Wall Associated Kinases; Subversion by Necrotrophs to Cause Disease in the Cereals

Plant resistance against biotrophic pathogens relies on immunity receptor-mediated programmed cell death (PCD) that sequesters the pathogen and inhibits further colonization. Necrotrophic pathogens evolved effectors to hijack these resistances to activate PCD and colonize the resulting dying and dead tissue. Thus, immunity receptors, including the wall-associated kinases (WAKs), effective against biotrophic pathogens can become necrotrophic pathogen dominant susceptibility targets, typically characterized as recessive resistances. Genetic and functional data determined that resistance to the necrotrophic fungal pathogen Cochliobolis sativus, previously designated the dominant Rcs5 gene, is actually recessive in nature. High-resolution mapping delimited rcs5 to ~0.32 cM representing an ~ 234kb physical region containing four WAK genes, designated HvWak2-5. Allele analysis determined that HvWak2 and 5 contain no amino acid polymorphism between resistant and susceptible alleles. However, HvWak3 has a 620 bp deletion in the resistant cv Morex allele eliminating a predicted Epidermal Growth Factor-Calcium binding domain, and HvWak4 has amino acid polymorphisms present in the N-terminal region making both strong candidate Rcs5 dominant susceptibility genes. Silencing the candidate genes using virus induced gene silencing (VIGS) in resistant and susceptible cultivars showed that specific silencing of both HvWak3 and HvWak4 resulted in susceptible cultivars having significantly higher levels of resistance as compared to the HvWak2, and HvWak5 silencing constructs and VIGS controls. The transcript analysis of HvWak3 and HvWak4 showed no expression in resistant and susceptible lines prior to pathogen exposure but HvWak3 was induced at 36 hours post inoculation (hpi) and HvWak4 at 12 and 72 hpi in the susceptible line Steptoe. Thus, the high resolution mapping, post-transcriptional genes silencing and expression analysis data suggests that HvWak3 and HvWak4 both function as susceptibility targets of C. sativus in an inverse gene-for-gene manor. This WAK gene function is similar to that shown for the Parastagonospora nodorum dominant susceptibility gene Snn1 that was recently reported in wheat.