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Background Under saline conditions, Suaeda salsa , as a typical halophyte, accumulates large amounts of Na + in its leaves during optimal growth. Key transporters involved in Na + accumulation in plants are HKT-type protein, the plasma membrane Na + /H + transporter SOS1, and the tonoplast Na + /H + antiporter NHX1. In this study, the function of SsHKT1;1 and its coordinate expression with SsSOS1 and SsNHX1 to regulate Na + homeostasis in S. salsa was investigated. Results We showed, by yeast complementation assays, that SsHKT1;1 encoded a Na + -selective transporter, which located to the plasma membrane and was preferentially expressed within the stele, and was particularly abundant in xylem parenchyma and pericycle cells. When compared with a treatment of 25 mM NaCl, 150 mM NaCl greatly decreased the transcripts of SsHKT1;1 , but maintained a relatively constant level of the expression of SsSOS1 in roots. Consequently, the synergistic effect of SsHKT1;1 and SsSOS1 would result in greater Na + loading into the xylem under 150 mM NaCl than 25 mM NaCl. In leaves, 150 mM NaCl up-regulated the abundance of SsNHX1 compared with levels in 25 mM NaCl. This enabled the permanent sequestering of Na + into leaf vacuoles. Conclusions Overall, SsHKT1;1 functioned in reducing Na + retrieval from the root xylem, and played an important role in coordinating with SsSOS1 and SsNHX1 to maintain Na + accumulation in S. salsa under saline conditions.

Sweet sorghum is an important bioenergy grass and valuable forage with a strong adaptability to saline environments. However, little is known about the mechanisms of sweet sorghum coping with ion toxicity under salt stresses. Here, we first evaluated the salt tolerance of a sweet sorghum cultivar “Lvjuren” and determined its ion accumulation traits under NaCl treatments; then, we explored key genes involved in Na+, Cl−, K+ and NO3− transport using transcriptome profiling and the qRT-PCR method. The results showed that growth and photosynthesis of sweet sorghum were unaffected by 50 and 100 mM NaCl treatments, indicative of a strong salt tolerance of this species. Under NaCl treatments, sweet sorghum could efficiently exclude Na+ from shoots and accumulate Cl− in leaf sheaths to avoid their overaccumulation in leaf blades; meanwhile, it possessed a prominent ability to sustain NO3− homeostasis in leaf blades. Transcriptome profiling identified several differentially expressed genes associated with Na+, Cl−, K+ and NO3− transport in roots, leaf sheaths and leaf blades after 200 mM NaCl treatment for 6 and 48 h. Moreover, transcriptome data and qRT-PCR results indicated that HKT1;5, CLCc and NPF7.3-1 should be key genes involved in Na+ retention in roots, Cl− accumulation in leaf sheaths and maintenance of NO3− homeostasis in leaf blades, respectively. Many TFs were also identified after NaCl treatment, which should play important regulatory roles in salt tolerance of sweet sorghum. In addition, GO analysis identified candidate genes involved in maintaining membrane stability and photosynthetic capacity under salt stresses. This work lays a preliminary foundation for clarifying the molecular basis underlying the adaptation of sweet sorghum to adverse environments.

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.

Background Atriplex canescens is a typical C 4 secretohalophyte with salt bladders on the leaves. Accumulating excessive Na + in tissues and salt bladders, maintaining intracellular K + homeostasis and increasing leaf organic solutes are crucial for A. canescens survival in harsh saline environments, and enhanced photosynthetic activity and water balance promote its adaptation to salt. However, the molecular basis for these physiological mechanisms is poorly understood. Four-week-old A. canescens seedlings were treated with 100 mM NaCl for 6 and 24 h, and differentially expressed genes in leaves and roots were identified, respectively, with Illumina sequencing. Results In A. canescens treated with 100 mM NaCl, the transcripts of genes encoding transporters/channels for important nutrient elements, which affect growth under salinity, significantly increased, and genes involved in exclusion, uptake and vacuolar compartmentalization of Na + in leaves might play vital roles in Na + accumulation in salt bladders. Moreover, NaCl treatment upregulated the transcripts of key genes related to leaf organic osmolytes synthesis, which are conducive to osmotic adjustment. Correspondingly, aquaporin-encoding genes in leaves showed increased transcripts under NaCl treatment, which might facilitate water balance maintenance of A. canescens seedlings in a low water potential condition. Additionally, the transcripts of many genes involved in photosynthetic electron transport and the C 4 pathway was rapidly induced, while other genes related to chlorophyll biosynthesis, electron transport and C 3 carbon fixation were later upregulated by 100 mM NaCl. Conclusions We identified many important candidate genes involved in the primary physiological mechanisms of A. canescens salt tolerance. This study provides excellent gene resources for genetic improvement of salt tolerance of important crops and forages.

