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Purpose: Atriplex canescens adapts to saline soils by sequestering excessive Na+ in salt bladders on the surface of aerial tissues, which is a complex comprising epidermal cells (ECs), stalk cells (SCs) and epidermal bladder cells (EBCs). However, the mechanism of how Na+ enters salt bladders of A. canescens is not yet clear. We previously identified two A. canescens HKT1 genes that might be related to Na+ sequestration in salt bladders. The aim of this study was to evaluate the function of AcHKT1 genes in Na+ secretion. Results: Two AcHKT1 genes were cloned; AcHKT1;1 was largely expressed in roots, while AcHKT1;2 was mainly expressed in shoots and strongly induced by NaCl. Heterologous expression of AcHKT1;2 aggravated the Na+-sensitive phenotype in yeast. This result was further confirmed in Xenopus system, in which AcHKT1;2 exerted high selectivity for Na+, indicating that AcHKT1;2 functions as a plasma membrane-localized Na+ transporter and mediates robust Na+ influx at the cellular level. Interestingly, AcHKT1;2 expression in leaves was significantly reduced once salt bladders were removed from the leaf surfaces; in particular, it had the greatest impact on the expression in mature leaves with the strongest activity toward ion secretion, suggesting that AcHKT1;2 was predominantly expressed in the EC-SC-EBC complex of A. canescens leaves. Conclusions: AcHKT1;2 is a key candidate transporter involved in mediating the entry of Na+ into A. canescens EBCs, thereby facilitating continuous Na+ sequestration in EBCs to ensure the survival of plants in harsh saline environments. [ABSTRACT FROM AUTHOR]

Purpose: Different from most crops and forages, the xerophyte Pugionium cornutum can absorb high quantities of Cl− and then efficiently transport them into shoots to enhance osmotic adjustment ability under salt stress. However, the molecular mechanism underlying long-distance Cl− transport remains unclear. SLAH1 is crucial for root-to-shoot Cl− transport in Arabidopsis only under non-saline conditions. This study aimed to evaluate the function of PcSLAH1 from P. cornutum in long-distance Cl− transport under salt stress. Methods: PcSLAH1 was cloned, its tissue and subcellular locations and expression patterns in response to salt treatments were analyzed. The effect of PcSLAH1 on ion accumulation and the expression of key genes involved in Na+ and Cl− transport were investigated by ectopically overexpressing in Arabidopsis driven by a root stelar-specific promoter. Results: PcSLAH1 was specifically expressed at root stelar cells and located on the plasma membrane. Opposing with AtSLAH1 in Arabidopsis, the relative expression level of PcSLAH1 in roots was significantly induced by high Cl− treatments. The root stelar-specific expression of PcSLAH1 in Arabidopsis resulted in a significant increase in shoot Cl− content under NaCl or KCl treatment. PcSLAH1 could coordinate with CLCg and NHX1 to regulate shoot Cl− and Na+ homeostasis under salt treatment. In addition, the root stelar-specific expression of PcSLAH1 conferred a stronger long-distance transport ability of Cl− than AtSLAH1. Conclusions: PcSLAH1 functions in facilitating the long-distance transport of Cl− and modulating shoot Na+ and Cl− homeostasis under salt stress. The up-regulated expression of PcSLAH1 is conducive to salt tolerance of P. cornutum. [ABSTRACT FROM AUTHOR]

Zygophyllum xanthoxylum, a succulent xerophyte, possesses excellent salt tolerance which is closely associated with vacuolar Na+ compartmentation via ZxNHX1. RNA interference (RNAi)-mediated ZxNHX1 silencing impaired the characteristics of Na+ accumulation and thus inhibited normal growth of Z. xanthoxylum. To explore the molecular mechanisms underlying the changed phenotype, here we compared the different expression of salt-responsive genes between ZxNHX1-RNAi line and wild type at transcriptome level. The result showed that vast genes were differently expressed in leaves or roots between ZxNHX1-RNAi line and wild type under control condition or salt treatment. Among them, 142 unigenes were differently expressed in both roots and leaves under 50 mM NaCl for 6 h and 24 h between ZxNHX1-RNAi line and WT. These differentially expressed genes (DEGs) were related to transmembrane transportation based on Gene Ontology (GO) annotations. In terms of ion transport, 49 DEGs were identified, in which some genes associated with Na+ and K+ homeostasis were down-regulated; meanwhile, silencing of ZxNHX1 triggered the differential expression of several genes involved in the transport of important nutrient elements including N, P, Ca, and Mg. Besides, silencing of ZxNHX1 affected the expression level of 18 and 26 genes related to photosynthesis and ROS scavenging, respectively. This study provided strong evidences to indicate the dominant role of ZxNHX1 in maintaining the characteristics of salt accumulation and regulating the salt tolerance of Z. xanthoxylum. [ABSTRACT FROM AUTHOR]

