CONTENIDO DE LA PROPUESTA

In order to adapt to changes in the environment, circadian clocks need to be ad- justed through the perception of environmental light-dark signals (Yanovsky and Kay, 2002; Thomas, 2006). The photoreceptor genes are responsible for perceiv- ing different light durations and qualities and initiating signals that interact with the circadian clock to entrain the circadian rhythm (Thomas, 2006). Mutations in photoreceptor genes have varied effects on flowering time. Some mutants flower early, whereas other flower late indicating that these genes are involved in both the promotion and inhibition of flowering (Chapter 1.5.2.7; Simpson and Dean, 2002). Physiological, biochemical and molecular genetic studies have led to the identifica- tion of four families of photoreceptors in higher plants: PHYs (Quail, 2002), CRYs (Lin, 2002), phototropins (PHOTs; Briggset al., 2001) and the unidentified ultra- violet B photoreceptors (UVB; Frohnmeyer and Staiger, 2003; Brosche and Strid, 2003; Jiao et al., 2007).

PHYs are photochromic proteins absorbing light in the red/far red end of the spectrum (600-700 nm). It has been suggested that PHYs evolved from bacterial bilinsensory proteins, a hypothesis that is supported by the discovery of PHY- like proteins in photosynthetic bacteria, nonphotosynthetic eubacteria and fungi (Montgomery and Lagarias, 2002). They are involved in a variety of plant functions including hypocotyl elongation, stem and leaf expansion, vegetative growth and development, and flowering (Ahmad, 1999). The PHYs exist in two reversible isomeric forms; Pr absorbs red-light and is converted into Pfr whereas Pfr absorbs far-red light and is converted back to Pr. Exposure to red light leads to a late flowering phenotype and thus, the activated Pfr form is inhibitory to floral induction (Goto et al., 1991; Bagnall, 1992, 1993). In Arabidopsis, five PHY genes (PHYA to E) have been isolated and sequenced (Sharrock and Quail, 1989; Clack et al., 1994). Sequence analysis showed thatPHYB, PHYD, and PHYE have 80% amino acid similarity and they all function as inhibitors of floral induction (Devlin et al., 1998; Neff and Chory, 1998; Devlin et al., 1999). In contrast, PHYA acts as a

promoter of floral initiation (Neff and Chory, 1998), whereas the role ofPHYC has not been established.

CRYs comprise flavoproteins and are able to detect blue light (400-500 nm). CRY- 1, together with CRY-2 (FHA1; FHA-1 is a CRY-2 allele in Landsberg erecta (Ler) background) are very important during de-etiolation, the transition of a dark grown seedling living from its seed reserves to a photoautotrophically competent seedling. This developmental transition includes a massive reorganization of the transcriptional program, inhibition of hypocotyl growth, promotion of cotyledon expansion, and synthesis of a number of pigments including chlorophyll and an- thocyanins (Lin, 2002; Liscum et al., 2003). In addition, these photoreceptors are important for photoperiod-dependent flowering induction and for resetting the cir- cadian oscillator(Cashmore et al., 1999; Yanovsky and Kay, 2002). Analysis of multiple PHY and CRY mutants has revealed that interactions exist between these photoreceptors (Lin, 2000). PHYB, PHYD andPHYE have redundant functions in directly mediating red-light inhibition of floral induction. PHYAand CRY-2, pro- mote floral induction by suppressing the effect of these inhibitory PHYs (Thomas, 2006).

The PHOTs (PHOT-1 and PHOT-2 in Arabidopsis) perceive blue and ultraviolet A (UVA; 340-480 nm) wavelengths and mediate a number of photomorphogenic responses. They are protein kinases harboring two light, oxygen or voltage (LOV) domains that can perceive light and in response, to control phototropism, stom- atal aperture and chloroplast movements (Briggs and Christie, 2002). However, their contribution to transcriptional regulation is relatively small, as only a limited number of genes are under their control (Jiao et al., 2007).

Ultraviolet B (UVB; 280–320 nm) light is sensed by plants as both an informa- tional signal and an environmental stress factor. Arabidopsis UVB RESISTANCE- 8 (UVR-8) is a UVB-specific signalling component that orchestrates the expression of a range of genes with vital UVB protective responses. Under low fluence UVB rates,UVR-8 regulates the expression ofELONGATED HYPOCOTYL-5 (HY-5),

a bZIP transcription factor which is involved in many light rsponsive pathways (Brown et al., 2005). In addition, COP-1, a negative regulator of the visible light response, is also regulated under low fluence UVB rates (Oravecz et al., 2006). However, it is not clear if both loci function as UVB receptors or are downstream parts of the mechanism which control plant development in response to UVB wave- lengths.

How photoreceptors initiate the signal transduction pathway for floral induction is still unclear. There have been reports that PHYs can act as protein kinases (Fankhauser et al., 1999) or they can interact with other nuclear proteins that modulate expression of light-regulated genes (Ni et al., 1998). Ahmad and Cash- more (1996a) isolated early flowering Arabidopsis mutant phytochromes-signalling early-flowering-1 (pef-1),pef-2 andpef-3 and these loci might code for proteins in- volved in the early steps of the PHY signalling pathway. Several studies have shown that multiple related bHLH (basic helix–loop–helix) class transcription factors play key roles in PHY signal transduction (Quail, 2002). Members of the bHLH family appear to be particularly important because several of them specifically interact with the light-activated Pfr (Quail, 2002). Surprisingly, these transcription factors primarily act as negative regulators of PHY signalling. Moreover, in some cases, the PHYs inhibit those negative regulators (Duek and Fankhauser, 2005). Pho- toreceptors may also affect flowering time indirectly through their effects on the biological clock. Clock entrainment is known to involve different photoreceptors in different light conditions (quality and irradiance) and novel blue light photorecep- tors of the ZEITLUPE/FLAVIN-BINDING, KELCH REPEAT, F-BOX-1/LOV KELCH PROTEIN-2 (ZTL/FKF-1/LKP-2) family have been demonstrated to regulate clock components Boss et al. (2004).