A bioassay-guided fractionation system to identify endogenous small molecules that activate plasma membrane H+-ATPase activity in Arabidopsis

Insights into plant salt stress signaling and tolerance

Soil salinization is an essential environmental stressor, threatening agricultural yield and ecological security worldwide. Saline soils accumulate excessive soluble salts which are detrimental to most plants by limiting plant growth and productivity. It is of great necessity for plants to efficiently deal with the adverse effects caused by salt stress for survival and successful reproduction. Multiple determinants of salt tolerance have been identified in plants, and the cellular and physiological mechanisms of plant salt response and adaption have been intensely characterized. Plants respond to salt stress signals and rapidly initiate signaling pathways to re-establish cellular homeostasis with adjusted growth and cellular metabolism. This review summarizes the advances in salt stress perception, signaling, and response in plants. A better understanding of plant salt resistance will contribute to improving crop performance under saline conditions using multiple engineering approaches. The rhizosphere microbiome-mediated plant salt tolerance as well as chemical priming for enhanced plant salt resistance are also discussed in this review.

Lipid Metabolomic and Transcriptomic Analyses Reveal That Phosphatidylcholine Enhanced the Resistance of Peach Seedlings to Salt Stress through Phosphatidic Acid

Soil salinity is a major conlinet limiting sustainable agricultural development in peach tree industry. In this study, lipid metabolomic pathway analysis indicated that phosphatidic acid is essential for root resistance to salt stress in peach seedlings. Through functional annotation analysis of differentially expressed genes in transcriptomics, we found that MAPK signaling pathway is closely related to peach tree resistance to salt stress, wherein PpMPK6 expression is significantly upregulated. Under salt conditions, the OE-PpMPK6 Arabidopsis thaliana (L.) Heynh. line showed higher resistance to salt stress than WT and KO-AtMPK6 lines. Furthermore, we found that the Na⁺ content in OE-PpMPK6 roots was significantly lower than that in WT and KO-AtMPK6 roots, indicating that phosphatidic acid combined with PpMPK6 activated the SOS1 (salt-overly-sensitive 1) protein to enhance Na⁺ efflux, thus alleviating the damage caused by NaCl in roots; these findings provide insight into the salt stress-associated transcriptional regulation.

New Insight into Plant Saline-Alkali Tolerance Mechanisms and Application to Breeding

Saline-alkali stress is a widespread adversity that severely affects plant growth and productivity. Saline-alkaline soils are characterized by high salt content and high pH values, which simultaneously cause combined damage from osmotic stress, ionic toxicity, high pH and HCO₃^-/CO₃^2- stress. In recent years, many determinants of salt tolerance have been identified and their regulatory mechanisms are fairly well understood. However, the mechanism by which plants respond to comprehensive saline-alkali stress remains largely unknown. This review summarizes recent advances in the physiological, biochemical and molecular mechanisms of plants tolerance to salinity or salt- alkali stress. Focused on the progress made in elucidating the regulation mechanisms adopted by plants in response to saline-alkali stress and present some new views on the understanding of plants in the face of comprehensive stress. Plants generally promote saline-alkali tolerance by maintaining pH and Na⁺ homeostasis, while the plants responding to HCO₃^-/CO₃^2- stress are not exactly the same as high pH stress. We proposed that pH-tolerant or sensitive plants have evolved distinct mechanisms to adapt to saline-alkaline stress. Finally, we highlight the areas that require further research to reveal the new components of saline-alkali tolerance in plants and present the current and potential application of key determinants in breed improvement and molecular breeding.

The Role of Taraxacum mongolicum in a Puccinellia tenuiflora Community under Saline-Alkali Stress

