Photosynthetically active radiation (PAR) is an important environmental cue inducing the production of many secondary metabolites involved in plant oxidative stress avoidance and tolerance. To examine the complex role of PAR irradiance and specific spectral components on the accumulation of phenolic compounds (PheCs), we acclimated spring barley (Hordeum vulgare) to different spectral qualities (white, blue, green, red) at three irradiances (100, 200, 400 µmol m⁻² s⁻¹). We confirmed that blue light irradiance is essential for the accumulation of PheCs in secondary barley leaves (in UV-lacking conditions), which underpins the importance of photoreceptor signals (especially cryptochrome). Increasing blue light irradiance most effectively induced the accumulation of B-dihydroxylated flavonoids, probably due to the significantly enhanced expression of the F3′H gene. These changes in PheC metabolism led to a steeper increase in antioxidant activity than epidermal UV-A shielding in leaf extracts containing PheCs. In addition, we examined the possible role of miRNAs in the complex regulation of gene expression related to PheC biosynthesis.

G-quadruplexes have long been perceived as rare and physiologically unimportant nucleic acid structures. However, several studies have revealed their importance in molecular processes, suggesting their possible role in replication and gene expression regulation. Pathways involving G-quadruplexes are intensively studied, especially in the context of human diseases, while their involvement in gene expression regulation in plants remains largely unexplored. Here, we conducted a bioinformatic study and performed a complex circular dichroism measurement to identify a stable G-quadruplex in the gene RPB1, coding for the RNA polymerase II large subunit. We found that this G-quadruplex-forming locus is highly evolutionarily conserved amongst plants sensu lato (Archaeplastida) that share a common ancestor more than one billion years old. Finally, we discussed a new hypothesis regarding G-quadruplexes interacting with UV light in plants to potentially form an additional layer of the regulatory network.

G-quadruplexes are four-stranded nucleic acid structures occurring in the genomes of all living organisms and viruses. It is increasingly evident that these structures play important molecular roles; generally, by modulating gene expression and overall genome integrity. For a long period, G-quadruplexes have been studied specifically in the context of human promoters, telomeres, and associated diseases (cancers, neurological disorders). Several of the proteins for binding G-quadruplexes are known, providing promising targets for influencing G-quadruplex-related processes in organisms. Nonetheless, in plants, only a small number of G-quadruplex binding proteins have been described to date. Thus, we aimed to bioinformatically inspect the available protein sequences to find the best protein candidates with the potential to bind G-quadruplexes. Two similar glycine and arginine-rich G-quadruplex-binding motifs were described in humans. The first is the so-called “RGG motif”-RRGDGRRRGGGGRGQGGRGRGGGFKG, and the second (which has been recently described) is known as the “NIQI motif”-RGRGRGRGGGSGGSGGRGRG. Using this general knowledge, we searched for plant proteins containing the above mentioned motifs, using two independent approaches (BLASTp and FIMO scanning), and revealed many proteins containing the G4-binding motif(s). Our research also revealed the core proteins involved in G4 folding and resolving in green plants, algae, and the key plant model organism, Arabidopsis thaliana. The discovered protein candidates were annotated using STRINGdb and sorted by their molecular and physiological roles in simple schemes. Our results point to the significant role of G4-binding proteins in the regulation of gene expression in plants.

Accumulation and metabolic profile of phenolic compounds (PheCs; serving as UV-screening pigments and antioxidants) as well as carbon fixation rate (A_n) and plant growth are sensitive to irradiance and temperature. Since these factors are naturally co-acting in the environment, it is worthy to study the combined effects of these environmental factors to assess their possible physiological consequences. We investigated how low and high irradiance in combination with different temperatures modify the metabolic profile of PheCs and expression of genes involved in the antioxidative enzyme and PheCs biosynthesis, in relation to photosynthetic activity and availability of non-structural carbohydrates (NSC) in spring barley seedlings. High irradiance positively affected A_n, NSC, PheCs content, and antioxidant activity (AOX). High temperature led to decreased A_n, NSC, and increased dark respiration, whilst low temperature was accompanied by reduction of UV-A shielding but increase of PheCs content and AOX. Besides that, irradiance and temperature caused changes in the metabolic profile of PheCs, particularly alteration in homoorientin/isovitexin derivatives ratio, possibly related to demands on AOX-based protection. Moreover, we also observed changes in the ratio of sinapoyl-/feruloyl- acylated flavonoids, the function of which is not yet known. The data also strongly suggested that the NSC content may support the PheCs production.

We examined the dynamics of phenolic compounds (PheCs) and carotenoids (Cars) in the leaves of wild type (WT) spring barley (Hordeum vulgare L.) and its mutant lacking chlorophyll b Chlorina f2 (Clo f2) grown from seeds at high irradiance (8 d at 1 000 μmol m^-2 s^-1; HI) during 9 d of acclimation to low irradiance (50 μmol m^-2 s^-1; LI). Our results showed that a leaf epidermal flavonoid UV-shielding index remained rather constant after transfer of plants from HI to LI conditions and that it was significantly lower in Clo f2 compared to WT plants. This suggests that HI pretreated plants can be well protected against excessive UV for at least 9 d, as supported also by the constant absorbance of leaf PheCs extracts in the UV-A region (at 335 nm). In contrast, absorbance in the UV-B region (at 270 nm) was reduced, particularly during the initial days of LI treatment, indicating specific changes in PheC profile. High-performance liquid chromatography of soluble PheCs revealed stable content of the major PheC saponarin during LI acclimation, whereas luteolin and feruloylquinic acid content decreased, particularly in WT plants. We also observed a pronounced decrease in Car relative content, particularly a reduction in the xanthophyll cycle pigments (violaxanthin, antheraxanthin, and zeaxanthin, VAZ) pool and diminution of their de-epoxidation state (DEPS) in dark-adapted leaves. As both VAZ and DEPS were higher in HI-acclimated Clo f2 plants than they were in WT plants, the presence of a significant VAZ pool within the lipid phase of thylakoid membrane is indicated. That can contribute to antioxidant capacity particularly in Clo f2 plants. We can therefore conclude that there is a tendency to retain the PheCs responsible for UV shielding during LI acclimation. Meanwhile, the accumulation of both PheCs and zeaxanthin serving as effective antioxidants is considerably downregulated within 9 d.

Phenolic compounds are a group of secondary metabolites responsible for several processes in plants—these compounds are involved in plant–environment interactions (attraction of pollinators, repelling of herbivores, or chemotaxis of microbiota in soil), but also have antioxidative properties and are capable of binding heavy metals or screening ultraviolet radiation. Therefore, the accumulation of these compounds has to be precisely driven, which is ensured on several levels, but the most important aspect seems to be the control of the gene expression. Such transcriptional control requires the presence and activity of transcription factors (TFs) that are driven based on the current requirements of the plant. Two environmental factors mainly affect the accumulation of phenolic compounds—light and temperature. Because it is known that light perception occurs via the specialized sensors (photoreceptors) we decided to combine the biophysical knowledge about light perception in plants with the molecular biology-based knowledge about the transcription control of specific genes to bridge the gap between them. Our review offers insights into the regulation of genes related to phenolic compound production, strengthens understanding of plant responses to environmental cues, and opens avenues for manipulation of the total content and profile of phenolic compounds with potential applications in horticulture and food production.