Title: Enhanced salt tolerance and photosynthetic performance: Implication of ɤ-amino butyric acid application in salt-exposed lettuce (Lactuca sativa L.) 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.

Microbial Biocontrol Agents and Natural Products Act as Salt Stress Mitigators in Lactuca sativa L

One of the major problems related to climate change is the increase in land area affected by higher salt concentrations and desertification. Finding economically and environmentally friendly sustainable solutions that effectively mitigate salt stress damage to plants is of great importance. In our work, some natural products and microbial biocontrol agents were evaluated for their long-term effectiveness in reducing salt stress in lettuce (Lactuca sativa L. var. romana) plants. Fourteen different treatments applied to soil pots, with and without salt stress, were analyzed using biometric (leaf and root length and width), physiological (chlorophyll and proline content), and morphological (microscopic preparations) techniques and NGS to study the microbial communities in the soil of plants subjected to different treatments. Under our long-term experimental conditions (90 days), the results showed that salt stress negatively affected plant growth. The statistical analysis showed a high variability in the responses of the different biostimulant treatments. Notably, the biocontrol agents Papiliotrema terrestris (strain PT22AV), Bacillus amyloliquefaciens (strain B07), and Rahnella aquatilis (strain 36) can act as salt stress mitigators in L. sativa. These findings suggest that both microbial biocontrol agents and certain natural products hold promise for reducing the adverse effects of salt stress on plants.

Mechanisms by Which Exogenous Substances Enhance Plant Salt Tolerance through the Modulation of Ion Membrane Transport and Reactive Oxygen Species Metabolism

Soil salinization is one of the major abiotic stresses affecting plant growth and development. Plant salt tolerance is controlled by complex metabolic pathways. Exploring effective methods and mechanisms to improve crop salt tolerance has been a key aspect of research on the utilization of saline soil. Exogenous substances, such as plant hormones and signal transduction substances, can regulate ion transmembrane transport and eliminate reactive oxygen species (ROS) to reduce salt stress damage by activating various metabolic processes. In this review, we summarize the mechanisms by which exogenous substances regulate ion transmembrane transport and ROS metabolism to improve plant salt tolerance. The molecular and physiological relationships among exogenous substances in maintaining the ion balance and enhancing ROS clearance are examined, and trends and research directions for the application of exogenous substances for improving plant salt tolerance are proposed.

Gamma-aminobutyric acid treatment promotes resistance against Sogatella furcifera in rice

The Sogatella furcifera (Horváth) (Homoptera: Delphacidae) is a white-backed planthopper (WBPH) that causes "hopper burn" in rice, resulting in severe yield loss. Gamma-aminobutyric acid (GABA) is a well-known neurotransmitter that inhibits neurotransmission in insects by binding to specific receptors. In this study, we investigated the potential role of GABA in modulating rice resistance to WBPH and evaluated possible defense mechanisms. The experiment was conducted in green house in pots consist of four groups: control, GABA-treated, WBPH-infested, and WBPH-infested treated with GABA. Among the various tested concentration of GABA, 15 mM GABA was applied as a single treatment in water. The treatment was administered one week before WBPH infestation. The results revealed that 15 mM GABA treatment strongly increased WBPH resistance. A plate-based assay indicated that direct application of 15 mM GABA increased the mortality rate of WBPH and increased the damage recovery rate in rice plants. We found that GABA treatment increased the activation of antioxidant enzymes and reduced the reactive oxygen species content and malondialdehyde contents, and reduced the damage rate caused by WBPH. Interestingly, GABA-supplemented plants infested with WBPH exhibited increased phenylalanine ammonia-lyase and pathogenesis-related (PR) genes expression levels. GABA induced the accumulation of abscisic acid (ABA) and salicylic acid (SA) and enhanced the stomata closure and reduced leaf vessels to reduce water conductance during WBPH stress. Furthermore, we found that GABA application to the plant induced the expression of Jasmonic acid (JA) biosynthesis genes (LOX, AOS, AOC, and OPR) and melatonin biosynthesis-related genes (TDC, T5H, ASMT, and SNAT). Our study suggested that GABA increases resistance against WBPH infestation by regulating antioxidant defense system, TCA cycle regulation, phytohormonal signaling, and PR gene regulation.

Improving postharvest quality and vase life of cut rose flowers by pre-harvest foliar co-applications of γ-aminobutyric acid and calcium chloride

Rose flowers (Rosa hybrida L.) are highly perishable and have a limited vase life. This study evaluated the effects of preharvest foliar applications of γ-aminobutyric acid (GABA) and calcium chloride (CaCl2), individually and combined, on antioxidant responses and vase life of cut Jumilia rose flowers. Treatments included foliar sprays of GABA at 0, 20, 40, and 60 mM and CaCl2 at 0, 0.75%, and 1.5%, applied in a factorial design within a completely randomized setup before harvest. Results showed GABA and CaCl2 interaction (especially, 60 mM GABA and 1.5% CaCl2) significantly increased enzymatic antioxidants including superoxide dismutase, catalase, and peroxidase, as well as non-enzymatic antioxidants such as flavonoids, carotenoids, phenolics, and antioxidant activity in petals compared to control. SOD activity in roses, treated with CaCl2 (1.5%) and GABA (60 mM), peaked at 7.86 units. mg-1 protein min-1, showing a nearly 2.93-fold increase over the control (2.68 units. mg-1 protein min-1). A parallel trend was observed for CAT activity. These treatments also reduced petal malondialdehyde content and polyphenol oxidase activity. Protein content and vase life duration increased in all treatments. Plants treated with a combination of GABA (20 mM) and CaCl2 (0.75%), GABA (60 mM) and CaCl2 (1.5%), or GABA (40 mM) individually exhibited the longest vase life duration. The co-application of GABA and CaCl2 improved the antioxidant activity and postharvest quality of cut roses by reducing PPO activity and MDA contents, increasing protein content and prolonging vase life. This treatment is a potential postharvest strategy to improve antioxidant capacity and delay senescence in cut roses.

Interplay among photoreceptors determines the strategy of coping with excess light in tomato

This study investigates photoreceptor's role in the adaption of photosynthetic apparatus to high light (HL) intensity by examining the response of tomato wild type (WT) (Solanum lycopersicum L. cv. Moneymaker) and tomato mutants (phyA, phyB1, phyB2, cry1) plants to HL. Our results showed a photoreceptor-dependent effect of HL on the maximum quantum yield of photosystem II (Fv/Fm) with phyB1 exhibiting a decrease, while phyB2 exhibiting an increase in Fv/Fm. HL resulted in an increase in the efficient quantum yield of photosystem II (ΦPSII) and a decrease in the non-photochemical quantum yields (ΦNPQ and ΦN0) solely in phyA. Under HL, phyA showed a significant decrease in the energy-dependent quenching component of NPQ (qE), while phyB2 mutants showed an increase in the state transition (qT) component. Furthermore, ΔΔFv/Fm revealed that PHYB1 compensates for the deficit of PHYA in phyA mutants. PHYA signaling likely emerges as the dominant effector of PHYB1 and PHYB2 signaling within the HL-induced signaling network. In addition, PHYB1 compensates for the role of CRY1 in regulating Fv/Fm in cry1 mutants. Overall, the results of this research provide valuable insights into the unique role of each photoreceptor and their interplay in balancing photon energy and photoprotection under HL condition.

Gamma-aminobutyric acid (GABA) improves salinity stress tolerance in soybean seedlings by modulating their mineral nutrition, osmolyte contents, and ascorbate-glutathione cycle

BACKGROUND

In plants, GABA plays a critical role in regulating salinity stress tolerance. However, the response of soybean seedlings (Glycine max L.) to exogenous gamma-aminobutyric acid (GABA) under saline stress conditions has not been fully elucidated.

