Rcs5 Is a Wall Associated Kinase Gene That Functions As a Dominant Susceptibility Factor in the Barley- Cochliobolus sativus Interaction to Produce Necroptosis

Spot blotch, caused by Cochliobolus sativus, is a devastating disease of barley and wheat. One of the major resistance QTL against spot blotch contains the Rcs5 gene and is located on the short arm of barley chr 7H. A high-resolution genetic map delimited this region to ~0.32cM between the flanking markers ctg1584 and ctg7. Previous attempts to identify the gene were unsuccessful as it was hypothesized to be a dominant resistance gene present in cultivar (cv) Morex, but our genetic analysis of the F₂ population between susceptible cv, Steptoe and resistant cv Morex showed a 1 (resistant):3 (susceptible) segregation ratio suggesting that the resistance is recessive in nature. Utilizing the barley genome sequence of Morex, four wall-associated kinase (WAK) genes were identified in the delimited region and designated as HvWAK2-5. Functional characterization of the candidate genes using BSMV-VIGS revealed that silencing either HvWAK3 or HvWAK4, shifted from susceptibility to resistance in the cvs Steptoe and Harrington and produced no response in resistant cv Morex. Thus, both genes encode dominant susceptibility factors. Genome walking and cDNA analysis revealed that only HvWAK3 is polymorphic, having a 620 bp unique sequence in the susceptible cvs. Absence of this unique segment results in the nonfunctional protein in Morex. Thus, we designate barley HvWAK3 as the spot blotch dominant susceptibility gene rcs5. We hypothesize that HvWAK3 encodes the Rcs5 dominant susceptibility protein targeted by the C. sativus necrotrophic pathogen to induce programmed cell death in an inverse-gene-for-gene manner facilitating disease in the susceptible lines.