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

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 NH-sensitive pathway dependent on SlCIPK23, and a second one sensitive to Ba, Ca, La, 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.