Doing ‘business as usual’ comes with a cost: evaluating energy cost of maintaining plant intracellular K⁺ homeostasis under saline conditions

Summary: 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.