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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]

Atriplex canescens is a representative halophyte with excellent tolerance to salt. Previous studies have revealed certain physiological mechanisms and detected functional genes associated with salt tolerance. However, knowledge on the ROS scavenging system and regulatory mechanisms in this species when adapting to salinity is limited. Therefore, this study further analyzed the transcriptional changes in genes related to the ROS scavenging system and important regulatory mechanisms in A. canescens under saline conditions using our previous RNA sequencing data. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotation revealed that the differentially expressed genes (DEGs) were highly enriched in signal transduction- and reactive oxygen species-related biological processes, including "response to oxidative stress", "oxidoreductase activity", "protein kinase activity", "transcription factor activity", and "plant hormone signal transduction". Further analyses suggested that the transcription abundance of many genes involved in SOD, the AsA-GSH cycle, the GPX pathway, PrxR/Trx, and the flavonoid biosynthesis pathway were obviously enhanced. These pathways are favorable for scavenging excessive ROS induced by salt and maintaining the integrity of the cell membrane. Meanwhile, many vital transcription factor genes (WRKY, MYB, ZF, HSF, DREB, and NAC) exhibited increased transcripts, which is conducive to dealing with saline conditions by regulating downstream salt-responsive genes. Furthermore, a larger number of genes encoding protein kinases (RLK, CDPK, MAPK, and CTR1) were significantly induced by saline conditions, which is beneficial to the reception/transduction of salt-related signals. This study describes the abundant genetic resources for enhancing the salt tolerance in salt-sensitive plants, especially in forages and crops. [ABSTRACT FROM AUTHOR]

Salinity and drought are major environmental factors limiting the growth and productivity of alfalfa worldwide as this economically important legume forage is sensitive to these kinds of abiotic stress. In this study, transgenic alfalfa lines expressing both tonoplast NXH and H+- PPase genes, Zx NHX and Zx VP1-1 from the xerophyte Zygophyllum xanthoxylum L., were produced via Agrobacterium tumefaciens-mediated transformation. Compared with wild-type ( WT) plants, transgenic alfalfa plants co-expressing Zx NHX and Zx VP1-1 grew better with greater plant height and dry mass under normal or stress conditions (NaCl or water-deficit) in the greenhouse. The growth performance of transgenic alfalfa plants was associated with more Na+, K+ and Ca2+ accumulation in leaves and roots, as a result of co-expression of Zx NHX and Zx VP1-1. Cation accumulation contributed to maintaining intracellular ions homoeostasis and osmoregulation of plants and thus conferred higher leaf relative water content and greater photosynthesis capacity in transgenic plants compared to WT when subjected to NaCl or water-deficit stress. Furthermore, the transgenic alfalfa co-expressing Zx NHX and Zx VP1-1 also grew faster than WT plants under field conditions, and most importantly, exhibited enhanced photosynthesis capacity by maintaining higher net photosynthetic rate, stomatal conductance, and water-use efficiency than WT plants. Our results indicate that co-expression of tonoplast NHX and H+- PPase genes from a xerophyte significantly improved the growth of alfalfa, and enhanced its tolerance to high salinity and drought. This study laid a solid basis for reclaiming and restoring saline and arid marginal lands as well as improving forage yield in northern China. [ABSTRACT FROM AUTHOR]

