Phytochromes exist in two stable conformers: a biologically inactive red-light absorbing form (Pr) and a biologically active far-red light-absorbing form (Pfr) ( Rockwell et al., 2006 Bae and Choi, 2008). The protein domains in plant phytochromes can be divided into two modules: the chromophore-bearing, N-terminal photosensory module, responsible for light perception and signaling, and the C-terminal module that directs nuclear localization, dimerization, and nuclear body formation ( Rockwell et al., 2006 Nagatani, 2010). Phytochromes are dimeric chromoproteins, with each monomer covalently attached to a light-absorbing linear tetrapyrrole chromophore, phytochromobilin ( Cornejo et al., 1992 Terry et al., 1995). Phytochrome A and B have overlapping and distinct photosensory roles in seedling development ( Reed et al., 1994). PhyA is mainly responsible for sensing and responding to far-red light, in addition to red light, whereas phyB-E are responsible for photomorphogenesis in response to red light and foliar shade ( Whitelam et al., 1992 McCormac et al., 1993 Nagatani et al., 1993 Parks and Quail, 1993 Whitelam et al., 1993 Reed et al., 1994 Paik and Huq, 2019).
PhyA is classified as light-labile and is the most abundant phytochrome in etiolated seedlings, whereas phyB-E are classified as light-stable ( Clack et al., 1994 Nagy and Schäfer, 2002). In the model plant Arabidopsis thaliana, phytochromes are a five-member family, phyA-phyE ( Jones and Quail, 1989 Somers et al., 1991 Quail, 1991 López-Juez et al., 1992 Clack et al., 1994 Mathews, 2010). Phytochromes perceive red/far-red light (600–750 nm) and regulate many aspects of plant development, including seed germination, de-etiolation, gravitropism, flowering, circadian rhythms, and senescence ( Bae and Choi, 2008 Franklin and Quail, 2010 Kami et al., 2010 Paik and Huq, 2019). To perceive light, plants have evolved different classes of photoreceptors that absorb light wavelengths from the UV to far-red (380–735 nm wavelengths), including UV RESISTANCE LOCUS 8 (UVR8), PHOTOTROPINS, CRYPTOCHROMES, LOV (Light, Oxygen, Voltage)-KELCH DOMAIN containing F-box proteins, and PHYTOCHROMES ( Butler et al., 1959 Gressel, 1979 Kandori et al., 1992 Jansen et al., 1998 Briggs and Huala, 1999 Cashmore et al., 1999 Lin, 2000 Nelson et al., 2000 Somers et al., 2000 Briggs et al., 2001 Jarillo et al., 2001 Kinoshita et al., 2001 Kagawa et al., 2001 Sakai et al., 2001 Schultz et al., 2001 Briggs and Christie, 2002 Kasahara et al., 2002 Lin, 2002 Frohnmeyer and Staiger, 2003 Imaizumi et al., 2003 Kleine et al., 2003 Ulm and Nagy, 2005 Kaiserli and Jenkins, 2007 Quail, 2010 Yu et al., 2010 Chaves et al., 2011 Rizzini et al., 2011 Jenkins and Brown, 2018).
Thus, it is critical for plants to appropriately detect, coordinate, and respond to light cues for their overall fitness and survival. Light is the most critical environmental stimulus for all plant development, serving as the energy source for photosynthesis and as an environmental cue to regulate growth and development. Understanding the molecular, cellular, and biophysical processes that shape how plants perceive light will help in engineering improved sunlight capture and fitness of important crops. Here, we explore photobodies as environmental sensors, examine the role of their protein constituents, and outline the biophysical perspective that photobodies may be undergoing liquid-liquid phase separation. Based on photobody cellular dynamics and the properties of internal components, photobodies have been suggested to undergo liquid-liquid phase separation, a process by which some membraneless compartments form. However, photobody function, composition, and biogenesis remain undefined since their discovery. PhyB photobodies regulate phytochrome-mediated signaling and physiological outputs. In response to red light, phytochromes convert to a biologically active form, translocating from the cytoplasm into the nucleus and further compartmentalizes into subnuclear compartments termed photobodies. In the model plant Arabidopsis thaliana, phytochrome B serves as a light and thermal sensor, mediating physiological processes such as seedling germination and establishment, hypocotyl growth, chlorophyll biogenesis, and flowering. Phytochromes are red-light photoreceptors that are highly conserved across kingdoms. Plants perceive light through various light-responsive proteins, termed photoreceptors. Light is a critical environmental stimulus for plants, serving as an energy source via photosynthesis and a signal for developmental programming. 2Division of Biology and Biomedical Sciences, Washington University in St.1Donald Danforth Plant Science Center, St.
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