Photoreactivation of UV‐lnactivated Spores of Trichoderma harzianum

Abstract

Abstract— Photoreactivation in the filamentous soil fungus Trichoderma harzianum is of interest because its blue, UVA photoreceptors (cryptochromes) may share homology with DNA photolyases. Furthermore, this organism antagonizes, by mycoparasitism, a number of soil‐borne pathogens. Photoreactivation is thus important as one of the factors that may contribute to survival in the field. Exposure of asexually produced spores (conidia) to UVC inhibits germination. Nongerminating spores either do not swell or are inhibited later in germination, swelling but failing to put out a germ tube. Both types of inhibition can be reversed by photoreactivation with visible and UVA (320‐400 nm) light, restoring high germination percentages. Conidia of mutants lacking the normal greenish pigmentation are more sensitive to UVC (200‐280 nm) than wild‐type conidia but photoreactivation still occurs. The action spectrum for photoreactivation indicates that T. harzianum has a DNA photolyase with a pterin as second chromophore. The most effective wavelengths for photoreactivation correspond to valleys, rather than peaks, in the action spectrum for photoinduction of sporulation. Furthermore, mutants with defects in photoinduction of sporulation (dimY) are not defective in photoreactivation. Induction of sporulation and DNA photorepair, while sharing parts of the blue/UVA spectrum, are different, by spectroscopic, kinetic and genetic criteria.