INDUCTION OF PLANT DEFENSE RESPONSE BY EXOENZYMES OF ERWINIA-CAROTOVORA SUBSP CAROTOVORA

Erwinia carotovora subsp. carotovora, the etiological agent of bacteri al soft rot, produces a variety of plant cell wall-degrading exoenzyme s that are the main virulence factors of this pathogen. To determine t he role of these enzymes in the plant-bacterium interaction, individua l exoenzymes were produced in Escherichia coli harboring cloned exoenz yme encoding genes from E. c. subsp. carotovora and applied to tobacco plants. The plant response was monitored by following the expression of a plant gene encoding a pathogenesis-related beta-1,3-glucanase. Th e transcript for beta-1,3-glucanase was shown to rapidly accumulate in plants treated with pectic enzymes, but not with a cellulase from E. c. subsp. carotovora. Both pectate lyase (Pel) and polygalacturonase ( Peh) were shown to increase the host beta-1,3-glucanase mRNA levels. I n addition, a similar plant response could be elicited by the applicat ion of polygalacturonase-treated polypectate. In planta analysis of to bacco seedlings inoculated with reduced virulence mutants of the patho gen that still produced pectic enzymes resulted in accumulation of bet a-1,3-glucanase mRNA. However, no accumulation of beta-1,3-glucanase m RNA was observed in plants inoculated with exoenzyme-negative mutants. These results indicate that pectic enzymes of E. c. subsp. carotovora probably elicit the plant defense response by releasing pectic fragme nts from the plant cell wall that may function as endogenous elicitors . Interestingly, infection of plants by the wild-type pathogen induced the plant response only weakly and transiently, suggesting that the w ild-type bacteria are able to suppress the plant response. Induction o f the plant defense by exoenzyme treatment conferred increased resista nce against subsequent infections by E. c. subsp. carotovora.