Arabidopsis root K+-efflux conductance activated by hydroxyl radicals: single-channel properties, genetic basis and involvement in stress-induced cell death

Waterlogging stress mechanism and membrane transporters in soybean (Glycine max (L.) Merr.)

An excess of water is more harmful to plant growth, root growth and the uniformity of the plant population than a water deficit. Water is a crucial factor in the three basic stages of soybean development: germination, emergence and flowering/seed filling. Waterlogging is one of the biggest constraints to crop production and productivity in India and can occur at any stage in soybean. However, seeds and seedlings are damaged by waterlogging resulting in a significant reduction in grain yield. Seed yield and growth are significantly correlated at the seedling stage. In addition, the plant is under constant pressure due to changing environmental conditions and has difficulty withstanding these harsh, unpredictable and difficult situations. Membrane transporters are essential and play fundamental roles during waterlogging thereby facilitating cellular homeostasis and gaseous exchange, which support plant growth and development. This review highlights the genetic basis and mechanism of waterlogging tolerance in soybean and the role of climate in influencing the genetic makeup of the crop, paving the way for further development of improved soybean varieties. Simultaneously, the article highlights membrane transporters' importance in water-mediated stress in soybeans.

Regulation of anaplerotic enzymes by melatonin enhances resilience to cadmium toxicity in Vigna radiata (L.) R. Wilczek

Melatonin (Mel) is a tryptophan-derived (N-acetyl-5-methoxytryptamine) molecule. In the present study, role of Mel in the regulation of various anaplerotic enzymes is discussed in relation to N metabolism and H+-ATPase activity in mung bean under Cd stress. The application of Mel to the Cd-stressed mung bean seedlings was remarkable in improving the activity of hexokinase (35.7%), pyruvate kinase (79.2%), phosphoenolpyruvate carboxylase (38.9%) pyruvate dehydrogenase (41.5%), malate dehydrogenase (49.2%), citrate synthase (37.7%), isocitrate dehydrogenase (33.1%), ATP synthase (63.6%), and ATPase (38.6%). Incubation of Cd-stressed seedlings with Mel also improved the activity of nitrate reductase by 89.4%, nitrite reductase by 78.2%, and glutamine synthetase by 35.3% that resulted in higher level of ammonium and their subsequent assimilation to amino acids and proteins. Activation of these enzymes was strongly associated with Mel-induced regulation of H+-ATPase activity that improved K+ retention and N assimilation capacity of the Cd-stressed seedlings of mung bean. The coordinated mechanism of action of tricarboxylic acid (TCA) cycle, N metabolism, and higher K+ levels were helpful in providing protection against detrimental effects of Cd toxicity through improving the defense system and energy level of the plants. However, inclusion of sodium orthovanadate (PM H+-ATPase inhibitor) to the incubation medium reversed the positive effect of Mel and suppressed the performance of plants under Cd-stress. The findings of the study indicate that under Cd stress, the regulatory mechanisms of anaplerotic enzymes and antioxidant defense are mediated by Mel, and this process is facilitated by the retention of K+ induced by H+-ATPase.

A comprehensive analytical method 'Regulatome' revealed a novel pathway for aerenchyma formation under waterlogging in wheat

Waterlogging is a major abiotic stress restricting crop yield globally, and aerenchyma formation is one of the most important adaptive strategies in waterlogging-tolerant plants. However, the conservation of this process remains poorly understood, and additional pathways are yet to be identified. Here, physiological, anatomical, transcriptomic, and metabolomic analyses were conducted on wheat seedlings under normal and waterlogging conditions. Waterlogging caused growth inhibition and physiological damage, as well as induced aerenchyma formation in roots. A total of 10,346 differentially expressed genes and 3,419 differential metabolites were identified in roots. In addition to the AP2/ERF (APETALA2/ETHYLENE RESPONSIVE FACTOR) gene family, integrating analyses also revealed the role of LOB/AS2 (LATERAL ORGAN BOUNDARIES/ASYMMETRIC LEAVES2) in aerenchyma formation under waterlogging. It was revealed that the classical pathway of aerenchyma formation mediated by ethylene response, as well as synergy of calcium ion and reactive oxygen species, was deeply conserved in both monocots and eudicots during 160 million years of evolution through gene co-expression networks of cross-species. The newly introduced concept 'Regulatome' supported the classical pathway of aerenchyma formation, with a proposed model of the jasmonic acid signalling pathway involved in waterlogging, suggesting its usefulness in gene identification and function exploration. These findings provide a novel insight into the regulatory mechanisms of aerenchyma formation and breeding approaches for developing wheat cultivars with high waterlogging tolerance.

L-histidine makes Ni2+ 'visible' for plant signalling systems: Shading the light on Ni2+-induced Ca2+ and redox signalling in plants

Nickel is both an important nutrient and an ecotoxicant for plants. Organic ligands, such as L-histidine (His), play a key role in Ni2+ detoxification. Here, we show that His (added together with 0.01-10 mM Ni2+) decreases Ni2+ toxicity to Arabidopsis thaliana roots not only as a result of a decrease in Ni2+ activity, but also via the induction of signalling phenomena important for adaptation such as the generation of reactive oxygen species (ROS) and cytosolic Ca2+ transients. With the use of EPR spectroscopy, we demonstrate that Ni-His complexes generate hydroxyl radicals that is not detected by the addition of Ni2+ or His separately. Similarly, Ni-His complexes, but not Ni2+, activate Ca2+ influx and K+ efflux currents in patch-clamped root protoplasts resulting in distinct cytosolic Ca2+ signals and a transient K+ release. His prevented programmed cell death symptoms (cytoplasm shrinkage, protease and endonuclease activation) induced by Ni2+ and inhibited Ni2+ accumulation at [Ni2+]>0.3 mM. Intriguingly, priming of roots with Ni-His stimulated plant resistance to Ni2+. Overall, these data show that His triggers ROS-Ca2+-mediated reactions making Ni2+ 'visible' for plant signalling machinery and facilitating adaptation to the excess Ni2+.

Physiological and molecular mechanisms of exogenous salicylic acid in enhancing salt tolerance in tobacco seedlings by regulating antioxidant defence system and gene expression

Introduction

Salt stress has emerged as a predominant abiotic factor that jeopardizes global crop growth and yield. The plant hormone salicylic acid (SA) has notable potential in mitigating salt toxicity, yet its mechanism in enhancing the salinity tolerance of tobacco plants is not well explored.

Methods

This study aimed to assess the potential benefits of exogenous SA application (1.0 mM) on tobacco seedlings subjected to saline soil conditions.

Results

The foliar spray of SA partially mitigated these salt-induced effects, as evidenced by a reduction of malondialdehyde content, and improvements of leaf K+/Na+ ratios, pigment biosynthesis, and electron transport efficiency under NaCl stress. Additionally, SA increased the contents of total phenolic compound and soluble protein by 16.2% and 28.7% to alleviate NaCl-induced oxidative damage. Under salt stressed conditions, the activities of antioxidant enzymes, including superoxide dismutase, ascorbate peroxidase, catalase, and peroxidase increased by 4.2%~14.4% in SA sprayed tobacco seedlings. Exogenous SA also increased ascorbate and glutathione levels and reduced their reduced forms by increasing the activities of glutathione reductase, ascorbate peroxidase, monodehydroascorbate reductase and dehydroascorbate reductase. qRT-PCR analysis revealed that the key genes regulating SA biosynthesis, carbon assimilation, the antioxidant system and the ascorbate-glutathione cycle were activated by SA under conditions of salt stress.

Discussion

Our study elucidates the physiological and molecular mechanisms of exogenous SA in enhancing plant salt tolerance and provides a practical basis for crop improvement in saline environments.

A mechanistic integration of hypoxia signaling with energy, redox, and hormonal cues

Oxygen deficiency (hypoxia) occurs naturally in many developing plant tissues but can become a major threat during acute flooding stress. Consequently, plants as aerobic organisms must rapidly acclimate to hypoxia and the associated energy crisis to ensure cellular and ultimately organismal survival. In plants, oxygen sensing is tightly linked with oxygen-controlled protein stability of group VII ETHYLENE-RESPONSE FACTORs (ERFVII), which, when stabilized under hypoxia, act as key transcriptional regulators of hypoxia-responsive genes (HRGs). Multiple signaling pathways feed into hypoxia signaling to fine-tune cellular decision-making under stress. First, ATP shortage upon hypoxia directly affects the energy status and adjusts anaerobic metabolism. Secondly, altered redox homeostasis leads to reactive oxygen and nitrogen species (ROS and RNS) accumulation, evoking signaling and oxidative stress acclimation. Finally, the phytohormone ethylene promotes hypoxia signaling to improve acute stress acclimation, while hypoxia signaling in turn can alter ethylene, auxin, abscisic acid, salicylic acid, and jasmonate signaling to guide development and stress responses. In this Update, we summarize the current knowledge on how energy, redox, and hormone signaling pathways are induced under hypoxia and subsequently integrated at the molecular level to ensure stress-tailored cellular responses. We show that some HRGs are responsive to changes in redox, energy, and ethylene independently of the oxygen status, and we propose an updated HRG list that is more representative for hypoxia marker gene expression. We discuss the synergistic effects of hypoxia, energy, redox, and hormone signaling and their phenotypic consequences in the context of both environmental and developmental hypoxia.

