PHOTOMORPHOGENESIS IN BEHAVIORAL AND COLOR MUTANTS OF PHYCOMYCES

Morphogenesis in the fungus Phycomyces blakesleeanus is governed by blue light, which stimulates the production of giant sporangiophores (macrophorogenesis) and inhibits the production of dwarf sporangiophores (microphorogenesis). We have measured these responses in colour mutants, defective in the biosynthesis of β-carotene or in its regulation, and in behavioural mutants defective in phototropism. Genetic blocks in the production of β-carotene (carb, carr and carRA mutants) severely inhibit both responses (threshold about 104J m-2 compared with 10-4 J m-2 for the wild type). Neither the presence of phytoene or lycopene nor the addition of retinol substitute for β-carotene. Partial defects (thresholds of 10-2 J m-2) are found in madA and madB mutants (defective in photoreception for phototropism) and in carA mutants (defective in the regulation of carotenogenesis). The carA defect is not related to variations in the β-carotene content. Other mutations affecting phototropism (genes madC to madH) and carotenogenesis (genes carC, carD, carS, picA and picB) have no effect on photomorphogenesis. Thus photomorphogenesis requires elements responsible for phototropism (the products of genes madA and madB) and elements unrelated to phototropism (β-carotene and the carA gene product, which may be β-carotene-binding protein). © 1990.