Coexisting salt and alkaline stresses seriously threaten plant survival. Most studies have focused on halophytes; however, knowledge on how plants defend against saline-alkali stress is limited. This study investigated the role of Taraxacum mongolicum in a Puccinellia tenuiflora community under environmental saline-alkali stress to analyse the response of elements and metabolites in T. mongolicum, using P. tenuiflora as a control. The results show that the macroelements Ca and Mg are significantly accumulated in the aboveground parts (particularly in the stem) of T. mongolicum. Microelements B and Mo are also accumulated in T. mongolicum. Microelement B can adjust the transformation of sugars, and Mo contributes to the improvement in nitrogen metabolism. Furthermore, the metabolomic results demonstrate that T. mongolicum leads to decreased sugar accumulation and increased amounts of amino acids and organic acids to help plants resist saline-alkali stress. The resource allocation of carbon (sugar) and nitrogen (amino acids) results in the accumulation of only a few phenolic metabolites (i.e., petunidin, chlorogenic acid, and quercetin-3-O-rhamnoside) in T. mongolicum. These phenolic metabolites help to scavenge excess reactive oxygen species. Our study primarily helps in understanding the contribution of T. mongolicum in P. tenuiflora communities on coping with saline-alkali stress.

Screening of protonstatin-1 (PS-1) analogs for improved inhibitors of plant plasma membrane H+-ATPase activity

We previously identified protonstatin-1 (PS-1) as a selective inhibitor of plasma membrane H⁺-ATPase (PM H⁺-ATPase) activity and used it as a tool to validate the chemiosmotic model for polar auxin transport. Here, to obtain compounds with higher affinity than PS-1 for PM H⁺-ATPase, we synthesized 34 PS-1 analogs and examined their ability to inhibit PM H⁺-ATPase activity. The 34 analogs showed varying inhibitory effects on the activity of this enzyme. The strongest effect was observed for the small molecule PS-2, which was approximately five times stronger than PS-1. Compared to PS-1, PS-2 was also a stronger inhibitor of auxin uptake as well as acropetal and basipetal polar auxin transport in Arabidopsis thaliana seedlings. Because PS-2 is a more potent inhibitor of PM H⁺-ATPase than PS-1, we believe that this compound could be used as a tool to study the functions of this key plant enzyme.

Exogenous hydrogen sulfide mediates Na+ and K+ fluxes of salt gland in salt-secreting mangrove plant Avicennia marina

Hydrogen sulfide (H2S), is a crucial biological player in plants. Here, we primarily explored the interaction between sodium hydrosulfide (NaHS, a H2S donor) and the fluxes of Na+ and K+ from the salt glands of mangrove species Avicennia marina (Forsk.) Vierh. with non-invasive micro-test technology (NMT) and quantitative real-time PCR (qRT-PCR) approaches under salinity treatments. The results showed that under 400-mM NaCl treatment, the addition of 200-μM NaHS markedly increased the quantity of salt crystals in the adaxial epidermis of A. marina leaves, accompanied by an increase in the K+/Na+ ratio. Meanwhile, the endogenous content of H2S was dramatically elevated in this process. The NMT result revealed that the Na+ efflux was increased from salt glands, whereas K+ efflux was decreased with NaHS application. On the contrary, the effects of NaHS were reversed by H2S scavenger hypotaurine (HT), and DL-propargylglycine (PAG), an inhibitor of cystathionine-γ-lyase (CES, a H2S synthase). Moreover, enzymic assay revealed that NaHS increased the activities of plasma membrane and tonoplast H+-ATPase. qRT-PCR analysis revealed that NaHS significantly increased the genes transcript levels of tonoplast Na+/H+ antiporter (NHX1), plasma membrane Na+/H+ antiporter (SOS1), plasma membrane H+-ATPase (AHA1) and tonoplast H+-ATPase subunit c (VHA-c1), while suppressed above-mentioned gene expressions by the application of HT and PAG. Overall, H2S promotes Na+ secretion from the salt glands of A. marina by up-regulating the plasma membrane and tonoplast Na+/H+ antiporter and H+-ATPase.

Molecular Mechanisms of Plant Responses to Salt Stress

Saline-alkali soils pose an increasingly serious global threat to plant growth and productivity. Much progress has been made in elucidating how plants adapt to salt stress by modulating ion homeostasis. Understanding the molecular mechanisms that affect salt tolerance and devising strategies to develop/breed salt-resilient crops have been the primary goals of plant salt stress signaling research over the past few decades. In this review, we reflect on recent major advances in our understanding of the cellular and physiological mechanisms underlying plant responses to salt stress, especially those involving temporally and spatially defined changes in signal perception, decoding, and transduction in specific organelles or cells.