RESULTS

This study investigated the effects of exogenous GABA (2 mM) on plant biomass and the physiological mechanism through which soybean plants are affected by saline stress conditions (0, 40, and 80 mM of NaCl and Na2SO4 at a 1:1 molar ratio). We noticed that increased salinity stress negatively impacted the growth and metabolism of soybean seedlings, compared to control. The root-stem-leaf biomass (27- and 33%, 20- and 58%, and 25- and 59% under 40- and 80 mM stress, respectively]) and the concentration of chlorophyll a and chlorophyll b significantly decreased. Moreover, the carotenoid content increased significantly (by 35%) following treatment with 40 mM stress. The results exhibited significant increase in the concentration of hydrogen peroxide (H2O2), malondialdehyde (MDA), dehydroascorbic acid (DHA) oxidized glutathione (GSSG), Na+, and Cl- under 40- and 80 mM stress levels, respectively. However, the concentration of mineral nutrients, soluble proteins, and soluble sugars reduced significantly under both salinity stress levels. In contrast, the proline and glycine betaine concentrations increased compared with those in the control group. Moreover, the enzymatic activities of ascorbate peroxidase, monodehydroascorbate reductase, glutathione reductase, and glutathione peroxidase decreased significantly, while those of superoxide dismutase, catalase, peroxidase, and dehydroascorbate reductase increased following saline stress, indicating the overall sensitivity of the ascorbate-glutathione cycle (AsA-GSH). However, exogenous GABA decreased Na+, Cl-, H2O2, and MDA concentration but enhanced photosynthetic pigments, mineral nutrients (K+, K+/Na+ ratio, Zn2+, Fe2+, Mg2+, and Ca2+); osmolytes (proline, glycine betaine, soluble sugar, and soluble protein); enzymatic antioxidant activities; and AsA-GSH pools, thus reducing salinity-associated stress damage and resulting in improved growth and biomass. The positive impact of exogenously applied GABA on soybean plants could be attributed to its ability to improve their physiological stress response mechanisms and reduce harmful substances.

CONCLUSION

Applying GABA to soybean plants could be an effective strategy for mitigating salinity stress. In the future, molecular studies may contribute to a better understanding of the mechanisms by which GABA regulates salt tolerance in soybeans.

Horticulture crop under pressure: Unraveling the impact of climate change on nutrition and fruit cracking

Climate change is increasingly affecting the nutritional content and structural integrity of horticultural crops, leading to challenges such as diminished fruit quality and the exacerbation of fruit cracking. This manuscript systematically explores the multifaceted impacts of these changes, with a particular focus on the nutritional quality and increased incidence of fruit cracking. An exhaustive review of current research identifies the critical role of transcription factors in mediating plant responses to climatic stressors, such as drought, temperature extremes, and saline conditions. The significance of transcription factors, including bHLH, bZIP, DOF, MDP, HD-ZIP, MYB, and ERF4, is highlighted in the development of fruit cracking, underscoring the genetic underpinnings behind stress-related phenotypic outcomes. The effectiveness of greenhouse structures in mitigating adverse climatic effects is evaluated, offering a strategic approach to sustain crop productivity amidst CO2 fluctuations and water scarcity, which are shown to influence plant physiology and lead to changes in fruit development, nutrient dynamics, and a heightened risk of cracking. Moreover, the manuscript delves into advanced breeding strategies and genetic engineering techniques, such as genome editing, to enhance crop resilience against climatic challenges. It also discusses adaptation strategies vital for sustainable horticulture, emphasizing the need to integrate novel genetic insights with controlled environment horticulture to counteract climate change's detrimental effects. The synthesis presented here underscores the urgent need for innovative breeding strategies aimed at developing resilient crop varieties that can withstand climatic uncertainty while preserving nutritional integrity.

"Deciphering the enigmatic role of gamma-aminobutyric acid (GABA) in plants: Synthesis, transport, regulation, signaling, and biological roles in interaction with growth regulators and abiotic stresses."

Gamma-aminobutyric acid (GABA) is an amino acid with a four-carbon structure, widely distributed in various organisms. It exists as a zwitterion, possessing both positive and negative charges, enabling it to interact with other molecules and participate in numerous physiological processes. GABA is widely distributed in various plant cell compartments such as cytoplasm mitochondria, vacuoles, peroxisomes, and plastids. GABA is primarily synthesized from glutamate using glutamate decarboxylase and participates in the GABA shunt within mitochondria, regulating carbon and nitrogen metabolism in plants The transport of GABA is regulated by several intracellular and intercellular transporters such as aluminium-activated malate transporters (ALMTs), GABA transporters (GATs), bidirectional amino acid transporters (BATs), and cationic amino acid transporters (CATs). GABA plays a vital role in cellular transformations, gene expression, cell wall modifications, and signal transduction in plants. Recent research has unveiled the role of GABA as a signaling molecule in plants, regulating stomatal movement and pollen tube growth. This review provides insights into multifaceted impact of GABA on physiological and biochemical traits in plants, including cellular communication, pH regulation, Krebs cycle circumvention, and carbon and nitrogen equilibrium. The review highlights involvement of GABA in improving the antioxidant defense system of plants, mitigating levels of reactive oxygen species under normal and stressed conditions. Moreover, the interplay of GABA with other plant growth regulators (PGRs) have also been explored.

Exogenous γ-Aminobutyric Acid Can Improve Seed Germination and Seedling Growth of Two Cotton Cultivars under Salt Stress

Excessive salt content in soil has adverse effects on cotton production, especially during the germination and seedling stages. γ-aminobutyric acid (GABA) is an important active substance that is expected to improve the resistance of plants to abiotic stresses. This study focused on two cotton cultivars (Gossypium hirsutum L.: Tahe 2 and Xinluzhong 62) and investigated the impact of exogenous GABA (0, 1, 2, 3, and 4 mM) on seed germination, seedling growth, and related morphological, physiological, and biochemical indicators under salt stress (150 mM NaCl). The results showed that salt stress significantly reduced the germination rate and germination index of cotton seeds (decreased by 20.34% and 32.14% for Tahe 2 and Xinluzhong 62, respectively), leading to decreased seedling height and biomass and causing leaf yellowing. Salt stress induced osmotic stress in seedlings, resulting in ion imbalance (marked reduction in K+/Na+ ratio) and oxidative damage. Under salt stress conditions, exogenous GABA increased the germination rate (increased by 10.64~23.40% and 2.63~31.58% for Tahe 2 and Xinluzhong 62, respectively) and germination index of cotton seeds, as well as plant height and biomass. GABA treatment improved leaf yellowing. Exogenous GABA treatment increased the content of proline and soluble sugars, with varying effects on betaine. Exogenous GABA treatment reduced the Na+ content in seedlings, increased the K+ content, and increased the K+/Na+ ratio (increased by 20.44~28.08% and 29.54~76.33% for Tahe 2 and Xinluzhong 62, respectively). Exogenous GABA treatment enhanced the activities of superoxide dismutase and peroxidase, and reduced the accumulation of hydrogen peroxide and malondialdehyde, but had a negative impact on catalase activity. In conclusion, exogenous GABA effectively improved cotton seed germination. By regulating osmoprotectant levels, maintaining ion homeostasis, and alleviating oxidative stress, GABA mitigated the adverse effects of salt stress on cotton seedling growth.

Optimizing supplemental light spectrum improves growth and yield of cut roses

During the seasons with limited light intensity, reductions in growth, yield, and quality are challenging for commercial cut rose production in greenhouses. Using artificial supplemental light is recommended for maintaining commercial production in regions with limited light intensity. Nowadays, replacing traditional lighting sources with LEDs attracted lots of attention. Since red (R) and blue (B) light spectra present the important wavelengths for photosynthesis and growth, in the present study, different ratios of supplemental R and B lights, including 90% R: B 10% (R90B10), 80% R: 20% B (R80B20), 70% R: 30% B (R70B30) with an intensity of 150 µmol m-2 s-1 together with natural light and without supplemental light (control) were applied on two commercial rose cultivars. According to the obtained results, supplemental light improved growth, carbohydrate levels, photosynthesis capacity, and yield compared to the control. R90B10 in both cultivars reduced the time required for flowering compared to the control treatment. R90B10 and R80B20 obtained the highest number of harvested flower stems in both cultivars. Chlorophyll and carotenoid levels were the highest under control. They had a higher ratio of B light, while carbohydrate and anthocyanin contents increased by having a high ratio of R light in the supplemental light. Analysis of chlorophyll fluorescence was indicative of better photosynthetic performance under a high ratio of R light in the supplemental light. In conclusion, the R90B10 light regime is recommended as a suitable supplemental light recipe to improve growth and photosynthesis, accelerate flowering, and improve the yield and quality of cut roses.

