Exogenous Kinetin Promotes the Nonenzymatic Antioxidant System and Photosynthetic Activity of Coffee (Coffea arabica L.) Plants Under Cold Stress Conditions

Adaptive roles of cytokinins in enhancing plant resilience and yield against environmental stressors

Innovative agricultural strategies are essential for addressing the urgent challenge of food security in light of climate change, population growth, and various environmental stressors. Cytokinins (CKs) play a pivotal role in enhancing plant resilience and productivity. These compounds, which include isoprenoid and aromatic types, are synthesized through pathways involving key enzymes such as isopentenyl transferase and cytokinin oxidase. Under abiotic stress conditions, CKs regulate critical physiological processes by improving photosynthetic efficiency, enhancing antioxidant enzyme activity, and optimizing root architecture. They also reduce the levels of reactive oxygen species and malondialdehyde, resulting in improved plant performance and yield. CKs interact intricately with other phytohormones, including abscisic acid, ethylene, salicylic acid, and jasmonic acid, to modulate stress-responsive pathways. This hormonal cross-talk is vital for finely tuning plant responses to stress. Additionally, CKs influence nutrient uptake and enhance responses to heavy metal stress, thereby bolstering overall plant resilience. The application of CKs helps plants maintain higher chlorophyll levels, boost antioxidant systems, and promote root and shoot growth. The strategic utilization of CKs presents an adaptive approach for developing robust crops capable of withstanding diverse environmental stressors, thus contributing to sustainable agricultural practices and global food security. Ongoing research into the mechanisms of CK action and their interactions with other hormones is essential for maximizing their agricultural potential. This underscores the necessity for continued innovation and research in agricultural practices, in alignment with global goals of sustainable productivity and food security.

Drought Protective Effects of Exogenous ABA and Kinetin on Lettuce: Sugar Content, Antioxidant Enzyme Activity, and Productivity

Drought is an environmental stressor that significantly impacts plant growth and development. Comprehending the complexity of drought stress and water utilization in the context of plant growth and development holds significant importance for sustainable agriculture. The aim of this study was to evaluate the effect of exogenously applied phytohormones on lettuce (Lactuca sativa L.) sugar content profiles and antioxidant enzyme activity and productivity. Lettuce plants were grown under normal and drought conditions in a growth chamber with a photoperiod of 14/10 h (day/night). Kinetin and abscisic acid were applied separately and in combinations when the second leaf was fully expanded. The results showed that sugar accumulation and productivity of the pretreated plants under drought were significantly higher than the controls. The perspective offered by this work showed that growth-related and stress-related phytohormones significantly influenced plant sugar metabolism, metabolic profiles, and productivity, thus enabling the control of yield and quality.

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.

Artificial Light for Improving Tomato Recovery Following Grafting: Transcriptome and Physiological Analyses

Grafting is widely used to enhance the phenotypic traits of tomatoes, alleviate biotic and abiotic stresses, and control soil-borne diseases of the scion in greenhouse production. There are many factors that affect the healing and acclimatization stages of seedlings after grafting. However, the role of light has rarely been studied. In this study, we compared the effects of artificial light and traditional shading (under shaded plastic-covered tunnels) on the recovery of grafted tomato seedlings. The results show that the grafted tomato seedlings recovered using artificial light had a higher healthy index, leaf chlorophyll content, shoot dry weight, and net photosynthetic rate (Pn) and water use efficiency (WUE) compared with grafted seedling recovered using the traditional shading method. Transcriptome analysis showed that the differentially expressed genes (DEGs) of grafted seedlings restored using artificial light were mainly enriched in the pathways corresponding to plant hormone signal transduction. In addition, we measured the endogenous hormone content of grafted tomato seedlings. The results show that the contents of salicylic acid (SA) and kinetin (Kin) were significantly increased, and the contents of indoleacetic acid (IAA) and jasmonic acid (JA) were decreased in artificial-light-restored grafted tomato seedlings compared with those under shading treatments. Therefore, we suggest that artificial light affects the morphogenesis and photosynthetic efficiency of grafted tomato seedlings, and it can improve the performance of tomato seedlings during grafting recovery by regulating endogenous hormone levels.

