PHOTOTROPISM IN <i>PHYCOMYCES</i> MUTANTS LACKING β‐CAROTENE

Abstract

Abstract. β‐carotene and riboflavin are considered as the major candidates for the photoreceptor for physiological responses to blue light in Phycomyces and a number of other organisms. Mutants of Phycomyces blocked in all six steps of the biosynthesis of β‐carotene from phytoene contain no detectable β‐carotene (less than 4 times 10 ‐5 of wild‐type amount) but exhibit phototropic responses identical to wild‐type. Moreover, wild‐type Phycomyces , while abundant in trans ‐β‐carotene, contains no detectable cis ‐β‐carotene, sometimes proposed as a photoreceptor candidate on the basis of the close similarity of the cis ‐species absorption spectrum in the near UV region to many action spectra for blue‐light responses. These results indicate that β‐carotene cannot be the photoreceptor for phototropism in Phycomyces.