Multiple plasmid‐borne virulence genes of Clavibacter michiganensis ssp. capsici critical for disease development in pepper

Abstract

Clavibacter michiganensis ssp. capsici is a Gram-positive plant-pathogenic bacterium causing bacterial canker disease in pepper. Virulence genes and mechanisms of C. michiganensis ssp. capsici in pepper have not yet been studied. To identify virulence genes of C. michiganensis ssp. capsici, comparative genome analyses with C. michiganensis ssp. capsici and its related C. michiganensis subspecies, and functional analysis of its putative virulence genes during infection were performed. The C. michiganensis ssp. capsici type strain PF008 carries one chromosome (3.056 Mb) and two plasmids (39 kb pCM1 and 145 kb pCM2 ). The genome analyses showed that this bacterium lacks a chromosomal pathogenicity island and celA gene that are important for disease development by C. michiganensis ssp. michiganensis in tomato, but carries most putative virulence genes in both plasmids. Virulence of pCM1 -cured C. michiganensis ssp. capsici was greatly reduced compared with the wild-type strain in pepper. The complementation analysis with pCM1 -located putative virulence genes showed that at least five genes, chpE, chpG, ppaA1, ppaB1 and pelA1, encoding serine proteases or pectate lyase contribute to disease development in pepper. In conclusion, C. michiganensis ssp. capsici has a unique genome structure, and its multiple plasmid-borne genes play critical roles in virulence in pepper, either separately or together.

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

“…Fifty-four C. capsici natural isolates were isolated from pepper plants in Korea, and their identity was confirmed by PCR with a speciesspecific primer set and DNA sequencing (Table S1) (Hwang et al, 2018). Their virulence was determined by checking their ability to cause necrosis in pepper leaves.…”

“…pCM Cc deletion by plasmid curing in C. capsici resulted in significantly reduced virulence compared with the virulence of the wild-type strain (Hwang et al, 2018). The introduction of pCM1 Cc -borne chpG, encoding a putative serine protease, into the pCM1 Cc -cured strain of C. capsici PF008 restored its virulence in pepper, indicating that chpG is an important virulence gene in C. capsici for infection.…”

“…type strain PF008 carries one chromosome and two plasmids, pCM Cc and pCM2 Cc (Oh et al, 2016). However, C. capsici PF008 lacks PAI, but carries a part of the C. michiganensis PAI region in both plasmids (Hwang et al, 2018). At least five virulence genes in the PAI region of C. michiganensis have been found in pCM1 Cc and pCM2 Cc of C. capsici.…”

“…Artificial curing of virulence plasmid(s) in either C. michiganensis or C. capsici causes significant reduction of virulence (Jahr et al, 2000;Hwang et al, 2018). In C. michiganensis and C. capsici, the ability of a pathogenic isolate to be converted into a nonvirulent or low-virulence variant depends on the presence/absence of plasmid(s), suggesting the evolutionary role of plasmid(s) in virulence in host plants (Vivian et al, 2001;Thapa et al, 2017).…”

A horizontally acquired expansin gene increases virulence of the emerging plant pathogenErwinia tracheiphila

“…Expansin function has also been investigated in several pathogens. In the bacterial plant pathogen Ralstonia solanacearum , an expansin deletion mutant has decreased virulence (24), and in Clavibacter michiganensis , studies have described contradictory expansin roles for virulence and ability to colonize xylem (24-28). Overall, fundamental questions surrounding how microbial expansins mediate plant colonization in divergent genetic backgrounds and variable ecological contexts, and the molecular mechanism(s) by which microbial expansins interact with plant structural carbohydrates remain enigmatic (7, 8, 10, 19, 20, 24, 26, 29, 30).…”