Microplastic stress in plants: effects on plant growth and their remediations

Microplastic (MP) pollution is becoming a global problem due to the resilience, long-term persistence, and robustness of MPs in different ecosystems. In terrestrial ecosystems, plants are exposed to MP stress, thereby affecting overall plant growth and development. This review article has critically analyzed the effects of MP stress in plants. We found that MP stress-induced reduction in plant physical growth is accompanied by two complementary effects: (i) blockage of pores in seed coat or roots to alter water and nutrient uptake, and (ii) induction of drought due to increased soil cracking effects of MPs. Nonetheless, the reduction in physiological growth under MP stress is accompanied by four complementary effects: (i) excessive production of ROS, (ii) alteration in leaf and root ionome, (iii) impaired hormonal regulation, and (iv) decline in chlorophyll and photosynthesis. Considering that, we suggested that targeting the redox regulatory mechanisms could be beneficial in improving tolerance to MPs in plants; however, antioxidant activities are highly dependent on plant species, plant tissue, MP type, and MP dose. MP stress also indirectly reduces plant growth by altering soil productivity. However, MP-induced negative effects vary due to the presence of different surface functional groups and particle sizes. In the end, we suggested the utilization of agronomic approaches, including the application of growth regulators, biochar, and replacing plastic mulch with crop residues, crop diversification, and biological degradation, to ameliorate the effects of MP stress in plants. The efficiency of these methods is also MP-type-specific and dose-dependent.

Artificial neural network (ANN)-based algorithms for high light stress phenotyping of tomato genotypes using chlorophyll fluorescence features

High light (HL) is a common environmental stress directly imposes photoinhibition on the photosynthesis apparatus. Breeding plants for tolerance against HL is therefore highly demanded. Chlorophyll fluorescence (ChlF) is a sensitive indicator of stress in plants and can be evaluated using OJIP transients. In this study, we compared the ChlF features of plants exposed to HL (1200 μmol m-2 s-1) with that of control plants (300 μmol m-2 s-1). To extract the most reliable ChlF features for discrimination between HL-stressed and non-stressed plants, we applied three artificial neural network (ANN)-based algorithms, namely, Boruta, Support Vector Machine (SVM), and Recursive Feature Elimination (RFE). Feature selection algorithms identified multiple features but only two features, namely the maximal quantum yield of PSII photochemistry (FV/FM) and quantum yield of energy dissipation (ɸD0), remained consistent across all genotypes in control conditions, while exhibited variation in HL. Therefore, considered reliable features for HL stress screening. The selected features were then used for screening 14 tomato genotypes for HL. Genotypes were categorized into three groups, tolerant, semi-tolerant, and sensitive genotypes. Foliar hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents were measured as independent proxies for benchmarking selected features. Tolerant genotypes were attributed with the lowest change in H2O2 and MDA contents, while the sensitive genotypes displayed the highest magnitude of increase in H2O2 and MDA by HL treatment compared to the control. Finally, a FV/FM higher than 0.77 and ɸD0 lower than 0.24 indicates a healthy electron transfer chain (ETC) when tomato plants are exposed to HL.

Role of Gamma-Aminobutyric Acid in Plant Defense Response

Gamma-aminobutyric acid (GABA) is a four-carbon non-protein amino acid that acts as a defense substance and a signaling molecule in various physiological processes, and which helps plants respond to biotic and abiotic stresses. This review focuses on the role of GABA's synthetic and metabolic pathways in regulating primary plant metabolism, redistributing carbon and nitrogen resources, reducing the accumulation of reactive oxygen species, and improving plants' tolerance of oxidative stress. This review also highlights the way in which GABA maintains intracellular pH homeostasis by acting as a buffer and activating H⁺-ATPase. In addition, calcium signals participate in the accumulation process of GABA under stress. Moreover, GABA also transmits calcium signals through receptors to trigger downstream signaling cascades. In conclusion, understanding the role of GABA in this defense response provides a theoretical basis for applying GABA in agriculture and forestry and feasible coping strategies for plants in complex and changeable environments.

Effects of foliage-applied exogenous γ-aminobutyric acid on seedling growth of two rice varieties under salt stress

Exogenous γ-aminobutyric acid (GABA) has been used and regarded as a potential enhancer for plant resistance against various biotic or abiotic attackers in the crop production, especially as a promising alleviator against salt stress. In order to determine whether GABA is truly effective in promoting rice resistance under a certain level of salt stress or not and to evaluate its effect on the growth and some physiological responses of two Japonica rice varieties under salt stress. 3-leaf rice seedlings germinated from seeds were cultivated in a separate hydroponic cup with a nutrient solution that was salinized with 0, 25, 50, or 75 mmol K+ of NaCl. A 4 mmol L-1 GABA solution or water were sprayed onto leaves once a day for 8 days prior to an assessment of the seedling growth, the growth indices, root activities and three antioxidant enzyme activities in leaves were measured. Data analyses indicated that as the salt concentration increased, the plant height and the leaf area of both rice varieties decreased, while the dead leaf rate, weight ratio of the dry- and fresh-roots, superoxide dismutase (SOD) and peroxidase (POD) activities increased. Under the same saline conditions, the root activities and the leaf ascorbate peroxidase (APX) activity were enhanced at a low NaCl concentration but reduced when the salt concentration was high. A foliar application of GABA daily on both rice varieties for over a week under 3 different salinized treatments as compared with the corresponding treatments sprayed with water resulted in an enhanced effect on plant height increment by 1.7-32.4%, a reduction of dead leaf rate by 1.6-36.4%, a decline of root dry weight by 9.3-30.9% respectively, and an increment in root activities by 8.1-114.5%, and POD, SOD and APX enzyme activities increased by 5.0-33.3%, 4.1-18.5%, and 7.2-64.4% respectively. However, two rice varieties showed a significant difference in response to various salinized levels. Overall results of this study demonstrate that the application of exogenous GABA on the leaves of rice seedlings under salt stress has improved rice salt tolerance, which should provide a sufficient information for ultimately making it possible to grow rice in salinized soil.

Exogenous γ-aminobutyric acid (GABA) mitigated salinity-induced impairments in mungbean plants by regulating their nitrogen metabolism and antioxidant potential

BACKGROUND

Increasing soil salinization has a detrimental effect on agricultural productivity.Therefore, strategies are needed to induce salinity-tolerance in crop species for sustainable foodproduction. γ-aminobutyric acid (GABA) plays a key role in regulating plant salinity stresstolerance. However, it remains largely unknown how mungbean plants (Vigna radiata L.) respondto exogenous GABA under salinity stress.

METHODS

Thus, we evaluated the effect of exogenous GABA (1.5 mM) on the growth and physiobiochemicalresponse mechanism of mungbean plants to saline stress (0-, 50-, and 100 mM [NaCland Na2SO4, at a 1:1 molar ratio]).

RESULTS

Increased saline stress adversely affected mungbean plants' growth and metabolism. Forinstance, leaf-stem-root biomass (34- and 56%, 31- and 53%, and 27- and 56% under 50- and 100mM, respectively]) and chlorophyll concentrations declined. The carotenoid level increased (10%)at 50 mM and remained unaffected at 100 mM. Hydrogen peroxide (H2O2), malondialdehyde(MDA), osmolytes (soluble sugars, soluble proteins, proline), total phenolic content, andenzymatic activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase(POD), glutathione reductase (GTR), and polyphenol oxidation (PPO) were significantlyincreased. In leaves, salinity caused a significant increase in Na+ concentration but a decrease inK+ concentration, resulting in a low K+/Na+ concentration (51- and 71% under 50- and 100- mMstress). Additionally, nitrogen concentration and the activities of nitrate reductase (NR) andglutamine synthetase (GS) decreased significantly. The reduction in glutamate synthase (GOGAT)activity was only significant (65%) at 100 mM stress. Exogenous GABA decreased Na+, H2O2,and MDA concentrations but enhanced photosynthetic pigments, K+ and K+/Na+ ratio, Nmetabolism, osmolytes, and enzymatic antioxidant activities, thus reducing salinity-associatedstress damages, resulting in improved growth and biomass.

