Evidence for the existence of membrane-associated phytochrome in the cell

Comparative fluorescence and photochemical studies of phytochrome in etiolated seedlings of maize and in soluble and membrane-containing fractions isolated from them were carried out. The membrane fractions prepared in the absence of Mg2+ from etiolated coleoptiles contained 13% of total photoreversible phytochrome, which was readily solubilized by mild detergents. Its molecular size was indistinguishable from soluble phytochrome and equal to nondegraded maize phytochrome. Low-temperature fluorescence studies with intact tissue found that the position of the emission maximum at 85 K (lambda max) and the extent of the phototransformation of the red-absorbing form (Pr) into the first stable photoproduct, lumi-R, at 85 K (gamma 1), varied in different parts of etiolated seedlings: lambda max and gamma 1 reached their maximum values in the tips of coleoptiles and roots, 686 nm and 0.30-0.40, whereas the lowest values, 682 nm and ca 0.05, were observed in the root base. These parameters correlated well with those obtained for the pigment in the soluble and membrane-containing fractions: 684 and 680 nm, and 0.33 and 0.06, respectively. The extent of the Pr phototransformation into the far red-absorbing form (Pfr) (gamma 2) did not differ much: values of 0.80-0.85 and 0.70-0.75 correlated with the high and low values of gamma 1. These variations of the parameters were interpreted in agreement with our previous observations in terms of two phytochrome A species whose relative concentrations vary depending on the experimental conditions--the longer wavelength bulk light-labile species with high gamma 1 (Pr'), and the shorter wavelength minor light-stable species with low gamma 1 (Pr").(ABSTRACT TRUNCATED AT 250 WORDS)