Background: The research on plant salt tolerance has mainly focused on Na+, but Cl− has been relatively neglected. Previous studies have found that the xerophyte Pugionium cornutum, an important forage grass in the arid and semi‐arid regions of northwestern China, could synergistically accumulate high quintiles of Na+ and Cl− in its shoots under NaCl treatments. However, the separate effects of these ions on the adaptation of P. cornutum to saline conditions have not been investigated. Aims: In this study, the response of P. cornutum to Na+ and Cl− was analyzed. Methods: Four‐week‐old seedlings were treated with additional 50 mM NaCl, Na+‐specific solution containing 50 mM Na+ with a mix of NO3-, H2PO4-, and SO42- as counter anions, and Cl−‐specific solution containing 50 mM Cl− with a mix of K+, Ca2+, and Mg2+ as counter cations. Results: Compared with the normal growth condition irrigated with Hoagland solution, the Na+‐specific solution severely impaired the growth and photosynthesis of P. cornutum due to the high accumulation of Na+ in shoots and the deterioration of tissue K+ homeostasis; while the Cl−‐specific solution significantly increased shoot fresh and dry biomass. The Cl−‐specific solution could also increase the turgor pressure in leaves for enhancing osmotic adjustment, which should be mainly attributed to the large accumulation of Cl−, since the concentrations of other ions, including K+, Mg2+, Ca2+, H2PO4-, and SO42-, in tissues under Cl−‐specific treatment were maintained at the same levels as those observed under the normal condition. Conclusions: P. cornutum displays an excellent tolerance to moderate Cl− but not to Na+, and the large accumulation of Cl− should play a positive role in stimulating the growth of P. cornutum under salt stress. [ABSTRACT FROM AUTHOR]

Chloride (Cl–) is pervasive in saline soils, and research on its influence on plants has mainly focused on its role as an essential nutrient and its toxicity when excessive accumulation occurs. However, the possible functions of Cl– in plants adapting to abiotic stresses have not been well documented. Previous studies have shown that the salt tolerance of the xerophytic species Pugionium cornutum might be related to high Cl– accumulation. In this study, we investigated the Cl–-tolerant characteristics and possible physiological functions of Cl– in the salt tolerance and drought resistance of P. cornutum. We found that P. cornutum can accumulate a large amount of Cl– in its shoots, facilitating osmotic adjustment and turgor generation under saline conditions. Application of DIDS (4,4´-diisothiocyanostilbene-2,2´-disulfonic acid), a blocker of anion channels, significantly inhibited Cl– uptake, and decreased both the Cl– content and its contribution to leaf osmotic adjustment, resulting in the exacerbation of growth inhibition in response to NaCl. Unlike glycophytes, P. cornutum was able to maintain NO3– homeostasis in its shoots when large amounts of Cl– were absorbed and accumulated. The addition of NaCl mitigated the deleterious effects of osmotic stress on P. cornutum because Cl– accumulation elicited a strong osmotic adjustment capacity. These findings suggest that P. cornutum is a Cl–-tolerant species that can absorb and accumulate Cl– to improve growth under salt and drought stresses. [ABSTRACT FROM AUTHOR]

Background and aims: Uncontrolled uptake of Na+ is the reason that many species are sensitive to salinity. Suberin is a protective barrier found in the walls of root endodermal cells that appears to be important for salt tolerance, yet its specific protective mechanism has not been fully elucidated. Methods: Here we investigated the role of aliphatic suberin in protecting plants against salt stress by using a mutant of Arabidopsis, cyp86a1, which exhibits a significant reduction of root aliphatic suberin. Results: We found that NaCl significantly increased suberization in roots of hydroponic-grown wild-type plants, but not in cyp86a1. Cyp86a1 exhibited a salt-sensitive phenotype. Compared with wild-type, Na+ accumulation in shoots was higher in cyp86a1. We provide evidence that increased Na+ uptake was via the root transcellular pathway. Furthermore, cyp86a1 accumulated less K+ in shoots than wild-type under NaCl stress, which was a consequence of increased K+ efflux from the root vasculature. Additionally, we provide evidence that aliphatic suberin reduces inflow of water across the root endodermis under non-stress conditions but reduces the backflow of water to the medium under salt stress. Conclusions: Finally, we propose a model for the role of aliphatic suberin in restricting Na+ influx, K+ efflux and water backflow in plants under saline conditions. [ABSTRACT FROM AUTHOR]