The plasma membrane Na[sup +]/H[sup +] antiporter (SOS1) was shown to be a Na[sup +] efflux protein and also involved in K[sup +] uptake and transport. PtSOS1 was characterised from Puccinellia tenuiflora (Griseb.) Scribn. et Merr., a monocotyledonous halophyte that has a high selectivity for K[sup +] over Na[sup +] by roots under salt stress. To assess the contribution of PtSOS1 to the selectivity for K[sup +] over Na[sup +], the expression levels of PtSOS1 and Na[sup +], K[sup +] accumulations in P. tenuiflora exposed to different concentrations of NaCl, KCl or NaCl plus KCl were analysed. Results showed that the expression levels of PtSOS1 in roots increased significantly with the increase of external NaCl (25-150 mM), accompanied by an increase of selective transport (ST) capacity for K[sup +] over Na[sup +] by roots. Transcription levels of PtSOS1 in roots and ST values increased under 0.1-1 mM KCl, then declined sharply under 5-10 mM KCl. Under 150 mM NaCl, PtSOS1 expression levels in roots and ST values at 0.1 mM KCl was significantly lower than that at 5 mM KCl with the prolonging of treatment time. A significant positive correlation was found between root PtSOS1 expression levels and ST values under various concentrations of NaCl, KCl or 150 mM NaCl plus 0.1 or 5 mM KCl treatments. Therefore, it is proposed that PtSOS1 is the major component of selective transport capacity for K[sup +] over Na[sup +] and hence, salt tolerance of P. tenuiflora. Finally, we hypothesise a function model of SOS1 in regulating K[sup +] and Na[sup +] transport system in the membrane of xylem parenchyma cells by sustaining the membrane integrity; it also appears that this model could reasonably explain the phenomenon of Na[sup +] retrieval from the xylem when plants are exposed to severe salt stress. The halophyte Puccinellia tenuiflora has a high selectivity for K[sup +] over Na[sup +] by roots under salt stress. Our aim was to investigate the molecular basis of this observation: we found that PtSOS1 is the major component of selective transport capacity for K[sup +] over Na[sup +] in P. tenuiflora under salt stress. This result will broaden the understanding of interplay between SOS1 and HKT as a factor depolarising xylem parenchyma, which leads to xylem K[sup +] loading via SKOR. [ABSTRACT FROM AUTHOR]

Abstract: Salinity and drought are two major abiotic factors limiting crop production. To generate the legume forage adapting to saline and arid soils, we had transformed alfalfa (Medicago sativa) with AVP1, a vacuolar H+-pyrophosphatase (H+-PPase) gene from Arabidopsis thaliana. In this paper, we report that overexpression of the AVP1 gene confers enhanced salt and drought tolerance to the transformed alfalfa. Transgenic alfalfa grows well in the presence of 200mM NaCl and also under a water-deprivation condition, while wild-type plants exhibit chlorosis and growth inhibition, even death. Compared with wild-type plants, transgenic plants accumulate more Na+, K+ and Ca2+ in leaves and roots. Moreover, the leaves of transgenic plants retain more water during drought stress than those of wild-type plants due to lower solute potential. Increased solute accumulation and water retention, and steady intracellular ion homeostasis might also confer other phenotypes of salt and drought tolerance in the transgenic plants, which include the higher photosynthesis capacity and the lesser cell membrane damage during salt or water-deficit stress. Furthermore, the increased potassium uptake and root activity in transgenic alfalfa may be the consequences of rhizosphere acidification resulting from expression of the AVP1. These results indicated that the expression of AVP1 confers enhanced salt and drought tolerance on alfalfa, a very important crop. This study provides a way for improving salt and drought tolerance in important legume forages. [Copyright &y& Elsevier]

Abstract: Sodium (Na+) has been found to play important roles in the adaptation of xerophytic species to drought conditions. The tonoplast Na+/H+ antiporter (NHX) proved to be involved in the compartmentalization of Na+ into vacuoles from the cytosol. In this study, a gene (ZxNHX) encoding tonoplast Na+/H+ antiporter was isolated and characterized in Zygophyllum xanthoxylum, a succulent xerophyte growing in desert areas of northwest China. The results revealed that ZxNHX consisted of 532 amino acid residues with a conserved binding domain (78LFFIYLLPPI87) for amiloride and shared high similarity (73–81%) with the identified tonoplast Na+/H+ antiporters in other plant species. Semi-quantitative RT-PCR analysis showed that the mRNA level of ZxNHX was significantly higher in the leaf than in stem or root. The transcript abundance of ZxNHX in Z. xanthoxylum subjected to salt (5–150mM NaCl) or drought (50–15% of field water capacity (FWC)) was 1.4–8.4 times or 2.3–4.4 times that of plants grown in the absence of NaCl or 70% of FWC, respectively. Leaf Na+ concentration in plants exposed to salt or drought was 1.7–5.2 times or 1.5–2.2 times that of corresponding control plants, respectively. It is clear that there is a positive correlation between up-regulation of ZxNHX and accumulation of Na+ in Z. xanthoxylum exposed to salt or drought. Furthermore, Z. xanthoxylum accumulated larger amounts of Na+ than K+ in the leaf under drought conditions, even in low salt soil. In summary, our results suggest that ZxNHX encodes a tonoplast Na+/H+ antiporter and plays important roles in Na+ accumulation and homeostasis of Z. xanthoxylum under salt and drought conditions. [Copyright &y& Elsevier]