Sae Shimizu and Hitoshi Mori .
Light is one of the most important external signals.
In the light, plant seedling shows photomorphogenesis resulting in the inhibition of hypocotyl elongation, opening of the apical hook of cotyledons, induction of greening, and elongation of leaf primordia. While in the absence of light, the elongation of hypocotyl and root is not inhibited, and the apical hook of cotyledons is maintained. Little is known about how light signals inhibit hypocotyl cell elongation and induce cellular differentiation in the hypocotyl and cotyledons. Many mutants have been screened in Arabidopsis in order to study on the developmental programs of higher plant. The data indicate that these genes are involved in the biosynthesis of active brassinosteroids, which may constitute a distinct class of phytohormones with an important role in cell elongation and light-regulated development of higher plants.