sequenced members of the Xanthomonas and Xylella genera based on their shared protein-
coding genes is shown in Fig. 2. The tree shows that the three organisms under study form a
well-defined group within the family Xanthomonadaceae. This tree is in agreement with trees of
the Xanthomonas genus based on multilocus sequence analysis [34-35].
At http://bioinfo.facom.ufms.br/xanthomonas we provide an interactive tool that allows user-
defined gene content comparisons among all sequenced Xanthomonas genomes.
Genes shared by all three strains but not present in other Xanthomonadaceae species
XAC, XauB, and XauC have 65 families of orthologous genes specific to them when compared
to all other fully sequenced members of the genera Xanthomonas and Xylella (Additional file 2:
Table S2). Among these 65 families we have identified 11 syntenic blocks. Not surprisingly,
almost half of these genes code for hypothetical proteins of unknown function. Of the genes with
a predicted function the genes encoding the predicted effectors XopE3 (XAC3224) and XopAl
(XAC3230) (discussed below) are especially noteworthy. The large number of genes coding for
various kinds of transporters are also worth mentioning: four TonB dependent receptors
including one with homology to the Escherichia coli receptor FepA, which is involved in transport
of siderophores across the bacterial membrane  and seven ABC transporters, which might
be used either for translocation of substrates from the citrus apoplast into the bacterial cell to
provide nutrients for the pathogen, or, alternatively, for secretion of toxins (either bacterial toxins
or expulsion of citrus metabolites toxic to the CC strains). An additional transporter specific to
the three CC genomes (XAC3198) is an alkanesulfonate transporter substrate-binding subunit,
which is reported to enable E. coli to use sulfonates other than taurine . Besides
transporters, several genes encode metabolic enzymes (an amidase, an urea amidolyase, a
peptidase, and a nitrilotriacetate monooxygenase). This conspicuous presence of transporters
and metabolic enzymes suggests that the CC strains might have adapted to specific metabolites
present in the citrus apoplast. However, this will need to be confirmed experimentally by
comparing growth of wild type strains and strains mutated in CC-specific genes in apoplastic
fluid of citrus species and of other plant species. One other interesting gene present in all CC
genomes is a gene coding for a methyl parathion hydrolase (XAC0726), predicted to degrade
the insecticide methyl parathion [38-39]. Orthologs of this gene and other genes that degrade
organophosphates are common in soil bacteria and in the soil-borne pathogen Ralstonia
solanacearum but have not yet been found in any other foliar plant pathogen.
Some of these syntenic CC-specific regions are anomalous in terms of nucleotide composition
as determined by the program AlienHunter  and may thus have been acquired by horizontal
The three CC genomes have important differences in regard to their repertoires of type III
The hrp/hrc genes encoding the T3SS are basically the same and found in the same order in all
three CC genomes. However, there are notable differences in the three putative T3SS-secreted
A list of twenty-seven T3SS effector genes predicted in the genomes of the CC strains is shown
in Table 2. Effectors are important determinants of virulence and host range in many plant
pathogenic bacteria, in particular in Xanthomonas sp. and Pseudomonas syringae .
Comparison of effector repertoires between the three CC genomes and all other Xanthomonas
genomes can thus give us important clues. The effector genes avrBs2, xopL, xopQ, and xopX
are present in all three CC genomes, in all sequenced genomes of other Xanthomonas species,
and in all X. citri and most Xanthomonas strains that were surveyed by PCR and hybridization
for these genes by Hajri et al. . These effectors thus belong to the Xanthomonas core set of
effectors possibly important for pathogenicity on all plants. The putative effector genes xopK,
xopR, and xopZ also belong to this group since they can be found in all sequenced
Xanthomonas genomes. However, no data exist for these effectors in regard to other
Xanthomonas strains . The effector genes xopl, xopV, xopAD, and xopAK are present in all
three CC genomes and in several, but not all, sequenced Xanthomonas genomes. These
effectors, therefore, might contribute to disease in some plant species while they might trigger
immunity in others.
As already mentioned, PthA is well known to be an important X. citri effector that plays an
essential role in citrus canker, while limiting the host range of CC strains to citrus because it
triggers immunity in all other tested plant species (see references above). The pthA gene is a
member of the avrBS3 family of effector genes, members of which are present in most
Xanthomonas genomes and in some R. solanacearum genomes . However, only PthA is
known to induce citrus canker. Besides pthA (XACb0065), three paralogs of pthA are also
present in the XAC genome (XACa0022, XACa0039, and XACb0015). All four copies are found
on plasmids. The three paralogs do not seem to play an important role in citrus canker . We
found two pthA homologs in the XauB genome (XAUB_40130 and XAUB_28490) and two in the
XauC genome (XAUC_22430 and XAUC_24060/XAUC_09900 [the latter is a single gene with
halves in different contigs]). Not all of these genes have been completely assembled due to the
repetitive regions found in avrBS3 family members. However, El Yacoubi et al.  previously
assembled a pthA homolog (pthB [GenBank: 2657482]) from the pXcB plasmid [GenBank:
NC_005240] of a XauB strain with the same repeat copy number (i.e. 17.5) as pthA, and Al-
Saadi et al.  sequenced and assembled another homolog (pthC [GenBank: EF473088]) from
a XauC strain. These genes functionally complemented a pthA deletion in XAC without affecting
host range . The XAUC_22430 gene has 99% nucleotide identity to pthC and thus probably
corresponds to pthC and would be the functional pthA homolog of XauC. We do not have
enough data to confidently report on the repeat copy number of the other three Xau pth
homologs, but a phylogenetic analysis (see below) suggests that XAUB_28490 is the functional
pthA homolog of XauB.
To get insight into the evolution of the pthA/avrBS3 family, a phylogenetic analysis of all
available avrBS3 family members in the genus Xanthomonas was performed (Fig. 3). The tree
shows that the four XAC pth genes group together, while the XauB and XauC genes form two
families, with one member from each strain participating in each family. Moreover, the XauB and
XauC pth genes group separately from the XAC genes, with good bootstrap support, which is a
surprising result when compared with the species tree (Fig. 2). The genes that flank the pth
copies in XauB do not have XAC genes as their best BLAST  hits. In particular, the two
genes upstream of XAUB_40130, XAUB_40110 and XAUB_40120, do not match any
Xanthomonas genes; instead, their best BLAST hits (e-value 10-102 and 10-99, respectively) are
gene sequences from Burkholderia pseudomallei NCTC 13177. (In XauC the pth regions are
too fragmented to allow genomic context determination.) We derive the following conclusions
from these results: An ancestor of the current XauB and XauC strains already had two different
copies of the pth genes; hence the existence of the two noted families. The fact that in XAC the
four copies are nearly identical, the fact that the Xau and XAC pth genes group in a way
distinctively different from the species grouping, and the fact that the genomic context in which
the XAC pth genes are found is different from that of XauB leads us to believe that XAC
acquired its pth genes by a different route and that their duplication is more recent when
compared to the XauB and XauC pth duplications. The ability to cause citrus canker may thus
have independently evolved by XAC and by XauB and XauC, and may represent an example of
convergent evolution. AI-Saadi et al. , based on a neighbor-joining phylogeny of pth genes
that included the pthB and pthC sequences, have come to the same conclusion.
