RESUMO
Citrus canker disease is caused by five groups of Xanthomonas citri strains that are distinguished primarily by host range: three from Asia (A, A*, and A(w)) and two that form a phylogenetically distinct clade and originated in South America (B and C). Every X. citri strain carries multiple DNA fragments that hybridize with pthA, which is essential for the pathogenicity of wide-host-range X. citri group A strain 3213. DNA fragments that hybridized with pthA were cloned from a representative strain from all five groups. Each strain carried one and only one pthA homolog that functionally complemented a knockout mutation of pthA in 3213. Every complementing homolog was of identical size to pthA and carried 17.5 nearly identical, direct tandem repeats, including three new genes from narrow-host-range groups C (pthC), A(w) (pthAW), and A* (pthA*). Every noncomplementing paralog was of a different size; one of these was sequenced from group A* (pthA*-2) and was found to have an intact promoter and full-length reading frame but with 15.5 repeats. None of the complementing homologs nor any of the noncomplementing paralogs conferred avirulence to 3213 on grapefruit or suppressed avirulence of a group A* strain on grapefruit. A knockout mutation of pthC in a group C strain resulted in loss of pathogenicity on lime, but the strain was unaffected in ability to elicit an HR on grapefruit. This pthC- mutant was fully complemented by pthA, pthB, or pthC. Analysis of the predicted amino-acid sequences of all functional pthA homologs and nonfunctional paralogs indicated that the specific sequence of the 17th repeat may be essential for pathogenicity of X. citri on citrus.
Assuntos
Proteínas de Bactérias/fisiologia , Citrus/microbiologia , Xanthomonas/patogenicidade , Sequência de Aminoácidos , Proteínas de Bactérias/química , Proteínas de Bactérias/genética , Dados de Sequência Molecular , Folhas de Planta/microbiologia , Alinhamento de Sequência , Análise de Sequência de Proteína , Homologia de Sequência de Aminoácidos , Especificidade da Espécie , Sequências de Repetição em Tandem , Virulência , Xanthomonas/genética , Xanthomonas/fisiologiaRESUMO
ABSTRACT Xanthomonas axonopodis pv. allii is phenotypically and genetically diverse and its relationship to other X. axonopodis pathovars within DNA homology group 9.2 is unknown. In growth chamber experiments, disease symptoms were produced on onion only by inoculation with X. axonopodis pv. allii. Citrus bacterial spot symptoms were induced by X. axonopodis pvs. alfalfae, itrumelo, and allii on Duncan grapefruit and key lime. X. axonopodis pv. allii multiplication and persistence in Duncan grapefruit were equal to those of an aggressive strain of X. axonopodis pv. citrumelo, but populations of X. axonopodis pvs. alfalfae, betlicola, citrumelo, phaseoli, and vesicatoria were 1.3 to 4.0 log units less than X. axonopodis pv. allii in onion. Genomic fingerprinting by repetitive sequence- based polymerase chain reaction demonstrated that X. axonopodis pvs. allii, alfalfae, and citrumelo are distinct from other Xanthomonas species and X. axonopodis pathovars, but these pathovars were indistinguishable from each other. Three genotype groups were apparent among DNA homology group 9.2 strains, and generally correspond to the aggressiveness and genotype groups previously described for X. axonopodis pv. citrumelo. X. axonopodis pvs. allii, alfalfae, and citrumelo appear to have recently diverged from a common ancestral strain.