Acinetobacter oleivorans IRS14 alleviates cold stress in wheat by regulating physiological and biochemical factors

AIMS

Climate change is responsible for extreme cold winters, causing a significant loss in crop yield and productivity due to chilling stress. This study aims to investigate the potential of psychrotrophic plant growth-promoting rhizobacteria (PGPR) strain to promote wheat growth under cold stress and explore the adaptive responses of wheat.

METHODS AND RESULTS

Wheat seeds and seedlings were inoculated with the psychrotrophic strain IRS14 and the plants were cultivated for five weeks at 6°C ± 2°C. The genetic, biochemical, physiological, and molecular analysis of the bacterium and plant was done to evaluate the effect of the PGPR strain in alleviating chilling stress. IRS14 possesses antioxidant activity and produced multiple phytohormones, which enhanced seed germination (∼50%) and plant growth (∼50%) during chilling stress.

CONCLUSIONS

Here, we reported that the application of IRS14 helps to regulate the biochemical and metabolic pathways in wheat plants. It alleviates chilling stress and increases plant growth rate and biomass. Strain IRS14 in wheat effectively increased chlorophyll content, antioxidants, carotenoid, proline, and endogenous phytohormones compared with untreated wheat.

Farm or Lab? A Comparative Study of Oregano's Leaf and Callus Volatile Isolates Chemistry and Cytotoxicity

Oregano (Origanum vulgare, Lamiaceae plant family) is a well-known aromatic herb with great commercial value, thoroughly utilized by food and pharmaceutical industries. The present work regards the comparative assessment of in vitro propagated and commercially available oregano tissue natural products. This study includes their secondary metabolites' biosynthesis, antioxidant properties, and anticancer activities. The optimization of callus induction from derived oregano leaf explants and excessive oxidative browning was performed using various plant growth regulators, light conditions, and antioxidant compounds. The determination of oregano callus volatiles against the respective molecules in maternal herbal material was performed using gas chromatography-mass spectrometry (GC/MS) analysis. In total, the presence of twenty-seven phytochemicals was revealed in both leaf and callus extracts, from which thirteen molecules were biosynthesized in both tissues studied, seven compounds were present only in callus extracts, and seven metabolites only in leaf extracts. Carvacrol and sabinene hydrate were the prevailing volatiles in all tissues exploited, along with alkanes octacosane and triacontane and the trimethylsilyl (TMS) derivative of carvacrol that were detected in significant amounts only in callus extracts. The MTT assay was employed to assess the in vitro cytotoxic properties of oregano extracts against the epithelial human breast cancer MDA-MB-231 and the human neuroblastoma SK-N-SH cell lines. The extracts displayed concentration and time-dependent responses in cell proliferation rates.

Cold stress triggers freezing tolerance in wheat (Triticum aestivum L.) via hormone regulation and transcription of related genes

Low temperatures limit the geographic distribution and yield of plants. Hormones play an important role in coordinating the growth and development of plants and their tolerance to low temperatures. However, the mechanisms by which hormones affect plant resistance to extreme cold stress in the natural environment are still unclear. In this study, two winter wheat varieties with different cold resistances, Dn1 and J22, were used to conduct targeted plant hormone metabolome analysis on the tillering nodes of winter wheat at 5 °C, -10 °C and -25 °C using an LC-ESI-MS/MS system. We screened 39 hormones from 88 plant hormone metabolites and constructed a partial regulatory network of auxin, jasmonic acid and cytokinin. GO analysis and enrichment of KEGG pathways in different metabolites showed that the 'plant hormone signal transduction' pathway was the most common. Our study showed that extreme low temperature increased the most levels of auxin, cytokinin and salicylic acid, and decreased levels of jasmonic acid and abscisic acid, and that levels of auxin, jasmonic acid and cytokinin in Dn1 were higher than those in J22. These changes in hormone levels were associated with changes in gene expression in synthesis, catabolism, transport and signal transduction pathways. These results differ from the previous hormone regulation mechanisms, which were mostly obtained at 4 °C. Our results provide a basis for further understanding the molecular mechanisms by which plant endogenous hormones regulate plant freezing stress tolerance.