Testing the polar auxin transport model with a selective plasma membrane H+ -ATPase inhibitor

Auxin is unique among plant hormones in that its function requires polarized transport across plant cells. A chemiosmotic model was proposed to explain how polar auxin transport is derived by the H⁺ gradient across the plasma membrane (PM) established by PM H⁺ -adenosine triphosphatases (ATPases). However, a classical genetic approach by mutations in PM H⁺ -ATPase members did not result in the ablation of polar auxin distribution, possibly due to functional redundancy in this gene family. To confirm the crucial role of PM H⁺ -ATPases in the polar auxin transport model, we employed a chemical genetic approach. Through a chemical screen, we identified protonstatin-1 (PS-1), a selective small-molecule inhibitor of PM H⁺ -ATPase activity that inhibits auxin transport. Assays with transgenic plants and yeast strains showed that the activity of PM H⁺ -ATPases affects auxin uptake as well as acropetal and basipetal polar auxin transport. We propose that PS-1 can be used as a tool to interrogate the function of PM H⁺ -ATPases. Our results support the chemiosmotic model in which PM H⁺ -ATPase itself plays a fundamental role in polar auxin transport.

The molecular mechanism of plasma membrane H+-ATPases in plant responses to abiotic stress

Plasma membrane H⁺-ATPases (PM H⁺-ATPases) are critical proton pumps that export protons from the cytoplasm to the apoplast. The resulting proton gradient and difference in electrical potential energize various secondary active transport events. PM H⁺-ATPases play essential roles in plant growth, development, and stress responses. In this review, we focus on recent studies of the mechanism of PM H⁺-ATPases in response to abiotic stresses in plants, such as salt and high pH, temperature, drought, light, macronutrient deficiency, acidic soil and aluminum stress, as well as heavy metal toxicity. Moreover, we discuss remaining outstanding questions about how PM H⁺-ATPases contribute to abiotic stress responses.

H+-ATPases in Plant Growth and Stress Responses

H⁺-ATPases, including the phosphorylated intermediate-type (P-type) and vacuolar-type (V-type) H⁺-ATPases, are important ATP-driven proton pumps that generate membrane potential and provide proton motive force for secondary active transport. P- and V-type H⁺-ATPases have distinct structures and subcellular localizations and play various roles in growth and stress responses. A P-type H⁺-ATPase is mainly regulated at the posttranslational level by phosphorylation and dephosphorylation of residues in its autoinhibitory C terminus. The expression and activity of both P- and V-type H⁺-ATPases are highly regulated by hormones and environmental cues. In this review, we summarize the recent advances in understanding of the evolution, regulation, and physiological roles of P- and V-type H⁺-ATPases, which coordinate and are involved in plant growth and stress adaptation. Understanding the different roles and the regulatory mechanisms of P- and V-type H⁺-ATPases provides a new perspective for improving plant growth and stress tolerance by modulating the activity of H⁺-ATPases, which will mitigate the increasing environmental stress conditions associated with ongoing global climate change.

Structure, Function, and Regulation of the Plasma Membrane Na+/H+ Antiporter Salt Overly Sensitive 1 in Plants

Physiological studies have confirmed that export of Na⁺ to improve salt tolerance in plants is regulated by the combined activities of a complex transport system. In the Na⁺ transport system, the Na⁺/H⁺ antiporter salt overly sensitive 1 (SOS1) is the main protein that functions to excrete Na⁺ out of plant cells. In this paper, we review the structure and function of the Na⁺/H⁺ antiporter and the physiological process of Na⁺ transport in SOS signaling pathway, and discuss the regulation of SOS1 during phosphorylation activation by protein kinase and the balance mechanism of inhibiting SOS1 antiporter at molecular and protein levels. In addition, we carried out phylogenetic tree analysis of SOS1 proteins reported so far in plants, which implied the specificity of salt tolerance mechanism from model plants to higher crops under salt stress. Finally, the high complexity of the regulatory network of adaptation to salt tolerance, and the feasibility of coping strategies in the process of genetic improvement of salt tolerance quality of higher crops were reviewed.

