Downy mildew (Peronospora parasitica) resistance genes in Arabidopsis varyin functional requirements for NDR1, EDS1, NPR1 and salicylic acid accumulation

To better understand the genetic requirements for R gene-dependent defense activation in Arabidopsis, we tested the effect of several defense response mutants on resistance specified by eight RPP genes (for resistance to Pero nospora parasitica) expressed in the Col-0 background. In most cases, resis tance was not suppressed by a mutation in the SAR regulatory gene NPR1 or b y expression of the NahG transgene. Thus, salicylic acid accumulation and N PR1 function are not necessary for resistance mediated by these RPP genes. In addition, resistance conferred by two of these genes, RPP7 and RPP8, was not significantly suppressed by mutations in either EDS1 or NDR1. RPP7 res istance was also not compromised by mutations in EIN2, JAR1 or COI1 which a ffect ethylene or jasmonic acid signaling. Double mutants were therefore te sted. RPP7 and RPP8 were weakly suppressed in an eds1-2/ndr1-1 background, suggesting that these RPP genes operate additively through EDS1, NDR1 and a s-yet-undefined signaling components. RPP7 was not compromised in coi1/npr1 or coi1/NahG backgrounds. These observations suggest that RPP7 initiates r esistance through a novel signaling pathway that functions independently of salicylic acid accumulation or jasmonic acid response components.