Update on Photobodies in Light Signaling Photobodies in Light Signaling

Plant development and growth are extremely plastic in response to changes in ambient light conditions. Light is not only the ultimate energy source for photosynthesis; its physical parameters, such as quality, intensity, direction, and duration, also serve as key environmental and time cues (Chen et al., 2004). Therefore, it is vital for plants to closely monitor and precisely respond to changes in light properties in order to optimize growth under a wide range of ecological environments and to synchronize developmental transitions with diurnal and seasonal time. Plants have evolved to “see” the light spectrum between 280 and 750 nm, which spans UV-B, UV-A, and visible light, through five classes of photoreceptors. In the reference plant Arabidopsis (Arabidopsis thaliana), they include the newly determined UV-B receptor UV RESISTANCE LOCUS8 (UVR8; Kaiserli and Jenkins, 2007; Rizzini et al., 2011); three types of UV-A/blue light receptors, including cryptochromes (cry1 to 3; Kleine et al., 2003; Yu et al., 2010; Chaves et al., 2011), phototropins (phot1 and 2; Briggs and Christie, 2002), and the family of ZEITLUPE/FLAVIN-BINDING, KELCH, F-BOX1 (FKF1)/LOV KELCH PROTEIN2 (LKP2; Nelson et al., 2000; Somers et al., 2000; Schultz et al., 2001); and the red and far-red sensing phytochromes (phyA to E; Quail, 2010). Collectively, these photoreceptors regulate almost every facet of plant development and growth from seed germination to floral initiation (Franklin and Quail, 2010; Kami et al., 2010). Many light responses are mediated by alterations in gene expression. Among the 15 photoreceptors discovered so far, 10 of them, UVR8 (Kaiserli and Jenkins, 2007; Favory et al., 2009), cry1 (Wu and Spalding, 2007), cry2 (Kleiner et al., 1999; Yu et al., 2009), FKF1 (Sawa et al., 2007; Fornara et al., 2009), LKP2 (Yasuhara et al., 2004), and phyA to E (Sakamoto and Nagatani, 1996; Yamaguchi et al., 1999; Kircher et al., 2002), have been shown to localize to the nucleus, where they regulate gene expression in a light-dependent manner (Jiao et al., 2007). An emerging common mechanism for such light-dependent gene expression is through regulating the stability of key transcriptional regulators (discussed below; Yi and Deng, 2005; Brown et al., 2009; Chen and Chory, 2011; Leivar and Quail, 2011). At the cellular level, photoactivation of photoreceptors triggers the rapid localization of a number of them, including phyA to E, cry2, and possibly cry1 and UVR8, to discrete subnuclear foci or nuclear bodies (Kleiner et al., 1999; Yamaguchi et al., 1999; Wang et al., 2001; Kircher et al., 2002; Favory et al., 2009; Yu et al., 2009; Gu et al., 2011; Lian et al., 2011; Liu et al., 2011). Nuclear bodies are morphologically distinct subnuclear domains that provide microenvironments for the regulation of protein dynamics, gene expression, and DNA replication and repair in both plant and animal cells (Shaw and Brown, 2004; Spector, 2006). The general principles of nuclear body function and assembly are still largely unknown. The photoreceptorcontaining nuclear bodies, or “photobodies,” found in plants are a unique type of subnuclear domain whose size, number, and potentially function are directly regulated by external light cues (Chen and Chory, 2011). Observations of these speckle-shaped photobodies have raised many questions. How is the formation of photobodies regulated? What are the functions of the photobodies? What are the factors required for photobody formation? These are some of the key questions that are being actively investigated. Further understanding of photobody function and regulation will not only be important for understanding the subcellular organization of light signaling events in plants but also could potentially uncover general principles governing subnuclear domains in higher eukaryotes. In this review, we will summarize recent developments related to phy-containing photobodies, touch briefly on photobodies containing crys, and discuss the potential functions of photobodies in relationship to protein degradation and gene expression. For more comprehensive coverage of light signaling in plants, the reader is referred to the following recent reviews (Henriques et al., 2009; Jenkins, 2009; Chory, 2010; Franklin and Quail, 2010; Kami et al., 2010; Lau and Deng, 2010; Möglich et al., 2010; Nagatani, 2010; Rockwell and Lagarias, 2010; Yu et al., 2010; Chaves et al., 2011; Chen and Chory, 2011; Leivar and Quail, 2011; Ulijasz and Vierstra, 2011).