Metabolic strategies in hypoxic plants

Complex multicellular organisms have evolved in an oxygen-enriched atmosphere. Oxygen is therefore essential for all aerobic organisms, including plants, for energy production through cellular respiration. However, plants can experience hypoxia following extreme flooding events and also under aerated conditions in proliferative organs or tissues characterized by high oxygen consumption. When oxygen availability is compromised, plants adopt different strategies to cope with hypoxia and limited aeration. A common feature among different plant species is the activation of an anaerobic fermentative metabolism to provide ATP to maintain cellular homeostasis under hypoxia. Fermentation also requires many sugar substrates, which is not always feasible, and alternative metabolic strategies are thus needed. Recent findings have also shown that the hypoxic metabolism is also active in specific organs or tissues of the plant under aerated conditions. Here, we describe the regulatory mechanisms that control the metabolic strategies of plants and how they enable them to thrive despite challenging conditions. A comprehensive mechanistic understanding of the genetic and physiological components underlying hypoxic metabolism should help to provide opportunities to improve plant resilience under the current climate change scenario.

Photochemical Hydroxyl Group Abstraction from N-Hydroxypyridine-2(1H)-thione Isolated in a Solid Hydrogen Matrix: Photogeneration of 2-Mercaptopyridine

Monomers of N-hydroxypyridine-2(1H)-thione were isolated in low-temperature matrices of solid normal hydrogen (n-H2). The matrix-isolated compound was irradiated with UV-B (λ = 305 nm) or UV-A (λ > 360 nm) light. Upon such irradiation, the initial form of N-hydroxypyridine-2(1H)-thione was completely consumed and converted into photoproducts. 2-Mercaptopyridine and water were identified as the main products of these photochemical transformations. Identification of photoproduced 2-mercaptopyridine is unquestionable. It is based on the identity of two sets of IR bands: (i) the bands observed in the IR spectrum recorded (in a separate experiment) for monomers of 2-mercaptopyridine trapped in an n-H2 matrix and (ii) a set of IR bands observed in the spectrum recorded after UV irradiation of N-hydroxypyridine-2(1H)-thione. It should be emphasized that the UV-induced processes, occurring for N-hydroxypyridine-2(1H)-thione isolated in an n-H2 matrix, lead to products that are significantly different from those generated from the compound trapped in solid Ar or in solid N2.

Boron confers salt tolerance through facilitating BnaA2.HKT1-mediated root xylem Na+ unloading in rapeseed (Brassica napus L.)

Boron (B) is an important limiting factor for plant growth and yield in saline soils, but the underlying molecular mechanisms remain poorly understood. In this study, we found that appropriate B supply obviously complemented rapeseed (Brassica napus L.) growth under salinity accompanied by higher biomass production and less reactive oxygen species accumulation. Determination of Na+ content in shoots and roots indicated that B significantly repressed root-to-shoot Na+ translocation, and non-invasive micro-tests of root xylem sap demonstrated that B increased xylem Na+ unloading in the roots of rapeseed plants under salinity. Comparative transcriptomic profiling revealed that B strongly upregulated BnaHKT1s expression, especially BnaA2.HKT1, in rapeseed roots exposed to salinity. In situ hybridizations analysis showed that BnaA2.HKT1 was significantly induced in root stelar tissues by high B (HB) under salinity. Green fluorescent protein and yeast heterologous expression showed that BnaA2.HKT1 functioned as a plasma membrane-localized Na+ transporter. Knockout of BnaA2.HKT1 by CRISPR/Cas9 resulted in hypersensitive of rapeseed plants to salinity even under HB condition, with higher shoot Na+ accumulation and lower biomass production. By contrast, overexpression of BnaA2.HKT1 ameliorated salinity-induced growth inhibition under B deficiency and salinity. Overall, our results proposed that B functioned as a positive regulator for the rapeseed growth and seed production under salt stress through facilitating BnaA2.HKT1-mediated root xylem Na+ unloading. This study may also provide an alternative strategy for the improvement of crop growth and development in saline soils.

Outward-rectifying potassium channels GORK and SKOR function in regulation of root growth under salt stress in Arabidopsis thaliana

Plants often face high salinity as a significant environmental challenge with roots being the first responders to this stress. Maintaining K+/Na+ ratio within plant cells is crucial for survival, as the intracellular K+ level decreases and the intracellular Na+ level increases under saline conditions. However, knowledge about the molecular regulatory mechanisms of K+ loss in response to salt stress through outward-rectifying K+ channels in plants is largely unknown. In this study, we found that the Arabidopsis double mutant gorkskor, in which the GORK and SKOR genes are disrupted, showed an improved primary root growth under salt stress compared to wild-type (WT) and the gork and skor single-mutant plants. No significant differences in the sensitivity to mannitol stress between the WT and gorkskor mutant were observed. Accumulation of ROS induced by salt stress was reduced in the gorkskor roots. The gorkskor mutant seedlings had significantly higher K+ content, lower Na+ content, and a greater resultant K+/Na+ ratio than the WT under salt stress. Moreover, salt-stress-induced elevation of cytosolic free Ca2+ concentration was reduced in the gorkskor roots. Taken together, these results suggest that Arabidopsis Shaker-type outward-rectifying K+ channels GORK and SKOR may redundantly function in regulation of primary root growth under salt stress and are involved in not only the late-stage response (e.g. K+ leakage) but also the early response including ROS production and [Ca2+]cyt elevation.

Chitosan-thiourea and their derivatives: Applications and action mechanisms for imparting drought tolerance

The increasing abiotic stresses from changing global climatic conditions, including drought, extreme temperatures, salinity, storms, pollutants, and floods, impend crop cultivation and sustainability. To mitigate these effects, numerous synthetic and non-synthetic chemicals or plant growth regulators are in practice. Chitosan, a natural organic substance rich in nitrogen and carbon, and thiourea, a synthetic plant growth regulator containing sulfur and nitrogen, have garnered significant interest for their roles in enhancing plant stress tolerance. Despite extensive use, the precise mechanisms of their actions remain unclear. Towards this endeavor, the present review examines how chitosan and thiourea contribute to stress tolerance in crop plants, particularly under drought conditions, to improve production and sustainability. It also explores thiourea's potential as a hydrogen sulfide (H2S) donor and the possible applications of thiolated chitosan derivatives and chitosan-thiourea combinations, emphasizing their biological functions and benefits for sustainable agriculture.

Cell death in bryophytes: emerging models to study core regulatory modules and conserved pathways

This review summarizes recent progress in our current understanding of the mechanisms underlying the cell death pathways in bryophytes, focusing on conserved pathways and particularities in comparison to angiosperms. Regulated cell death (RCD) plays key roles during essential processes along the plant life cycle. It is part of specific developmental programmes and maintains homeostasis of the organism in response to unfavourable environments. Bryophytes could provide valuable models to study developmental RCD processes as well as those triggered by biotic and abiotic stresses. Some pathways analogous to those present in angiosperms occur in the gametophytic haploid generation of bryophytes, allowing direct genetic studies. In this review, we focus on such RCD programmes, identifying core conserved mechanisms and raising new key questions to analyse RCD from an evolutionary perspective.

Understanding Ameliorating Effects of Boron on Adaptation to Salt Stress in Arabidopsis

When faced with salinity stress, plants typically exhibit a slowdown in their growth patterns. Boron (B) is an essential micronutrient for plants that are known to play a critical role in controlling cell wall properties. In this study, we used the model plant Arabidopsis thaliana Col-0 and relevant mutants to explore how the difference in B availability may modulate plant responses to salt stress. There was a visible root growth suppression of Col-0 with the increased salt levels in the absence of B while this growth reduction was remarkably alleviated by B supply. Pharmacological experiments revealed that orthovanadate (a known blocker of H+-ATPase) inhibited root growth at no B condition, but had no effect in the presence of 30 μM B. Salinity stress resulted in a massive K+ loss from mature zones of A. thaliana roots; this efflux was attenuated in the presence of B. Supplemental B also increased the magnitude of net H+ pumping by plant roots. Boron availability was also essential for root halotropism. Interestingly, the aha2Δ57 mutant with active H+-ATPase protein exhibited the same halotropism response as Col-0 while the aha2-4 mutant had a stronger halotropism response (larger bending angle) compared with that of Col-0. Overall, the ameliorative effect of B on the A. thaliana growth under salt stress is based on the H+-ATPase stimulation and a subsequent K+ retention, involving auxin- and ROS-pathways.

Phytofabricated gold nanoparticles as modulators of salt stress responses in spinach: implications for redox homeostasis, biochemical and physiological adaptation

Introduction

The utilization of plant material for synthesizing nanoparticles effectively triggers physiological and biochemical responses in plants to combat abiotic stresses. Salt stress, particularly caused by NaCl, significantly affects plant morphology and physiology, leading to reduced crop yields. Understanding the mechanisms of salt tolerance is crucial for maintaining crop productivity.

