Nitric oxide functions as a signal and acts upstream of AtCaM3 in thermotolerance in Arabidopsis seedlings

The interaction of nutrient uptake with biotic and abiotic stresses in plants

Plants depend heavily on efficient nutrient uptake and utilization for optimal growth and development. However, plants are constantly subjected to a diverse array of biotic stresses, such as pathogen infections, insect pests, and herbivory, as well as abiotic stress like drought, salinity, extreme temperatures, and nutrient imbalances. These stresses significantly impact the plant's ability to take up nutrient and use it efficiency. Understanding how plants maintain nutrient uptake and use efficiency under biotic and abiotic stress conditions is crucial for improving crop resilience and sustainability. This review explores the recent advancements in elucidating the mechanisms underlying nutrient uptake and utilization efficiency in plants under such stress conditions. Our aim is to offer a comprehensive perspective that can guide the breeding of stress-tolerant and nutrition-efficient crop varieties, ultimately contributing to the advancement of sustainable agriculture.

Nitric oxide-mediated thermomemory: a new perspective on plant heat stress resilience

In the intricate world of plant responses to environmental stress, the concept of thermomemory has emerged as a fascinating and complex phenomenon. Plants, as sessile organisms, continually face the challenge of adapting to fluctuating climates, and the ability to "remember" prior heat stress encounters, a phenomenon known as thermomemory is a testament to their remarkable adaptability. Nitric oxide (NO), a versatile signaling molecule in plant physiology, has been implicated in a myriad of cellular processes crucial for stress adaptation. From its involvement in stomatal regulation to its influence on gene expression and antioxidant defense mechanisms, NO emerges as a central orchestrator in the plant's response to elevated temperatures. Exploration of NO-mediated pathways provides insights into how plants not only cope with immediate heat stress but also retain a memory of these encounters. Unraveling the molecular intricacies of NO's involvement in thermomemory enhances our understanding of the sophisticated strategies employed by plants to navigate a changing climate, offering potential avenues for innovative approaches to enhancing crop resilience and sustainable agriculture.

Functional validation of AaCaM3 response to high temperature stress in Amorphophallus albus

Amorphophallus is a perennial monocotyledonous herbaceous plant native to the southwestern region of China, widely used in various fields such as food processing, biomedicine and chemical agriculture. However, Amorphophallus is a typical thermolabile plant, and the continuous high temperature in summer have seriously affected the growth, development and economic yield of Amorphophallus in recent years. Calmodulin (CaM), a Ca2+ sensor ubiquitous in eukaryotes, is the most important multifunctional receptor protein in plant cells, which affects plant stress resistance by participating in the activities of a variety of signaling molecules. In this study, the key gene AaCaM3 for the Ca2+-CaM regulatory pathway was obtained from A. albus, the sequence analysis confirmed that it is a typical calmodulin. The qRT-PCR results demonstrated that with the passage of heat treatment time, the expression of AaCaM3 was significantly upregulated in A. albus leaves. Subcellular localization analysis revealed that AaCaM3 localized on the cytoplasm and nucleus. Meanwhile, heterologous transformation experiments have shown that AaCaM3 can significantly improve the heat tolerance of Arabidopsis under heat stress. The promoter region of AaCaM3 was sequenced 1,338 bp by FPNI-PCR and GUS staining assay showed that the promoter of AaCaM3 was a high-temperature inducible promoter. Yeast one-hybrid analysis and Luciferase activity reporting system analysis showed that the AaCaM3 promoter may interact with AaHSFA1, AaHSFA2c, AaHSP70, AaDREB2a and AaDREB2b. In conclusion, this study provides new ideas for further improving the signal transduction network of high-temperature stress in Amorphophallus.

Early signaling enhance heat tolerance in Arabidopsis through modulating jasmonic acid synthesis mediated by HSFA2

Given the detrimental impact of global warming on crop production, it is particularly important to understand how plants respond and adapt to higher temperatures. Using the non-invasive micro-test technique and laser confocal microscopy, we found that the cascade process of early signals (K+, H2O2, H+, and Ca2+) ultimately resulted in an increase in the cytoplasmic Ca2+ concentration when Arabidopsis was exposed to heat stress. Quantitative real-time PCR demonstrated that heat stress significantly up-regulated the expression of CAM1, CAM3 and HSFA2; however, after CAM1 and CAM3 mutation, the upregulation of HSFA2 was reduced. In addition, heat stress affected the expression of LOX3 and OPR3, which was not observed when HSFA2 was mutated. Luciferase reporter gene expression assay and electrophoretic mobility shift assay showed that HSFA2 regulated the expression of both genes. Determination of jasmonic acid (JA) content showed that JA synthesis was promoted by heat stress, but was damaged when HSFA2 and OPR3 were mutated. Finally, physiological experiments showed that JA reduced the relative electrical conductivity of leaves, enhanced chlorophyll content and relative water content, and improved the survival rate of Arabidopsis under heat stress. Together, our results reveal a new pathway for Arabidopsis to sense and transmit heat signals; HSFA2 is involved in the JA synthesis, which can act as a defensive compound improving Arabidopsis heat tolerance.

Drought priming-induced stress memory improves subsequent drought or heat tolerance via activation of γ-aminobutyric acid-regulated pathways in creeping bentgrass

Recurrent drought can induce stress memory in plants to induce tolerance to subsequent stress, such as high temperature or drought. Drought priming (DP) is an effective approach to improve tolerance to various stresses; however, the potential mechanism of DP-induced stress memory has not been fully resoved. We examined DP-regulated subsequent drought tolerance or thermotolerance associated with changes in physiological responses, GABA and NO metabolism, heat shock factor (HSF) and dehydrin (DHN) pathways in perennial creeping bentgrass. Plants can recover after two cycle of DP, and DP-treated plants had significantly higher tolerance to subsequent drought or heat stress, with higher leaf RWC, Chl content, photochemical efficiency, and cell membrane stability. DP significantly alleviated oxidative damage through enhancing total antioxidant capacity in response to subsequent drought or heat stress. Endogenous GABA was significantly increased by DP through activating glutamic acid decarboxylase activity and inhibiting GABA transaminase activity. DP also enhanced accumulation of NO, depending on NOS activity, under subsequent drought or heat stress. Transcript levels of multiple transcription factors, heat shock proteins, and DHNs in the HSF and DHN pathways were up-regulated by DP under drought or heat stress, but there were differences between DP-regulated heat tolerance and drought tolerance in these pathways. The findings indicate that under recurrent moderate drought, DP improves subsequent tolerance to drought or heat stress in relation to GABA-regulated pathways, providing new insight into understanding of the role of stress memory in plant adaptation to complex environmental stresses.

Appraisal of the Role of Gaseous Signaling Molecules in Thermo-Tolerance Mechanisms in Plants

A significant threat to the ongoing rise in temperature caused by global warming. Plants have many stress-resistance mechanisms, which is responsible for maintaining plant homeostasis. Abiotic stresses largely increase gaseous molecules' synthesis in plants. The study of gaseous signaling molecules has gained attention in recent years. The role of gaseous molecules, such as nitric oxide (NO), hydrogen sulfide (H2S), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), and ethylene, in plants under temperature high-temperature stress are discussed in the current review. Recent studies revealed the critical function that gaseous molecules play in controlling plant growth and development and their ability to respond to various abiotic stresses. Here, we provide a thorough overview of current advancements that prevent heat stress-related plant damage via gaseous molecules. We also explored and discussed the interaction of gaseous molecules. In addition, we provided an overview of the role played by gaseous molecules in high-temperature stress responses, along with a discussion of the knowledge gaps and how this may affect the development of high-temperature-resistant plant species.

Calcium Signaling and the Response to Heat Shock in Crop Plants

Climate change and the increasing frequency of high temperature (HT) events are significant threats to global crop yields. To address this, a comprehensive understanding of how plants respond to heat shock (HS) is essential. Signaling pathways involving calcium (Ca2+), a versatile second messenger in plants, encode information through temporal and spatial variations in ion concentration. Ca2+ is detected by Ca2+-sensing effectors, including channels and binding proteins, which trigger specific cellular responses. At elevated temperatures, the cytosolic concentration of Ca2+ in plant cells increases rapidly, making Ca2+ signals the earliest response to HS. In this review, we discuss the crucial role of Ca2+ signaling in raising plant thermotolerance, and we explore its multifaceted contributions to various aspects of the plant HS response (HSR).

