Role of nitric oxide synthase in the light-induced development of sporangiophores in Phycomyces blakesleeanus

Blue light controls the development of sporangiophores in the zygomycete Ph ycomyces blakesleennus Burgeff. Light represses the production of microspor angiophores and enhances the development of macrosporangiophores. Inhibitio n of the biosynthesis of tetrahydrobiopterin, a cofactor of NO synthase, in hibits this photomorphogenesis. Light induces production of citrulline from arginine in the mycelium and in sporangiophores. The citrulline-forming ac tivity is dependent on NADPH, independent of calcium, and inhibited by NO s ynthase inhibitors. It is reduced in tetrahydrobiopterin-depleted mycelium. Light induces emission of NO from the developing fungus in the same order of magnitude as citrulline formation from arginine. The NO donor sodium nit roprusside can replace the light effect on sporangiophore development, and inhibitors of NO synthase repress it. We suggest that a fungal NO synthase is involved in sporangiophore development and propose its participation in light signaling.