Methods

In this study, we examined the effects of 150 μM spinach-assisted gold nanoparticles (S-AuNPs) on various parameters related to seed germination, growth attributes, photosynthetic pigments, stomatal traits, ion concentrations, stress markers, antioxidants, metabolites, and nutritional contents of spinach plants irrigated with 50 mM NaCl.

Results

Results showed that S-AuNPs enhanced chlorophyll levels, leading to improved light absorption, increased photosynthates production, higher sugar content, and stimulated plant growth under NaCl stress. Stomatal traits were improved, and partially closed stomata were reopened with S-AuNPs treatment, possibly due to K+/Na+ modulation, resulting in enhanced relative water content and stomatal conductance. ABA content decreased under S-AuNPs application, possibly due to K+ ion accumulation. S-AuNPs supplementation increased proline and flavonoid contents while reducing ROS accumulation and lipid peroxidation via activation of both non-enzymatic and enzymatic antioxidants. S-AuNPs also regulated the ionic ratio of K+/Na+, leading to decreased Na+ accumulation and increased levels of essential ions in spinach plants under NaCl irrigation.

Discussion

Overall, these findings suggest that S-AuNPs significantly contribute to salt stress endurance in spinach plants by modulating various physiological attributes.

Is plant acoustic communication fact or fiction?

In recent years, the idea has flourished that plants emit and perceive sound and could even be capable of exchanging information through the acoustic channel. While research into plant bioacoustics is still in its infancy, with potentially fascinating discoveries awaiting ahead, here we show that the current knowledge is not conclusive. While plants do emit sounds under biotic and abiotic stresses such as drought, these sounds are high-pitched, of low intensity, and propagate only to a short distance. Most studies suggesting plant sensitivity to airborne sound actually concern the perception of substrate vibrations from the soil or plant part. In short, while low-frequency, high-intensity sounds emitted by a loudspeaker close to the plant seem to have tangible effects on various plant processes such as growth - a finding with possible applications in agriculture - it is unlikely that plants can perceive the sounds they produce, at least over long distances. So far, there is no evidence of plants communicating with each other via the acoustic channel.

Physiological and molecular mechanisms of plant-root responses to iron toxicity

The chemical form and physiological activity of iron (Fe) in soil are dependent on soil pH and redox potential (Eh), and Fe levels in soils are frequently elevated to the point of causing Fe toxicity in plants, with inhibition of normal physiological activities and of growth and development. In this review, we describe how iron toxicity triggers important physiological changes, including nitric-oxide (NO)-mediated potassium (K+) efflux at the tips of roots and accumulation of reactive oxygen species (ROS) and reactive nitrogen (RNS) in roots, resulting in physiological stress. We focus on the root system, as the first point of contact with Fe in soil, and describe the key processes engaged in Fe transport, distribution, binding, and other mechanisms that are drawn upon to defend against high-Fe stress. We describe the root-system regulation of key physiological processes and of morphological development through signaling substances such as ethylene, auxin, reactive oxygen species, and nitric oxide, and discuss gene-expression responses under high Fe. We especially focus on studies on the physiological and molecular mechanisms in rice and Arabidopsis under high Fe, hoping to provide a valuable theoretical basis for improving the ability of crop roots to adapt to soil Fe toxicity.

SOG1 and BRCA1 Interdependently Regulate RAD54 Expression for Repairing Salinity-Induced DNA Double-Strand Breaks in Arabidopsis

As sessile organisms, land plants experience various forms of environmental stresses throughout their life span. Therefore, plants have developed extensive and complicated defense mechanisms, including a robust DNA damage response (DDR) and DNA repair systems for maintaining genome integrity. In Arabidopsis, the NAC [NO APICAL MERISTEM (NAM), ARABIDOPSIS TRANSCRIPTION ACTIVATION FACTOR (ATAF), CUP-SHAPED COTYLEDON (CUC)] domain family transcription factor SUPPRESSOR OF GAMMA RESPONSE 1 (SOG1) plays an important role in regulating DDR. Here, we show that SOG1 plays a key role in regulating the repair of salinity-induced DNA double-strand breaks (DSBs) via the homologous recombination (HR) pathway in Arabidopsis. The sog1-1 mutant seedlings display a considerably slower rate of repair of salinity-induced DSBs. Accumulation of SOG1 protein increases in wild-type Arabidopsis under salinity stress, and it enhances the expression of HR pathway-related genes, including RAD51, RAD54 and BReast CAncer gene 1 (BRCA1), respectively, as found in SOG1 overexpression lines. SOG1 binds specifically to the AtRAD54 promoter at the 5'-(N)4GTCAA(N)3C-3' consensus sequence and positively regulates its expression under salinity stress. The phenotypic responses of sog1-1/atrad54 double mutants suggest that SOG1 functions upstream of RAD54, and both these genes are essential in regulating DDR under salinity stress. Furthermore, SOG1 interacts directly with BRCA1, an important component of the HR-mediated DSB repair pathway in plants, where BRCA1 appears to facilitate the binding of SOG1 to the RAD54 promoter. At the genetic level, SOG1 and BRCA1 function interdependently in modulating RAD54 expression under salinity-induced DNA damage. Together, our results suggest that SOG1 regulates the repair of salinity-induced DSBs via the HR-mediated pathway through genetic interactions with RAD54 and BRCA1 in Arabidopsis.

Exploring Natural Variations in Arabidopsis thaliana: Plant Adaptability to Salt Stress

The increase in soil salinization represents a current challenge for plant productivity, as most plants, including crops, are mainly salt-sensitive species. The identification of molecular traits underpinning salt tolerance represents a primary goal for breeding programs. In this scenario, the study of intraspecific variability represents a valid tool for investigating natural genetic resources evolved by plants in different environmental conditions. As a model system, Arabidopsis thaliana, including over 750 natural accessions, represents a species extensively studied at phenotypic, metabolic, and genomic levels under different environmental conditions. Two haplogroups showing opposite root architecture (shallow or deep roots) in response to auxin flux perturbation were identified and associated with EXO70A3 locus variations. Here, we studied the influence of these genetic backgrounds on plant salt tolerance. Eight accessions belonging to the two haplogroups were tested for salt sensitivity by exposing them to moderate (75 mM NaCl) or severe (150 mM NaCl) salt stress. Salt-tolerant accessions were found in both haplogroups, and all of them showed efficient ROS-scavenging ability. Even if an exclusive relation between salt tolerance and haplogroup membership was not observed, the modulation of root system architecture might also contribute to salt tolerance.

Electrophysiological techniques in marine microalgae study: A new perspective for harmful algal bloom (HAB) research

Electrophysiological techniques, by measuring bioelectrical signals and ion channel activities in tissues and cells, are now widely utilized to study ion channel-related physiological functions and their underlying mechanisms. Electrophysiological techniques have been extensively employed in the investigation of animals, plants, and microorganisms; however, their application in marine algae lags behind that in other organisms. In this paper, we present an overview of current electrophysiological techniques applicable to algae while reviewing the historical usage of such techniques in this field. Furthermore, we explore the potential specific applications of electrophysiological technology in harmful algal bloom (HAB) research. The application prospects in the studies of stress tolerance, competitive advantage, nutrient absorption, toxin synthesis and secretion by HAB microalgae are discussed and anticipated herein with the aim of providing novel perspectives on HAB investigations.

Application of thiourea ameliorates drought induced oxidative injury in Linum usitatissimum L. by regulating antioxidant defense machinery and nutrients absorption

Thiourea (TU) is considered an essential and emerging biostimulant against the negative impacts of severe environmental stresses, including drought stress in plants. However, the knowledge about the foliar application of TU to mitigate drought stress in Linum usitatissimum L., has yet to be discovered. The present study was designed to assess the impact of foliar application of TU for its effects against drought stress in two flax cultivars. The study comprised two irrigation regimes [60% field capacity (FC) and the control (100% FC)], along with TU (0, 500, 1000 mg L-1) application at the vegetative stage. The findings indicated that drought stress reduced the shoot fresh weight (44.2%), shoot dry weight (67.5%), shoot length (41.5%), total chlorophyll (51.6%), and carotenoids (58.8%). Drought stress increased both cultivars' hydrogen peroxide (H2O2) and malondialdehyde (MDA). Foliar application of TU (1000 mg L-1) enhanced the growth and chlorophyll contents with or without drought stress. Under drought stress (60% FC), TU decreased MDA and H2O2 contents up to twofold. Moreover, TU application increased catalase (40%), peroxidase (13%), superoxide dismutase (30%), and total soluble protein contents (32.4%) differentially in both cultivars. Nevertheless, TU increased calcium (Ca2+) (42.8%), potassium (K+) (33.4%), and phosphorus (P) (72%) in shoots and decreased the elevated sodium (Na+) (28.2%) ions under drought stress. It is suggested that TU application (1000 mg L-1) enhances the growth potential of flax by enhancing photosynthetic pigment, nutrient uptake, and antioxidant enzymes under drought stress. Research outcomes, therefore, recommend that TU application can ameliorate drought-induced negative effects in L. usitatissimum L. seedlings, resulting in improved plant growth and mineral composition, as depicted by balanced primary and secondary metabolite accumulation.