Effectors XopAl and XopE3 may play a role in citrus canker
A comparison of effectors present in all three CC strains with those present in fully sequenced
Xanthomonas species, and data from the study by Hajri et al. , suggest that two additional
putative effectors may play a special role in citrus canker. These are XopAl and XopE3. Both
are present in all three CC genomes.
The putative effector xopAl is not found in any other sequenced Xanthomonas species and it
was not included in the Hajri et al.  analysis. We do have evidence that it is present in
Xanthomonas vesicatoria str. 1111 (Potnis et al., unpublished). Interestingly, the C-terminal
region of XopAl has similarity to predicted ADP-ribosyl transferase domains of the effector
HopO1-1 of Pseudomonas syringae and of hypothetical proteins in Acidovorax citrulli, Ralstonia
solanacearum, and other bacteria. The N-terminus has high similarity to the N-terminus of the
effector XopE2 of X. campestris pv. vesicatoria 85-10 as well the N-termini of a number of other
Xanthomonas and Pseudomonas syringae effectors (more on the N-terminal region of xopAl
XopE3 belongs to the HopX/AvrPphE family of effectors. Effectors belonging to this family have
been found in diverse phytopathogenic bacteria including Ralstonia, Pseudomonas, Acidovorax,
and Xanthomonas, suggesting their conserved role in virulence on a wide range of hosts.
Sequences from this family have similarity to the transglutaminase superfamily of enzymes,
which are responsible for modification of host proteins . The HopX/AvrPphE effector from
Pseudomonas syringae has been shown to be involved in host protein proteolysis, thereby
suppressing host defenses [46-47]. In xanthomonads, multiple effectors belonging to this group
have been found, such as xopEl, xopE2, xopE3, xopE4. XopE1 and xopE2 have been found in
most of the xanthomonads. XopE3 effector gene homologs have been found by PCR and dot-
blot hybridization methods in some Xanthomonas axonopodis strains belonging to the alfalfae,
anacardii, glycines, phaseoli, malvacearum, fuscans, mangiferae, indicate, and citrumelo
pathovars . However, sequences of xopE3 from these strains could not be compared
against homologs from CC strains since sequence data from the X. axonopodis strains
mentioned are not currently available. Phylogenetic analysis of hopX orthologs shows that the
xopE3 effector genes found in the CC strains group together with hopX1 effector genes from
pseudomonads (data not shown).
Although all hopXorthologs show conservation of the catalytic triad (Cys, His, Asp residues) as
well as the conserved domain "GRGN" N-terminal to the triad, the region C-terminal to the triad
shows high degree of variability. This variable region has been hypothesized to be responsible
for targeting different host proteins . In fact, while some AvrPphE (hopX) alleles from P.
syringae pv. phaseolicola strains trigger gene for gene disease resistance in some bean
cultivars, other alleles were shown to be virulent on these same cultivars. Amino acid
differences in the C-terminal region of AvrPphE were identified between alleles . Similarly,
comparing XopE3 homologs from different strains at the amino acid level and their
corresponding reactions on different hosts might give clues regarding the variable C-terminal
domains of XopE3 family members and might determine whether this variability is responsible
for targeting different proteins in different host species.
Both xopE3 and xopAl belong to an interesting XAC chromosomal region of approximately 15
kbp in size (Fig. 4) that has been hypothesized to be a genomic island . An alignment of the
XAC chromosome sequence with the chromosome sequences of X. campestris pv. campestris
str. ATCC33913 and X. oryzae pv. oryzae str. PX099A strongly suggests that this region is an
insertion (data not shown). The presence of three transposase genes and two phage-related
genes in the region provides additional evidence for this hypothesis. The central part of this
region (7 kbp) duplicates a region found in XAC plasmid pXAC64 (Fig. 4), suggesting a
chromosome-plasmid DNA exchange. In the plasmid we find the effector gene xopE2
(XACb001 1), which as described above shares its N-terminal region with xopAl (XAC3230)
(Fig. 4). Transposons and phage elements in this region might thus have been responsible for
a shuffling process, described as terminal reassortment , resulting in the novel effector gene
xopAl. Although we can characterize this region completely only in XAC, XauB and XauC
contigs contain the most important elements of this region (Fig. 4). Next to xopE3 (XAC3224)
we find gene XAC3225, whose product is annotated as tranglycosylase mrtB. This gene has
strong similarity (e-value 10-133, 100% coverage) to hopAJi from P. syringae pv. tomato strain
DC3000, where it is annotated as a T3SS helper protein. Although the hopAJi gene is not itself
a T3SS substrate, it contributes to effector translocation . A mutant with a deletion of
XAC3225 has reduced ability to cause canker (mutant phenotypes include a reduction in water
soaking, hyperplasia, and necrosis compared to wild type) . We thus conclude that the
effector and effector-related genes in this region probably play an important role in citrus canker.
Additional differences in effector repertoires among CC genomes
In addition to the pth differences noted above, other effectors that distinguish the XAC genome
from the two Xau genomes are XopB, XopE4, XopJ (AvrXccB), XopAF (avrXv3), and XopAG,
which are all present in both Xau genomes but absent from XAC strain 306. (AvrXccB homologs
were found in two XAC strains by Hajri et al. .) The absence of these effectors from XAC
strain 306 raises the possibility that these effectors might be responsible for limiting the host
range of both B and C strains. Interestingly, XauB and XauC strains both contain xopAG, an
effector gene belonging to the same effector family as avrGfl from X. citri Aw, which has been
shown to be responsible for triggering a hypersensitive defense response in C. paradisi
(grapefruit) . The xopAG gene from the B and C genomes shows 44% identity to avrGfl at
the amino acid level. The XauB and and XauC genes are almost identical to each other, with
one important difference: in XauB xopAG is interrupted by a transposon. Therefore, the
incompatibility between XauC and grapefruit and the ability of XauB to cause disease in
grapefruit could be explained by this single gene difference. The xopE4 DNA sequence is
identical in the two Xau genomes and has similarity to avrXacE3 but only with 31% identity at
the amino acid level; this is why we named this gene xopE4 instead of xopE2. Unlike other
XopE family members, XopE4 does not have a predicted myristoylation site, suggesting that it
may not be targeted to the cell membrane as the other XopE family members.
Presence of an additional effector gene, the avirulence gene avrXccA2, has been shown in
some X. aurantifolii B (CFBP3528, CFBP3530) and X. aurantifolii C (CFBP2866) strains by
hybridization and PCR analysis . However, this avirulence gene was not found in the two
sequenced Xau genomes. A homolog of the effector xopFl (XAUC_20070) was found only in
the XauC strain. It is located in a 5-kbp region that lies between the T3SS genes hrpW
(XAUC_20020) and hpa3 (XAUC_20080). The same two genes are adjacent in XauB. Two
transposases are present in this region, and the sequence of xopFl has a frameshift,
suggesting that this gene is likely the result of a recent insertion and is not active.
There are four effector genes present in the XAC and XauB genomes that have not been found
in the genome of XauC: xopE2, xopN, xopP, and xopAE. These effectors could explain the
wider host range of XAC and XauB compared to XauC, assuming a virulence activity of these
effectors on citrus species. XopN has been shown to interact with the plant protein TARK1 and
to interfere with immunity triggered by pathogen-associated molecular patterns (PAMP-triggered
immunity) . Further experiments are required to determine the possible role of XopN in
extending host range to lemon, grapefruit and sweet orange. Another effector that could have a
similar role is XopAE (a hpaF/PopC homolog) [52-53].
