From Darkness into Light: Factors Controlling Photomorphogenesis

A seedling that emerges in darkness follows a developmental program known as skotomorphogenesis (dark development), which is characterized by etiolation, that is, a long, spindly hypocotyl and pale cotyledons and inhibition of chlorophyll and anthocyanin biosynthesis and true leaf development. Upon exposure to light, the seedling switches rapidly to photomorphogenesis (light development); the hypocotyl ceases rapid elongation and becomes thicker, the shoot apical meristem is activated, chlorophyll and anthocyanin biosynthesis is initiated, and true leaves begin to develop. It has long been known that this process is triggered directly by light perception and signal transduction, because developmental changes in the hypocotyl and cotyledons can be detected within minutes of exposure to a light source. Plants have an array of light-sensing molecules that function in association with numerous downstream targets to allow for the precise determination of light quality, quantity, and duration. This issue of The Plant Cell includes two papers related to light perception and photomorphogenesis. On pages 425–436, Kohchi et al. report the isolation of Arabidopsis HY2 and its identification as the gene encoding phytochromobilin synthase. The HY2 protein catalyzes the last step in the production of the chromophore (phytochromobilin) entity of phytochromes, the red light photoreceptors that are essential for photomorphogenesis and many other light responses of plants. Meanwhile, Yamamoto et al. (pages 399–411) report on the characterization of a novel protein, CIP4, which acts downstream of the photoreceptors and is required for the promotion of photomorphogenesis via From Darkness into Light: Factors Controlling Photomorphogenesis