Induction of Ca2+-Calmodulin Signaling by Hard-Surface Contact Primes Colletotrichum gloeosporioides Conidia To Germinate and Form Appressoria

Conidia of many plant-pathogenic fungi sense physical or chemical signals from the plant surface to trigger germination and differentiation into an infection structure called the appressorium that is required to successfully penetrate into the host plant (10, 30). In anthracnose fungi belonging to the genus Colletotrichum, several species have been reported to produce appressoria in response to physical signals related to the topography of the leaf surface (17, 20, 31). Chemical signals have also been suggested to induce appressorium formation (14, 25). Germination and appressorium formation by Colletotrichum gloeosporioides were found to be induced specifically by the surface wax of its host but not by other plant waxes (27). Recently ethylene, the host ripening hormone, was found to signal germination and appressorium formation and thus help this fungus to time its infection to coincide with the ripening of the host fruit (11). The signal transduction pathways involved in the perception of the various signals leading to infection structure formation are unclear. The use of inhibitors of protein kinases and protein phosphatase suggested that protein phosphorylation is involved in the induction of appressorium formation by Colletotrichum (12). A calmodulin (CaM) antagonist inhibited both germination and appressorium formation in Colletotrichum trifolii (9), implying that Ca2+ and CaM could function in the infection process. In C. gloeosporioides the response to host wax and ethylene requires the contact of conidia with a hard surface for a 2-h period (12). This hard-surface contact may be a touch-like response, which has been found to induce CaM-like proteins in Arabidopsis thaliana (2). On the basis of these observations we postulate that the hard-surface contact which primes the conidia to respond to chemical signals might use the CaM and CaM kinase (CaMK) signaling pathway in this early phase of interaction with the host. Here we present evidence that strongly supports this hypothesis. We also present evidence that a selective inhibitor of CaMK inhibits the melanization of the appressorium and that scytalone, an intermediate in melanin synthesis, can partially overcome this inhibition, suggesting that the CaMK inhibitor affects melanin synthesis at a step prior to that involving scytalone.