Light signaling as cellular integrator of multiple environmental cues in plants

Plants being sessile need to rapidly adapt to the constantly changing environment through modifications in their internal clock, metabolism, and gene expression. They have evolved an intricate system to perceive and transfer the signals from the primary environmental factors namely light, temperature and water to regulate their growth development and survival. Over past few decades rigorous research using molecular genetics approaches, especially in model plant Arabidopsis, has resulted in substantial progress in discovering various photoreceptor systems and light signaling components. In parallel several molecular pathways operating in response to other environmental cues have also been elucidated. Interestingly, the studies have shown that expression profiles of genes involved in photomorphogenesis can undergo modulation in response to other cues from the environment. Recently, the photoreceptor, PHYB, has been shown to function as a thermosensor. Downstream components of light signaling pathway like COP1 and PIF have also emerged as integrating hubs for various kinds of signals. All these findings indicate that light signaling components may act as central integrator of various environmental cues to regulate plant growth and development processes. In this review, we present a perspective on cross talk of signaling mechanisms induced in response to myriad array of signals and their integration with the light signaling components. By putting light signals on the central stage, we propose the possibilities of enhancing plant resilience to the changing environment by fine-tuning the genetic manipulation of its signaling components in the future.

Transcriptome Analysis Reveals the Dynamic and Rapid Transcriptional Reprogramming Involved in Heat Stress and Identification of Heat Response Genes in Rice

High temperature is one of the most important environmental factors influencing rice growth, development, and yield. Therefore, it is important to understand how rice plants cope with high temperatures. Herein, the heat tolerances of T2 (Jinxibai) and T21 (Taizhongxianxuan2hao) were evaluated at 45 °C, and T21 was found to be sensitive to heat stress at the seedling stage. Analysis of the H2O2 and proline content revealed that the accumulation rate of H2O2 was higher in T21, whereas the accumulation rate of proline was higher in T2 after heat treatment. Meanwhile, transcriptome analysis revealed that several pathways participated in the heat response, including "protein processing in endoplasmic reticulum", "plant hormone signal transduction", and "carbon metabolism". Additionally, our study also revealed that different pathways participate in heat stress responses upon prolonged stress. The pathway of "protein processing in endoplasmic reticulum" plays an important role in stress responses. We found that most genes involved in this pathway were upregulated and peaked at 0.5 or 1 h after heat treatment. Moreover, sixty transcription factors, including the members of the AP2/ERF, NAC, HSF, WRKY, and C2H2 families, were found to participate in the heat stress response. Many of them have also been reported to be involved in biotic or abiotic stresses. In addition, through PPI (protein-protein interactions) analysis, 22 genes were identified as key genes in the response to heat stress. This study improves our understanding of thermotolerance mechanisms in rice, and also lays a foundation for breeding thermotolerant cultivars via molecular breeding.

Multiple Ways of Nitric Oxide Production in Plants and Its Functional Activity under Abiotic Stress Conditions

Nitric oxide (NO) is an endogenous signaling molecule that plays an important role in plant ontogenesis and responses to different stresses. The most widespread abiotic stress factors limiting significantly plant growth and crop yield are drought, salinity, hypo-, hyperthermia, and an excess of heavy metal (HM) ions. Data on the accumulation of endogenous NO under stress factors and on the alleviation of their negative effects under exogenous NO treatments indicate the perspectives of its practical application to improve stress resistance and plant productivity. This requires fundamental knowledge of the NO metabolism and the mechanisms of its biological action in plants. NO generation occurs in plants by two main alternative mechanisms: oxidative or reductive, in spontaneous or enzymatic reactions. NO participates in plant development by controlling the processes of seed germination, vegetative growth, morphogenesis, flower transition, fruit ripening, and senescence. Under stressful conditions, NO contributes to antioxidant protection, osmotic adjustment, normalization of water balance, regulation of cellular ion homeostasis, maintenance of photosynthetic reactions, and growth processes of plants. NO can exert regulative action by inducing posttranslational modifications (PTMs) of proteins changing the activity of different enzymes or transcriptional factors, modulating the expression of huge amounts of genes, including those related to stress tolerance. This review summarizes the current data concerning molecular mechanisms of NO production and its activity in plants during regulation of their life cycle and adaptation to drought, salinity, temperature stress, and HM ions.

Identification and functional analysis of calcium sensor calmodulins from heavy metal hyperaccumulator Noccaea caerulescens

Noccaea caerulescens (J. Presl & C. Presl) F. K. Mey. is a heavy metal hyperaccumulator exhibiting extreme tolerance to various environmental stresses. To date, the functional role of Ca2+ -binding protein in this plant is largely unknown. To investigate the function of calmodulins (CaMs) in N. caerulescens , CaM2 , a Ca2+ sensor encoding gene, was identified and functionally characterised. Protein structure analysis showed that NcCaM2 contains four classic exchange factor (EF)-hand motifs with high sequence similarity to the CaM proteins from model plant Arabidopsis thaliana L. Tissue specific expression analysis showed that NcCaM2 is constitutively expressed in stems, leaves, and roots. Expression level of NcCaM2 was significantly upregulated under various environmental stimulus, indicating a potential involvement of NcCaM2 in the tolerance to abiotic stresses. The heterologous expression of NcCaM2 in a yeast mutant strain increased the heavy metal tolerance in yeast cells. Furthermore, the constitutive expression of NcCaM2 enhanced the heavy metal tolerance capability of transgenic tobacco (Nicotiana tabacum L.) plants. Our data suggested an important role of NcCaM2 in the responses to environmental stresses and provided a potential target gene to enhance of the ability to hyperaccumulate metals.

Functions of nitric oxide-mediated post-translational modifications under abiotic stress

Environmental conditions greatly impact plant growth and development. In the current context of both global climate change and land degradation, abiotic stresses usually lead to growth restriction limiting crop production. Plants have evolved to sense and respond to maximize adaptation and survival; therefore, understanding the mechanisms involved in the different converging signaling networks becomes critical for improving plant tolerance. In the last few years, several studies have shown the plant responses against drought and salinity, high and low temperatures, mechanical wounding, heavy metals, hypoxia, UV radiation, or ozone stresses. These threats lead the plant to coordinate a crosstalk among different pathways, highlighting the role of phytohormones and reactive oxygen and nitrogen species (RONS). In particular, plants sense these reactive species through post-translational modification (PTM) of macromolecules such as nucleic acids, proteins, and fatty acids, hence triggering antioxidant responses with molecular implications in the plant welfare. Here, this review compiles the state of the art about how plant systems sense and transduce this crosstalk through PTMs of biological molecules, highlighting the S-nitrosylation of protein targets. These molecular mechanisms finally impact at a physiological level facing the abiotic stressful traits that could lead to establishing molecular patterns underlying stress responses and adaptation strategies.

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.

Say "NO" to plant stresses: Unravelling the role of nitric oxide under abiotic and biotic stress

Nitric oxide (NO) is a diatomic gaseous molecule, which plays different roles in different strata of organisms. Discovered as a neurotransmitter in animals, NO has now gained a significant place in plant signaling cascade. NO regulates plant growth and several developmental processes including germination, root formation, stomatal movement, maturation and defense in plants. Due to its gaseous state, it is unchallenging for NO to reach different parts of cell and counterpoise antioxidant pool. Various abiotic and biotic stresses act on plants and affect their growth and development. NO plays a pivotal role in alleviating toxic effects caused by various stressors by modulating oxidative stress, antioxidant defense mechanism, metal transport and ion homeostasis. It also modulates the activity of some transcriptional factors during stress conditions in plants. Besides its role during stress conditions, interaction of NO with other signaling molecules such as other gasotransmitters (hydrogen sulfide), phytohormones (abscisic acid, salicylic acid, jasmonic acid, gibberellin, ethylene, brassinosteroids, cytokinins and auxin), ions, polyamines, etc. has been demonstrated. These interactions play vital role in alleviating plant stress by modulating defense mechanisms in plants. Taking all these aspects into consideration, the current review focuses on the role of NO and its interaction with other signaling molecules in regulating plant growth and development, particularly under stressed conditions.

