Nitrogen starvation of the rice blast fungus Magnaporthe grisea may act as an environmental cue for disease symptom expression

The effect of nitrogen limitation on the rice blast fungus Magnaporthe grisea was examined. Nitrogen starvation-stress induced expression of a large number of genes expressed during growth of the fungus in plant tissue, particularly during disease symptom outbreak. Nitrogen starvation-stress elicited production of secreted products which were isolated from culture filtrates of M. grisea, and caused senescence of rice leaves within 48 h of application. Leaf senescence was associated with uptake of the secreted product(s) and rapid leaf water loss. The senescence-promoting activity was restricted to a high molecular weight culture filtrate Fraction and was eliminated by boiling or prolonged treatment with proteinase K. The production of the activity was conserved in different host-limited forms of the fungus and was nor associated with melanin or MPG1 hydrophobin production. Strains carrying mutations at the NUT1 locus controlling nitrogen source utilization, or the NPR1 and NPR2 loci which regulate nitrogen metabolism and pathogenicity, produced only residual senescent activity. We conclude that nitrogen starvation-stress may act as one of the inductive cues for disease symptom expression during rice blast infections. (C) 1997 Academic Press Limited.