Stimulation of ipt overexpression as a tool to elucidate the role of cytokinins in high temperature responses of Arabidopsis thaliana

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.

Overexpression of MtIPT gene enhanced drought tolerance and delayed leaf senescence of creeping bentgrass (Agrostis stolonifera L.)

BACKGROUND

Isopentenyltransferases (IPT) serve as crucial rate-limiting enzyme in cytokinin synthesis, playing a vital role in plant growth, development, and resistance to abiotic stress.

RESULTS

Compared to the wild type, transgenic creeping bentgrass exhibited a slower growth rate, heightened drought tolerance, and improved shade tolerance attributed to delayed leaf senescence. Additionally, transgenic plants showed significant increases in antioxidant enzyme levels, chlorophyll content, and soluble sugars. Importantly, this study uncovered that overexpression of the MtIPT gene not only significantly enhanced cytokinin and auxin content but also influenced brassinosteroid level. RNA-seq analysis revealed that differentially expressed genes (DEGs) between transgenic and wild type plants were closely associated with plant hormone signal transduction, steroid biosynthesis, photosynthesis, flavonoid biosynthesis, carotenoid biosynthesis, anthocyanin biosynthesis, oxidation-reduction process, cytokinin metabolism, and wax biosynthesis. And numerous DEGs related to growth, development, and stress tolerance were identified, including cytokinin signal transduction genes (CRE1, B-ARR), antioxidase-related genes (APX2, PEX11, PER1), Photosynthesis-related genes (ATPF1A, PSBQ, PETF), flavonoid synthesis genes (F3H, C12RT1, DFR), wax synthesis gene (MAH1), senescence-associated gene (SAG20), among others.

CONCLUSION

These findings suggest that the MtIPT gene acts as a negative regulator of plant growth and development, while also playing a crucial role in the plant's response to abiotic stress.

Two Carya Species, Carya hunanensis and Carya illinoinensis, Used as Rootstocks Point to Improvements in the Heat Resistance of Carya cathayensis

Grafting as a crucial horticultural technique has been widely used in the cultivation of Carya cathayensis (Chinese hickory), which is a unique and important economic tree in the northeast of Zhejiang Province and the south of Anhui Province. However, the existing literature lacks research on the potential impact of various rootstocks on the thermal tolerance of Chinese hickory. The objectives of this study were to evaluate heat tolerance in four distinct groups of Chinese hickory, including C. cathayensis grafted onto Carya hunanensis and Carya illinoinensis, one self-grafted group (C. cathayensis grafted onto C. cathayensis), and one non-grafted group (C. cathayensis). We examined photosynthesis parameters, phytohormones, and differentially expressed genes in the four various hickory groups subjected to 25 °C, 35 °C, and 40 °C heat stress (HS). The results demonstrated that grafting onto C. hunanensis and C. illinoinensis exhibited a higher net photosynthetic rate and stomatal conductance, lower intercellular CO2 concentration, and smaller changes in plant hormone content compared to self-grafted and non-grafted group under HS. The transcriptome results revealed that the majority of differentially expressed genes (DEGs) associated with photosynthetic pathways exhibited downregulation under HS, while the degree of variation in grafted groups using C. hunanensis and C. illinoinensis as rootstocks was comparatively lower than that observed in self-grafted and non-grafted groups. The alteration in the expression patterns of DEGs involved in plant hormone synthesis and metabolism under HS corresponded to changes in plant hormone contents. Overall, Chinese hickory grafted onto C. hunanensis and C. illinoinensis exhibited enhanced resistance to high-temperature stress at the juvenile stage.

Abiotic Stress in Crop Production

The vast majority of agricultural land undergoes abiotic stress that can significantly reduce agricultural yields. Understanding the mechanisms of plant defenses against stresses and putting this knowledge into practice is, therefore, an integral part of sustainable agriculture. In this review, we focus on current findings in plant resistance to four cardinal abiotic stressors—drought, heat, salinity, and low temperatures. Apart from the description of the newly discovered mechanisms of signaling and resistance to abiotic stress, this review also focuses on the importance of primary and secondary metabolites, including carbohydrates, amino acids, phenolics, and phytohormones. A meta-analysis of transcriptomic studies concerning the model plant Arabidopsis demonstrates the long-observed phenomenon that abiotic stressors induce different signals and effects at the level of gene expression, but genes whose regulation is similar under most stressors can still be traced. The analysis further reveals the transcriptional modulation of Golgi-targeted proteins in response to heat stress. Our analysis also highlights several genes that are similarly regulated under all stress conditions. These genes support the central role of phytohormones in the abiotic stress response, and the importance of some of these in plant resistance has not yet been studied. Finally, this review provides information about the response to abiotic stress in major European crop plants—wheat, sugar beet, maize, potatoes, barley, sunflowers, grapes, rapeseed, tomatoes, and apples.

Abscisic Acid and Cytokinins Are Not Involved in the Regulation of Stomatal Conductance of Scots Pine Saplings during Post-Drought Recovery

Delayed or incomplete recovery of gas exchange after water stress relief limits assimilation in the post-drought period and can thus negatively affect the processes of post-drought recovery. Abscisic acid (ABA) accumulation and antagonistic action between ABA and cytokinins (CKs) play an important role in regulation of stomatal conductance under water deficit. Specifically, in pine species, sustained ABA accumulation is thought to be the main cause of delayed post-drought gas exchange recovery, although the role of CKs is not yet known. Therefore, we aimed to study the effects of ABA and CKs on recovery of stomatal conductance in greenhouse-grown 3-year-old Scots pine saplings recovering from water stress. We analysed both changes in endogenous ABA and CK contents and the effects of treatment with exogenous CK on stomatal conductance. Drought stress suppressed stomatal conductance, and post-drought stomatal conductance remained suppressed for 2 weeks after plant rewatering. ABA accumulated during water stress, but ABA levels decreased rapidly after rewatering. Additionally, trans-zeatin/ABA and isopentenyladenine/ABA ratios, which were decreased in water-stressed plants, recovered rapidly in rewatered plants. Spraying plants with 6-benzylaminopurine (0.1–100 µM) did not influence recovery of either stomatal conductance or needle water status. It can be concluded that the delayed recovery of stomatal conductance in Scots pine needles was not due to sustained ABA accumulation or a sustained decrease in the CK/ABA ratio, and CK supplementation was unable to overcome this delayed recovery.

