| To identify Pseudomonas syringae pv. tomato strain DC3000 genes required for pathogenesis, we screened 947 Tn 5 mutants for reduced virulence phenotypes on Arabidopsis thaliana. Mutants defining both known and novel virulence loci were identified. Thirty-seven mutants exhibited reduced virulence. Seven mutants contained insertions in biosynthetic genes for the phytotoxin coronatine (COR). COR consists of two distinct moieties that are joined by an amide linkage: the polyketide coronafacic acid (CFA), and coronamic acid (CMA), an ethylcyclopropyl amino acid. COR, especially the CFA moiety, shares structural and functional relatedness with jasmonates (JAs), endogenous plant signaling molecules that are produced under conditions of biological stress.; Although COR is required for pathogen virulence, its role in pathogenesis is not understood. The DC3000 COR genes are chromosomally encoded and arranged in two separate clusters, which encode enzymes responsible for the synthesis of CFA and CMA. All COR biosynthetic mutants were impaired in their ability to cause disease on A. thaliana. To assess the relative contributions of CFA, CMA and COR in virulence, I constructed a cfa6 cmaA double mutant. This exhibited reduced virulence phenotypes identical to those observed for the cmaA or cfa6 single mutants, suggesting that disease formation requires the entire COR molecule. COW mutants failed to induce jasmonate-responsive genes during infection whereas wild-type DC3000 did so vigorously, indicating that COR functions as a molecular mimic of JA. COR− mutants multiplied to wild-type levels on plants compromised for the salicylic acid (SA) defense pathway. This suggests that COR− functions to overcome SA-dependent host defenses responsible for limiting growth of COR− mutants in wild-type plants. Although the COR− mutants were able to cause disease symptoms on SA-compromised plants, the severity of these symptoms were reduced when compared to wild-type DC3000. Thus, COR is also required for wild-type disease symptom development. Unexpectedly, I observed no difference in the expression of SA-dependent defense-related genes during infection of A. thaliana. This leads us to conclude that, although COR is required to overcome plant SA dependent responses, this occurs via the suppression or evasion of unknown host defenses. |