Comparative transcriptome analysis of the mechanism difference in heat stress response between indica rice cultivar "IR64" and japonica cultivar "Koshihikari" at the seedling stage

Heat stress (HS) has become a major abiotic stress in rice, considering the frequency and intensity of extreme hot weather. There is an urgent need to explore the differences in molecular mechanisms of HS tolerance in different cultivars, especially in indica and japonica. In this study, we investigated the transcriptome information of IR64 (indica, IR) and Koshihikari (japonica, Kos) in response to HS at the seedling stage. From the differentially expressed genes (DEGs) consistently expressed at six time points, 599 DEGs were identified that were co-expressed in both cultivars, as well as 945 and 1,180 DEGs that were specifically expressed in IR and Kos, respectively. The results of GO and KEGG analysis showed two different HS response pathways for IR and Kos. IR specifically expressed DEGs were mainly enriched in chloroplast-related pathways, whereas Kos specifically expressed DEGs were mainly enriched in endoplasmic reticulum and mitochondria-related pathways. Meanwhile, we highlighted the importance of NO biosynthesis genes, especially nitrate reductase genes, in the HS response of IR based on protein-protein interaction networks. In addition, we found that heat shock proteins and heat shock factors play very important roles in both cultivars. This study not only provides new insights into the differences in HS responses between different subspecies of rice, but also lays the foundation for future research on molecular mechanisms and breeding of heat-tolerant cultivars.

Brassinosteroids promote thermotolerance through releasing BIN2-mediated phosphorylation and suppression of HsfA1 transcription factors in Arabidopsis

High temperature adversely affects plant growth and development. The steroid phytohormones brassinosteroids (BRs) are recognized to play important roles in plant heat stress responses and thermotolerance, but the underlying mechanisms remain obscure. Here, we demonstrate that the glycogen synthase kinase 3 (GSK3)-like kinase BRASSINOSTEROID INSENSITIVE2 (BIN2), a negative component in the BR signaling pathway, interacts with the master heat-responsive transcription factors CLASS A1 HEAT SHOCK TRANSCRIPTION FACTORS (HsfA1s). Furthermore, BIN2 phosphorylates HsfA1d on T263 and S56 to suppress its nuclear localization and inhibit its DNA-binding ability, respectively. BR signaling promotes plant thermotolerance by releasing the BIN2 suppression of HsfA1d to facilitate its nuclear localization and DNA binding. Our study provides insights into the molecular mechanisms by which BRs promote plant thermotolerance by strongly regulating HsfA1d through BIN2 and suggests potential ways to improve crop yield under extreme high temperatures.

Guvermectin, a novel plant growth regulator, can promote the growth and high temperature tolerance of maize

Guvermectin is a recently discovered microbial N9-glucoside cytokinin compound extracted from Streptomyces sanjiangensis NEAU6. Although some research has reported that N9-glucoside cytokinin compounds do not have the activity of cytokinin, it has been noted that guvermectin can promote growth and antifungal activity in Arabidopsis. Maize is an important food crop in the world and exploring the effect of guvermectin on this crop could help its cultivation in regions with adverse environmental conditions such as a high temperature. Here, we investigated the effects of guvermectin seed soaking treatment on the growth of maize at the seedlings stage and its yield attributes with different temperature stresses. The maize (cv. Zhengdan 958) with guvermectin seed soaking treatment were in two systems: paper roll culture and field conditions. Guvermectin seed soaking treated plants had increased plant height, root length, and mesocotyl length at the seedlings stage, and spike weight at maturity in the field. But only root length was increased at the paper roll culture by guvermectin seed soaking treatment. Guvermectin seed soaking treatment reduced the adverse effects on maize seedling when grow at a high temperature. Further experiments showed that, in high temperature conditions, guvermectin treatment promoted the accumulation of heat shock protein (HSP) 17.0, HSP 17.4 and HSP 17.9 in maize roots. Comparative transcriptomic profiling showed there were 33 common differentially expressed genes (DEGs) in guvermectin treated plants under high temperature and room temperature conditions. The DEGs suggested that guvermectin treatment led to the differential modulation of several transcripts mainly related with plant defense, stress response, and terpenoid biosynthesis. Taken together, these results suggested that the guvermectin treatment promoted the growth and tolerance of high temperature stresses, possibly by activation of related pathways. These results show that guvermectin is a novel plant growth regulator and could be developed as an application to maize seeds to promote growth in high temperature environments.

Gene Networks Involved in Plant Heat Stress Response and Tolerance

Global warming is an environmental problem that cannot be ignored. High temperatures seriously affect the normal growth and development of plants, and threaten the development of agriculture and the distribution and survival of species at risk. Plants have evolved complex but efficient mechanisms for sensing and responding to high temperatures, which involve the activation of numerous functional proteins, regulatory proteins, and non-coding RNAs. These mechanisms consist of large regulatory networks that regulate protein and RNA structure and stability, induce Ca²⁺ and hormone signal transduction, mediate sucrose and water transport, activate antioxidant defense, and maintain other normal metabolic pathways. This article reviews recent research results on the molecular mechanisms of plant response to high temperatures, highlighting future directions or strategies for promoting plant heat tolerance, thereby helping to identify the regulatory mechanisms of heat stress responses in plants.

Hydrogen Sulfide, Ethylene, and Nitric Oxide Regulate Redox Homeostasis and Protect Photosynthetic Metabolism under High Temperature Stress in Rice Plants

Rising temperatures worldwide due to global climate change are a major scientific issue at present. The present study reports the effects of gaseous signaling molecules, ethylene (200 µL L⁻¹; 2-chloroethylphosphonic acid; ethephon, Eth), nitric oxide (NO; 100 µM sodium nitroprusside; SNP), and hydrogen sulfide (H₂S; 200 µM sodium hydrosulfide, NaHS) in high temperature stress (HS) tolerance, and whether or not H₂S contributes to ethylene or NO-induced thermo-tolerance and photosynthetic protection in rice (Oryza sativa L.) cultivars, i.e., Taipei-309, and Rasi. Plants exposed to an HS of 40 °C for six h per day for 15 days caused a reduction in rice biomass, associated with decreased photosynthesis and leaf water status. High temperature stress increased oxidative stress by increasing the content of hydrogen peroxide (H₂O₂) and thiobarbituric acid reactive substance (TBARS) in rice leaves. These signaling molecules increased biomass, leaf water status, osmolytes, antioxidants, and photosynthesis of plants under non-stress and high temperature stress. However, the effect was more conspicuous with ethylene than NO and H₂S. The application of H₂S scavenger hypotaurine (HT) reversed the effect of ethylene or NO on photosynthesis under HS. This supports the findings that the ameliorating effects of Eth or SNP involved H₂S. Thus, the presence of H₂S with ethylene or NO can enhance thermo-tolerance while also protecting plant photosynthesis.

The calcium signaling module CaM-IQM destabilizes IAA-ARF interaction to regulate callus and lateral root formation

Induction of a pluripotent cell mass, called callus, from detached organs is an initial step in in vitro plant regeneration, during which phytohormone auxin-induced ectopic activation of a root developmental program has been shown to be required for subsequent de novo regeneration of shoots and roots. However, whether other signals are involved in governing callus formation, and thus plant regeneration capability, remains largely unclear. Here, we report that the Arabidopsis calcium (Ca²⁺) signaling module CALMODULIN IQ-MOTIF CONTAINING PROTEIN (CaM-IQM) interacts with auxin signaling to regulate callus and lateral root formation. We show that disruption of IQMs or CaMs retards auxin-induced callus and lateral root formation by dampening auxin responsiveness, and that CaM-IQM complexes physically interact with the auxin signaling repressors INDOLE-3-ACETIC ACID INDUCIBLE (IAA) proteins in a Ca²⁺-dependent manner. We further provide evidence that the physical interaction of CaM6 with IAA19 destabilizes the repressive interaction of IAA19 with AUXIN RESPONSE FACTOR 7 (ARF7), and thus regulates auxin-induced callus formation. These findings not only define a critical role of CaM-IQM-mediated Ca²⁺ signaling in callus and lateral root formation, but also provide insight into the interplay of Ca²⁺ signaling and auxin actions during plant regeneration and development.

