γ-Aminobutyric acid plays a key role in plant acclimation to a combination of high light and heat stress

Characterization of the Calmodulin-Like Protein Family in Chara braunii and their Conserved Interaction with the Calmodulin-Binding Transcription Activator Family

Calcium sensor proteins play important roles by detecting changes in intracellular calcium and relaying that information onto downstream targets through protein-protein interaction. Very little is known about calcium sensors from plant species that predate land colonization and the evolution of embryophytes. Here, we examined the genome of the multicellular algae, Chara braunii, for orthologs to the evolutionarily conserved calcium sensor calmodulin (CaM) and for CaM-like (CML) proteins. We identified one CaM and eight CML isoforms that range in size from 16.4 to 21.3 kDa and are predicted to have between two to four calcium-binding (EF-hand) domains. Using recombinant protein, we tested whether CbCaM and CbCML1-CbCML7 possess biochemical properties of typical calcium sensors. CbCaM and the CbCMLs all displayed high-affinity calcium binding with estimated global KD,app values in the physiological µM range. In response to calcium binding, CbCaM and the CbCMLs exhibited varying degrees of increase in exposed hydrophobicity, suggesting that different calcium-induced conformational changes occur among isoforms. We found many examples of putative CaM targets encoded in the C. braunii genome and explored the ability of CbCaM and CbCMLs to interact in planta with a representative putative target, a C. braunii CaM-binding transcription factor (CbCAMTA1). CbCaM, CbCML2 and CbCML4 each associated with the C-terminal region of CbCAMTA1. Collectively, our data support the hypothesis that complex calcium signaling and sensing networks involving CaM and CMLs evolved early in the green lineage. Similarly, it seems likely that calcium-mediated regulation of transcription occurs in C. braunii via CAMTAs and is an ancient trait predating embryophytic emergence.

Gamma-aminobutyric acid as a regulator of astaxanthin production in Haematococcus lacustris under salinity: Exploring physiology, signaling, autophagy, and multi-omics landscape

Haematococcus lacustris-derived natural astaxanthin has significant commercial value, but stressful conditions alone impair cell growth and reduce the total productivity of astaxanthin in industrial settings. This study used gamma-aminobutyric acid (GABA) to increase biomass, astaxanthin productivity, and tolerance to salinity. GABA under NaCl stress enhanced the biomass to 1.76 g/L, astaxanthin content to 30.37 mg g-1, and productivity to 4.10 mg/L d-1, outperforming the control. Further analysis showed GABA enhanced nitrogen assimilation, Ca2+ level, and cellular GABA content, boosting substrate synthesis, energy metabolism, osmoregulation, autophagy, and antioxidant defenses. GABA also activated signaling pathways involving phytohormones, cAMP, cGMP, and MAPK, aiding astaxanthin synthesis. The application of biomarkers (ethylene, salicylic acid, trans-zeatin) and an autophagy inhibitor cooperated with GABA to further enhance the total astaxanthin productivity under NaCl stress. Combining GABA with 25 μM salicylic acid maximized astaxanthin yield at 4.79 mg/L d-1, offering new strategies for industrial astaxanthin production.

Advances in Plant GABA Research: Biological Functions, Synthesis Mechanisms and Regulatory Pathways

The γ-aminobutyric acid (GABA) is a widely distributed neurotransmitter in living organisms, known for its inhibitory role in animals. GABA exerts calming effects on the mind, lowers blood pressure in animals, and enhances stress resistance during the growth and development of plants. Enhancing GABA content in plants has become a focal point of current research. In plants, GABA is synthesized through two metabolic pathways, the GABA shunt and the polyamine degradation pathway, with the GABA shunt being the primary route. Extensive studies have investigated the regulatory mechanisms governing GABA synthesis. At the genetic level, GABA production and degradation can be modulated by gene overexpression, signaling molecule-induced expression, transcription factor regulation, and RNA interference. Additionally, at the level of transporter proteins, increased activity of GABA transporters and proline transporters enhances the transport of glutamate and GABA. The activity of glutamate decarboxylase, a key enzyme in GABA synthesis, along with various external factors, also influences GABA synthesis. This paper summarizes the biological functions, metabolic pathways, and regulatory mechanisms of GABA, providing a theoretical foundation for further research on GABA in plants.