Redox Signaling in Plant Heat Stress Response

The increase in environmental temperature due to global warming is a critical threat to plant growth and productivity. Heat stress can cause impairment in several biochemical and physiological processes. Plants sense and respond to this adverse environmental condition by activating a plethora of defense systems. Among them, the heat stress response (HSR) involves an intricate network of heat shock factors (HSFs) and heat shock proteins (HSPs). However, a growing amount of evidence suggests that reactive oxygen species (ROS), besides potentially being responsible for cellular oxidative damage, can act as signal molecules in HSR, leading to adaptative responses. The role of ROS as toxic or signal molecules depends on the fine balance between their production and scavenging. Enzymatic and non-enzymatic antioxidants represent the first line of defense against oxidative damage and their activity is critical to maintaining an optimal redox environment. However, the HS-dependent ROS burst temporarily oxidizes the cellular environment, triggering redox-dependent signaling cascades. This review provides an overview of the redox-activated mechanisms that participate in the HSR.

Cytokinin Deficiency Alters Leaf Proteome and Metabolome during Effector-Triggered Immunity in Arabidopsis thaliana Plants

The involvement of cytokinins (CK) in biotic stresses has been recognized, while knowledge regarding the effects of CK deficiency on plant response against pathogens is less abundant. Thus, the purpose of this study was to reveal the effects of CK deficiency on proteomics and metabolomic responses of flg22-triggered immunity. We conducted a series of histochemical assays to investigate the activity of the downstream pathways caused by flg22, such as accumulation of ROS, induction of defence genes, and callose deposition, that occurred in Arabidopsis thaliana transgenic lines overexpressing the Hordeum vulgare CKX2 gene (HvCKX2), which are therefore CK-deficient. We also used GC and LC-MS-based technology to quantify variations in stress hormone levels and metabolomic and proteomic responses in flg22-treated HvCKX2 and wild-type Arabidopsis plants. We found that CK deficiency alters the flg22-triggered plant defence response, especially through induction of callose deposition, upregulation of defence response-related proteins, increased amino acid biosynthesis, and regulation of plant photosynthesis. We also indicated that JA might be an important contributor to immune response in plants deficient in CKs. The present study offers new evidence on the fundamental role of endogenous CK in the response to pathogens, as well as the possibility of altering plant biotic tolerance by manipulating CK pools.

Cytokinin and abiotic stress tolerance -What has been accomplished and the way forward?

More than a half-century has passed since it was discovered that phytohormone cytokinin (CK) is essential to drive cytokinesis and proliferation in plant tissue culture. Thereafter, cytokinin has emerged as the primary regulator of the plant cell cycle and numerous developmental processes. Lately, a growing body of evidence suggests that cytokinin has a role in mitigating both abiotic and biotic stress. Cytokinin is essential to defend plants against excessive light exposure and a unique kind of abiotic stress generated by an altered photoperiod. Secondly, cytokinin also exhibits multi-stress resilience under changing environments. Furthermore, cytokinin homeostasis is also affected by several forms of stress. Therefore, the diverse roles of cytokinin in reaction to stress, as well as its interactions with other hormones, are discussed in detail. When it comes to agriculture, understanding the functioning processes of cytokinins under changing environmental conditions can assist in utilizing the phytohormone, to increase productivity. Through this review, we briefly describe the biological role of cytokinin in enhancing the performance of plants growth under abiotic challenges as well as the probable mechanisms underpinning cytokinin-induced stress tolerance. In addition, the article lays forth a strategy for using biotechnological tools to modify genes in the cytokinin pathway to engineer abiotic stress tolerance in plants. The information presented here will assist in better understanding the function of cytokinin in plants and their effective investigation in the cropping system.

Cytokinin and Its Key Role to Enrich the Plant Nutrients and Growth Under Adverse Conditions-An Update

Among the field crops, wheat is regarded as one of the most paramount cereal crops because it is widely grown, consumed as food across the world, and also known as the staple food for nearly 35 per cent of the world population. However, it is threatened by spot blotch disease causing considerable yield loss, with existing genotypes lacking the resistance and the necessary nutrients. Cytokinins (CKs) are key phytohormones that not only regulate the plant growth/development but also play an important role during stress and in the nutrient metabolic pathway of crop plants. Deficiency of important nutrients like zinc, iron, and vitamin A causes irreparable damage to the body, pressing the need to increase the accumulation of such micronutrients in the edible parts of the plant. Crop bio-fortification is one of the emerging approaches through which the quantities of these nutrients could be increased to an advisable amount. Cytokinin is observed to have a pivotal role in managing environmental stress/climate change and defense systems of plants, and apart from this, it is also found that it has an impact over Zn accumulation in cereal crops. Manipulation of the cytokine dehydrogenase (CKX) enzyme that degrades cytokinin could affect the yield, root growth, and important nutrients. Several instances revealed that an increment in the contents of Zn, S, Fe, and Mn in the seeds of cereals is a reflection of increasing the activity of CKX enzyme resulting the enhancement of the root system which not only helps in the absorption of water in a drought prone area but is also beneficial for scavenging nutrients to the deeper ends of the soil. Exploring micronutrients from the lithosphere via the root system helps in the uptake of the micronutrients and transporting them via the vascular system to the sink of crop plants, therefore, identification and incorporation of CKs/CKX linked gene(s) into targeted crop plants, exploring a bio-fortification approach including CRISPR-Cas9 through conventional and molecular breeding approaches could be the most paramount job for improving the important traits and stress management in order to enhance the plant growth, productivity, and nutritional value of the wheat crops, which would be useful for mankind.

Thermo-Priming Mediated Cellular Networks for Abiotic Stress Management in Plants

Plants can adapt to different environmental conditions and can survive even under very harsh conditions. They have developed elaborate networks of receptors and signaling components, which modulate their biochemistry and physiology by regulating the genetic information. Plants also have the abilities to transmit information between their different parts to ensure a holistic response to any adverse environmental challenge. One such phenomenon that has received greater attention in recent years is called stress priming. Any milder exposure to stress is used by plants to prime themselves by modifying various cellular and molecular parameters. These changes seem to stay as memory and prepare the plants to better tolerate subsequent exposure to severe stress. In this review, we have discussed the various ways in which plants can be primed and illustrate the biochemical and molecular changes, including chromatin modification leading to stress memory, with major focus on thermo-priming. Alteration in various hormones and their subsequent role during and after priming under various stress conditions imposed by changing climate conditions are also discussed.

