Photoregulation of plant gene expression.

Light is the ultimate energy source for all living organisms and is utilized primarily through photosynthesis by green plants. However there is a separate requirement of plants to be illuminated in which light acts not as an energy source but as an environmental trigger of developmental processes. This decisive role of light in controlling the growth of plants is described by the term "photomorphogenesis'. Photomorphogenesis is an essential feature of all land plants; in fact the normal development of higher plants is photomorphogenesis (1). Thus photomorphogenetic responses include the germination of seeds, the elongation and direction of growth of the main axis, the expansion of leaves, the development of chloroplasts, the synthesis of pigments and the formation of flowers. Seedlings grown from seed entirely in darkness lack chlorophyll and differ greatly in morphology from those grown under a normal photoperiod; the leaves remain unexpanded while the stems elongate greatly. Such plants are termed "etiolated" and the etiolated response is regarded as an adaptive one; the energy reserves from the seed are used to promote growth in length at the expense of growth in area, so as to increase the chance of bringing the embryonic leaves above the soil surface before the reserves are exhausted. This chance is greatly enhanced by the phototropic response, which ensures that the etiolated stem grows towards the light source and not away from it. Exposure of etiolated seedlings to light slows down the rate of stem elongation and triggers the rapid formation of photosynthetically-active chloroplasts in expanding leaves. This "de-etiolation" process provides a useful experimental system which has provided much of the biochemical evidence that we have regarding photomorphogenesis. The morphogenetic effects of light are not related to the use of light as an energy source. For example, the effects of light on the development of etiolated seedlings are initiated before the pathway of photosynthesis is operational, and require amounts of