Ethylene: Symptom, Not Signal for the Induction of Chitinase and β-1,3-Glucanase in Pea Pods by Pathogens and Elicitors

Abstract

Infection of immature pea pods with Fusarium solani f.sp. phascoli (a non-pathogen of peas) or f.sp. pisi (a pea pathogen) resulted in induction of chitinase and 8-1,3-glucanase. Within 30 hours, activities of the two enzymes increased 9-fold and 4-fold, respectively. Chitinase and j8-1,3-glucanase were also induced by autoclaved spores of the two F. solani strains and by the known elicitors of phytoalexins in pea pods, cadmium ions, actinomycin D, and chitosan. Furthermore, exogenously applied ethylene caused an increase of chitinase and ,-1,3-gucanase in uninfected pods. Fungal infection or treatment with elicitors strongly increased ethylene production by immature pea pods. Infected or elicitor-treated pea pods were incubated with aminoethoxyvinylglycine, a specific inhibitor of ethylene biosynthesis. This lowered stress ethylene production to or below the level of uninfected controls; however, chitinase and P-1,3-glucanase were still strongly induced. It is concluded that ethylene and fungal infection or elicitors are separate, independent signals for the induction of chitinase and 8B-1,3-glucanase.Many plants respond to an attack by pathogens with an enhanced ethylene production (20,23,26). Exogenously applied ethylene has been found to activate or enhance biochemical defenses against potential pathogens in a number of cases (4). It has been hypothesized, therefore, that the endogenously produced stress ethylene may function as a signal for the plant to enhance or activate its defenses against pathogens (4,20,26).Two recent studies tested this hypothesis by manipulation of stress ethylene biosynthesis in diseased plants (18,23). In soybean cotyledons treated with a fungal elicitor, suppression of ethylene biosynthesis by AVG2 did not reduce phytoalexin production (18), while in diseased melon seedlings, an AVG treatment only slightly reduced the biosynthesis of hydroxyproline-rich cell wall glycoprotein, a substance implicated in defense (23).These studies indicate that endogenous stress ethylene has little importance in the induction of biochemical defenses against pathogens. However, interpretation of their results is difficult since exogenously applied ethylene had only a small effect (9,23) or none at all (18) on the defense reaction in question.Exogenously applied ethylene induces a large increase of chitinase and f-1,3-glucanase in a number of plants (1, 5). These

Plant defence genes are induced in the pathogenic interaction between bean roots and Fusarium solani, but not in the symbiotic interaction with the arbuscular mycorrhizal fungus Glomus mosseae

“…pisi and F. solani f. sp. phaseoli, both strains were able to elicit increased chitinase activity (Mauch, Hadwiger & Boller, 1984) and increased accumulation of class I chitinase transcripts (Chang et al, 1995) with basically identical kinetics. It should be noted however, that Fusarium solani is a soil-borne fungus which, under natural conditions, might not get in contact with seed pods.…”

Xylanase, a Novel Elicitor of Pathogenesis-Related Proteins in Tobacco, Uses a Non-Ethylene Pathway for Induction

“…1 B, lane 4) (25). In contrast, ethylene was considered to be a concomitant symptom, not signal, for chitinase and (1-3)-3-glucanase synthesis in pea seedlings treated with elicitor (chitosan) or inoculated with Fusarium solani (19). Nonetheless, ethylene treatment of the seedlings can by itself coordinately induce both enzymatic activities (32).…”

Calcium Requirement for Ethylene-Dependent Responses.

“…It also includes elicitors, such as a-aminobutyric acid (a-AB), that promote ethylene production and require ethylene in their mode of action (Lotan and Fluhr, 1990b;Eyal and Fluhr, 1992). The other pathway is ethylene independent (Mauch et al, 1984) and is exemplified by the fungal endoxylanase. The enzyme apparently degrades P-1,4-xylan linkages in the plant cell wall and is an exceptionally potent inducer of chitinase accumulation.…”