Relevance of the SlCIPK23 kinase in Na+ uptake and root morphology in K+-starved tomato plants

Abstract

The beneficial effects of Na⁺ as a substitute for K⁺ have been well-documented at the physiological level. However, the transport systems and regulatory mechanisms that allow Na⁺ acquisition under K⁺ deficiency remain poorly understood in the majority of land plants. In tomato, SlCIPK23 kinase was involved in Na⁺ accumulation in K⁺-starved plants, in addition to activating the LKT1 K⁺ channel and the K⁺ transporter SlHAK5. We used the central role of SlCIPK23 in K⁺ and Na⁺ acquisition to study which molecular entities mediate Na⁺ uptake with knockout tomato mutants and expression in heterologous systems. Two main pathways for Na⁺ uptake were deduced in tomato plants: an NH₄⁺-sensitive pathway dependent on SlCIPK23, and a second one sensitive to Ba²⁺, Ca²⁺, La³⁺, and Li⁺. The addition of Na⁺ (10 mM) to lkt1, slhak5, or slcipk23 mutant KO lines produced interesting changes in root morphology. In particular, the roots of slcipk23 plants were longer and lighter than those of the WT under K⁺-deficient conditions and this effect was reversed by the addition of 10 mM Na⁺. These results provide a stimulating perspective for the study of the beneficial effects of Na⁺ in crops.