Regulation of Coronatine Biosynthesis in Pseudomonas syringae

Abstract

Coronatine (COR) is a chlorosis-inducing phytotoxin produced by Pseudomonas syringae pv. tomato DC3000 and P. s. pv. glycinea PG4180. The cfl gene, which encodes coronafacate ligase, is required for COR production in both strains. Sequence analysis indicated that the cfl gene is conserved in DC3000 and PG4180 strains, but the cfl upstream regions diverge in the two strains. The cfl promoter regions in DC3000 and PG4180 were fused to uidA (encoding glucuronidase), and transcriptional fusions were used to investigate the regulation of COR production. In vitro assays indicated that the DC3000 cfl promoter was activated within 6 to 24 h after incubation at 21^°C in COR-inducing media, with expression declining thereafter. However, expression of cfl in strain PG4180 increased gradually beginning at 12 h, and continued to increase throughout the sampling period. Histochemical staining for GUS activity in planta demonstrated that cfl transcription is activated prior to symptom development in tomato and collard plants inoculated with strain DC3000. However, cfl expression in soybeans inoculated with PG4180 occurred more slowly and was most evident at the onset of chlorosis. Therefore, the temporal expression of cfl activity in planta is consistent with results obtained in vitro. The implication of these results with respect to the role of COR in disease development and symptom manifestation is discussed.