Water scarcity is a major environmental constraint on plant growth in arid regions. Soluble sugars and amino acids are essential osmolytes for plants to cope with osmotic stresses. Sweet sorghum is an important bioenergy crop and forage with strong adaptabilities to adverse environments; however, the accumulation pattern and biosynthesis basis of soluble sugars and amino acids in this species under osmotic stresses remain elusive. Here, we investigated the physiological responses of a sweet sorghum cultivar to PEG-induced osmotic stresses, analyzed differentially accumulated soluble sugars and amino acids after 20% PEG treatment using metabolome profiling, and identified key genes involved in the biosynthesis pathways of soluble sugars and amino acids using transcriptome sequencing. The results showed that the growth and photosynthesis of sweet sorghum seedlings were significantly inhibited by more than 20% PEG. After PEG treatments, the leaf osmotic adjustment ability was strengthened, while the contents of major inorganic osmolytes, including K+ and NO3−, remained stable. After 20% PEG treatment, a total of 119 and 188 differentially accumulated metabolites were identified in the stems and leaves, respectively, and the accumulations of soluble sugars such as raffinose, trehalose, glucose, sucrose, and melibiose, as well as amino acids such as proline, leucine, valine, serine, and arginine were significantly increased, suggesting that these metabolites should play key roles in osmotic adjustment of sweet sorghum. The transcriptome sequencing identified 1711 and 4978 DEGs in the stems, as well as 2061 and 6596 DEGs in the leaves after 20% PEG treatment for 6 and 48 h, respectively, among which the expressions of genes involved in biosynthesis pathways of sucrose (such as SUS1, SUS2, etc.), trehalose (including TPS6), raffinose (such as RAFS2 and GOLS2, etc.), proline (such as P5CS2 and P5CR), leucine and valine (including BCAT2), and arginine (such as ASS and ASL) were significantly upregulated. These genes should be responsible for the large accumulation of soluble sugars and amino acids under osmotic stresses. This study deepens our understanding of the important roles of individual soluble sugars and amino acids in the adaptation of sweet sorghum to water scarcity.

Kentucky bluegrass (Poa pratensis L.), a widely used cool-season turfgrass, shows a high sensitivity to soil salinity. Clarifying the adaptative mechanisms of Kentucky bluegrass that serve to improve its salt tolerance in saline environments is urgent for the application of this turfgrass in salt-affected regions. In this study, physiological responses of the Kentucky bluegrass cultivars “Explorer” and “Blue Best” to NaCl treatment, as well as gene expressions related to photosynthesis, ion transport, and ROS degradation, were analyzed. The results showed that the growth of “Explorer” was obviously better compared to “Blue Best” under 400 mM NaCl treatment. “Explorer” exhibited a much stronger photosynthetic capacity than “Blue Best” under NaCl treatment, and the expression of key genes involved in chlorophyll biosynthesis, photosystem II, and the Calvin cycle in “Explorer” was greatly induced by salt treatment. Compared with “Blue Best”, “Explorer” could effectively maintain Na+/K+ homeostasis in its leaves under NaCl treatment, which can be attributed to upregulated expression of genes, such as HKT1;5, HAK5, and SKOR. The relative membrane permeability and contents of O2− and H2O2 in “Explorer” were significantly lower than those in “Blue Best” under NaCl treatment, and, correspondingly, the activities of SOD and POD in the former were significantly higher than in the latter. Moreover, the expression of genes involved in the biosynthesis of enzymes in the ROS-scavenging system of “Explorer” was immediately upregulated after NaCl treatment. Additionally, free proline and betaine are important organic osmolytes for maintaining hydration status in Kentucky bluegrass under NaCl treatment, as the contents of these metabolites in “Explorer” were significantly higher than in “Blue Best”. This work lays a theoretical basis for the improvement of salt tolerance in Kentucky bluegrass.

