Two novel genes induced by hard-surface contact of Colletotrichum gloeosporioides conidia.

Germinating conidia of many phytopathogenic fungi must differentiate into an infection structure called the appressorium in order to penetrate into their hosts. This differentiation is known to require contact with a hard surface. However, the molecular basis for this requirement is not known. Induction of this differentiation in the avocado pathogen, Colletotrichum gloeosporioides, by chemical signals such as the host's surface wax or the fruit-ripening hormone, ethylene, requires contact of the conidia with a hard surface for about 2 h. To study molecular events triggered by hard-surface contact, we isolated several genes expressed during the early stage of hard-surface treatment by a differential-display method. The genes that encode Colletotrichum hard-surface induced proteins are designated chip genes. In this study, we report the characterization of CHIP2 and CHIP3 genes that would encode proteins with molecular masses of 65 and 64 kDa, respectively, that have no homology to any known proteins. The CHIP2 product would contain a putative nuclear localization signal, a leucine zipper motif, and a heptad repeat region which might dimerize into coiled-coil structure. The CHIP3 product would be a nine-transmembrane-domain-containing protein. RNA blots showed that CHIP2 and CHIP3 are induced by a 2-h hard-surface contact. However, disruption of these genes did not affect the appressorium-forming ability and did not cause a significant decrease in virulence on avocado or tomato fruits suggesting that C. gloeosporioides might have genes functionally redundant to CHIP2 and CHIP3 or that these genes induced by hard-surface contact control processes not directly involved in pathogenesis.