Winning by Losing: Exploiting Modified Plant Susceptibility Genes to Counteract Necrotrophic Fungal Pathogens

The infection of plants by pathogens is an intricate process in which genes from both the host and pathogen contribute to the infection process. Susceptibility (S) genes have been defined as plant genes that encode functions that are exploited by pathogens to invade and reproduce in host plants. Mutations in S-genes therefore result in reduced susceptibility to the pathogen. The identity and mode of action of S-genes in interactions of plants with biotrophic fungi have been studied since the cloning of the barley mlo gene in the late 1990s. The infection strategy of necrotrophic fungi, however, substantially differs from that of biotrophic fungi and therefore is likely to be facilitated by distinct physiological processes in a host plant. There is a rapidly increasing amount of information about S-genes in plants that facilitate the infection by necrotrophic fungi and their mode of action. In this review, we summarise the current knowledge on plant S-genes for susceptibility to necrotrophic fungi and categorise them based on cellular compartments and physiological processes. With recent examples, we subsequently discuss the challenges and opportunities of exploiting impaired plant S-genes for breeding crops with disease resistance.