Role of Reactive Oxygen Species and Hormones in Plant Responses to Temperature Changes

Temperature stress is one of the major abiotic stresses that adversely affect agricultural productivity worldwide. Temperatures beyond a plant's physiological optimum can trigger significant physiological and biochemical perturbations, reducing plant growth and tolerance to stress. Improving a plant's tolerance to these temperature fluctuations requires a deep understanding of its responses to environmental change. To adapt to temperature fluctuations, plants tailor their acclimatory signal transduction events, and specifically, cellular redox state, that are governed by plant hormones, reactive oxygen species (ROS) regulatory systems, and other molecular components. The role of ROS in plants as important signaling molecules during stress acclimation has recently been established. Here, hormone-triggered ROS produced by NADPH oxidases, feedback regulation, and integrated signaling events during temperature stress activate stress-response pathways and induce acclimation or defense mechanisms. At the other extreme, excess ROS accumulation, following temperature-induced oxidative stress, can have negative consequences on plant growth and stress acclimation. The excessive ROS is regulated by the ROS scavenging system, which subsequently promotes plant tolerance. All these signaling events, including crosstalk between hormones and ROS, modify the plant's transcriptomic, metabolomic, and biochemical states and promote plant acclimation, tolerance, and survival. Here, we provide a comprehensive review of the ROS, hormones, and their joint role in shaping a plant's responses to high and low temperatures, and we conclude by outlining hormone/ROS-regulated plant responsive strategies for developing stress-tolerant crops to combat temperature changes.

Crosstalk between Light- and Temperature-Mediated Processes under Cold and Heat Stress Conditions in Plants

Extreme temperatures are among the most important stressors limiting plant growth and development. Results indicate that light substantially influences the acclimation processes to both low and high temperatures, and it may affect the level of stress injury. The interaction between light and temperature in the regulation of stress acclimation mechanisms is complex, and both light intensity and spectral composition play an important role. Higher light intensities may lead to overexcitation of the photosynthetic electron transport chain; while different wavelengths may act through different photoreceptors. These may induce various stress signalling processes, leading to regulation of stomatal movement, antioxidant and osmoregulation capacities, hormonal actions, and other stress-related pathways. In recent years, we have significantly expanded our knowledge in both light and temperature sensing and signalling. The present review provides a synthesis of results for understanding how light influences the acclimation of plants to extreme low or high temperatures, including the sensing mechanisms and molecular crosstalk processes.

Isopentenyltransferases as master regulators of crop performance: their function, manipulation, and genetic potential for stress adaptation and yield improvement

Isopentenyltransferase (IPT) in plants regulates a rate-limiting step of cytokinin (CTK) biosynthesis. IPTs are recognized as key regulators of CTK homeostasis and phytohormone crosstalk in both biotic and abiotic stress responses. Recent research has revealed the regulatory function of IPTs in gene expression and metabolite profiles including source-sink modifications, energy metabolism, nutrient allocation and storage, stress defence and signalling pathways, protein synthesis and transport, and membrane transport. This suggests that IPTs play a crucial role in plant growth and adaptation. In planta studies of IPT-driven modifications indicate that, at a physiological level, IPTs improve stay-green characteristics, delay senescence, reduce stress-induced oxidative damage and protect photosynthetic machinery. Subsequently, these improvements often manifest as enhanced or stabilized crop yields and this is especially apparent under environmental stress. These mechanisms merit consideration of the IPTs as 'master regulators' of core cellular metabolic pathways, thus adjusting plant homeostasis/adaptive responses to altered environmental stresses, to maximize yield potential. If their expression can be adequately controlled, both spatially and temporally, IPTs can be a key driver for seed yield. In this review, we give a comprehensive overview of recent findings on how IPTs influence plant stress physiology and yield, and we highlight areas for future research.

Cytokinins are involved in drought tolerance of Pinus radiata plants originating from embryonal masses induced at high temperatures

Vegetative propagation through somatic embryogenesis is an effective method to produce elite varieties and can be applied as a tool to study the response of plants to different stresses. Several studies show that environmental changes during embryogenesis could determine future plant development. Moreover, we previously reported that physical and chemical conditions during somatic embryogenesis can determine the protein, hormone and metabolite profiles, as well as the micromorphological and ultrastructural organization of embryonal masses and somatic embryos. In this sense, phytohormones are key players throughout the somatic embryogenesis process as well as during numerous stress-adaptation responses. In this work, we first applied different high-temperature regimes (30 °C, 4 weeks; 40 °C, 4 days; 50 °C, 5 min) during induction of Pinus radiata D. Don somatic embryogenesis, together with control temperature (23 °C). Then, the somatic plants regenerated from initiated embryogenic cell lines and cultivated in greenhouse conditions were subjected to drought stress and control treatments to evaluate survival, growth and several physiological traits (relative water content, water potential, photosynthesis, stomatal conductance and transpiration). Based on those preliminary results, even more extreme high-temperature regimes were applied during induction (40 °C, 4 h; 50 °C, 30 min; 60 °C, 5 min) and the corresponding cytokinin profiles of initiated embryonal masses from different lines were analysed. The results showed that the temperature regime during induction had delayed negative effects on drought resilience of somatic plants as indicated by survival, photosynthetic activity and water- use efficiency. However, high temperatures for extended periods of time enhanced subsequent plant growth in well-watered conditions. High-temperature regime treatments induced significant differences in the profile of total cytokinin bases, N6-isopentenyladenine, cis-zeatin riboside and trans-zeatin riboside. We concluded that phytohormones could be potential regulators of stress-response processes during initial steps of somatic embryogenesis and that they may have delayed implications in further developmental processes, determining the performance of the generated plants.

Coping with inclement weather conditions due to high temperature and water deficit in rice: An insight from genetic and biochemical perspectives

Climatic fluctuations, temperature extremes, and water scarcity are becoming increasingly unpredictable with the passage of time. Such environmental atrocities have been the scourge of agriculture over the ages, bringing with them poor harvests and threat of famine. Rice production, owing to its high-water requirement for cultivation, is highly vulnerable to the threat of changing climate, particularly prolonged drought and high temperature, individually or in combination. Amidst all the abiotic stresses, heat and drought are considered as the most important concurrent stressors, largely affecting rice yield and productivity under the current scenario. Such threats heighten the need for new breeding and cultivation strategies in generating abiotic stress-resilient rice varieties with better yield potential. Responses of rice to these stresses can be categorized at the morphological, physiological and biochemical levels. This review examines the physiological and molecular mechanism, in the form of up regulation of several defense machineries of rice varieties to cope with drought stress (DS), high temperature stress (HTS), and their combination (DS-HTS). Genotypic differences among rice varieties in their tolerance ability have also been addressed. The review also appraises research studies conducted in rice regarding various phenotypic traits, genetic loci and response mechanisms to stress conditions to help craft new breeding strategies for improved tolerance to DS and HTS, singly or in combination. The review also encompasses the gene regulatory networks and transcription factors, and their cross-talks in mediating tolerance to such stresses. Understanding the epigenetic regulation, involving DNA methylation and histone modification during such hostile situations, will also play a crucial role in our comprehensive understanding of combinatorial stress responses. Taken together, this review consolidates current research and available information on promising rice cultivars with desirable traits as well as advocates synergistic and complementary approaches in molecular and systems biology to develop new rice breeds that favorably respond to DS-HTS-induced abiotic stress.

