Effects of soaking seeds in exogenous vitamins on active oxygen metabolism and seedling growth under low-temperature stress

Revelation of mechanisms associated with strengthening plant cold tolerance through using exogenous substances

Cold stress (CS) is one of the main factors that limits the crop or plant growth and development in many regions of the world. Many researchers have been endeavoring to break the natural temperature barrier to grow plants in extremely cold areas or to alleviate erratic cold devastation on crops in temperate or subtropical regions for years. Numerous studies and research papers published recently for the last two decades have proven that exogenous substances (ESs) are effective and practical in helping plants tolerate CS. Here, we systematically summarize and characterize all 72 ESs that have been tried against CS, analyze research hotspots in the Web of Science database from 2000 to 2024 using VOSviewer with the keywords "cold stress" and "exogenous substances", and grouped them accordingly. Based on their underlying mechanisms, five categories of ESs are clearly defined, described and discussed: 1) enhancement of cell osmotic adjustment, 2) improvement of antioxidant pathways, 3) involvement in phytohormone regulation, 4) promotion of photosynthesis; 5) enrichment of nutritional status. After clarifying these five categories, a detailed plant responses and their possible interactions through a signal cross-talk are proposed and followed by discussions on future perspectives on using ESs to fortify plants against CS. The accumulative knowledge and information provided here will be ultimately used to increase plant productivity and agricultural sustainability under CS through chemical and microbial approaches.

Multi-scale characterisation of cold response reveals immediate and long-term impacts on cell physiology up to seed composition in sunflower

Early sowing can help summer crops escape drought and can mitigate the impacts of climate change on them. However, it exposes them to cold stress during initial developmental stages, which has both immediate and long-term effects on development and physiology. To understand how early night-chilling stress impacts plant development and yield, we studied the reference sunflower line XRQ under controlled, semi-controlled and field conditions. We performed high-throughput imaging of the whole plant parts and obtained physiological and transcriptomic data from leaves, hypocotyls and roots. We observed morphological reductions in early stages under field and controlled conditions, with a decrease in root development, an increase in reactive oxygen species content in leaves and changes in lipid composition in hypocotyls. A long-term increase in leaf chlorophyll suggests a stress memory mechanism that was supported by transcriptomic induction of histone coding genes. We highlighted DEGs related to cold acclimation such as chaperone, heat shock and late embryogenesis abundant proteins. We identified genes in hypocotyls involved in lipid, cutin, suberin and phenylalanine ammonia lyase biosynthesis and ROS scavenging. This comprehensive study describes new phenotyping methods and candidate genes to understand phenotypic plasticity better in response to chilling and study stress memory in sunflower.

Proteomics- and metabolomics-based analysis of the regulation of germination in Norway maple and sycamore embryonic axes

Norway maple and sycamore belong to the Acer genus and produce desiccation-tolerant and desiccation-sensitive seeds, respectively. We investigated the seed germination process at the imbibed and germinated stages using metabolomic and proteomic approaches to determine why sycamore seeds germinate earlier and are more successful at establishing seedlings than Norway maple seeds under controlled conditions. Embryonic axes and embryonic axes with protruded radicles were analyzed at the imbibed and germinated stages, respectively. Among the 212 identified metabolites, 44 and 67 differentially abundant metabolites were found at the imbibed and germinated stages, respectively, in both Acer species. Higher levels of amines, growth and defense stimulants, including B vitamins, were found in sycamore. We identified 611 and 447 proteins specific to the imbibed and germinated stages, respectively, in addition to groups of proteins expressed at different levels. Functional analysis of significantly regulated proteins revealed that proteins with catalytic and binding activity were enriched during germination, and proteins possibly implicated in nitrogen metabolism and metabolite interconversion enzymes were the predominant classes. Proteins associated with the control of plant growth regulation and seed defense were observed in both species at both germination stages. Sycamore proteins possibly involved in abscisic acid signal transduction pathway, stress tolerance and alleviation, ion binding and oxygenase activities appeared to accompany germination in sycamore. We identified peptides containing methionine (Met) oxidized to methionine sulfoxide (MetO), and functional analyses of proteins with significantly regulated MetO sites revealed that translation, plant growth and development and metabolism of nitrogen compounds were the main processes under Met/MetO redox control. We propose that higher levels of storage proteins and amines, together with higher levels of B vitamins, supported more efficient nitrogen utilization in sycamore, resulting in faster seedling growth. In conclusion, omic signatures identified in sycamore seem to predispose germinated sycamore seeds to better postgerminative growth.

