Halophytic NHXs confer salt tolerance by altering cytosolic and vacuolar K super(+) and Na super(+) in Arabidopsis root cell

While the role of the vacuolar NHX Na super(+)/H super(+) exchangers in plant salt tolerance has been demonstrated on numerous occasions, their control over cytosolic ionic relations has never been functionally analysed in the context of subcellular Na super(+) and K super(+) homeostasis. In this work, PutNHX1 and SeNHX1 were cloned from halophytes Puccinellia tenuiflora and Salicornia europaea and transiently expressed in Arabidopsis wild type Col-0 and the nhx1 mutant. Phylogentic analysis, topological prediction, analysis of evolutionary conservation, the topology structure and analysis of hydrophobic or polar regions of PutNHX1 and SeNHX1 indicated that they are unique tonoplast Na super(+)/H super(+) antiporters with characteristics for salt tolerance. As a part of the functional assessment, cytosolic and vacuolar Na super(+) and K super(+) in different root tissues and ion fluxes from root mature zone of Col-0, nhx1 and their transgenic lines were measured. Transgenic lines sequestered large quantity of Na super(+) into root cell vacuoles and also promoted high cytosolic and vacuolar K super(+) accumulation. Expression of PutNHX1 and SeNHX1 led to significant transient root Na super(+) uptake in the four transgenic lines upon recovery from salt treatment. In contrast, the nhx1 mutant maintained a prolonged Na super(+) efflux and the nhx1:PutNHX1 and nhx1:SeNHX1 lines started to actively pump Na super(+) out of the cell. Overall, our findings suggest that PutNHX1 and SeNHX1 improve Na super(+) sequestration in the vacuole and K super(+) retention in the cytosol and vacuole of root cells of Arabidopsis, and that they interact with other regulatory mechanisms to provide a highly orchestrated regulation of ionic relations among intracellular cell compartments.