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

Erwinia carotovora subsp. carotovora, the etiological agent of bacterial soft rot, produces a variety of plant cell wall-degrading exoenzymes that are the main virulence factors of this pathogen. To determine the role of these enzymes in the plant-bacterium interaction, individual exoenzymes were produced in Escherichia coli harboring cloned exoenzyme 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. The 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. In addition, a similar plant response could be elicited by the application of polygalacturonase-treated polypectate. In planta analysis of tobacco seedlings inoculated with reduced virulence mutants of the pathogen that still produced pectic enzymes resulted in accumulation of beta-1,3-glucanase mRNA. However, no accumulation of beta-1,3-glucanase mRNA 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 fragments 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 wild-type bacteria are able to suppress the plant response. Induction of the plant defense by exoenzyme treatment conferred increased resistance against subsequent infections by E. c. subsp. carotovora.