A Neofunctionalized Partial Gene Duplicate of a Plant U-Box E3-Ligase Confers Quantitative Powdery Mildew Resistance in Barley and Wheat

The U-box E3 ligase is a key regulatory component of plant ubiquitination machinery affecting almost all aspects of plants growth and development. Because of its central role in degradation and modification of target proteins, E3 ligases are also preferred targets of pathogen-encoded effectors for manipulation of host’s defense responses. Plants on the other hand have evolved several strategies to circumvent such manipulations by effectors and thereby to arrest the growth of pathogens. Here we report on a partial gene duplicate with a probable function as a decoy for a functional E3 ligase which may be an effector target.  

Using a high-throughput transient-induced gene silencing (TIGS) screen we identified a partial gene duplicate of the HvPUB15 U-box/armadillo-repeat E3 ligase designated as HvARM1 (for barley Armadillo 1) affecting the interaction with the biotrophic powdery mildew fungal pathogen. Further functional characterization of HvARM1 by transient silencing and in stable transgenic RNAi barley resulted in enhanced susceptibility to Blumeria graminis f.sp. hordei (Bgh) and  transient over-expression in barley and wheat triggered resistance to Bgh. These two related proteins HvPUB15 and HvARM1 were also found to physically interact with the plastid-localized barley homologue of TaToxABP1 (for wheat toxin A binding protein 1) and with HvClpS1 (for Clp-protease adaptor S) in Y2H assay. Transient silencing and overexpression of HvToxABP1 revealed this putative HvPUB15-client protein as susceptibility-related plant factor for Bgh interaction. Putting together, the data suggests a neofunctionalized role of HvARM1 as a factor functioning as a decoy for the susceptibility-related HvPUB15 protein, which might be involved in regulating downstream plastid-localized responses to support powdery mildew infection.