Separate and Combined Response to UV-B Radiation and Jasmonic Acid on Photosynthesis and Growth Characteristics of Scutellaria baicalensis

Melatonin alleviates UV-B stress and enhances phenolic biosynthesis in rosemary (Rosmarinus officinalis) callus

Although used in in vitro culture to boost secondary metabolite production, UV-B radiation can seriously affect plant growth if not properly dosed. Rosemary callus can be used as an important source of effective ingredients in the food and medicine industry. To balance the positive and negative effects of UV-B on rosmary callus, this study investigated the effects of melatonin on rosemary callus under UV-B radiation. The results showed that melatonin improved rosemary callus growth, with fresh weight and dry weight increased by 15.81% and 8.30%, respectively. The addition of 100 μM melatonin increased antioxidant enzyme activity and NO content in rosemary callus. At the same time, melatonin also significantly reduced membrane lipid damage and H2O2 accumulation in rosemary callus under UV-B stress, with malondialdehyde (MDA) and H2O2 contents reduced by 13.03% and 14.55%, respectively. In addition, melatonin increased the total phenol and rosmarinic acid contents in rosemary callus by 19% and 54%, respectively. Melatonin significantly improved the antioxidant activity of the extracts from rosemary callus. These results suggest that exogenous melatonin can alleviate the adverse effects of UV-B stress on rosemary callus by promoting NO accumulation while further enhancing phenolic accumulation and biological activity.

Exogenous Cytokinin 4PU-30 Modulates the Response of Wheat and Einkorn Seedlings to Ultraviolet B Radiation

Abiotic stress is responsible for a significant reduction in crop plant productivity worldwide. Ultraviolet (UV) radiation is a natural component of sunlight and a permanent environmental stimulus. This study investigated the distinct responses of young wheat and einkorn plants to excessive UV-B radiation (180 min at λmax 312 nm) following foliar pretreatment with 1 µM synthetic cytokinin 4PU-30. Results demonstrated that UV radiation significantly amplified hydrogen peroxide levels in both wheat and einkorn, with einkorn exhibiting a more pronounced increase compared to wheat. This elevation indicated the induction of oxidative stress by UV radiation in the two genotypes. Intensified antioxidant enzyme activities and the increased accumulation of typical stress markers and non-enzyme protectants were evidenced. Transcriptional activity of genes encoding the key antioxidant enzymes POX, GST, CAT, and SOD was also investigated to shed some light on their genetic regulation in both wheat and einkorn seedlings. Our results suggested a role for POX1 and POX7 genes in the UV-B tolerance of the two wheat species as well as a cytokinin-stimulated UV-B stress response in einkorn involving the upregulation of the tau subfamily gene GSTU6. Based on all our findings, it could be concluded that 4PU-30 had the potential of alleviating oxidative stress by attenuating the symptoms of superfluous UV-B illumination in the two examined plant species.

UVR8-TCP4-LOX2 module regulates UV-B tolerance in Arabidopsis

The phytohormone jasmonate (JA) coordinates stress and growth responses to increase plant survival in unfavorable environments. Although JA can enhance plant UV-B stress tolerance, the mechanisms underlying the interaction of UV-B and JA in this response remain unknown. In this study, we demonstrate that the UV RESISTANCE LOCUS 8 - TEOSINTE BRANCHED1, Cycloidea and PCF 4 - LIPOXYGENASE2 (UVR8-TCP4-LOX2) module regulates UV-B tolerance dependent on JA signaling pathway in Arabidopsis thaliana. We show that the nucleus-localized UVR8 physically interacts with TCP4 to increase the DNA-binding activity of TCP4 and upregulate the JA biosynthesis gene LOX2. Furthermore, UVR8 activates the expression of LOX2 in a TCP4-dependent manner. Our genetic analysis also provides evidence that TCP4 acts downstream of UVR8 and upstream of LOX2 to mediate plant responses to UV-B stress. Our results illustrate that the UV-B-dependent interaction of UVR8 and TCP4 serves as an important UVR8-TCP4-LOX2 module, which integrates UV-B radiation and JA signaling and represents a new UVR8 signaling mechanism in plants.

Berry secondary metabolites and leaf physiological parameters are independently regulated by exogenous methyl jasmonate application in Sangiovese grapevines (Vitis vinifera L.)