The harpin-like protein HrpW with a pectate lyase domain is present in all CC strains. In the
sequenced XAC genome, it is not associated with the T3SS gene cluster, whereas in the
genomes of XauB and XauC it is. The role of harpin-like proteins like HrpW as virulence factors
or T3SS accessory proteins has not yet been determined in the Xanthomonas genus.
Experiments will need to be performed to confirm translocation of the above putative effectors
and their putative function as virulence or avirulence genes.
XAC-specific genes and genomic regions with respect to XauB and XauC
We compared the XAC genome to the XauB and XauC genomes both computationally and by
doing experimental whole genome hybridizations. Results are summarized in Fig. 5. The results
of the two methods were consistent. In the following sections we focus on regions and genes
specific to XAC with respect to XauB and XauC according to these results.
We have identified 25 groups of at least four consecutive genes that we term XAC-specific
regions (XACSR) [Fig. 5, additional file 3 (Table S3), and additional file 4 (Fig. S4)]. Nearly all
regions contain or are flanked by transposition elements or phage-related genes, suggesting
that they could be the result of lateral transfer.
Several genomic differences are related to biofilm formation and quorum sensing
Xanthan gum is an exopolysaccharide that plays an important role in biofilm formation and
hence in virulence of pathogens of the Xanthomonadaceae family [54-56]. Moreover, the
synthesis of xanthan gum is regulated by variation in sugar concentration in the culture medium
and by the activation of regulatory rpf genes [57-58]. These genes are also responsible for the
synthesis of diffusible signal factors, fundamental molecules for quorum sensing processes [59-
60]. Both Xau genomes contain an identical xanthan gum operon (XauC: XAUC_26940-27060;
XauB: XAUB_007400-007410 and XAUB_10560-10450). The Xau genomes contain gene rpfH
(XAUB_10500 and XAUC_27010), but this gene is not found in XAC. Gene rpfl, present in the
xanthan gum operon of X. campestris pv. campestris strain ATCC33913, is absent from all three
XAC and the two Xau genomes contain the xanthomonadin biosynthesis regulon as well as
sugar metabolism genes. XauB however presents some important variations that may explain
its in plant and in culture fastidious phenotype, when compared to both XAC and XauC.
Differences were found in the phosphotransferase system (PTS-Fru), which specializes in
internalization of fructose, and in the rpfN gene, a sugar porin, which is regulated by rpf genes,
which also regulate xanthan gum synthesis . The importance of the PTS-Fru system and of
the sugar porin encoded by the rpfN gene for growth and pathogenicity of certain bacteria has
been reported in the literature: PTS-Fru mutants of Spiroplasma citri, causative agent of citrus
stubborn disease, are reduced in virulence [61-64], and rpfN mutants of Xanthomonas
campestris pv. campestris show an increase in the level of polygalacturonate lyase , an
important pathogenicity factor in bacterial plant pathogens. This result shows that the absence
of the sugar porin could cause lack of carbohydrate uptake, therefore inducing the synthesis of
cell wall degrading enzymes, in order to increase sugar supply [58, 66]. In XAC the PTS system,
encoded by genes fruBKA and rpfN, is organized in one single genomic region (XAC2501-2504)
(Fig. 6). XauC presents the same organization (XAUC_01750-01780). In XauB fruBKA
corresponds to XAUB_05120, XAUB_05110 and XAUB_05100 respectively, but rpfN was not
found. In addition, the fruA gene sequence contains a frameshift, indicating that it may have
become a pseudogene. XauB needs a culture medium with glutamic acid , possibly because
it is used by the bacterium as an alternative carbon source. Xylella fastidiosa 9a5c, which also
lacks the PTS system and the rpfN gene, is also fastidious .
Experiments were carried out to compare cellular growth and xanthan gum production in all
three organisms under study. Results (Table 3) confirm the fastidiousness of XauB, whereas
XAC and XauC have similar cellular mass values. We also determined that XAC produces more
than twice as much gum as XauC and almost three times as much as XauB. We believe this
variation is due to the XauB deficiencies in the rpfN and PTS-Fru genes discussed above. On
the other hand, this result also suggests that the lack of the rpfH and rpfl genes in XAC, caused
by the presence of transposition elements , does not influence the gum production capability
of XAC. The fastidiousness of XauB would thus be related to the low gum production and the
consequent decrease in biofilm formation together with the fact that it needs an additional
substrate source (glutamate) to support its growth in culture.
In addition to the differences noted above, five additional XAC-specific regions (XACSR7,
XACSR9, XACSR10, XACSR14 and XACSR17) may be related to its greater biofilm-formation
capability when compared to XauB and XauC. Several of the genes in these regions facilitate
adhesion in a process mediated by hemagglutinin .
XACSR7 contains two hemin storage system genes, hmsF and hmsH, and hemagglutinin
coding genes (XAC1811-1816). Genes involved with acquisition and storage of hemin groups
(hmsRFH) and type I secretion system genes (fhaC), and their secreted hemagglutinin (fhaB),
are found in tandem and flanked by a tRNAR in the XAC genome (Fig. 6). In Yersinia pestis the
hms genes are present in a cluster (the pgm cluster) related to temperature-dependent storage
of hemin as well as expression of a number of other physiological characteristics . Mutations
in these genes cause drastic decrease in Yersinia growth, preventing it from colonizing its point
of entrance in infected flies (bucal orifice) [71-72]. These genes also play a role in
exopolysacharide synthesis, and reduction in biolfilm formation has also been observed in these
mutants in Yersinia . Among all sequenced xanthomonads only XAC and X. oryzae have
these genes. In both cases they are similar to (35 to 53% identity at the amino acid level) and
syntenic with their homologs in Yersinia, E.coli K-12 and Erwinia carotovora (data not shown).
Recent work  describing mutations in the genes that code for hemagglutinin in Xylella
fastidiosa strain Temecula (Pierce's disease) has shown that biofilm composition and virulence
(adhesion and colonization) were affected in the mutants. This is consistent with results in XAC
. This is evidence that the apparent absence of these genes in XauB and XauC might have
the same effect (Fig. 6).
XACSR9 contains 19 genes. One of them (XAC1918) is a hemolysin-related gene. In
enterobacteria hemolysins are an important virulence factor that are associated with proteins
related to biofilm formation [75-76]. The protein encoded by this particular gene interacts with
VirD4, a Type IV secretion system component , which in turn may play a role in biofilm
formation and cell aggregation, as observed for E coli .
XACSR10 contains several noteworthy genes. Gene XAC2151 codes for the YapH protein. Its
homolog (XOO02380, 84% identity at the amino acid level) in X. oryzae pv. oryzae KACC10331
when mutated drastically reduced the pathogen adhesion to plant tissue, thus decreasing its
virulence ; in addition a homolog of this gene in X. fuscans subsp. fuscans CFBP4834-R
(the causative agent of bacterial blight of bean, Phaseolus vulgaris), was required for adhesion
to seed, leaves, and abiotic surfaces . XAC2197 and XAC2198 code for hemolysin-type
calcium binding proteins, whereas XAC2201 and XAC2202 code for hemolysin secretion protein
D (HIyD) and hemolysin secretion protein B (HlyB), respectively. The latter four genes do not
have Xanthomonas matches in the sequence databases; they are similar instead to protein
sequences from Acidovorax, Xylella and Pseudomonas species. The best hits are from
Acidovorax avenae subsp. avenae ATCC19860, which is also a plant pathogen.
