Jin-Lin Zhang

Mechanisms of sodium uptake by roots of higher plants

The negative impact of soil salinity on agricultural yields is significant. For agricultural plants, sensitivity to salinity is commonly (but not exclusively) due to the abundance of Na+ in the soil as excess Na+ is toxic to plants. We consider reducing Na+ uptake to be the key, as well as the most efficient approach, to control Na+ accumulation in crop plants and hence to improve their salt resistance. Understanding the mechanism of Na+ uptake by the roots of higher plants is crucial for manipulating salt resistance. Hence, the aim of this review is to highlight and discuss recent advances in our understanding of the mechanisms of Na+ uptake by plant roots at both physiological and molecular levels. We conclude that continued efforts to investigate the mechanisms of root Na+ uptake in higher plants are necessary, especially that of low-affinity Na+ uptake, as it is the means by which sodium enters into plants growing in saline soils.

Low-Affinity Na+ Uptake in the Halophyte Suaeda maritima

Na+ uptake by plant roots has largely been explored using species that accumulate little Na+ into their shoots. By way of contrast, the halophyte Suaeda maritima accumulates, without injury, concentrations of the order of 400 mm NaCl in its leaves. Here we report that cAMP and Ca2+ (blockers of nonselective cation channels) and Li+ (a competitive inhibitor of Na+ uptake) did not have any significant effect on the uptake of Na+ by the halophyte S. maritima when plants were in 25 or 150 mm NaCl (150 mm NaCl is near optimal for growth). However, the inhibitors of K+ channels, TEA+ (10 mm), Cs+ (3 mm), and Ba2+ (5 mm), significantly reduced the net uptake of Na+ from 150 mm NaCl over 48 h, by 54%, 24%, and 29%, respectively. TEA+ (10 mm), Cs+ (3 mm), and Ba2+ (1 mm) also significantly reduced 22Na+ influx (measured over 2 min in 150 mm external NaCl) by 47%, 30%, and 31%, respectively. In contrast to the situation in 150 mm NaCl, neither TEA+ (1–10 mm) nor Cs+ (0.5–10 mm) significantly reduced net Na+ uptake or 22Na+ influx in 25 mm NaCl. Ba2+ (at 5 mm) did significantly decrease net Na+ uptake (by 47%) and 22Na+ influx (by 36% with 1 mm Ba2+) in 25 mm NaCl. K+ (10 or 50 mm) had no effect on 22Na+ influx at concentrations below 75 mm NaCl, but the influx of 22Na+ was inhibited by 50 mm K+ when the external concentration of NaCl was above 75 mm. The data suggest that neither nonselective cation channels nor a low-affinity cation transporter are major pathways for Na+ entry into root cells. We propose that two distinct low-affinity Na+ uptake pathways exist in S. maritima: Pathway 1 is insensitive to TEA+ or Cs+, but sensitive to Ba2+ and mediates Na+ uptake under low salinities (25 mm NaCl); pathway 2 is sensitive to TEA+, Cs+, and Ba2+ and mediates Na+ uptake under higher external salt concentrations (150 mm NaCl). Pathway 1 might be mediated by a high-affinity K transporter-type transporter and pathway 2 by an AKT1-type channel.

Physiological and molecular mechanisms of plant salt tolerance

Salt tolerance is an important economic trait for crops growing in both irrigated fields and marginal lands. The plant kingdom contains plant species that possess highly distinctive capacities for salt tolerance as a result of evolutionary adaptation to their environments. Yet, the cellular mechanisms contributing to salt tolerance seem to be conserved to some extent in plants although some highly salt-tolerant plants have unique structures that can actively excrete salts. In this review, we begin by summarizing the research in Arabidopsis with a focus on the findings of three membrane transporters that are important for salt tolerance: SOS1, AtHKT1, and AtNHX1. We then review the recent studies in salt tolerance in crops and halophytes. Molecular and physiological mechanisms of salt tolerance in plants revealed by the studies in the model plant, crops, and halophytes are emphasized. Utilization of the Na+ transporters to improve salt tolerance in plants is also summarized. Perspectives are provided at the end of this review.