Sweet sorghum is an important sugar crop and forage with a strong tolerance to soil salinity. We have previously analyzed the ion accumulation traits and transcriptome of a sweet sorghum cultivar under NaCl treatments. However, the mechanisms underlying Na+, K+, Cl−, and NO3− transports and the osmotic adjustment of sweet sorghum under salt stresses need further investigations. In this study, the growth, photosynthesis, inorganic ion and organic solute contents, and leaf osmotic adjustment ability of the sweet sorghum cultivars “Lvjuren” and “Fengtian” under NaCl treatments were determined; meanwhile, the expressions of key genes associated with the Na+, K+, Cl−, and NO3− transport were analyzed using the qRT-PCR method. The results showed that NaCl treatments more severely inhibited the growth and photosynthesis of “Lvjuren” than those of “Fengtian”. After NaCl treatments, “Fengtian” could more efficiently restrict the overaccumulation of Na+ and Cl− in leaf blades than “Lvjuren” by withholding large amounts of Na+ in the roots or reserving high quantities of Cl− in the leaf sheaths, which could be attributed to the upregulated expressions of SbNHX2, SbHKT1;4, SbHKT1;5, SbCLCc, and SbCLCg or the downregulated expression of SbNPF6.4. “Fengtian” exhibited significantly lower leaf osmotic potential but higher leaf water potential and turgor pressure under NaCl treatments, suggesting that the former possessed a stronger osmotic ability than the latter. The contents of K+, NO3−, soluble sugar, and betaine in leaf blades, as well as the contributions of these osmolytes to the leaf osmotic potential, in “Fengtian” were significantly higher than those in “Lvjuren”. In addition, the upregulated expressions of SbAKT1, SbHAK5, SbSKOR, SbNPF3.1, SbNPF6.3, and SbNPF7.3 should be responsible for maintaining K+ and NO3− homeostasis under NaCl treatment. These results lay a foundation for uncovering the salt tolerance mechanisms of sweet sorghum and large-scale cultivation of this species in saline areas.

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.

PurposeThe xerophyte Pugionium cornutum is a salt-tolerant species that can accumulate high amounts of Cl− in shoots for osmotic adjustment under saline condition. However, the molecular basis underlying how P. cornutum uses Cl− as a beneficial osmoticum is not clear yet. In this study, we evaluated the function of a chloride channel PcCLCg from P. cornutum in vacuolar Cl− compartmentalization and plant salt tolerance.MethodsPcCLCg was cloned; its subcellular localization and expression patterns were analyzed; its function in vacuolar Cl− compartmentalization, ion homeostasis and plant salt tolerance were investigated by expression in yeast and Arabidopsis.ResultsPcCLCg was located on the tonoplast, the encoding gene was mainly expressed in shoots, and the expression level was induced by Cl−-salts. The expression of PcCLCg could improve the growth and increase the Cl− content of the yeast mutant Δgef1, in which a chloride channel gene ScGEF1 is deleted, under Cl−-salt treatments. The overexpression of PcCLCg in wild-type Arabidopsis or atclcg mutant alleviated the detrimental effects of NaCl stress on plant growth and significantly increased shoot Cl− and Na+ content under NaCl treatments. Interestingly, PcCLCg-overexpressing lines showed an increased expression of SLAH1 and NHX1 in roots and shoots, respectively, while a decreased expression of SOS1 in roots than wild-type under salt stress.ConclusionsThe upregulated expression of PcCLCg in shoots is conducive to vacuolar Cl− compartmentalization and regulation of Na+ and Cl− accumulation, thus enhancing the osmotic adjustment capacity in the shoot of P. cornutum under saline conditions.

PurposeAtriplex 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.ResultsTwo 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.ConclusionsAcHKT1;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.