The bZIP transcription factor ELONGATED HYPOCOTYL5 (HY5) represents a major hub in the light-signaling cascade both under visible and UV-B light. The mode of transcriptional regulation of HY5, especially under UV-B light, is not well characterized. B-BOX (BBX) transcription factors regulate HY5 transcription and also posttranscriptionally modulate HY5 to control photomorphogenesis under white light. Here, we identify BBX31 as a key signaling intermediate in visible and UV-B light signal transduction in Arabidopsis (Arabidopsis thaliana). BBX31 expression is induced by UV-B radiation in a fluence-dependent manner. HY5 directly binds to the promoter of BBX31 and regulates its transcript levels. Loss- and gain-of-function mutants of BBX31 indicate that it acts as a negative regulator of photomorphogenesis under white light but is a positive regulator of UV-B signaling. Genetic interaction studies suggest that BBX31 regulates photomorphogenesis independent of HY5. We found no evidence for a direct BBX31-HY5 interaction, and they primarily regulate different sets of genes in white light. Under high doses of UV-B radiation, BBX31 promotes the accumulation of UV-protective flavonoids and phenolic compounds. It enhances tolerance to UV-B radiation by regulating genes involved in photoprotection and DNA repair in a HY5-dependent manner. Under UV-B radiation, overexpression of BBX31 enhances HY5 transcriptional levels in a UV RESISTANCE LOCUS8-dependent manner, suggesting that BBX31 might regulate HY5 transcription.

Photomorphogenesis is an important developmental process that helps the seedlings adapt to external light conditions. B-Box proteins are a family of transcription factors that regulate photomorphogenic responses. BBX31 negatively regulates photomorphogenesis under visible light. In contrast, it promotes photomorphogenesis under UV-B and enhances tolerance to high doses of UV-B radiation. BBX31 and HY5 independently and oppositely regulate the ability of seedlings to adapt to varying light intensities. BBX31 also regulates primary root elongation under low intensities of white light. GC-MS and HPLC-based metabolite profiling identified differential accumulation of multiple primary and secondary metabolites in 35S:BBX31 that might enhance tolerance to UV-B.

Plants use complex photoreceptors, including phytochromes, to detect and respond to light. Phytochromes detect red and far-red light, triggering light-dependent activities from seed germination to blooming. Phytochromes fluctuate between active (Pfr) and inactive (Pr) forms in response to red and far-red wavelengths, affecting gene expression and physiological reactions. Phytochrome interacting factors (PIFs) are basic helix loop helix (bHLH) transcription factors that play important roles in light signaling, growth, and development. They act as cellular signaling hubs, integrating external and internal signals in plants ( Krahmer and Fankhauser 2023). PIFs regulate gene expression by binding to the promoter DNA of target genes. They play different functions in seedling growth, germination, and temperature response. The Arabidopsis genome encodes 8 PIFs (PIF1-8) based on the presence of an Active Phytochrome B binding (APB) motif; however, PIF1 and PIF3 also possess an Active Phytochrome A binding motif in addition to the APB ( Khanna et al. 2004).

A new class of amphiphilic triazolyl peptides was designed and synthesized from peptide-based building blocks containing alkyne and azide functional groups namely linear (W(pG))₃, cyclic[W(pG)]₃, and Ac-K(N₃)R-NH₂,where W, R, K, and pG represent tryptophan, arginine, lysine, and propargylglycine residues, respectively. The linear (W(pG))₃ and cyclic [W(pG)]₃ peptides containing alkyne residues were conjugated with Ac-K(N₃)R-NH₂ functionalized with azide group through click chemistry in the presence of CuSO₄.5H₂O, Cu (powder), sodium ascorbate, and N,N-disopropylethylamine in methanol:water to afford amphiphilic triazolyl linear-linear (WG(triazole-KR-NH₂))₃ and cyclic-linear [WG(triazole-KR-NH₂)]₃ peptides, respectively. The secondary structures of both peptides were similar to a distorted α-helix as shown by CD spectroscopy. TEM imaging showed that linear-linear (WG(triazole-KR-NH₂))₃ and cyclic-linear [WG(triazole-KR-NH₂)]₃ peptides formed nano-sized structures in the size range of 50-100 nm and 50-80 nm, respectively.