Transcription factors BBX11 and HY5 interdependently regulate the molecular and metabolic responses to UV-B

BBX22 enhances the accumulation of antioxidants to inhibit DNA damage and promotes DNA repair under high UV-B

Under changing climatic conditions, plant exposure to high-intensity UV-B can be a potential threat to plant health and all plant-derived human requirements, including food. It's crucial to understand how plants respond to high UV-B radiation so that proper measures can be taken to enhance tolerance towards high UV-B stress. We found that BBX22, a B-box protein-coding gene, is strongly induced within one hour of exposure to high-intensity UV-B. Our metabolomics data indicated that BBX22 promotes the accumulation of antioxidants like ascorbic acid and proline. These antioxidants play a vital role in shielding plants exposed to high UV-B from the detrimental effects of Reactive Oxygen Species (ROS), including DNA damage. Additionally, BBX22 promotes DNA damage repair by inducing the expression of DNA repair genes like UVR1 and UVR3. BBX22 directly binds to the promoter of UVR1 to regulate its expression. Furthermore, BBX22 indirectly induces the expression of UVR1 and UVR3 by enhancing the binding of HY5 to their promoters. Together, these results suggest a multi-pronged role of BBX22 in protection against high-intensity UV-B. Enhancing BBX22 levels or its orthologs in different plant species can potentially offer DNA damage protection and tolerance against intense UV radiation.

The homeostasis of AtMYB4 is maintained by ARA4, HY5, and CAM7 during Arabidopsis seedling development

Calmodulin7 (CAM7) is a key transcription factor of Arabidopsis seedling development. CAM7 works together with HY5 bZIP protein to promote photomorphogenesis at various wavelengths of light. In this study, we show that AtMYB4, identified from a yeast two-hybrid screen, physically interacts with CAM7 and works as a positive regulator of photomorphogenesis at various wavelengths of light. CAM7 and HY5 directly bind to the promoter of AtMYB4 to promote its expression for photomorphogenic growth. On the other hand, ARA4, identified from the same yeast two-hybrid screen, works as a negative regulator of photomorphogenic growth specifically in white light. The double mutant analysis reveals that the altered hypocotyl elongation of atmyb4 and ara4 is either partly or completely suppressed by additional loss of function of CAM7. Furthermore, ARA4 genetically interacts with AtMYB4 in an antagonistic manner to suppress the elongated hypocotyl phenotype of atmyb4. The transactivation studies reveal that while CAM7 activates the promoter of AtMYB4 in association with HY5, ARA4 negatively regulates AtMYB4 expression. Taken together, these results demonstrate that working as a negative regulator of photomorphogenesis, ARA4 plays a balancing act on CAM7 and HY5-mediated regulation of AtMYB4.

The B-box protein BBX13/COL15 suppresses photoperiodic flowering by attenuating the action of CONSTANS in Arabidopsis

The optimal timing of transition from vegetative to floral reproductive phase is critical for plant productivity and agricultural yields. Light plays a decisive role in regulating this transition. The B-box (BBX) family of transcription factors regulates several light-mediated developmental processes in plants, including flowering. Here, we identify a previously uncharacterized group II BBX family member, BBX13/COL15, as a negative regulator of flowering under long-day conditions. BBX13 is primarily expressed in the leaf vasculature, buds, and flowers, showing a similar spatial expression pattern to the major flowering time regulators CO and FT. bbx13 mutants flower early, while BBX13-overexpressors exhibit delayed flowering under long days. Genetic analyses showed that BBX13 acts upstream to CO and FT and negatively regulates their expression. BBX13 physically interacts with CO and inhibits the CO-mediated transcriptional activation of FT. In addition, BBX13 directly binds to the CORE2 motif on the FT promoter, where CO also binds. Chromatin immunoprecipitation data indicates that BBX13 reduces the in vivo binding of CO on the FT promoter. Through luciferase assay, we found that BBX13 inhibits the CO-mediated transcriptional activation of FT. Together, these findings suggest that BBX13/COL15 represses flowering in Arabidopsis by attenuating the binding of CO on the FT promoter.

