School of Biological Sciences, Royal Holloway, University of London, Egham TW20 0EX, UK
Abstract Light is the source of energy for plants, and as a result is a key environmental cue controlling their growth. The need to reach the appropriate light environment, and then to maximise its capture, leads to different organs showing contrasting growth regulation by light: generally negative in aerial elongating organs (hypocotyl, internodes, petioles) and positive in photosynthetic ones (leaf blades), as well as in roots. The basis of the growth repression of elongating organs by photoreceptors appears to involve the suppression of hormone signalling pathways, particularly those of auxin and gibberellin, although the mechanisms of control prior or subsequent to light exposure differ. Less is known about growth promotion by light in cotyledons, shoot meristem and leaves, but this promotion is closely associated both with the repression of elongation growth in hypocotyls and with the differentiation of photosynthetic cells: molecular interference with one process generally affects the others. The possible nature of these links is discussed. Lastly, light fluence rate controls the internal anatomy of leaves, and this highlights a role for chloroplasts themselves as sources of growth-regulatory signals.
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