Light perception in plants
Light is detected by photoreceptors. The three classes of photoreceptor described in plants are the phytochromes, which predominantly detect red/far-red light; the cryptochromes, which detect light in blue/UV-A wavelengths, and the phototropins, which also detect in the blue region of the spectrum. Phytochromes and cryptochromes have established roles in flowering, whereas phototropins do not influence flowering time. In Arabidopsis, exposure to blue or far-red light promotes flowering, whereas exposure to red light has an inhibitory effect. Nevertheless, exposure to light is not essential for flowering to occur and Arabidopsis plants grown in darkness on sucrose-containing medium will flower (Bodson and Outlaw, 1985) . It is thought that by regulating its development by use of blue and red light photoreceptors that a plants development will proceed under optimal conditions (Lin, 2000b; Lin, 2000d) .
Phytochromes
There are five genes encoding phytochrome isoforms in Arabidopsis. These genes are referred to as PhyA to PhyE. The PhyA and PhyB genes have been extensively analysed and have a greater influence upon flowering than the others. The analysis of mutations in these two genes demonstrated that PhyA functions as a far-red light photoreceptor during seedling de-etiolation, and in particular to repress hypocotyl elongation in response to exposure to light. In contrast PhyB controls the same responses in response to red light.
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