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