Correlation of defense gene induction defects with powdery mildew susceptibility in Arabidopsis enhanced disease susceptibility mutants

We investigated the relative importance of specific Arabidopsis thaliana ge nes in conferring resistance to bacterial versus fungal pathogens'. We firs t developed a patho-system involving the infection of Arabidopsis accession Columbia with a Virulent isolate of the obligate biotrophic fungal pathoge n Erysiphe orontii. E. orontii elicited the accumulation of mRNAs correspon ding to the defense-related genes PR1, BGL2 (PR2), PR5 and GST1, but did no t elicit production of the phytoalexin camalexin or the accumulation of def ensin (PDF1.2) or thionin (THI2.1) mRNAs. We tested a set of 15 previously isolated Arabidopsis phytoalexin deficient (pad), non-expresser of PR (npr) and enhanced disease susceptibility (eds) mutants that are more susceptibl e to Pseudomonas syringae for their susceptibility to E. orontii. Four of t hese mutants (pad4-1, npr1-1, eds5-1 and a double npr1-1 eds5-1 mutant) as well as Arabidopsis lines carrying a nahG transgene exhibited enhanced susc eptibility to E. orontii and reduced levels of PR gene expression. Comparis on of the PR gene induction patterns in response to E. orontii in the vario us mutants and in the nahG transgenics suggests the existence of NPR1-indep endent salicylate-dependent and NPR1-independent salicylate-in dependent de fense gene activation pathways. Eleven other eds and pad mutants did not sh ow measurable enhanced susceptibility to E. orontii, suggesting that these mutants are defective in factors that are not important for the limitation of E. orontii growth.