Roles of Phytohormones and Their Signaling Pathways in Leaf Development and Stress Responses

Phytohormones participate in various processes over the course of a plant's lifecycle. In addition to the five classical phytohormones (auxins, cytokinins, gibberellins, abscisic acid, and ethylene), phytohormones such as brassinosteroids, jasmonic acid, salicylic acid, strigolactones, and peptides also play important roles in plant growth and stress responses. Given the highly interconnected nature of phytohormones during plant development and stress responses, it is challenging to study the biological function of a single phytohormone in isolation. In the current Review, we describe the combined functions and signaling cascades (especially the shared points and pathways) of various phytohormones in leaf development, in particular, during leaf primordium initiation and the establishment of leaf polarity and leaf morphology as well as leaf development under various stress conditions. We propose a model incorporating the roles of multiple phytohormones in leaf development and stress responses to illustrate the underlying combinatorial signaling pathways. This model provides a reference for breeding stress-resistant crops.

Plant Hormone-Mediated Regulation of Heat Tolerance in Response to Global Climate Change

Agriculture is largely dependent on climate and is highly vulnerable to climate change. The global mean surface temperatures are increasing due to global climate change. Temperature beyond the physiological optimum for growth induces heat stress in plants causing detrimental and irreversible damage to plant development, growth, as well as productivity. Plants have evolved adaptive mechanisms in response to heat stress. The classical plant hormones, such as auxin, abscisic acid (ABA), brassinosteroids (BRs), cytokinin (CK), salicylic acid (SA), jasmonate (JA), and ethylene (ET), integrate environmental stimuli and endogenous signals to regulate plant defensive response to various abiotic stresses, including heat. Exogenous applications of those hormones prior or parallel to heat stress render plants more thermotolerant. In this review, we summarized the recent progress and current understanding of the roles of those phytohormones in defending plants against heat stress and the underlying signal transduction pathways. We also discussed the implication of the basic knowledge of hormone-regulated plant heat responsive mechanism to develop heat-resilient plants as an effective and efficient way to cope with global warming.

Heat Stress Targeting Individual Organs Reveals the Central Role of Roots and Crowns in Rice Stress Responses

Inter-organ communication and the heat stress (HS; 45°C, 6 h) responses of organs exposed and not directly exposed to HS were evaluated in rice (Oryza sativa) by comparing the impact of HS applied either to whole plants, or only to shoots or roots. Whole-plant HS reduced photosynthetic activity (F _v /F _m and QY_{_Lss} ), but this effect was alleviated by prior acclimation (37°C, 2 h). Dynamics of HSFA2d, HSP90.2, HSP90.3, and SIG5 expression revealed high protection of crowns and roots. Additionally, HSP26.2 was strongly expressed in leaves. Whole-plant HS increased levels of jasmonic acid (JA) and cytokinin cis-zeatin in leaves, while up-regulating auxin indole-3-acetic acid and down-regulating trans-zeatin in leaves and crowns. Ascorbate peroxidase activity and expression of alternative oxidases (AOX) increased in leaves and crowns. HS targeted to leaves elevated levels of JA in roots, cis-zeatin in crowns, and ascorbate peroxidase activity in crowns and roots. HS targeted to roots increased levels of abscisic acid and auxin in leaves and crowns, cis-zeatin in leaves, and JA in crowns, while reducing trans-zeatin levels. The weaker protection of leaves reflects the growth strategy of rice. HS treatment of individual organs induced changes in phytohormone levels and antioxidant enzyme activity in non-exposed organs, in order to enhance plant stress tolerance.

Hydrogen peroxide regulates antioxidant responses and redox related proteins in drought stressed wheat seedlings

Hydrogen peroxide plays pivotal role as a potent regulator in signalling pathways when the plant is under stress. The current study appraised the potential of hydrogen peroxide through seed pre-treatment on the seedling growth and defense responses of three wheat cultivars i.e. PBW 644 (tolerant), PBW 621 and HD 2967 (sensitive) grown under drought stress. Imposition of drought stress reduced seedling growth of all the three wheat cultivars. Pre-treatment of seeds with 60 mM H₂O₂ alleviated water stress induced growth inhibition in all the three wheat cultivars. Further, it enhanced the drought tolerance of PBW 644 by upregulating SOD, POX, APX and GR enzymes accompanied by an increase in total phenols and ascorbate content. H₂O₂ treatment also protected the sensitive cultivars from drought stress by increasing CAT, POX, APX, MDHAR and GR enzymes. The contents of osmolytes were comparable or slightly higher as compared to stressed seedlings. The levels of MDA content were reduced in the treated seedlings of all the cultivars which further revealed the role of H₂O₂ pre-treatment in alleviating membrane damage. The comprehensive scrutiny of proteins differentially expressed in control, stressed and H₂O₂ primed stressed seedlings revealed that drought stress enhanced the expression of proteins involved in photosynthesis, protein biosynthesis and degradation, carbohydrate metabolism, fatty acid metabolism, nucleic acid metabolism, phytohormone response, defense and regulation, whereas H₂O₂ pre-treatment led to over expression of proteins which had functions in processes such as defense, redox homeostasis and photosynthesis.

SUPPLEMENTARY INFORMATION

The online version of this article (10.1007/s12298-021-00937-z).

