11 Photomorphogenesis and Gravitropism in Fungi

Abstract

Light and gravity are environmental stimuli responsible for the final appearance of fungi. Research on fungal photomorphogenesis and gravitropic responses have concentrated on the description and characterization of the responses to light and gravity. Genes that are required for photoreception and photomorphogenesis have been identified and, in some cases, isolated and characterized. The identification of the WCC in N. crassa has served as a model for other fungi and in most cases mutants in the homologs of the WCC result in blind phenotypes. A number of WCC homologs have been identified with the increased number of fungal genome projects, and the reduction in the cost of genome sequences allows us to predict that the number of fungal photoreceptors will soar in the near future. Genome projects allowed the identification of additional photoreceptor genes in fungal genomes, many of them unexpected and most of them without a clear function in the fungal cell. Understanding the role and the coordination of fungal photoreceptors will be a major avenue of research in the future. N. crassa has been established as a model system for fungal photobiology, but there are indications that other fungi, even other ascomycetes, have molecular mechanisms for photoreception that are different from the one in N. crassa. It is becoming clear that there are limits to the use of N. crassa as a model for fungal photobiology. The molecular basis of fungal gravitropism, however, is still rudimentary. Several examples of gravisusceptors and mechanisms for gravity sensing have been suggested and presented here. We hope that the combination of genetics and molecular biology with detailed cytological characterization of model systems will help to unravel the complexities of fungal gravitropism. Light and gravity interact in complex ways to direct sporangiophore growth in Phycomyces, possibly at the level of the photoreceptor system itself. It is thus possible that light and gravity will share some of the elements of the transduction chain in fungi. Only future research will confirm the possible relationship between light and gravity as major signals for fungal development, growth, and appearance.