Exogenous 24-Epibrassinolide Improves Low-Temperature Tolerance of Maize Seedlings by Influencing Sugar Signaling and Metabolism

Low-temperature (LT) stress seriously affects the distribution, seedling survival, and grain yield of maize. At the seedling emergence stage, maize's coleoptile is one of the most sensitive organs in sensing LT signaling and, in general, it can envelop young leaves to protect them from LT damage. In addition, brassinolides (BRs) have been shown to enhance LT tolerance from various species, but the effects of BRs on coleoptiles in maize seedlings under LT stress are unclear. Therefore, in this study, the pre-cultured coleoptiles of Zheng58 seedlings were treated with or without 2.0 μM 24-epibrassinolide (EBR) at 25 °C and 10 °C environments for five days to analyze their physiological and transcriptomic changes. Physiological analysis showed that a 10°C LT stress increased the content of glucose (0.43 mg g-1 FW), sucrose (0.45 mg g-1 FW), and starch (0.76 mg g-1 FW) of Zheng58 coleoptiles compared to a 25°C environment. After the coleoptiles were exposed to a 2.0 μM EBR application under 10°C temperature for five days, the contents of these three sugars continued to increase, and reached 2.68 mg g-1 FW, 4.64 mg g-1 FW, and 9.27 mg g-1 FW, respectively, indicating that sugar signaling and metabolism played key roles in regulating LT tolerance in the coleoptiles of maize seedlings. Meanwhile, a transcriptome analysis showed that 84 and 15 differentially expressed genes (DEGs) were enriched in the sucrose and starch metabolism and photosynthesis pathways, respectively, and multiple DEGs involved in these pathways were significantly up-regulated under LT stress and EBR stimulation. Further analysis speculated that the four DEGs responsible for sucrose-phosphate synthetase (SPS, i.e., Zm00001d048979, probable sucrose-phosphate synthase 5 and Zm00001d012036, sucrose-phosphate synthase 1), sucrose synthase (SUS, Zm00001d029091, sucrose synthase 2 and Zm00001d029087, sucrose synthase 4) were crucial nodes that could potentially link photosynthesis and other unknown pathways to form the complex interaction networks of maize LT tolerance. In conclusion, our findings provide new insights into the molecular mechanisms of exogenous EBR in enhancing LT tolerance of maize seedlings and identified potential candidate genes to be used for LT tolerance breeding in maize.

Insights on the enhancement of chilling tolerance in Rice through over-expression and knock-out studies of OsRBCS3

Chilling stress is an important environmental factor that affects rice (Oryza sativa L.) growth and yield, and the booting stage is the most sensitive stage of rice to chilling stress. In this study, we focused on OsRBCS3, a rice gene related to chilling tolerance at the booting stage, which encodes the key enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit in photosynthesis. The aim of this study was to elucidate the role and mechanism of OsRBCS3 in rice chilling tolerance at the booting stage. The expression levels of OsRBCS3 under chilling stress were compared in two japonica rice cultivars with different chilling tolerances: Kongyu131 (KY131) and Longjing11 (LJ11). A positive correlation was found between OsRBCS3 expression and chilling tolerance. Over-expression (OE) and knock-out (KO) lines of OsRBCS3 were constructed using over-expression and CRISPR/Cas9 technology, respectively, and their chilling tolerance was evaluated at the seedling and booting stages. The results showed that OE lines exhibited higher chilling tolerance than wild-type (WT) lines at both seedling and booting stages, while KO lines showed lower chilling tolerance than WT lines. Furthermore, the antioxidant enzyme activities, malondialdehyde (MDA) content and Rubisco activity of four rice lines under chilling stress were measured, and it was found that OE lines had stronger antioxidant and photosynthetic capacities, while KO lines had the opposite effects. This study validated that OsRBCS3 plays an important role in rice chilling tolerance at the booting stage, providing new molecular tools and a theoretical basis for rice chilling tolerance breeding.

Combined Physiological and Transcriptomic Analysis Reveals Key Regulatory Networks and Potential Hub Genes Controlling Chilling Tolerance During Soybean Germination

Chilling is an important limiting factor for seed germination of soybean (Glycine max [L.] Merr.). To reveal the regulatory mechanism of chilling tolerance during the soybean germination stage, based on previous studies, the chilling tolerance line R48 and chilling sensitive line R89 in chromosome segment substitution lines were selected for physiological index determination and transcriptome sequencing. It was found that reactive oxygen species (ROS) scavenging system related enzymes, ROS, and osmotic regulators were significantly different between the two lines. Gene Ontology enrichment and Kyoto Encyclopedia of Genes and Genomes enrichment were performed on the differentially expressed genes obtained by transcriptome sequencing. It was found that terms or pathways related to flavonoids, unsaturated fatty acids, and abscisic acid were highly enriched. In addition, weighted gene coexpression network analysis (WGCNA) method was used to analyze the physiological index data and transcriptome sequencing data. Four main coexpression modules significantly related to physiological indicators were obtained, and the hub genes in each module were screened according to eigengene-based connectivity value. Haplotype analysis of important candidate genes using soybean germplasm resources showed that there were significant differences in germination indexes between different major haplotypes of Glyma.17G163200. Based on the results of enrichment analysis and WGCNA, the regulation model of low temperature tolerance during soybean germination was preliminarily drawn. This study will provide theoretical guidance for analyzing the molecular regulation mechanism of cold tolerance in soybean germination stage.

