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 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.