Combined Phosphorus and Potassium Starvation Upregulates Sodium Accumulation by HKT1;1 in Tomato Plants

Na⁺ can substitute K⁺ under K⁺ deficiency conditions, which is denominated as beneficial Na⁺. Despite its relevance at the physiological level and its interest in agriculture, the mechanisms allowing its uptake and use by plants remain unclear. The accumulation of beneficial Na⁺ has been associated with K⁺ deficiency, but the deficiency of other mineral nutrients could also affect this process. In this study, we observed that the combined deficiency of K⁺ and P (-K -P) resulted in a 2-fold increase in the root Na⁺ content in tomato plants with respect to the single K⁺ deficiency (-K). Such a response was observed also in quinoa and barley but not in Arabidopsis, wheat, rice, soybean and Solanum pennellii, evidencing natural variability for this trait. Pi uptake in -K -P plants was not coupled to Na⁺ as deduced from electrophysiology and plant growth experiments. Finally, we show that the Na⁺ channel HKT1;1, but not HKT1;2, contributes to Na⁺ uptake in tomato plants, in particular, in the -K -P condition where a 40-45% reduction in root Na⁺ content is observed in HKT1;1-silenced lines. Therefore, the information acquired on HKT1;1 is crucial for boosting Na⁺ uptake in plants.