In some LDPs, such as Arabidopsis, blue light can promote flowering, suggesting the possible participation of a blue-light photoreceptor in the control of flowering. The role of blue light in flowering and its relationship to circadian rhythms have been investigated by use of the luciferase reporter gene construct mentioned in Web Topic 24.6. In continuous white light, the cyclic luminescence has a period of 24.7 hours, but in constant darkness the period lengthens to 30 to 36 hours. Either red or blue light, given individually, shortens the period to 25 hours.
To distinguish between the effects of phytochrome and a blue-light photoreceptor, researchers transformed phy-tochrome-deficient hy1 mutants, which are defective in chromophore synthesis and are therefore deficient in all phytochromes (see Chapter 17), with the luciferase construct to determine the effect of the mutation on the period length (Millar et al. 1995).
Under continuous white light, the hy1 plants had a period similar to that of the wild type, indicating that little or no phytochrome is required for white light to affect the period. Furthermore, under continuous red light, which would be perceived only by PHYB (see Chapter 17), the period of hy1 was significantly lengthened (i.e., it became more like constant darkness), whereas the period was not lengthened by continuous blue light. These results indicate that both phytochrome and a blue-light photoreceptor are involved in period control.
FIGURE 24.25 Vernalization induces flowering in the winter-annual types of Arabidopsis thaliana. The plant on the left is a winter-annual type that has not been exposed to cold. The plant on the right is a genetically identical winter-
The role of blue light in regulating both circadian rhyth-micity and flowering is also supported by studies with an Arabidopsis flowering-time mutant: elf3 (earlyflowering 3) (see Web Topics 24.5 and 24.6). Confirmation that a blue-light photoreceptor is involved in sensing inductive photoperiods in Arabidopsis was recently provided by experiments demonstrating that mutations in one of the cryptochrome genes, CRY2 (see Chapter 18), caused a delay in flowering and an inability to perceive inductive photoperiods (Guo et al. 1998). As discussed in Chapter 18, CRY1 encodes a blue-light photoreceptor controlling seedling growth in Arabidopsis. Thus, various CRY family members have, through evolution, become specialized for different functions in the plant. As noted earlier, the CRY protein has also been implicated in the entrainment of the circadian oscillator (see Chapter 17).
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