SURFACE SIGNALING IN PATHOGENESIS

Surface signaling plays a major role in fungal infection. Topographica l features of the plant surface and chemicals on the surface can trigg er germination of fungal spores and differentiation of the germ tubes into appressoria. Ethylene, the fruit-ripening hormone, triggers germi nation of conidia, branching of hyphae, and multiple appressoria forma tion in Colletotrichum, thus allowing fungi to time their infection to coincide with ripening of the host. Genes uniquely expressed during a ppressoria formation induced by topography and surface chemicals have been isolated. Disruption of some of them has been shown to decrease v irulence on the hosts. Penetration of the cuticle by the fungus is ass isted by fungal cutinase secreted at the penetration structure of the fungus. Disruption of cutinase gene in Fusarium solani pisi drasticall y decreased its virulence. Small amounts of cutinase carried by spores of virulent pathogens, upon contact with plant surface, release small amounts of cutin monomers that trigger cutinase gene expression. The promoter elements involved in this process in F. solani pisi sere iden tified, and transcription factors that bind these elements were cloned . One of them, cutinase transcription factor 1, expressed in Escherich ia coli, is phosphorylated. Several protein kinases from F. solani pis i were cloned. The kinase involved in phosphorylation of specific tran scription factors and the precise role of phosphorylation in regulatin g cutinase gene transcription remain to be elucidated.