Puccinellia tenuiflora maintains a low Na+ level under salinity by limiting unidirectional Na+ influx resulting in a high selectivity for K+ over Na+

Puccinellia tenuiflora is a useful monocotyledonous halophyte that might be used for improving salt tolerance of cereals. This current work has shown that P. tenuiflora has stronger selectivity for K+ over Na+ allowing it to maintain significantly lower tissue Na+ and higher K+ concentration than that of wheat under short- or long-term NaCl treatments. To assess the relative contribution of Na+ efflux and influx to net Na+ accumulation, unidirectional 22Na+ fluxes in roots were carried out. It was firstly found that unidirectional 22Na+ influx into root of P. tenuiflora was significantly lower (by 31-37%) than in wheat under 100 and 150 mM NaCl. P. tenuiflora had lower unidirectional Na+ efflux than wheat; the ratio of efflux to influx was similar between the two species. Leaf secretion of P. tenuiflora was also estimated, and found the loss of Na+ content from leaves to account for only 0.0006% of the whole plant Na+ content over 33 d of NaCl treatments. Therefore, it is proposed that neither unidirectional Na+ efflux of roots nor salt secretion by leaves, but restricting unidirectional Na+ influx into roots with a strong selectivity for K+ over Na+ seems likely to contribute to the salt tolerance of P. tenuiflora.

The ZxNHX gene encoding tonoplast Na+/H+ antiporter from the xerophyte Zygophyllum xanthoxylum plays important roles in response to salt and drought

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 ((78)LFFIYLLPPI(87)) 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 (5-150 mM 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.

Beneficial soil bacterium Bacillus subtilis (GB03) augments salt tolerance of white clover

Soil salinity is an increasingly serious problem worldwide that reduces agricultural output potential. Selected beneficial soil bacteria can promote plant growth and augment tolerance to biotic and abiotic stresses. Bacillus subtilis strain GB03 has been shown to confer growth promotion and abiotic stress tolerance in the model plant Arabidopsis thaliana. Here we examined the effect of this beneficial soil bacterium on salt tolerance in the legume forage crop, white clover. Plants of white clover (Trifolium repens L. cultivar Huia) were grown from seeds with or without soil inoculation of the beneficial soil bacterium Bacillus subtilis GB03 supplemented with 0, 50, 100, or 150 mM NaCl water into soil. Growth parameters, chlorophyll content, malondialdehyde (MDA) content and osmotic potential were monitored during the growth cycle. Endogenous Na+ and K+ contents were determined at the time of harvest. White clover plants grown in GB03-inoculated soil were significantly larger than non-inoculated controls with respect to shoot height, root length, plant biomass, leaf area and chlorophyll content; leaf MDA content under saline condition and leaf osmotic potential under severe salinity condition (150 mM NaCl) were significantly decreased. Furthermore, GB03 significantly decreased shoot and root Na+ accumulation and thereby improved K+/Na+ ratio when GB03-inoculated plants were grown under elevated salt conditions. The results indicate that soil inoculation with GB03 promotes white clover growth under both non-saline and saline conditions by directly or indirectly regulating plant chlorophyll content, leaf osmotic potential, cell membrane integrity and ion accumulation.

Selective transport capacity for K+ over Na+ is linked to the expression levels of PtSOS1 in halophyte Puccinellia tenuiflora

The plasma membrane Na+/H+ antiporter (SOS1) was shown to be a Na+ efflux protein and also involved in K+ uptake and transport. PtSOS1 was characterised from Puccinellia tenuiflora (Griseb.) Scribn. et Merr., a monocotyledonous halophyte that has a high selectivity for K+ over Na+ by roots under salt stress. To assess the contribution of PtSOS1 to the selectivity for K+ over Na+, the expression levels of PtSOS1 and Na+, K+ 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 (25-150mM), accompanied by an increase of selective transport (ST) capacity for K+ over Na+ by roots. Transcription levels of PtSOS1 in roots and ST values increased under 0.1-1mM KCl, then declined sharply under 5-10mM KCl. Under 150mM NaCl, PtSOS1 expression levels in roots and ST values at 0.1mM KCl was significantly lower than that at 5mM 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 150mM NaCl plus 0.1 or 5mM KCl treatments. Therefore, it is proposed that PtSOS1 is the major component of selective transport capacity for K+ over Na+ and hence, salt tolerance of P. tenuiflora. Finally, we hypothesise a function model of SOS1 in regulating K+ and Na+ 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+ retrieval from the xylem when plants are exposed to severe salt stress.

