Mutants of Phycomyces with enhanced tropisms

Seven mutants of Phycomyces which exhibit phototropism at high intensity of blue light (10 W/m²) where the wild-type strain is unresponsive have been isolated. These mutants have the same absolute threshold for phototropism as wild type (10^-9 W/m²). In comparison to wild type in the region just above this threshold, the mutants respond more strongly to light than to gravity, as determined by photogeotropic equilibrium experiments. The kinetics of phototropism, avoidance, and geotropism are also enhanced in the mutants. Therefore these mutants are designated phenotypically by the term "hypertropic." The phenotype of these mutants is in many ways opposite to that of the so-called "stiff" mutants, which have slow tropisms. The rapid bending rates of the hypertropic mutants may be associated in part with the smaller diameter of their sporangiophores, but not according to a simple proportional relationship between phototropic bending rate and inverse diameter that applies for wild type and the hypergeotropic mutant C5. For photophorogenesis, which is mediated by the mycelium, one hypertropic mutant studied responds similarly to wild type. The pleiotropic character of these mutants suggests that they are affected near the output of the common sensory transduction pathway for these responses.