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 plan t tissue, particularly during disease symptom outbreak. Nitrogen starv ation-stress elicited production of secreted products which were isola ted from culture filtrates of M. grisea, and caused senescence of rice leaves within 48 h of application. Leaf senescence was associated wit h uptake of the secreted product(s) and rapid leaf water loss. The sen escence-promoting activity was restricted to a high molecular weight c ulture filtrate Fraction and was eliminated by boiling or prolonged tr eatment with proteinase K. The production of the activity was conserve d in different host-limited forms of the fungus and was nor associated with melanin or MPG1 hydrophobin production. Strains carrying mutatio ns at the NUT1 locus controlling nitrogen source utilization, or the N PR1 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 sy mptom expression during rice blast infections. (C) 1997 Academic Press Limited.