BBX9 forms feedback loops with PIFs and BBX21 to promote photomorphogenic development

Light is one of the most essential environmental factors that tightly and precisely control various physiological and developmental processes in plants. B-box CONTAINING PROTEINs (BBXs) play central roles in the regulation of light-dependent development. In this study, we report that BBX9 is a positive regulator of light signaling. BBX9 interacts with the red light photoreceptor PHYTOCHROME B (phyB) and transcription factors PHYTOCHROME-INTERACTING FACTORs (PIFs). phyB promotes the stabilization of BBX9 in light, while BBX9 inhibits the transcriptional activation activity of PIFs. In turn, PIFs directly bind to the promoter of BBX9 to repress its transcription. On the other hand, BBX9 associates with the positive regulator of light signaling, BBX21, and enhances its biochemical activity. BBX21 associates with the promoter regions of BBX9 and transcriptionally up-regulates its expression. Collectively, this study unveiled that BBX9 forms a negative feedback loop with PIFs and a positive one with BBX21 to ensure that plants adapt to fluctuating light conditions.

A dose-dependent effect of UV-328 on photosynthesis: Exploring light harvesting and UV-B sensing mechanisms

Benzotriazole ultraviolet (UV) stabilizers (BUVs) have emerged as significant environmental contaminants, frequently detected in various ecosystems. While the toxicity of BUVs to aquatic organisms is well-documented, studies on their impact on plant life are scarce. Plants are crucial as they provide the primary source of energy and organic matter in ecosystems through photosynthesis. This study investigated the effects of UV-328 (2-(2-hydroxy-4',6'-di-tert-amylphenyl) benzotriazole) on plant growth indices and photosynthesis processes, employing conventional physiological experiments, RNA sequencing (RNA-seq) analysis, and computational methods. Results demonstrated a biphasic response in plant biomass and the maximum quantum yield of PS II (Fv/Fm), showing improvement at a 50 μM UV-328 treatment but reduction under 150 μM UV-328 exposure. Additionally, disruption in thylakoid morphology was observed at the higher concentration. RNA-seq and qRT-PCR analysis identified key differentially expressed genes (light-harvesting chlorophyll-protein complex Ⅰ subunit A4, light-harvesting chlorophyll b-binding protein 3, UVR8, and curvature thylakoid 1 A) related to photosynthetic light harvesting, UV-B sensing, and chloroplast structure pathways, suggesting they may contribute to the observed alterations in photosynthesis activity induced by UV-328 exposure. Molecular docking analyses further supported the binding affinity between these proteins and UV-328. Overall, this study provided comprehensive physiological and molecular insights, contributing valuable information to the evaluation of the potential risks posed by UV-328 to critical plant physiological processes.

A birch ELONGATED HYPOCOTYL 5 gene enhances UV-B and drought tolerance

UV-B radiation and drought majorly restrict plant growth, particularly in summer. ELONGATED HYPOCOTYL 5 (HY5), a bZIP transcription factor (TF), has a beneficial impact on photomorphogenesis. However, the sequence of HY5 from Betula platyphylla (BpHY5) has not been identified and the gene functions remain unclarified. We cloned the sequence of BpHY5, which was targeted to the nucleus. The hypocotyl phenotypes of heterologous expression in Arabidopsis thaliana and reverse mutation showed that BpHY5 is homologous to AtHY5. The expression of BpHY5 was increased in response to UV-B radiation, drought conditions, and the presence of abscisic acid (ABA). The overexpression of BpHY5 resulted in increased tolerance to UV-B radiation and drought and decreased ABA sensitivity with higher germination and greening rate, more developmental root system, stronger reactive oxygen species scavenging ability, and lower damage degree. The lignin content under UV-B condition of BpHY5/Col was higher than that of Col. Furthermore, overexpressing BpHY5 up-regulated the expression of genes related to tolerance (NCED3/9, ABI5, DREB2A, RD20, ERF4, NDB2, and APX2). In brief, the study suggests that BpHY5 from birch serves as a beneficial modulator of plant responses to UV-B radiation and drought stress.

