| Summary: | 碩士 === 國立中興大學 === 植物病理學系所 === 97 === Bacterial spot disease of pepper and tomato caused by Xanthomonas campestris
pv. vesicatoria (Xcv) is a major problem where high temperatures and high humidity
occur simultaneously. The pathogenicity of Xanthomonas spp. depends on a
functional Type III secretion system (T3SS) and an array of effectors translocating
through the T3SS. One effector of interest found in nearly all examined Xanthomonas
spp. is AvrBs2. AvrBs2 is readily recognized by the R protein of Bs2 in resistant
plants and is involved in promoting virulence in susceptible plants by unknown
mechanisms. Sequence analysis reveals the amino acid residues 271-422 of Xv
AvrBs2 are homologous to the catalytic domain residing in agrocinopine synthase
(encoded by acs) of Agrobacterium tumefaciens and glycerophosphoryl diester
phosphodiesterase (GDPD, encoded by ugpQ) of E. coli. In addition, diverse avrBs2
alleles identified in other Xanthomonas spp. are highly conserved in the catalytic
domain, suggesting it is important for AvrBs2 activities in planta. Genetic studies of
AvrBs2 function are done using gain-of-function and loss-of-function strategies in X.
vesicatoria Taiwan strain Xvt147 and Xanthomonas campestris pv. raphani (cxsp3)
Taiwan strain cxsp3, respectively. Xvt147 avrBs2 is mutated by marker exchange and
the resultant mutant phenotypes are assayed in tomato. The degrees of symptom
development and bacterial multiplication of Xvt147 avrBs2 mutant are comparable
to the wild type strain. Nevertheless, overexpression of AvrBs2 in Xvt147 elicits
severe symptoms on tomato when infected plants are subjected to short-term water
deficiency. Heterologous expression of AvrBs2 in cxsp3 lacking the avrBs2 gene
slightly increases bacterial growth at 1 dpi and delays the onset of symptom
development in cabbage, indicating Xvt147 AvrBs2 might affect plant biochemical
activities for symptom production. Interestingly, the expression of AvrBs2 in cxsp3
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prevents the formation of bacterial aggregates that are usually seen in the stationary
phase-grown bacteria, suggesting the functions of AvrBs2 might be important for
bacterial adaptation in vitro and in planta.
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