Analysis Of Flax and Stem Rust Effectors

Stem rust caused by the fungus Puccinia graminis tritici is one of the most serious diseases in wheat and is combated by the use of resistant varieties. Because the fungus evolves increased virulence towards resistant varieties, continuous breeding is necessary to keep apace of rust epidemics. Rust resistance in wheat depends on resistance proteins that recognize rust effector proteins called avirulence proteins. To understand how effectors are recognized by resistance proteins and how they contribute to disease in the absence of host resistance, we are using genome information to predict effector candidates genes encoding small proteins with a secretory signal that are expressed in haustoria (infection structure) in the Australian stem rust strain 21-0. We have predicted 400 candidate genes. To identify specific effectors we use mutant analysis and functional assays based on effector protein delivery to wheat plants carrying one or more of 27 different resistance genes. For the mutation analysis we compare the effector complement and its transcription in 21-0 to mutant derivatives of this strain that have occurred in the field or in induced mutation experiments and that have overcome the resistance genes Sr5, Sr11, Sr27 and SrSatu. For the functional assay we use Type III Secretion System (TTSS)-based delivery of effectors from the non-pathogen Pseudomonas fluorescens strain Pfo that has been engineered for a TTSS (Thomas et al 2009, Plant J.60:919). Among 46 candidates tested, one protein induces a rapid cell death response specifically on a wheat genotype carrying Sr22.