Integration of reactive oxygen species and hormone signaling during abiotic stress

Each year, abiotic stress conditions such as drought, heat, salinity, cold and particularly their different combinations, inflict a heavy toll on crop productivity worldwide. The effects of these adverse conditions on plant productivity are becoming ever more alarming in recent years in light of the increased rate and intensity of global climatic changes. Improving crop tolerance to abiotic stress conditions requires a deep understanding of the response of plants to changes in their environment. This response is dependent on early and late signal transduction events that involve important signaling molecules such as reactive oxygen species (ROS), different plant hormones and other signaling molecules. It is the integration of these signaling events, mediated by an interplay between ROS and different plant hormones that orchestrates the plant response to abiotic stress and drive changes in transcriptomic, metabolic and proteomic networks that lead to plant acclimation and survival. Here we review some of the different studies that address hormone and ROS integration during the response of plants to abiotic stress. We further highlight the integration of ROS and hormone signaling during early and late phases of the plant response to abiotic stress, the key role of respiratory burst oxidase homologs in the integration of ROS and hormone signaling during these phases, and the involvement of hormone and ROS in systemic signaling events that lead to systemic acquired acclimation. Lastly, we underscore the need to understand the complex interactions that occur between ROS and different plant hormones during stress combinations.

Molecular investigation of Tuscan sweet cherries sampled over three years: gene expression analysis coupled to metabolomics and proteomics

Sweet cherry (Prunus avium L.) is a stone fruit widely consumed and appreciated for its organoleptic properties, as well as its nutraceutical potential. We here investigated the characteristics of six non-commercial Tuscan varieties of sweet cherry maintained at the Regional Germplasm Bank of the CNR-IBE in Follonica (Italy) and sampled ca. 60 days post-anthesis over three consecutive years (2016-2017-2018). We adopted an approach merging genotyping and targeted gene expression profiling with metabolomics. To complement the data, a study of the soluble proteomes was also performed on two varieties showing the highest content of flavonoids. Metabolomics identified the presence of flavanols and proanthocyanidins in highest abundance in the varieties Morellona and Crognola, while gene expression revealed that some differences were present in genes involved in the phenylpropanoid pathway during the 3 years and among the varieties. Finally, proteomics on Morellona and Crognola showed variations in proteins involved in stress response, primary metabolism and cell wall expansion. To the best of our knowledge, this is the first multi-pronged study focused on Tuscan sweet cherry varieties providing insights into the differential abundance of genes, proteins and metabolites.

Signal Integration in Plant Abiotic Stress Responses via Multistep Phosphorelay Signaling

Plants growing in any particular geographical location are exposed to variable and diverse environmental conditions throughout their lifespan. The multifactorial environmental pressure resulted into evolution of plant adaptation and survival strategies requiring ability to integrate multiple signals that combine to yield specific responses. These adaptive responses enable plants to maintain their growth and development while acquiring tolerance to a variety of environmental conditions. An essential signaling cascade that incorporates a wide range of exogenous as well as endogenous stimuli is multistep phosphorelay (MSP). MSP mediates the signaling of essential plant hormones that balance growth, development, and environmental adaptation. Nevertheless, the mechanisms by which specific signals are recognized by a commonly-occurring pathway are not yet clearly understood. Here we summarize our knowledge on the latest model of multistep phosphorelay signaling in plants and the molecular mechanisms underlying the integration of multiple inputs including both hormonal (cytokinins, ethylene and abscisic acid) and environmental (light and temperature) signals into a common pathway. We provide an overview of abiotic stress responses mediated via MSP signaling that are both hormone-dependent and independent. We highlight the mutual interactions of key players such as sensor kinases of various substrate specificities including their downstream targets. These constitute a tightly interconnected signaling network, enabling timely adaptation by the plant to an ever-changing environment. Finally, we propose possible future directions in stress-oriented research on MSP signaling and highlight its potential importance for targeted crop breeding.

Arabidopsis Response to Inhibitor of Cytokinin Degradation INCYDE: Modulations of Cytokinin Signaling and Plant Proteome

Cytokinins are multifaceted plant hormones that play crucial roles in plant interactions with the environment. Modulations in cytokinin metabolism and signaling have been successfully used for elevating plant tolerance to biotic and abiotic stressors. Here, we analyzed Arabidopsis thaliana response to INhibitor of CYtokinin DEgradation (INCYDE), a potent inhibitor of cytokinin dehydrogenase. We found that at low nanomolar concentration, the effect of INCYCE on seedling growth and development was not significantly different from that of trans-Zeatin treatment. However, an alteration in the spatial distribution of cytokinin signaling was found at low micromolar concentrations, and proteomics analysis revealed a significant impact on the molecular level. An in-depth proteome analysis of an early (24 h) response and a dose-dependent response after 168 h highlighted the effects on primary and secondary metabolism, including alterations in ribosomal subunits, RNA metabolism, modulations of proteins associated with chromatin, and the flavonoid and phenylpropanoid biosynthetic pathway. The observed attenuation in stress-response mechanisms, including abscisic acid signaling and the metabolism of jasmonates, could explain previously reported positive effects of INCYDE under mild stress conditions.

Research Progress on the Roles of Cytokinin in Plant Response to Stress

Cytokinins promote plant growth and development under normal plant growth conditions and also play an important role in plant resistance to stress. Understanding the working mechanisms of cytokinins under adverse conditions will help to make full use of cytokinins in agriculture to increase production and efficiency of land use. In this article, we review the progress that has been made in cytokinin research in plant response to stress and propose its future application prospects.

Steady-State Levels of Cytokinins and Their Derivatives May Serve as a Unique Classifier of Arabidopsis Ecotypes

We determined steady-state (basal) endogenous levels of three plant hormones (abscisic acid, cytokinins and indole-3-acetic acid) in a collection of thirty different ecotypes of Arabidopsis that represent a broad genetic variability within this species. Hormone contents were analysed separately in plant shoots and roots after 21 days of cultivation on agar plates in a climate-controlled chamber. Using advanced statistical and machine learning methods, we tested if basal hormonal levels can be considered a unique ecotype-specific classifier. We also explored possible relationships between hormone levels and the prevalent environmental conditions in the site of origin for each ecotype. We found significant variations in basal hormonal levels and their ratios in both root and shoot among the ecotypes. We showed the prominent position of cytokinins (CK) among the other hormones. We found the content of CK and CK metabolites to be a reliable ecotype-specific identifier. Correlation with the mean temperature at the site of origin and the large variation in basal hormonal levels suggest that the high variability may potentially be in response to environmental factors. This study provides a starting point for ecotype-specific genetic maps of the CK metabolic and signalling network to explore its contribution to the adaptation of plants to local environmental conditions.

