Signaling pathways involved in preinfection development of Colletotrichum gloeosporioides, C. coccodes, and C. dematium pathogenic on red pepper

Abstract

Colletotrichum gloeosporioides, C. coccodes, and C. dematium are responsible for pepper anthracnose and they infect pepper as tissue-specific manner. They form a specialized infection structure, an appressorium, to infect the host. Contacting hard surface induces appressorium formation in these fungi, whereas hydrophobicity of the contact surface does not affect this infection-related differentiation. To investigate the common signal transduction pathways involved in conidial germination and appressorium formation, several pharmacological agents thought to act on critical steps in cAMP- and calcium-dependent signaling pathways were tested. Exogenous addition of cAMP did not alter both differentiations of all 3 anthracnose pathogens tested. Two phosphodiesterase inhibitors, caffeine and isobutylmethylxanthine (IBMX), specifically diminished appressorium formation of C. gloeosporioides and conidial germination of C. dematium. Addition of Ca 2+ specifically reduced appressorium formation of C. coccodes and C. dematium, whereas EGTA (calcium chelator) and neomycin (phospholipase C inhibitor) showed nonspecific inhibitory effects on both developments. Calcium channel blockers and calmodulin antagonists showed species-specific inhibitory effects. These results suggest that calcium/calmodulin-dependent signaling pathway plays an essential role on conidial germination and appressorium formation in pepper anthracnose pathogens.