Green analytical assay for the viability assessment of single maize seeds using double-threshold strategy for catalase activity and malondialdehyde content

The development of nondestructive technology for the detection of seed viability is challenging. In this study, to establish a green and effective method for the viability assessment of single maize seeds, a two-stage seed viability detection method was proposed. The catalase (CAT) activity and malondialdehyde (MDA) content were selected as the most key biochemical components affecting maize seed viability, and regression prediction models were developed based on their hyperspectral information and a data fusion strategy. Qualitative discrimination models for seed viability evaluation were constructed based on the predicted response values of the selected key biochemical components. The results showed that the double components thresholds strategy achieved the highest discrimination accuracy (92.9%), providing a crucial approach for the rapid and environmentally friendly detection of seed viability.

Application of Exogenous Ascorbic Acid Enhances Cold Tolerance in Tomato Seedlings through Molecular and Physiological Responses

Ascorbic acid (AsA), an essential non-enzymatic antioxidant in plants, regulates development growth and responses to abiotic and biotic stresses. However, research on AsA's role in cold tolerance remains largely unknown. Here, our study uncovered the positive role of AsA in improving cold stress tolerance in tomato seedlings. Physiological analysis showed that AsA significantly enhanced the enzyme activity of the antioxidant defense system in tomato seedling leaves and increased the contents of proline, sugar, abscisic acid (ABA), and endogenous AsA. In addition, we found that AsA is able to protect the photosynthetic system of tomato seedlings, thereby relieving the declining rate of chlorophyll fluorescence parameters. qRT-PCR analysis indicated that AsA significantly increased the expression of genes encoding antioxidant enzymes and involved in AsA synthesis, ABA biosynthesis/signal transduction, and low-temperature responses in tomato. In conclusion, the application of exogenous AsA enhances cold stress tolerance in tomato seedlings through various molecular and physiological responses. This provides a theoretical foundation for exploring the regulatory mechanisms underlying cold tolerance in tomato and offers practical guidance for enhancing cold tolerance in tomato cultivation.

Transcriptomic and Physiological Studies Unveil that Brassinolide Maintains the Balance of Maize's Multiple Metabolisms under Low-Temperature Stress

Low-temperature (LT) is one of the major abiotic stresses that restrict the growth and development of maize seedlings. Brassinolides (BRs) have been shown to enhance LT tolerance in several plant species; the physiological and molecular mechanisms by which BRs enhance maize tolerance are still unclear. Here, we characterized changes in the physiology and transcriptome of N192 and Ji853 seedlings at the three-leaf stage with or without 2 μM 2,4-epibrassinolide (EBR) application at 25 and 15 °C environments via high-performance liquid chromatography and RNA-Sequencing. Physiological analyses revealed that EBR increased the antioxidant enzyme activities, enhanced the cell membrane stability, decreased the malondialdehyde formation, and inhibited the reactive oxygen species (ROS) accumulation in maize seedlings under 15 °C stress; meanwhile, EBR also maintained hormone balance by increasing indole-3-acetic acid and gibberellin 3 contents and decreasing the abscisic acid level under stress. Transcriptome analysis revealed 332 differentially expressed genes (DEGs) enriched in ROS homeostasis, plant hormone signal transduction, and the mitogen-activated protein kinase (MAPK) cascade. These DEGs exhibited synergistic and antagonistic interactions, forming a complex LT tolerance network in maize. Additionally, weighted gene co-expression network analysis (WGCNA) revealed that 109 hub genes involved in LT stress regulation pathways were discovered from the four modules with the highest correlation with target traits. In conclusion, our findings provide new insights into the molecular mechanisms of exogenous BRs in enhancing LT tolerance of maize at the seedling stage, thus opening up possibilities for a breeding program of maize tolerance to LT stress.