Heat Acclimation and Inhibition of Cytokinin Degradation Positively Affect Heat Stress Tolerance of Arabidopsis

In order to pinpoint phytohormone changes associated with enhanced heat stress tolerance, the complex phytohormone profiles [cytokinins, auxin, abscisic acid (ABA), jasmonic acid (JA), salicylic acid and ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC)] were compared in Arabidopsis thaliana after direct heat shock (45°C, 3 h) and in heat-stressed pre-acclimated plants (1 h at 37°C followed by 2 h at optimal temperature 20°C). Organ-specific responses were followed in shoot apices, leaves, and roots immediately after heat shock and after 24-h recovery at 20°C. The stress strength was evaluated via membrane ion leakage and the activity of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOX) and antioxidant enzymes [superoxide dismutases, guaiacol peroxidases (POD), catalases, ascorbate peroxidases (APX)]. Heat acclimation diminished negative effects of heat stress, especially in apices and roots, no significant differences being observed in leaves. Low NOX1-3 activities indicated diminished production of reactive oxygen species. Higher activity of APX, POD1, and the occurrence of POD3-4 reflected acclimation-stimulated readiness of the antioxidant system. Acclimation diminished heat shock-induced changes of ABA, JA, cytokinin, and auxin levels in apices. Excess of ABA catabolites suggested an early stress response. The strong up-regulation of ABA and ACC in roots indicated defense boost in roots of acclimated plants compared to the non-acclimated ones. To evaluate the possibility to enhance stress tolerance by cytokinin pool modulation, INCYDE-F, an inhibitor of cytokinin oxidase/dehydrogenase, was applied. As cytokinin effects on stress tolerance may depend on timing of their regulation, INCYDE was applied at several time-points. In combination with acclimation, INCYDE treatment had a slight positive effect on heat stress tolerance, mainly when applied after 2-h period of the optimal temperature. INCYDE increased contents of cytokinins trans-zeatin and cis-zeatin in roots and auxin in all tissues after heat shock. INCYDE also helped to suppress the content of ABA in leaves, and ethylene in apices and roots. INCYDE application to non-acclimated plants (applied before or after heat shock) strengthened negative stress effects, probably by delaying of the repair processes. In conclusion, pre-treatment with moderately elevated temperature enhanced heat stress tolerance and accelerated recovery after stress. Inhibition of cytokinin degradation by INCYDE slightly improved recovery of acclimated plants.

Plant Hormone-Mediated Regulation of Heat Tolerance in Response to Global Climate Change

Agriculture is largely dependent on climate and is highly vulnerable to climate change. The global mean surface temperatures are increasing due to global climate change. Temperature beyond the physiological optimum for growth induces heat stress in plants causing detrimental and irreversible damage to plant development, growth, as well as productivity. Plants have evolved adaptive mechanisms in response to heat stress. The classical plant hormones, such as auxin, abscisic acid (ABA), brassinosteroids (BRs), cytokinin (CK), salicylic acid (SA), jasmonate (JA), and ethylene (ET), integrate environmental stimuli and endogenous signals to regulate plant defensive response to various abiotic stresses, including heat. Exogenous applications of those hormones prior or parallel to heat stress render plants more thermotolerant. In this review, we summarized the recent progress and current understanding of the roles of those phytohormones in defending plants against heat stress and the underlying signal transduction pathways. We also discussed the implication of the basic knowledge of hormone-regulated plant heat responsive mechanism to develop heat-resilient plants as an effective and efficient way to cope with global warming.

Light Regulates the Cytokinin-Dependent Cold Stress Responses in Arabidopsis

To elucidate the effect of light intensity on the cold response (5°C; 7 days) in Arabidopsis thaliana, we compared the following parameters under standard light (150 μmol m^-2 s^-1), low light (20 μmol m^-2 s^-1), and dark conditions: membrane damage, photosynthetic parameters, cytokinin oxidase/dehydrogenase (CKX) activity, phytohormone levels, and transcription of selected stress- and hormone-related genes and proteome. The impact of cytokinins (CKs), hormones directly interacting with the light signaling pathway, on cold responses was evaluated using transformants overexpressing CK biosynthetic gene isopentenyl transferase (DEX:IPT) or CK degradation gene HvCKX2 (DEX:CKX) under a dexamethasone-inducible promoter. In wild-type plants, cold treatment under light conditions caused down-regulation of CKs (in shoots) and auxin, while abscisic acid (ABA), jasmonates, and salicylic acid (SA) were up-regulated, especially under low light. Cold treatment in the dark strongly suppressed all phytohormones, except ABA. DEX:IPT plants showed enhanced stress tolerance associated with elevated CK and SA levels in shoots and auxin in apices. Contrarily, DEX:CKX plants had weaker stress tolerance accompanied by lowered levels of CKs and auxins. Nevertheless, cold substantially diminished the impact from the inserted genes. Cold stress in dark minimized differences among the genotypes. Cold treatments in light strongly up-regulated stress marker genes RD29A, especially in roots, and CBF1-3 in shoots. Under control conditions, their levels were higher in DEX:CKX plants, but after 7-day stress, DEX:IPT plants exhibited the highest transcription. Transcription of genes related to CK metabolism and signaling showed a tendency to re-establish, at least partially, CK homeostasis in both transformants. Up-regulation of strigolactone-related genes in apices and leaves indicated their role in suppressing shoot growth. The analysis of leaf proteome revealed over 20,000 peptides, representing 3,800 proteins and 2,212 protein families (data available via ProteomeXchange, identifier PXD020480). Cold stress induced proteins involved in ABA and jasmonate metabolism, antioxidant enzymes, and enzymes of flavonoid and glucosinolate biosynthesis. DEX:IPT plants up-regulated phospholipase D and MAP-kinase 4. Cold stress response at the proteome level was similar in all genotypes under optimal light intensity, differing significantly under low light. The data characterized the decisive effect of light-CK cross-talk in the regulation of cold stress responses.

Barley Root Proteome and Metabolome in Response to Cytokinin and Abiotic Stimuli

Cytokinin is a phytohormone involved in the regulation of diverse developmental and physiological processes in plants. Its potential in biotechnology and for development of higher-yield and more resilient plants has been recognized, yet the molecular mechanisms behind its action are far from understood. In this report, the roots of barley seedlings were explored as a new source to reveal as yet unknown cytokinin-responsive proteins for crop improvement. Here we found significant differences reproducibly observed for 178 proteins, for which some of the revealed cytokinin-responsive pathways were confirmed in metabolome analysis, including alterations phenylpropanoid pathway, amino acid biosynthesis and ROS metabolism. Bioinformatics analysis indicated a significant overlap between cytokinin response and response to abiotic stress. This was confirmed by comparing proteome and metabolome profiles in response to drought, salinity or a period of temperature stress. The results illustrate complex abiotic stress response in the early development of model crop plant and confirm an extensive crosstalk between plant hormone cytokinin and response to temperature stimuli, water availability or salinity stress.

