ISOLATION OF ARABIDOPSIS MUTANTS WITH ENHANCED DISEASE SUSCEPTIBILITYBY DIRECT SCREENING

To discover which components of plant defense responses make significa nt contributions to limiting pathogen attack, we screened a mutagenize d population of Arabidopsis thaliana for individuals that exhibit incr eased susceptibility to the moderately virulent bacterial pathogen Pse udomonas synigae pv. maculicola ES4326 (Psm ES4326). The 12 enhanced d isease Susceptibility (eds) mutants isolated included alleles of two g enes involved in phytoalexin biosynthesis (pad2, which had been identi fied previously, and pad4, which had not been identified previously), two alleles of the previously identified npr1 gene, which affects expr ession of other defense genes, and alleles of seven previously unident ified genes of unknown function. The npr1 mutations caused greatly red uced expression of the PR1 gene in response to PsmES4326 infection, bu t had little effect on expression of two other defense genes, BGL2 and PR5, suggesting that PR1 expression may be important for limiting gro wth of PsmES4326. While direct screens for mutants with quantitative p athogen-susceptibility phenotypes have not been reported previously, o ur finding that mutants isolated in this way include those affected in known defense responses supports the notion that this type of screeni ng strategy allows genetic dissection of the roles of various plant de fense responses in disease resistance.