Transcriptome Analysis Reveals the Important Role of Vitamin B12 in the Response of Natronorubrum daqingense to Salt Stress

Natronorubrum daqingense JX313T is an extremely halophilic archaea that can grow in a NaCl-saturated environment. The excellent salt tolerance of N. daqingense makes it a high-potential candidate for researching the salt stress mechanisms of halophilic microorganisms from Natronorubrum. In this study, transcriptome analysis revealed that three genes related to the biosynthesis of vitamin B12 were upregulated in response to salt stress. For the wild-type (WT) strain JX313T, the low-salt adaptive mutant LND5, and the vitamin B12 synthesis-deficient strain ΔcobC, the exogenous addition of 10 mg/L of vitamin B12 could maximize their cell survival and biomass in both optimal and salt stress environments. Knockout of cobC resulted in changes in the growth boundary of the strain, as well as a significant decrease in cell survival and biomass, and the inability to synthesize vitamin B12. According to the HPLC analysis, when the external NaCl concentration (w/v) increased from 17.5% (optimal) to 22.5% (5% salt stress), the intracellular accumulation of vitamin B12 in WT increased significantly from (11.54 ± 0.44) mg/L to (15.23 ± 0.20) mg/L. In summary, N. daqingense is capable of absorbing or synthesizing vitamin B12 in response to salt stress, suggesting that vitamin B12 serves as a specific compatible solute effector for N. daqingense during salt stress.

Melatonin Application Alleviates Stress-Induced Photosynthetic Inhibition and Oxidative Damage by Regulating Antioxidant Defense System of Maize: A Meta-Analysis

Melatonin is effective in modulating metabolism and regulating growth and development in many plants under biotic and abiotic stress. However, there is no systematic quantification of melatonin effects on maize growth, gas exchange, chlorophyll content, and the antioxidant defense system. A meta-analysis was conducted on thirty-two currently available published articles to evaluate the effect of stress types, study types, and maize varieties on response ratio (lnRR₊₊) of “melatonin” to “control (no melatonin)” on plant growth, enzyme activities, gas exchange parameters, and photosynthetic pigments. Our findings revealed that melatonin application overall increased plant height, leaf area, root length, fresh and dry root weight and shoot weight, superoxide dismutase (SOD), peroxide (POD), catalase (CAT), ascorbate peroxidase (APX), soluble sugar and protein, photosynthetic rate, stomatal conductance, transpiration rate, chlorophyll, and carotenoid in maize leaf under stress conditions. In contrast, melatonin application decreased the levels of hydrogen peroxide (H₂O₂), superoxide anion (O_2⁻), malondialdehyde (MDA), and electrolyte leakage. The categorical meta-analysis demonstrated that melatonin application to chilling stress resulted in higher SOD activity followed by salt stress. Melatonin application to all stress types resulted in higher POD, CAT and APX activities, except Cd stress, which had no effect on POD and decreased CAT by 38% compared to control. Compared to control, melatonin resulted in lower reactive oxygen species (ROS) and electrolyte leakage under no stress, Cd, drought, salt, lead, heat, and chilling stress in all study types (pot, growth chamber, hydroponic, and field), except O₂ content which was not affected in pot and growth chamber studies. It was concluded that melatonin alleviates oxidative damage by improving stress tolerance, regulating the antioxidant defense system, and increasing leaf chlorophyll content compared to control.

Psychrophilic Bacterial Phosphate-Biofertilizers: A Novel Extremophile for Sustainable Crop Production under Cold Environment

Abiotic stresses, including low-temperature environments, adversely affect the structure, composition, and physiological activities of soil microbiomes. Also, low temperatures disturb physiological and metabolic processes, leading to major crop losses worldwide. Extreme cold temperature habitats are, however, an interesting source of psychrophilic and psychrotolerant phosphate solubilizing bacteria (PSB) that can ameliorate the low-temperature conditions while maintaining their physiological activities. The production of antifreeze proteins and expression of stress-induced genes at low temperatures favors the survival of such organisms during cold stress. The ability to facilitate plant growth by supplying a major plant nutrient, phosphorus, in P-deficient soil is one of the novel functional properties of cold-tolerant PSB. By contrast, plants growing under stress conditions require cold-tolerant rhizosphere bacteria to enhance their performance. To this end, the use of psychrophilic PSB formulations has been found effective in yield optimization under temperature-stressed conditions. Most of the research has been done on microbial P biofertilizers impacting plant growth under normal cultivation practices but little attention has been paid to the plant growth-promoting activities of cold-tolerant PSB on crops growing in low-temperature environments. This scientific gap formed the basis of the present manuscript and explains the rationale for the introduction of cold-tolerant PSB in competitive agronomic practices, including the mechanism of solubilization/mineralization, release of biosensor active biomolecules, molecular engineering of PSB for increasing both P solubilizing/mineralizing efficiency, and host range. The impact of extreme cold on the physiological activities of plants and how plants overcome such stresses is discussed briefly. It is time to enlarge the prospects of psychrophilic/psychrotolerant phosphate biofertilizers and take advantage of their precious, fundamental, and economical but enormous plant growth augmenting potential to ameliorate stress and facilitate crop production to satisfy the food demands of frighteningly growing human populations. The production and application of cold-tolerant P-biofertilizers will recuperate sustainable agriculture in cold adaptive agrosystems.