INDUCTION OF Trichoderma SPORULATION BY NANOSECOND LASER PULSES: EVIDENCE AGAINST CRYPTOCHROME CYCLING

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Abstract
— An important question in the study of photoreceptor action in morphogenesis is whether the chromophore is unidirectionally photobleached, or whether it is recycled, allowing each receptor molecule to be counted more than once. The common soil fungus Trichoderma harzianum grows vegetatively in the dark and sporulates in response to a pulse of blue or UV-A light. Colonies were grown at 26oC, transferred to 3oC, illuminated with non-saturating light, and then put back at 26oC to sporulate. The fluence-response curves for photoinduction in the cold and at 26oC were identical, indicating that there are no enzymatic transduction processes during irradiation. Regions of the perimeter of dark-grown colonies were given single pulses (maximum duration, 30 ns) at 355 nm with a neodymium laser. We obtained a complete fluence-response curve for the laser pulses, which agreed with data for irradiations in the second to minute range. Photoinduction at 3oC, and validity of Bunsen-Roscoe reciprocity from nanoseconds to minutes, support the hypothesis that the inductive event is a simple first-order photobleaching reaction.