Reactive oxygen species signaling in melatonin-mediated plant stress response

Reactive oxygen species (ROS) are crucial signaling molecules in plants that play multifarious roles in prompt response to environmental stimuli. Despite the classical thoughts that ROS are toxic when accumulate in excess, recent advances in plant ROS signaling biology reveal that ROS participate in biotic and abiotic stress perception, signal integration, and stress-response network activation, hence contributing to plant defense and stress tolerance. ROS production, scavenging and transport are fine-tuned by plant hormones and stress-response signaling pathways. Crucially, the emerging plant hormone melatonin attenuates excessive ROS accumulation under stress, whereas ROS signaling mediates melatonin-induced plant developmental response and stress tolerance. In particular, RESPIRATORY BURST OXIDASE HOMOLOG (RBOH) proteins responsible for apoplastic ROS generation act downstream of melatonin to mediate stress response. In this review, we discuss promising developments in plant ROS signaling and how ROS might mediate melatonin-induced plant resilience to environmental stress.

Cyclic guanosine monophosphate improves salt tolerance in Solanum lycopersicum

The cyclic nucleotide cyclic guanosine monophosphate (cGMP) is a powerful cell signaling molecule involved in biotic and abiotic stress perception and signal transduction. In the model plant Arabidopsis thaliana, salt and osmotic stress rapidly induce increase in cGMP which plays role by modulating the activity of monovalent cation transporters, possibly by direct binding to these proteins and by altering the expression of many abiotic stress responsive genes. In a recent study, a membrane permeable analogue of cGMP (8-bromo-cGMP) was found to have a promotive effect on soluble sugar, flavonoids and lignin content, and membrane integrity in Solanum lycopersicum seedlings under salt stress. However, it remains to be elucidated how salt stress affects the endogenous cGMP level in S. lycopersicum and if Br-cGMP-induced improvement in salt tolerance in S. lycopersicum involves altered cation fluxes. The current study was conducted to answer these questions. A rapid increase (within 30 s) in endogenous cGMP level was determined in S. lycopersicum roots after treatment with 100 mM NaCl. Addition of membrane permeable Br-cGMP in growth medium remarkably ameliorated the inhibitory effects of NaCl on seedlings' growth parameters, chlorophyll content and net photosynthesis rate. In salt stressed plants, Br-cGMP significantly decreased Na+ content by reducing its influx and increasing efflux while it improved plants K+ content by reducing its efflux and enhancing influx. Furthermore, supplementation with Br-cGMP improved plant's proline content and total antioxidant capacity, resulting in markedly decreased electrolyte leakage under salt stress. Br-cGMP increased the expression of Na+/H+ antiporter genes in roots and shoots of S. lycopersicum growing under salt stress, potentially enhancing plant's ability to sequester Na+ into the vacuole. The findings of this study provide insights into the mechanism of cGMP-induced salt stress tolerance in S. lycopersicum.

The IAA17.1/HSFA5a module enhances salt tolerance in Populus tomentosa by regulating flavonol biosynthesis and ROS levels in lateral roots

Auxin signaling provides a promising approach to controlling root system architecture and improving stress tolerance in plants. However, how the auxin signaling is transducted in this process remains unclear. The Aux indole-3-acetic acid (IAA) repressor IAA17.1 is stabilized by salinity, and primarily expressed in the lateral root (LR) primordia and tips in poplar. Overexpression of the auxin-resistant form of IAA17.1 (IAA17.1m) led to growth inhibition of LRs, markedly reduced salt tolerance, increased reactive oxygen species (ROS) levels, and decreased flavonol content. We further identified that IAA17.1 can interact with the heat shock protein HSFA5a, which was highly expressed in roots and induced by salt stress. Overexpression of HSFA5a significantly increased flavonol content, reduced ROS accumulation, enhanced LR growth and salt tolerance in transgenic poplar. Moreover, HSFA5a could rescue the defective phenotypes caused by IAA17.1m. Expression analysis showed that genes associated with flavonol biosynthesis were altered in IAA17.1m- and HAFA5a-overexpressing plants. Furthermore, we identified that HSFA5a directly activated the expression of key enzyme genes in the flavonol biosynthesis pathway, while IAA17.1 suppressed HSFA5a-mediated activation of these genes. Collectively, the IAA17.1/HSFA5a module regulates flavonol biosynthesis, controls ROS accumulation, thereby modulating the root system of poplar to adapt to salt stress.

Ion Changes and Signaling under Salt Stress in Wheat and Other Important Crops

High concentrations of sodium (Na+), chloride (Cl-), calcium (Ca2+), and sulphate (SO42-) are frequently found in saline soils. Crop plants cannot successfully develop and produce because salt stress impairs the uptake of Ca2+, potassium (K+), and water into plant cells. Different intracellular and extracellular ionic concentrations change with salinity, including those of Ca2+, K+, and protons. These cations serve as stress signaling molecules in addition to being essential for ionic homeostasis and nutrition. Maintaining an appropriate K+:Na+ ratio is one crucial plant mechanism for salt tolerance, which is a complicated trait. Another important mechanism is the ability for fast extrusion of Na+ from the cytosol. Ca2+ is established as a ubiquitous secondary messenger, which transmits various stress signals into metabolic alterations that cause adaptive responses. When plants are under stress, the cytosolic-free Ca2+ concentration can rise to 10 times or more from its resting level of 50-100 nanomolar. Reactive oxygen species (ROS) are linked to the Ca2+ alterations and are produced by stress. Depending on the type, frequency, and intensity of the stress, the cytosolic Ca2+ signals oscillate, are transient, or persist for a longer period and exhibit specific "signatures". Both the influx and efflux of Ca2+ affect the length and amplitude of the signal. According to several reports, under stress Ca2+ alterations can occur not only in the cytoplasm of the cell but also in the cell walls, nucleus, and other cell organelles and the Ca2+ waves propagate through the whole plant. Here, we will focus on how wheat and other important crops absorb Na+, K+, and Cl- when plants are under salt stress, as well as how Ca2+, K+, and pH cause intracellular signaling and homeostasis. Similar mechanisms in the model plant Arabidopsis will also be considered. Knowledge of these processes is important for understanding how plants react to salinity stress and for the development of tolerant crops.

Understanding plant responses to saline waterlogging: insights from halophytes and implications for crop tolerance

MAIN CONCLUSION

Saline and wet environments stress most plants, reducing growth and yield. Halophytes adapt with ion regulation, energy maintenance, and antioxidants. Understanding these mechanisms aids in breeding resilient crops for climate change. Waterlogging and salinity are two abiotic stresses that have a major negative impact on crop growth and yield. These conditions cause osmotic, ionic, and oxidative stress, as well as energy deprivation, thus impairing plant growth and development. Although few crop species can tolerate the combination of salinity and waterlogging, halophytes are plant species that exhibit high tolerance to these conditions due to their morphological, anatomical, and metabolic adaptations. In this review, we discuss the main mechanisms employed by plants exposed to saline waterlogging, intending to understand the mechanistic basis of their ion homeostasis. We summarize the knowledge of transporters and channels involved in ion accumulation and exclusion, and how they are modulated to prevent cytosolic toxicity. In addition, we discuss how reactive oxygen species production and cell signaling enhance ion transport and aerenchyma formation, and how plants exposed to saline waterlogging can control oxidative stress. We also address the morphological and anatomical modifications that plants undergo in response to combined stress, including aerenchyma formation, root porosity, and other traits that help to mitigate stress. Furthermore, we discuss the peculiarities of halophyte plants and their features that can be leveraged to improve crop yields in areas prone to saline waterlogging. This review provides valuable insights into the mechanisms of plant adaptation to saline waterlogging thus paving the path for future research on crop breeding and management strategies.

Melatonin enhances KCl salinity tolerance by maintaining K+ homeostasis in Malus hupehensis

Large amounts of potash fertilizer are often applied to apple (Malus domestica) orchards to enhance fruit quality and yields, but this treatment aggravates KCl-based salinity stress. Melatonin (MT) is involved in a variety of abiotic stress responses in plants. However, its role in KCl stress tolerance is still unknown. In the present study, we determined that an appropriate concentration (100 μm) of MT significantly alleviated KCl stress in Malus hupehensis by enhancing K+ efflux out of cells and compartmentalizing K+ in vacuoles. Transcriptome deep-sequencing analysis identified the core transcription factor gene MdWRKY53, whose expression responded to both KCl and MT treatment. Overexpressing MdWRKY53 enhanced KCl tolerance in transgenic apple plants by increasing K+ efflux and K+ compartmentalization. Subsequently, we characterized the transporter genes MdGORK1 and MdNHX2 as downstream targets of MdWRKY53 by ChIP-seq. MdGORK1 localized to the plasma membrane and enhanced K+ efflux to increase KCl tolerance in transgenic apple plants. Moreover, overexpressing MdNHX2 enhanced the KCl tolerance of transgenic apple plants/callus by compartmentalizing K+ into the vacuole. RT-qPCR and LUC activity analyses indicated that MdWRKY53 binds to the promoters of MdGORK1 and MdNHX2 and induces their transcription. Taken together, our findings reveal that the MT-WRKY53-GORK1/NHX2-K+ module regulates K+ homeostasis to enhance KCl stress tolerance in apple. These findings shed light on the molecular mechanism of apple response to KCl-based salinity stress and lay the foundation for the practical application of MT in salt stress.

