Understanding resistance gene mediated recognition of stem rust in wheat
Stem rust caused by Puccinia graminis tritici (Pgt) is one of the most serious diseases in wheat and is combated mainly through the use of resistant varieties. Because the fungus evolves virulence towards previously resistant varieties, continuous breeding and identification of new sources of resistance are necessary to combat the threat of rust epidemics. Our work on the flax rust model system has provided insights into how the plant immune system recognises and responds to rust pathogens. We have been extending this work to wheat stem rust by targeted cloning of resistance (R) genes in wheat and corresponding Avr genes in Pgt. Plant R genes encode immune receptors that recognise and respond to pathogen effector proteins delivered into host cells from haustoria. We recently isolated the Sr33 and Sr50 resistance genes from wheat and have begun functional analyses to determine how they trigger defense responses. We are also targeting effectors from Pgt that are recognised by wheat R genes. We used genome and transcriptome sequencing to predict ~400 candidate effector genes from Australian Pgt race 21- 0. To screen for recognition of these proteins by wheat R genes, we developed a bacterial Type III Secretion System delivery assay using Pseudomonas fluorescens to inject the effector candidates into wheat leaf cells. We are screening candidate effectors on a set of 18 wheat cultivars carrying 22 different R genes and have so far identified one effector that induces a cell death response specifically on a wheat genotype carrying Sr22. Understanding the nature of wheat R genes and the Avr proteins that they recognize will allow better prediction of R gene durability and enable the possibility of rational design of novel R genes. We are also developing techniques for stacking R genes in cassettes for deployment of multiple genes at a single locus in wheat.