Spectrophotometric evidence for the presence of phytochrome in the envelope membranes of barley etioplasts

Abstract

PHYTOCHROME is a chromoprotein photoreceptor, existing in two photoconvertible forms (Pr and Pfr), and is responsible for various metabolic and developmental responses of higher plants to light¹. The location of phytochrome in plant cells and its mechanism of action are matters of debate. Many physiological responses to light have been adduced as evidence for phytochrome either being a component of certain cellular membranes, or acting on them, bringing about rapid changes in cell metabolism which lead to developmental changes². There have been claims that phytochrome, in the Pfr form only, binds specifically to as yet uncharacterised membranous fractions^3–6 and the data have formed the basis for allosteric models of the action of phytochrome^3–7. Further progress requires a demonstration of the precise location of phytochrome with relation to a well characterised membrane where it is known to direct a process in vitro. We demonstrated^8,9 the specific association of a proportion of total cellular phytochrome with etioplasts isolated from 6-d-old etiolated barley leaves. Etioplast phytochrome, as a result of its interconversions, was shown to regulate, in vitro, the efflux of endogenous etioplast gibberellins across the envelope membranes into the surrounding medium. Confirmatory results have been obtained with etiolated wheat leaves^10,11. Although phytochrome was detected spectrophotometrically in the purified etioplast preparations from barley, no data were obtained for the suborganellar location of the photoreceptor or for the amount present in the etioplasts. We report here that although only a small proportion of the total phytochrome is associated with etioplasts, it seems to be concentrated in the envelope membranes. These data support the view that phytochrome is located in specific cellular membranes where it regulates transmembrane transport¹².