Relationship between the GABA Pathway and Signaling of Other Regulatory Molecules

GABA (gamma-aminobutyric acid) is an amino acid whose numerous regulatory functions have been identified in animal organisms. More and more research indicate that in plants, this molecule is also involved in controlling basic growth and development processes. As recent studies have shown, GABA plays an essential role in triggering plant resistance to unfavorable environmental factors, which is particularly important in the era of changing climate. The main sources of GABA in plant cells are glutamic acid, converted in the GABA shunt pathway, and polyamines subjected to oxidative degradation. The action of GABA is often related to the activity of other messengers, including phytohormones, polyamines, NO, H2O2, or melatonin. GABA can function as an upstream or downstream element in the signaling pathways of other regulators, acting synergistically or antagonistically with them to control cellular processes. Understanding the role of GABA and its interactions with other signaling molecules may be important for developing crop varieties with characteristics that enable adaptation to a changing environment.

MdSINA2-MdNAC104 Module Regulates Apple Alkaline Resistance by Affecting γ-Aminobutyric Acid Synthesis and Transport

Soil alkalization is an adverse factor limiting plant growth and yield. As a signaling molecule and secondary metabolite, γ-aminobutyric acid (GABA) responds rapidly to alkaline stress and enhances the alkaline resistance of plants. However, the molecular mechanisms by which the GABA pathway adapts to alkaline stress remain unclear. In this study, a transcription factor, MdNAC104 is identified, from the transcriptome of the alkaline-stressed roots of apple, which effectively reduces GABA levels and negatively regulates alkaline resistance. Nevertheless, applying exogenous GABA compensates the negative regulatory mechanism of overexpressed MdNAC104 on alkaline resistance. Further research confirms that MdNAC104 repressed the GABA biosynthetic gene MdGAD1/3 and the GABA transporter gene MdALMT13 by binding to their promoters. Here, MdGAD1/3 actively regulates alkaline resistance by increasing GABA synthesis, while MdALMT13 promotes GABA accumulation and efflux in roots, resulting in an improved resistance to alkaline stress. This subsequent assays reveal that MdSINA2 interacts with MdNAC104 and positively regulates root GABA content and alkaline resistance by ubiquitinating and degrading MdNAC104 via the 26S proteasome pathway. Thus, the study reveals the regulation of alkaline resistance and GABA homeostasis via the MdSINA2-MdNAC104-MdGAD1/3/MdALMT13 module in apple. These findings provide novel insight into the molecular mechanisms of alkaline resistance in plants.

Surviving a Double-Edged Sword: Response of Horticultural Crops to Multiple Abiotic Stressors

Climate change-induced weather events, such as extreme temperatures, prolonged drought spells, or flooding, pose an enormous risk to crop productivity. Studies on the implications of multiple stresses may vary from those on a single stress. Usually, these stresses coincide, amplifying the extent of collateral damage and contributing to significant financial losses. The breadth of investigations focusing on the response of horticultural crops to a single abiotic stress is immense. However, the tolerance mechanisms of horticultural crops to multiple abiotic stresses remain poorly understood. In this review, we described the most prevalent types of abiotic stresses that occur simultaneously and discussed them in in-depth detail regarding the physiological and molecular responses of horticultural crops. In particular, we discussed the transcriptional, posttranscriptional, and metabolic responses of horticultural crops to multiple abiotic stresses. Strategies to breed multi-stress-resilient lines have been presented. Our manuscript presents an interesting amount of proposed knowledge that could be valuable in generating resilient genotypes for multiple stressors.

Exogenous treatment with N-acetylglutamic acid confers tolerance to heat stress in plants

Heat stress, which occurs when temperatures exceed the optimal range for growth, challenges the maintenance of crop yield because it disrupts plant homeostasis at the cellular and developmental levels. Chemical priming, which can activate the response to environmental stress using chemical compounds, is a promising method of maintaining plant growth under stressful conditions. Recently, we found that the non-proteogenic amino acid N-acetylglutamic acid (NAG) confers tolerance to oxidative stress through the activation of genes related to scavenging reactive oxygen species in plants. However, it has been unknown whether NAG alleviates environmental stress except oxidative stress. Here, we revealed that the response to heat stress was enhanced by exogenous treatment with NAG in plants. NAG alleviated the reduction in chlorophyll content induced by heat stress in Arabidopsis thaliana. Gene expression analysis showed that NAG activates the transcription factor HSFA2, which is regarded as a master regulator of the transcriptional cascade in response to heat stress. NAG induces histone H4 acetylation, an active histone modification, at the HSFA2 locus, suggesting that NAG could activate the expression of HSFA2 based on epigenetic modifications such as histone acetylation. Additionally, we found that Oryza sativa treated with NAG showed tolerance to heat stress. These results suggest that NAG could be used for chemical priming in the maintenance of plant growth under heat-stress conditions.

