ENZYME-ACTIVITIES IN SOFT ROT PATHOGENESIS OF POTATO-TUBERS - EFFECTS OF CALCIUM, PH, AND DEGREE OF PECTIN ESTERIFICATION ON THE ACTIVITIES OF POLYGALACTURONASE AND PECTATE LYASE

The activities of cell wall degrading enzymes were monitored during the maceration of potato tuber tissue by Erwinia carotovora subsp. atroseptica. Pectin-methylesterase activity was present in uninoculated tuber tissues. Polygalacturonase (PG), pectate lyase (PL), cellulase, protease, and xylanase activities appeared in a sequential manner 10, 14, 16, 19, and 22 h, respectively after inoculation. The pH of crude tissue extracts made over this time period changed from pH 6·3 to 6·7. The depolymerizing pectic enzymes produced in the tissues exhibited different characteristics. PG activity was inhibited by the divalent cations Ca2+, Sr2+, Ba2+, Mg2+ and its optimum pH was around 6·0. PL activity was enhanced by Ca2+ and Sr2+, but not by Mg2+ and Ba2+ and its optimum pH was around 8·5. At their optimal pHs both PG and PL enzymes attacked polygalacturonic acid in preference to pectin, but at pH 6·5 PL activity was greater on pectin. Both PGs and PLs generated a series of reaction products including galacturonic acid suggesting both exo- and endo-activity. At the beginning of the maceration process, between 12 and 14 h after inoculation, PG played the dominant role but thereafter the increasing levels of calcium and pH reduced activity. By 22 h after inoculation the pH and free Ca2+ levels in the macerate enhanced PL activity. At pH 6·5 both a high level of free Ca2+ and pectin, favoured PL activity. Thirty hours after inoculation six multiple forms of PL were detected in macerated tissues, with pIs between 8·0 and 10·7. PLs 2, 5, and 6 were the first formed of the PL isozymes during tissue maceration. Only PLs 1, 2, and 5 were detected in cultures of E. carotovora subsp. atroseptica. © 1990.