Photoperception and de-etiolation

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Abstract
In seedlings or sprouts of higher plants, photomorphogenesis is the strategy of development if and as long as abundant light is available, and scotomorphogenesis (etiolation) is the developmental strategy of choice as long as light is not yet, or no longer, available. The transition from scotomorphogenesis to photomorphogenesis (called de-etiolation) can be considered a process in which a single, well defined environmental factor causes a plant to change its pattern of gene expression. The present article focuses on the question: what is the photosensory system, including photoreception and signal transduction, through which a plant can detect those light conditions that justify the (gradual) shift from scotomorphogenesis to photomorphogenesis, i.e. de-etiolation, which implies a strong and partly irreversible investment of m atter and energy? The significance of phytochrome for signal reception, the mode of signal expression, and the time course of signal transduction in phytochromemediated responses are reviewed briefly. The emphasis is on amplification of the phytochrome signal by red, blue and ultraviolet light (measured as responsivity amplification) because these recent findings may lead to a better understanding of the responses of plants under natural light conditions.