Timing of fungal invasion using host's ripening hormone as a signal.

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Abstract
In many postharvest fruit diseases, fungi remain latent until the fruit ripens. How the fungus times its infection at ripening of the host is not known. We have found that the volatiles produced by the climacteric tomato, avocado, and banana fruits induce germination and appressorium formation in Colletotrichum gloeosporioides and Colletotrichum musae. Exposure of the spores of these fungi to ethylene, the host's ripening hormone, at </=1 microl/liter, caused germination, branching of the germ tube, and formation of up to six appressoria from a single spore. Propylene, an ethylene analog, but not the hydrocarbon gas methane was able to induce spore germination and multiple appressorium formation. The ethylene effect on the fungi appears to be a plant-like response as it was inhibited by silver ion and 2,5-norbornadiene; the inhibition by the latter could be reversed by higher ethylene concentrations. Ethylene induced germination and appressorium formation in the Colletotrichum sp. penetrating climacteric fruit but not in other Colletotrichum strains. That the ethylene induction of multiple appressorium formation could be relevant to postharvest infection was indicated by the observation that C. gloeosporioides spores formed multiple appressoria on normally ripening tomato that produces ethylene, whereas on transgenic tomato and orange, fruits incapable of producing ethylene, exogenous ethylene was required to induce multiple appressorium formation and lesion formation. These results strongly suggest that these fungi must have coevolved to develop a mechanism to use the host's ripening hormone as a signal to differentiate into multiple infection structure and thus time the infection process.