Biology of the Photoreceptors

Plants possess three main classes of photomorphogenic photoreceptors: phytochromes, cryptochromes and phototropins (see Jiao et al. 2007). Those first identified, the phytochromes, act "photochromically" in the red and far-red region of the spectrum, being reversibly activated by red wavelengths and inactivated by far-red. This "ancestral" type of phytochrome is also termed type II or light stable, and it is encoded by four genes in Arabidopsis, PHYB-PHYE (Clack et al. 1994), or tomato, PHYB1, PHYB2, PHYE, PHYF (Hauser et al. 1997), of which PHYB is most highly expressed. In angiosperms, however, a further type of phytochrome, type I or light labile, is encoded by PHYA, accumulates in the dark, absorbs a broad range of wavelengths, notably far-red, is exquisitely light sensitive and is not photoreversible (Shi-nomura et al. 1996). A second class of photoreceptors are the blue light-absorbing cryptochromes, in Arabidopsis the products of CRY1 and CRY2. The phototropins, in Arabidopsis the products of PHOT1 and PHOT2, also absorb blue light. The phototropins harbour a flavin chromophore in a LOV domain. Plant genomes also encode other LOV domain-containing proteins, at least some of which have recently been found to be capable of photoperception (Kim et al. 2007).

During the initial transition from darkness to light, a dramatic modification of plant growth programme takes place, from skoto-morphogenesis (dark form) to photo-morphogenesis (light form), and this is accompanied by a massive transcriptional reprogramming (Jiao et al. 2007). The vast majority of such gene expression changes are controlled by the phytochromes and the cryptochromes (Ma et al. 2001). Phototropins, associated with plasma membrane, primarily control short-term responses to light, often directional light, for example tropisms, stomatal opening and chloroplast relocation. An exception is leaf lamina expansion, seemingly a developmental response, but strongly dependent on blade tropism towards the light (Takemiya et al. 2005). At least in the first few hours after light exposure, cryptochromes mediate the majority of gene expression changes initiated by blue light (Ohgishi et al. 2004), while phototropins mediate their actions through cytoskeletal or membrane-based responses.

The transduction of phytochrome and cryptochrome signals, which often interact, occurs through multiple pathways, many of them based in the nucleus. A key theme is the proteolysis-mediated removal of repressors for photomorphogenesis in the light that are otherwise active in the dark. Those repressors include COP1, DET1 and its associated complex, and the COP9 signalosome (CSN). DET1 and its partner DDB1 can modify chromatin structure and also enhance ubiquitin conjugating enzymes (Yanagawa et al. 2004). COP1 has ubiquitin ligase activity, and the CSN regulates the activity of another type of ubiquitin ligases (SCF) by post-translational modification (de-rubylation, Wei and Deng 2003). Among the positive regulators of photo-morphogenesis targeted by these repressors, transcription factors of the bZIP (HY5 and HYH) and bHLH (PIF3 and PIL1) classes are the best known.

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