Bacterial endopectate lyase: evidence that plant cell wall pH prevents tissue maceration and increases the half-life of elicitor-active oligogalacturonides

Abstract

The abilities of endopectate lyase (α-1,4-poly- d-galacturonide lyase), purified from the bacterial phytopathogen Erwinia carotovora, to macerate potato tissue and to produce elicitor-active oligogalacturonides from polygalacturonic acid were studied at various pH values. The rate at which the lyase depolymerized polygalacturonic acid was maximum (100%) at pH 9·0, 10% at pH 8·0, and 3% at pH 7·0. Endopectate lyase macerated potato medullary tissue discs extensively at pH 9·0, to a lesser extent between pH 8·0 and 7·0, and did not detectably macerate the discs at pH values lower than 7·0 or in water (pH approx. 6·0). Digestion products of polygalacturonic acid treated with endopectate lyase at pH 5·75, the experimentally determined pH of soybean cotyledon exudates, and at pH 8·5, optimum for polygalacturonic acid depolymerization, were tested for phytoalexin elicitor activity in the soybean cotyledon bioassay. The elicitor activity of the digestion products obtained at pH 5·75 increased during the first 40 h of incubation and slowly decreased thereafter as degradation to short elicitor-inactive oligomers progressed. On the other hand, the activity of the digestion products obtained at pH 8·5 dropped to a very low level in less than 20 min. The stability of the oligogalacturonides with degrees of polymerization between 10 and 20, i.e. the oligogalacturonides which exhibit high elicitor activity, was longer than 24 h when the digestion of polygalacturonic acid was carried out at pH 5·75, but was less than 20 min when digestions were carried out at pH 8·5. These studies suggest that, in vivo, endopectate lyases are unlikely to hydrolyse plant cell wall pectin extensively or cause maceration at early stages of pathogenesis, but that the lyases are able to generate elicitor-active oligogalacturonides.