XACSR14 contains genes related to the type IV pilus-dependent system (Fig. 6). This system
takes part in several processes, including adhesion, motility, microcolony formation, and
protease secretion . In Xylella fastidiosa functional studies of these genes have shown that
they are crucial for the host colonization process [82-86].
XACSR17 is also related to biofilm formation. Laia et al.  observed decrease in biofilm
activity and virulence in four mutants with changes in this region (XAC3245-14G01/14G12,
XAC3263-10G07/10G09, XAC3285-10F02 and XAC3294-17B04) (Fig. 6). The only mutated
gene with functional assignment is rhsD (XAC3245). This gene has been described as coding
for a membrane protein related to adhesion . In Xanthomonas campestris pv. campestris a
RhsD protein has been found in the outer membrane vesicle associated with other virulence-
associated proteins, such as HrpA/F/X/B4, HrcU, AvrBsl and AvrBs2 . XAC contains a
paralog of this gene (XAC2529), but it was not found in the Xau genomes either.
Gene wapA (XAC1305), not part of any XAC-specific region, and which is an adhesion facilitator
by way of hemagglutinins , was not found in either of the Xau genomes.
XauB contains T4SS gene clusters similar to those found in Ralstonia solanacearum and
in Agrobacterium tumefaciens
In addition to the type III secretion system, the type IV secretion system (T4SS) also plays a role
in pathogenicity. For example, this system has been shown to contribute to full virulence in the
phytopathogen X. campestris pv. campestris strain 8004 . Genome clusters containing
T4SS genes are found in several bacterial species but with marked differences in terms of gene
presence/absence and organization, which relate to the system's function in the organism
where it is present [89-90]. All three genomes under study contain T4SS genes, arrayed in
clusters. In order to better understand these gene clusters we have classified the T4SS genes
found in the three CC genomes as well as T4SS gene clusters from other organisms into four
groups, using as criteria presence/absence of genes and synteny (Fig. 7). We have found
important differences between XAC and XauB using this classification; data for XauC was too
fragmented to allow a similar general observation.
As reported by da Silva et al.  XAC has two T4SS clusters, one in the chromosome and the
other in a plasmid (pXAC64). In XauB we found three clusters. Only one of them is similar to a
XAC cluster (the pXAC64 cluster), and was therefore placed in group II, along with the T4SS
cluster found on plasmid pXcB (already mentioned above in the context of the pth gene
discussion). The other two XauB clusters were placed in groups I and III, respectively, while the
XAC chromosome cluster was placed in group IV (Fig. 7A).
The XauB cluster placed in group III is similar to a cluster found in Ralstonia solanacearum
, both in terms of organization as well as individual gene sequence similarity. This
organization is quite different from those found in other bacterial species. This XauB cluster is
found in a region containing 45 genes, all of which are XauB-specific when compared to other
Xanthomonas genomes. Moreover this cluster is flanked by insertion elements. This evidence
suggests that this cluster was likely acquired by lateral transfer.
The third XauB cluster was placed in group I, which contains T4SS clusters similar to those
found in Agrobacterium tumefaciens C58 plasmid Ti , Xylella fastidiosa 9a5c plasmid pXF51
, and rhizosphere plasmids plPO2T and pSB102 [94-95]. This XauB cluster is most similar
to those found in the rhizosphere plasmids, in particular to plPO2T. The XauC draft genome
sequence did allow the identification of one T4SS gene cluster, and it belongs to this group (Fig.
The XAC chromosomal cluster belongs to group IV (Fig. 7A). It is flanked by regions XACSR12
and XACSR13 (Fig. 8A). In addition, some of the genes in this T4SS cluster had uncertain
hybridizations (zoomed-in portion of Fig. 5). This adds evidence that this cluster, as a specific
unit, is indeed absent from both Xau genomes. XACSR12 contains genes that code for
transporters, alpha-glucosidases, hypothetical genes and transposases, as well as a virB6-
related gene. XACSR13 contains only hypothetical genes and transposases.
One of the members of this T4SS cluster is virD4 (XAC2623). Alegria et al.  have identified
12 XAC proteins that interact with VirD4. Six of these (XAC0096, 3266, 0151, 4264, 2609, 1918)
are apparently absent from the Xau genomes, and five belong to XAC-specific regions (Fig. 8B).
These genes might thus contribute to the increased virulence of XAC when compared to XauB
Flagellum and motility
The XAC genome contains all genes known to be needed for flagellum synthesis [12-14]. Three
of these clusters (which we call F1, F2, and F3) are close to the terminus of replication and are
spread out over a 126 kbp region (Fig. 9A). A fourth cluster contains just two genes, coding for
flagellar motor proteins A and B (motA and motB). All four clusters are found in both XauB and
XauC with one exception: in XauB we did not find cluster F2 (XAC1930-XAC1955, 31 kbp),
which contains 26 genes, coding for flagellum proteins that interact with bacterial membranes
and coding for flagellar motor components (Fig. 9CD). On the other hand we have observed
(microscopically) that XauB does have a flagellar structure similar to those of XAC and XauC,
and motility tests have shown that XauB is able to move (Fig. 9B). Although our genome
sequence data for XauB is incomplete, we believe it unlikely that a 31 kbp long fragment would
be entirely missed; moreover, the hybridization experiment did not indicate presence of this
fragment. These results suggest that the absence of genes in cluster F2 is compensated in
XauB by other as yet undetermined genes.
We did not find in XauB nor in XauC genes that lie between clusters F, and F2; this region is
XACSR9. In this region the gene XAC1927 has been shown to be important for citrus canker
since mutations in it significantly decreased virulence  (Fig. 9A).
LPS and O-antigen genes
Two of the XAC-specific regions contain genes related to lipopolysaccharides (LPS) that may
play a role in the differing phenotypes between XAC and the two Xau strains. Region XACSR18
(XAC3596-3599) (which contains gene rfbC, a truncated O-antigen biosynthesis protein) is
immediately downstream of a CC-specific region (XAC3588-3595) (Fig. 10). An alignment with
X. campestris pv. campestris ATCC33913 shows that these two regions together occupy
roughly the same genomic locus as a 25-gene region related to LPS synthesis in X. campestris
pv. campestris  (Fig. 10). Immediately downstream of XACSR18 we find genes wzt
(XAC3600) and wzm (XAC3601), also described as part of the LPS synthesis cluster in X.
campestris . Laia et al.  obtained significant phenotypic differences in XAC (less
necrosis, more water soaking and more hyperplasia, as compared to the wild type) by mutating
gene XAC3600 (which codes for an ABC transporter ATP-binding protein). The wzt gene is
present in both Xau genomes (XAUB_16610 and XAUC_09380), although both genes have lost
the C-terminus when compared to the XAC wzt gene (Fig. 10). The wzm gene is also present in
both Xau genomes (XAUB_ 16600 and XAUC_09370).
