Phy-Gene Structure, Evolution, and Expression

The fundamental notion that phytochrome controls plant development through differential regulation of gene expression (Mohr, 1966) is now well supported by direct experimental evidence (Benfey and Chua, 1989; Gilmartin et al., 1990; Kuhlemeier et al., 1987; Nagy et al., 1988; Tobin and Silverthorne, 1985). However, the molecular mechanism by which the photoreceptor transduces its regulatory signal to genes under its control remains unknown. For some time we have approached this question by simultaneously studying the properties of the photoreceptor molecule and the negative autoregulation of phytochrome (phy) genes as a paradigm of phytochrome-regulated gene expression (Colbert, 1988; Lissemore and Quail, 1988; Quail et al., 1987b, 1990). Recent molecular-genetic studies have revealed that phytochrome is encoded by a small family of divergent and differentially regulated genes (Dehesh et al., 1990b; Sharrock and Quail, 1989); have shown that overexpression of the photoreceptor in heterologous, transgenic plants provides a system for directed mutational analysis of functional regions of the polypeptide (Boylan and Quail, 1989; Kay et al., 1989c; Keller et al., 1989); and have begun to provide insight into the cis-regulatory elements and trans-acting factors involved in phy gene transcription (Brace et al., 1989, 1990; Dehesh et al., 1990a).