SOS1, HKT1;5, and NHX1 Synergistically Modulate Na+ Homeostasis in the Halophytic Grass Puccinellia tenuiflora

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.

Adaptation of Plants to Salt Stress: Characterization of Na+ and K+ Transporters and Role of CBL Gene Family in Regulating Salt Stress Response

“…This is one of the mechanisms to maintain a higher K + /Na + ratio in shoots during salt stress in plants [155]. Synergistic effects of SOS1, HKT1;5, and NHX1 have been proposed to regulate Na + homeostasis in Puccinellia tenuiflora, a halophytic plant [156]. The NaCl stress-induced vacuolar compartmentalization of its xylem load has been attributed to regulation by the differential expression of NHX1 and HKT1;5.…”

Xylem Ion Loading and Its Implications for Plant Abiotic Stress Tolerance

“…After completion of osmotic adjustment, the maintenance of non-toxic shoot Na + levels and a favourable Na + /K + ratio is crucial for plant survival under prolonged salinity. The expression levels of PtSOS1 and PtHKT1;5 in Puccinellia tenuiflora (halophytic grass) depend on the capacity of the vacuole to sequester Na + , mediated by PtNHX1 (Zhang et al, 2017). PtSOS1 expression is enhanced under mild salinity; a condition under which PtNHX would efficiently sequester Na + to the vacuole.…”

“…PtSOS1 expression is enhanced under mild salinity; a condition under which PtNHX would efficiently sequester Na + to the vacuole. Under more severe conditions, when the vacuolar Na + capability is exceeded, strong expression of PtHKT1;5 (mediating xylem Na + unloading) is observed (Zhang et al, 2017).…”

A salinity‐tolerant japonica cultivar has Na+ exclusion mechanism at leaf sheaths through the function of a Na+ transporter OsHKT1;4 under salinity stress

“…Potential relevance of the aquaporin for Na + entry into roots is also discussed (Byrt et al., ). A model of synergistic modulation of Na + homoeostasis through Na + transporters was proposed in a halophytic grass (Zhang et al., ). Besides these transporters, transcription factors also participate in regulation of Na + accumulation through transcriptional activation (Almeida, Gregorio, Oliveira, & Saibo, ).…”