Abstract
Salinization of agricultural lands is a major threat to agriculture. Many different factors affect and determine plant salt tolerance. Nonetheless, there is a consensus on the relevance of maintaining an optimal cytosolic potassium:sodium ion (K+ :Na+ ) ratio for salinity tolerance in plants. This ratio depends on the operation of plasma membrane and tonoplast transporters. In the present review we focus on some aspects related to the energetic cost of maintaining that K+ :Na+ ratio. One of the factors that affect the cost of the first step of K+ acquisition - root K+ uptake through High Affinity K+ transporter and Arabidopsis K+ transport system 1 transport systems - is the value of the plasma membrane potential of root cells, a parameter that may differ amongst plant species. In addition to its role in nutrition, cytosolic K+ also is important for signalling, and K+ efflux through gated outward-rectifying K+ and nonselective cation channels can be regarded as a switch to redirect energy towards defence reactions. In maintaining cytosolic K+ , the great buffer capacity of the vacuole should be considered. The possible role of high-affinity K+ transporters (HKT)2s in mediating K+ uptake under saline conditions and the importance of cycling of K+ throughout the plant also are discussed.
Highlights
Soil salinization, with NaCl as the predominant salt, is a major threat to agriculture lands
Amongst these, maintaining the optimal cytosolic K+/Na+ ratio is often named as a critical feature conferring overall salinity tolerance in plants
We focus on two dilemmas that face plant cells when exposed to salinity: (i) the maintenance of cytoplasmic K+ for nutrition or for signalling and (ii) having a type of transporters, High-affinity K+ transporters (HKT), that are permeable to Na+
Summary
Many different factors affect and determine plant salt tolerance. There is a consensus on the relevance of maintaining an optimal cytosolic K+/Na+ ratio for salinity tolerance in plants. This ratio depends on the operation of plasma membrane and tonoplast transporters. One of the factors that affect the cost of the first step of K+ acquisition, i.e. root K+ uptake through HAK and AKT1 transport systems, is the value of the plasma membrane potential of root cells, a parameter that may differ amongst plant species. The possible role of HKT2s transporters to mediate K+ uptake under salinity and the importance of cycling of K+ throughout the plant are discussed
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