Salt Stress in Thellungiella halophila Activates Na⁺ Transport Mechanisms Required for Salinity Tolerance

Salinity is considered one of the major limiting factors for plant growth and agricultural productivity. We are using salt cress (Thellungiella halophila) to identify biochemical mechanisms that enable plants to grow in saline conditions. Under salt stress, the major site of Na⁺ accumulation occurred in old leaves, followed by young leaves and taproots, with the least accumulation occurring in lateral roots. Salt treatment increased both the H⁺ transport and hydrolytic activity of salt cress tonoplast (TP) and plasma membrane (PM) H⁺-ATPases from leaves and roots. TP Na⁺/H⁺ exchange was greatly stimulated by growth of the plants in NaCl, both in leaves and roots. Expression of the PM H⁺-ATPase isoform AHA3, the Na⁺ transporter HKT1, and the Na⁺/H⁺ exchanger SOS1 were examined in PMs isolated from control and salt-treated salt cress roots and leaves. An increased expression of SOS1, but no changes in levels of AHA3 and HKT1, was observed. NHX1 was only detected in PM fractions of roots, and a salt-induced increase in protein expression was observed. Analysis of the levels of expression of vacuolar H⁺-translocating ATPase subunits showed no major changes in protein expression of subunits VHA-A or VHA-B with salt treatment; however, VHA-E showed an increased expression in leaf tissue, but not in roots, when the plants were treated with NaCl. Salt cress plants were able to distribute and store Na⁺ by a very strict control of ion movement across both the TP and PM.