Functional Analysis On A RAC1 Homolog Of Rho Family Proteins In Magnaporthe Grisea And Exploration In Related Signal Pathway

Rac1 makes up a major branch of the Rho-family proteins (also called RhoGTPases). Rho GTPases play key role in fungal development, and almost all the RhoGTPases are shown to be involved in the regulation of polarity growth. In filamentousfungi, Rho GTPases also control the pathogenicity. The rice blast fungus,Magnaporthe grisea, is taken as a model for the study of filamentous fungi. Itsinfection process involves a complex cell morphogenesis in response to host defensemechanism. We expected that Rho GTPases are involved in regulation of thiscomplex process. Therefore, we started functional analysis of Rho GTPases of thefungus at whole genome level. In this study, we characterized the biological functionof MgRac1, a gene encoding Rac1 homolog in M. grisea. First we conducted the genedeletion through homologous recombination. The conidiation of the mgrac1 mutantsreduced dramatically while malformed conidia was also defective in appressoriumformation. And the mycelia was a little delay and branched more frequently at theends. Moreover, this mutant lost the pathogenicity completely. Expression ofconstitutively activated allele of the gene reduced conidiation of the fungus andcaused dramatic defects in infectious related growth. Moreover, through RT-PCRanalysis of interactional expression pattern between MgRac1 and other Rho GTPases,it may be helpful to the dissect of the Rho GTPases protein network in M. grisea. In ayeast two-hybrid assay, Rac1 physically interacts with the PAK kinase Chm1．Deletion of the PBD domain in Chm1 gene fully restores the conidiation of mgrac1mutant, but these conidia could not develop appressoria normally and are notpathogenic to plant. These data suggested that the small G protein MgRac1 interactswith downstream Chm1 to control conidial development in fungus, and dynamicallyon and off of the molecular switch is necessary to the fungal pathogenicity.