Molecular light switches for plant genes.

Light is essential for normal plant growth and development not only as a source of energy but also as a stimulus that regulates numerous developmental and metabolic proc? esses. The plant's responses are varied and complex and dependent upon the quality and quantity of ambient light. The initial requirement for light is as a signal for germination in many plant species. After germination in complete dark? ness, seedlings have a morphology distinct from lightgrown ones and do not express light-inducible genes. Upon illumination of these etiolated seedlings, modifications in the transcription of light-responsive genes occur and rapid light-induced morphological changes ensue. Adaptation of plants in continuous darkness for 2 to 3 days does not cause dramatic morphological changes but results in alter? ations of specific transcript levels (for recent reviews, see Ellis, 1986; Kendrick and Kronenberg, 1986; Cuozzo et al., 1987; Kuhlemeier etal., 1987b; Silverthorne and Tobin, 1987; Jenkins, 1988; Nagy et al., 1988). The most extensively studied light-responsive genes are those encoding the small subunit of ribulose-1,5-bisphos? phate carboxylase-oxygenase (rbcS) and the chlorophyll a/? >-binding proteins (cab) (see Tobin and Silverthorne, 1985; Manzara and Gruissem, 1988; Dean et al., 1989c). In several plant species an increase in the transcript levels from these genes occurs in etiolated seedlings and darkadapted plants in response to light. This increase is me? diated by the photoreceptor phytochrome and is regulated at the transcriptional level (Gallagher and Ellis, 1982; Sil? verthorne and Tobin, 1984; Berry-Lowe and Meagher, 1985; Mosinger et al., 1985). Phytochrome is the best characterized of the three known photoreceptors. The other two, cryptochrome and the UV-B photoreceptor, mediate their effects in response to blue and UV light, respectively (see Kendrick and Kronenberg, 1986). The expression of many light-responsive genes is modulated by more than one wavelength of light (see Tobin and Silverthorne, 1984; Ellis, 1986; Kuhlemeier et al., 1987b). The observation that different light responses are mediated through distinct photoreceptors raises the question of whether genes that respond to more than one wavelength do so through distinct c/s-acting elements or whether the signal transduction pathways converge to act upon the same regulatory sequence. Analyses of the kinetics of light-responsive gene induc? tion show that the rate of mRNA accumulation is variable among genes and can be dependent on the developmental state of the plant (Gallagher et al., 1985; Fluhr and Chua, 1986). This variation may be due to a requirement for distinct regulatory factors or because the genes have different thresholds for a specific regulator. Several genes are down-regulated by light, specifically those encoding phytochrome (Lissemore and Quail, 1988; Kay et al., 1989), NADPH-protochlorophyllide reductase (Batschauer and Apel, 1984; Darrah et al., 1990), and asparagine synthetase (Tsai and Coruzzi, 1990). For each of these genes, the photoresponse is mediated by phyto? chrome. The ability of one photoreceptor to mediate op? posite patterns of expression implies that there is a branch point in the signal transduction pathway leading to these different responses. Studies of many light-regulated genes from different species demonstrate that DNA elements responsible for light-responsive expression are located within 5' upstream sequences (see Kuhlemeier et al., 1987b; Silverthorne and Tobin, 1987; Jenkins, 1988; Benfey and Chua, 1989; Dean et al., 1989a; Stockhaus et al., 1989). However, there is evidence that other regions of the gene can mediate changes in transcript abundance in response to light. For example, in the case of a pea gene encoding ferredoxin, sequences within the transcribed region modulate mRNA levels by affecting transcript stability (Elliot et al., 1989a). In addition, nuclear run-on experiments with petunia rbcS show that both upstream and downstream sequences play a role in the transcriptional regulation of these genes (Dean et al., 1989b). In contrast, downstream sequences of pea rbcS do not affect steady-state transcript abundance (Kuhlemeier et al., 1988b). These differences may reflect subtle variations in the mechanisms that operate to regu? late rbcS expression in different plant species. 1 Current address: Department of Plant Molecular Biology, Leiden University, Clusius Laboratory, Lassenaarseweg 64, 2333 AL Leiden, The Netherlands. 2 To whom correspondence should be addressed.