Comparative analysis of physiological variations and genetic architecture for cold stress response in soybean germplasm

Soybean (Glycine max L.) is susceptible to low temperatures. Increasing lines of evidence indicate that abiotic stress-responsive genes are involved in plant low-temperature stress response. However, the involvement of photosynthesis, antioxidants and metabolites genes in low temperature response is largely unexplored in Soybean. In the current study, a genetic panel of diverse soybean varieties was analyzed for photosynthesis, chlorophyll fluorescence and leaf injury parameters under cold stress and control conditions. This helps us to identify cold tolerant (V100) and cold sensitive (V45) varieties. The V100 variety outperformed for antioxidant enzymes activities and relative expression of photosynthesis (Glyma.08G204800.1, Glyma.12G232000.1), GmSOD (GmSOD01, GmSOD08), GmPOD (GmPOD29, GmPOD47), trehalose (GmTPS01, GmTPS13) and cold marker genes (DREB1E, DREB1D, SCOF1) than V45 under cold stress. Upon cold stress, the V100 variety showed reduced accumulation of H₂O₂ and MDA levels and subsequently showed lower leaf injury compared to V45. Together, our results uncovered new avenues for identifying cold tolerant soybean varieties from a large panel. Additionally, we identified the role of antioxidants, osmo-protectants and their posttranscriptional regulators miRNAs such as miR319, miR394, miR397, and miR398 in Soybean cold stress tolerance.

Mitigation of Negative Effects of Chromium (VI) Toxicity in Faba Bean (Vicia faba) Plants through the Supplementation of Kinetin (KN) and Gibberellic Acid (GA3)

The present study was carried out to explore the possible role of kinetin and gibberellic acid (GA3) on faba bean under chromium (Cr) stress. Cr treatment negatively affected growth and biomass production, reduced photosynthetic pigments, and inhibited photosynthesis, gas exchange parameters, antioxidant enzymes, and the glyoxylase cycle. Moreover, Cr stress enhanced the production of malondialdehyde (MDA, 216.11%) and hydrogen peroxide (H₂O₂, 230.16%), electrolyte leakage (EL, 293.30%), and the accumulation of proline and glycine betaine. Exogenous application of kinetin and GA3 increased growth and biomass, improved pigment contents and photosynthesis, as well as up-regulated the antioxidant system by improving the antioxidant enzyme activities and the content of nonenzymatic components, and the glyoxylase cycle. Additionally, kinetin and GA3 application displayed a considerable enhancement in proline (602.61%) and glycine betaine (423.72), which help the plants to maintain water balance under stress. Furthermore, a decline in Cr uptake was also observed due to kinetin and GA3 application. Exogenous application of kinetin and GA3 ameliorated the toxic effects of Cr in faba bean plants, up-shooting the tolerance mechanisms, including osmolyte metabolism and the antioxidant system.

Appraisal of kinetin spraying strategy to alleviate the harmful effects of UVC stress on tomato plants

Increasing ultraviolet (UV) radiation is causing oxidative stress that accounts for growth and yield losses in the present era of climate change. Plant hormones are useful tools for minimizing UV-induced oxidative stress in plants, but their putative roles in protecting tomato development under UVC remain unknown. Therefore, we investigated the underlying mechanism of pre-and post-kinetin (Kn) treatments on tomato plants under UVC stress. The best dose of Kn was screened in the preliminary experiments, and this dose was tested in further experiments. UVC significantly decreases growth traits, photosynthetic pigments, protein content, and primary metabolites (proteins, carbohydrates, amino acids) but increases oxidative stress biomarkers (lipid peroxidation, lipoxygenase activity, superoxide anion, hydroxyl radical, and hydrogen peroxide) and proline content. Treatment of pre-and post-kinetin spraying to tomato plants decreases UVC-induced oxidative stress by restoring the primary and secondary metabolites' (phenolic compounds, flavonoids, and anthocyanins) status and upregulating the antioxidant defense systems (non-enzymatic antioxidants as ascorbate, reduced glutathione, α-tocopherol as well as enzymatic antioxidants as superoxide dismutase, catalase, ascorbate peroxidase, glutathione peroxidase, glutathione-S-transferase, and phenylalanine ammonia-lyase). Thus, the application of Kn in optimum doses and through different modes can be used to alleviate UVC-induced negative impacts in tomato plants.

