Arabidopsis <i>dth9</i> Mutation Identifies a Gene Involved in Regulating Disease Susceptibility without Affecting Salicylic Acid–Dependent Responses

To determine which components of the plant defense response make important contributions to limiting pathogen attack, an M₂ mutagenized population of a transgenic Arabidopsis line was screened for mutants showing constitutive expression of ß-glucuronidase activity driven by the promoter region of the <i>CEVI-1</i> gene. The <i>CEVI-1</i> gene originally was isolated from tomato plants and has been shown to be induced in susceptible varieties of tomato plants by virus infection in a salicylic acid–independent manner. We report here the characterization of a recessive mutant, <i>detachment9 (dth9)</i>. This mutant is more susceptible to both virulent and avirulent forms of the oomycete Peronospora and also exhibits increased susceptibility to the moderately virulent bacterial pathogen <i>Pseudomonas syringae</i> pv <i>maculicola</i> ES4326. However, this mutant is not affected in salicylic acid metabolism and shows normal expression of pathogenesis-related (<i>PR</i>) genes after pathogen attack. Furthermore, after inoculation with avirulent pathogens, the dth9 mutant shows a compromised systemic acquired resistance response that cannot be complemented by exogenous application of salicylic acid, although this molecule is able to promote normal activation of PR genes. Therefore, the <i>dth9</i> mutation defines a regulator of disease susceptibility that operates upstream or independently of salicylic acid. Pleiotropy is also evident in the <i>dth9</i> mutant in the sense that the shoots of <i>dth9</i> plants are insensitive to the exogenously applied auxin analog 2,4-dichlorophenoxyacetic acid.