Plasmid composition and the chpG gene determine the virulence level of Clavibacter capsici natural isolates in pepper

摘要

The gram‐positive bacterial species causes necrosis and canker in pepper plants. Genomic and functional analyses of type strain PF008 have shown that multiple virulence genes exist in its two plasmids. We aimed to identify the key determinants that control the virulence of . Pepper leaves inoculated with 54 natural isolates exhibited significant variation in the necrosis. Six isolates showed very low virulence, but their population titres in plants were not significantly different from those of the highly virulent isolates. All six isolates lacked the pCM1 plasmid that carries , which has been shown to be required for virulence and encodes a putative serine protease, but two of them, isolates 1,106 and 1,207, had the intact elsewhere in the genome. Genomic analysis of these two isolates revealed that was located in the pCM2 plasmid, and two highly homologous regions were present next to the locus. The expression in isolate 1,106 was not induced in plants. Introduction of of the PF008 strain into the six low‐virulence isolates restored their virulence to that of PF008. Our findings indicate that there are at least three different variant groups of and that the plasmid composition and the gene are critical for determining the virulence level. Moreover, our findings also indicate that the virulence level of does not directly correlate with bacterial titres in plants.