Suo-Min Wang

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.

The characteristics of Na+, K+ and free proline distribution in several drought-resistant plants of the Alxa Desert, China

The distribution characteristics of Na+, K+ and free proline were investigated in succulent xerophytes-Haloxylon ammodendron and Zygophyllum xanthoxylum; xerophytes-Artemisia sphaerocephala and Caragana korshinskii; and mesophytes-Agriophyllum squarrosum and Corispermum mongolicum in the Alxa Desert of China. The results showed that mesophytes and xerophytes were salt excluding species, and the concentrations of Na+ in these species were 1.5% to 3.8% of those in succulent xerophytes. Concentrations of K+ in Agriophyllum squarrosum and Corispermum mongolicum were 1.3-2.7 times those in Artemisia sphaerocephala and Caragana korshinskii. Concentrations of K+ in the stems of Agriophyllum squarrosum and Corispermum mongolicum were 1.8 and 2.2 times those in their roots, respectively, For mesophytes, accumulating large quantities of K+ in their stems may facilitate water movement along a soil-plant gradient. The xerophytes accumulated large quantities of K+ and free proline. Their proline concentrations in the whole plant were 6.0-16.0 times higher than those of mesophytes, and were 1.8-25.0 times higher than those of succulent xerophytes. In Artemisia sphaerocephala, the concentrations of proline increased by 3.1- and 10.5-fold from roots to stems, and from stems to leaves, respectively. A similar trend was observed in Caragana korshinskii. Therefore, accumulating K+ and free proline may play a role in drought adaptation in xerophytes. Succulent xerophytes Haloxylon ammodendron and Zygophyllum xanthoxylum were identified as salt diluting species that absorbed much Na+ by roots, and the Na+ was transported to the leaves and photosynthesizing branches. The succulent xerophytes accumulated larger quantities of Na+ than K+ for osmotic adjustment even at low soil salinities, resulting in the lowest values of selective absorption and selective transport capacities of the root systems. Our data suggest that Na+ accumulation rather than exclusion may be one of the most effective strategies for adaptation of succulent xerophytes to and environments

Effects of Silicon on Growth of Wheat Under Drought

Abstract Plants of wheat growing in pots with silicon (Si) applied before sowing had greater plant height, leaf area, and dry materials compared to those without Si applied in well watering conditions. Drought stress was applied by withholding watering for 12 days from 26‐day old seedlings. In the stress conditions, plants growing in Si‐applied soil could maintain higher relative water content (RWC), water potential and leaf area compared to those without Si applied. Moreover, the Si applied plant dry materials were not significantly changed by drought while those of plants growing in pots without Si applied were significantly decreased, and this was mainly due to growth inhibition of the shoots. Drought stressed wheat growing in pots with Si applied had a significantly greater leaf weight ratio (LWR) and lower specific leaf area (SLA) compared to those of stressed plants in the absence of applied Si. This demonstrates that the leaves of stressed plants growing in pots with Si applied were thicker compared to those without Si applied. This may have a beneficial effect by reducing the transpirational loss of water and maintain high RWC and water potential. Therefore, application of Si may be one of the available pathways to improve growth of this crop and increase its production in arid or semi arid areas.

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.

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.

Evidence for the involvement of nitric oxide and reactive oxygen species in osmotic stress tolerance of wheat seedlings: Inverse correlation between leaf abscisic acid accumulation and leaf water loss

Nitric oxide (NO) and reactive oxygen species (ROS) play important roles in both abscisic acid (ABA) signaling and stress-induced ABA accumulation. However, little is known about their physiological roles in the whole plant. In this study, the effects of NO and ROS on leaf water control and the roles of ABA were determined using wheat (Triticum aestivum L.) seedlings. As compared with the control, osmotic stress reduced leaf water loss (LWL) while it increased leaf ABA content. The effects of osmotic stress on LWL and ABA contents were partially reversed by NO scavengers or NO synthase (NOS) inhibitors. Furthermore, sodium nitroprusside (SNP) at concentrations between 0.01 and 10 mM all reduced LWL efficiently and induced ABA accumulation in a dose-dependent manner. When ABA synthesis was inhibited by fluridone or actidione, the effects of SNP on LWL were partially reversed. These results suggest that NO is involved in leaf water maintenance of wheat seedlings under osmotic stress, and one of the possible mechanisms is by stimulating ABA synthesis. The ROS scavengers used in our experiments had no effects on either LWL or ABA accumulation induced by osmotic stress. However, all ROS induced LWL reduction and ABA accumulation significantly. Hydrogen peroxide had the same effects as SNP on LWL and induced ABA accumulation in a dose-dependent manner but had a maximal effect at 1 mM. Fluridone reversed the effects of H2O2 on both LWL reduction and ABA accumulation, while actidione had no effect. These results suggest that ROS are also involved in leaf water maintenance of wheat seedlings by stimulating ABA biosynthesis, but with a different mechanism to that of NO. The ABA-independent mechanism in NO/ROS regulation of leaf water balance is discussed, in relation to our results.

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.

Gradual Drought Under Field Conditions Influences the Glutathione Metabolism, Redox Balance and Energy Supply in Spring Wheat

Glutathione (GSH) metabolism, redox balance and energy supply in spring wheat (Triticum aestivum L.) during gradual drought stress under field conditions were investigated. Although levels of total and reduced GSH were decreased, the ratio of GSH/GSSG (glutathione disulfide) was markedly increased by drought. Levels of GSH biosynthetic precursors, cysteine (Cys) and γ-glutamylcysteine (γ-GC), and the activities of their biosynthetic enzymes, γ-glutamylcysteine synthetase (γ-GCS) and glutathione synthetase (GSHS) were also significantly increased in stressed plants. Glutathione reductase (GR) activity, which is responsible for the conversion of GSSG to GSH, was also increased under this field stress. However, two other important enzymes in GSH metabolism, glutathione peroxidase (GP) and glutathione S-transferase (GST), showed decreased activity in the droughted plants. These results suggest that the higher ratio of GSH/GSSG, the rate of GSH biosynthesis and the capacity of its redox cycling rather than GSH accumulation might be essential for drought resistance of plants. Activities of the two key Calvin-cycle enzymes possessing exposed sulfhydryl groups, NADP+-dependent glyceraldehydes-3-phosphate dehydrogenase (G3PD) and fructose-1,6-bisphosphatase (FBPase) were not affected by drought stress, whereas, activity of the key enzyme in the pentose-phosphate pathway (PPP), 6-phosphogluconate dehydrogenase (6-PGD), increased in the droughted plants. The ratios of NADPH/NADP+, NADH/NAD+ and ATP/ADP increased in the droughted plants, indicating that an up-regulation of the reduced redox state and the energy supply in the plant cells might be an important physiological strategy for plants responding to drought stress. A simple correlation between the high ratio of GSH/GSSG, the rate of GSH biosynthesis and the redox cycle and the high reduction states of redox status in the plant cells was also observed under field drought.

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.