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A cellulase as an essential virulence factor of Clavibacter michiganensis subsp. michiganensis causing bacterial canker in tomato

In Sun Hwang: Department of Horticultural Biotechnology, Kyung Hee University

<div><em>Clavibacter michiganensis</em> (<em>Cm</em>) subsp. <em>michiganensis</em> causes bacterial canker in tomato plants. This bacterial pathogen carries the plasmid (pCM1)-borne cellulase gene, <em>celA</em> that is required for its pathogenicity in plants. CelA consists of the cellulase domain, the cellulose binding domain, and an expansin domain in its C-terminus. Based on the previous reports, the removal of pCM1 from <em>Cm</em> subsp. <em>michiganensis</em> strain NCPPB382 caused very mild and delayed wilting symptoms in tomato. In addition, this strain carries the second cellulase gene, <em>celB</em>, which has not been functionally characterized. To better understand how cellulase genes contribute to the pathogenicity of <em>Cm</em> subsp. <em>michiganensis</em>, <em>celA </em>mutants of <em>Cm</em> subsp. <em>michiganensis</em> type strain LMG7333 were generated by transposon mutagenesis, and these mutants almost lost the ability to cause severe wilting symptoms in tomato. This strain was complemented with full or truncated forms of <em>celA</em> or <em>celB</em> genes. The truncated <em>celA</em> gene only carrying two domains, cellulase and cellulose binding domains, was enough to recover the ability to cause severe wilting symptoms in tomato. Moreover, the functional <em>celB</em> genes could complement <em>celA</em> mutants only partially. The <em>celA </em>overexpression in <em>Cm</em> subsp. <em>capsici</em> strain that originally lacks <em>celA</em> and almost non-pathogenic in tomato was able to cause disease symptom in tomato. These results indicate that the <em>celA</em> gene acts as a major virulence factor in a host-specific manner.</div>