WRKY48 negatively regulates plant acclimation to a combination of high light and heat stress

Plants growing under natural conditions experience high light (HL) intensities that are often accompanied by elevated temperatures. These conditions could affect photosynthesis, reduce yield, and negatively impact agricultural productivity. The combination of different abiotic challenges creates a new type of stress for plants by generating complex environmental conditions that often exceed the impact of their individual parts. Transcription factors (TFs) play a key role in integrating the different molecular signals generated by multiple stress conditions, orchestrating the acclimation response of plants to stress. In this study, we show that the TF WRKY48 negatively controls the acclimation of Arabidopsis thaliana plants to a combination of HL and heat stress (HL + HS), and its expression is attenuated by jasmonic acid under HL + HS conditions. Using comparative physiological and transcriptomic analyses between wild-type and wrky48 mutants, we further demonstrate that under control conditions, WRKY48 represses the expression of a set of transcripts that are specifically required for the acclimation of plants to HL + HS, hence its suppression during the HL + HS stress combination contributes to plant survival under these conditions. Accordingly, mutants that lack WRKY48 are more resistant to HL + HS, and transgenic plants that overexpress WRKY48 are more sensitive to it. Taken together, our findings reveal that WRKY48 is a negative regulator of the transcriptomic response of Arabidopsis to HL + HS and provide new insights into the complex regulatory networks of plant acclimation to stress combination.

The GABA shunt contributes to ROS homeostasis in guard cells of Arabidopsis

γ-Aminobutyric acid (GABA) accumulates rapidly under stress via the GABA shunt pathway, which has been implicated in reducing the accumulation of stress-induced reactive oxygen species (ROS) in plants. γ-Aminobutyric acid has been demonstrated to act as a guard-cell signal in Arabidopsis thaliana, modulating stomatal opening. Knockout of the major GABA synthesis enzyme Glutamate Decarboxylase 2 (GAD2) increases the aperture of gad2 mutants, which results in greater stomatal conductance and reduces water-use efficiency compared with wild-type plants. Here, we found that the additional loss of GAD1, GAD4, and GAD5 in gad2 leaves increased GABA deficiency but abolished the more open stomatal pore phenotype of gad2, which we link to increased cytosolic calcium (Ca2+ ) and ROS accumulation in gad1/2/4/5 guard cells. Compared with wild-type and gad2 plants, glutamate was ineffective in closing gad1/2/4/5 stomatal pores, whereas lowering apoplastic calcium, applying ROS inhibitors or complementation with GAD2 reduced gad1/2/4/5 guard-cell ROS, restored the gad2-like greater stomatal apertures of gad1/2/4/5 beyond that of wild-type. We conclude that GADs are important contributors to ROS homeostasis in guard cells likely via a Ca2+ -mediated pathway. As such, this study reveals greater complexity in GABA's role as a guard-cell signal and the interactions it has with other established signals.

γ-Aminobutyric acid enhances salt tolerance by sustaining ion homeostasis in apples

Soil salinization had become a global ecological problem, which restricts the plant growth, and the quantity and quality of fruits. As a signaling molecule, γ-Aminobutyric acid (GABA) mediates a series of physiological processes and stress responses. Our previous research showed that GABA could alleviate drought, low phosphorus, cadmium stresses in apples, but the further research about its physiological mechanisms under salt stress was even more needed. The present study showed that the inhibition of salt stress on plant growth might be effectively alleviated by the treatment of 0.5 mM GABA, and the osmotic balance and photosynthetic capacity of plants could be maintained. Exogenous GABA could effectively inhibit the enrichment of reactive oxygen species and the uptake of Na+, while maintaining ion homeostasis. The experiment results indicated GABA could markedly promote the expression amount of Na+ and K+ transport-related genes (e.g., HKT1, AKT1, NHX1, SOS1, SOS2, and SOS3) in apples under salt stress. Overexpression and interference (RNAi) of MdGAD1 in apple roots, which is a crucial enzyme in the GABA biosynthesis, affected the salt tolerance of plants. Transgenic apple plants with roots of overexpression MdGAD1 showed less relative electrolyte leakage and more expression level of related ion transport genes than CK group, but RNAi MdGAD1 led to the opposite results. These results indicated that GABA accumulation could effectively strengthen the resistance of apple plants to salt stress and alleviate the injury of apple seedlings resulted from salinity.

