Salt Stress in Thellungiella halophila Activates Na1 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 Na1 accumulation occurred in old leaves, followed by young leaves and taproots, with the least accumulation occurring in lateral roots. Salt treatment increased both the H1 transport and hydrolytic activity of salt cress tonoplast (TP)and plasma membrane (PM)H1-ATPases fromleaves and roots. TP Na1/H1 exchange was greatly stimulated by growth of the plants in NaCl, both in leaves and roots. Expression of the PM H1-ATPase isoform AHA3, the Na1 transporter HKT1, and the Na1/H1 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 inPMfractions of roots, and a saltinduced increase in protein expression was observed. Analysis of the levels of expression of vacuolar H1-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 Na1 by a very strict control of ion movement across both the TP and PM