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 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.