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.
Photoreceptors have opposing roles in the transition to flowering
PhyA and PhyB photoreceptors appear to have opposing roles in the floral-transition. PhyA promotes flowering under long days, so that the phyA mutant flowers later than wild-type under long but not short days (Johnson et al., 1994; Neff and Chory, 1998) . Conversely, PhyB delays the floral transition, and phyB mutants flower much earlier than wild-type plants under both long and short days (Goto et al., 1991) . PhyB represses flowering in response to a reduction in the ratio of Red to far-red light; a phenomenon that occurs when a plant is shaded by other plants (Halliday et al., 1994) . PhyB partially represses flowering through down-regulation of genes in the photoperiod pathway (Halliday et al., 2003) . PhyD and PhyE also repress flowering, but only in the absence of PhyB, strongly suggesting that the genes act redundantly (Aukerman et al., 1997; Devlin et al., 1998; Franklin et al., 2003; Halliday and Whitelam, 2003).
Cryptochromes - blue light
There are at least two cryptochromes present in Arabidopsis, CRY1 and CRY2. The genes encoding the photoreceptors were originally known as HY4 and CRY2/FHA respectively. Both of these genes promote flowering (Lin, 2000a, b) . For example, cry2 mutants flower later than wild-type under long, but not short photoperiods (Koornneef et al., 1998b; Guo et al., 1999) . Surprisingly, cry2 mutants flower at approximately the same time as wild-type plants when grown in continuous blue light. However, when grown in blue plus red light, flowering time of the mutant is delayed. This indicates that CRY2 may promote flowering by overcoming the inhibitory effect of red light (Guo et al., 1998; Mockler et al., 1999).
Light and the circadian rhythm
Light perception also plays a role in the control of flowering time by day length. The duration of the photoperiod is measured by an interaction between light signalling and a circadian rhythm in light signalling (see section 1.4.3.1). In this process, described as the external coincidence model, light perception plays two roles. First, it acts to entrain (or synchronise) circadian rhythms to the daily cycles of light and dark. Secondly, light interacts with a circadian rhythm promoting or repressing flowering if the plant is exposed to light at a particular phase of the rhythm (reviewed by Samach and Coupland, 2000; Roden et al., 2002; Yanovsky and Kay, 2003; Searle and Coupland, 2004).