GABA: a key player of abiotic stress regulation

Abiotic stress is considered as the main culprit for reduction of global food production. Recent studies have reported GABA as a major regulator of abiotic stress and thus opening new avenues in research on emerging roles of GABA in abiotic stress acclimation in plants.

Enhancing crop resilience by harnessing the synergistic effects of biostimulants against abiotic stress

Plants experience constant exposed to diverse abiotic stresses throughout their growth and development stages. Given the burgeoning world population, abiotic stresses pose significant challenges to food and nutritional security. These stresses are complex and influenced by both genetic networks and environmental factors, often resulting in significant crop losses, which can reach as high as fifty percent. To mitigate the effects of abiotic stresses on crops, various strategies rooted in crop improvement and genomics are being explored. In particular, the utilization of biostimulants, including bio-based compounds derived from plants and beneficial microbes, has garnered considerable attention. Biostimulants offer the potential to reduce reliance on artificial chemical agents while enhancing nutritional efficiency and promoting plant growth under abiotic stress condition. Commonly used biostimulants, which are friendly to ecology and human health, encompass inorganic substances (e.g., zinc oxide and silicon) and natural substances (e.g., seaweed extracts, humic substances, chitosan, exudates, and microbes). Notably, prioritizing environmentally friendly biostimulants is crucial to prevent issues such as soil degradation, air and water pollution. In recent years, several studies have explored the biological role of biostimulants in plant production, focusing particularly on their mechanisms of effectiveness in horticulture. In this context, we conducted a comprehensive review of the existing scientific literature to analyze the current status and future research directions concerning the use of various biostimulants, such as plant-based zinc oxide, silicon, selenium and aminobutyric acid, seaweed extracts, humic acids, and chitosan for enhancing abiotic stress tolerance in crop plants. Furthermore, we correlated the molecular modifications induced by these biostimulants with different physiological pathways and assessed their impact on plant performance in response to abiotic stresses, which can provide valuable insights.

Rapid systemic responses of Arabidopsis to waterlogging stress

Waterlogging stress (WLS) negatively impacts the growth and yield of crops resulting in heavy losses to agricultural production. Previous studies have revealed that WLS induces a systemic response in shoots that is partially dependent on the plant hormones ethylene and abscisic acid. However, the role of rapid cell-to-cell signaling pathways, such as the reactive oxygen species (ROS) and calcium waves, in systemic responses of plants to WLS is unknown at present. Here, we reveal that an abrupt WLS treatment of Arabidopsis (Arabidopsis thaliana) plants growing in peat moss triggers systemic ROS and calcium wave responses and that the WLS-triggered ROS wave response of Arabidopsis is dependent on the ROS-generating RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD), calcium-permeable channels GLUTAMATE-LIKE RECEPTOR 3.3 and 3.6 (GLR3.3 and GLR3.6), and aquaporin PLASMA MEMBRANE INTRINSIC PROTEIN 2;1 (PIP2;1) proteins. We further show that WLS is accompanied by a rapid systemic transcriptomic response that is evident as early as 10 min following waterlogging initiation, includes many hypoxia-response transcripts, and is partially dependent on RBOHD. Interestingly, the abrupt WLS of Arabidopsis resulted in the triggering of a rapid hydraulic wave response and the transient opening of stomata on leaves. In addition, it induced in plants a heightened state of tolerance to a subsequent submergence stress. Taken together, our findings reveal that the initiation of WLS in plants is accompanied by rapid systemic physiological and transcriptomic responses that involve the ROS, calcium, and hydraulic waves, as well as the induction of hypoxia acclimation mechanisms in systemic tissues.

