triggers the production of carotenoids (Middleton and Teramura, 1993), and some of the same high-light photoprotective mechanisms can also protect plants from UV radiation. For example, it was demonstrated that zeaxanthin contributes to UV stress protection and damage prevention in tobacco (Götz et al., 2002). Additionally, some plants have evolved UV-absorbing chemical sunscreens such as flavonoids that reduce the amount of UV reaching sensitive molecules (Tohge and Fernie, 2017).
Exposure to UV radiation does not always yield a negative effect for photosynthetic organisms, however. Recently there has been a paradigm shift in understanding UV as a regulatory signal rather than solely a stressor, as UV perception is involved in critical metabolic functions (Rozema et al., 1997; Davey et al., 2012; Hideg et al., 2013; Morales et al., 2013; Singh et al., 2014; Williamson et al., 2014; Neugart and Schreiner, 2018). Researchers have begun to instead classify UV radiation as a “eustress,” (Hideg et al., 2013). In this framework, UV-B is understood to stimulate a state of alert that includes activation defenses, especially if the radiation is experienced in small doses. For example, UVR8, the UV-B receptor in plants, mediates the accumulation of transcripts encoding early light-inducible proteins (ELIPs) (Singh et al., 2014), which function in photoprotection (Hutin et al., 2003). Furthermore, low doses of UV radiation can induce protective responses that increase a plant’s tolerance to other abiotic and biotic stressors (Frohnmeyer and Staiger, 2003). For instance, ELIPs are also important for desiccation tolerance in resurrection plants (Zeng, 2002; Oliver et al., 2004; Van Buren et al., 2019).
Although many mosses are found in cool, low light environments, several species are abundant in drylands where they are common and important members of biological soil crusts (biocrusts). Biocrusts are complex communities of bryophytes, lichens, fungi, cyanobacteria, and other microorganisms living on the surface of soil in drylands
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