Abstract
Light plays a crucial role in plant growth and development, and light signaling is integrated with various stress responses to adapt to different environmental changes. During this process, excessive protein synthesis overwhelms the protein-folding ability of the endoplasmic reticulum (ER), causing ER stress. Although crosstalk between light signaling and ER stress response has been reported in plants, the molecular mechanisms underlying this crosstalk are poorly understood. Here, we demonstrate that the photoreceptor phytochrome B (phyB) induces the expression of ER luminal protein chaperones as well as that of unfolded protein response (UPR) genes. The phyB-5 mutant was less sensitive to tunicamycin (TM)-induced ER stress than were the wild-type plants, whereas phyB-overexpressing plants displayed a more sensitive phenotype under white light conditions. ER stress response genes (BiP2 and BiP3), UPR-related bZIP transcription factors (bZIP17, bZIP28, and bZIP60), and programmed cell death (PCD)-associated genes (OXI1, NRP1, and MC8) were upregulated in phyB-overexpressing plants, but not in phyB-5, under ER stress conditions. The ER stress-sensitive phenotype of phyB-5 under red light conditions was eliminated with a reduction in photo-equilibrium by far-red light and darkness. The N-terminal domain of phyB is essential for signal transduction of the ER stress response in the nucleus, which is similar to light signaling. Taken together, our results suggest that phyB integrates light signaling with the UPR to relieve ER stress and maintain proper plant growth.
Highlights
Light is essential for plants as an energy source as well as a critical signal that influences plant developmental processes, including germination, de-etiolation, phototropism, vegetative growth, and reproductive development, during the entire lifecycle (Kami et al, 2010)
The complemented transgenic plants in phytochrome B (phyB)-5 with the N- or C-terminal domain fused with GFP (NG, CG), NG fused with nuclear localization signal (NG-NLS), and PBG, which is transformed by Agrobacterium-mediated floral dip method with the fragments encoding the full length, N- and C-terminal domains of phyB were inserted between the 35S promoter and the Nos terminator of pPZP211/35SNosT vector used in the previous study (Matsushita et al, 2003)
To test whether phyB-dependent signaling is associated with endoplasmic reticulum (ER) stress response, we observed the phenotypes of Ler, phyB5, and PBG under TM-induced ER stress conditions
Summary
Light is essential for plants as an energy source as well as a critical signal that influences plant developmental processes, including germination, de-etiolation, phototropism, vegetative growth, and reproductive development, during the entire lifecycle (Kami et al, 2010). A high R:FR ratio increases the photo-equilibrium, that is, the ratio of active form (Pfr) to total phytochrome (Pr + Pfr), whereas the photoequilibrium is decreased under a low R:FR ratio condition (Legris et al, 2019) Some environmental factors, such as far-red light, darkness, and high temperatures, accelerate reversion in the Pr form (Legris et al, 2016, 2019). Type II phytochromes are activated under relatively high red/far-red ratio (FR low fluence response, LFR) light conditions and red light-dependent high irradiance responses (R-HIRs; Shinomura et al, 1996; Hirschfeld et al, 1998). Because of these differences, the action spectra are different, the absorption spectra of types I and II are similar (Eichenberg et al, 2000; Klose et al, 2015). The far-red light-induced leaf senescence is regulated by antagonistic roles of phyA and phyB (Lim et al, 2018)
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