Bacterial Wilt of Forage Grasses: Insights Into Plant Resistance and Pathogen Virulence.

Bacterial wilt is one of the most serious diseases of temperate forage and turf grasses, such as ryegrasses and fescues, leading to significant losses in forage yield and may drastically reduce turf 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 optimise breeding procedures. Although a major QTL explaining up to 84% of resistance was reported, no major host genotype – pathogen isolate interactions were identified using phenotypic and SSR marker analysis. This suggests that Xtg resistance, so far observed, is effective across a broad range of different bacterial isolates and plant genotypes. In order to gain further insight into this complex host-pathogen interaction, factors influencing pathogen virulence were investigated in Xtg using conserved primer approaches and whole genome sequencing. 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. In conclusion, our investigations on the Xtg x Lm interactions provide fundamental insights for the development of innovative resistance breeding approaches.