Expression of sucrose metabolizing enzymes in different sugarcane varieties under progressive heat stress

Studying the thermal stress effect on sucrose-metabolizing enzymes in sugarcane is of great importance for understanding acclimation to thermal stress. In this study, two varieties, S2003-US-633 and SPF-238, were grown at three different temperatures ( ± 2°C): 30°C as a control, 45°C for various episodes of high temperature treatments and recovery conditions at 24, 48 and 72 hours. Data showed that reducing sugar content increased until the grand growth stage but sharply declined at the maturity stage in both cultivars. On the other hand, sucrose is enhanced only at the maturity stage. The expression of all invertase isozymes declined prominently; however, the expression of SPS was high at the maturity stage. Hence, the sucrose accumulation in mature cane was due to increased SPS activity while decreased invertase isozymes (vacuolar, cytoplasmic and cell wall) activities at maturity stage in both cultivars. Heat shock decreased the sucrose metabolizing enzymes, sucrose content and sugar recovery rate in both cultivars. In contrast, heat-shock treatments induced maximum proline, MDA, H2O2 and EC in both cultivars. Notably, this is the first report of diverse invertase isozyme molecular weight proteins, such as those with 67, 134 and 160 kDa, produced under heat stress, suggesting that these enzymes have varied activities at different developmental stages. Overall, S2003-US-633 performs better than the cultivar SPF-238 under heat stress conditions at all development stages, with increased sucrose content, enzyme expression, proline and sugar recovery rate. This work will provide a new avenue regarding sugarcane molecular breeding programs with respect to thermal stress.

Reactive oxygen species- and nitric oxide-dependent regulation of ion and metal homeostasis in plants

Deterioration and impoverishment of soil, caused by environmental pollution and climate change, result in reduced crop productivity. To adapt to hostile soils, plants have developed a complex network of factors involved in stress sensing, signal transduction, and adaptive responses. The chemical properties of reactive oxygen species (ROS) and reactive nitrogen species (RNS) allow them to participate in integrating the perception of external signals by fine-tuning protein redox regulation and signal transduction, triggering specific gene expression. Here, we update and summarize progress in understanding the mechanistic basis of ROS and RNS production at the subcellular level in plants and their role in the regulation of ion channels/transporters at both transcriptional and post-translational levels. We have also carried out an in silico analysis of different redox-dependent modifications of ion channels/transporters and identified cysteine and tyrosine targets of nitric oxide in metal transporters. Further, we summarize possible ROS- and RNS-dependent sensors involved in metal stress sensing, such as kinases and phosphatases, as well as some ROS/RNS-regulated transcription factors that could be involved in metal homeostasis. Understanding ROS- and RNS-dependent signaling events is crucial to designing new strategies to fortify crops and improve plant tolerance of nutritional imbalance and metal toxicity.

Modulation of potassium transport to increase abiotic stress tolerance in plants

Potassium is the major cation responsible for the maintenance of the ionic environment in plant cells. Stable potassium homeostasis is indispensable for virtually all cellular functions, and, concomitantly, viability. Plants must cope with environmental changes such as salt or drought that can alter ionic homeostasis. Potassium fluxes are required to regulate the essential process of transpiration, so a constraint on potassium transport may also affect the plant's response to heat, cold, or oxidative stress. Sequencing data and functional analyses have defined the potassium channels and transporters present in the genomes of different species, so we know most of the proteins directly participating in potassium homeostasis. The still unanswered questions are how these proteins are regulated and the nature of potential cross-talk with other signaling pathways controlling growth, development, and stress responses. As we gain knowledge regarding the molecular mechanisms underlying regulation of potassium homeostasis in plants, we can take advantage of this information to increase the efficiency of potassium transport and generate plants with enhanced tolerance to abiotic stress through genetic engineering or new breeding techniques. Here, we review current knowledge of how modifying genes related to potassium homeostasis in plants affect abiotic stress tolerance at the whole plant level.

Ion Channels in Electrical Signaling in Higher Plants

Electrical signals (ESs) appearing in plants under the action of various external factors play an important role in adaptation to changing environmental conditions. Generation of ES in higher plant cells is associated with activation of Ca2+, K+, and anion fluxes, as well as with changes in the activity of plasma membrane H+-ATPase. In the present review, molecular nature of the ion channels contributing to ESs transmission in higher plants is analyzed based on comparison of the data from molecular-genetic and electrophysiological studies. Based on such characteristics of ion channels as selectivity, activation mechanism, and intracellular and tissue localization, those ion channels that meet the requirements for potential participation in ES generation were selected from a wide variety of ion channels in higher plants. Analysis of the data of experimental studies performed on mutants with suppressed or enhanced expression of a certain channel gene revealed those channels whose activation contributes to ESs formation. The channels responsible for Ca2+ flux during generation of ESs include channels of the GLR family, for K+ flux - GORK, for anions - MSL. Consideration of the prospects of further studies suggests the need to combine electrophysiological and genetic approaches along with analysis of ion concentrations in intact plants within a single study.

Long-Distance Electrical and Calcium Signals Evoked by Hydrogen Peroxide in Physcomitrella

Electrical and calcium signals in plants are some of the basic carriers of information that are transmitted over a long distance. Together with reactive oxygen species (ROS) waves, electrical and calcium signals can participate in cell-to-cell signaling, conveying information about different stimuli, e.g. abiotic stress, pathogen infection or mechanical injury. There is no information on the ability of ROS to evoke systemic electrical or calcium signals in the model moss Physcomitrella nor on the relationships between these responses. Here, we show that the external application of hydrogen peroxide (H2O2) evokes electrical signals in the form of long-distance changes in the membrane potential, which transmit through the plant instantly after stimulation. The responses were calcium-dependent since their generation was inhibited by lanthanum, a calcium channel inhibitor (2 mM), and EDTA, a calcium chelator (0.5 mM). The electrical signals were partially dependent on glutamate receptor (GLR) ion channels since knocking-out the GLR genes only slightly reduced the amplitude of the responses. The basal part of the gametophyte, which is rich in protonema cells, was the most sensitive to H2O2. The measurements carried out on the protonema expressing fluorescent calcium biosensor GCaMP3 proved that calcium signals propagated slowly (>5 µm/s) and showed a decrement. We also demonstrate upregulation of a stress-related gene that appears in a distant section of the moss 8 min after the H2O2 treatment. The results help understand the importance of both types of signals in the transmission of information about the appearance of ROS in the plant cell apoplast.

Evaluating the genotoxicity of salinity stress and secondary products gene manipulation in lime, Citrus aurantifolia, plants

Salinity is a significant abiotic stress that has a profound effect on growth, the content of secondary products, and the genotoxicity of cells. Lime, Citrus aurantifolia, is a popular plant belonging to the family Rutaceae. The interest in cultivating this plant is due to the importance of its volatile oil, which is included in many pharmaceutical industries, but C. aurantifolia plants are affected by the NaCl salinity levels. In the present study, a comet assay test has been applied to evaluate the genotoxic impact of salinity at 0, 50, 100, and 200 mM of NaCl on C. aurantifolia tissue-cultured plants. Furthermore, terpene gene expression was investigated using a semi-quantitative real-time polymerase chain reaction. Results from the two analyses revealed that 200 mM of NaCl stress resulted in high levels of severe damage to the C. aurantifolia plants' DNA tail 21.8%, tail length 6.56 µm, and tail moment 3.19 Unit. The relative highest expression of RtHK and TAT genes was 2.08, and 1.693, respectively, when plants were exposed to 200 mM of NaCl, whereas pv4CL2RT expressed 1.50 in plants subjected to 100 mM of NaCl. The accumulation of transcripts for the RTMYB was 0.951 when plants were treated with NaCl at 50 mM, and RtGPPS gene was significantly decreased to 0.446 during saline exposure at 100 mM. We conclude that the comet assay test offers an appropriate tool to detect DNA damage as well as RtHK, TAT, and pv4CL2RT genes having post-transcriptional regulation in C. aurantifolia plant cells under salinity stress. Future studies are needed to assess the application of gene expression and comet assay technologies using another set of genes that show vulnerability to different stresses on lime and other plants.