Reproductive-Stage Heat Stress in Cereals: Impact, Plant Responses and Strategies for Tolerance Improvement

Reproductive-stage heat stress (RSHS) poses a major constraint to cereal crop production by damaging main plant reproductive structures and hampering reproductive processes, including pollen and stigma viability, pollination, fertilization, grain setting and grain filling. Despite this well-recognized fact, research on crop heat stress (HS) is relatively recent compared to other abiotic stresses, such as drought and salinity, and in particular, RSHS studies in cereals are considerably few in comparison with seedling-stage and vegetative-stage-centered studies. Meanwhile, climate change-exacerbated HS, independently or synergistically with drought, will have huge implications on crop performance and future global food security. Fortunately, due to their sedentary nature, crop plants have evolved complex and diverse transient and long-term mechanisms to perceive, transduce, respond and adapt to HS at the molecular, cell, physiological and whole plant levels. Therefore, uncovering the molecular and physiological mechanisms governing plant response and tolerance to RSHS facilitates the designing of effective strategies to improve HS tolerance in cereal crops. In this review, we update our understanding of several aspects of RSHS in cereals, particularly impacts on physiological processes and yield; HS signal perception and transduction; and transcriptional regulation by heat shock factors and heat stress-responsive genes. We also discuss the epigenetic, post-translational modification and HS memory mechanisms modulating plant HS tolerance. Moreover, we offer a critical set of strategies (encompassing genomics and plant breeding, transgenesis, omics and agronomy) that could accelerate the development of RSHS-resilient cereal crop cultivars. We underline that a judicious combination of all of these strategies offers the best foot forward in RSHS tolerance improvement in cereals. Further, we highlight critical shortcomings to RSHS tolerance investigations in cereals and propositions for their circumvention, as well as some knowledge gaps, which should guide future research priorities. Overall, our review furthers our understanding of HS tolerance in plants and supports the rational designing of RSHS-tolerant cereal crop cultivars for the warming climate.

Molecular Bases of Heat Stress Responses in Vegetable Crops With Focusing on Heat Shock Factors and Heat Shock Proteins

The effects of the climate change including an increase in the average global temperatures, and abnormal weather events such as frequent and severe heatwaves are emerging as a worldwide ecological concern due to their impacts on plant vegetation and crop productivity. In this review, the molecular processes of plants in response to heat stress-from the sensing of heat stress, the subsequent molecular cascades associated with the activation of heat shock factors and their primary targets (heat shock proteins), to the cellular responses-have been summarized with an emphasis on the classification and functions of heat shock proteins. Vegetables contain many essential vitamins, minerals, antioxidants, and fibers that provide many critical health benefits to humans. The adverse effects of heat stress on vegetable growth can be alleviated by developing vegetable crops with enhanced thermotolerance with the aid of various genetic tools. To achieve this goal, a solid understanding of the molecular and/or cellular mechanisms underlying various responses of vegetables to high temperature is imperative. Therefore, efforts to identify heat stress-responsive genes including those that code for heat shock factors and heat shock proteins, their functional roles in vegetable crops, and also their application to developing vegetables tolerant to heat stress are discussed.

A nitric oxide burst at the shoot apex triggers a heat-responsive pathway in Arabidopsis

When confronted with heat stress, plants depend on the timely activation of cellular defences to survive by perceiving the rising temperature. However, how plants sense heat at the whole-plant level has remained unanswered. Here we demonstrate that shoot apical nitric oxide (NO) bursting under heat stress as a signal triggers cellular heat responses at the whole-plant level on the basis of our studies mainly using live-imaging of transgenic plants harbouring pHsfA2::LUC, micrografting, NO accumulation mutants and liquid chromatography-tandem mass spectrometry analysis in Arabidopsis. Furthermore, we validate that S-nitrosylation of the trihelix transcription factor GT-1 by S-nitrosoglutathione promotes its binding to NO-responsive elements in the HsfA2 promoter and that loss of function of GT-1 disrupts the activation of HsfA2 and heat tolerance, revealing that GT-1 is the long-sought mediator linking signal perception to the activation of cellular heat responses. These findings uncover a heat-responsive mechanism that determines the timing and execution of cellular heat responses at the whole-plant level.

Calmodulin and calmodulin-like gene family in barley: Identification, characterization and expression analyses

Calmodulin (CaM) and calmodulin-like (CML) proteins are Ca²⁺ relays and play diverse and multiple roles in plant growth, development and stress responses. However, CaM/CML gene family has not been identified in barley (Hordeum vulgare). In the present study, 5 HvCaMs and 80 HvCMLs were identified through a genome-wide analysis. All HvCaM proteins possessed 4 EF-hand motifs, whereas HvCMLs contained 1 to 4 EF-hand motifs. HvCaM2, HvCaM3 and HvCaM5 coded the same polypeptide although they differed in nucleotide sequence, which was identical to the polypeptides coded by OsCaM1-1, OsCaM1-2 and OsCaM1-3. HvCaMs/CMLs were unevenly distributed over barley 7 chromosomes, and could be phylogenetically classified into 8 groups. HvCaMs/CMLs differed in gene structure, cis-acting elements and tissue expression patterns. Segmental and tandem duplication were observed among HvCaMs/CMLs during evolution. HvCML16, HvCML18, HvCML50 and HvCML78 were dispensable genes and the others were core genes in barley pan-genome. In addition, 14 HvCaM/CML genes were selected to examine their responses to salt, osmotic and low potassium stresses by qRT-PCR, and their expression were stress-and time-dependent. These results facilitate our understanding and further functional identification of HvCaMs/CMLs.

TypiCal but DeliCate Ca++re: Dissecting the Essence of Calcium Signaling Network as a Robust Response Coordinator of Versatile Abiotic and Biotic Stimuli in Plants

Plant growth, development, and ultimately crop productivity are largely impacted by the interaction of plants with different abiotic and biotic factors throughout their life cycle. Perception of different abiotic stresses, such as salt, cold, drought, heat, and heavy metals, and interaction with beneficial and harmful biotic agents by plants lead to transient, sustained, or oscillatory changes of [calcium ion, Ca²⁺]_cyt within the cell. Significant progress has been made in the decoding of Ca²⁺ signatures into downstream responses to modulate differential developmental and physiological responses in the whole plant. Ca²⁺ sensor proteins, mainly calmodulins (CaMs), calmodulin-like proteins (CMLs), and others, such as Ca²⁺-dependent protein kinases (CDPKs), calcineurin B-like proteins (CBLs), and calmodulin-binding transcription activators (CAMTAs) have played critical roles in coupling the specific stress stimulus with an appropriate response. This review summarizes the current understanding of the Ca²⁺ influx and efflux system in plant cells and various Ca²⁺ binding protein-mediated signal transduction pathways that are delicately orchestrated to mitigate abiotic and biotic stresses. The probable interactions of different components of Ca²⁺ sensor relays and Ca²⁺ sensor responders in response to various external stimuli have been described diagrammatically focusing on established pathways and latest developments. Present comprehensive insight into key components of the Ca²⁺ signaling toolkit in plants can provide an innovative framework for biotechnological manipulations toward crop improvability in near future.

Electrical Signaling of Plants under Abiotic Stressors: Transmission of Stimulus-Specific Information

Plants have developed complex systems of perception and signaling to adapt to changing environmental conditions. Electrical signaling is one of the most promising candidates for the regulatory mechanisms of the systemic functional response under the local action of various stimuli. Long-distance electrical signals of plants, such as action potential (AP), variation potential (VP), and systemic potential (SP), show specificities to types of inducing stimuli. The systemic response induced by a long-distance electrical signal, representing a change in the activity of a complex of molecular-physiological processes, includes a nonspecific component and a stimulus-specific component. This review discusses possible mechanisms for transmitting information about the nature of the stimulus and the formation of a specific systemic response with the participation of electrical signals induced by various abiotic factors.