XACSR1 (XAC0037-0063) contains several genes related to LPS synthesis, including two
copies of asnB (XAC0051 and XAC0059), which codes for an asparagine synthase. A homolog
of this gene in Pseudomonas aeruginosa has been implicated in O-antigen biosynthesis .
The gene xacPNP
Gottig et al.  have identified a plant natriuretic peptide-like protein in XAC (xacPNP),
encoded by gene XAC2654. They have shown that a XAC2654-deletion mutant resulted in
more necrotic tissues and earlier bacterial cell death than in the wild type. XAC2654 lies
between regions XACSR13 and XACSR14, and is flanked on both sides by phage-related
genes. We have experimentally verified that neither XauB nor XauC appears to contain a
homolog of XAC2654 (Additional file 5: Fig. S5). Therefore, xacPNP might be another gene
contributing to the higher virulence displayed by XAC as compared to XauB and XauC.
Citrus canker continues to be an economically important disease. The publication of the XAC
strain 306 genome in 2002 opened up new avenues of research, and several important insights
into the genetics of canker have been obtained since then, most of them cited here. Yet,
understanding the genomic basis of a bacterial plant disease is a complex undertaking. By
obtaining the genome sequences of two additional citrus canker strains we have uncovered
several new clues towards a thorough understanding of this disease.
We have approached the problem from two basic perspectives. The first was to determine
commonalities among the three CC strains that were not found in other Xanthomonas genomes.
Such traits are excellent candidates for the general genomic basis of canker and/or adaptation
to citrus hosts. The second was to carefully compare the three CC genomes to one another,
with special attention to genes that XAC has that the others (apparently) do not, as well as
genes present in the Xau genomes but absent in XAC. Because of the draft nature of the Xau
genomes here presented, all results concerning gene absence in their sequences are tentative.
However, our hybridization platform provided additional evidence for the specificity of XAC
genes and regions with respect to the other two strains.
Our most important findings are related to presence/absence of effector genes. In addition to
the already known pthA gene, the genes xopE3 and xopAl deserve special attention in future
studies. Moreover, we have identified several genes (such as xacPNP) that differentiate XAC
from XauB and/or XauC. These genes or their homologs in other bacterial plant pathogens have
demonstrated roles in virulence and/or host specificity. Hypotheses on their role in citrus canker
and in host range differences between CC strains can now be tested experimentally.
We anticipate that knowledge in regard to CC-specific effectors and other CC-specific genes will
be used in the future to engineer citrus species with durable resistance to citrus canker, thus
reducing the economic impact of this disease on the citrus industry worldwide. Such knowledge
will also be crucial for dealing with new canker variants that may emerge in the field, exemplified
by the recent detection of what appears to be a new variant of Xanthomonas fuscans subsp.
aurantifolii in single citrumelo .
Bacterial strains and DNA sequencing
The Xanthomonas fuscans subsp. aurantifolii type B genome sequenced was strain 11122 (B-
69), isolated from a Citrus limon tree in Argentina. The Xanthomonas fuscans subsp. aurantifolii
type C genome sequenced was strain 10535 (IBSBF338), isolated from a Mexican lime tree in
S.o Paulo state in Brazil. We sequenced the genomes using the Sanger technique as
described previously , with sequencers ABI 3700 and ABI 3100. For XauB we generated
both shotgun and cosmid libraries; for XauC only shotgun libraries were created. For XauB we
obtained 114,874 reads; for XauC we obtained 114,805 reads. We estimate this provided about
15X average coverage for each genome.
Microscopy and motility tests
Initial attempts to visualize the flagellum of the two Xau strains with a scanning electron
microscope by the normal routine (adhesion of the cultured bacteria on a cover slip with the help
of the cationic compound poly-L-lysine, fixation, dehydration and critical point drying) failed
because apparently attachment of the flagellum is very weak and it tends to fall off easily. To
circumvent the problem, a diluted suspension of the bacterial culture was directly transferred
onto a cover slip and excess of liquid eliminated. A moist chamber was made in a Petri dish
where the cover slip was inserted together with a small plastic vial containing about 1 ml of 2%
aqueous osmium tetroxide. The Petri dish was sealed and wrapped with aluminium foil and left
overnight. The next morning the cover slip was removed, air dried and sputter-coated with gold,
mounted on the stub and examined in a LEO 435 VP scanning electron microscope. Results are
shown in Fig. 9B.
For motility tests TSA and DYGS media were used . The Agar concentration of both media
was changed (0.7%) so that the media were semisolid. For better growth visualization a phenol
dye of 1% was added. Strains were placed in plates with solid media (TSA and DYGS) where
isolated colonies were grown. The colonies were then placed in semisolid media. After 96 hours
of incubation at 280 C, bacterial growth was observed in the test tubes, and results are given in
the table in Fig. 9B.
Gum production determination
Strains were maintained both in autoclaved tap water at room temperature and at -80 C in NA
medium (3 g/l meat extract and 5 g/l peptone) containing 25% glycerol. The three strains were
picked from -80 C stock, streaked on solid TSA medium (10 g/l tryptone, 10 g/l sucrose, 1 g/l
sodium glutamate, and 15 g/l agar) and grown overnight at 28 C. One single isolated colony
from each strain was streaked again on solid TSA medium and grown overnight at 28 C. For
XAC, a single colony was inoculated into 20 ml of liquid TSA medium (10 g/l tryptone, 10 g/l
sucrose, 1 g/l sodium glutamate) in a 125 ml Erlenmeyer flask and incubated at 28 C in a rotary
shaker at 180 rpm for 17 hours (1.100 OD at 600 nm). A 125 ml Erlenmeyer flask containing 50
ml of liquid TSA medium was inoculated with 1 ml of XAC culture and incubated at 28 C in a
rotary shaker at 180 rpm for 5 hours (0.300 OD at 600 nm). This XAC culture was used as
inoculum for xanthan gum production. For XauB and XauC, an inoculating loop was used to
inoculate the bacteria from the solid TSA medium plates into 20 ml of liquid TSA medium in a 50
ml Falcon tube, followed by incubation at 28 C in a rotary shaker at 180 rpm for 24.5 hours
(0.260 and 0.550 OD at 600 nm for XauB and XauC, respectively). One ml of XauB and XauC
culture was inoculated into separate 125 ml Erlenmeyer flasks containing 50 ml of liquid TSA
medium and incubated at 28 C in a rotary shaker at 180 rpm for 15 hours (0.300 OD at 600
nm). The XauB and XauC cultures were used as inoculum for xanthan gum production.
For gum production, 2.5 ml of each bacterial strain in liquid TSA medium (0.300 OD at 600 nm)
was inoculated in three (triplicate) 250 ml Erlenmeyer flasks containing 100 ml of media for
xanthan gum production (25 g/l glucose, 3 g/l yeast extract, 2 g/l K2HPO4, 0.1 g/1 MgSO4.7H20,
pH 7.0 with 4 M HCI ) and incubated at 28 C in a rotary shaker at 178 rpm for 96 h. The
cells from the 96 h culture were centrifuged at 9.666 g for 40 min. The bacterial pellets were
stored at -20 C and the supernatants were transferred to 500 ml beakers. The gum was
recovered from the supernatants by alcohol precipitation. Four grams of KCI were added to
each beaker followed by agitation at room temperature for 15 min. Two volumes of cold
isopropyl alcohol were added and the gum from each beaker was removed to pre-weighted
plastic discs. After 16 h at 37 C the discs were weighted again and the gum amount was
calculated. The bacterial pellet from each culture was also obtained. For this, each pellet was
transferred to a pre-weighted beaker and weighted again after 14 h at 70 C.
