Phototropism mutants of Phycomyces blakesleeanus isolated at low light intensity

Mutants of Phycomyces have played a major role in the analysis of phototropism and other responses. Fifteen new mutants of Phycomyces with abnormal phototropism (genotype mad) have been isolated on the basis of their inability to bend toward dim unilateral blue light (fluence rate 5 × 10−7 W m−2), a protocol different from those employed in previous mutant hunts. One mutant resulted from chemical mutagenesis with ICR-170, and the other 14 were induced with N-methyl-N′-nitro-N-nitro-soguanidine. Seven of the mutants are “night blind”; six have phototropic thresholds intermediate between those of wild type (10−9 W m−2) and madA strains (∼ 10−4 W m−2); and one has a threshold similar to that of night-blind madB and madC mutants. The other eight mutants are “stiff”, with various reductions of tropic responsiveness. Two of them, when compared to previously isolated stiff mutants, show unusually weak responses to light, barriers, and gravity.