PHYTOCHROME A controls the DNA damage response and cell death tolerance within the Arabidopsis root meristem

The DNA damage response avoids mutations into dividing cells. Here, we analysed the role of photoreceptors on the restriction of root growth imposed by genotoxic agents and its relationship with cell viability and performance of meristems. Comparison of root growth of Arabidopsis WT, phyA-211, phyB-9, and phyA-211phyB-9 double mutants unveiled a critical role for phytochrome A (PhyA) in protecting roots from genotoxic stress, regeneration and cell replenishment in the meristematic zone. PhyA was located on primary root tips, where it influences genes related to the repair of DNA, including ERF115 and RAD51. Interestingly, phyA-211 mutants treated with zeocin failed to induce the expression of the repressor of cell cycle MYB3R3, which correlated with expression of the mitotic cyclin CycB1, suggesting that PhyA is required for safeguarding the DNA integrity during cell division. Moreover, the growth of the primary roots of PhyA downstream component HY5 and root growth analyses in darkness suggest that cell viability and DNA damage responses within root meristems may act independently from light and photomorphogenesis. These data support novel roles for PhyA as a key player for stem cell niche maintenance and DNA damage responses, which are critical for proper root growth.

OsbZIP18 Is a Positive Regulator of Phenylpropanoid and Flavonoid Biosynthesis under UV-B Radiation in Rice

In plants exposed to ultraviolet B radiation (UV-B; 280-315 nm), metabolic responses are activated, which reduce the damage caused by UV-B. Although several metabolites responding to UV-B stress have been identified in plants, the accumulation of these metabolites at different time points under UV-B stress remains largely unclear, and the transcription factors regulating these metabolites have not been well characterized. Here, we explored the changes in metabolites in rice after UV-B treatment for 0 h, 6 h, 12 h, and 24 h and identified six patterns of metabolic change. We show that the rice transcription factor OsbZIP18 plays an important role in regulating phenylpropanoid and flavonoid biosynthesis under UV-B stress in rice. Metabolic profiling revealed that the contents of phenylpropanoid and flavonoid were significantly reduced in osbzip18 mutants compared with the wild-type plants (WT) under UV-B stress. Further analysis showed that the expression of many genes involved in the phenylpropanoid and flavonoid biosynthesis pathways was lower in osbzip18 mutants than in WT plants, including OsPAL5, OsC4H, Os4CL, OsCHS, OsCHIL2, and OsF3H. Electrophoretic mobility shift assays (EMSA) revealed that OsbZIP18 bind to the promoters of these genes, suggesting that OsbZIP18 function is an important positive regulator of phenylpropanoid and flavonoid biosynthesis under UV-B stress. In conclusion, our findings revealed that OsbZIP18 is an essential regulator for phenylpropanoid and flavonoid biosynthesis and plays a crucial role in regulating UV-B stress responses in rice.

Cross-Species Metabolomic Analyses in the Brassicaceae Reveals Common Responses to Ultraviolet-B Exposure

