High Ca2+ reverts the repression of high-affinity K+ uptake produced by Na+ in Solanum lycopersycum L. (var. microtom) plants

Abstract Potassium (K + ) is an essential nutrient for plants which is acquired by plant roots through the operation of specific transport systems. Abiotic stress conditions such as salinity impair K + nutrition because, in addition to other effects, high salt concentrations in the solution bathing the roots inhibit K + uptake systems. This detrimental effect of salinity is exacerbated when external K + is very low and the only system capable of mediating K + uptake is one with high-affinity for K + , as that mediated by transporters of the HAK5 type. Increasing external Ca 2+ has been shown to improve K + nutrition under salinity and, although the specific mechanisms for this beneficial effect are largely unknown, they are beginning to be understood. The genes encoding the HAK5 transporters are induced by K + starvation and repressed by long-term exposure to high Na + . This occurs in parallel with the hyperpolarization and depolarization of root cell membrane potential. In the present study it is shown in tomato plants that the presence of high Ca 2+ during the K + starvation period that leads to LeHAK5 induction, counteracts the repression exerted by high Na + . High Ca 2+ reduces the Na + -induced plasma membrane depolarization of root cells, resorting one of the putative first steps in the low-K + signal cascade. This allows proper LeHAK5 expression and functional high-affinity K + uptake at the roots. Thus, the maintenance of HAK5-mediated K + nutrition under salinity by high Ca 2+ can be regarded as a specific beneficial effect of Ca 2+ contributing to salt tolerance in plants.