Plants use light and dark cycles to predict environmental change and to regulate their developmental processes. They measure changes in day length using their circadian clocks and by sensing the surrounding light environment. The circadian clock is an endogenous molecular oscillator with period of approximately 24 hour. The clock itself regulates light resetting by limiting the timing of maximum responsiveness to light to specific day periods, a phenomenon commonly referred to as gating (Millar and Kay, 1996). Rice is a short day plant and uses two classes of photoreceptors, the red/far-red light sensing phytochromes (Phy) and the blue/UVA-light sensing cryptochromes (CRY), enable clock resetting to day-night cycles. Clock resetting by day length is mediated by members of the ZEITLUPE (ZTL); FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (FKF1); and LOV, KELCH PROTEIN 2 (LKP2) protein family ( Nelson et al., 2000, Schultz et al., 2001 and Somers et al., 2000). ZTL and FKF1 act as blue light receptors that interact with GIGANTEA (GI), a clock-associated protein whose accumulation is tightly controlled at both transcriptional and post-translational levels (David et al., 2006, Fowler et al., 1999, Kim et al., 2007 and Sawa et al., 2007). Indeed, blue light stabilizes the FKF-GI and ZTL-GI interactions, allowing these complexes to regulate flowering time and circadian rhythms by controlling protein stability of a floral repressor CYCLING DOF FACTOR 1 (CDF1) and a circadian oscillator component TIMING OF CAB EXPRESSION 1 (TOC1), respectively (Imaizumi et al., 2005, Kim et al., 2007, Más et al., 2003 and Sawa et al., 2007). EARLY FLOWERING 3 (ELF3) plays a pivotal role in the circadian gating pathway ( Hicks et al., 1996 and McWatters et al., 2000) thus, rhythmically inhibits the activity of the light input pathways around dusk. CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) is also related to the control of light input to the clock, participating in the modulation of circadian rhythms and flowering. COP1 acts as a RING-type E3 ubiquitin-ligase that mediates ubiquitination and targeted degradation of positive regulators of light signal transduction in dark conditions (Yi and Deng, 2005). COP1 and ELF3 function toward GI destabilization plays an important role in the regulation of light input signaling to the clock and the control of the expression modes of flowering-time genes. There are a wide range of processes in plants that show circadian rhythms, including the movement of leaves, stomatal opening, stem elongation, metabolic processes, such as photorespiration and photosynthesis, and the expression of a large set of genes. Circadian clock plays an important role as a pacemaker for various physiological events that occur throughout the day. Several positive regulators and multiple intertwined negative-feedback loops exist in the architecture of the circadian clock.
