Involvement of the Light Signalling Components HY5 and BIC1,2 and SPA1 in Plant Responses to Elevated Daytime UV-B Doses

Plants respond to ultraviolet B radiation (280-320 nm) with an integrated reaction that includes the reception of the acting stress factor, followed by the generation of reactive oxygen species and damage to macromolecules and membrane structures, as well as changes in cellular metabolism and the formation of protective systems. However, the involvement of key UV-B-related signalling components such as HY5, SPA1 and BIC1 or BIC2 proteins in physiological, biochemical and molecular responses remains insufficiently understood. The effects of 8, 16 and 24 h of UV-B exposure (within an 8 h photoperiod over three days) on the net photosynthetic rate (Pn), chlorophyll fluorescence parameters Y(II) and F_v/F_m, reflecting the functional state of PSII, nonphotochemical quenching (NPQ), pigment contents (Chl(<i>a</i>+<i>b</i>), carotenoids, anthocyanins and UV-absorbing pigments (UAPs) and the expression of key light-induced genes in wild-type <i>Arabidopsis thaliana</i> and <i>spa1</i>, <i>bic1</i>,<i>2</i> and <i>hy5</i> mutants were studied. UV-B irradiation resulted in a gradual reduction in the Pn, Y(II), F_v/F_m values and Chl(<i>a</i>+<i>b</i>) but caused a marked increase in the anthocyanin and UAP contents and only minor changes in the carotenoid content. The <i>hy5</i> mutant presented the lowest net photosynthetic rate (Pn), chlorophyll fluorescence parameters, and chlorophyll and carotenoid contents under all the UV-B exposures. In addition, the accumulation of anthocyanins and UAPs during UV-B treatment was consistently the lowest in <i>hy5</i>. After any UV-B exposure, the highest accumulation of UAPs and anthocyanins was observed in the <i>spa1</i> mutant, whereas the highest Pn values were detected after 24 h in <i>bic1</i>,<i>2</i>. One of the reasons for the reduced photosynthetic activity and antioxidant capacity in <i>hy5</i> may be the lower expression levels of <i>CHS</i> and <i>PAL</i> in this variety than in the other genotypes. Our results indicate that HY5 is required to maintain antioxidant responses and photosynthetic performance under repeated daytime UV-B exposure (16.8 kJ m^-2 per day). In contrast, BIC1, BIC2, and SPA1 also contribute to UV-B tolerance, but through distinct regulatory mechanisms and to a lesser extent.