Abscisic acid promotes plant acclimation to the combination of salinity and high light stress

Plants encounter combinations of different abiotic stresses such as salinity (S) and high light (HL). These environmental conditions have a detrimental effect on plant growth and development, posing a threat to agricultural production. Metabolic changes play a crucial role in enabling plants to adapt to fluctuations in their environment. Furthermore, hormones such as abscisic acid (ABA), jasmonic acid (JA) and salicylic acid (SA) have been previously identified as regulators of plant responses to different abiotic stresses. Here we studied the response of Arabidopsis wild type (Col and Ler) plants and mutants impaired in hormone biosynthesis (aba2-11 and aba1-1 in ABA, aos in JA and sid2 in SA) to the combination of S and HL (S + HL). Our findings showed that aba2-11 plants displayed reduced growth, impaired photosystem II (PSII) function, increased leaf damage, and decreased survival compared to Col when subjected to stress combination. However, aos and sid2 mutants did not display significant changes in response to S + HL compared to Col, indicating a key role for ABA in promoting plant tolerance to S + HL and suggesting a marginal role for JA and SA in this process. In addition, we revealed differences in the metabolic response of plants to S + HL compared to S or HL. The analysis of altered metabolic pathways under S + HL suggested that the accumulation of flavonoids is ABA-dependent, whereas the accumulation of branched-chain amino acids (BCAAs) and proline is ABA-independent. Therefore, our study uncovered a key function for ABA in regulating the accumulation of different flavonoids in plants during S + HL.

Metabolic pathways engineering for drought or/and heat tolerance in cereals

Drought (D) and heat (H) are the two major abiotic stresses hindering cereal crop growth and productivity, either singly or in combination (D/+H), by imposing various negative impacts on plant physiological and biochemical processes. Consequently, this decreases overall cereal crop production and impacts global food availability and human nutrition. To achieve global food and nutrition security vis-a-vis global climate change, deployment of new strategies for enhancing crop D/+H stress tolerance and higher nutritive value in cereals is imperative. This depends on first gaining a mechanistic understanding of the mechanisms underlying D/+H stress response. Meanwhile, functional genomics has revealed several stress-related genes that have been successfully used in target-gene approach to generate stress-tolerant cultivars and sustain crop productivity over the past decades. However, the fast-changing climate, coupled with the complexity and multigenic nature of D/+H tolerance suggest that single-gene/trait targeting may not suffice in improving such traits. Hence, in this review-cum-perspective, we advance that targeted multiple-gene or metabolic pathway manipulation could represent the most effective approach for improving D/+H stress tolerance. First, we highlight the impact of D/+H stress on cereal crops, and the elaborate plant physiological and molecular responses. We then discuss how key primary metabolism- and secondary metabolism-related metabolic pathways, including carbon metabolism, starch metabolism, phenylpropanoid biosynthesis, γ-aminobutyric acid (GABA) biosynthesis, and phytohormone biosynthesis and signaling can be modified using modern molecular biotechnology approaches such as CRISPR-Cas9 system and synthetic biology (Synbio) to enhance D/+H tolerance in cereal crops. Understandably, several bottlenecks hinder metabolic pathway modification, including those related to feedback regulation, gene functional annotation, complex crosstalk between pathways, and metabolomics data and spatiotemporal gene expressions analyses. Nonetheless, recent advances in molecular biotechnology, genome-editing, single-cell metabolomics, and data annotation and analysis approaches, when integrated, offer unprecedented opportunities for pathway engineering for enhancing crop D/+H stress tolerance and improved yield. Especially, Synbio-based strategies will accelerate the development of climate resilient and nutrient-dense cereals, critical for achieving global food security and combating malnutrition.

Proposing Signaling Molecules as Key Optimization Targets for Intensifying the Phytochemical Biosynthesis Induced by Emerging Nonthermal Stress Pretreatments of Plant-Based Foods: A Focus on γ-Aminobutyric Acid

Emerging evidence has confirmed the role of emerging nonthermal stressors (e.g., electromagnetic fields, ultrasonication, plasma) in accumulating bioactive metabolites in plant-based food. However, the signal decoding mechanisms behind NonTt-driven phytochemical production remain unclear, hindering postharvest bioactive component intensification. This study aims to summarize the association between signaling molecules and bioactive secondary metabolite production under nonthermal conditions, demonstrating the feasibility of enhancing phytochemical accumulation through signaling molecule crosstalk manipulation. Nonthermal elicitors were found to be capable of inducing stress metabolisms and activating various signaling molecules, similar to conventional abiotic stress. A simplified pathway model for nonthermally induced γ-aminobutyric acid accumulation was proposed with reactive oxygen species and calcium signaling being versatile pathways responsive to nonthermal elicitors. Manipulating signal molecules/pathways under nonthermal conditions can intensify phytochemical biosynthesis. Further research is needed to integrate signaling molecule responses and metabolic network shifts in nonthermally stressed plant-based matrices, balancing quality modifications and intensification of food functionality potential.

