Arabidopsis dth9 mutation identifies a gene involved in regulating diseasesusceptibility without affecting salicylic acid-dependent responses

To determine which components of the plant defense response make important contributions to limiting pathogen attack, an Mp mutagenized population of a transgenic Arabidopsis line was screened for mutants showing constitutive expression of P-glucuronidase activity driven by the promoter region of th e CEVI-1 gene. The CEVI-1 gene originally was isolated from tomato plants a nd has been shown to be induced in susceptible varieties of tomato plants b y virus infection in a salicylic acid-independent manner. We report here th e characterization of a recessive mutant, detachment9 (dth9). This mutant i s more susceptible to both virulent and avirulent forms of the oomycete Per onospora and also exhibits increased susceptibility to the moderately virul ent bacterial pathogen Pseudomonas syringae pv maculicola ES4326. However, this mutant is not affected in salicylic acid metabolism and shows normal e xpression of pathogenesis-related (PR) genes after pathogen attack. Further more, after inoculation with avirulent pathogens, the dth9 mutant shows a c ompromised systemic acquired resistance response that cannot be complemente d by exogenous application of salicylic acid, although this molecule is abl e to promote normal activation of PR genes. Therefore, the dth9 mutation de fines a regulator of disease susceptibility that operates upstream or indep endently of salicylic acid. Pleiotropy is also evident in the dth9 mutant i n the sense that the shoots of dth9 plants are insensitive to the exogenous ly applied auxin analog 2,4-dichlorophenoxyacetic acid.