Determining the basis of nonhost resistance in rice to cereal rusts

Cereal rusts are a constant disease threat that limits the production of almost all agricultural cereals. Rice is atypical in that it is an intensively grown agricultural cereal that is immune to rust pathogens. This immunity is manifested by nonhost resistance (NHR), the mechanisms of which are poorly understood. As part of the Borlaug Global Rust Initiative (BGRI), studies are being undertaken to dissect the molecular mechanisms that provide rust immunity in rice and determine if they can be transferred to wheat via transgenesis. Microscopic analyses showed that cereal rusts are capable of entering the rice leaf via formation of an appressorium over a stomate and subsequent infection of underlying mesophyll cells. However, there is considerable variation in the extent of colonization at each infection site. Our research effort has focused on screening for increased growth of cereal rust using natural and induced variants of rice. Two collections of rice mutants, T-DNA insertional mutants and chemical/irradiation-induced mutants, and diverse germplasm accessions are being screened for compromised NHR to cereal rusts. Preliminary screening with stripe rust identified several potential mutants that allow increased fungal growth. The confirmation of these lines will serve as the foundation for the isolation of gene(s) responsible for this compromised resistance. Details of the strategies being undertaken and progress to date are provided.