Comparison of metabolomic reconfiguration between Columbia and Landsberg ecotypes subjected to the combination of high salinity and increased irradiance

BACKGROUND

Plants growing in the field are subjected to combinations of abiotic stresses. These conditions pose a devastating threat to crops, decreasing their yield and causing a negative economic impact on agricultural production. Metabolic responses play a key role in plant acclimation to stress and natural variation for these metabolic changes could be key for plant adaptation to fluctuating environmental conditions.

RESULTS

Here we studied the metabolomic response of two Arabidopsis ecotypes (Columbia-0 [Col] and Landsberg erecta-0 [Ler]), widely used as genetic background for Arabidopsis mutant collections, subjected to the combination of high salinity and increased irradiance. Our findings demonstrate that this stress combination results in a specific metabolic response, different than that of the individual stresses. Although both ecotypes displayed reduced growth and quantum yield of photosystem II, as well as increased foliar damage and malondialdehyde accumulation, different mechanisms to tolerate the stress combination were observed. These included a relocation of amino acids and sugars to act as potential osmoprotectants, and the accumulation of different stress-protective compounds such as polyamines or secondary metabolites.

CONCLUSIONS

Our findings reflect an initial identification of metabolic pathways that differentially change under stress combination that could be considered in studies of stress combination of Arabidopsis mutants that include Col or Ler as genetic backgrounds.

γ-Aminobutyric acid plays a key role in alleviating Glomerella leaf spot in apples

The fungal disease Glomerella leaf spot (GLS) seriously impacts apple production. As a nonprotein amino acid, γ-aminobutyric acid (GABA) is widely involved in biotic and abiotic stresses. However, it is not clear whether GABA is involved in a plant's response to GLS, nor is its molecular mechanism understood. Here, we found that exogenous GABA could significantly alleviate GLS, reduce lesion lengths, and increase antioxidant capacity. MdGAD1 was identified as a possible key gene for GABA synthesis in apple. Further analysis indicated that MdGAD1 promoted antioxidant capacity to improve apple GLS resistance in transgenic apple calli and leaves. Yeast one-hybrid analysis identified the transcription factor MdWRKY33 upstream of MdGAD1. Electrophoretic mobility shift assay, β-glucuronidase activity, and luciferase activity further supported that MdWRKY33 bound directly to the promoter of MdGAD1. The content of GABA and the transcription level of MdGAD1 in the MdWRKY33 transgenic calli were higher than that of the wild type. When MdWRKY33 transgenic calli and leaves were inoculated with GLS, MdWKRY33 positively regulated resistance to GLS. These results explained the positive regulatory effects of GABA on apple GLS and provided insight into the metabolic regulatory network of GABA.

Exogenous spermine alleviates the negative effects of combined salinity and paraquat in tomato plants by decreasing stress-induced oxidative damage

Plants are frequently exposed to different combinations of soil constraints including salinity and different herbicides. These abiotic conditions negatively affect photosynthesis, growth and plant development resulting in limitations in agriculture production. To respond to these conditions, plants accumulate different metabolites that restore cellular homeostasis and are key for stress acclimation processes. In this work, we analyzed the role of exogenous spermine (Spm), a polyamine involved in plant tolerance to abiotic stress, in tomato responses to the combination of salinity (S) and the herbicide paraquat (PQ). Our findings showed that application of Spm reduced leaf damage and enhanced survival, growth, photosystem II function and photosynthetic rate of tomato plants subjected to the combination of S and PQ. In addition, we revealed that exogenous Spm reduced H2O2 and malondialdehyde (MDA) accumulation in plants subjected to S+PQ, suggesting that the role of exogenous Spm in alleviating the negative effects of this stress combination could be attributed to a decrease in stress-induced oxidative damage in tomato plants. Taken together, our results identify a key role for Spm in improving plant tolerance to combined stress.

