Abstract
Puccinellia tenuiflora is a typical salt-excluding halophytic grass with excellent salt tolerance. Plasma membrane Na+/H+ transporter SOS1, HKT-type protein and tonoplast Na+/H+ antiporter NHX1 are key Na+ transporters involved in plant salt tolerance. Based on our previous research, we had proposed a function model for these transporters in Na+ homeostasis according to the expression of PtSOS1 and Na+, K+ levels in P. tenuiflora responding to salt stress. Here, we analyzed the expression patterns of PtSOS1, PtHKT1;5, and PtNHX1 in P. tenuiflora under 25 and 150 mM NaCl to further validate this model by combining previous physiological characteristics. Results showed that the expressions of PtSOS1 and PtHKT1;5 in roots were significantly induced and peaked at 6 h under both 25 and 150 mM NaCl. Compared to the control, the expression of PtSOS1 significantly increased by 5.8-folds, while that of PtHKT1;5 increased only by 1.2-folds in roots under 25 mM NaCl; on the contrary, the expression of PtSOS1 increased by 1.4-folds, whereas that of PtHKT1;5 increased by 2.2-folds in roots under 150 mM NaCl. In addition, PtNHX1 was induced instantaneously under 25 mM NaCl, while its expression was much higher and more persistent in shoots under 150 mM NaCl. These results provide stronger evidences for the previous hypothesis and extend the model which highlights that SOS1, HKT1;5, and NHX1 synergistically regulate Na+ homeostasis by controlling Na+ transport systems at the whole-plant level under both lower and higher salt conditions. Under mild salinity, PtNHX1 in shoots compartmentalized Na+ into vacuole slowly, and vacuole potential capacity for sequestering Na+ would enhance Na+ loading into the xylem of roots by PtSOS1 through feedback regulation; and consequently, Na+ could be transported from roots to shoots by transpiration stream for osmotic adjustment. While under severe salinity, Na+ was rapidly sequestrated into vacuoles of mesophyll cells by PtNHX1 and the vacuole capacity became saturated for sequestering more Na+, which in turn regulated long-distance Na+ transport from roots to shoots. As a result, the expression of PtHKT1;5 was strongly induced so that the excessive Na+ was unloaded from xylem into xylem parenchyma cells by PtHKT1;5.
Highlights
Soil salinity is one of the major environmental factors restricting agricultural productivity worldwide (Venema et al, 2002; Zhang et al, 2010; Flowers et al, 2015; Gu et al, 2016)
The primers of PtACTIN, PtSOS1, PtHKT1;5, and PtNHX1 were designed by using the Primer 5.0 program (Premier Biosoft International, Palo Alto, CA, USA; Table 1)
Our results further confirmed this opinion: shoot Na+ accumulation in P. tenuiflora was significantly increased in P. tenuiflora under 150 mM NaCl (Wang et al, 2009), which in turn strongly induced the expression of PtHKT1;5, facilitated excessive Na+ unloading into xylem parenchyma cells (XPCs) of roots (Figure 2B), and alleviated Na+ toxicity in plants
Summary
Soil salinity is one of the major environmental factors restricting agricultural productivity worldwide (Venema et al, 2002; Zhang et al, 2010; Flowers et al, 2015; Gu et al, 2016). Our previous study showed that P. tenuiflora could maintain significantly lower net Na+ uptake rates than wheat, especially under 150 and 200 mM NaCl; the accumulation of Na+ in P. tenuiflora was increased, but was significantly lower than that in wheat under 50–200 mM NaCl. P. tenuiflora maintained significantly higher tissue K+ concentrations under various concentrations of NaCl, indicating that restricting unidirectional Na+ influx in roots and maintaining a high selectivity for K+ over Na+ is a major salt-tolerance mechanism of P. tenuiflora (Wang et al, 2009). SOS1 functions mainly in loading Na+ from xylem parenchyma cells (XPCs) into xylem in roots and plays an important role in maintaining Na+ homeostasis in whole plant (Shi et al, 2002b; Qi and Spalding, 2004). In A. thaliana, AtHKT1;1 unloads Na+ from xylem vessels to XPCs, thereby reducing Na+ content in leaves, and the overexpression of AtHKT1;1 in mature root stele increased the influx of Na+
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