Roles of phytohormone changes in the grain yield of rice plants exposed to heat: a review

During its reproductive phase, rice is susceptible to heat stress. Heat events will occur at all stages during the reproductive phase of rice as a result of global warming. Moreover, rice yield traits respond differently to heat stress during panicle initiation, flowering and grain filling. The reduction in the number of spikelets per panicle of heat-stressed plants is due to the attenuated differentiation of secondary branches and their attached florets as well as the promotion of their degradation during the panicle-initiation stage but is not affected by heat stress thereafter. Spikelet sterility as a result of heat stress is attributed not only to physiological abnormalities in the reproductive organs during the flowering stage but also to structural and morphological abnormalities in reproductive organs during the panicle-initiation stage. The reduced grain weight of heat-stressed plants is due to a reduction in nonstructural carbohydrates, undeveloped vascular bundles, and a reduction in glume size during the panicle-initiation stage, while a shortened grain-filling duration, reduced grain-filling rate, and decreased grain width contribute to reduced grain weight during the grain-filling stage. Thus, screening and breeding rice varieties that have comprehensive tolerance to heat stress at all time points during their reproductive stage may be possible to withstand unpredictable heat events in the future. The responses of yield traits to heat stress are regulated by phytohormone levels, which are determined by phytohormone homeostasis. Currently, the biosynthesis and transport of phytohormones are the key processes that determine phytohormone levels in and grain yield of rice under heat stress. Studies on phytohormone homeostatic responses are needed to further reveal the key processes that determine phytohormone levels under heat conditions.

The role of cytokinin in selenium stress response in Arabidopsis

Cytokinins (CKs) regulate many developmental processes and environmental stress responses in plants. In this study, our data provide evidence that CK negatively regulates Arabidopsis selenium (Se) stress response. CK-deficient plant ipt1 3 5 7 exhibited enhanced Se tolerance which was abolished by exogenous benzylaminopurine (BA) application, while CK- receptor -deficient mutants ahk2 and ahk3 were sensitive to Se stress. Further investigation suggested that CK regulated Se tolerance of ipt1 3 5 7 through reduction of Se uptake and activation of metabolism detoxification, which had significantly lower transcriptions of high-affinity transporters PHT1;1, PHT1;8, PHT1;9 and the higher transcription of selenocysteine methyltransferase (SMT) respectively. Moreover, Se tolerance of ipt1 3 5 7 was associated with the enhanced antioxidant levels which had the higher catalase (CAT), ascorbate peroxidase (APX) and glutathione peroxidase (GPX) activities as well as the higher glutathione (GSH) content. On the other hand, loss-of-function mutations in single CK receptor genes could increase Se uptake and reactive oxygen species (ROS) accumulation, which caused Se sensitivity in ahk2 and ahk3 mutants. Taken together, these findings provide new insights to the role of CK in Se stress response in Arabidopsis.

Cytokinin action in response to abiotic and biotic stresses in plants

The phytohormone cytokinin was originally discovered as a regulator of cell division. Later, it was described to be involved in regulating numerous processes in plant growth and development including meristem activity, tissue patterning, and organ size. More recently, diverse functions for cytokinin in the response to abiotic and biotic stresses have been reported. Cytokinin is required for the defence against high light stress and to protect plants from a novel type of abiotic stress caused by an altered photoperiod. Additionally, cytokinin has a role in the response to temperature, drought, osmotic, salt, and nutrient stress. Similarly, the full response to certain plant pathogens and herbivores requires a functional cytokinin signalling pathway. Conversely, different types of stress impact cytokinin homeostasis. The diverse functions of cytokinin in responses to stress and crosstalk with other hormones are described. Its emerging roles as a priming agent and as a regulator of growth-defence trade-offs are discussed.

Cytokinin at the Crossroads of Abiotic Stress Signalling Pathways

Cytokinin is a multifaceted plant hormone that plays major roles not only in diverse plant growth and development processes, but also stress responses. We summarize knowledge of the roles of its metabolism, transport, and signalling in responses to changes in levels of both macronutrients (nitrogen, phosphorus, potassium, sulphur) and micronutrients (boron, iron, silicon, selenium). We comment on cytokinin's effects on plants' xenobiotic resistance, and its interactions with light, temperature, drought, and salinity signals. Further, we have compiled a list of abiotic stress-related genes and demonstrate that their expression patterns overlap with those of cytokinin metabolism and signalling genes.

Cytokinins: Their Impact on Molecular and Growth Responses to Drought Stress and Recovery in Arabidopsis

Our phenotyping and hormonal study has characterized the role of cytokinins (CK) in the drought and recovery responses of Arabidopsis thaliana. CK down-regulation was achieved by overexpression of the gene for CK deactivating enzyme cytokinin oxidase/dehydrogenase (CKX): constitutive (35S:CKX) or at the stress onset using a dexamethasone-inducible pOp/LhGR promoter (DEX:CKX). The 35S:CKX plants exhibited slow ontogenesis and higher expression levels of stress-associated genes, e.g., AtP5CS1, already at well-watered conditions. CK down-regulation resulted during drought in higher stress tolerance (indicated by relatively low up-regulation of the expression of drought stress marker gene AtRD29B) accompanied with lower leaf water loss. Nevertheless, these plants exhibited slow and delayed recovery after re-watering. CK levels were increased at the stress onset by stimulation of the expression of CK biosynthetic gene isopentenyl transferase (ipt) (DEX:IPT) or by application of exogenous CK meta-topolin. After water withdrawal, long-term CK elevation resulted in higher water loss in comparison with CKX transformants as well as with plants overexpressing ipt driven by senescence-inducible SAG12 promoter (SAG:IPT), which gradually enhanced CKs during the stress progression. In all cases, CK up-regulation resulted in fast and more vigorous recovery. All drought-stressed plants exhibited growth suppression associated with elevation of abscisic acid and decrease of auxins and active CKs (with the exception of SAG:IPT plants). Apart from the ipt overexpressers, also increase of jasmonic and salicylic acid was found.

