Deetiolation

Deetiolation is the series of rapid morphological, physiological, and molecular changes that occur when a plant seedling or shoot emerging from darkness is exposed to light. In darkness, etiolation produces elongated, pale stems and underdeveloped, chlorophyll-deficient leaves. Deetiolation reverses these traits, promoting chlorophyll biosynthesis, chloroplast development, leaf expansion, and the onset of photosynthesis.

Morphological changes include shorter hypocotyls, expanded cotyledons, and greening as chloroplasts mature and photosynthetic machinery assembles. Biochemical shifts involve increased chlorophyll content, carotenoids, and a transition from skotomorphogenic to photomorphogenic metabolism, with enhanced carbon fixation and energy capture.

Molecularly, light perception by photoreceptors such as phytochromes (red/far-red), cryptochromes, and phototropins initiates signaling cascades that regulate gene expression. Key regulators include HY5, a transcription factor promoting photomorphogenesis, and the COP1/SPA E3 ligase complex, which targets HY5 for degradation in darkness. Light inactivates COP1, stabilizing HY5 and increasing expression of photosynthesis genes; PIF transcription factors are degraded in light, further promoting deetiolation.

The timing of deetiolation depends on light quality and intensity and proceeds within hours to days. Deetiolation is a central topic in studies of photomorphogenesis and plant development, providing insights into how plants integrate environmental light cues to optimize growth and survival.