Salt‑responsive transcriptome analysis of canola roots reveals candidate genes involved in the key metabolic pathway in response to salt stress

Salinity is a major constraint on crop growth and productivity, limiting sustainable agriculture in arid regions. Understanding the molecular mechanisms of salt-stress adaptation in canola is important to improve salt tolerance and promote its cultivation in saline lands. In this study, roots of control (no salt) and 200 mM NaCl-stressed canola seedlings were collected for RNA-Seq analysis and qRT-PCR validation. A total of 5385, 4268, and 7105 DEGs at the three time points of salt treatment compared to the control were identified, respectively. Several DEGs enriched in plant signal transduction pathways were highly expressed under salt stress, and these genes play an important role in signaling and scavenging of ROS in response to salt stress. Transcript expression in canola roots differed at different stages of salt stress, with the early-stages (2 h) of salt stress mainly related to oxidative stress response and sugar metabolism, while the late-stages (72 h) of salt stress mainly related to transmembrane movement, amino acid metabolism, glycerol metabolism and structural components of the cell wall. Several families of TFs that may be associated with salt tolerance were identified, including ERF, MYB, NAC, WRKY, and bHLH. These results provide a basis for further studies on the regulatory mechanisms of salt stress adaptation in canola.

Glycine betaine increases salt tolerance in maize (Zea mays L.) by regulating Na+ homeostasis

Improving crop salt tolerance is an adaptive measure to climate change for meeting future food demands. Previous studies have reported that glycine betaine (GB) plays critical roles as an osmolyte in enhancing plant salt resistance. However, the mechanism underlying the GB regulating plant Na⁺ homeostasis during response to salinity is poorly understood. In this study, hydroponically cultured maize with 125 mM NaCl for inducing salinity stress was treated with 100 μM GB. We found that treatment with GB improved the growth of maize plants under non-stressed (NS) and salinity-stressed (SS) conditions. Treatment with GB significantly maintained the properties of chlorophyll fluorescence, including Fv/Fm, ΦPSII, and ΦNPQ, and increased the activity of the antioxidant enzymes for mitigating salt-induced growth inhibition. Moreover, GB decreased the Na⁺/K⁺ ratio primarily by reducing the accumulation of Na⁺ in plants. The results of NMT tests further confirmed that GB increased Na⁺ efflux from roots under SS condition, and fluorescence imaging of cellular Na⁺ suggested that GB reduced the cellular allocation of Na⁺. GB additionally increased Na⁺ efflux in leaf protoplasts under SS condition, and treatment with sodium orthovanadate, a plasma membrane (PM) H⁺-ATPase inhibitor, significantly alleviated the positive effects of GB on Na⁺ efflux under salt stress. GB significantly improved the vacuolar activity of NHX but had no significant effects on the activity of V type H⁺-ATPases. In addition, GB significantly upregulated the expression of the PM H⁺-ATPase genes, ZmMHA2 and ZmMHA4, and the Na⁺/H⁺ antiporter gene, ZmNHX1. While, the V type H⁺-ATPases gene, ZmVP1, was not significantly regulated by GB. Altogether these results indicate that GB regulates cellular Na⁺ homeostasis by enhancing PM H+-ATPases gene transcription and protein activities to improve maize salt tolerance. This study provided an extended understanding of the functions of GB in plant responses to salinity, which can help the development of supportive measures using GB for obtaining high maize yield in saline conditions.

Dynamic changes of phosphatidylinositol and phosphatidylinositol 4-phosphate levels modulate H+-ATPase and Na+/H+ antiporter activities to maintain ion homeostasis in Arabidopsis under salt stress

Plant metabolites are dynamically modified and distributed in response to environmental changes. However, it is poorly understood how metabolic change functions in plant stress responses. Maintaining ion homeostasis under salt stress requires coordinated activation of two types of central regulators: plasma membrane (PM) H⁺-ATPase and Na⁺/H⁺ antiporter. In this study, we used a bioassay-guided isolation approach to identify endogenous small molecules that affect PM H⁺-ATPase and Na⁺/H⁺ antiporter activities and identified phosphatidylinositol (PI), which inhibits PM H⁺-ATPase activity under non-stress conditions in Arabidopsis by directly binding to the C terminus of the PM H⁺-ATPase AHA2. Under salt stress, the phosphatidylinositol 4-phosphate-to-phosphatidylinositol (PI4P-to-PI) ratio increased, and PI4P bound and activated the PM Na⁺/H⁺ antiporter. PI prefers binding to the inactive form of PM H⁺-ATPase, while PI4P tends to bind to the active form of the Na⁺/H⁺ antiporter. Consistent with this, pis1 mutants, with reduced levels of PI, displayed increased PM H⁺-ATPase activity and salt stress tolerance, while the pi4kβ1 mutant, with reduced levels of PI4P, displayed reduced PM Na⁺/H⁺ antiporter activity and salt stress tolerance. Collectively, our results reveal that the dynamic change between PI and PI4P in response to salt stress in Arabidopsis is crucial for maintaining ion homeostasis to protect plants from unfavorable environmental conditions.