SfGPX regulates low-temperature tolerance by affecting velocity and intensity of Ca2+ transfer and photosynthetic characteristics in interfered Spiraea fritschiana and overexpressed Spiraea japonica 'Gold Mound'

Genes play regulatory roles in plants' response to low-temperature stress. Our understanding of the mechanism of plants' response to low-temperature stress can be expanded by studying the functions of these genes. SfGPX was cloned from Spiraea fritschiana (S. fritschiana) with the highest low-temperature tolerance, to explore the molecular mechanisms of SfGPX in response to low-temperature stress and the physiological mechanisms involved in the regulation of SfGPX to adapt to low temperature, in two species of Spiraea. SfGPX, which was localized in the cytoplasm, was induced by low temperature. The low-temperature tolerance of Spiraea fritschiana was decreased via the interference of SfGPX, and the low-temperature tolerance of Spiraea japonica 'Gold Mound' (S. japonica 'Gold Mound') was elevated via the overexpression of SfGPX. Under low-temperature stress, the photosynthetic capacity of two species of Spiraea was affected by SfGPX; it was higher in the cold-tolerant plants and lower in the cold-intolerant plants. Under low-temperature stress, the transfer intensity of Ca²⁺ was affected by SfGPX. The transfer intensity of cold-tolerant plants with lower influx level of Ca²⁺ kinetics was weaker than that of cold-intolerant plants. Under low-temperature stress, the transfer velocity of Ca²⁺ was affected by SfGPX, and there were slower effluxes of Ca²⁺ from Ca²⁺ reservoir in cold-tolerant plants than in cold-intolerant plants. The above results indicate that the response of Spiraea to low temperature is regulated by SfGPX through affecting photosynthetic capacity as well as intensity and velocity of Ca²⁺ transfer in response to low temperature in Spiraea.

Cytokinin and gibberellic acid-mediated waterlogging tolerance of mungbean (Vigna radiata L. Wilczek)

BACKGROUND

Mungbean (Vigna radiata L. Wilczek) is one of the most important pulse crops, well-known for its protein-rich seeds. Growth and productivity are severely undermined by waterlogging.

METHODS

In this study, we aim to evaluate how two promising phytohormones, namely cytokinin (CK) and gibberellic acid (GA₃), can improve waterlogging tolerance in mungbean by investigating key morphological, physiological, biochemical, and yield-related attributes.

RESULTS

Our results showed that foliar application of CK and GA₃ under 5-day of waterlogged conditions improved mungbean growth and biomass, which was associated with increased levels of photosynthetic rate and pigments. Waterlogged-induced accumulation of reactive oxygen species and the consequently elevated levels of malondialdehyde were considerably reduced by CK and GA₃ treatments. Mungbean plants sprayed with either CK or GA₃ suffered less oxidative stress due to the enhancement of total phenolics and flavonoids levels. Improvement in the contents of proline and total soluble sugars indicated a better osmotic adjustment following CK and GA₃ treatments in waterlogged-exposed plants. Most fundamentally, CK or GA₃-sprayed waterlogged-stressed mungbean plants demonstrated better performance in the aforementioned parameters after the 15-day recovery period as compared to water-sprayed waterlogged-exposed plants. Our results also revealed that CK and GA₃ treatments increased yield-associated features in the waterlogged-stressed plant. Here, both phytohormones are efficient in improving mungbean resistance to waterlogging. However, CK was found to be more effective. Overall, our findings suggested that CK or GA₃ could be used for managing waterlogging-induced damage to mungbean and perhaps in other cash crops.

