Interactions of the Arabidopsis Type II Phytochromes

Abstract

The need to distinguish between two different “pools” or “types” of phytochrome in higher plants dates to the early 1960s, when efforts to correlate plant physiological light responses with the spectroscopic properties of phytochrome were sometimes confounding and inconsistent with the activity of a single molecular species of the photoreceptor ([Pratt 1995]). Moreover, it progressively became clear that phytochrome regulates responses to remarkably different parameters of the light environment (LFRs,VLFRs, HIRs and responses to ratios of R :FR) and the likelihood that all of these responses were mediated by one pigment seemed remote. In 1985, the three groups led by Furuya, Pratt, and Quail independently described forms of phytochrome that were present at low levels in extracts of both light-grown and dark-grown plants. These appeared to be distinct from the form of phytochrome that was abundant in etiolated plant tissues but rapidly degraded upon exposure to light ([Furuya 1993]).The distinction made by all three of these groups between a pool of phytochrome called “etiolatedtissue,” “light-labile,” or “type I” phytochrome and a pool called “green-tissue,” “light-stable,” or “type II” phytochrome has largely been supported by the identification and analysis of PHY gene families and the phytochrome apoproteins encoded by those genes, and by characterization of phy mutants that illuminate the individual activities of those genes.