Activation of PR-la Promoter by Rhizobacteria That Induce Systemic Resistance in Tobacco against Pseudomonas syringae pv. tabaci

Investigations were conducted to determine whether induction of PR-1a gene promoter was correlated with systemic resistance, induced by rhizobacteria, against wildfire disease of tobacco. Ten different strains of plant growth-promoting rhizobacteria (PGPR), including the species Bacillus pumilus, Serratia marcescens, Pseudomonas fluorescens, P. putida, Curtobacterium flaccumfaciens, and Bukholderia gladioli, were tested. Induction of PR-1a gene activity was assessed using transgenic tobacco plants expressing the ß-glucuronidase (GUS) gene fused to the PR-la gene promoter. In a microtiter plate assay, GUS activity was significantly enhanced, compared to that of water-treated controls by salicylic acid (SA), four PGPR strains with known induced systemic resistance activity, and one endophytic bacterium previously shown to lack induced systemic resistance activity in cucumber. No enhanced GUS activity was noted with three control bacteria (two plant-associated strains and Escherichia coli strain HB-101). In a separate assay, infiltration of greenhouse-grown tobacco leaves with these same bacteria resulted in significant increases in GUS activity compared to that of the water-treated control, for all strains which induced GUS activity in the microtiter plate assay. The two plant-associated bacterial controls did not affect GUS activity. Treatment of tobacco with SA and all bacterial strains which enhanced GUS activity in the microtiter and leaf assays led to reduced symptoms of wildfire disease caused by Pseudomonas syringae pv. tabaci in the greenhouse, whereas none of the three control bacterial strains significantly affected disease. These results support the conclusion that induction of PR-1a promoter activity and PGPR-mediated induced systemic disease resistance are linked events for the PGPR strains studied.