Virulence Factors and Genome Sequencing of the Bacterial Wilt Causing Pathogen X. translucens pv. graminis

Bacterial wilt is one of the most serious diseases of temperate forage grasses, such as ryegrasses and fescues, leading to significant losses in forage yield and quality. The disease is caused by X. translucens pv. graminis (Xtg), a gram-negative bacterium that enters the plants primarily through wounded tissue, multiply in intercellular spaces and spread through the xylem. Infected, susceptible plants wilt within a few days after infection. A detailed understanding of the genetic control of this complex host-pathogen interaction is indispensible for the further development of Lm cultivars with increased resistance to bacterial wilt and to refine and optimize breeding procedures. In order to gain further insight into this complex host-pathogen interaction, factors influencing pathogen virulence were investigated based on whole genome sequencing of two Xtg isolates and one isolate of the closely related pathovar arrhenatheri (Xta). Genomes were compared for differences in genes encoding virulence factors and effector proteins. Knock-out mutation of several components of the type three secretion system (T3SS), a major virulence factor in many Xanthomonas spp., showed that the T3SS is important for Xtg virulence but not for in planta multiplication. Analysis of the draft genome sequences revealed a substantial number of effectors apparently characteristic for Xtg and considerable differences to Xta, such as the absence of genes encoding flagella or the only partially represented type four secretion system (T4SS). In conclusion, our investigations on virulence factors of Xtg provide fundamental insights for the development of innovative resistance breeding approaches.