Phytoalexin induction and β-1,3-glucanase activities in Colletotrichum trifolii infected leaves of alfalfa (Medicago sativa L.)

Abstract

Defence responses were measured in leaves of alfalfa (Medicago sativa L.) after infection by virulent (compatible) and avirulent (incompatible) races of the anthracnose fungusColletotrichum trifolii . In the incompatible interaction, the isoflavonoid phytoalexin medicarpin and its conjugates started to accumulate 24 and 48h after inoculation, respectively, whereas their appearance was delayed to 48 and 72h, respectively, in the compatible interaction. No differences in the activities of β-1,3-glucanase isoforms were detected between infected and mock-inoculated tissues; expression of these defence-related hydrolases was constitutive. Constitutive overexpression of the alfalfa acidic glucanase Aglu1 in Regen SY, an alfalfa cultivar susceptible to C. trifolii, did not result in a greater or more rapid phytoalexin response, nor did it reduce the symptoms following pathogen attack, even though the majority of transgenic plants expressing the aglu1 transgene produced other glucanase isoforms that were not expressed in the non-transformed plants. The results do not support the hypothesis that glucanases may function in host defence to release elicitors of the phytoalexin response in the alfalfa/ C. trifolii interaction.