Exposure to UV-B radiation, an intrinsic component of solar light, is detrimental to all living organisms as chromophore units of DNA, RNA and proteins readily absorb high-energy photons. Indirect damage to the same molecules and lipids is mediated by elevated reactive oxygen species (ROS) levels, a side effect of exposure to UV-B stress. To protect themselves from UV-B radiation, plants produce phytochemical sunscreens, among which flavonoids have shown to be particularly effective. The core aglycone of flavonoid molecules is subjected to chemical decoration, such as glycosylation and acylation, further improving sunscreen properties. In particular, acylation, which adds a phenolic ring to flavonoid molecules, enhances the spectral absorption of UV-A and UV-B rays, providing to this class of compounds exceptional shielding power. In this study, we comprehensively analyzed the responses to UV-B radiation in four Brassicaceae species, including Arabidopsis thaliana, Brassica napus, Brassica oleracea, and Brassica rapa. Our study revealed a complete reprogramming of the central metabolic pathway in response to UV-B radiation characterized by increased production of functional precursors of specialized metabolites with UV-B shielding properties, indicating a targeted effort of plant metabolism to provide increased protection. The analysis of specialized metabolites and transcripts revealed the activation of the phenylpropanoid-acetate pathway, leading to the production of specific classes of flavonoids and a cross-species increase in phenylacylated-flavonoid glucosides with synapoyl glycoside decorations. Interestingly, our analysis also revealed that acyltransferase genes of the class of serine carboxypeptidase-like (SCPLs) proteins are costitutively expressed, but downregulated in response to UV-B radiation, possibly independently of the ELONGATED HYPOCOTYL 5 (HY5) signaling pathway.

Coordinated regulation of the mitochondrial retrograde response by circadian clock regulators and ANAC017

Mitochondrial retrograde signaling (MRS) supports photosynthetic function under a variety of conditions. Induction of mitochondrial dysfunction with myxothiazol (a specific inhibitor of the mitochondrial bc₁ complex) or antimycin A (an inhibitor of the mitochondrial bc₁ complex and cyclic electron transport in the chloroplast under light conditions) in the light and dark revealed diurnal control of MRS. This was evidenced by (1) significantly enhanced binding of ANAC017 to promoters in the light compared with the dark in Arabidopsis plants treated with myxothiazol (but not antimycin A), (2) overlap in the experimentally determined binding sites for ANAC017 and circadian clock regulators in the promoters of ANAC013 and AOX1a, (3) a diurnal expression pattern for ANAC017 and transcription factors it regulates, (4) altered expression of ANAC017-regulated genes in circadian clock mutants with and without myxothiazol treatment, and (5) a decrease in the magnitude of LHY and CCA1 expression in an ANAC017-overexpressing line and protein-protein interaction between ANAC017 and PIF4. This study also shows a large difference in transcriptome responses to antimycin A and myxothiazol in the dark: these responses are ANAC017 independent, observed in shoots and roots, similar to biotic challenge and salicylic acid responses, and involve ERF and ZAT transcription factors. This suggests that antimycin A treatment stimulates a second MRS pathway that is mediated or converges with salicylic acid signaling and provides a merging point with chloroplast retrograde signaling.

Plant responses to UV-B radiation: signaling, acclimation and stress tolerance

Ultraviolet-B (UV-B) light is an intrinsic part of sunlight that reaches the earth's surface, and affects plant survival and adaptation. How plants respond to UV-B light is regulated by the wavelength, intensity and duration of UV-B radiation, and is also regulated by photosynthetically active radiation perceived by phytochrome and cryptochrome photoreceptors. Non-damaging UV-B light promotes plant photomorphogenesis and UV-B acclimation which enhances plant tolerance against UV-B stress. However, high-level UV-B radiation induces DNA damage, generates reactive oxygen species (ROS) and impairs photosynthesis. Plants have evolved efficient mechanisms to utilize informational UV-B signal, and protect themselves from UV-B stress. UV RESISTANCE LOCUS8 (UVR8) is a conserved plant-specific UV-B photoreceptor. It interacts with CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) to initiate UV-B-specific light signaling and regulate UV-B responsive gene expression. A set of transcription factors such as ELONGATED HYPOCOTYL5 (HY5) function downstream of the UVR8-COP1 module to promote seedling de-etiolation for photomorphogenic development and biosynthesis of sunscreen flavonoids for UV-B stress tolerance. In addition to UVR8 signaling pathways, plants subjected to damaging UV-B radiation initiate stress protection and repair mechanisms through UVR8-independent pathways. In this review, we summarize the emerging mechanisms underlying UV-B stress acclimation and protection in plants, primarily revealed in the model plant Arabidopsis thaliana.