The role of carbohydrates in the pathogenicity of the rice blast fungus Magnaporthe grisea

摘要

To understand, and ultimately control, the ability of Magnaporthe grisea to infect rice it will be necessary to dissect the underlying genetic and biochemical adaptations associated. Using molecular genetic, cytological and biochemical approaches there has been significant progress in unravelling the basis of appressorium formation and function (Dean, 1997). In contrast relatively little is known of the characteristic biology associated with growth in planta. Such growth will depend on the ability of the fungus to assimilate photosynthetically derived sugars and to convert these into forms useful for its own nutrition, or for storage and subsequent mobilisation. Most plant sugars are unavailable to the fungus prior to penetration such that the forms of nutrient available will change qualitatively and quantitatively through the stages of pathogenic development. Different enzymes for carbohydrate storage and mobilisation might therefore be expected act at each stage and these enzymes, and the proteins which control their activity, may be required for successful plant colonisation. Identification of proteins involved in nutrition during pathogenesis will indicate which metabolic pathways are involved and may identify novel fungicide targets. The first clues as to what these factors might be involved have come with the recent identification, via insertional mutagenesis, of the PTHI and PTH9 genes (Sweigard et al., 1998). These have homology to GRR1, a yeast gene involved glucose regulation of transcription, (catabolite repression), and coupling the nutritional status of the cell to the progression of the cell cycle (Flick and Johnston, 1991; Li and Johnston, 1997), and NTHI, a neutral trehalase encoding gene (Kopp et al., 1993; Kopp et al., 1994) respectively. Mutant strains of M. grisea which carry an insertion within the respective homologues of these genes show reduced pathogenicity toward susceptible hosts suggesting that regulation of carbohydrate metabolism, possibly involving catabolite repression, and trehalose mobilisation both play a role in pathogenesis. As a first step toward understanding the carbohydrate fluxes which occur during pathogenesis we are characterising the PTH9 neutral trehalase encoding gene in greater detail. In this review we consider the storage carbohydrates present in fungi generally and reflect on WnlCn or mese may haw particular significance for pathogenesis of the rice blast fungus as well as the possible regulation of pathogenicity associated metabolic processes.