Novel insights into the genomic basis of citrus canker based on the genome sequences of two strains of Xanthomonas fuscans subsp. aurantifolii

Abstract

BackgroundCitrus 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.ResultsWe 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 effector genes, xopE3 and xopAI, are also present in all three strains and are both located on the same putative genomic island. These two effector genes, along with one other effector-like gene 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.ConclusionWe 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 canker control.

Lateral organ boundaries 1 is a disease susceptibility gene for citrus bacterial canker disease

“…Genome sequences are available for representative strains of Xcc type A, Xcc type A w , Xfa type B, and Xfa type C (9)(10)(11). Both Xcc and Xfa contain type III secretion systems (T3SS), although the contributions, biochemically and functionally, of the individual substrate effectors are unknown.…”

“…Both Xcc and Xfa contain type III secretion systems (T3SS), although the contributions, biochemically and functionally, of the individual substrate effectors are unknown. Xcc-A and Xcc-A w contain 24 and 30 putative type 3 secretion (T3S) effectors, respectively, whereas Xfa-B and Xfa-C contain 27 and 26 T3S effectors, respectively (10). Loss of T3SS function in Xcc results in complete loss of disease symptoms and reduced bacterial populations in host tissue.…”

Identification of New Genes Related to Virulence of <i>Xanthomonas axonopodis</i> Pv. <i>Citri</i> during Citrus Host Interactions

“…After inoculation, plants were grown in a room at 28˚C with artificial light 3000 lux with 12 h photoperiod by 16 days. Evaluation of symptoms was carried out over 16 days (4,8,12 and 16 days after inoculation) and documented by digital photography. Those mutants that were unable to induce the pathogenic process or led to a difference in the symptoms, compared to that observed for the wt strain were selected.…”

“…citri A* and A W , as well as X. fuscans subsp. aurantifolli strains B and C also cause citrus canker [4] [5] [6]. Their importance is attributed to their ability to infect only specific species of citrus, since they are restricted to certain regions in the world or vary considerably in their virulence induction profiles which are generally weaker than that observed for canker A.…”

“…Their importance is attributed to their ability to infect only specific species of citrus, since they are restricted to certain regions in the world or vary considerably in their virulence induction profiles which are generally weaker than that observed for canker A. For these reasons, the genomes of these bacteria were also completely or partially sequenced [4] [7] [8], thus enabling a comparative genomic analysis would guide the search for genes that possibly explain the role of biological variants, leading to a better characterization of the pathosystem as well as its relationships with respective hosts.…”