N-acetylglutamic acid alleviates oxidative stress based on histone acetylation in plants

Oxidative stress causes cellular damage and genomic instability through the accumulation of reactive oxygen species (ROS) in plants, resulting in reduced crop production. Chemical priming, which can enhance plant tolerance to environmental stress using functional chemical compounds, is expected to improve agricultural yield in various plants without genetic engineering. In the present study, we revealed that non-proteogenic amino acid N-acetylglutamic acid (NAG) can alleviate oxidative stress damage in Arabidopsis thaliana (Arabidopsis) and Oryza sativa (rice). Exogenous treatment with NAG prevented chlorophyll reduction induced by oxidative stress. The expression levels of ZAT10 and ZAT12, which are regarded as master transcriptional regulators in response to oxidative stress, increased following NAG treatment. Additionally, Arabidopsis plants treated with NAG showed enhanced levels of histone H4 acetylation at ZAT10 and ZAT12 with the induction of histone acetyltransferases HAC1 and HAC12. The results suggest that NAG could enhance tolerance to oxidative stress through epigenetic modifications and contribute to the improvement of crop production in a wide variety of plants under environmental stress.

Exogenous γ-aminobutyric acid (GABA) improves salt-inhibited nitrogen metabolism and the anaplerotic reaction of the tricarboxylic acid cycle by regulating GABA-shunt metabolism in maize seedlings

Salinity stress hampers the growth of most crop plants and reduces yield considerably. In addition to its role in metabolism, γ-aminobutyric acid (GABA) plays a special role in the regulation of salinity stress tolerance in plants, though the underlying physiological mechanism remains poorly understood. In order to study the physiological mechanism of GABA pathway regulated carbon and nitrogen metabolism and tis relationship with salt resistance of maize seedlings, we supplemented seedlings with exogenous GABA under salt stress. In this study, we showed that supplementation with 0.5 mmol·L^-1 (0.052 mg·g^-1) GABA alleviated salt toxicity in maize seedling leaves, ameliorated salt-induced oxidative stress, and increased antioxidant enzyme activity. Applying exogenous GABA maintained chloroplast structure and relieved chlorophyll degradation, thus improving the photosynthetic performance of the leaves. Due to the improvement in photosynthesis, sugar accumulation also increased. Endogenous GABA content and GABA transaminase (GABA-T) and succinate semialdehyde dehydrogenase (SSADH) activity were increased, while glutamate decarboxylase (GAD) activity was decreased, via the exogenous application of GABA under salt stress. Meanwhile, nitrogen metabolism and the tricarboxylic acid (TCA) cycle were activated by the supply of GABA. In general, through the regulation of GABA-shunt metabolism, GABA activated enzymes related to nitrogen metabolism and replenished the key substrates of the TCA cycle, thereby improving the balance of carbon and nitrogen metabolism of maize and improving salt tolerance.

The Role of γ-Aminobutyric Acid (GABA) in the Occurrence of Adventitious Roots and Somatic Embryos in Woody Plants

The occurrence of adventitious roots and somatic embryos is a crucial step in micropropagation that frequently limits the application of this technique in woody plants. Recent studies demonstrated that they can be negatively or positively regulated with γ-aminobutyric acid (GABA), which is a four-carbon non-proteinous amino acid that not only acts as a main inhibitory neurotransmitter in mammals. It has been reported that GABA affects plant growth and their response to stress although its mode of action is still unclear. This review dealt with the effects of GABA on adventitious root formation and growth as well as on somatic embryogenesis. Furthermore, we focused on discussing the interaction of GABA with phytohormones, such as auxin, ethylene, abscisic acid, and gibberellin, as well as with the carbon and nitrogen metabolism during adventitious root development. We suggested that research on GABA will contribute to the application of micropropagation in the recalcitrant fruit and forest species.

Exogenous GABA improves the resistance of apple seedlings to long-term drought stress by enhancing GABA shunt and secondary cell wall biosynthesis

Drought stress is an important factor limiting apple production. γ-Aminobutyric acid (GABA) exists widely in plants and participates in the response to abiotic stress as a metabolite or signaling molecule. The role of exogenous GABA in apple plants, response to long-term drought stress remains unclear. Our study confirmed that exogenous GABA affects the drought resistance of apple plants under long-term drought stress. We found that 1 mM exogenous GABA improved the resistance of apple seedlings to long-term drought stress. The plants showed better growth, less reactive oxygen radical accumulation, less damage to cell membranes and greater active photosynthetic capacity. Under long-term drought stress, exogenous GABA facilitated GABA shunt, resulting in more accumulation of organic acids, namely citric acid, succinic acid and malic acid, in roots and stems of apple seedlings. In addition, exogenous GABA upregulated the expression of cellulose-related genes and lignin-related genes, and activated secondary cell wall-related transcription factors to synthesize more cellulose and lignin. A multiple factorial analysis confirmed that the GABA shunt and the biosynthesis of cellulose and lignin substantially contributed to the growth of apple seedlings with the application of exogenous GABA under long-term drought stress. Our results suggested that exogenous GABA improved the resistance of apple seedlings to long-term drought stress by enhancing GABA shunt and secondary cell wall biosynthesis.