Co-expression of xerophyte Zygophyllum xanthoxylum ZxNHX and ZxVP1-1 enhances salt and drought tolerance in transgenic Lotus corniculatus by increasing cations accumulation

Lotus corniculatus L. is an important legume for forage, but is sensitive to salinity and drought. To develop salt- and drought-resistant L. corniculatus, ZxNHX and ZxVP1-1 genes encoding tonoplast Na+/H+ antiporter and H+-pyrophosphatase (H+-PPase) from a succulent xerophyte Zygophyllum xanthoxylum L., which is well adapted to arid environments through accumulating Na+ in its leaves, were transferred into this forage. We obtained the transgenic lines co-expressing ZxNHX and ZxVP1-1 genes (VX) as well as expressing ZxVP1-1 gene alone (VP). Compared with wild-type, both VX and VP transgenic lines grew better at 200mM NaCl, and also exhibited higher tolerance and faster recovery from water-deficit stress: these performances were associated with more Na+, K+ and Ca2+ accumulation in their leaves and roots, which caused lower leaf solute potential and thus retained more water. Moreover, the transgenic lines maintained lower relative membrane permeability and higher net photosynthesis rate under salt or water-deficit stress. These results indicate that expression of tonoplast Na+/H+ antiporter and H+-PPase genes from xerophyte enhanced salt and drought tolerance of L. corniculatus. Furthermore, compared with VP, VX showed higher shoot biomass, more cations accumulation, higher water retention, lesser cell membrane damage and higher photosynthesis capacity under salt or water-deficit condition, suggesting that co-expression of ZxVP1-1 and ZxNHX confers even greater performance to transgenic L. corniculatus than expression of the single ZxVP1-1.

Augmenting Sulfur Metabolism and Herbivore Defense in Arabidopsis by Bacterial Volatile Signaling

Sulfur is an element necessary for the life cycle of higher plants. Its assimilation and reduction into essential biomolecules are pivotal factors determining a plant’s growth and vigor as well as resistance to environmental stress. While certain soil microbes can enhance ion solubility via chelating agents or oxidation, microbial regulation of plant-sulfur assimilation has not been reported. With an increasing understanding that soil microbes can activate growth and stress tolerance in plants via chemical signaling, the question arises as to whether such beneficial bacteria also regulate sulfur assimilation. Here we report a previously unidentified mechanism by which the growth-promoting rhizobacterium Bacillus amyloliquefaciens (GB03) transcriptionally activates genes responsible for sulfur assimilation, increasing sulfur uptake and accumulation in Arabidopsis. Transcripts encoding for sulfur-rich aliphatic and indolic glucosinolates are also GB03 induced. As a result, GB03-exposed plants with elevated glucosinolates exhibit greater protection against the generalist herbivore, Spodoptera exigua (beet armyworm, BAW). In contrast, a previously characterized glucosinolate mutant compromised in the production of both aliphatic and indolic glucosinolates is also compromised in terms of GB03-induced protection against insect herbivory. As with in vitro studies, soil-grown plants show enhanced glucosinolate accumulation and protection against BAW feeding with GB03 exposure. These results demonstrate the potential of microbes to enhance plant sulfur assimilation and emphasize the sophisticated integration of microbial signaling in plant defense.

Ultrastructural and physiological responses of potato (Solanum tuberosum L.) plantlets to gradient saline stress

Salinity is one of the major abiotic stresses that impacts plant growth and reduces the productivity of field crops. Compared to field plants, test tube plantlets offer a direct and fast approach to investigate the mechanism of salt tolerance. Here we examined the ultrastructural and physiological responses of potato (Solanum tuberosum L. c.v. “Longshu No. 3”) plantlets to gradient saline stress (0, 25, 50, 100, and 200 mM NaCl) with two consequent observations (2 and 6 weeks, respectively). The results showed that, with the increase of external NaCl concentration and the duration of treatments, (1) the number of chloroplasts and cell intercellular spaces markedly decreased, (2) cell walls were thickened and even ruptured, (3) mesophyll cells and chloroplasts were gradually damaged to a complete disorganization containing more starch, (4) leaf Na and Cl contents increased while leaf K content decreased, (5) leaf proline content and the activities of catalase (CAT) and superoxide dismutase (SOD) increased significantly, and (6) leaf malondialdehyde (MDA) content increased significantly and stomatal area and chlorophyll content decline were also detected. Severe salt stress (200 mM NaCl) inhibited plantlet growth. These results indicated that potato plantlets adapt to salt stress to some extent through accumulating osmoprotectants, such as proline, increasing the activities of antioxidant enzymes, such as CAT and SOD. The outcomes of this study provide ultrastructural and physiological insights into characterizing potential damages induced by salt stress for selecting salt-tolerant potato cultivars.