FHY1: a phytochrome A-specific signal transducer

Phytochromes are plant photoreceptors that regulate plant growth and develo pment with respect to the light environment. Following the initial light-pe rception event, the phytochromes initiate a signal-transduction process tha t eventually results in alterations in cellular behavior, including gene ex pression. Here we describe the molecular cloning and functional characteriz ation of Arabidopsis FHY1. FHY1 encodes a product (FHY1) that specifically transduces signals downstream of the far-red (FR) light-responsive phytochr ome A (PHYA) photoreceptor. We show that FHY1 is a novel light-regulated pr otein that accumulates in dark (D)-grown but not in FR-grown hypocotyl cell s. In addition, FH-YI transcript levels are regulated by light, and by the product of FHY3, another gene implicated in FR signaling. These observation s indicate that FHY1 function is both FR-signal transducing and FR-signal r egulated, suggesting a negative feedback regulation of FHY1 function. Seedl ings homozygous for loss-of-function fhy1 alleles are partially blind to FR , whereas seedlings overexpressing FHY1 exhibit increased responses to FR, but not to white (WL) or red (R) light. The increased FR-responses conferre d by overexpression of FHY1 are abolished in a PHYA-deficient mutant backgr ound, showing that FHY1 requires a signal from PHYA for function, and canno t modulate growth independently of PHYA.