Defective cytokinin signaling reprograms lipid and flavonoid gene-to-metabolite networks to mitigate high salinity in Arabidopsis

Cytokinin (CK) in plants regulates both developmental processes and adaptation to environmental stresses. Arabidopsis histidine phosphotransfer ahp2,3,5 and type-B Arabidopsis response regulator arr1,10,12 triple mutants are almost completely defective in CK signaling, and the ahp2,3,5 mutant was reported to be salt tolerant. Here, we demonstrate that the arr1,10,12 mutant is also more tolerant to salt stress than wild-type (WT) plants. A comprehensive metabolite profiling coupled with transcriptome analysis of the ahp2,3,5 and arr1,10,12 mutants was conducted to elucidate the salt tolerance mechanisms mediated by CK signaling. Numerous primary (e.g., sugars, amino acids, and lipids) and secondary (e.g., flavonoids and sterols) metabolites accumulated in these mutants under nonsaline and saline conditions, suggesting that both prestress and poststress accumulations of stress-related metabolites contribute to improved salt tolerance in CK-signaling mutants. Specifically, the levels of sugars (e.g., trehalose and galactinol), amino acids (e.g., branched-chain amino acids and γ-aminobutyric acid), anthocyanins, sterols, and unsaturated triacylglycerols were higher in the mutant plants than in WT plants. Notably, the reprograming of flavonoid and lipid pools was highly coordinated and concomitant with the changes in transcriptional levels, indicating that these metabolic pathways are transcriptionally regulated by CK signaling. The discovery of the regulatory role of CK signaling on membrane lipid reprogramming provides a greater understanding of CK-mediated salt tolerance in plants. This knowledge will contribute to the development of salt-tolerant crops with the ability to withstand salinity as a key driver to ensure global food security in the era of climate crisis.

Phospholipids in Salt Stress Response

High salinity threatens crop production by harming plants and interfering with their development. Plant cells respond to salt stress in various ways, all of which involve multiple components such as proteins, peptides, lipids, sugars, and phytohormones. Phospholipids, important components of bio-membranes, are small amphoteric molecular compounds. These have attracted significant attention in recent years due to the regulatory effect they have on cellular activity. Over the past few decades, genetic and biochemical analyses have partly revealed that phospholipids regulate salt stress response by participating in salt stress signal transduction. In this review, we summarize the generation and metabolism of phospholipid phosphatidic acid (PA), phosphoinositides (PIs), phosphatidylserine (PS), phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), as well as the regulatory role each phospholipid plays in the salt stress response. We also discuss the possible regulatory role based on how they act during other cellular activities.

Membrane dynamics during individual and combined abiotic stresses in plants and tools to study the same

The plasma membrane (PM) is possibly the most diverse biological membrane of plant cells; it separates and guards the cell against its external environment. It has an extremely complex structure comprising a mosaic of lipids and proteins. The PM lipids are responsible for maintaining fluidity, permeability and integrity of the membrane and also influence the functioning of membrane proteins. However, the PM is the primary target of environmental stress, which affects its composition, conformation and properties, thereby disturbing the cellular homeostasis. Maintenance of integrity and fluidity of the PM is a prerequisite for ensuring the survival of plants during adverse environmental conditions. The ability of plants to remodel membrane lipid and protein composition plays a crucial role in adaptation towards varying abiotic environmental cues, including high or low temperature, drought, salinity and heavy metals stress. The dynamic changes in lipid composition affect the functioning of membrane transporters and ultimately regulate the physical properties of the membrane. Plant membrane-transport systems play a significant role in stress adaptation by cooperating with the membrane lipidome to maintain the membrane integrity under stressful conditions. The present review provides a holistic view of stress responses and adaptations in plants, especially the changes in the lipidome and proteome of PM under individual or combined abiotic stresses, which cause alterations in the activity of membrane transporters and modifies the fluidity of the PM. The tools to study the varying lipidome and proteome of the PM are also discussed.