Heat stress in macrofungi: effects and response mechanisms

Temperature is one of the key factors that affects the growth and development of macrofungi. Heat stress not only negatively affects the morphology and growth rate of macrofungi, but also destroys cell structures and influences cell metabolism. Due to loosed structure of cell walls and increased membrane fluidity, which caused by heat stress, the outflow of intracellular nutrients makes macrofungi more vulnerable to invasion by pathogens. Macrofungi accumulate reactive oxygen species (ROS), Ca²⁺, and nitric oxide (NO) when heat-stressed, which transmit and amplify the heat stimulation signal through intracellular signal transduction pathways. Through regulation of some transcription factors including heat response factors (HSFs), POZCP26 and MYB, macrofungi respond to heat stress by different mechanisms. In this paper, we present mechanisms used by macrofungi to adapt and survive under heat stress conditions, including antioxidant defense systems that eliminate the excess ROS, increase in trehalose levels that prevent enzymes and proteins deformation, and stabilize cell structures and heat shock proteins (HSPs) that repair damaged proteins and synthesis of auxins, which increase the activity of antioxidant enzymes. All of these help macrofungi resist and adapt to heat stress. KEY POINTS: • The effects of heat stress on macrofungal growth and development were described. • The respond mechanisms to heat stress in macrofungi were summarized. • The further research directions of heat stress in macrofungi were discussed.

Crosstalk between abscisic acid and nitric oxide under heat stress: exploring new vantage points

Heat stress adversely affects plants growth potential. Global warming is reported to increase in the intensity, frequency, and duration of heatwaves, eventually affecting ecology, agriculture and economy. With an expected increase in average temperature by 2-3 °C over the next 30-50 years, crop production is facing a severe threat to sub-optimum growth conditions. Abscisic acid (ABA) and nitric oxide (NO) are growth regulators that are involved in the adaptation to heat stress by affecting each other and changing the adaptation process. The interaction between these molecules has been discussed in various studies in general or under stress conditions; however, regarding high temperature, their interaction has little been worked out. In the present review, the focus is shifted on the role of these molecules under heat stress emphasizing the different possible interactions between ABA and NO as both regulate stomatal closure and other molecules including hydrogen peroxide (H₂O₂), hydrogen sulfide (H₂S), antioxidants, proline, glycine betaine, calcium (Ca²⁺) and heat shock protein (HSP). Exploring the crosstalk between ABA and NO with other molecules under heat stress will provide us with a comprehensive knowledge of plants mechanism of heat tolerance which could be useful to develop heat stress-resistant varieties.

Insights into heat response mechanisms in Clematis species: physiological analysis, expression profiles and function verification

Our results provide insights into heat response mechanisms among Clematis species. Overexpressing CvHSFA2 enhanced the heat resistance of yeast and silencing NbHSFA2 reduced the heat resistance of tobacco. Clematis species are commonly grown in western and Japanese gardens. Heat stress can inhibit many physiological processes mediating plant growth and development. The mechanism regulating responses to heat has been well characterized in Arabidopsis thaliana and some crops, but not in horticultural plants, including Clematis species. In this study, we found that Clematis alpina 'Stolwijk Gold' was heat-sensitive whereas Clematis vitalba and Clematis viticella 'Polish Spirit' were heat-tolerant based on the physiological analyses in heat stress. Transcriptomic profiling identified a set of heat tolerance-related genes (HTGs). Consistent with the observed phenotype in heat stress, 41.43% of the differentially expressed HTGs between heat treatment and control were down-regulated in heat-sensitive cultivar Stolwijk Gold, but only 9.80% and 20.79% of the differentially expressed HTGs in heat resistant C. vitalba and Polish Spirit, respectively. Co-expression network, protein-protein interaction network and phylogenetic analysis revealed that the genes encoding heat shock transcription factors (HSFs) and heat shock proteins (HSPs) may played an essential role in Clematis resistance to heat stress. Two clades of heat-induced CvHSFs were further identified by phylogenetic tree, motif analysis and qRT-PCR. Ultimately, we proposed that overexpressing CvHSFA2-2 could endow yeast with high temperature resistance and silencing its homologous gene NbHSFA2 reduced the heat resistance of tobacco. This study provides first insights into the diversity of the heat response mechanisms among Clematis species.

Magneto-priming promotes nitric oxide via nitric oxide synthase to ameliorate the UV-B stress during germination of soybean seedlings

We have evaluated the contribution of nitric oxide (NO) in static magnetic field (SMF-200 mT for 1h) induced tolerance towards UV-B stress in soybean seedlings using various NO modulators like sodium nitroprusside (SNP), inhibitor of nitrate reductase (NR) sodium tungstate (ST), NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) and diphenylene iodonium (DPI) a NADPH oxidase inhibitor. The UV-B exposure significantly reduced germination, seedling growth together with activities of total amylase, NOS and NR in seedlings from un-primed seeds whereas SMF-primed seedlings showed significant enhancement in all these parameters along with higher level of NO/ROS. The supply of NO donor, SNP further improved all the seedlings parameters in un-primed and SMF-primed seeds after UV-B exposure. While ST, L-NAME and DPI significantly reduced the SMF-induced seedling performance after UV-B exposure. The gene expression study also showed significant up-regulation of α-amylase (GmAMY1, GmAMY2), nitric oxide synthase (GmNOS2) and nitrate reductase (GmNR2) encoding genes in UV-B exposed SMF-primed seedlings over un-primed seedlings. In particular, SNP+UV-B treatment enhanced the GmNOS2 expression in both unprimed (31.9-fold) and SMF-primed (93.2-fold) seedlings in comparison to their respective controls of CK+UV-B. In contrast, L-NAME+UV-B treatment reduced the SMF-induced GmNOS2 expression (4.8-fold) and NOS activity (76%). It confirmed that NO may be the key signaling molecule in SMF stimulated tolerance towards UV-B stress during early seedling growth and NOS may possibly be accountable for SMF-triggered NO production in soybean seedlings exposed to UV-B irradiations.

Mitogen-activated protein kinase action in plant response to high-temperature stress: a mini review

In recent years, extreme weather events such as high temperature (HT) are becoming more frequent. HT has become one of the main environmental factors affecting crop growth and development. In nature, plant cells initiate corresponding tolerant mechanisms by sensing and transducing HT signals. The mitogen-activated protein kinase (MAPK) cascade is widely involved in the signal transduction of plants to various environmental stresses. MAPK-mediated HT responses have attracted more and more attention. We herein focus on the current state of knowledge of MAPK in the plant under HT stress and summarize the mechanisms of MAPK in HT response from Ca²⁺ signal, reactive oxygen species (ROS) signal, heat shock transcription factor and heat shock protein, antioxidant system, and the direct downstream targets of MAPK. This review encapsulates the known plant MAPK cascade and provides prospects for ongoing research on HT response.

Dual Roles of GSNOR1 in Cell Death and Immunity in Tetraploid Nicotiana tabacum

S-nitrosoglutathione reductase 1 (GSNOR1) is the key enzyme that regulates cellular homeostasis of S-nitrosylation. Although extensively studied in Arabidopsis, the roles of GSNOR1 in tetraploid Nicotiana species have not been investigated previously. To study the function of NtGSNOR1, we knocked out two NtGSNOR1 genes simultaneously in Nicotiana tabacum using clustered regularly interspaced short palindromic repeats (CRISPR)/caspase 9 (Cas9) technology. To our surprise, spontaneous cell death occurred on the leaves of the CRISPR/Cas9 lines but not on those of the wild-type (WT) plants, suggesting that NtGSNOR1 negatively regulates cell death. The natural cell death on the CRISPR/Cas9 lines could be a result from interactions between overaccumulated nitric oxide (NO) and hydrogen peroxide (H₂O₂). This spontaneous cell death phenotype was not affected by knocking out two Enhanced disease susceptibility 1 genes (NtEDS11a/1b) and thus was independent of the salicylic acid (SA) pathway. Unexpectedly, we found that the NtGSNOR1a/1b knockout plants displayed a significantly (p < 0.001) enhanced resistance to paraquat-induced cell death compared to WT plants, suggesting that NtGSNOR1 functions as a positive regulator of the paraquat-induced cell death. The increased resistance to the paraquat-induced cell death of the NtGSNOR1a/1b knockout plants was correlated with the reduced level of H₂O₂ accumulation. Interestingly, whereas the N gene-mediated resistance to Tobacco mosaic virus (TMV) was significantly enhanced (p < 0.001), the resistance to Pseudomonas syringae pv. tomato DC3000 was significantly reduced (p < 0.01) in the NtGSNOR1a/1b knockout lines. In summary, our results indicate that NtGSNOR1 functions as both positive and negative regulator of cell death under different conditions and displays distinct effects on resistance against viral and bacterial pathogens.

