Involvement of Phytochrome-Interacting Factors in High-Irradiance Adaptation

Phytochrome-interacting factors (PIFs) are key transcriptional regulators of phytochrome signalling that coordinate photomorphogenesis and photosynthesis under different environmental conditions. PIFs play an important role in this regulation and act mainly as negative regulators of photomorphogenesis, but under high-intensity light (HIL), their functions can also include adaptive roles. We investigated the contribution of individual PIFs to the adaptation of the photosynthetic apparatus in wild-type A. thaliana and pif4, pif5, pif4pif5, and pif1pif3pif4pif5 mutants exposed to HIL for 0, 16, 32, or 48 h. Chlorophyll fluorescence parameters (Y(II), F_v/F_m, NPQ), net photosynthesis (Pn), transpiration rates, stomatal conductance (g_S), pigment contents and the expression of key genes were evaluated. The response of plants to HIL varied depending on the duration of exposure. After 16 h of irradiation, the greatest reductions in Pn and g_S were observed in the pif4pif5 and pif1pif3pif4pif5 mutants, whereas after 48 h, the decreases were most pronounced in the pif4, pif5, and pif4pif5 mutants. After 16 h of HIL exposure, the absence of pif4 and pif5 did not substantially alter the chlorophyll fluorescence parameters. However, after 48 h, both Y(II) and Fv/Fm were lower in these mutants than in the wild type, indicating changes in PSII functional status rather than direct reductions in photochemical quantum efficiency. At 16 h, chlorophyll levels were the highest in pif5 and WT, whereas anthocyanin and UV-absorbing pigment (UAP) levels were the highest in pif4, pif5 and WT. After 48 h, the highest levels of any pigments were detected in the WT and the pif1pif3pif4pif5 mutant. These results suggest that the accumulation of anthocyanins and UAPs under HIL is likely associated with the regulation of transcription factors, such as PIFs, de-etiolated 1 (DET1), constitutive photomorphogenic 1 (COP1), and elongated hypocotyl 5 (HY5). During prolonged HIL exposure, the absence of PIF4 and PIF5 has a critical impact on photosynthesis and the accumulation of photosynthetic pigments, whereas the simultaneous loss of PIF1, PIF3, PIF4, and PIF5 is less detrimental. This finding likely indicates opposite roles of PIF1 and PIF3 in the above-described processes, on the one hand, and PIF4 and PIF5, on the other hand, under HIL conditions.