Abstract
Magnaporthe oryzae is the most important fungal pathogen of rice (Oryza sativa). Under laboratory conditions, it is able to colonize both aerial and underground plant organs using different mechanisms. Here, we characterize an infection-related development in M. oryzae produced on hydrophilic polystyrene (PHIL-PS) and on roots. We show that fungal spores develop preinvasive hyphae (pre-IH) from hyphopodia (root penetration structures) or germ tubes and that pre-IH also enter root cells. Changes in fungal cell wall structure accompanying pre-IH are seen on both artificial and root surfaces. Using characterized mutants, we show that the PMK1 (for pathogenicity mitogen-activated protein kinase 1) pathway is required for pre-IH development. Twenty mutants with altered pre-IH differentiation on PHIL-PS identified from an insertional library of 2885 M. oryzae T-DNA transformants were found to be defective in pathogenicity. The phenotypic analysis of these mutants revealed that appressorium, hyphopodium, and pre-IH formation are genetically linked fungal developmental processes. We further characterized one of these mutants, M1373, which lacked the M. oryzae ortholog of exportin-5/Msn5p (EXP5). Mutants lacking EXP5 were much less virulent on roots, suggesting an important involvement of proteins and/or RNAs transported by EXP5 during M. oryzae root infection.