From the shotgun libraries made for the sequencing of XAC strain 306 in 2001, 2,653 clones
were selected for the design of a glass slide hybridization array. Inserts from selected clones
were amplified by PCR with the M13-R and M13-F universal oligonucleotides. Products were
purified and placed in duplicate slides, resulting in 6,144 probes, with 768 positive controls and
624 negative controls. Experimental validation was done by hybridization of total XAC DNA
probes differentially stained, with 86% of all products with a hybridization signal greater than the
average value for the noise signal plus two standard deviations. The final XACarray contains
targets for the identification of 2,760 putative coding sequences (61% of all annotated protein-
coding genes). The XACarray was used to compare XAC and XauC, and XAC and XauB, with
XAC itself as a positive control. The results were similar (128 CDSs considered XAC-specific
had similar ratios for both the XauB and XauC experiments, and 101 of these are in XACSRs);
we report here detailed results only for the first experiment. Details of the array construction are
described elsewhere (Moreira LM, Laia ML, de Souza RF, Zaini PA, da Silva ACR, Ferro JA, da
Silva AM: Development and validation of a Xanthomonas citri subsp. citri DNA microarray
platform (XACarray) generated from the shotgun libraries previously used in the sequencing of
this bacterial genome, submitted).
Evidence for the absence of genes coding for XacPNP homologs in XauB and XauC was
obtained by PCR using gene-specific primers (Forward: GGACCAACAACGAATATC; Reverse:
ATGGGAATAGTCATGAAAC). XAC was used as positive control.
Assembly and genome annotation
Base calling, genome assembly and visualization were done with the phred-phrap-consed
package [99-101]. Contigs were trimmed to remove low phred quality regions at both ends. For
both XauB and XauC all contigs larger than 1 kbp have average phred quality greater than 20,
and nearly all (99% for XauB and 96% for XauC) these contigs have average phred quality
greater than or equal to 40 (i.e. accuracy equal to or better than 1 error in 10,000 bp). To
improve assembly many transposase sequences were masked, and reads were re-assembled
to obtain the final result. We estimated the fraction obtained of the complete genome of XauB by
dividing the total length of contigs by the total genome size of XAC, and by adding 1% because
of the removed transposases. We did the same with XauC. To estimate the average gap size
we divided the number of contigs by the estimated size of un-sequenced genome.
Using paired-end reads scaffolds for the XauB and XauC chromosomes were obtained. The
overall correctness of each scaffold was validated using a sliding-window GC-skew
computation. Each scaffold was aligned against the XAC chromosome using MUMmer .
Results are presented in additional file 1 (Fig. Sl).
The XauB and XauC genomes were automatically annotated with the Genome Reverse
Compiler , with a few manual refinements. Ortholog groups were built using OrthoMCL
For determination of XAC-specific regions we relied on published data about putative genomic
islands [14, 31, 105] and on AlienHunter results . Islands whose genes were not found in the
XauB and XauC genomes by BLAST  analysis and with differential hybridization signals in
the XACarray were considered XAC-specific regions.
Species tree. We used a supermatrix approach as in previous work . Protein sequences of
eleven Xanthomonas genomes ingroupp) and four Xylella genomes (outgroup) were clustered in
6,375 families using OrthoMCL . We then selected families with one and only one
representative from each of the ingroup genomes and at least one outgroup protein, resulting in
1,666 families. Their sequences were aligned using MUSCLE  and the resulting alignments
were concatenated. Non-informative columns were removed using Gblocks , resulting in
596,246 positions. RAxML  with the PROTGAMMAWAGF model was used to build the final
Pth tree. The same methodology as above was used, with the following differences.
Representative pth nucleotide sequences from Xanthomonas species were retrieved from
GenBank, and added to the set of XauB and XauC pth nucleotide sequences. A pth gene from
Ralstonia solanacearum [GenBank: CAD1557.1] was used as an outgroup in a preliminary
round of tree construction to ascertain root position. The Tandem Repeat Finder program 
with parameters 2,7,7,80,10,50,500,1 was used to mask the internal repeats. The masked
regions were removed and the resulting sequences were aligned with MUSCLE. A few manual
adjustments to the multiple alignment were made before running Gblocks, which yielded 1,679
positions. RAxML with the GTRGAMMA model was used to build the final tree.
The candidate T3SS effectors in the XauB and XauC genomes were identified using tBLASTn
 analysis and Pfam domain  searches. For tBLASTn analysis, all known plant and
animal pathogen effectors were used as query with an e-value threshold < 0.00001. Pfam
domains were searched for possible domains found in known effectors in the predicted set of
ORFs of draft genome sequences. Candidate effectors were classified according to the
nomenclature and classification scheme for effectors in xanthomonads recently described by
White et al. .
The draft genome sequences of XauB and XauC are available at GenBank under accession
numbers ACPX00000000 and ACPY00000000, respectively.
Abbreviations. CC: citrus canker; CDS: coding sequence; ORF: Open Reading Frame; PCR:
polymerase chain reaction; T3SS: Type III secretion system; T4SS: Type IV secretion system;
XAC: Xanthomonas citri subsp. citri; XACSR: XAC-specific region; Xau: Xanthomonas fuscans
subsp. aurantifolii; XauB: Xanthomonas fuscans subsp. aurantifolii strain B; XauC:
Xanthomonas fuscans subsp. aurantifolii strain C.
JAF and ACRS conceived the project and oversaw genomic sequencing. RPL provided the
strains. JCS oversaw all bioinformatics work, including assembly, annotation and other
specialized analyses. MYN, EHO, and LAD helped with genome assembly. LAD and EHO did
computational analyses. RIDT maintained the computational infrastructure. LMM conceived the
XACarray. LMM and ALJF carried out hybridization experiments. HAP and LASN did the motility
tests. EWK did microscopy analysis. DFRJ did the xanthan gum production experiment. JAF
oversaw final PCR experiments. APF ran PCR analyses. NFA helped with automated genome
annotation and manual refinement, created ortholog families, and generated the GenBank files.
NFA, SSA and JCS did the phylogenetic analyses. APF, AMS, HAP, FEM, JCB, JCFO, LMM,
LASN, LRF, MLL, MITF, RFS, and RIDT helped with data analyses. DJN and BJS helped create
an effector database. NP did the effector analysis. NP, JBJ, BAV, LMM, and JCS interpreted
effector analysis. LMM created all figures except Figs. 2 and 3. MLL, LRF, and JRN provided
preliminary sections of the manuscript. LMM, BAV, and JCS wrote the final manuscript. All
authors approved the final manuscript.
This work was supported by Fundag.o de Amparo a Pesquisa do Estado de Sao Paulo
(FAPESP), by Conselho Nacional de Desenvolvimento Cientlfico e Tecnol6gico (CNPq), by
Coordenag.o para Aperfeigoamento de Pessoal de Ensino Superior (CAPES), and by
FUNDECITRUS, all of them Brazilian funding agencies. We thank the following students for
their help: Aloisio J. Almeida Jr., Maria L. R. Carrera, Gustavo G. L. Costa, Daniel M. de Faria,
Daniel P. Gasparotto, Rafael A. Homem, Vanessa C. Morgan, Julia M. Perdigueiro, Marcelo C.
