Abstract
We examined the impact of UV-B irradiation on chloroplast movements in Arabidopsis leaves. Directional chloroplast movements induced by blue light have been described in multiple plant species. In weak light, chloroplasts accumulate at periclinal cell walls to increase light capture. In strong light, chloroplasts exhibit the avoidance response, as they move towards anticlinal walls to protect the photosynthetic apparatus from light-induced damage. In Arabidopsis, chloroplast movements are triggered by phototropins, phot1 and phot2, which are known as blue/UV-A photoreceptors. We found that irradiation with UV-B of 3.3 µmol·m−2·s−1 induced chloroplast accumulation in wild-type plants. UV-B-triggered accumulation was dependent on the presence of phototropins, especially phot1, but not on UVR8 (the canonical UV-B photoreceptor). Irradiation with strong UV-B of 20 µmol·m−2·s−1 did not induce substantial chloroplast relocations in wild-type leaves. However, in the jac1 mutant, which is defective in chloroplast accumulation, strong UV-B elicited chloroplast avoidance. This indicated that UV-B can also activate signaling to the avoidance response. To assess the possibility of indirect effects of UV-B on chloroplast movements, we examined the impact of UV-B on the actin cytoskeleton, which serves as the motile system for chloroplast movements. While irradiation with UV-B of 3.3 µmol·m−2·s−1 did not affect the actin cytoskeleton, strong UV-B disrupted its structure as shown using an Arabidopsis line expressing Lifeact-green fluorescent protein (GFP). In wild-type plants, pretreatment with strong UV-B attenuated chloroplast responses triggered by subsequent blue light irradiation, further indicating that this UV-B intensity also indirectly affects chloroplast movements. Taken together, our results suggest that the effect of UV-B on chloroplast movement is twofold: it directly induces phototropin-mediated movements; however, at higher intensities, it attenuates the movements in a nonspecific manner.
Highlights
Apart from visible light, the solar spectrum contains ultraviolet radiation, by convention divided into the UV-A (315–400 nm), UV-B (280–315 nm), and UV-C range (200–280 nm)
To investigate the impact of UV-B on chloroplast movements in the model plant Arabidopsis thaliana, detached leaves of dark-adapted plants were irradiated for 1 h with UV-B of 3.3 μmol·m−2·s−1 (1.3 W·m−2) or strong UV-B of 20 μmol·m−2·s−1 (7.9 W·m−2)
A substantial (~2%) decrease of transmittance, indicative of chloroplast accumulation, was observed in the WT and uvr8 leaves irradiated for 1 h with UV-B of 3.3 μmol·m−2·s−1
Summary
Apart from visible light, the solar spectrum contains ultraviolet radiation, by convention divided into the UV-A (315–400 nm), UV-B (280–315 nm), and UV-C range (200–280 nm). The main plant photoreceptor for UV-B is UVR8 (Jenkins, 2017). Upon UV-B exposure, it monomerizes (Rizzini et al, 2011) and moves from the cytoplasm to the nucleus (Yin et al, 2016). It is involved in protection against damage caused by excessive UV-B radiation through regulation of gene expression related to flavonoid biosynthesis, antioxidant activities, and DNA repair (Yin and Ulm, 2017). The main receptors for UV-B-induced phototropism are phototropins (Vandenbussche et al, 2014; Vanhaelewyn et al, 2016)
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