8.4  Photoreceptors and light cues

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Light is the energy source that drives plant life so it is no surprise that plants generally maximise the interception of solar radiation. These strategies range from the complexities of chloroplast ultrastructure to tree architecture. Energy for photosynthesis is harvested by chlorophyll and accessory pigments (Section 2.3), but plants also possess other light-absorbing molecules that have evolved to sense light intensity, light duration, light direction and spectral composition. These photoreceptors are coupled to many developmental processes. For example, the developmental strategy of a seed on the soil surface with immediate access to sunlight is quite different from one buried under several centimetres of soil. The initial growth phase of the latter needs to be rapid and upwards and to consume as little of the seed’s resources as possible. That is why seeds germinated in the dark have spindly stems, aren’t green (because there is no possible photosynthesis) and don’t expand their leaves (because this is unnecessary and they would cause friction as the shoot grows through the soil). When the shoot tip does reach light, there is a complete reassignment of priorities resulting in assembly of functional chloroplasts, expansion of leaves and reduction in stem elongation. These processes are coordinated by two main classes of photo-receptor: phytochromes and blue-light receptors (also known as cryptochromes). Here, we consider briefly the operation of these light sensors at the molecular and physiological levels.