Metabolome Integrated Analysis of High-Temperature Response in Pinus radiata

The integrative omics approach is crucial to identify the molecular mechanisms underlying high-temperature response in non-model species. Based on future scenarios of heat increase, Pinus radiata plants were exposed to a temperature of 40°C for a period of 5 days, including recovered plants (30 days after last exposure to 40°C) in the analysis. The analysis of the metabolome using complementary mass spectrometry techniques (GC-MS and LC-Orbitrap-MS) allowed the reliable quantification of 2,287 metabolites. The analysis of identified metabolites and highlighter metabolic pathways across heat time exposure reveal the dynamism of the metabolome in relation to high-temperature response in P. radiata, identifying the existence of a turning point (on day 3) at which P. radiata plants changed from an initial stress response program (shorter-term response) to an acclimation one (longer-term response). Furthermore, the integration of metabolome and physiological measurements, which cover from the photosynthetic state to hormonal profile, suggests a complex metabolic pathway interaction network related to heat-stress response. Cytokinins (CKs), fatty acid metabolism and flavonoid and terpenoid biosynthesis were revealed as the most important pathways involved in heat-stress response in P. radiata, with zeatin riboside (ZR) and isopentenyl adenosine (iPA) as the key hormones coordinating these multiple and complex interactions. On the other hand, the integrative approach allowed elucidation of crucial metabolic mechanisms involved in heat response in P. radiata, as well as the identification of thermotolerance metabolic biomarkers (L-phenylalanine, hexadecanoic acid, and dihydromyricetin), crucial metabolites which can reschedule the metabolic strategy to adapt to high temperature.

Plants under Stress: Involvement of Auxin and Cytokinin

Plant growth and development are critically influenced by unpredictable abiotic factors. To survive fluctuating changes in their environments, plants have had to develop robust adaptive mechanisms. The dynamic and complementary actions of the auxin and cytokinin pathways regulate a plethora of developmental processes, and their ability to crosstalk makes them ideal candidates for mediating stress-adaptation responses. Other crucial signaling molecules responsible for the tremendous plasticity observed in plant morphology and in response to abiotic stress are reactive oxygen species (ROS). Proper temporal and spatial distribution of ROS and hormone gradients is crucial for plant survival in response to unfavorable environments. In this regard, the convergence of ROS with phytohormone pathways acts as an integrator of external and developmental signals into systemic responses organized to adapt plants to their environments. Auxin and cytokinin signaling pathways have been studied extensively. Nevertheless, we do not yet understand the impact on plant stress tolerance of the sophisticated crosstalk between the two hormones. Here, we review current knowledge on the function of auxin and cytokinin in redirecting growth induced by abiotic stress in order to deduce their potential points of crosstalk.

The "STAY-GREEN" trait and phytohormone signaling networks in plants under heat stress

The increasing demand for food and the heavy yield losses in primary crops due to global warming mean that there is an urgent need to improve food security. Therefore, understanding how plants respond to heat stress and its consequences, such as drought and increased soil salinity, has received much attention in plant science community. Plants exhibit stress tolerance, escape or avoidance via adaptation and acclimatization mechanisms. These mechanisms rely on a high degree of plasticity in their cellular metabolism, in which phytohormones play an important role. "STAY-GREEN" is a crucial trait for genetic improvement of several crops, which allows plants to keep their leaves on the active photosynthetic level under stress conditions. Understanding the physiological and molecular mechanisms concomitant with "STAY-GREEN" trait or delayed leaf senescence, as well as those regulating photosynthetic capability of plants under heat stress, with a certain focus on the hormonal pathways, may be a key to break the plateau of productivity associated with adaptation to high temperature. This review will discuss the recent findings that advance our understanding of the mechanisms controlling leaf senescence and hormone signaling cascades under heat stress.

Heat-Induced Cytokinin Transportation and Degradation Are Associated with Reduced Panicle Cytokinin Expression and Fewer Spikelets per Panicle in Rice

Cytokinins (CTKs) regulate panicle size and mediate heat tolerance in crops. To investigate the effect of high temperature on panicle CTK expression and the role of such expression in panicle differentiation in rice, four rice varieties (Nagina22, N22; Huanghuazhan, HHZ; Liangyoupeijiu, LYPJ; and Shanyou63, SY63) were grown under normal conditions and subjected to three high temperature treatments and one control treatment in temperature-controlled greenhouses for 15 days during the early reproductive stage. The high temperature treatments significantly reduced panicle CTK abundance in heat-susceptible LYPJ, HHZ, and N22 varieties, which showed fewer spikelets per panicle in comparison with control plants. Exogenous 6-benzylaminopurine application mitigated the effect of heat injury on the number of spikelets per panicle. The high temperature treatments significantly decreased the xylem sap flow rate and CTK transportation rate, but enhanced cytokinin oxidase/dehydrogenase (CKX) activity in heat-susceptible varieties. In comparison with the heat-susceptible varieties, heat-tolerant variety SY63 showed less reduction in panicle CTK abundance, an enhanced xylem sap flow rate, an improved CTK transport rate, and stable CKX activity under the high temperature treatments. Enzymes involved in CTK synthesis (isopentenyltransferase, LONELY GUY, and cytochrome P450 monooxygenase) were inhibited by the high temperature treatments. Heat-induced changes in CTK transportation from root to shoot through xylem sap flow and panicle CTK degradation via CKX were closely associated with the effects of heat on panicle CTK abundance and panicle size. Heat-tolerant variety SY63 showed stable panicle size under the high temperature treatments because of enhanced transport of root-derived CTKs and stable panicle CKX activity. Our results provide insight into rice heat tolerance that will facilitate the development of rice varieties with tolerance to high temperature.

Proteomic Analysis Reveals the Positive Effect of Exogenous Spermidine in Tomato Seedlings' Response to High-Temperature Stress

Polyamines are phytohormones that regulate plant growth and development as well as the response to environmental stresses. To evaluate their functions in high-temperature stress responses, the effects of exogenous spermidine (Spd) were determined in tomato leaves using two-dimensional electrophoresis and MALDI-TOF/TOF MS. A total of 67 differentially expressed proteins were identified in response to high-temperature stress and/or exogenous Spd, which were grouped into different categories according to biological processes. The four largest categories included proteins involved in photosynthesis (27%), cell rescue, and defense (24%), protein synthesis, folding and degradation (22%), and energy and metabolism (13%). Exogenous Spd up-regulated most identified proteins involved in photosynthesis, implying an enhancement in photosynthetic capacity. Meanwhile, physiological analysis showed that Spd could improve net photosynthetic rate and the biomass accumulation. Moreover, an increased high-temperature stress tolerance by exogenous Spd would contribute to the higher expressions of proteins involved in cell rescue and defense, and Spd regulated the antioxidant enzymes activities and related genes expression in tomato seedlings exposed to high temperature. Taken together, these findings provide a better understanding of the Spd-induced high-temperature resistance by proteomic approaches, providing valuable insight into improving the high-temperature stress tolerance in the global warming epoch.