An Anti-hydrotactic Response and Solid Surface Recognition of Germ Tubes of the Rice Blast Fungus,Magnaporthe gvisea

Magnaporthe grisea, the causal agent of rice blast disease, differentiates an appressorium to penetrate through the host cuticle with an infection peg elaborated from the appressorium. Similar reaction is observed on various synthetic substrata. By using a hardness-adjustable material, Hycel A-342, the correlation between appressorium formation and substratum hardness was evaluated. The results suggested that substratum hardness played an important role in triggering appressorium formation of M. grisea. Furthermore, growth orientation of germ tubes was coerced by the hydrophobicity of the contact surfaces. When conidia germinated at the interface of two differently hydrophobic phases, the germ tubes grew preferentially towards the more hydrophobic phase. Mutagenesis studies suggested that loss of this anti-hydrotactic behavior impaired appressorium formation. Since water is a pre-requisite for germination, we propose that the anti-hydrotactic response initiates attempted penetration into the plants by germ tubes, then triggers appressorium formation by the surface hardness.