Further evidence for the involvement of a preformed antifungal compound in the latency of Colletotrichum Gloeosporioides on unripe avocado fruits

Abstract

Peel of unripe avocado fruits contained a preformed antifungal compound, cis, cis 1-acetoxy-2-hydroxy-4-oxo-heneicosa-12,15-diene. Concentrations of the compound decreased during fruit ripening, coincident with the renewed development of previously latent peel infections by Colletotrichum gloeosporioides . The purified diene was oxidized in vitro by lipoxygenase extracted from avocado peel and by soybean lipoxygenase. The specific activity of lipoxygenase in peel extracts also increased by 80% during the climacteric stage, coincident with a rapid decrease of the diene in fruit peel, and before symptoms of C. gloeosporioides infections were expressed. Butylated hydroxy anisole and α-tocopherol acetate inhibited oxidation of linoleic acid and the avocado diene by lipoxygenases from avocado and soybean. Infiltration of fungus-inoculated fruits with α-tocopherol acetate partially eliminated reduction of the diene compound during fruit ripening and fungus development in the peel was delayed. On the other hand, infiltration with 0·25 M CaCl 2 prevented softening of the fruits, but did not affect the decrease of the diene compound or decay of the peel by C. gloeosporioides . The evidence supports the hypothesis that the preformed antifungal compound causes C. gloeosporioides infections to remain latent in unripe avocado peel and that subsequent active infections result from metabolism of the diene compound during ripening, probably mediated by lipoxygenase.