ROLE OF EXTERNAL SIGNALS IN REGULATING THE PRE-PENETRATION PHASE OF INFECTION BY THE RICE BLAST FUNGUS, MAGNAPORTHE-GRISEA

The role of external signals (particularly the substratum surface and light) in regulating the pre-penetration phase of Magnaporthe grisea (Herbert) Barr [anamorph, Pyricularia grisea Sacc.] were analysed on rice (Oryza sativa L.) leaves, artificial substrata and in liquid suspension. Surface contact was found to be essential for appressorium induction but not conidium germination. Both a high surface hydrophobicity and light favoured the formation of short differentiated germ tubes and large numbers of appressoria, but neither factor was essential for their induction. Light intensity had a graded effect on the lengths of differentiated germ tubes but not on the number of appressoria formed. Higher numbers of appressoria differentiated on rice leaves than on artificial substrata suggesting that the host provides additional factors, and thus a more conducive environment, for promoting appressorium formation. Our study indicates that the pre-penetration phase of rice blast infection involves a programme of growth and differentiation triggered at conidium germination and regulated by multiple signals from the host and environment. No evidence was found for a single, external signal which initiates appressorium formation. Starvation is suggested as providing the necessary intracellular signal.