6-Benzylaminopurine Alleviates the Impact of Cu2+ Toxicity on Photosynthetic Performance of Ricinus communis L. Seedlings

Copper (Cu) is an essential element involved in various metabolic processes in plants, but at concentrations above the threshold level, it becomes a potential stress factor. The effects of two different cytokinins, kinetin (KIN) and 6-benzylaminopurine (BAP), on chlorophyll a fluorescence parameters, stomatal responses and antioxidation mechanisms in castor (Ricinus communis L.) under Cu²⁺ toxicity was investigated. Ricinus communis plants were exposed to 80 and 160 μM CuSO₄ added to the growth medium. Foliar spraying of 15 μM KIN and BAP was carried out on these seedlings. The application of these cytokinins enhanced the tissue water status, chlorophyll contents, stomatal opening and photosynthetic efficiency in the castor plants subjected to Cu²⁺ stress. The fluorescence parameters, such as Fm, Fv/Fo, Sm, photochemical and non-photochemical quantum yields, energy absorbed, energy trapped and electron transport per cross-sections, were more efficiently modulated by BAP application than KIN under Cu²⁺ toxicity. There was also effective alleviation of reactive oxygen species by enzymatic and non-enzymatic antioxidation systems, reducing the membrane lipid peroxidation, which brought about a relative enhancement in the membrane stability index. Of the various treatments, 80 µM CuSO₄ + BAP recorded the highest increase in photosynthetic efficiency compared to other cytokinin treatments. Therefore, it can be concluded that BAP could effectively alleviate the detrimental effects of Cu²⁺toxicity in cotyledonary leaves of R. communis by effectively modulating stomatal responses and antioxidation mechanisms, thereby enhancing the photosynthetic apparatus’ functioning.

Exogenous kinetin and putrescine synergistically mitigate salt stress in Luffa acutangula by modulating physiology and antioxidant defense

Salinity is one of the most vicious environmental constraints that hamper agricultural production. Experiments were done to explore the significant role of sole and synergistic supplementation of kinetin (100 µM KN) and putrescine (100 µM PUT) on Luffa acutangula in NaCl (100 mM) treatment. The harmful effects of salinity on growth were manifested by decreased seedling length, biomass, and pigment contents. We studied the effect of KN, and PUT in preventing salt (NaCl) induced physiological disorders and oxidative damages in 20-day-old Luffa acutangula seedlings. The individual application of KN and PUT increased growth and biochemical parameters, whereas combined KN + PUT treatment showed significant enhancement in growth, photosynthetic pigment content, and osmolyte accumulation in salt-affected plants. Application of KN and PUT also prevented hydrogen peroxide and superoxide production as confirmed by inhibition in electrolyte leakage and lipid peroxidation. Kinetin and PUT application upregulated the antioxidant defense system by enhancing antioxidant enzymes and non-enzymatic contents. Luffa seedlings treated with NaCl + KN + PUT showed 79, 26, 74, and 73% rise in superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase enzymes, respectively, in comparison to NaCl-stressed Luffa acutangula. Findings revealed that synergistic utilization of KN and PUT modulate growth and biochemical processes in seedlings efficaciously in comparison to the individual application under salt stress, and it may be due to a regulatory crosstalk mechanism.

The Roles of Cytokinins in Plants and Their Response to Environmental Stimuli

Cytokinins (CKs) are adenine-derived, small-molecule plant growth regulators that control aspects of almost all plant growth and development processes. Internally, CKs play significant roles in plant cell division, nutrient allocation, and photosynthetic performance, and they are also detection and signaling agents for plant responses to the environmental challenges. CK functions in plant metabolism include plant adaptations to various abiotic stresses as well as their regulatory role in plant interactions with biotic components of the environment. Interestingly, CK biosynthesis is not exclusive to plants. New genetic and chemical approaches have revealed that both beneficial (symbiotic microorganisms) and detrimental (pathogenic bacteria, fungi, or insects) non-plant biota can secrete these phytohormones to purposefully modify plant metabolism. Therefore, while many open questions remain about how CKs are actively utilized by plants and plant-interacting organisms, CK roles should be seen more broadly, as signaling molecules for which effects range from within cells to as far as interkingdom relationships. The papers in this Special Issue highlight several aspects of CK biosynthesis, metabolism, and functions within plants and among plant-associated organisms, typifying the extensive range of roles played by these signaling molecules. The collection of papers represents new examples for CK researchers to consider advancing the growing range of topics related to how CKs mediate responses to many kinds of environmental stimuli and stresses.