Reuse of treated urban wastewater on the growth and physiology of Medicago sativa L. cv. Gea and Petroselinum crispum L. cv. Commun: correlation with oxydative stress and DNA damage

The freshwater scarcity is one of the major environmental problems, which is why the water reuse has become a possible remedy to cope with the shortage of water needed for agriculture irrigation. This study focuses on the evaluation of the irrigation effect with treated effluent from wastewater treatment plant in Tunisia on parsley (Petroselinum crispum L. cv. Commun) used as human food and alfalfa (Medicago sativa L. cv. Gea) as animal food. In vitro germination test was conducted at different dilution levels of wastewater as rejected into the environment (25, 50, and 100%) and wastewater with further treatment (TWW). Results have shown that wastewater with dilution of 25% as well as TWW positively affected the physiological parameters in comparison with the dilutions 50 and 100%. However, the tap water (TW) applied as control treatment has shown the best effects. Oxidative stress evaluated by malondialdehyde (MDA) content was in agreement with the physiological results and showed that the most stressed seeds were those treated with the dilutions 50 and 100%. A pot trial was also conducted to evaluate the suitability of WW and TWW in comparison to TW. Results have shown that TWW is more adapted than WW for irrigation as an improvement of growth and physiological parameters was recorded. Oxidative stress assessed with MDA and proline content has shown that plants irrigated with WW significantly accumulate MDA and proline compared to TWW. The TW has shown the lowest values. DNA damage was evaluated by extraction and agarose gel electrophoresis. It has revealed degradation of DNA for plants irrigated with WW. According to these results, it can be concluded that TWW can be used for irrigation of plants destined for human or animal foods. So, it can be a hydric alternative to resolve the problem of water deficit in semi-arid countries.

DORN1 and GORK regulate stomatal closure in Arabidopsis mediated by volatile organic compound ethyl vinyl ketone

Evk (ethyl vinyl ketone) is a signal substance for plant defense, but little is known about how evk mediates stomatal closure. Through stomatal biology experiments, we found that evk can mediate stomatal closure, and stomatal closure is weakened when DORN1 (DOES NOT RESPOND TO NUCLEOTIDES 1) and GORK (GATED OUTWARDLY-RECTIFYING K+ CHANNEL) are mutated. In addition, it was found by non-invasive micro-test technology (NMT) that the K⁺ efflux mediated by evk was significantly weakened when DORN and GORK were mutated. Yeast two-hybrid (Y2H), firefly luciferase complementation imaging (LCI), and in vitro pull-down assays demonstrated that DORN1 and GORK could interact in vitro and in vivo. It was found by molecular docking that evk could combine with MRP (Multidrug Resistance-associated Protein), thus affecting ATP transport, promoting eATP (extracellular ATP) concentration increase and realizing downstream signal transduction. Through inoculation of botrytis cinerea, it was found that evk improved the antibacterial activity of Arabidopsis thaliana. As revealed by reverse transcription quantitative PCR (RT-qPCR), the expression of defense related genes was enhanced by evk treatment. Evk is a potential green antibacterial drug.

The UDP-glucuronic acid decarboxylase OsUXS3 regulates Na+ ion toxicity tolerance under salt stress by interacting with OsCATs in rice

Overly Na⁺ ion in soil caused by salt stress has a significant negative impact on the growth and production of crops, especially rice (Oryza sativa L.). Therefore, it is vital for us to clarify how salt stress tolerance in rice is caused by Na⁺ ion toxicity. The UDP-glucuronic acid decarboxylase (UXS) is a critical enzyme in the biosynthesis of UDP-xylose, which is the key substrate of cytoderm synthesis in plants. In this study, we found that OsUXS3, a rice UXS, is a positive regulator to regulate Na⁺ ion toxicity under salt stress by interacting with OsCATs (Oryza sativa catalase; OsCAT). The expression of OsUXS3 was significantly up-regulated under NaCl and NaHCO₃ treatments of rice seedlings. Meanwhile, by the genetic and biochemical evidence, knockout of OsUXS3 significantly increased reactive oxygen species (ROS) levels and decreased CAT activity under NaCl and NaHCO₃ treatments in tissue. Furthermore, knockout of OsUXS3 caused excessive accumulation of Na ⁺ ion and rapid loss of K⁺ ion and disrupts Na⁺/K⁺ homeostasis under NaCl and NaHCO₃ treatments. Based on the results above, we can conclude that OsUXS3 might regulate CAT activity by interacting with OsCATs, which is not only characterized for the first time but also regulating Na⁺/K⁺ homeostasis, positively regulating the Na⁺ ion toxicity tolerance under salt stress in rice.

Comparative effects of copper nanoparticles and copper oxide nanoparticles on physiological characteristics and mineral element accumulation in Brassica chinensis L

In this study, the short-term toxicity of Cu NPs, CuO NPs, and CuSO₄ on bok choy (Brassica chinensis L.) under hydroponic conditions was evaluated using indicators such as biomass, net photosynthesis rate, root morphology, enzyme activity, and Cu accumulation and subcellular distribution. Results showed that CuO NPs exposure notably increased the biomass, root length, and root tip number by 22.0%, 22.7%, and 82.9%, respectively, whereas Cu NPs and CuSO₄ significantly reduced root biomass, net photosynthetic rate (P_N), and root length by 31.2% and 44.2%, 24.5% and 32.2%, and 43.4% and 40.6%, respectively. In addition, Cu NPs, CuO NPs and CuSO₄ exposure increased the distribution of Cu in soluble component and cell wall. Moreover, short-term exposure to different Cu forms significantly affected mineral element accumulation in bok choy. For instance, Cu NPs exposure reduced the concentrations of Mg, Ca and Mn in edible part by 21.7%, 16.1% and 23.2%, respectively. CuSO₄ exposure reduced the concentrations of Mg and Ca in edible part by 12.3% and 50.1%, respectively. CuO NPs caused a significant increase of 30.4% for Ca concentration in root, 34.5% and 34.5% for K and Mn concentration in edible part. Over all, CuO NPs exposure was beneficial for plant growth. These findings help understand the phytotoxic effect of different Cu forms on bok choy, and CuO NPs has the potential to be applied to improve nutrition and prompt growth in edible plants.

Metabolic imprint induced by seed halo-priming promotes a differential physiological performance in two contrasting quinoa ecotypes

"Memory imprint" refers to the process when prior exposure to stress prepares the plant for subsequent stress episodes. Seed priming is a strategy to change the performance of seedlings to cope with stress; however, mechanisms associated with the metabolic response are fragmentary. Salinity is one of the major abiotic stresses that affect crop production in arid and semiarid areas. Chenopodium quinoa Willd. (Amaranthaceae) is a promising crop to sustain food security and possesses a wide genetic diversity of salinity tolerance. To elucidate if the metabolic memory induced by seed halo-priming (HP) differs among contrasting saline tolerance plants, seeds of two ecotypes of Quinoa (Socaire from Atacama Salar, and BO78 from Chilean Coastal/lowlands) were treated with a saline solution and then germinated and grown under different saline conditions. The seed HP showed a more positive impact on the sensitive ecotype during germination and promoted changes in the metabolomic profile in both ecotypes, including a reduction in carbohydrates (starch) and organic acids (citric and succinic acid), and an increase in antioxidants (ascorbic acid and α-tocopherol) and related metabolites. These changes were linked to a further reduced level of oxidative markers (methionine sulfoxide and malondialdehyde), allowing improvements in the energy use in photosystem II under saline conditions in the salt-sensitive ecotype. In view of these results, we conclude that seed HP prompts a "metabolic imprint" related to ROS scavenger at the thylakoid level, improving further the physiological performance of the most sensitive ecotype.

Melatonin delays ABA-induced leaf senescence via H2 O2 -dependent calcium signalling

Precocious leaf senescence can reduce crop yield and quality by limiting the growth stage. Melatonin has been shown to delay leaf senescence; however, the underlying mechanism remains obscure. Here, we show that melatonin offsets abscisic acid (ABA) to protect photosystem II and delay the senescence of attached old leaves under the light. Melatonin induced H₂ O₂ accumulation accompanied by an upregulation of melon respiratory burst oxidase homolog D (CmRBOHD) under ABA-induced stress. Both melatonin and H₂ O₂ induced the accumulation of cytoplasmic-free Ca²⁺ ([Ca²⁺ ]_cyt ) in response to ABA, while blocking of Ca²⁺ influx channels attenuated melatonin- and H₂ O₂ -induced ABA tolerance. CmRBOHD overexpression induced [Ca²⁺ ]_cyt accumulation and delayed leaf senescence, whereas deletion of Arabidopsis AtRBOHD, a homologous gene of CmRBOHD, compromised the melatonin-induced [Ca²⁺ ]_cyt accumulation and delay of leaf senescence in Arabidopsis under ABA stress. Furthermore, melatonin, H₂ O₂  and Ca²⁺ attenuated ABA-induced K⁺ efflux and subsequent cell death. CmRBOHD overexpression and AtRBOHD deletion alleviated and aggravated the ABA-induced K⁺ efflux, respectively. Taken together, our study unveils a new mechanism by which melatonin offsets ABA action to delay leaf senescence via RBOHD-dependent H₂ O₂ production that triggers [Ca²⁺ ]_cyt accumulation and subsequently inhibits K⁺ efflux and delays cell death/leaf senescence in response to ABA.

