The influence of genes regulating transmembrane transport of Na+ on the salt resistance of Aeluropus lagopoides

Plantlets of Aeluropus lagopoides (Linn.) Trin. Ex Thw. were grown at different NaCl concentrations (26, 167, 373 and 747 mM) for 3, 7 and 15 days; their growth, osmotic adjustment, gas exchange, ion compartmentalisation and expression of various genes related to Na+ flux was studied. Plantlets showed optimal growth in non-saline (control; 26mM NaCl) solutions, whereas CO2/H2O gas exchange, leaf water concentration and water use efficiency decreased under all salinity treatments, accompanied by increased leaf senescence, root ash, sodium content and leaf osmolality. A decrease in malondialdehyde (MDA) content with time was correlated with Na+ accumulation in the leaf apoplast and a concomitant increase in Na+ secretion rate. A. lagopoides accumulated a higher concentration of Na+ in root than in leaf vacuoles, corresponding with higher expression of V-NHX and lower expression of PM-NHX in root than leaf tissue. It appears that V-ATPase plays a vital role during Na+ transport by producing an electromotive force, driving ion transport. Leaf calcium increased with increasing salinity, with more rapid accumulation at high salinity than at low salinity, indicating a possible involvement of Ca2+ in maintaining K+ : Na+ ratio. Our results suggest that A. lagopoides successfully compartmentalised Na+ at salinities up to 373mM NaCl by upregulating the gene expression of membrane linked transport proteins (V-NHX and PM-NHX). At higher salinity (747mMNaCl), a reduction in the expression of V-NHX and PM-NHX in leaves without any change in the rate of salt secretion, is a possible cause of the toxicity of NaCl.