Systematic Characterization Of Homeobox Genes In Magnaporthe Grisea

Homeobox genes,encoding proteins with homeodomain,function as a transcriptional regulatory factor,and play an important role in regulating the ontogeny development and cell differentiation in eukaryotes.For animals,homeobox genes play an important role in embryonic development,gene transcription,and cell differentiation.In plants,homeobox genes regulate embryonic development,leaf and flower development.In fungi,only a few studies on homeobox genes have indicated that they participate in the regulation ofmycelial growth,sporulation,pathogenicity,and the sexual reproduction.The rice blast fungus,Magnaporthe grisea,infects many economically important cereal crops,particularly rice.The complicated infection process starts when the fungal conidia land on and attach to a suitable surface,such as a rice leaf,germ tubes arise when moist condition is suitable.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 and eventurally conidiation of the fungus.Thus the host invasion by the rice blast fungus involves a complex cell morphogenesis.The interaction of M.grisea and rice is also taken as a model for the study of fungus-plant interaction.Based on the M.grisea genome,there are seven homeobox genes(HTF1,HTF2,HTF3, HTF4,HTF5,HTF6,PTH12) in M.grisea.In this study,we have systematiely characterized these homeobox transcription factors in M.grisea.In addition,we have also analysed the regulation of conidiogenesis by a homeobox transcription factor gene(HTF1) in the rice blast fungus.The functional studies showed that the growth rate of htf1,htf2,htf5,and htf4 deletion mutant did not change significantly.However,the growth of htf3 knockout mutant significantly slowed down,and its colony became more tight compared with that of the wild type control and the htf6 knockout mutant growed slightly slow.Interestingly,Htf1 knockout mutant lost sporulation ability,other mutant sporulation ability and spores' morphology were normal.Further studies proved that HTF1 controls the conidiation,and HTF3 controls mycerial growth in M.grisea.HTF1 did not affect the development of conidiophore,but specifically blocked the condium development.GFP signals were observed in nuclei of conidiophore in transformants expressing the HTF1-GFP fusion construct.Quantitative RT-PCR results showed that HTF1 is involved in the conidiation pathway,Htf1 mutant is not de-repressed in conidiophore development under non-conducive conditions.The observed phenotype of enhanced conidiophore development in the htf1 mutant may result from its defect in conidium development.It is possible that some feedback signal from conidia may inhibit further conidiophore development in M.grisea.Although in this study,we have systematically characterized homeobox transcription factors in growth,conidiation and pathogenic infection in M.grisea,but the signal transduction pathway of these homeobox gene are still unclear.Therefore,further analysis on the signal transduction pathway may be helpful to understand the molecular regulation mechanism of growth development,conidiation and the pathogenesis in the fungus.