The disruption of calcium and hydrogen ion homeostasis of submerged macrophyte Vallisneria natans (Lour.) Hara caused by microcystin-LR

Aquatic plants play an important role in maintaining lake water status and ecosystem stability, but the effect of the cyanotoxin microcystin (MC) on ion homeostasis in aquatic plants and the resulting adverse consequences remains unclear. This study used non-invasive micro-test technology to detect the effect of MC-LR on homeostasis of calcium (Ca²⁺) and hydrogen ions (H⁺) in Vallisneria natans (Lour.) Hara, and examined the relationship between ion homeostasis and physiological indicators. Results showed that 1) MC-LR was enriched in V. natans tissues, with greater absorption in roots than in leaves, and 2) MC-LR induced a sustained and dose-dependent Ca²⁺ efflux from leaves and recoverable Ca²⁺ efflux from roots. Although H⁺-ATPase of leaves and roots was activated by MC-LR, the effluent of H⁺ from roots and influent of H⁺ into leaves was enhanced. By affecting the homeostasis of Ca²⁺ and H⁺, MC-LR directly or indirectly affected accumulation of nutrients essential for maintaining normal growth: accumulation of nitrogen, magnesium, phosphorus, calcium, iron, and zinc decreased in leaves; calcium, magnesium, and zinc decreased in roots; and potassium showed an increase in both leaves and roots. Microscopy revealed MC-LR results in leaf swelling and reduced accumulation of protein and starch, presumably due to changes in nutrient processes. In addition, efflux of Ca²⁺ and reduced accumulation of transition metals resulted in decreased ROS levels in leaves and roots. The disruption of ionic homeostasis in aquatic plants can be caused by as small a concentration as 1 μg/L MC-LR, indicating potential ecological impacts caused by microcystin need greater attention.

Salt-Induced Changes in Cytosolic pH and Photosynthesis in Tobacco and Potato Leaves

Salinity is one of the most common factors limiting the productivity of crops. The damaging effect of salt stress on many vital plant processes is mediated, on the one hand, by the osmotic stress caused by large concentrations of Na⁺ and Cl^- outside the root and, on the other hand, by the toxic effect of these ions loaded in the cell. In our work, the influence of salinity on the changes in photosynthesis, transpiration, water content and cytosolic pH in the leaves of two important crops of the Solanaceae family-tobacco and potato-was investigated. Salinity caused a decrease in photosynthesis activity, which manifested as a decrease in the quantum yield of photosystem II and an increase in non-photochemical quenching. Along with photosynthesis limitation, there was a slight reduction in the relative water content in the leaves and a decrease in transpiration, determined by the crop water stress index. Furthermore, a decrease in cytosolic pH was detected in tobacco and potato plants transformed by the gene of pH-sensitive protein Pt-GFP. The potential mechanisms of the salinity influence on the activity of photosynthesis were analyzed with the comparison of the parameters' dynamics, as well as the salt content in the leaves.

To Be or Not to Be? Are Reactive Oxygen Species, Antioxidants, and Stress Signalling Universal Determinants of Life or Death?

In the environmental and organism context, oxidative stress is complex and unavoidable. Organisms simultaneously cope with a various combination of stress factors in natural conditions. For example, excess light stress is accompanied by UV stress, heat shock stress, and/or water stress. Reactive oxygen species (ROS) and antioxidant molecules, coordinated by electrical signalling (ES), are an integral part of the stress signalling network in cells and organisms. They together regulate gene expression to redirect energy to growth, acclimation, or defence, and thereby, determine cellular stress memory and stress crosstalk. In plants, both abiotic and biotic stress increase energy quenching, photorespiration, stomatal closure, and leaf temperature, while toning down photosynthesis and transpiration. Locally applied stress induces ES, ROS, retrograde signalling, cell death, and cellular light memory, then acclimation and defence responses in the local organs, whole plant, or even plant community (systemic acquired acclimation, systemic acquired resistance, network acquired acclimation). A simplified analogy can be found in animals where diseases vs. fitness and prolonged lifespan vs. faster aging, are dependent on mitochondrial ROS production and ES, and body temperature is regulated by sweating, temperature-dependent respiration, and gene regulation. In this review, we discuss the universal features of stress factors, ES, the cellular production of ROS molecules, ROS scavengers, hormones, and other regulators that coordinate life and death.

β-Cyclocitral: Emerging Bioactive Compound in Plants

β-cyclocitral (βCC), a main apocarotenoid of β-carotene, increases plants’ resistance against stresses. It has recently appeared as a novel bioactive composite in a variety of organisms from plants to animals. In plants, βCC marked as stress signals that accrue under adverse ecological conditions. βCC regulates nuclear gene expression through several signaling pathways, leading to stress tolerance. In this review, an attempt has been made to summarize the recent findings of the potential role of βCC. We emphasize the βCC biosynthesis, signaling, and involvement in the regulation of abiotic stresses. From this review, it is clear that discussing compound has great potential against abiotic stress tolerance and be used as photosynthetic rate enhancer. In conclusion, this review establishes a significant reference base for future research.

Establishment and preliminary study of electrophysiological techniques in a typical red tide species

Electrophysiology studies the electrical properties of cells and tissues including bioelectrical signals and membrane ion channel activities. As an important means to reveal ion channel related physiological functions and the underlying mechanisms, electrophysiological techniques have been widely used in studies of animals, higher plants and algae that are closely related to higher plants. However, few electrophysiological studies have been carried out in red tide organisms, especially in dinoflagellates, which is mainly due to the complex surface structure of dinoflagellate amphiesma. In this study, the surface amphiesma of Alexandrium pacificum, a typical red tide species, was removed by centrifugation, low-temperature treatment and enzymatic treatment. In all three treatments, low-temperature treatment with 4 °C for 2 h had high ecdysis rate and high fixation rate, and the treated cells were easy to puncture, so low-temperature treatment was used as a preprocessing treatment for subsequent current recording. Acquired protoplasts of A. pacificum were identified by calcofluor fluorescence and immobilized by poly-lysine. A modified "puncture" single-electrode voltage-clamp recording was first applied to dinoflagellates, and voltage-gated currents, which had the characteristics of outward K⁺ current and inward Cl^- current, were recorded and confirmed by ion replacement, indicating the voltage-gated currents were mixed. This method can be used as a technical basis for the electrophysiological study of dinoflagellates and provides a new perspective for the study of stress tolerance, red tide succession, and the regulation of physiological function of dinoflagellates.

Calcium channels and transporters: Roles in response to biotic and abiotic stresses

Calcium (Ca2+) serves as a ubiquitous second messenger by mediating various signaling pathways and responding to numerous environmental conditions in eukaryotes. Therefore, plant cells have developed complex mechanisms of Ca2+ communication across the membrane, receiving the message from their surroundings and transducing the information into cells and organelles. A wide range of biotic and abiotic stresses cause the increase in [Ca2+]cyt as a result of the Ca2+ influx permitted by membrane-localized Ca2+ permeable cation channels such as CYCLIC NUCLEOTIDE-GATE CHANNELs (CNGCs), and voltage-dependent HYPERPOLARIZATION-ACTIVATED CALCIUM2+ PERMEABLE CHANNELs (HACCs), as well as GLUTAMATE RECEPTOR-LIKE RECEPTORs (GLRs) and TWO-PORE CHANNELs (TPCs). Recently, resistosomes formed by some NUCLEOTIDE-BINDING LEUCINE-RICH REPEAT RECEPTORs (NLRs) are also proposed as a new type of Ca2+ permeable cation channels. On the contrary, some Ca2+ transporting membrane proteins, mainly Ca2+-ATPase and Ca2+/H+ exchangers, are involved in Ca2+ efflux for removal of the excessive [Ca2+]cyt in order to maintain the Ca2+ homeostasis in cells. The Ca2+ efflux mechanisms mediate the wide ranges of cellular activities responding to external and internal stimuli. In this review, we will summarize and discuss the recent discoveries of various membrane proteins involved in Ca2+ influx and efflux which play an essential role in fine-tuning the processing of information for plant responses to abiotic and biotic stresses.

