8.4.5  Blue-light receptors and responses

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Figure 8.35 A blue-light receptor is responsible for phototropism. Action spectra for monocotyledons (Avena, oat) and dicotyledons (alfalfa) are very similar, and suggest that a flavin is part of the chromophore.

(Based on Baskin and Iino 1987)



Figure 8.36   Responses to blue light can be very rapid, often much faster than those mediated by phytochrome. Here, growth rate (a) decreases after 30s and plasma membrane electric potential (b) changes even sooner, within 15s, when blue light (10 µmol m-2 s-1) is applied to hypocotyls of dark-grown cucumber seedlings.

(Based on Spalding and Cosgrove 1988; reproduced with permission of Springer-Verlag)

Although Julius von Sachs in the 1860s discovered that blue light caused phototropism, photomorphogenesis under blue-light control has long been the poor cousin of studies on phytochrome. However, since the 1980s, enormous progress has been made, leading to characterisation of a blue-light receptor, sometimes called cryptochrome, that is quite unrelated to phytochrome. Responses to blue light require relatively high light intensities, but can occur extremely fast — electrical potentials across the plasma membrane can alter within 15s, and cucumber seedling growth can be reduced within 30s of transferring from dark to blue light (Figure 8.35). Speeds of this order tell us that some blue-light responses are initiated without any need for a change in gene expression. Although blue light is also the prime causative agent in phototropism (Figure 8.36 and see Section 8.2.5), this differential growth response has a much longer lag time, usually around 30 min, than in the straight growth inhibition mentioned above. As with phytochrome, we now know that there are multiple forms and genes for the blue-light receptor (Cashmore 1997), each comprising a protein and two chromophores, one of which is flavin adenine dinucleotide (FAD) and the other possibly a pterin. However, it is not clear which of these functions as the receptor for phototropism. Briggs and Liscum (1997) concluded from studies with the hy4 (hypocotyl length) and nph (non-phototropic hypocotyl) mutants of Arabodopsis that elongation growth and phototropism are under genetically independent control.