The role of jasmonates as elicitor of secondary metabolites is well known, and many experiments have been conducted in grapevine to evaluate their effects on berry and wine quality. Even though most of these studies used foliar jasmonates applications, little investigations have been done to assess the effects on leaves which, in turn, may indirectly affect grape metabolism potentially involving a long distance signaling or crosstalk. In this experiment we jointly investigated the specific effect of jasmonates on grape berry secondary metabolites and leaf physiological parameters to better comprehend their elicitation mechanisms in grapevine. A 10 mM methyl jasmonate (MeJA) solution was applied during the lag-phase only on the leaves or only on the clusters and compared to an untreated control. The MeJA specifically affected leaf physiological parameters and berry metabolism in the treated area. When applied only on the leaves, gas exchange parameters and leaf efficiency were reduced, stimulating the senescence mechanisms, without affecting berry metabolism. On the contrary, MeJA applied on the clusters significantly delayed berry ripening, leading to hypothesize a re-route of the berry carbon resources through the biosynthesis of volatile organic compounds which were strongly increased, especially the monoterpenes in their glycosylated form.

The effect of exogenous plant growth regulators on elevated Cd phytoremediation by Solanum nigrum L. in contaminated soil

Maximizing amendment potential is an emphasis in the HM-contaminated field of phytoremediation by hyperaccumulators due to the low bioavailability of HMs in soils and small biomass yields of plants. This study investigated the influence of different types and concentrations of plant growth regulators on Cd phytoremediation by Solanum nigrum in contaminated soil. Our conclusions showed that the shoot Cd extractions (μg plant-1) and the root and shoot biomasses at all the treatments remarkedly increased compared with that of the CK (p < 0.05), while the Cd concentrations at root and aboveground parts by S. nigrum, the extractable Cd concentrations, and pH value of soils did not change significantly compared with the CK (p < 0.05). Furthermore, correlation analysis showed that the shoot Cd phytoaccumulation and the root and aboveground biomasses of S. nigrum were particularly dependent upon the application of CTK and GA3 concentration gradient (p < 0.05). Moreover, some related physicochemical indexes were determined for supervising the growth conditions of plants, and these results pointed out that after exogenous PGRs treatments, the chlorophyll content and antioxidative enzymes POD and SOD activities in vivo of plants clearly advanced, while the H2O2 and MDA contents and CAT apparently declined. These consequence demonstrated that the exogenous PGR addition prominently reinforced the Cd phytoextraction capacity of S. nigrum in contaminated soil by stimulating plant growth and increasing shoot yields.

Effect of enhanced UV-B radiation on growth and photosynthetic physiology of Iris tectorum maxim

Iris tectorum Maxim. is an important plant that plays a very crucial role in the ecological welfare of wetlands. In this study, the effects of different intensities of UV-B radiation on the growth, photosynthetic pigment content, chlorophyll fluorescence characteristics, chloroplast ultrastructure, and gas exchange parameters of Iris tectorum Maxim. were studied. The results showed that enhanced UV-B radiation had a significant influence on the above-mentioned parameters of iris. Compared with the control, enhanced UV-B radiation caused certain damage to the leaf appearance. With the increasing intensity of radiation, the apparent damage degree became more serious. Enhanced UV-B radiation significantly decreased leaf chlorophyll contents, and the effect accumulated with the exposure time. Enhanced UV-B radiation increased Fo, significantly increased the non-photochemical quenching coefficient NPQ, reduced PSII and Qp, and significantly decreased the Fm, Fv/Fm, and Fv/Fo in leaves. The effect of UV-B radiation on PSII destruction of Iris tectorum Maxim. increased as the radiation intensity increased and the exposure time prolonged. The chloroplast structure was damaged under the enhanced UV-B radiation. More specifically, thylakoid lamellae were distorted, swelling and even blurred, and a large number of starch granules appeared. The effect of the high intensity of radiation on chloroplast ultrastructure was greater than that of lower intensity. Enhanced UV-B radiation reduced significantly the net photosynthetic rate, stomatal conductance, and transpiration rate, and the degree of degradation increased with the increasing irradiation intensity. However, the intercellular CO₂ content increased, which suggests that the main reason for the decrease of photosynthetic rate was the non-stomatal factors.

