Elevated Riboflavin Requirement for Postphotoinductive Events in Sporulation of a Trichoderma Auxotroph

Abstract

A riboflavin auxotroph of Trichoderma required more riboflavin for bluelight-induced conidiation than for growth. Colonies transferred after Mumination from 0.2 micromolar (limiting conidiation, not growth) to 3.5 micromolar riboflavin (nonlimiting), responded the same as those grown at 3.5 micromolar. The additional riboflavin is therefore not required as a photoreceptor. The data do not rule out the hypothesis that cryptochrome(s) is/are flavin(s).A flavin or flavoprotein is a favorite candidate for the elusive blue-light photoreceptor 'cryptochrome.' Carotenoids and other pigments cannot, however, be ruled out. Also, there might well be a variety of receptors triggering different responses in different organisms (2,6,(19)(20)(21)(22)(23). The action spectra of blue-light photoresponses can be approximated by flavin absorption spectra, provided one assumes that the molecular environment of the chromophore alters its energy levels enough to account for the fine structure of the peaks. Indirect evidence for a flavin chromophore is accumulating. Delbriick et al. (3) found, in Phycomyces, a minor action spectrum peak at 600 nm that they attributed to direct excitation of the flavin triplet state. Loser and Schafer (14) suggest that at least two distinct absorbing species (perhaps a photochromic pigment) are involved in Phycomyces. A shift in the Phycomyces phototropic action spectrum was caused by culturing in the presence of roseoflavin, a riboflavin analog with longer wavelength absorption bands (17). Although this result certainly fits the idea of a flavin photoreceptor, roseoflavin may have acted as an artificial photosensitizer, as do redox dyes such as methylene blue (12).Isolation of mutants specifically defective in photoreceptor would provide an additional tool for identifying cryptochrome(s). This approach is being applied to Phycomyces phototropism (1, 13), to blue light responses of Neurospora (10, 18), and to photoinduced conidiation in Trichoderma (Horwitz, Malkin, Gressel, unpublished). If the chromophore is a flavin, however, a mutation causing complete lack of photoreceptor would be lethal. One way to circumvent this problem partially is to work with a riboflavin-E

Effect of Light on the Concentration of Adenine Nucleotides in Trichoderma viride

“…The explanation for the observed photostimulated increase of the ATP concentration in T. viride is, at this stage of the work, purely speculative. By analogy with Neurospora crassa (Muiioz et a/., 1974;Paietta & Sargent, 1983), the photoreceptor could be a flavin or a flavoprotein (Horowitz & Gressel, 1983). If, as suggested by Muiioz et al (1974), the light causes flavinmediated reduction of cytochrome b, the oxidation of the latter coupled with phosphorylation should lead to an increase of the intracellular ATP content, as has indeed been observed in our experiments.…”

ROSEOFLAVIN INHIBITION OF PHOTOCONIDIATION IN A Trichoderma RIBOFLAVIN AUXOTROPH: INDIRECT EVIDENCE FOR FLAVIN REQUIREMENT for PHOTOREACTIONS

“…Most of the riboflavin may be needed for conidiation, and less for growth, and even less at the time of photoinduction of non-growing cultures. This would be supported by our previous findings (Horwitz and Gressel, 1983): ten fold more riboflavin was needed for development of conidia than to give maximal growth.…”

“…Conidia were removed into water with a rubber-tipped glass rod ("rubber policeman") and filtered through 35-pm nylon mesh to remove mycelial debris. The number of conidia was determined by the turbidity of the filtered suspension at 550 nm (Horwitz and Gressel, 1983). Each symbol in the figures is an average of suspensions from three replicate colonies, unless otherwise noted in the legends.…”

“…Low riboflavin may have inhibited photoreactions, dark reactions, or both in phase shifting of the circadian rhythm and carotenogenesis in Neurospora (Paietta and Sargent, 1981). Extra flavin was required for post-photoinductive events in Trichoderma conidiation but not for the light reactions themselves (Horwitz and Gressel, 1983). ' In Trichoderma, there is a clear separation between light-requiring processes (photoinduction), and the subsequent development of conidia in the dark.…”

Genetic Analysis of Phototropism of Neurospora crassa Perithecial Beaks Using White Collar and Albino Mutants

“…Genetic studies have been carried out with N. crassa (9,17,19,20), Phycomyces blakesleeanus (2,3,18,25), and Trichoderma (11) which are aimed at eventually identifying the blue light photoreceptor(s) (designated 'cryptochrome' [6]). In N. crassa, photoinduced carotenoid biosynthesis was investigated using wc and al mutants (9).…”