CONCLUSION

Exogenous GABA may have improved the salinity tolerance of mungbean plants by maintaining their morpho-physiological responses and reducing the accumulation of harmfulsubstances under salinity. Future molecular studies can contribute to a better understanding of themolecular mechanisms by which GABA regulates mungbean salinity tolerance.

Physiological effects of γ-aminobutyric acid application on cold tolerance in Medicago ruthenica

Low temperatures in the seedling stage during early spring limit Medicago ruthenica germination and seedling growth. Elucidating the physiological mechanism of γ-aminobutyric acid (GABA)-regulated cold tolerance in M. ruthenica could provide a reference for alleviating the harmful effects of low temperatures on legumes in alpine meadows. The regulatory effects of GABA on M. ruthenica physiological parameters were explored by simulating the ground temperatures in the alpine meadow area of Tianzhu, China, in early May (2 h at 7°C; 6 h at 15°C; 4 h at 12°C; 2 h at 7°C; 10 h at 3°C). Our results showed that 15 mmol/l GABA was the optimal spray concentration to promote growth in the aboveground and belowground parts and increase the fresh and dry weights of seedlings. At this concentration, GABA enhanced the activities of catalase, peroxidase, superoxide dismutase, and ascorbate peroxidase; increased the osmotic balance; and inhibited the production of harmful substances in the cells under low-temperature conditions. GABA also regulated the tissue structure of leaves, increased the cell tense ratio, maintained photochemical activity, increased the amount of light energy to the photochemical reaction center, and improved the photosynthetic rate. Furthermore, exogenous GABA application increased the endogenous GABA content by promoting GABA synthesis in the early stages of low-temperature stress but mainly participated in low-temperature stress mitigation via GABA degradation in the late stages. Our results show that GABA can improve the cold tolerance of M. ruthenica by promoting endogenous GABA metabolism, protecting the membrane system, and improving the leaf structure.

Novel Approaches for Sustainable Horticultural Crop Production: Advances and Prospects

Reduction of plant growth, yield and quality due to diverse environmental constrains along with climate change significantly limit the sustainable production of horticultural crops. In this review, we highlight the prospective impacts that are positive challenges for the application of beneficial microbial endophytes, nanomaterials (NMs), exogenous phytohormones strigolactones (SLs) and new breeding techniques (CRISPR), as well as controlled environment horticulture (CEH) using artificial light in sustainable production of horticultural crops. The benefits of such applications are often evaluated by measuring their impact on the metabolic, morphological and biochemical parameters of a variety of cultures, which typically results in higher yields with efficient use of resources when applied in greenhouse or field conditions. Endophytic microbes that promote plant growth play a key role in the adapting of plants to habitat, thereby improving their yield and prolonging their protection from biotic and abiotic stresses. Focusing on quality control, we considered the effects of the applications of microbial endophytes, a novel class of phytohormones SLs, as well as NMs and CEH using artificial light on horticultural commodities. In addition, the genomic editing of plants using CRISPR, including its role in modulating gene expression/transcription factors in improving crop production and tolerance, was also reviewed.

Toxic effects of microplastics in plants depend more by their surface functional groups than just accumulation contents

Differentially charged microplastics (MPs) engendered by plastic aging (e.g., plastic film) widely existed in the agricultural ecosystem, yet minimal was known about the toxic effects of MPs on plants and their absorption and accumulation characteristics. Root absorption largely determined the migration and accumulation risks of MPs in the soil-crop food chain. Here, five types of MPs exposure experiments of leaf lettuce were implemented to simulate root absorption by hydroponics. MPs exposure caused different degrees of growth inhibition, root lignification, root cell apoptosis, and oxidative stress responses; accelerated chlorophyll decomposition and hampered normal electron transfer within the PSII photosystem. Moreover, the uptake of essential elements by roots was inhibited to varying degrees due to the pore blockage in the cell wall and the hetero-aggregation of opposite charges after MPs exposure. MPs exposure observably up-regulated the organic metabolic pathways in roots, thus affecting MPs mobility and absorption through the electrostatic and hydrophobic interactions between the root exudations and MPs. Importantly, MPs penetrated the root extracellular cortex into the stele and were transported to the shoots by transpiration through xylem vessels based on confocal laser scanning microscopy and scanning electron microscopy images. Quantitative analysis of MPs indicated that their toxic effects on plants were determined to a greater extent by the types of surface functional groups than just their accumulation contents, that is, MPs were confirmed edible risks through crop food chain transfer, but bioaccumulation varied by surface functional groups.

Elevated light intensity compensates for nitrogen deficiency during chrysanthemum growth by improving water and nitrogen use efficiency

Identifying environmental factors that improve plant growth and development under nitrogen (N) constraint is essential for sustainable greenhouse production. In the present study, the role of light intensity and N concentrations on the biomass partitioning and physiology of chrysanthemum was investigated. Four light intensities [75, 150, 300, and 600 µmol m^-2 s^-1 photosynthetic photon flux density (PPFD)] and three N concentrations (5, 10, and 15 mM N L^-1) were used. Vegetative and generative growth traits were improved by increase in PPFD and N concentration. High N supply reduced stomatal size and g_s in plants under lowest PPFD. Under low PPFD, the share of biomass allocated to leaves and stem was higher than that of flower and roots while in plants grown under high PPFD, the share of biomass allocated to flower and root outweighed that of allocated to leaves and stem. As well, positive effects of high PPFD on chlorophyll content, photosynthesis, water use efficiency (WUE), Nitrogen use efficiency (NUE) were observed in N-deficient plants. Furthermore, photosynthetic functionality improved by raise in PPFD. In conclusion, high PPFD reduced the adverse effects of N deficiency by improving photosynthesis and stomatal functionality, NUE, WUE, and directing biomass partitioning toward the floral organs.

The regulatory role of γ-aminobutyric acid in chickpea plants depends on drought tolerance and water scarcity level

γ-Aminobutyric acid (GABA) is a non-protein amino acid with multifunctional roles in dynamic plant responses. To determine the effects of exogenous GABA application (0, 25 and 50 µM) on drought response, two chickpea cultivars with contrasting tolerance to water deficit were examined. Plants were exposed to four irrigation levels (irrigation to 100, 60, 40 and 20% field capacity). Water deficit decreased growth, chlorophyll content, and photosynthetic efficiency. It increased electrolyte leakage and lipid peroxidation owing to both higher ROS accumulation and lower antioxidant enzyme activity. These negative effects of water deficit and the alleviating role of GABA application were more prominent in the sensitive, as compared to the tolerant cultivar. Water deficit also increased proline and GABA contents more in the tolerant cultivar, whereas their content was more enhanced by GABA application in the sensitive one. This may confer an additional level of regulation that results in better alleviation of drought damage in tolerant chickpea cultivars. In conclusion, the stimulatory effect of GABA on growth and physiological modulation depends on both the water stress severity and the cultivar sensitivity to it, implying a probable unknown GABA-related mechanism established by tolerant chickpea cultivars; a lost or not gained mechanism in susceptible ones.

