Proper expression of AS1 is required for RPW8.1-mediated defense against powdery mildew in Arabidopsis

• Screening mutants impairing cell death mediated by RPW8.1 identified alleles of AS1.
• Mutation in AS1 compromised RPW8.1-mediated defense against powdery mildew.
• Ectopic expression of AS1-eYFP partially restored defects of as1 mutant.
• AS1 may positively regulates RPW8.1-mediated defense functions.

The ectopic expression of Arabidopsis gene RESISTANCE TO POWDERY MILDEW 8.1 (R1Y4 plant line) triggers a hypersensitive response (HR) providing defense potentially effective against different pathogens. In order to understand the regulation of RPW8.1-mediated signaling, an approach based on ethyl methanesulfonate (EMS) mutagenesis was adopted to identify mutants that enhance or impair RPW8.1-mediated cell death. Here we describe the identification of mutants b10-6 and b9-1, point mutations that were induced in the At2g37630 locus resulting in an amino acid substitution or a premature stop codon in ASYMMETRIC LEAVES 1 (AS1), respectively. Loss-of-function in AS1 altered the transcription of RPW8.1, RPW8.1-dependent necrotic lesions in leaves and resistance to powdery mildew. In the mutant b10-6, RPW8.1-mediated cell death and disease resistance was compromised, however. in b9-1 RPW8.1-mediated cell death and resistance to powdery mildew was slightly enhanced. Expression of AS1-eYFP within b10-6 background suppressed RPW8.1-mediated cell death and disease resistance in the short-day condition, but largely restored the phenotypes of b10-6 to that of R1Y4 in the long-day condition. These data imply that proper expression of AS1 is required for full function of RPW8.1 in triggering cell death and resistance to powdery mildew.