A mechanism for ethylene-induced disease resistance in soybean: enhanced synthesis of an elicitor-releasing factor, β-1,3-endoglucanase

Abstract

Exogenous ethylene increased the level of β-1,3-endoglucanase (EC 3.2.1.39) activity and the level ofβ-1,3-endoglucanase protein 2- to 4-fold higher than constitutive levels in hypocotyls of soybean seedlings. Ethylene-treated soybean hypocotyls showed partial resistance upon infection with a compatible race of Phytophthora megasperma f. sp. glycinea, accompanied with higher levels of the phytoalexin, glyceollin. Purified soybean β-1,3-endoglucanase had no direct fungitoxic activity to the fungus. The purified glucanase, however, released soluble elicitors from cell walls of the fungus. Higher levels of glyceollin accumulated in hypocotyls of ethylene-treated soybean seedlings than in control hypocotyls when they were treated with native fungal cell walls. Similar levels of glyceollin accumulated when ethylene-treated and control hypocotyls were treated with soluble elicitor released by the glucanase. These results indicate that more efficient elicitor release occurred in the ethylene-treated soybean tissue. In addition, co-treatment of hypocotyls with the purified glucanase and the native cell walls, but not the glucanase-released elicitor, resulted in greater glyceollin accumulation. Exogenous application of the purified glucanase to inoculation sites also increased resistance of soybean hypocotyls against attack by the fungus. In contrast, resistance of soybean hypocotyls to an incompatible race of P. megasperma f. sp. glycinea was partially negated when the seedlings were absorbed with laminarin, which had been shown to inhibit the reaction of elicitor release from fungus cell walls. These results support the idea that the increase in resistance in ethylene-treated soybean tissue may be due, in part, to enhanced β-1, 3-endoglucanase activity which may release elicitor-active molecules from fungal cell walls. The results also suggest that the process of elicitor release mediated by host β-1,3-endoglucanase is an important process leading to active disease resistance in soybean.