Nanoalgosomes: Introducing extracellular vesicles produced by microalgae

Cellular, inter-organismal and cross kingdom communication via extracellular vesicles (EVs) is intensively studied in basic science with high expectation for a large variety of bio-technological applications. EVs intrinsically possess many attributes of a drug delivery vehicle. Beyond the implications for basic cell biology, academic and industrial interests in EVs have increased in the last few years. Microalgae constitute sustainable and renewable sources of bioactive compounds with a range of sectoral applications, including the formulation of health supplements, cosmetic products and food ingredients. Here we describe a newly discovered subtype of EVs derived from microalgae, which we named nanoalgosomes. We isolated these extracellular nano-objects from cultures of microalgal strains, including the marine photosynthetic chlorophyte Tetraselmis chuii, using differential ultracentrifugation or tangential flow fractionation and focusing on the nanosized small EVs (sEVs). We explore different biochemical and physical properties and we show that nanoalgosomes are efficiently taken up by mammalian cell lines, confirming the cross kingdom communication potential of EVs. This is the first detailed description of such membranous nanovesicles from microalgae. With respect to EVs isolated from other organisms, nanoalgosomes present several advantages in that microalgae are a renewable and sustainable natural source, which could easily be scalable in terms of nanoalgosome production.

An improved protein lipid overlay assay for studying lipid-protein interactions

BACKGROUND

Lipids perform multiple functions in the cell, and lipid-protein interactions play a key role in metabolism. Although various techniques have been developed to study lipid-protein interactions, the interacting protein partners that bind to most lipids remain unknown. The protein lipid overlay (PLO) assay has revealed numerous lipid-protein interactions, but its application suffers from unresolved technical issues.

RESULTS

Herein, we found that blocking proteins may interfere with interactions between lipids and their binding proteins if a separate blocking step is carried out before the incubation step in the PLO assay. To overcome this, we modified the PLO assay by combining an incubation step alongside the blocking step. Verification experiments included phosphatidylinositol-3-phosphate (PI3P) and its commercially available interacting protein G302, C18:1, C18:2, C18:3 and the Arabidopsis plasma membrane H⁺-ATPase (PM H⁺-ATPase) AHA2 C-terminus, phosphatidylglycerol (PG) and AtROP6, and phosphatidylserine (PS) and the AHA2 C-terminus. The lipid-protein binding signal in the classical PLO (CPLO) assay was weak and not reproducible, but the modified PLO (MPLO) assay displayed significantly improved sensitivity and reproducibility.

CONCLUSIONS

This work identified a limitation of the CPLO assay, and both sensitivity and reproducibility were improved in the modified assay, which could prove to be more effective for investigating lipid-protein interactions.

Involvement of Phosphatidylserine and Triacylglycerol in the Response of Sweet Potato Leaves to Salt Stress

Lipid remodeling plays an important role in the adaptation of plants to environmental factors, but the mechanism by which lipid remodeling mediates salt stress response remains unclear. In this study, we compared the root and leaf lipidome profiles of salt-tolerant and salt-sensitive sweet potato cultivars (Xu 22 and Xu 32, respectively) under salinity stress. After salt treatment, the leaf lipidome showed more significant remodeling than the root lipidome in both cultivars. Compared with Xu 32 leaves, Xu 22 leaves generally maintained higher abundance of phospholipids, glycolipids, sphingolipids, sterol derivatives, and diacylglycerol under salinity conditions. Interestingly, salinity stress significantly increased phosphatidylserine (PS) abundance in Xu 22 leaves by predominantly triggering the increase of PS (20:5/22:6). Furthermore, Xu 32 leaves accumulated higher triacylglycerol (TG) level than Xu 22 leaves under salinity conditions. The exogenous application of PS delayed salt-induced leaf senescence in Xu 32 by reducing salt-induced K+ efflux and upregulating plasma membrane H+-ATPase activity. However, the inhibition of TG mobilization in salinized-Xu 22 leaves disturbed energy and K+/Na+ homeostasis, as well as plasma membrane H+-ATPase activity. These results demonstrate alterations in the leaf lipidome of sweet potato under salinity condition, underscoring the importance of PS and TG in mediating salt-defensive responses in sweet potato leaves.

