The endosomal LeNHX2 ion transporter exchanges H
+ with K
+ and, to lesser extent, Na
+. Here, we investigated the response to NaCl supply and K
+ deprivation in transgenic tomato (
Solanum lycopersicum L.) overexpressing LeNHX2 and show that transformed tomato plants grew better in saline conditions than untransformed controls, whereas in the absence of K
+ the opposite was found. Analysis of mineral composition showed a higher K
+ content in roots, shoots and xylem sap of transgenic plants and no differences in Na
+ content between transgenic and untransformed plants grown either in the presence or the absence of 120 mm NaCl. Transgenic plants showed higher Na
+/H
+ and, above all, K
+/H
+ transport activity in root intracellular membrane vesicles. Under K
+ limiting conditions, transgenic plants enhanced root expression of the high‐affinity K
+ uptake system HAK5 compared to untransformed controls. Furthermore, tomato overexpressing LeNHX2 showed twofold higher K
+ depletion rates and half cytosolic K
+ activity than untransformed controls. Under NaCl stress, transgenic plants showed higher uptake velocity for K
+ and lower cytosolic K
+ activity than untransformed plants. These results indicate the fundamental role of K
+ homeostasis in the better performance of LeNHX2 overexpressing tomato under NaCl stress.
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