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Figure 1: Phenotype and Genotype. (A) Disease symptoms caused by XAC, XauB, and XauC
on leaves of three different citrus species. The lesions caused by XAC and XauB are similar but
differ in size. XauC causes a hypersensitive response in C. limonia and C. sinensis, with C.
aurantifolia as a true host. The pictures were taken 21 days after inoculation. (B) Venn diagram
showing the numbers of protein-coding orthologous genes shared among the three strains.
Figure 2: Maximum likelihood tree of fully sequenced Xanthomonas and Xylella
genomes. The bootstrap support is 100% for all branches (100 bootstrap runs). Bar, number of
amino acid substitutions per site.
Figure 3: Maximum likelihood tree of Xanthomonas pth genes. The sequences used to
build the tree had their tandem repeat portions masked before alignment. The root position was
obtained from a preliminary tree that included as an outgroup a Ralstonia solanacearum pth
gene [GenBank: CAD15517.1]. Full information about the pth gene sequences used is given in
additional file 6 (Table S6). The numbers on branches represent bootstrap support (from 100
bootstrap runs). The bootstrap values obtained for the clade indicated by the asterisk are
given in additional file 7 (Fig. S7). Bar, number of amino acid substitutions per site.
Figure 4: Region containing citrus-canker specific effector genes xopE3 and xopAI. The
XAC region depicted is hypothesized to be a genomic island . The central parts of this
region (gray areas) are CC-specific, being present in both XauB and XauC as well as in XAC
plasmid pXAC64. Gene mrntB is not an effector but plays a role in Type III secretion.
Figure 5: Genomotyping and similarity analysis between the genomes of XAC and XauC
based on DNA hybridization and matching of contig sequences. (A) The hybridization
results showed that 2,486 CDSs (out of a total 2,760) gave a hybridization signal greater than
the estimated background noise. Of the 2,486 CDSs, 2,341 (94.2%) seem to be present in
XauC (ratio Cy3/Cy5 between 0 and 1.5). The remaining 145 CDSs (5.8%) seem specific to
XAC. (B) Of these 145, most (101 = 70%) belong to regions previously described as putative
genomic islands in XAC (black ovals: ; green ovals: ; yellow ovals: ; the oval
numbering correspond to the original publication numbering). Pink ovals mark other nonsimilar
regions between the genomes of XAC and XauC. (C) The two bottommost graphs (G+C and
codon bias) show variation of these two metrics along the XAC genome, thus presenting
evidence for the putative genomic islands denoted by ovals. The numbered triangles
correspond to the XAC-specific regions based on DNA hybridization. Blue horizontal bars simply
denote regions in these graphs that correspond to ovals not associated with XAC-specific
regions. The Heat Map shows gene groups that yielded differential hybridization signals, the
vast majority of which correspond to regions marked by ovals. The zoomed-in regions in the
upper diagram show genes with differential hybridization signals but that are in regions shared
by the two genomes. The XAC and XauC genes in these regions have less similarity (from 42 to
68% identity) between them than other shared genes.
Figure 6: Models for sugar acquisition, quorum sensing modulation, and biofilm
formation and the XAC-specific genes and regions related to these processes. Region
XACSR17 (lower right corner) contains four genes that when mutated (separately) cause
decrease in biofilm activity and virulence, as indicated in the leaf photos. The photo labels are
mutant strain identifiers.
Figure 7: Schematic representation of Type IV Secretion System Genes in diverse
bacteria, grouped by their genomic architecture. The colors used for gene arrows
correspond to the colors used for their respective protein products in the secretion system
representation on the right. At: Agrobacterium tumefaciens C58; Lp: Legionella pneumophila;
Xfa: Xylella fastidiosa 9a5c; Rs: Ralstonia solanacearum; Xcc: Xanthomonas campestris pv.
campestris ATCC33913; Xoo: Xanthomonas oryzae pv. oryzae. References not cited in the text
are: Goodner et al. (2001) ; Segal et al. (1999) ; Cazalet et al. (2004) ; Bolland et
al. (1990) ; Marques et al. (2001) ; Lee et al. (2005) 
Figure 8: The chromosomal T4SS gene cluster in XAC. (A) This T4SS cluster is located
between regions XACSR12 and XACSR13. Genes shared with XauB and XauC are shown with
a black asterisk. Two of the genes are shared only by XAC and XauB (red asterisk). (B)
Interactions among T4SS proteins, based on data presented by Alegria et al. . Proteins
specific to XAC are represented in yellow, and proteins shared by XAC, XauB and XauC are
represented in orange. Six of the proteins colored in yellow are in XAC-specific regions. Genes
XAC0095 and XAC0096, although not part of XAC-specific regions, seem to play an especially
important role in pathogenicity. Under this model the protein coded for by XAC0095 interacts
with HrpG, a protein that participates in the T3SS apparatus , and that causes phenotypic
changes when its gene is mutated . The product of gene XAC0096, which is next to
XAC0095, interacts wirth VirD4, a component of the T4SS .
Figure 9: Genes related to flagellum synthesis and regulation. (A) Synteny and
hybridization results between XAC e XauB/XauC. In the upper left diagram, F1, F2 and F3 denote
gene regions related to flagellar functions, whereas gene groups XACSR9 and Y appear to
contain unrelated genes. Dashed boxes denote regions apparently absent from the XauB and
XauC genomes. Horizontal green bars represent hybridization signals for probes marked with
Cy3 and which are XAC-specific; yellow horizontal bars represent regions with positive
hybridization results. The zoomed-in region XACSR9 shows that it contains four transposases
(in blue), twelve hypothetical genes (in gray), and one gene with assigned function (in yellow).
That gene (XAC1927) codes for a Fe-S-oxidoreductase that contains a RADICAL SAM domain.
XAC mutants in this particular region (14H02 and 25D11 ) presented decreased virulence
phenotypes. (B) Microscopy and motility test of each organism. These results validate flagellum
presence and functionality in two different culture media (DYGS and TSA). (C) Flaggellum gene
organization in gene clusters F1, F2 and F3, as given in panel A. The color coding is the same for
panel D. (D) Representation of the flagellum proteins using the same color coding as in panel C.
Figure 10: LPS and O-antigen synthesis. (A) Model representing LPS and O-antigen
synthesis in Xanthomonas. Each of the numbers in this panel correspond to the same numbers
in panel B. The model is based on the one described by Vorholter et al.  for X. campestris.
Genes for products 12, 14, 17 and 18 are absent from XAC, XauB and XauC. (B) Genomic
context of the LPS synthesis-related genes in XAC and X. campestris. The flanking regions (in
pink) are conserved. The central region is not conserved between X. campestris on the one
hand and XAC, XauB and XauC on the other. Moreover there is a group of genes that is shared
and syntenic between the CC-species (in orange) and another (in green) that is specific to XAC
(XACSR18). (C) A XAC mutant for gene wzt showed a different virulence pattern . The wzt
homolog in XauB and XauC does not have the C-terminal portion when compared to the XAC
wzt gene. (D) Region XACSR1 contains several genes related to LPS biosynthesis. Numbers
above gene arrows correspond to the annotations immediately below in this panel.
