Far red and blue light-induced binding of phytochrome to a subcellular fraction of maize coleoptiles

Abstract

Summary Irradiation of excised maize coleoptiles with 660 nm light caused phytochrome to become associated with a crude membrane fraction when the tissues were subsequently fractioned in the presence of added Mg++ ion. Over 85% of the particle-associated phytochrome was found to be in the far red absorbing form (Pfr). Wavelengths in the blue and far red ranges were found to be effective in inducing high levels of particle-associated phytochrome, though less efficiently than red light. Relatively longer irradiations with blue or far red light were therefore required to effect an equivalent degree of phytochrome-particle association when compared with 660 nm light. Examination of the phytochrome state in the crude membrane fraction of coleoptiles irradiated with large doses of far red light showed no detectable amount of Pfr. In the case of coleoptiles irradiated with blue light, about 40–50 % of the particle bound phytochrome was in the Pfr form. The data are interpreted as supporting a working hypothesis that any phytochrome molecule (irrespective whether Pfr or Pr) can be activated to associate with its putative binding site by absorbing sufficient energy and that this can be considered a primary photoreaction.