Characterization of GABA-Transaminase Gene from Mulberry (Morus multicaulis) and Its Role in Salt Stress Tolerance

Gamma-aminobutyric acid (GABA) has been reported to accumulate in plants when subjected to salt stress, and GABA-transaminase (GABA-T) is the main GABA-degrading enzyme in the GABA shunt pathway. So far, the salt tolerance mechanism of the GABA-T gene behind the GABA metabolism remains unclear. In this study, the cDNA (designated MuGABA-T) of GABA-T gene was cloned from mulberry, and our data showed that MuGABA-T protein shares some conserved characteristics with its homologs from several plant species. MuGABA-T gene was constitutively expressed at different levels in mulberry tissues, and was induced substantially by NaCl, ABA and SA. In addition, our results demonstrated that exogenous application of GABA significantly reduced the salt damage index and increased plant resistance to NaCl stress. We further performed a functional analysis of MuGABA-T gene and demonstrated that the content of GABA was reduced in the transgenic MuGABA-T Arabidopsis plants, which accumulated more ROS and exhibited more sensitivity to salt stress than wild-type plants. However, exogenous application of GABA significantly increased the activities of antioxidant enzymes and alleviated the active oxygen-related injury of the transgenic plants under NaCl stress. Moreover, the MuGABA-T gene was overexpressed in the mulberry hairy roots, and similar results were obtained for sensitivity to salt stress in the transgenic mulberry plants. Our results suggest that the MuGABA-T gene plays a pivotal role in GABA catabolism and is responsible for a decrease in salt tolerance, and it may be involved in the ROS pathway in the response to salt stress. Taken together, the information provided here is helpful for further analysis of the function of GABA-T genes, and may promote mulberry resistance breeding in the future.

Comprehensive Genome-Wide Identification and Transcript Profiling of GABA Pathway Gene Family in Apple (Malus domestica)

γ-Aminobutyric Acid (GABA), a four-carbon non-protein amino acid, is a significant component of the free amino acid pool in most prokaryotic and eukaryotic organisms. GABA is involved in pH regulation, maintaining C/N balance, plant development and defence, as well as a compatible osmolyte and an alternative pathway for glutamate utilization via anion flux. Glutamate decarboxylase (GAD, EC 4.1.1.15) and GABA transaminase (GABA-T, EC 2.6.1.19) are two key enzymes involved in the synthesis and metabolism of GABA. Recently, GABA transporters (GATs), protein and aluminium-activated malate transporter (ALMT) proteins which function as GABA receptors, have been shown to be involved in GABA regulation. However, there is no report on the characterization of apple GABA pathway genes. In this study, we performed a genome-wide analysis and expression profiling of the GABA pathway gene family in the apple genome. A total of 24 genes were identified including five GAD genes (namely MdGAD 1–5), two GABA-T genes (namely MdGABA-T 1,2), 10 GAT genes (namely GAT 1–10) and seven ALMT genes (namely MdALMT1–7). These genes were randomly distributed on 12 chromosomes. Phylogenetic analyses grouped GABA shunt genes into three clusters—cluster I, cluster II, and cluster III—which had three, four, and five genes, respectively. The expression profile analysis revealed significant MdGAD4 expression levels in both fruit and flower organs, except pollen. However, there were no significant differences in the expression of other GABA shunt genes in different tissues. This work provides the first characterization of the GABA shunt gene family in apple and suggests their importance in apple response to abiotic stress. These results can serve as a guide for future studies on the understanding and functional characterization of these gene families.

Supplementary Light with Increased Blue Fraction Accelerates Emergence and Improves Development of the Inflorescence in Aechmea, Guzmania and Vriesea

In protected cultivation, increasing the light level via supplementary lighting (SL) is critical to improve external quality, especially in periods with low light availability. Despite wide applications, the effect of light quality remains understated. In this study, the effect of SL quality and nutrient solution electrical conductivity (EC) on growth and flowering of three bromeliad species was investigated. Treatments included solar light, and this supplemented with R90B10 [90% red (R) and 10% blue (B)], R80B20 (80% R and 20% B), and R70B30 (70% R and 30% B). These were combined with an EC of 1 and 2 dS m-l. Irrespective of the light treatment, the higher EC promoted growth, inflorescence emergence, and development in Aechmea fasciata (Lindl.) Baker, whereas adverse effects were noted in Guzmania and Vriesea. The higher EC-induced negative effect in Guzmania and Vriesea was slightly alleviated by SL. With few notable exceptions, SL exerted limited effects on photosynthetic functionality. Depending on the species, SL improved external quality traits. In all species, SL increased root and inflorescence weight and stimulated biomass allocation to generative organs. It also accelerated inflorescence emergence and promoted inflorescence development. In this way, the time to commercial development stage was considerably shortened. These effects were more prominent at R80B20 and R70B30. Under those conditions, for instance, inflorescence emergence occurred 3–5 weeks earlier than in the control, depending on the species. In conclusion, SL with increased B proportion leads to shorter production period owing to faster emergence and improved development of the inflorescence and is recommended for commercial use.

Metabolomics and Physiological Insights into the Ability of Exogenously Applied Chlorogenic Acid and Hesperidin to Modulate Salt Stress in Lettuce Distinctively

Recent studies in the agronomic field indicate that the exogenous application of polyphenols can provide tolerance against various stresses in plants. However, the molecular processes underlying stress mitigation remain unclear, and little is known about the impact of exogenously applied phenolics, especially in combination with salinity. In this work, the impacts of exogenously applied chlorogenic acid (CA), hesperidin (HES), and their combination (HES + CA) have been investigated in lettuce (Lactuca sativa L.) through untargeted metabolomics to evaluate mitigation effects against salinity. Growth parameters, physiological measurements, leaf relative water content, and osmotic potential as well as gas exchange parameters were also measured. As expected, salinity produced a significant decline in the physiological and biochemical parameters of lettuce. However, the treatments with exogenous phenolics, particularly HES and HES + CA, allowed lettuce to cope with salt stress condition. Interestingly, the treatments triggered a broad metabolic reprogramming that involved secondary metabolism and small molecules such as electron carriers, enzyme cofactors, and vitamins. Under salinity conditions, CA and HES + CA distinctively elicited secondary metabolism, nitrogen-containing compounds, osmoprotectants, and polyamines.

γ-Aminobutyrate (GABA) Regulated Plant Defense: Mechanisms and Opportunities

Global climate change and associated adverse abiotic and biotic stress conditions affect plant growth and development, and agricultural sustainability in general. Abiotic and biotic stresses reduce respiration and associated energy generation in mitochondria, resulting in the elevated production of reactive oxygen species (ROS), which are employed to transmit cellular signaling information in response to the changing conditions. Excessive ROS accumulation can contribute to cell damage and death. Production of the non-protein amino acid γ-aminobutyrate (GABA) is also stimulated, resulting in partial restoration of respiratory processes and energy production. Accumulated GABA can bind directly to the aluminum-activated malate transporter and the guard cell outward rectifying K+ channel, thereby improving drought and hypoxia tolerance, respectively. Genetic manipulation of GABA metabolism and receptors, respectively, reveal positive relationships between GABA levels and abiotic/biotic stress tolerance, and between malate efflux from the root and heavy metal tolerance. The application of exogenous GABA is associated with lower ROS levels, enhanced membrane stability, changes in the levels of non-enzymatic and enzymatic antioxidants, and crosstalk among phytohormones. Exogenous GABA may be an effective and sustainable tolerance strategy against multiple stresses under field conditions.

Functional Characterization of the Stipa purpurea P5CS Gene under Drought Stress Conditions

Free proline has multiple functions in plant cells, such as regulating osmotic potential and protecting both proteins and cell membranes. The expression of Δ1-Pyrroline-5-carboxylate synthase (P5CS), a key enzyme in the proline biosynthetic pathway, increases under drought, salt and cold stress conditions, causing plant cells to accumulate large amounts of proline. In this study, we cloned and identified the P5CS gene from Stipa purpurea, which has a full-length of 2196 bp and encodes 731 amino acids. A subcellular localization analysis indicated that SpP5CS localized to the cytoplasm. The ectopic overexpression of SpP5CS in Arabidopsis thaliana resulted in higher proline contents, longer roots, higher survival rates and less membrane damage under drought stress conditions compared with wild-type controls. SpP5CS-overexpressing A. thaliana was more resistant to drought stress than the wild type, whereas the deletion mutant sp5cs was less resistant to drought stress. Thus, SpP5CS may be a potential candidate target gene for increasing plant resistance to drought stress.

