Chemical signals responsible for appressorium formation in the rice blast fungus Magnaporthe grisea

Magnaporthe grisea, the causative agent of rice blast disease, penetrates the host cuticle directly by way of a highly melanized attachment cell called an appressorium. The signals responsible for appressorium formation are not well known. We report on the sufficiency of a hydrophobic surface to induce appressorium formation in vitro for a collection of M. grisea isolates. Chemical cues, similar to those available on the host surface, were also found to induce appressorium formation selectively using a novel in vitro assay. The plant cutin monomers cis-9,10-epoxy-18-hydroxyoctadecanoic acid and cis-9-octadecen-1-ol were found to induce infection structure formation in the nanomolar range, while other monomers were effective only at the micromolar range. No differences were detected for appressorium formation rates on various host and nonhost plants suggesting appressorium initiation is not host specific. Plant lipid and wax compounds were also tested; the two most effective inducers were found to be 1,16-hexadecanedial and 1,16-hexadecanediol which had AID(60)s (chemical dose at which appressoria induction equals 50% for germinated conidia) of 3.86 nM and 7.25 nM respectively. Structural studies of the inducing compounds were performed, and terminal hydrogen bonds and chain length were found to be important for high activity. The thigmotropic effects and chemical cues are discussed in the context of possible mechanisms involved and a working model of infection related morphogenesis of the plant surface is presented. (C) 1996 Academic Press Limited