Reduction of heat stress pressure and activation of photosystem II repairing system are crucial for citrus tolerance to multiple abiotic stress combination

Drought, heat and high irradiance are abiotic stresses that negatively affect plant development and reduce crop productivity. The confluence of these three factors is common in nature, causing extreme situations for plants that compromise their viability. Drought and heat stresses increase the saturation of the photosystem reaction centers, increasing sensitivity to high irradiance. In addition, these stress conditions affect photosystem II (PSII) integrity, alter redox balance of the electron transport chain and decrease the photosynthetic rate. Here, we studied the effect of the stress combinations on the photosynthetic apparatus of two citrus genotypes, Carrizo citrange (Citrus sinensis × Poncirus trifoliata) and Cleopatra mandarin (Citrus reshni). Results obtained showed that physiological responses, such as modulation of stomatal aperture and transpiration rate, aimed to reduce leaf temperature, are key to diminishing heat impact on photosynthetic apparatus and increasing tolerance to double and triple combinations of drought, high irradiance and high temperatures. By using transcriptomic and proteomic analyses, we have demonstrated that under these abiotic stress combinations, Carrizo plants were able to increase expression of genes and proteins related to the photosystem repairing machinery (which better maintained the integrity of PSII) and other components of the photosynthetic apparatus. Our findings reveal crucial physiological and genetic responses in citrus to increase tolerance to the combination of multiple abiotic stresses that could be the basis for breeding programs that ensure a sustainable citrus production.

Unraveling metabolic patterns and molecular mechanisms underlying storability in sugar beet

BACKGROUND

Sugar beet is an important crop for sugar production. Sugar beet roots are stored up to several weeks post-harvest waiting for processing in the sugar factories. During this time, sucrose loss and invert sugar accumulation decreases the final yield and processing quality. To improve storability, more information about post-harvest metabolism is required. We investigated primary and secondary metabolites of six sugar beet varieties during storage. Based on their variety-specific sucrose loss, three storage classes representing well, moderate, and bad storability were compared. Furthermore, metabolic data were visualized together with transcriptome data to identify potential mechanisms involved in the storage process.

RESULTS

We found that sugar beet varieties that performed well during storage have higher pools of 15 free amino acids which were already observable at harvest. This storage class-specific feature is visible at harvest as well as after 13 weeks of storage. The profile of most of the detected organic acids and semi-polar metabolites changed during storage. Only pyroglutamic acid and two semi-polar metabolites, including ferulic acid, show higher levels in well storable varieties before and/or after 13 weeks of storage. The combinatorial OMICs approach revealed that well storable varieties had increased downregulation of genes involved in amino acid degradation before and after 13 weeks of storage. Furthermore, we found that most of the differentially genes involved in protein degradation were downregulated in well storable varieties at both timepoints, before and after 13 weeks of storage.

CONCLUSIONS

Our results indicate that increased levels of 15 free amino acids, pyroglutamic acid and two semi-polar compounds, including ferulic acid, were associated with a better storability of sugar beet taproots. Predictive metabolic patterns were already apparent at harvest. With respect to elongated storage, we highlighted the role of free amino acids in the taproot. Using complementary transcriptomic data, we could identify potential underlying mechanisms of sugar beet storability. These include the downregulation of genes for amino acid degradation and metabolism as well as a suppressed proteolysis in the well storable varieties.

The effects of secondary bacterial metabolites on photosynthesis in microalgae cells

Secondary metabolites of bacteria are regulatory molecules that act as "info-chemicals" that control some metabolic processes in the cells of microorganisms. These molecules provide the function of bacteria communication in microbial communities. As primary producers of organic matter in the biosphere, microalgae play a central ecological role in various ecosystems. Photosynthesis is a central process in microalgae cells, and it is exposed to various biotic and abiotic factors. Various secondary metabolites of bacteria confer a noticeable regulatory effect on photosynthesis in microalgae cells. The main purpose of this review is to highlight recent experimental results that demonstrate the impact of several types of common bacterial metabolites (volatile organic compounds, non-protein amino acids, and peptides) on photosynthetic activity in cells of microalgae. The use of these molecules as herbicides can be of great importance both for practical applications and for basic research.

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.