Stress response of lettuce (Lactuca sativa) to environmental contamination with selected pharmaceuticals: A proteomic study

Pharmaceutical compounds have been found in rivers and treated wastewaters. They often contaminate irrigation waters and consequently accumulate in edible vegetables, causing changes in plants metabolism. The main objective of this work is to understand how lettuce plants cope with the contamination from three selected pharmaceuticals using a label free proteomic analysis. A lettuce hydroponic culture, grown for 36 days, was exposed to metformin, acetaminophen and carbamazepine (at 1 mg/L), during 8 days, after which roots and leaves were sampled and analysed using a liquid chromatography-mass spectrometry proteomics-based approach. In roots, a total of 612 proteins showed differentially accumulation while in leaves 237 proteins were identified with significant differences over controls. Carbamazepine was the contaminant that most affected protein abundance in roots, while in leaves the highest number of differentially accumulated proteins was observed for acetaminophen. In roots under carbamazepine, stress related protein species such as catalase, superoxide dismutase and peroxidases presented higher abundance. Ascorbate peroxidase increased in roots under metformin. Cell respiration protein species were affected by the presence of the three pharmaceuticals suggesting possible dysregulation of the Krebs cycle. Acetaminophen caused the main differences in respiration pathways, with more emphasis in leaves. Lettuce plants revealed different tolerance levels when contaminants were compared, being more tolerant to metformin presence and less tolerant to carbamazepine. SIGNIFICANCE: The significant increase of emerging contaminants in ecosystems makes essential to understand how these compounds may affect the metabolism of different organisms. Our study contributes with a detailed approach of the main interactions that may occur in plant metabolism when subjected to the stress induced by three different pharmaceuticals (acetaminophen, carbamazepine and metformin).

Crucial Cell Signaling Compounds Crosstalk and Integrative Multi-Omics Techniques for Salinity Stress Tolerance in Plants

In the era of rapid climate change, abiotic stresses are the primary cause for yield gap in major agricultural crops. Among them, salinity is considered a calamitous stress due to its global distribution and consequences. Salinity affects plant processes and growth by imposing osmotic stress and destroys ionic and redox signaling. It also affects phytohormone homeostasis, which leads to oxidative stress and eventually imbalances metabolic activity. In this situation, signaling compound crosstalk such as gasotransmitters [nitric oxide (NO), hydrogen sulfide (H2S), hydrogen peroxide (H2O2), calcium (Ca), reactive oxygen species (ROS)] and plant growth regulators (auxin, ethylene, abscisic acid, and salicylic acid) have a decisive role in regulating plant stress signaling and administer unfavorable circumstances including salinity stress. Moreover, recent significant progress in omics techniques (transcriptomics, genomics, proteomics, and metabolomics) have helped to reinforce the deep understanding of molecular insight in multiple stress tolerance. Currently, there is very little information on gasotransmitters and plant growth regulator crosstalk and inadequacy of information regarding the integration of multi-omics technology during salinity stress. Therefore, there is an urgent need to understand the crucial cell signaling crosstalk mechanisms and integrative multi-omics techniques to provide a more direct approach for salinity stress tolerance. To address the above-mentioned words, this review covers the common mechanisms of signaling compounds and role of different signaling crosstalk under salinity stress tolerance. Thereafter, we mention the integration of different omics technology and compile recent information with respect to salinity stress tolerance.

Blue Light Improves Photosynthetic Performance during Healing and Acclimatization of Grafted Watermelon Seedlings

To investigate the importance of light on healing and acclimatization, in the present study, grafted watermelon seedlings were exposed to darkness (D) or light, provided by blue (B), red (R), a mixture of R (68%) and B (RB), or white (W; 35% B, 49% intermediate spectra, 16% R) LEDs for 12 days. Survival ratio, root and shoot growth, soluble carbohydrate content, photosynthetic pigments content, and photosynthetic performance were evaluated. Seedling survival was not only strongly limited in D but the survived seedlings had an inferior shoot and root development, reduced chlorophyll content, and attenuated photosynthetic efficiency. RB-exposed seedlings had a less-developed root system. R-exposed seedlings showed leaf epinasty, and had the smallest leaf area, reduced chlorophyll content, and suppressed photosynthetic apparatus performance. The R-exposed seedlings contained the highest amount of soluble carbohydrate and together with D-exposed seedlings the lowest amount of chlorophyll in their scions. B-exposed seedlings showed the highest chlorophyll content and improved overall PSII photosynthetic functioning. W-exposed seedling had the largest leaf area, and closely resembled the photosynthetic properties of RB-exposed seedlings. We assume that, during healing of grafted seedlings monochromatic R light should be avoided. Instead, W and monochromatic B light may be willingly adopted due to their promoting effect on shoot, pigments content, and photosynthetic efficiency.

Host Gamma-Aminobutyric Acid Metabolic Pathway Is Involved in Resistance Against Rhizoctonia solani

Rhizoctonia solani is a highly destructive necrotrophic fungal pathogen having a diverse host range, including rice and tomato. Previously R. solani infection has been found to cause large-scale readjustment in host primary metabolism and accumulation of various stress-associated metabolites such as gamma-aminobutyric acid (GABA) in rice. In this study, we report upregulation of GABA pathway genes during pathogenesis of R. solani in rice and tomato. The exogenous application of GABA provided partial resistance against R. solani infection in both the hosts. Furthermore, by using the virus-induced gene silencing approach, we knocked down the expression of some of the tomato genes involved in GABA biosynthesis (glutamate decarboxylase) and GABA catabolism (GABA-transaminase and succinic semialdehyde dehydrogenase) to study their role in host defense against R. solani infection. The silencing of each of these genes increased disease susceptibility in tomato. Overall the results from gene expression analysis, exogenous chemical application, and gene silencing studies suggest that the GABA pathway plays a positive role in plant defense against necrotrophic pathogen R. solani.

Synergistic Effects of Melatonin and Gamma-Aminobutyric Acid on Protection of Photosynthesis System in Response to Multiple Abiotic Stressors

GABA (gamma-aminobutyric acid) and melatonin are endogenous compounds that enhance plant responses to abiotic stresses. The response of Vicia faba to different stressors (salinity (NaCl), poly ethylene glycol (PEG), and sulfur dioxide (SO₂)) was studied after priming with sole application of GABA and melatonin or their co-application (GABA + melatonin). Both melatonin and GABA and their co-application increased leaf area, number of flowers, shoot dry and fresh weight, and total biomass. Plants treated with GABA, melatonin, and GABA + melatonin developed larger stomata with wider aperture compared to the stomata of control plants. The functionality of the photosynthetic system was improved in primed plants. To investigate the photosynthetic functionality in details, the leaf samples of primed plants were exposed to different stressors, including SO₂, PEG, and NaCl. The maximum quantum yield of photosystem II (PS II) was higher in the leaf samples of primed plants, while the non-photochemical quenching (NPQ) of primed plants was decreased when leaf samples were exposed to the stressors. Correlation analysis showed the association of initial PI_abs with post-stress F_V/F_M and NPQ. Stressors attenuated the association of initial PI_abs with both F_V/F_M and NPQ, while priming plants with GABA, melatonin, or GABA + melatonin minimized the effect of stressors by attenuating these correlations. In conclusion, priming plants with both GABA and melatonin improved growth and photosynthetic performance of Vicia faba and mitigated the effects of abiotic stressors on the photosynthetic performance.

Hypoxia-induced increase in GABA content is essential for restoration of membrane potential and preventing ROS-induced disturbance to ion homeostasis

When plants are exposed to hypoxic conditions, the level of γ-aminobutyric acid (GABA) in plant tissues increases by several orders of magnitude. The physiological rationale behind this elevation remains largely unanswered. By combining genetic and electrophysiological approach, in this work we show that hypoxia-induced increase in GABA content is essential for restoration of membrane potential and preventing ROS-induced disturbance to cytosolic K⁺ homeostasis and Ca²⁺ signaling. We show that reduced O₂ availability affects H⁺-ATPase pumping activity, leading to membrane depolarization and K⁺ loss via outward-rectifying GORK channels. Hypoxia stress also results in H₂O₂ accumulation in the cell that activates ROS-inducible Ca²⁺ uptake channels and triggers self-amplifying "ROS-Ca hub," further exacerbating K⁺ loss via non-selective cation channels that results in the loss of the cell's viability. Hypoxia-induced elevation in the GABA level may restore membrane potential by pH-dependent regulation of H⁺-ATPase and/or by generating more energy through the activation of the GABA shunt pathway and TCA cycle. Elevated GABA can also provide better control of the ROS-Ca²⁺ hub by transcriptional control of RBOH genes thus preventing over-excessive H₂O₂ accumulation. Finally, GABA can operate as a ligand directly controlling the open probability and conductance of K⁺ efflux GORK channels, thus enabling plants adaptation to hypoxic conditions.

