Characterization of a foxtail mosaic virus vector for rust fungus avirulence gene expression in wheat

The wheat fungal pathogens, Puccinia graminis f.sp. tritici (Pgt), P. striiformis f.sp. tritici, and P. triticina, causing stem, stripe and leaf rust respectively, pose a threat to global wheat production. Genetic resistance in the form of resistance (R) genes provides the best protection but rust fungal populations change frequently by mutating avirulence (AVR) effectors matching specific R genes, thereby defeating resistance. Hence, characterization of AVR effectors is needed to understand the evolution of pathogen populations, provide insights for extending the effectiveness of R genes, and yield tools for the identification and isolation of R genes. Functional characterization of Avr genes in rust fungi is challenging in these biotrophic pathogens that lack a reliable and efficient transformation system. Studies indicate that the recently engineered foxtail mosaic virus (FoMV) shows promise as an expression system in cereals. In this study, we utilized two confirmed AVR effectors from Pgt, AVRSr35 and AVRSr50, to assess the applicability of FoMV for investigating rust fungus Avr genes. We showed that vector FoMV PV101 carrying PgtAvrSr35 induced a hypersensitive response (HR) in wheat having the corresponding Sr35 resistance gene. However, when carrying PgtAvrSr50, no HR or even mild viral symptoms were seen in a wheat line having Sr50, as this particular wheat line was not susceptible to FoMV. Several wheat cultivars did not support FoMV replication. Results here show that FoMV PV101 is effective as an expression vector for studying rust fungi AVR effectors, but its applicability relies on the susceptibility of wheat cultivars to FoMV.