Breeding for Resistance to Bacterial Wilt in Ryegrass: Insights into the Genetic Control of Plant Resistance and Pathogen Virulence

Abstract

Bacterial wilt caused by Xanthomonas translucens pv. graminis (Xtg) is a severe disease of forage grasses leading to drastic losses in pure and mixed stands. Italian ryegrass (Lolium multiflorum) is particularly susceptible to bacterial wilt and breeding for resistance is the only practicable means of disease control. A detailed understanding of the genetic control of this complex host-pathogen interaction is indispensible for the further development of L. multiflorum cultivars with increased resistance to bacterial wilt and to refine and optimise breeding procedures. While several recent studies have revealed novel insights on plant resistance, little is known about the processes involved in host-colonization and disease development. Therefore, factors influencing pathogen virulence were investigated in Xtg using conserved primer approaches and whole genome sequencing. Knock-out mutation of 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 conlusion, our investigations on the Xtg x L. multiflorum interactions provide fundamental insights for the development of innovative resistance breeding approaches. IntroductionBacterial wilt caused by Xanthomonas translucens pathovars is estimated to account for annual forage yield losses of up to 20 % in swards (Schmidt and Nuesch 1980), but yield losses of up to 80 % have been reported from experiments involving artificial leaf inoculation (Wang and Sletten 1995