Effects of filipin and steroids on phytochrome pelletability.

Red light given to dark-grown etiolated leaves of Hordeum vulgare L. in vivo or to crude homogenates increases the phytochrome content of the 20,000 g pellet on centrifugation. The steroids cholesterol and stigmasterol inhibit this red light-induced phytochrome pelletability. Filipin (a polyene antibiotic, which is known to combine with steroids) inhibits red light-induced phytochrome pelletability. Filipin and steroids at the appropriate concentration applied together prevent the inhibition caused by either when applied alone. These results suggest that phytochrome may bind to a steroid component of membranes. The phospholipid phosphatidyl choline dipalmitoyl has no effect on red light-induced phytochrome pelletability. Preliminary evidence demonstrates a direct association of soluble phytochrome in its active form and steroids. The physiological significance of red light-induced pelletability and the primary mechanism of phytochrome action are discussed in terms of a hypothetical steroid-binding site.