Monochromatic red light during plant growth decreases the size and improves the functionality of stomata in chrysanthemum

Light emitting diodes (LEDs) now enable precise light quality control. Prior to commercialisation however, the plant response to the resultant light quality regime ought to be addressed. The response was examined here in chrysanthemum by evaluating growth, chlorophyll fluorescence (before and following water deficit), as well as stomatal anatomy (density, size, pore dimensions and aperture heterogeneity) and closing ability. Plants were grown under blue (B), red (R), a mixture of R (70%) and B (RB), or white (W; 41% B, 39% intermediate spectrum, 20% R) light LEDs. Although R light promoted growth, it also caused leaf deformation (epinasty) and disturbed the photosynthetic electron transport system. The largest stomatal size was noted following growth under B light, whereas the smallest under R light. The largest stomatal density was observed under W light. Monochromatic R light stimulated both the rate and the degree of stomatal closure in response to desiccation compared with the other light regimes. We conclude that stomatal size is mainly controlled by the B spectrum, whereas a broader spectral range is important for determining stomatal density. Monochromatic R light enhanced stomatal ability to regulate water loss upon desiccation.

Adaptability to abiotic stress regulated by γ-aminobutyric acid in relation to alterations of endogenous polyamines and organic metabolites in creeping bentgrass

The frequency and severity of global abiotic stresses such as heat, drought, and salt stress are increasing due to climate changes. Objectives of this study were to investigate effects of γ-aminobutyric acid (GABA) priming on inducing plants' acclimation to abiotic stress associated with alterations of endogenous polyamines (PAs), amino acids, and sugars in creeping bentgrass (Agrostis stolonifera). The pretreatment with GABA fertigation significantly alleviated heat-, drought-, and salt-induced declines in leaf relative water content, chlorophyll content, cell membrane stability, photochemical efficiency (Fv/Fm), and performance index on absorption basis (PIABS), and also further decreased stress-caused decline in osmotic potential in leaves. The GABA priming uniformly increased total PAs, spermidine, amino acids involved in GABA shunt (GABA, glutamic acid, and alanine), and other amino acids (phenylalanine, aspartic acid, and glycine) accumulation under heat, drought, and salt stress. The GABA priming also significantly improved methionine content under heat and drought stress, maltose, galactose, and talose content under heat and salt stress, or cysteine, serine, and threonine content under drought and salt stress. Interestingly, the GABA priming uniquely led to significant accumulation of spermine, fructose, and glucose under heat stress, putrescine, proline, and mannose under drought stress, or arginine, trehalose and xylose under salt stress, respectively. These particular PAs, sugars, and amino acids differentially or commonly regulated by GABA could play critical roles in osmotic adjustment, osmoprotection, antioxidant, energy source, and signal molecular for creeping bentgrass to acclimate diverse abiotic stresses.

Gamma-aminobutyric acid (GABA) alleviates salt damage in tomato by modulating Na+ uptake, the GAD gene, amino acid synthesis and reactive oxygen species metabolism

BACKGROUND

Salt stress is a serious abiotic stress that caused crop growth inhibition and yield decline. Previous studies have reported on the the synthesis of gamma-aminobutyric acid (GABA) and its relationship with plant resistance under various abiotic stress. However, the relationship between exogenous GABA alleviating plant salt stress damage and ion flux, amino acid synthesis, and key enzyme expression remains largely unclear. We investigated plant growth, Na⁺ transportation and accumulation, reactive oxygen species (ROS) metabolism and evaluated the effect of GABA on amino acids, especially SlGADs gene expression and the endogenous GABA content of tomato (Solanum lycopersicum L.) seedlings treated with or without 5 mmol·L^- 1 GABA under 175 mmol·L^- 1 NaCl stress.

RESULTS

Exogenous application of GABA significantly reduced the salt damage index and increased plant height, chlorophyll content and the dry and fresh weights of tomato plants exposed to NaCl stress. GABA significantly reduced Na⁺ accumulation in leaves and roots by preventing Na⁺ influx in roots and transportation to leaves. The transcriptional expression of SlGAD1-3 genes were induced by NaCl stress especially with GABA application. Among them, SlGAD1 expression was the most sensitive and contributed the most to the increase in glutamate decarboxylase (GAD) activity induced by NaCl and GABA application; Exogenous GABA increased GAD activity and amino acid contents in tomato leaves compared with the levels under NaCl stress alone, especially the levels of endogenous GABA, proline, glutamate and eight other amino acids. These results indicated that SlGADs transcriptional expression played an important role in tomato plant resistance to NaCl stress with GABA application by enhancing GAD activity and amino acid contents. GABA significantly alleviated the active oxygen-related injury of leaves under NaCl stress by increasing the activities of antioxidant enzymes and decreasing the contents of active oxygen species and malondialdehyde.

CONCLUSION

Exogenous GABA had a positive effect on the resistance of tomato seedlings to salt stress, which was closely associated with reducing Na⁺ flux from root to leaves, increasing amino acid content and strengthening antioxidant metabolism. Endogenous GABA content was induced by salt and exogenous GABA at both the transcriptional and metabolic levels.

Strigolactone mediates jasmonic acid-induced lipid production in microalga Monoraphidium sp. QLY-1 under nitrogen deficiency conditions

The roles of jasmonic acid (JA) in the regulation of cell growth and lipid biosynthesis under the combination of strigolactone (SL) treatment and nitrogen deficiency (ND) were investigated. In this work, the optimised ND condition (46.18%) and ND combined with SL treatment (53.71%) showed 1.11- and 1.29-fold increases in lipid content in Monoraphidium sp. QLY-1 compared with the control condition (41.57%). The levels of JA, glutathione (GSH), and γ-aminobutyric acid (GABA) and lipogenic genes expression were upregulated by the combination of SL and ND, but the ROS level was decreased. Furthermore, exogenous JA supplementation induced the highest lipid content (57.12%) and productivity (312.35 mg L-1 d-1) under ND combined with SL treatment. This study provided a combined strategy for enhancing lipid production and supplied novel insights into the role of JA signalling in regulating lipid synthesis and oxidative stress in microalgae by combining SL treatment with ND.

Blue Light Improves Vase Life of Carnation Cut Flowers Through Its Effect on the Antioxidant Defense System

Improving marketability and extension of vase life of cut flowers has practical significance for the development of the cut flower industry. Although considerable efforts have been made over many years to improve the vase life of cut flowers through controlling the immediate environment and through post-harvest use of floral preservatives, the impact of lighting environment on vase life has been largely overlooked. In the current study, the effect of three LED light spectra [white (400-730 nm), blue (peak at 460 nm), and red (peak at 660 nm)] at 150 μmol m-2 s-1 on vase life and on physiological and biochemical characteristics of carnation cut flowers was investigated. Exposure to blue light (BL) considerably delayed senescence and improved vase life over that of flowers exposed to red light (RL) and white light (WL). H2O2 and malondialdehyde (MDA) contents in petals gradually increased during vase life; the increase was lowest in BL-exposed flowers. As a consequence, BL-exposed flowers maintained a higher membrane stability index (MSI) compared to RL- and WL-exposed flowers. A higher activity of antioxidant enzymes [superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX)] was detected in petals of BL-exposed flowers, compared to their activities in RL- and WL-exposed flowers. In BL-exposed flowers, the decline in petal carotenoid contents was delayed in comparison to RL- and WL-exposed flowers. Maximum quantum efficiency of photosystem II (Fv/Fm) and a higher percentage of open stomata were observed in leaves of BL-exposed flowers. Sucrose and glucose contents accumulated in petals during vase life; sugar concentrations were higher in BL-exposed flowers than in RL- and WL-exposed flowers. It is concluded that BL exposure improves the vase life of carnation cut flowers through its effect on the antioxidant defense system in petals and on photosynthetic performance in the leaves.