Modulations in Chlorophyll a Fluorescence Based on Intensity and Spectral Variations of Light

Photosynthetic efficiency is significantly affected by both qualitative and quantitative changes during light exposure. The properties of light have a profound effect on electron transport and energy absorption in photochemical reactions. In addition, fluctuations in light intensity and variations in the spectrum can lead to a decrease in photosystem II efficiency. These features necessitate the use of a simple and suitable tool called chlorophyll a fluorescence to study photosynthetic reactions as a function of the aforementioned variables. This research implies that chlorophyll a fluorescence data can be used to determine precise light conditions that help photoautotrophic organisms optimally function.

Enhanced antioxidant capacity and upregulated transporter genes contribute to the UV-B-induced increase in blinin in Conyza blinii

Conyza blinii (C. blinii) is a traditional Chinese medicinal plant mainly grown in Sichuan, China. C. blinii is suitable for studying the mechanism of plant tolerance to UV-B due to its living conditions, characterized by a high altitude and exposure to strong ultraviolet radiation. Our results showed that the growth and photosynthetic activity of C. blinii were improved under a specific intensity of UV-B, rather than being significantly inhibited. Although UV-B increased the content of reactive oxygen species (ROS) in C. blinii, the activities of antioxidative enzymes were elevated, including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), which contributed to the elimination of ROS. Additionally, the content of blinin, the characteristic diterpene in C. blinii, was markedly increased by UV-B. Furthermore, RNA sequencing analyses were used to explore the molecular mechanism of UV-B tolerance in C. blinii. According to the results, most of the key enzyme genes in the blinin synthesis pathway were upregulated by UV-B. In addition, 23 upregulated terpene transporter genes were identified, and these genes might participate in blinin transport during the response to UV-B. Taken together, these results implied that enhanced antioxidant capacity and upregulated transporter genes contributed to increased synthesis of blinin in response to UV-B in C. blinii.

Effects of exogenous methyl jasmonate on the synthesis of endogenous jasmonates and the regulation of photosynthesis in citrus

Methyl jasmonate (MeJA) is an airborne signaling phytohormone that can induce changes in endogenous jasmonates (JAs) and cause photosynthetic responses. However, the response of these two aspects of citrus plants at different MeJA concentrations is still unclear. Four MeJA concentrations were used in two citrus varieties, Huangguogan (C. reticulata × C. sinensis) and Shiranuhi [C. reticulata × (C. reticulata × C. sinensis)], to investigate the effects of MeJA dose on the endogenous JAs pathway and photosynthetic capacity. We observed that MeJA acted in a dose-dependent manner, and its stimulation in citrus leaves showed a bidirectional character at different concentrations. This work demonstrates that MeJA at only a concentration of 2.2 mM or less contributed to the activation of magnesium protoporphyrin IX methyltransferase (ChlM, EC 2.1.1.11) and protochlorophyllide oxidoreductase (POR, EC 1.3.1.11) and the simultaneous accumulation of Chl a and Chl b, which in turn contributed to an improved photosynthetic capacity and PSII photochemistry efficiency of citrus. Meanwhile, the inhibition of endogenous JAs synthesis by exogenous MeJA was observed. This was achieved by reducing the ratio of monogalactosyl diacylglycerol (MGDG) to diagalactosyl diacylglycerol (DGDG) and inhibiting the activities of key enzymes in JAs synthesis, especially 12-oxo-phytodienoic acid reductase (OPR, EC 1.3.1.42). Another noteworthy finding is that there may exist a JA-independent pathway that could regulate 12-oxo-phytodienoic acid (OPDA) synthesis. This study jointly analyzed the internal hormone regulation mechanism and the external physiological response, as well as revealed the effects of exogenous MeJA on promoting the photosynthesis and inhibiting the endogenous JAs synthesis.

Light signaling and UV-B-mediated plant growth regulation

Light plays an important role in plants' growth and development throughout their life cycle. Plants alter their morphological features in response to light cues of varying intensity and quality. Dedicated photoreceptors help plants to perceive light signals of different wavelengths. Activated photoreceptors stimulate the downstream signaling cascades that lead to extensive gene expression changes responsible for physiological and developmental responses. Proteins such as ELONGATED HYPOCOTYL5 (HY5) and CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) act as important factors which modulate light-regulated gene expression, especially during seedling development. These factors function as central regulatory intermediates not only in red, far-red, and blue light pathways but also in the UV-B signaling pathway. UV-B radiation makes up only a minor fraction of sunlight, yet it imparts many positive and negative effects on plant growth. Studies on UV-B perception, signaling, and response in plants has considerably surged in recent times. Plants have developed different strategies to use UV-B as a developmental cue as well as to withstand high doses of UV-B radiation. Plants' responses to UV-B are an integration of its cross-talks with both environmental factors and phytohormones. This review outlines the current developments in light signaling with a major focus on UV-B-mediated plant growth regulation.

