The role of the COP/DET/FUS genes in light control of arabidopsis seedling development.

Light is vital to plant life, not only as an energy source for photosynthesis but also as an important environmental signal regulating development and growth. Light affects almost every stage of plant development (Kendrick and Kronenberg, 1994), including seedling development, which represents one of the most dramatic and best characterized processes (von Arnim and Deng, 1996). In Arabidopsis tkaliana, for example, the morphology of the embryo in the imbibing seed (d 1), as well as the emerging seedling from the seed coat (d Z), are minimally affected by light conditions (Wei et al., 199417). Soon after, however, seedling morphogenesis differs drastically, depending on the light environment (Fig. 1). Light-grown seedlings exhibit short hypocotyls and open and expanded cotyledons. Cell-type differentiation and chloroplast development are soon established, and photosynthetically related genes are highly expressed. The shoot apical meristem is activated to produce true leaves and the plants proceed with further vegetative and reproductive growth soon thereafter. This development pattern in light is known as photomorphogenesis. In contrast, when seedlings are grown in complete darkness, they undergo a developmental program known as skotomorphogenesis or etiolation, in which the cotyledons remain folded and undeveloped, while the hypocotyls rapidly elongate. The apical hook serves to protect cotyledons and the quiescent shoot meristems as the seedling elongates rapidly to reach for the light. Instead of developing chloroplasts, the cotyledon cells form etioplasts that can readily convert into chloroplasts when exposed to light. This process is known as greening or de-etiolation. In addition, etiolated seedlings display a very different gene expression pattern from that determined by light. After the initial elongating growth, the seedlings come to a developmental arrest in the continuous absence of light. In higher plants, light-controlled physiological and developmental responses are mediated through at least three families of photoreceptors: phytochromes, cryptochromes, or blue-light receptors, and UV-B receptors, depending on the wavelengths of light to which they are most sensitive.