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 processes. 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 darkness, 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 alterations of specific transcript levels (for recent reviews, see Ellis, 1986; Kendrick and Kronenberg, 1986; Cuozzo et al., 1987; Kuhlemeier et al., 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 5bisphosphate carboxylase-oxygenase (rbcS) and the chlorophyll a/b-binding proteins (cab) (see Tobin and Silverthorne, 1985; Manzara and Gruissem, 1988; Dean et al., 1989~). In severa1 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 mediated by the photoreceptor phytochrome and is regulated at the transcriptional leve1 (Gallagher and Ellis, 1982; Silverthorne 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).