Investigation of urediospore morphology, histopathology and epidemiological components on wheat plants infected with UV-B-induced mutant strains of Puccinia striiformis f. sp. tritici

Planting resistant cultivars is the most economical and effective measure to control wheat stripe rust caused by Puccinia striiformis f. sp. tritici (Pst), but the cultivars often lose their resistance due to the emergence of new physiological races. The UV-B-irradiated urediospores of the Pst physiological race CYR32 in China were inoculated on wheat cultivar Guinong 22 for screening virulence-mutant strains. CYR32 and mutant strains (CYR32-5 and CYR32-61) before and after UV-B radiation were used to conduct urediospore morphological and histopathological observations and an investigation of epidemiological components. The results showed that UV-B radiation affected the urediospore morphology of each strain. UV-B radiation inhibited urediospore invasion and hyphal elongation, which mainly manifested as decreases in germination rate, quantities of hyphal branches, haustorial mother cells and haustoria and hyphal length. After wheat cultivar Mingxian 169 was inoculated with the UV-B-irradiated urediospores, the incubation period was prolonged, and the infection efficiency, lesion expansion rate, total sporulation quantity and area under the disease progress curve were reduced. The results demonstrated that CYR32-5 and CYR32-61 may have more tolerance to UV-B radiation than CYR32. The results are significant for understanding mechanisms of Pst virulence variations and implementing sustainable management of wheat stripe rust.

Effects of elevated ultraviolet-B radiation on root growth and chemical signaling molecules in plants

Ozone layer depletion leads to elevated ultraviolet-B (UV-B) radiation, which affects plant growth; however, little is known about the relationship between root growth and signaling molecules in roots. Therefore, in this work, simulated UV-B radiation was used to study the effects of elevated UV-B radiation on root growth of soybean seedlings and changes in the content of signaling molecules in roots. The results showed that compared with the control, the 2.63 kJ m-2 d-1 and 6.17 kJ m-2 d-1 elevated UV-B radiation treatments inhibited root growth, and root growth parameters (total root length, root surface area, root volume, average diameter, root tip number, and root dry weight) all decreased. For root signaling molecules, the content of nitric oxide, reactive oxygen species, abscisic acid, salicylic acid, and jasmonic acid increased, and the content of auxin, cytokinin, and gibberellin decreased. The above indices changed more significantly under the 6.17 kJ m-2 d-1 treatment. After withdrawal of the exposure, the above indices could be restored to a certain extent. These data indicated that UV-B radiation interfered with root growth by affecting the content of signaling molecules in roots, and the degree of the effects was related to the intensity of UV-B radiation. The results from this study provide a theoretical basis for studying the preliminary mechanism of elevated UV-B radiation on root growth and possible pathways that can mitigate UV-B radiation damage for root growth. ONE SENTENCE SUMMARY: The effects of elevated UV-B on root growth of soybean seedlings were regulated by signaling molecules, and the degree of the effects was related to the intensity of UV-B radiation.

Sedoheptulose-1,7-Bisphosphatase is Involved in Methyl Jasmonate- and Dark-Induced Leaf Senescence in Tomato Plants

Leaf senescence represents the final stage of leaf development and is regulated by diverse internal and environmental factors. Jasmonates (JAs) have been demonstrated to induce leaf senescence in several species; however, the mechanisms of JA-induced leaf senescence remain largely unknown in tomato plants (Solanum lycopersicum). In the present study, we tested the hypothesis that sedoheptulose-1,7-bisphosphatase (SBPase), an enzyme functioning in the photosynthetic carbon fixation in the Calvin⁻Benson cycle, was involved in methyl jasmonate (MeJA)- and dark-induced leaf senescence in tomato plants. We found that MeJA and dark induced senescence in detached tomato leaves and concomitantly downregulated the expression of SlSBPASE and reduced SBPase activity. Furthermore, CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9)-mediated mutagenesis of SlSBPASE led to senescence-associated characteristics in slsbpase mutant plants, including loss of chlorophyll, repressed photosynthesis, increased membrane ion leakage, and enhanced transcript abundance of senescence-associated genes. Collectively, our data suggest that repression of SBPase by MeJA and dark treatment plays a role in JA- and dark-induced leaf senescence.