Table 1. General features of XAC, XauB, and XauC genomes.
Feature XAC XauB* XauC*
size (bp) 5,274,174 4,877,808 5,012,633
# contigs 3 239 351
%GC 64.7 64.9 64.8
total 4,427 3,804 3,921
with functional assignment 2,779 2,694 2,728
conserved hypothetical 1,386 993 1,009
hypothetical 262 117 184
rRNA operons 2 2 2
tRNAs 54 51 51
For each strain the numbers are totals for the chromosome plus plasmids. The asterisk (*) denotes that
the numbers for XauB and XauC come from their respective incompletely sequenced genomes.
Table 2: Putative effectors found in the XAC, XauB, and XauC genome sequences.
Effector family XAC XauB XauC Pfam: functional/ Effectors
Candidate effectors common to XAC, XauB, and XAUC
AvrBs2 XAC0076 XAUB_16770 XAUC_23650 Glycerophosphoryl AvrBs2 from
diester X. campestris
AvrBs3 XACa0022 XAUB_40130 XAUC_22430 Transcriptional PthA 
(pthAl) XAUB_28490 XAUC_24060 activator, nuclear
XACa0039 XAUC 09900 localization
XopE1 XAC0286 XAUB_37010 XAUC_37580 Putative AvrXacE1,
(avrXacEl, transglutaminase XopE1 from X.
hopX, avrPphE) campestris pv.
XopE3 XAC3224 XAUB_14680 XAUC_00040 Putative AvrXacE2
(avrXacE2, transglutaminase 
XopF2 XAC2785 XAUB_07540 XAUC_21000 XopF2 
XopI XAC0754 XAUB_39080 XAUC_07100 F-box protein X. campestris
XopK XAC3085 XAUB_34090 XAUC_12520 Identified in
Xoo by cya
XopL XAC3090 XAUB_34130 XAUC_02900/ LRR protein 
XopQ(hopQl) XAC4333 XAUB_10220 XAUC_14670 Inosine uridine 
XopR XAC0277 XAUB_36920 XAUC_37490 Identified in
Xoo by cya
XopV XAC0601 XAUB_23140 XAUC_21260 Identified in
Xoo by cya
XopX XAC0543 XAUB_14760 XAUC_20690 
XopZ (HopAS, XAC2009 XAUB_ 11532/ XAUC_25915 
AWR) 13710 LP
XopAD (skwp, XAC4213 XAUB_02510 XAUC_34870 SKWP repeat Skwp from
RSc3401) protein Ralstonia 
XopAI (HopO1 XAC3230 XAUB_26830 XAUC_23780 ADP- 
XopAK (HopAK1 XAC3666 XAUB_02580 XAUC_32490 Not confirmed
(HopPtoK, to be effector
terminal domain) Xanthomonas;
HrpW (PopW) XAC2922 XAUB_19460 XAUC_20020 Pectate lyase, may 
(associated (associated not be T3SE
with hrp with hrp
Candidate effectors present in XAC and XauB BUT ABSENT in XauC
XopE2 XACb0011 XAUB_31660 Putative XopE2 found
(avrXacE3, transglutaminase in another C
avrXccE1) strain 
XopN (hopAU1) XAC2786 XAUB_07520 ARM/HEAT repeat 
XopP XAC1208 XAUB_06720 
XopAE XAC0393 XAUB_19500 LRR protein Xcv8510 
Candidate effectors present in XauB and XauC BUT ABSENT from XAC
XopB (hopD1, XAUB_09070/ XAUC_00260 
avrPphDl) 14842 LP
XopE4(HopX) XAUB_23330 XAUC_31730 New class
XopJ (AvrXccB) XAUB_20830 XAUC_08850 C55-family cysteine 
protease or Ser/Thr
XopAF (avrXv3, XAUB_02310 XAUC_00300 
XopAG (AvrGfl, XAUB_03570 XAUC_04910 AvrGfl 
Candidate effectors present only in XauC
XopF1 (Hpa4) XAUC_20060 
Some of the effectors appear to be pseudogenes (indicated by a L). In column effectorr family' we
provide the standard effector name along with the names of related proteins belonging to the same
family; in column effectorss' we provide the reference where the effector indicated was characterized.
Table 3: Xanthan gum production.
Strain Pellet weight Gum production
mg/ml (sd) mg/ml (sd)
XAC 2,57 ( 0,25) 3,93 ( 0,83)
XauB 1,70 (0,20) 1,62 (0,50)
XauC 2,35 ( 0,92) 1,82 ( 0,33)
sd Standard deviation
Additional file 1 Figure S1: whole chromosome alignments of XauB and XauC scaffolds
Additional file 2 Table S2: genes shared by XAC, XauB, and XauC but that are not found in
other fully sequenced Xanthomonas and Xylella genomes.
Additional file 3 Table S3: List of XAC-specific regions.
Additional file 4 Figure S4: gene diagrams showing the contents of all 25 XAC-specific
Additional file 5 Figure S5: PCR results for gene xacPNP.
Additional file 6 Table S6: names, organisms, locus tags, and accession numbers for the
nucleotide sequences used to build the pth gene phylogeny (Fig. 3).
Additional file 7 Figure S7: topology of clade in Fig. 3 with bootstrap values.
I 0.05 1
Xanthomonas fuscans subsp. aurantifolii B
Xanthomonas fuscans subsp. aurantifolii C
Xanthomonas citri subsp. citri
Xanthomonas campestris vesicatoria 85-10
Xanthomonas oryzae PXO99A
Xanthomonas oryzae MAFF 311018
Xanthomonas oryzae KACC 10331
Xanthomonas campestris B100
Xanthomonas campestris 8004
Xanthomonas campestris ATCC 33913
-Xylella fastidiosa 9a5c
Xylella fastidiosa M12
Xylella fastidiosa M23
Xylella fastidiosa Temeculal
I 0.01 I
XAUC 24060 09900
- XAUC 22430
- avrB6 Xoo
7 260 bp (identities = 95%)
m I I
m I I
34 5 6 789 10 11 121314 15 16 17
1 2 1 3 4 2 34 5
1 2 3 4 5 6 78 9 10 11 1213
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)008 0009/0010 0011 0012 0013 0014 0015 0016 0017
F:H K L M-------------- 0 ---------------
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:..... H. .-----.. ---------------------------
104 105 106 107 108 109 110 111 112
B56 --B8 9 -------------------------
B5 B6 >--
0040 0039 0038 0037 0036
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37050 37040 37030 37020 37010
SB8 B91 BlOlBli B1 ---------------------------
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I I I I I I I I
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' *.*. "" ,* ^ *.*: '.- it, **'.* .'.'*. / .'' i
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CITRUS CANKER STRAIN
11 12 13
Additional files provided with this submission:
Additional file 1: figS1 .ppt, 153K
Additional file 2: tableS2.doc, 137K
Additional file 3: tableS3.doc, 93K
Additional file 4: figS4.ppt, 401 K
Additional file 5: figS5.ppt, 230K
Additional file 6: tableS6.doc, 46K
Additional file 7: figS7.pdf, 23K