Regulation of redox homeostasis in cadmium stressed rice field cyanobacteria by exogenous hydrogen peroxide and nitric oxide

In the present study, defensive strategies of H2O2 mediated NO signaling were analyzed in Cd stressed Nostoc muscorum and Anabaena sp. Exogenously supplied SNP (10 µM) and H2O2 (1 µM) lessen the toxicity of Cd (6 µM) but without NO; H2O2 was unable to release the stress from cyanobacterial cells potentially. The reduced contents of exopolysaccharide, protein content, endogenous NO and enzymatic antioxidants (SOD, POD, CAT, and GST) due to Cd toxicity, were found increased significantly after exogenous application of H2O2 and SNP thereafter, cyanobacterial calls flourished much better after releasing toxic level of Cd. Moreover, increased level of ROS due to Cd stress also normalized under exogenous application of H2O2 and SNP. However, chelation of NO hindered the signaling mechanism of H2O2 that diminished its potential against Cd stress while signaling of NO has not been hindered by chelation of H2O2 and NO potentially released the Cd stress from cyanobacterial cells. In conclusion, current findings demonstrated the synergistic signaling between H2O2 and NO towards the improvement of cyanobacterial tolerance to Cd stress, thereby enhancing the growth and antioxidant defense system of test cyanobacteria that improved fertility and productivity of soil even under the situation of metal contamination.

Nitric Oxide and Hydrogen Sulfide Coordinately Reduce Glucose Sensitivity and Decrease Oxidative Stress via Ascorbate-Glutathione Cycle in Heat-Stressed Wheat (Triticum aestivum L.) Plants

The involvement of nitric oxide (NO) and hydrogen sulfide (H₂S) in countermanding heat-inhibited photosynthetic features were studied in wheat (Triticum aestivum L.). Heat stress (HS) was employed at 40 °C after establishment for 6 h daily, and then plants were allowed to recover at 25 °C and grown for 30 days. Glucose (Glc) content increased under HS and repressed plant photosynthetic ability, but the application of sodium nitroprusside (SNP, as NO donor) either alone or with sodium hydrosulfide (NaHS, as H₂S donor) reduced Glc-mediated photosynthetic suppression by enhancing ascorbate-glutathione (AsA-GSH) metabolism and antioxidant system, which reduced oxidative stress with decreased H₂O₂ and TBARS content. Oxidative stress reduction or inhibiting Glc repression was maximum with combined SNP and NaHS treatment, which was substantiated by 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) and hypotaurine (HT), scavengers for NO and H₂S, respectively. The scavenge of H₂S reduced NO-mediated alleviation of HS suggesting of its downstream action in NO-mediated heat-tolerance. However, a simultaneous decrease of both (NO and H₂S) led to higher Glc-mediated repression of photosynthesis and oxidative stress in terms of increased H₂O₂ content that was comparable to HS plants. Thus, NO and H₂S cooperate to enhance photosynthesis under HS by reducing H₂O₂-induced oxidative stress and excess Glc-mediated photosynthetic suppression.

Cyclic Nucleotide-Gated Ion Channel 6 Mediates Thermotolerance in Arabidopsis Seedlings by Regulating Hydrogen Peroxide Production via Cytosolic Calcium Ions

We previously reported the involvement of cyclic nucleotide-gated ion channel 6 (CNGC6) and hydrogen peroxide (H₂O₂) in plant responses to heat shock (HS). To demonstrate their relationship with plant thermotolerance, we assessed the effect of HS on several groups of Arabidopsis (Arabidopsis thaliana) seedlings: wild-type, cngc6 mutant, and its complementation line. Under exposure to HS, the level of H₂O₂ was lower in the cngc6 mutant seedlings than in the wild-type (WT) seedlings but obviously increased in the complementation line. The treatment of Arabidopsis seeds with calcium ions (Ca²⁺) increased the H₂O₂ levels in the seedlings under HS treatment, whereas treatment with a Ca²⁺ chelator (EGTA) inhibited it, indicating that CNGC6 may stimulate the accumulation of H₂O₂ in a manner dependent on an increase in cytosolic Ca²⁺ ([Ca²⁺]_cyt). This point was verified by phenotypic observations and thermotolerance testing with transgenic plants overexpressing AtRbohB and AtRbohD (two genes involved in HS-responsive H₂O₂ production), respectively, in a cngc6 background. Real-time reverse transcription-polymerase chain reactions and Western blotting suggested that CNGC6 enhanced the gene transcription of HS factors (HSFs) and the accumulation of HS proteins (HSPs) via H₂O₂. These upon results indicate that H₂O₂ acts downstream of CNGC6 in the HS signaling pathway, increasing our understanding of the initiation of plants responses to high temperatures.

Putrescine regulates nitric oxide accumulation in Ganoderma lucidum partly by influencing cellular glutamine levels under heat stress

When fungi are subjected to abiotic stresses, the polyamines (PAs) level alter significantly. Here, we reveal that the polyamine putrescine (Put) could play an important role in alleviating heat stress(HS)-induced accumulation of nitric oxide (NO). Ornithine decarboxylase (ODC)-silenced mutants that were defective in Put biosynthesis exhibited significantly lower NO levels than the wild type (WT) when subjected to HS. With addition of 5 mM exogenous Put, the ODC-silenced mutant endogenous Put obviously increased under HS. At the same time, the contents of NO in the ODC-silenced mutants recovered to approximately WT levels after the administration of exogenous Put. However, the elevated NO content in the ODC-silenced mutants disappeared when exogenous Put and carboxy-PTIO (PTIO is a specific scavenger of NO) were added. Intriguingly, the content of glutamine (Gln) was significantly increased in the ODC-silenced strains. When exogenous Put was added to the WT, the Gln content was significantly decreased. The appearance of a high level of Gln was accompanied by nitrate reductase (NR) activity reduction. Further studies showed that Put influenced ganoderic acids (GAs) biosynthesis by regulating NO content, possibly through NR, under HS. Our work reported that Put regulates HS-induced NO accumulation by changing the cellular Gln level in filamentous fungi. IMPORTANCE: In our present work, it was HS as an ubiquitous environmental stress that affects the important pharmacological secondary metabolite (GAs) content in G. lucidum. Afterwards, we began to explore the network formed between multiple substances to jointly reduce the massive accumulation of GAs content caused by HS. We firstly focused on Put, a substance that enhances resistance to multiple stresses. Further, we discovered an influence on Put could changing the NO content, which has been shown to decrease the accumulation of GAs via HS. Then, we also found the change of NO content may be due to Put level that would affect intracellular Gln content. It has never been reported. And ultimately, it is Put related network that could reduce HS-inducing secondary metabolite mess in fungi.

Regulation of physiological aspects in plants by hydrogen sulfide and nitric oxide under challenging environment

Plants are exposed to a plethora of abiotic stresses such as drought, salinity, heavy metal and temperature stresses at different stages of their life cycle, from germination to seedling till the reproductive phase. As protective mechanisms, plants release signaling molecules that initiate a cascade of stress-signaling events, leading either to programmed cell death or plant acclimation. Hydrogen sulfide (H₂ S) and nitric oxide (NO) are considered as new 'gasotransmitter' molecules that play key roles in regulating gene expression, posttranslational modification (PTM), as well as cross-talk with other hormones. Although the exact role of NO in plants remains unclear and is species dependent, various studies have suggested a positive correlation between NO accumulation and environmental stress in plants. These molecules are also involved in a large array of stress responses and act synergistically or antagonistically as signaling components, depending on their respective concentration. This study provides a comprehensive update on the signaling interplay between H₂ S and NO in the regulation of various physiological processes under multiple abiotic stresses, modes of action and effects of exogenous application of these two molecules under drought, salt, heat and heavy metal stresses. However, the complete picture of the signaling cascades mediated by H₂ S and NO is still elusive. Recent researches indicate that during certain plant processes, such as stomatal closure, H₂ S could act upstream of NO signaling or downstream of NO in response to abiotic stresses by improving antioxidant activity in most plant species. In addition, PTMs of antioxidative pathways by these two molecules are also discussed.