The xerophyte Pugionium cornutum adapts to salt stress by accumulating inorganic ions (e.g., Cl−) for osmotic adjustment and enhancing the activity of antioxidant enzymes, but the associated molecular basis remains unclear. In this study, we first found that P. cornutum could also maintain cell membrane stability due to its prominent ROS-scavenging ability and exhibits efficient carbon assimilation capacity under salt stress. Then, the candidate genes associated with the important physiological traits of the salt tolerance of P. cornutum were identified through transcriptomic analysis. The results showed that after 50 mM NaCl treatment for 6 or 24 h, multiple genes encoding proteins facilitating Cl− accumulation and NO3− homeostasis, as well as the transport of other major inorganic osmoticums, were significantly upregulated in roots and shoots, which should be favorable for enhancing osmotic adjustment capacity and maintaining the uptake and transport of nutrient elements; a large number of genes related to ROS-scavenging pathways were also significantly upregulated, which might be beneficial for mitigating salt-induced oxidative damage to the cells. Meanwhile, many genes encoding components of the photosynthetic electron transport pathway and carbon fixation enzymes were significantly upregulated in shoots, possibly resulting in high carbon assimilation efficiency in P. cornutum. Additionally, numerous salt-inducible transcription factor genes that probably regulate the abovementioned processes were found. This work lays a preliminary foundation for clarifying the molecular mechanism underlying the adaptation of xerophytes to harsh environments. [ABSTRACT FROM AUTHOR]

HKT1 and SOS1 are two key Na+ transporters that modulate salt tolerance in plants. Although much is known about the respective functions of HKT1 and SOS1 under salt conditions, few studies have examined the effects of HKT1 and SOS1 mutations on the expression of other important Na+ and K+ transporter genes. This study investigated the physiological parameters and expression profiles of AtHKT1;1, AtSOS1, AtHAK5, AtAKT1, AtSKOR, AtNHX1, and AtAVP1 in wild-type (WT) and athkt1;1 and atsos1 mutants of Arabidopsis thaliana under 25 mM NaCl. We found that AtSOS1 mutation induced a significant decrease in transcripts of AtHKT1;1 (by 56–62% at 6–24 h), AtSKOR (by 36–78% at 6–24 h), and AtAKT1 (by 31–53% at 6–24 h) in the roots compared with WT. This led to an increase in Na+ accumulation in the roots, a decrease in K+ uptake and transportation, and finally resulted in suppression of plant growth. AtHKT1;1 loss induced a 39–76% (6–24 h) decrease and a 27–32% (6–24 h) increase in transcripts of AtSKOR and AtHAK5, respectively, in the roots compared with WT. At the same time, 25 mM NaCl decreased the net selective transport capacity for K+ over Na+ by 92% in the athkt1;1 roots compared with the WT roots. Consequently, Na+ was loaded into the xylem and delivered to the shoots, whereas K+ transport was restricted. The results indicate that AtHKT1;1 and AtSOS1 not only mediate Na+ transport but also control ion uptake and the spatial distribution of Na+ and K+ by cooperatively regulating the expression levels of relevant Na+ and K+ transporter genes, ultimately regulating plant growth under salt stress. [ABSTRACT FROM AUTHOR]

Atriplex canescens is a C 4 shrub with excellent adaptation to saline and arid environments. Our previous study showed that the secretion of excessive Na+ into leaf salt bladders is a primary strategy in salt tolerance of A. canescens and external 100 mM NaCl can substantially stimulate its growth. To investigate whether NaCl could facilitate Atriplex canescens response to drought stress, five-week-old seedlings were subjected to drought stress (30% of field water capacity) in the presence or absence of additional 100 mM NaCl. The results showed that, under drought stress, the addition of NaCl could substantially improve the growth of A. canescens by increasing leaf relative water content, enhancing photosynthetic activity and inducing a significant declined leaf osmotic potential (Ψs). The addition of NaCl significantly increased Na+ concentration in leaf salt bladders and the Na+ contribution to leaf Ψs , while had no adverse effects on K+ accumulation in leaf laminae. Therefore, the large accumulation of Na+ in salt bladders for enhancing osmotic adjustment (OA) ability is a vital strategy in A. canescens responding to drought stress. In addition, the concentration of free proline, bataine and soluble sugars exhibited a significant increase in the presence of NaCl under drought stress, and the betaine contribution to leaf Ψs was significantly increased by additional NaCl compared with that under drought treatment alone, suggesting that compatible solutes are also involved in OA in addition to functioning as protectants to alleviate water deficit injury. • A. canescens could grow well in drought soils that contain certain salt. • Na+ secretion and K+ retention are vital strategies for A. canescens to cope with water deficit. • NaCl improves photosynthesis and hydration of A. canescens under drought stress. [ABSTRACT FROM AUTHOR]