The protein kinase SlCIPK23 boosts K+ and Na+ uptake in tomato plants.

Regulation of root transport systems is essential under fluctuating nutrient supply. In the case of potassium (K+ ), HAK/KUP/KT K+ transporters and voltage-gated K+ channels ensure root K+ uptake in a wide range of K+ concentrations. In Arabidopsis, the CIPK23/CBL1-9 complex regulates both transporter- and channel-mediated root K+ uptake. However, research about K+ homeostasis in crops is in demand due to species-specific mechanisms. In the present manuscript, we studied the contribution of the voltage-gated K+ channel LKT1 and the protein kinase SlCIPK23 to K+ uptake in tomato plants by analysing gene-edited knockout tomato mutant lines, together with two-electrode voltage-clamp experiments in Xenopus oocytes and protein-protein interaction analyses. It is shown that LKT1 is a crucial player in tomato K+ nutrition by contributing approximately 50% to root K+ uptake under K+ -sufficient conditions. Moreover, SlCIPK23 was responsible for approximately 100% of LKT1 and approximately 40% of the SlHAK5 K+ transporter activity in planta. Mg+2 and Na+ compensated for K+ deficit in tomato roots to a large extent, and the accumulation of Na+ was strongly dependent on SlCIPK23 function. The role of CIPK23 in Na+ accumulation in tomato roots was not conserved in Arabidopsis, which expands the current set of CIPK23-like protein functions in plants.