Arabidopsis cyclic nucleotide-gated channel 6 is negatively modulated by multiple calmodulin isoforms during heat shock

An increased concentration of cytosolic Ca2+ is an early response of plant cells to heat shock. Arabidopsis cyclic nucleotide-gated ion channel 6 (CNGC6) mediates heat-induced Ca2+ influx and is activated by cAMP. However, it remains unclear how the Ca2+ conductivity of CNGC6 is negatively regulated under the elevated cytosolic Ca2+ concentration. In this study, Arabidopsis calmodulin isoforms CaM1/4, CaM2/3/5, CaM6, and CaM7 were found to bind to CNGC6 to varying degrees, and this binding was dependent on the presence of Ca2+ and IQ6, an atypical isoleucine-glutamine motif in CNGC6. Knockout of CaM2, CaM3, CaM5, and CaM7 genes led to a marked increase in plasma membrane inward Ca2+ current under heat shock conditions; however, knockout of CaM1, CaM4, and CaM6 genes had no significant effect on plasma membrane Ca2+ current. Moreover, the deletion of IQ6 from CNGC6 led to a marked increase in plasma membrane Ca2+ current under heat shock conditions. Taken together, the data suggest that CNGC6-mediated Ca2+ influx is likely to be negatively regulated by CaM2/3/5 and CaM7 isoforms under heat shock conditions, and that IQ6 plays an important role in CaM binding and the feedback regulation of the channel.

Nitric oxide molecular targets: reprogramming plant development upon stress

Plants are sessile organisms that need to complete their life cycle by the integration of different abiotic and biotic environmental signals, tailoring developmental cues and defense concomitantly. Commonly, stress responses are detrimental to plant growth and, despite the fact that intensive efforts have been made to understand both plant development and defense separately, most of the molecular basis of this trade-off remains elusive. To cope with such a diverse range of processes, plants have developed several strategies including the precise balance of key plant growth and stress regulators [i.e. phytohormones, reactive nitrogen species (RNS), and reactive oxygen species (ROS)]. Among RNS, nitric oxide (NO) is a ubiquitous gasotransmitter involved in redox homeostasis that regulates specific checkpoints to control the switch between development and stress, mainly by post-translational protein modifications comprising S-nitrosation of cysteine residues and metals, and nitration of tyrosine residues. In this review, we have sought to compile those known NO molecular targets able to balance the crossroads between plant development and stress, with special emphasis on the metabolism, perception, and signaling of the phytohormones abscisic acid and salicylic acid during abiotic and biotic stress responses.

The function of S-nitrosothiols during abiotic stress in plants

Nitric oxide (NO) is an active redox molecule involved in the control of a wide range of functions integral to plant biology. For instance, NO is implicated in seed germination, floral development, senescence, stomatal closure, and plant responses to stress. NO usually mediates signaling events via interactions with different biomolecules, for example the modulation of protein functioning through post-translational modifications (NO-PTMs). S-nitrosation is a reversible redox NO-PTM that consists of the addition of NO to a specific thiol group of a cysteine residue, leading to formation of S-nitrosothiols (SNOs). SNOs are more stable than NO and therefore they can extend and spread the in vivo NO signaling. The development of robust and reliable detection methods has allowed the identification of hundreds of S-nitrosated proteins involved in a wide range of physiological and stress-related processes in plants. For example, SNOs have a physiological function in plant development, hormone metabolism, nutrient uptake, and photosynthesis, among many other processes. The role of S-nitrosation as a regulator of plant responses to salinity and drought stress through the modulation of specific protein targets has also been well established. However, there are many S-nitrosated proteins that have been identified under different abiotic stresses for which the specific roles have not yet been identified. In this review, we examine current knowledge of the specific role of SNOs in the signaling events that lead to plant responses to abiotic stress, with a particular focus on examples where their functions have been well characterized at the molecular level.

Alternative oxidase is an important player in the regulation of nitric oxide levels under normoxic and hypoxic conditions in plants

Plant mitochondria possess two different pathways for electron transport from ubiquinol: the cytochrome pathway and the alternative oxidase (AOX) pathway. The AOX pathway plays an important role in stress tolerance and is induced by various metabolites and signals. Previously, several lines of evidence indicated that the AOX pathway prevents overproduction of superoxide and other reactive oxygen species. More recent evidence suggests that AOX also plays a role in regulation of nitric oxide (NO) production and signalling. The AOX pathway is induced under low phosphate, hypoxia, pathogen infections, and elicitor treatments. The induction of AOX under aerobic conditions in response to various stresses can reduce electron transfer through complexes III and IV and thus prevents the leakage of electrons to nitrite and the subsequent accumulation of NO. Excess NO under various stresses can inhibit complex IV; thus, the AOX pathway minimizes nitrite-dependent NO synthesis that would arise from enhanced electron leakage in the cytochrome pathway. By preventing NO generation, AOX can reduce peroxynitrite formation and tyrosine nitration. In contrast to its function under normoxia, AOX has a specific role under hypoxia, where AOX can facilitate nitrite-dependent NO production. This reaction drives the phytoglobin-NO cycle to increase energy efficiency under hypoxia.

Cyclic nucleotide-gated ion channel 6 mediates thermotolerance in Arabidopsis seedlings by regulating nitric oxide production via cytosolic calcium ions

BACKGROUND

We previously reported the involvement of nitric oxide (NO) and cyclic nucleotide-gated ion channel 6 (CNGC6) in the responses of plants to heat shock (HS) exposure. To elucidate their relationship with heat tolerance in Arabidopsis thaliana, we examined the effects of HS on several groups of seedlings: wild type, cngc6, and cngc6 complementation and overexpression lines.

RESULTS

After HS exposure, the level of NO was lower in cngc6 seedlings than in wild-type seedlings but significantly elevated in the transgenic lines depending on CNGC6 expression level. The treatment of seeds with calcium ions (Ca²⁺) enhanced the NO level in Arabidopsis seedlings under HS conditions, whereas treatment with EGTA (a Ca²⁺ chelator) reduced it, implicating that CNGC6 stimulates the accumulation of NO depending on an increase in cytosolic Ca²⁺ ([Ca²⁺]_cyt). This idea was proved by phenotypic observations and thermotolerance testing of transgenic plants overexpressing NIA2 and NOA1, respectively, in a cngc6 background. Western blotting indicated that CNGC6 stimulated the accumulation of HS proteins via NO.

CONCLUSION

These data indicate that CNGC6 acts upstream of NO in the HS pathway, which improves our insufficient knowledge of the initiation of plant responses to high temerature.

A role for S-nitrosylation of the SUMO-conjugating enzyme SCE1 in plant immunity

SUMOylation, the covalent attachment of the small ubiquitin-like modifier (SUMO) to target proteins, is emerging as a key modulator of eukaryotic immune function. In plants, a SUMO1/2-dependent process has been proposed to control the deployment of host defense responses. The molecular mechanism underpinning this activity remains to be determined, however. Here we show that increasing nitric oxide levels following pathogen recognition promote S-nitrosylation of the Arabidopsis SUMO E2 enzyme, SCE1, at Cys139. The SUMO-conjugating activities of both SCE1 and its human homolog, UBC9, were inhibited following this modification. Accordingly, mutation of Cys139 resulted in increased levels of SUMO1/2 conjugates, disabled immune responses, and enhanced pathogen susceptibility. Our findings imply that S-nitrosylation of SCE1 at Cys139 enables NO bioactivity to drive immune activation by relieving SUMO1/2-mediated suppression. The control of global SUMOylation is thought to occur predominantly at the level of each substrate via complex local machineries. Our findings uncover a parallel and complementary mechanism by suggesting that total SUMO conjugation may also be regulated directly by SNO formation at SCE1 Cys139. This Cys is evolutionary conserved and specifically S-nitrosylated in UBC9, implying that this immune-related regulatory process might be conserved across phylogenetic kingdoms.

