Blue Light Effects

Early studies using action spectroscopy which implicated phytochrome as the photoreceptor for photoperiodism were based on night-break experiments in which relatively short light treatments were involved. When more extended light treatments were used to test the responses of LDP it was noted that blue was generally of low effectiveness, except in members of the Cruciferae (Wassink et al., 1950; Stolwijk, 1952a,b). Interpretation of the response to blue light is problematic because phytochrome absorbs in this region of the spectrum, albeit weakly, and the low Pfr/Ptot established in blue light would be expected to be favourable for induction in LDP (see Chapter 5). However, the strong sensitivity of the members of the Cruciferae, but not other LDP, to induction by blue light implied the action of a separate blue-absorbing photoreceptor. Support for this idea comes from work with Arabidopsis, a typical cruciferous LDP, where dichromatic irradiation with blue and monochromatic light at 589 nm was shown to be inductive, whereas irradiation with 589 nm light alone was ineffective. The Pfr/Ptot ratios and cycling rates between Pr and Pfr established by the two treatments are approximately equal and thus the blue response cannot be due to phytochrome (Mozley and Thomas, 1995). Although phytochromes are probably the most important photoperiodic photoreceptors in higher plants, blue light is frequently the most effective waveband in lower plants.

Speculation about the nature of the blue-absorbing photoreceptor has occupied photobiologists for several decades. Action spectra for responses sensitive to blue light, such as phototropism, are characterised by an action maximum at about 450 nm, with shoulders at about 420 and 490 nm, accompanied by a broad action peak near 370 nm. Candidates for the blue photoreceptor are flavoproteins and carotenoproteins; both flavins and carotenoids are capable of generating an absorbance spectrum to match the action spectrum under appropriate conditions. The blue-absorbing photoreceptor has yet to be isolated from plants, but the gene corresponding to the HY4 locus of Arabidopsis has recently been cloned and sequenced (Ahmad and Cashmore, 1993). Hy4 mutants show greatly reduced sensitivity to blue light for inhibition of hypocotyl extension and are thus postulated to be deficient in a blue-absorbing photoreceptor. The HY4 gene codes for a protein which shows significant homology to microbial DNA photolyases, which are flavoprotein enzymes activated by blue light. This offers very strong evidence that one higher plant blue-absorbing photoreceptor has now been identified. The hy4 mutant shows reduced sensitivity for long-day photoperiodic induction (Mozley and Thomas, 1995), suggesting that the same photoreceptor is responsible for blue-light regulation of hypocotyl growth and photoperiodic effects in Arabidopsis.

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