DEVELOPMENTALLY-REGULATED EXPRESSION OF HEAT-SHOCK GENES IN LEPTOSPHAERIA-MACULANS

To determine whether heat shock genes (hsp) were expressed in a stage- specific manner, total RNA was isolated from dormant pycnidiospores, g erminating spores, and mature mycelium of representatives of two patho types of Leptosphaeria maculans: the weakly virulent (or avirulent) an d the virulent. Northern blots prepared by using total RNA isolated fr om normally,grown and heat-shocked samples were hybridized with DNA of hsp genes of Neurospora crassa, hsp70, hsp80, and the heat shock indu cible manganese peroxidase cDNA of Phanerochaete chrysosporium. No hyb ridization signal was apparent in RNA from dormant spores in the absen ce of heat shock treatment, while heat shock treatment resulted in the induction of mRNA corresponding to these three hsp genes. In contrast , substantial amounts of hsp transcripts were observed in germinating spores, even in the complete absence of externally applied stress. The mature mycelium failed to show these transcripts under normal growth conditions, but following exposure to hyperthermal treatment, a charac teristic set of heat shock specific transcripts was witnessed. The two strains exhibited a similar pattern. Expression of the fungal hsp gen es was also detectable in the plant tissue, following infection by bot h the virulent and weakly virulent strain. These data suggest that pro ducts of stress-responsive genes may have a role in an early event dur ing spore germination.