P-172 - How Does Wheat Powdery Mildew Evade from Recognition by Major Resistance Proteins?

Canonical gene-for-gene interactions, in which a resistance (R) protein from a host plant recognises an avirulence (Avr) protein secreted by a pathogen, have been well studied in many pathosystems. In wheat powdery mildew, at least eight R-Avr pairs have been cloned, providing a basis for understanding the genetic and molecular aspects of this interaction. These studies often reveal a more complex system that deviates from the classical gene-for-gene model. The discovery of SvrPm3, a suppressor of AvrPm3 recognition, was the first indication that suppressors of avirulence serve as a recognition evasion strategy in wheat powdery mildew. We confirmed this with the identification of SvrPm4, which suppresses AvrPm4 recognition by the non-canonical R protein Pm4. Furthermore, through a mutagenesis screen, we described Bgt-646, a positive regulator of Avr genes, whose disruption leads to loss of avirulence on the R genes Pm3a and WTK4, two very different types of immune receptors. Finally, we also showed that not only sequence diversification and the presence of additional genetic components, but also transposable element insertions near Avrs, with consequent reduction in gene expression, are common mechanisms for evading recognition. Our studies of Avr determinants show that wheat powdery mildew employs a wide range of evasion strategies, suggesting that complex, multi-component interactions are the rule rather than the exception in this pathosystem.