Optical properties of piloboloid and β-carotene-overproducing mutants of phycomyces blakesleeanus responsible for their negative phototropism

Abstract

Sporangiophores (spphs) of the piloboloid mutant (genotype pil) of Phycomyces, the extension zone of which gradually ceases elongation but expands spherically shortly after sporangium formation (stage IVb), and of the β-carotene-overproducing mutant (genotype carS) grown on medium containing vitamin A show negative phototropism toward unilateral visible (blue) light, in contrast with the wild-type spph. We hypothesized that the phototropic orientation of spph, positive or negative, is determined by the ratio of the maximal light fluence rate of the proximal side ( I Pmax) to that of the distal side ( I Dmax) of spph. The reversal of phototropism in the pil and car mutants probably occurs because of the expansion of the extension zone (increase in length of intracellular light path) and the accumulation of excess β-carotene in the extension zone, respectively. These characteristics of the mutants reduce the transmission of incident light to the distal side of the spph cell, and cause a reverse in the I Pmax/ I Dmax ratio as a result of a decrease in I Dmax. The information obtained on the optical properties of the extension zones of these mutants supports our hypothesis.