Integrated physiologic, proteomic, and metabolomic analyses of Malus halliana adaptation to saline-alkali stress

Saline-alkali stress is a severely adverse abiotic stress limiting plant growth. Malus halliana Koehne is an apple rootstock that is tolerant to saline-alkali stress. To understand the molecular mechanisms underlying the tolerance of M. halliana to saline-alkali stress, an integrated metabolomic and proteomic approach was used to analyze the plant pathways involved in the stress response of the plant and its regulatory mechanisms. A total of 179 differentially expressed proteins (DEPs) and 140 differentially expressed metabolites (DEMs) were identified. We found that two metabolite-related enzymes (PPD and PAO) were associated with senescence and involved in porphyrin and chlorophyll metabolism; six photosynthesis proteins (PSAH2, PSAK, PSBO2, PSBP1, and PSBQ2) were significantly upregulated, especially PSBO2, and could act as regulators of photosystem II (PSII) repair. Sucrose, acting as a signaling molecule, directly mediated the accumulation of D-phenylalanine, tryptophan, and alkaloid (vindoline and ecgonine) and the expression of proteins related to aspartate and glutamate (ASP3, ASN1, NIT4, and GLN1-1). These responses play a central role in maintaining osmotic balance and removing reactive oxygen species (ROS). In addition, sucrose signaling induced flavonoid biosynthesis by activating the expression of CYP75B1 to regulate the homeostasis of ROS and promoted auxin signaling by activating the expression of T31B5_170 to enhance the resistance of M. halliana to saline-alkali stress. The decrease in peroxidase superfamily protein (PER) and ALDH2C4 during lignin synthesis further triggered a plant saline-alkali response. Overall, this study provides an important starting point for improving saline-alkali tolerance in M. halliana via genetic engineering.

Root-zone-specific sensitivity of K+-and Ca2+-permeable channels to H2O2 determines ion homeostasis in salinized diploid and hexaploid Ipomoea trifida

Polyploids generally possess superior K+/Na+ homeostasis under saline conditions compared with their diploid progenitors. In this study, we identified the physiological mechanisms involved in the ploidy-related mediation of K+/Na+ homeostasis in the roots of diploid (2x) and hexaploid (6x; autohexaploid) Ipomoea trifida, which is the closest relative of cultivated sweet potato. Results showed that 6x I. trifida retained more K+ and accumulated less Na+ in the root and leaf tissues under salt stress than 2x I. trifida. Compared with its 2x ancestor, 6x I. trifida efficiently prevents K+ efflux from the meristem root zone under salt stress through its plasma membrane (PM) K+-permeable channels, which have low sensitivity to H2O2. Moreover, 6x I. trifida efficiently excludes Na+ from the elongation and mature root zones under salt stress because of the high sensitivity of PM Ca2+-permeable channels to H2O2. Our results suggest the root-zone-specific sensitivity to H2O2 of PM K+- and Ca2+-permeable channels in the co-ordinated control of K+/Na+ homeostasis in salinized 2x and 6x I. trifida. This work provides new insights into the improved maintenance of K+/Na+ homeostasis of polyploids under salt stress.

Elucidating the molecular mechanisms mediating plant salt-stress responses

Contents Summary 523 I. Introduction 523 II. Sensing salt stress 524 III. Ion homeostasis regulation 524 IV. Metabolite and cell activity responses to salt stress 527 V. Conclusions and perspectives 532 Acknowledgements 533 References 533 SUMMARY: Excess soluble salts in soil (saline soils) are harmful to most plants. Salt imposes osmotic, ionic, and secondary stresses on plants. Over the past two decades, many determinants of salt tolerance and their regulatory mechanisms have been identified and characterized using molecular genetics and genomics approaches. This review describes recent progress in deciphering the mechanisms controlling ion homeostasis, cell activity responses, and epigenetic regulation in plants under salt stress. Finally, we highlight research areas that require further research to reveal new determinants of salt tolerance in plants.