Diverse Responses to Blue Light via LOV Photoreceptors

Abstract

Plants utilize light not only as an energy source for photosynthesis, but also as an environmental cue to direct numerous developmental and physiological processes. For this purpose, plants have evolved two main types of photoreceptors: the phytochromes (Franklin and Quail 2010), which sense red/ far-red light, and the ultraviolet (UV)-A/blue light (BL)-sensing photoreceptors, which include cryptochrome (Liu et al. 2011), phototropin (Christie 2007) and other BL receptors described below. In addition, a third class of UV-B receptor, UVR8, was recently identified (Rizzini et al. 2011). The long history of research on BL receptors dates back to the initial observation made by the famous Charles Darwin, in which he found that plants were able to move towards light (Darwin and Darwin 1881). For a long time, the nature of the chromophore in BL receptors was much disputed as to whether it was a flavin or a carotenoid (Senger 1980). This issue was partially resolved through the identification of the first BL receptor, which corresponded to a flavin protein named cryptochrome (Ahmad and Cashmore 1993). Shortly after this discovery, the Briggs group demonstrated that the disrupted gene in Arabidopsis non-phototropic hypocotyl 1 (nph1) mutants encoded a BL receptor required for phototropic response (Huala et al. 1997, Christie et al. 1998). A homolog of NPH1, NON-PHOTOTROPIC HYPOCOTYL-LIKE 1 (NPL1), was soon identified (Kagawa et al. 2001); thereafter, NPH1 and NPL1 were renamed phototropin 1 (phot1) and phot2, respectively. Phototropins mediate diverse plant responses, such as chloroplast relocation movements (Kagawa et al. 2001, Sakai et al. 2001), stomatal opening (Kinoshita et al. 2001), early hypocotyl growth inhibition (Folta and Spaulding 2001), leaf flattening (Sakamoto and Briggs 2002), leaf positioning (Inoue et al. 2005), nuclear positioning (Iwabuchi et al. 2007, Tsuboi et al. 2007), sun tracking (Inoue et al. 2008b) and leaf photomorphogenesis (Kozuka et al. 2011), in which phot1 acts over a broad range of light intensities, whereas phot2 acts as a high-light sensor. These physiological responses contribute towards enhancing photosynthesis in the plant by increasing the absorption of light and CO2 (Takemiya et al. 2005). In addition to the two phototropins, a BL receptor family of proteins characteristically containing one light, oxygen or voltage (LOV) domain, an F-box and a Kelch repeat has been identified in Arabidopsis. These proteins include ZEITLUPE (ZTL), FLAVIN-BINDING KELCH REPEAT F-BOX 1 (FKF1) and LOV KELCH PROTEIN 2 (LKP2), and are proposed to regulate the circadian clock and photoperiodic flowering by controlling BL-dependent protein degradation (Ito et al. 2012). A third type of LOV protein, aureochrome (AUREO), was identified in the stramenopile alga, Vaucheria frigida (Takahashi et al. 2007). AUREO has a basic region/leucine zipper (bZIP) domain as well as a LOV domain, and is thought to act as a BL-regulated transcription factor. Thus, the LOV BL receptor family in plants has divergent members who have a variety of physiological functions triggered by the photochemical reactions of FMN in the LOV domain.