Studies On The Functions Of Rho GTPases In Magnaporthe Grisea

The Rho-family proteins make up a major branch of the Ras superfamily of small GTPases. To date, more than 100 members of Rho-family proteins have been described, and they belong to Rho, Rac and Cdc42 subfamily. Rho GTPases has been fuelled by the realization that they regulate many other signal transduction pathways in addition to those linked to the actin cytoskeleton. They participate in the regulation of cell polarity, gene transcription, G1 cell cycle progression, microtubule dynamics, vesicular transport pathways and a variety of enzymatic activities ranging from an NADPH oxidase in phagocytes to a glucan synthase in yeast. In fungi, almost all the Rho GTPases are shown to be involved in the regulateon of polarity growth , the synthesis of cell wall, cell development and exocytosis; In filamentous fungi, Rho proteins also control the pathogenicity.The rice blast fungus, Magnaporthe grisea, infects many economically important cereal crops, particularly rice. The interaction of M. grisea and rice is taken as a model for the study of fungus-plant interaction.When the fungal conidia land on and attach to a suitable surface, such as a rice leaf, a genn tube arises. Subsequently, germ elongation ceases and the tip of the germ tube swells to form an appressorium surrounded by a thick melanized cell wall. Turgor pressure builds up inside the appressorium, forcing a penetration peg into the plant tissues; this is then followed by invasive growth of the fungus. Thus, the host invasion by the rice blast fungus involves a complex cell morphogenesis requiring dynamic changes in polarity in response to host factors.7 putative Rho GTPases were found on the M. grisea genome based on the bioinformation analysis. RT-PCR assay showed that all of the 7 Rho GTPases were expressed in M. grisea. The results reminded us that these 7 Rho GTPases may all play an important role in control of the growth, development and pathogenecity of M. grisea. In this paper, the functions of MgCdc42 and MgRho3, two of the 7 Rho GTPases were analysed.The highest expression level of MgCdc42 was observed in conidia, and the function of MgCdc42 was dissected by analysis of MgCdc42 knock-out, constitutive activated, dominant negative and overexpressed mutants. The results showed that the MgCdc42 deletion led to the deficiency of penetration pegs, which decreased the pathogenicity of M. griease. MgCdc42 knock out mutant was also decreased in the conidiation and aerial hypha, delayed on the germination and appressoria formation, and was developed into abnormal conidia and appressoria. The similar results were found from the dominant negative mutation of MgCdc42. Although the conidia of the constitutive activated mutant were not changed, the mycelia branched more frequently at the ends, and mutants were decreased in the apressoria formation and pathogenicity. While overexpression of MgCdc42 had no significant effect on...