γ-Aminobutyric acid and related amino acids in plant immune responses: Emerging mechanisms of action

The entanglement between primary metabolism regulation and stress responses is a puzzling and fascinating theme in plant sciences. Among the major metabolites found in plants, γ-aminobutyric acid (GABA) fulfils important roles in connecting C and N metabolic fluxes through the GABA shunt. Activation of GABA metabolism is known since long to occur in plant tissues following biotic stresses, where GABA appears to have substantially different modes of action towards different categories of pathogens and pests. While it can harm insects thanks to its inhibitory effect on the neuronal transmission, its capacity to modulate the hypersensitive response in attacked host cells was proven to be crucial for host defences in several pathosystems. In this review, we discuss how plants can employ GABA's versatility to effectively deal with all the major biotic stressors, and how GABA can shape plant immune responses against pathogens by modulating reactive oxygen species balance in invaded plant tissues. Finally, we discuss the connections between GABA and other stress-related amino acids such as BABA (β-aminobutyric acid), glutamate and proline.

γ-Aminobutyric acid confers cadmium tolerance in maize plants by concerted regulation of polyamine metabolism and antioxidant defense systems

Gamma-Aminobutyric acid (GABA) accumulates in plants following exposure to heavy metals. To investigate the role of GABA in cadmium (Cd) tolerance and elucidate the underlying mechanisms, GABA (0, 25 and 50 µM) was applied to Cd-treated maize plants. Vegetative growth parameters were improved in both Cd-treated and control plants due to GABA application. Cd uptake and translocation were considerably inhibited by GABA. Antioxidant enzyme activity was enhanced in plants subjected to Cd. Concurrently GABA caused further increases in catalase and superoxide dismutase activities, which led to a significant reduction in hydrogen peroxide, superoxide anion and malondealdehyde contents under stress conditions. Polyamine biosynthesis-responsive genes, namely ornithine decarboxylase and spermidine synthase, were induced by GABA in plants grown under Cd shock. GABA suppressed polyamine oxidase, a gene related to polyamine catabolism, when plants were exposed to Cd. Consequently, different forms of polyamines were elevated in Cd-exposed plants following GABA application. The maximum quantum efficiency of photosystem II (Fv/Fm) was decreased by Cd-exposed plants, but was completely restored by GABA to the same value in the control. These results suggest a multifaceted contribution of GABA, through regulation of Cd uptake, production of reactive oxygen species and polyamine metabolism, in response to Cd stress.

γ-Aminobutyric acid (GABA) regulates lipid production and cadmium uptake by Monoraphidium sp. QLY-1 under cadmium stress

This study explored the effects of γ-aminobutyric acid (GABA) on the production of biomass and lipids and on the uptake of Cd²⁺ by microalgae under cadmium (Cd) stress. Compared with the control and Cd stress alone, 2.5 mM GABA increased the maximum lipid content (55.37%) by 49.37% and 9.42%, respectively. GABA application resulted in increased contents of protein and glutathione (GSH) and in upregulated activity of α-amylase but decreased contents of starch, reactive oxygen species (ROS) and Cd²⁺, with no effect on subsequent biodiesel quality. Additional analysis of GABA further indicated that increased cellular GABA contents could promote lipid synthesis and reduce Cd accumulation by regulating the expression levels of lipogenesis genes, ROS signalling and mineral nutrient uptake under Cd stress. Collectively, these findings show that GABA not only increases lipid production in microalgae but also is involved in the mechanisms by which microalgae respond to Cd stress.

Calcium signaling and salt tolerance are diversely entwined in plants

In plants dehydration imposed by salinity can invoke physical changes at the interface of the plasma membrane and cell wall. Changes in hydrostatic pressure activate ion channels and cause depolarization of the plasma membrane due to disturbance in ion transport. During the initial phases of salinity stress, the relatively high osmotic potential of the rhizosphere enforces the plant to use a diverse spectrum of strategies to optimize water and nutrient uptake. Signals of salt stress are recognized by specific root receptors that activate an osmosensing network. Plant response to hyperosmotic tension is closely linked to the calcium (Ca2+) channels and interacting proteins such as calmodulin. A rapid rise in cytosolic Ca2+ levels occurs within seconds of exposure to salt stress. Plants employ multiple sensors and signaling components to sense and respond to salinity stress, of which most are closely related to Ca2+ sensing and signaling. Several tolerance strategies such as osmoprotectant accumulation, antioxidant boosting, polyaminses and nitric oxide (NO) machineries are also coordinated by Ca2+ signaling. Substantial research has been done to discover the salt stress pathway and tolerance mechanism in plants, resulting in new insights into the perception of salt stress and the downstream signaling that happens in response. Nevertheless, the role of multifunctional components such as Ca2+ has not been sufficiently addressed in the context of salt stress. In this review, we elaborate that the salt tolerance signaling pathway converges with Ca2+ signaling in diverse pathways. We summarize knowledge related to different dimensions of salt stress signaling pathways in the cell by emphasizing the administrative role of Ca2+ signaling on salt perception, signaling, gene expression, ion homeostasis and adaptive responses.

Diverse role of γ-aminobutyric acid in dynamic plant cell responses

Gamma-aminobutyric acid (GABA), a four-carbon non-protein amino acid, is found in most prokaryotic and eukaryotic organisms. Although, ample research into GABA has occurred in mammals as it is a major inhibitory neurotransmitter; in plants, a role for GABA has often been suggested as a metabolite that changes under stress rather than as a signal, as no receptor or motif for GABA binding was identified until recently and many aspects of its biological function (ranging from perception to function) remain to be answered. In this review, flexible properties of GABA in regulation of plant responses to various environmental biotic and abiotic stresses and its integration in plant growth and development either as a metabolite or a signaling molecule are discussed. We have elaborated on the role of GABA in stress adaptation (i.e., salinity, hypoxia/anoxia, drought, temperature, heavy metals, plant-insect interplay and ROS-related responses) and its contribution in non-stress-related biological pathways (i.e., involvement in plant-microbe interaction, contribution to the carbon and nitrogen metabolism and governing of signal transduction pathways). This review aims to represent the multifunctional contribution of GABA in various biological and physiological mechanisms under stress conditions; the objective is to review the current state of knowledge about GABA role beyond stress-related responses. Our effort is to place findings about GABA in an organized and broader context to highlight its shared metabolic and biologic functions in plants under variable conditions. This will provide potential modes of GABA crosstalk in dynamic plant cell responses.

Photosynthetic and growth responses of green and purple basil plants under different spectral compositions

Light spectrum of growing environment is a determinant factor for plant growth and photosynthesis. Plants under different light spectra exhibit different growth and photosynthetic behaviors. To unravel the effects of light spectra on plant growth, photosynthetic pigments and electron transport chain reactions, purple and green basil varieties were grown under five different light spectra including white (W: 400-730 nm), blue (B: 400-500 nm), red (R: 600-700 nm) and two combinations of R and B lights (R50B50 and R70B30), with same PPFD (photosynthetic photon flux density). Almost all values for shoot and root growth traits were higher in purple variety and were improved by combinational R and B lights (especially under R70B30), while they were negatively influenced by B monochromatic light when compared to growth traits of W-grown plants. Highest concentration of photosynthetic pigments was detected in R70B30. Biophysical properties of photosynthetic electron transport chain showed higher florescence intensity at all steps of OJIP kinetics in plants grown under R light in both varieties. Oxygen evolving complex activity (F_v/F_o) and PSII maximum quantum efficiency (F_v/F_m) in R-grown plants were lower than plants grown under other light spectra. Values for parameters related to specific energy fluxes per reaction center (ABS/RC, TR_o/RC, ET_o/RC and DI_o/RC) were increased under R light (especially for purple variety). Performance index was significantly decreased under R light in both varieties. In conclusion, light spectra other than RB combination, induced various limitations on pigmentations, efficiency of electron transport and growth of basil plants and the responses were cultivar specific.