The nitric oxide suppressed Arabidopsis mutants- Atnoa1 and Atnia1nia2noa1-2 produce nitric oxide in MS growth medium and on uranium exposure

The mutants Atnoa1 and Atnia1nia2noa1-2 having a defective chloroplast developmental process, showed enhanced chlorophyll levels when they were grown on Murashige and Skoog (MS) medium and on exposure with uranium (U) on Hoagland medium. Thus we hypothesized that these mutants probably produced NO in MS medium and on exposure with U. Wild-type Col-0, Atnoa1, Atnia1nia2noa1-2 plants were cultured on modified Hoagland and 1/10 MS media and NO generation in the roots of these mutants was monitored using NO selective fluorescent dyes, DAF-2DA and Fl2E. Both Atnoa1 and Atnia1nia2noa1-2 triple mutants produced NO as observed by increases in DAF-2T and Fl2E fluorescence when these mutants were grown on MS medium but not on Hoagland medium. In presence of NO scavenger, methylene blue (MB, 200 μM), DAF-2T and Fl2E fluorescence was completely abolished. On the other hand treatment of the plants with 25 μM U triggered NO generation. U-treated Atnoa1 and Atnia1nia2noa1-2 plants upregulated genes (POR B, POR D, CHL D) involved in the chlorophyll biosynthesis. From these results it was concluded that Atnoa1 and Atnia1nia2noa1-2 are conditional NO producers and it appears that NO generation in plants substantially depends on growth medium and NIA1, NIA2 or NOA1 does not appear to be really involved in NO generation in MS medium or after U exposure.

A Gene Regulatory Network Controlled by BpERF2 and BpMYB102 in Birch under Drought Conditions

Gene expression profiles are powerful tools for investigating mechanisms of plant stress tolerance. Betula platyphylla (birch) is a widely distributed tree, but its drought-tolerance mechanism has been little studied. Using RNA-Seq, we identified 2917 birch genes involved in its response to drought stress. These drought-responsive genes include the late embryogenesis abundant (LEA) family, heat shock protein (HSP) family, water shortage-related and ROS-scavenging proteins, and many transcription factors (TFs). Among the drought-induced TFs, the ethylene responsive factor (ERF) and myeloblastosis oncogene (MYB) families were the most abundant. BpERF2 and BpMYB102, which were strongly induced by drought and had high transcription levels, were selected to study their regulatory networks. BpERF2 and BpMYB102 both played roles in enhancing drought tolerance in birch. Chromatin immunoprecipitation combined with qRT-PCR indicated that BpERF2 regulated genes such as those in the LEA and HSP families, while BpMYB102 regulated genes such as Pathogenesis-related Protein 1 (PRP1) and 4-Coumarate:Coenzyme A Ligase 10 (4CL10). Multiple genes were regulated by both BpERF2 and BpMYB102. We further characterized the function of some of these genes, and the genes that encode Root Primordium Defective 1 (RPD1), PRP1, 4CL10, LEA1, SOD5, and HSPs were found to be involved in drought tolerance. Therefore, our results suggest that BpERF2 and BpMYB102 serve as transcription factors that regulate a series of drought-tolerance genes in B. platyphylla to improve drought tolerance.

Interdependent nitric oxide and hydrogen peroxide independently regulate the coix seed oil-induced triterpene acid accumulation in Ganoderma lingzhi

Recent progress has been made in adding exogenous vegetable oils in culture media to promote bioactive metabolite production in several medicinal mushrooms, but the mechanism is still unclear. In this study, we found that the vegetable oil coix seed oil (CSO) could induce the biosynthesis of triterpene acids (TAs) and also significantly increase cytoplasmic nitric oxide (NO) and hydrogen peroxide (H₂O₂) concentrations in the mycelium of Ganoderma lingzhi. The change in TA biosynthesis caused by CSO could be reversed by adding NO scavenger or H₂O₂ scavenger, and adding NO scavenger or H₂O₂ scavenger resulted in the reduction of the cytoplasmic H₂O₂ or NO concentration under CSO treatment, respectively. Moreover, adding NO scavenger or H₂O₂ scavenger reversed TA biosynthesis, which could be rescued by H₂O₂ or NO donor, respectively. Taken together, our study indicated that both NO and H₂O₂ were involved in the regulation of TA biosynthesis, and CSO-activated NO and H₂O₂ were interdependent but independently regulated the TA biosynthesis under CSO treatment in G. lingzhi.

Can wheat survive in heat? Assembling tools towards successful development of heat stress tolerance in Triticum aestivum L

Wheat is an important cereal crop that fulfils the calorie demands of the global humanity. Rapidly expanding populations are exposed to a fast approaching acute shortage in the adequate supply of food and fibre from agricultural resources. One of the significant threats to food security lies in the constantly increasing global temperatures which inflict serious injuries to the plants in terms of various physiological, biochemical and molecular processes. Wheat being a cool season crop is majorly impacted by the heat stress which adversely affects crop productivity and yield. These challenges would be potentially defeated with the implementation of genetic engineering strategies coupled with the new genome editing approaches. Development of transgenic plants for various crops has proved very effective for the incorporation of improved varietal traits in context of heat stress. With a similar approach, we need to target for the generation of heat stress tolerant wheat varieties which are capable of survival in such adverse conditions and yet produce well. In this review, we enumerate the current status of research on the heat stress responsive genes/factors and their potential role in mitigating heat stress in plants particularly in wheat with an aim to help the researchers get a holistic view of this topic. Also, we discuss on the prospective signalling pathway that is triggered in plants in general under heat stress.

AtCaM4 interacts with a Sec14-like protein, PATL1, to regulate freezing tolerance in Arabidopsis in a CBF-independent manner

Calmodulin (CaM), a multifunctional Ca2+ sensor, mediates multiple reactions involved in regulation of plant growth and responses to environmental stress. In this study, we found that AtCaM4 plays a negative role in freezing tolerance in Arabidopsis. The deletion of AtCaM4 resulted in enhanced freezing tolerance in cam4 mutant plants. Although AtCaM4 and AtCaM1 were cold-induced isoforms, cam4/cam1Ri double-mutant and cam4 single-mutant plants exhibited similar improvements in freezing tolerance, indicating that AtCaM4 plays major role. Furthermore, we found that AtCaM4 may influence freezing tolerance in a C-repeat binding factor (CBF)-independent manner as cold-induced expression patterns of CBFs did not change in the cam4/cam1Ri mutant. In addition, among the cold-responsive (COR) genes detected, KIN1, COR15b, and COR8.6 exhibited clearly enhanced expression over the long term in cam4/cam1Ri mutant plants exposed to cold stress. Using immunoprecipitation and mass spectrometry, we identified multiple candidate AtCaM4-interacting proteins. Co-immunoprecipitation assays confirmed the interaction of AtCaM4 with PATL1 in vivo and a phenotype analysis showed that patl1 mutant plants exhibited enhanced freezing tolerance. Thus, we conclude that AtCaM4 negatively regulates freezing tolerance in Arabidopsis by interacting with the novel CaM-binding protein PATL1.

Molecular mechanisms governing plant responses to high temperatures

The increased prevalence of high temperatures (HTs) around the world is a major global concern, as they dramatically affect agronomic productivity. Upon HT exposure, plants sense the temperature change and initiate cellular and metabolic responses that enable them to adapt to their new environmental conditions. Decoding the mechanisms by which plants cope with HT will facilitate the development of molecular markers to enable the production of plants with improved thermotolerance. In recent decades, genetic, physiological, molecular, and biochemical studies have revealed a number of vital cellular components and processes involved in thermoresponsive growth and the acquisition of thermotolerance in plants. This review summarizes the major mechanisms involved in plant HT responses, with a special focus on recent discoveries related to plant thermosensing, heat stress signaling, and HT-regulated gene expression networks that promote plant adaptation to elevated environmental temperatures.