ERWINIA-CHRYSANTHEMI EC16 PRODUCES A 2ND SET OF PLANT-INDUCIBLE PECTATE LYASE ISOZYMES

The enterobacterium Erwinia chrysanthemi causes soft-rot diseases invo lving extensive tissue maceration in a wide variety of plants and secr etes multiple pectic enzymes that degrade plant cell walls and middle lamellae. An E. chrysanthemi mutant with directed deletions or inserti ons in genes pehX, pelX, pelA, pelB, pelC, and pelE, which encode exo- poly-alpha-D-galacturonosidase, exopolygalacturonate lyase, and four i sozymes of pectate lyase, respectively, was constructed by the marker exchange of a cloned pehX.:TnphoA fragment into E. chrysanthemi CUCPB5 010, a DELTA(pelA pelE) DELTA(p B pelC)::28bp DELTA(pelX)DELTA4bp deri vative of strain EC16. This mutant, E. chrysanthemi CUCPB5012, no long er caused pitting in a standard pectate semisolid agar medium used to detect pectolytic activity in bacteria. Nevertheless, the mutant still macerated leaves of chrysanthemum (Chrysanthemum morifolium), althoug h with reduced virulence. The mutant was found to produce significant pectate lyase activity in rotting chrysanthemum tissue and in minimal media containing chrysanthemum extracts or cell walls as the sole carb on source. Activity-stained, ultra-thin-layer isoelectric focusing gel s revealed the presence in these preparations of several pectate lyase isozymes with pIs ranging from highly acidic to highly alkaline. Ster ile culture fluids containing these isozymes were able to macerate chr ysanthemum leaf tissue. Unlike the products of the pelA, pelB, pelC, a nd pelE genes in E. chrysanthemi EC16, these plant-inducible pectate l yase isozymes were not produced in minimal medium containing pectate. The results suggest that E. chrysanthemi produces two sets of independ ently regulated pectate lyase isozymes that are capable of macerating plant tissues.