Melanized appressoria are highly specialized
infection structures formed by germ tubes of the rice blast fungus Magnaporthe oryzae for plant
infection. M. oryzae also forms appressorium-like structures on hyphal
tips. Whereas appressorium formation by conidial germ tubes has been well characterized, formation of appressorium-like structures by hyphal
tips is under-investigated. In a previous study, we found that the chs7 deletion mutant failed to form appressoria on germ tubes but were normal in the development of appressorium-like structures on artificial hydrophobic surfaces. In this study, we compared the differences between the formation of appressoria by germ tubes and appressorium-like structures by hyphal
tips in M. oryzae. Structurally, both appressoria and appressorium-like structures had a
melanin layer that was absent in the pore region. In general, the latters were 1.4-fold larger in size but had lower turgor pressure than appressoria, which is consistent with its lower efficiency in plant penetration. Treatments with cAMP,
IBMX, or a
cutin monomer efficiently induced appressorium formation but not the development of appressorium-like structures. In contrast, coating surfaces with
waxes stimulated the formation of both
infection structures. Studies with various signaling mutants indicate that Osm1 and Mps1 are dispensable but Pmk1 is essential for both appressorium formation and development of appressorium-like structures on hyphal
tips. Interestingly, the cpkA mutant was reduced in the differentiation of appressorium-like structures but not appressorium formation. We also observed that the con7 mutant generated in our lab failed to form appressorium-like structures on hyphal
tips but still produced appressoria by germ tubes on hydrophobic surfaces. Con7 is a
transcription factor regulating the expression of CHS7. Overall, these results indicate that the development of appressorium-like structures by hyphal
tips and formation of appressoria by germ tubes are not identical differentiation processes in M. oryzae and may involve different molecular mechanisms.