P-123 - Effector-mediated suppression of effector-triggered cell death

The biotrophic smut fungus Ustilago maydis infects maize and causes major transcriptional, metabolic and developmental changes including gall formation in its host. The fungus secretes effector proteins during infection, with functional roles in plant immunity suppression and cell growth and division deregulation for several of these now described. Surprisingly on the other hand, recent studies demonstrate that some effector proteins also trigger programmed cell death (PCD) when expressed alone in plant systems, a defence response preventing the spread of biotrophs. This implicates that effector functions need to be thought in the context of the whole effectorome. At the same time, many U. maydis secreted proteins have no obvious functional role, yet are co-expressed during infection and often genomically linked to PCD-inducing effectors. We hypothesised some of these have evolved to suppress effector-triggered immunity. To test this, we co-expressed a library of secreted proteins alongside characterised PCD-inducing effectors in Nicotinia benthamiana. Results indicate several candidates have the potential to suppress PCD. Whether this is through direct interaction with plant immune-system components or through binding and shielding the recognised fungal-effector domain/motif is currently under investigation. How U. maydis evades plant immunity has been a longstanding question and the results of the ongoing research will be presented and discussed in this context.