Combined transcriptome and metabolome reveal glutathione metabolism plays a critical role in resistance to salinity in rice landraces HD961

Rice (Oryza sativa) is one of the most important food crops around the world, which is sensitive to salt stress, especially in the seedling and booting stage. HD961 is a salt-tolerant rice landrace that grows along coastal beaches and has disease and insect pest resistance, salt tolerance, and vigorous growth characteristics. We performed a combined transcriptome and metabolome analysis to clarify salinity resistance mechanisms in cultivar HD961, which has adapted to salinity soil at the early seedling stage. The results showed that the growth and antioxidant capacity of HD961 were stronger than 9311 under salt stress (SS). Transcriptomic analysis showed that a total of 6,145, 3,309, 1,819, and 1,296 differentially expressed genes (DEGs) were identified in the groups of TH60 (control group vs. 60 mM group of HD961 for transcriptome), TH120 (control group vs. 120 mM group of HD961 for transcriptome), T60 (control group vs. 60 mM group of 9311 for transcriptome), and T120 (control group vs. 120 mM group of 9311 for transcriptome), respectively. Starch and sucrose metabolism and phenylpropanoid biosynthesis were shared in the four treatment groups based on a KEGG enrichment analysis of DEGs. In addition, alpha-linolenic acid metabolism, plant hormone signal transduction, plant-pathogen interaction, and fatty acid elongation were specific and significantly different in HD961. A total of 92, 158, 151, and 179 significantly regulated metabolites (SRMs) responded to SS in MH60 (control group vs. 60 mM group of HD961 for metabolome), MH120 (control group vs. 120 mM group of HD961 for metabolome), M60 (control group vs. 60 mM group of 9311 for metabolome), and M120 (control group vs. 120 mM group of 9311 for metabolome), respectively. The KEGG analysis showed that eight common metabolic pathways were identified in the four treatment groups, of which biosynthesis of amino acids was the most significant. Three specific metabolic pathways were identified in the HD961, including glutathione metabolism, ascorbate and aldarate metabolism, and pantothenate and CoA biosynthesis. Integrative analysis between the transcriptome and metabolome showed that glutathione metabolism was specific and significantly affected under SS in HD961. A total of seven SRMs and 48 DEGs and four SRMs and 15 DEGs were identified in the glutathione metabolism pathway in HD961 and 9311, respectively. The Pearson correlation analysis showed a significant correlation between reduced glutathione and 16 genes (12 upregulated and four downregulated genes), suggesting these genes could be candidates as salt-tolerance regulation genes. Collectively, our data show that glutathione metabolism plays a critical role in response to SS in rice. Moreover, the stronger regulative ability of related common genes and metabolites might contribute to salt resistance in HD961.

Persistence behavior of chlorpyrifos and biological toxicity mechanism to cucumbers under greenhouse conditions

Chlorpyrifos, a broadly utilized insecticide, inhibits many cellular and physiological processes in plants. Here, the phyto-toxicity of chlorpyrifos on cucumber plants, as well as the dissipation kinetics of chlorpyrifos in leaves, were investigated. Those results showed that chlorpyrifos accumulated primarily in the leaves under normal agrochemical spraying conditions with the half-lives among 2.48-4.59 days. Residues of the primary metabolite, 3,5,6-trichloro-2-pyridinol (TCP), rapidly accumulated in plant tissues and soil with chlorpyrifos degradation. The application amount of chlorpyrifos had a significant effect on the persistence of chlorpyrifos and TCP in both plant and soil environments. Chlorpyrifos generated excessive reactive oxygen species (ROS) and malondialdehyde (MDA), which led to oxidative damage. High chlorpyrifos stress even inhibited antioxidant enzymes. The photosynthetic system and gas exchange were suppressed, which ultimately lead to inefficient light use under chlorpyrifos stress. Morphological results revealed that chlorpyrifos induced membrane damage and harmed organelles such as mitochondria and chloroplast. Noninvasive micro-test technology (NMT) showed that chlorpyrifos promoted intracellular Ca²⁺ influx and efflux of H⁺ and K⁺. The Ca²⁺ influx was significantly stimulated after both high and low chlorpyrifos treatment with the minimum value of - 336.33 pmol·cm^-2·s^-1 at 258 s and - 155.68 pmol·cm^-2·s^-1 at 288 s, respectively. Chlorpyrifos stress reversed the H⁺ influx to an efflux in cucumber mesophyll with the mean value of 0.45 ± 0.03 pmol·cm^-2·s^-1 and 0.19 ± 0.03 pmol·cm^-2·s^-1 in cucumber plants under low and high chlorpyrifos stress. High chlorpyrifos stress dramatically increase K⁺ efflux in cucumber leaves by 13.68 times higher than the control. We suggest that ion homeostasis destruction, accompanied by ROS, resulted in oxidative damage to the mesophyll cell of cucumber seedlings.

Ethyl Vinyl Ketone Activates K+ Efflux to Regulate Stomatal Closure by MRP4-Dependent eATP Accumulation Working Upstream of H2O2 Burst in Arabidopsis

Plants regulate stomatal mobility to limit water loss and improve pathogen resistance. Ethyl vinyl ketone (evk) is referred to as a reactive electrophilic substance (RES). In this paper, we found that evk can mediate stomatal closure and that evk-induced stomatal closure by increasing guard cell K⁺ efflux. To investigate the role of eATP, and H₂O₂ in evk-regulated K⁺ efflux, we used Arabidopsis wild-type (WT), mutant lines of mrp4, mrp5, dorn1.3 and rbohd/f. Non-invasive micro-test technology (NMT) data showed that evk-induced K⁺ efflux was diminished in mrp4, rbohd/f, and dorn1.3 mutant, which means eATP and H₂O₂ work upstream of evk-induced K⁺ efflux. According to the eATP content assay, evk stimulated eATP production mainly by MRP4. In mrp4 and mrp5 mutant groups and the ABC transporter inhibitor glibenclamide (Gli)-pretreated group, evk-regulated stomatal closure and eATP buildup were diminished, especially in the mrp4 group. According to qRT-PCR and eATP concentration results, evk regulates both relative gene expressions of MRP4/5 and eATP concentration in rbohd/f and WT group. According to the confocal data, evk-induced H₂O₂ production was lower in mrp4, mrp5 mutants, which implied that eATP works upstream of H₂O₂. Moreover, NADPH-dependent H₂O₂ burst is regulated by DORN1. A yeast two-hybrid assay, firefly luciferase complementation imaging assay, bimolecular fluorescence complementation assay, and pulldown assay showed that the interaction between DORN1 and RBOHF can be realized, which means DORN1 may control H₂O₂ burst by regulating RBOHF through interaction. This study reveals that evk-induced stomatal closure requires MRP4-dependent eATP accumulation and subsequent H₂O₂ accumulation to regulate K⁺ efflux.

A Multifactorial Regulation of Glutathione Metabolism behind Salt Tolerance in Rice

Knowledge of the stress-induced metabolic alterations in tolerant and sensitive plants is pivotal for identifying interesting traits that improve plant resilience toward unfavorable environmental conditions. This represents a hot topic area of plant science, particularly for crops, due to its implication in food security. Two rice varieties showing dissimilar resistance to salt, Baldo and Vialone Nano, have been studied to investigate the mechanisms underpinning tolerance toward salinity, and these studies have focused on the root system. A detailed analysis of the salt stress-dependent modulation of the redox network is here presented. The different phenotype observed after salt exposure in the two rice varieties is coherent with a differential regulation of cell-cycle progression and cell-death patterns observed at root level. Baldo, the tolerant variety, already showed a highly responsive antioxidative capacity in control conditions. Consistently, stressed Baldo plants showed a different pattern of H2O2 accumulation compared to Vialone Nano. Moreover, glutathione metabolism was finely modulated at transcriptional, post-transcriptional, and post-translational levels in Baldo. These results contribute to highlight the role of ROS and antioxidative pathways as a part of a complex redox network activated in rice toward salt stress.

The role of NADPH oxidases in regulating leaf gas exchange and ion homeostasis in Arabidopsis plants under cadmium stress

NADPH oxidase, an enzyme associated with the plasma membrane, constitutes one of the main sources of reactive oxygen species (ROS) which regulate different developmental and adaptive responses in plants. In this work, the involvement of NADPH oxidases in the regulation of photosynthesis and cell ionic homeostasis in response to short cadmium exposure was compared between wild type (WT) and three RBOHs (Respiratory Burst Oxidase Homologues) Arabidopsis mutants (AtrbohC, AtrbohD, and AtrbohF). Plants were grown under hydroponic conditions and supplemented with 50 µM CdCl₂ for 24 h. Cadmium treatment differentially affected photosynthesis, stomatal conductance, transpiration, and antioxidative responses in WT and Atrbohs mutants. The loss of function of RBOH isoforms resulted in higher Cd²⁺ influx, mainly in the elongation zone of roots, which was more evident in AtrbohD and AtrbohF mutants. In the mature zone, the highest Cd²⁺ influx was observed in rbohC mutant. The lack of functional RBOH isoforms also resulted in altered patterns of net K⁺ transport across cellular membranes, both in the root epidermis and leaf mesophyll. The analysis of expression of metal transporters by qPCR demonstrated that a loss of functional RBOH isoforms has altered transcript levels for metal NRAMP3, NRAMP6 and IRT1 and the K⁺ transporters outward-rectifying K⁺ efflux GORK channel, while RBOHD specifically regulated transcripts for high-affinity K⁺ transporters KUP8 and HAK5, and IRT1 and RBOHD and F regulated the transcription factors TGA3 and TGA10. It is concluded that RBOH-dependent H₂O₂ regulation of ion homeostasis and Cd is a highly complex process involving multilevel regulation from transpirational water flow to transcriptional and posttranslational modifications of K/metals transporters.