Responses of Phycomyces to environmental signals - How and why do Phycomyces sporangiophores bend in response to light? -

Abstract

　Phycomyces is a unicellular fungus with numerous responses to blue light, including phototropism by the giant sporangiophores. The sporangiophores show a pronounced positive phototropism when illuminated with unilateral blue light. The orientation of the phototropism, positive or negative, is determined by the ratio of the maximal light fluence rate on the proximal side to that on the distal side of the sporangiophore cell. Based on this hypothesis we found that the maximal bending angle was larger in thin sporangiophores than in thick ones, and larger in the sporangiophores with lower contents of cell components such as β-carotene than in those with higher contents. Several parameters influence the determination of the phototropic orientation and the maximal bending angles. To elucidate the photoreceptor and signal transduction systems, genetic and physiological analyses have been carried out using many mutants with abnormal phototropism. Possible photoperception and transduction systems of light signals were discussed.