Comparative analysis of the avirulence effectors produced by the fungal stem rust pathogen of wheat.

Crops are constantly exposed to pathogenic microbes. Rust fungi are examples of these harmful microorganisms which have a major economic impact on wheat production. To protect themselves from pathogens like rust fungi, plants employ a multi-layered immune system that includes immunoreceptors encoded by resistance genes. Significant efforts have led to the isolation of numerous resistance genes against rust fungi in cereals, especially in wheat. However, the evolution of virulence of rust fungi hinders the durability of resistance genes as a strategy for crop protection. Rust fungi like other biotrophic pathogens secrete an arsenal of effectors to facilitate infection, and these are the molecules that plant immunoreceptors target for pathogen recognition and mounting defence responses. When recognized these effector proteins are referred to as avirulence (Avr) effectors. Despite the many predicted effectors in wheat rust fungi only five Avr genes have been identified, all from wheat stem rust. Knowledge of the Avr genes and their variation in the fungal population will inform deployment of the most appropriate wheat disease resistance genes for breeding and farming. The review provides an overview of methodologies as well as the validation techniques that have been used to characterize Avr effectors from wheat stem rust.