Rectification of the Water Permeability in COS-7 Cells at 22, 10 and 0°C

The osmotic and permeability parameters of a cell membrane are essential physico-chemical properties of a cell and particularly important with respect to cell volume changes and the regulation thereof. Here, we report the hydraulic conductivity, Lp, the non-osmotic volume, Vb, and the Arrhenius activation energy, Ea, of mammalian COS-7 cells. The ratio of Vb to the isotonic cell volume, Vc iso, was 0.29. Ea, the activation energy required for the permeation of water through the cell membrane, was 10,700, and 12,000 cal/mol under hyper- and hypotonic conditions, respectively. Average values for Lp were calculated from swell/shrink curves by using an integrated equation for Lp. The curves represented the volume changes of 358 individually measured cells, placed into solutions of nonpermeating solutes of 157 or 602 mOsm/kg (at 0, 10 or 22°C) and imaged over time. Lp estimates for all six combinations of osmolality and temperature were calculated, resulting in values of 0.11, 0.21, and 0.10 µm/min/atm for exosmotic flow and 0.79, 1.73 and 1.87 µm/min/atm for endosmotic flow (at 0, 10 and 22°C, respectively). The unexpected finding of several fold higher Lp values for endosmotic flow indicates highly asymmetric membrane permeability for water in COS-7. This phenomenon is known as rectification and has mainly been reported for plant cell, but only rarely for animal cells. Although the mechanism underlying the strong rectification found in COS-7 cells is yet unknown, it is a phenomenon of biological interest and has important practical consequences, for instance, in the development of optimal cryopreservation

Wheat grain yield on saline soils is improved by an ancestral Na+ transporter gene

The ability of wheat to maintain a low sodium concentration ([Na+]) in leaves correlates with improved growth under saline conditions1,2. This trait, termed Na+ exclusion, contributes to the greater salt tolerance of bread wheat relative to durum wheat3,4. To improve the salt tolerance of durum wheat, we explored natural diversity in shoot Na+ exclusion within ancestral wheat germplasm. Previously, we showed that crossing of Nax2, a gene locus in the wheat relative Triticum monococcum into a commercial durum wheat (Triticum turgidum ssp. durum var. Tamaroi) reduced its leaf [Na+] (ref. 5). Here we show that a gene in the Nax2 locus, TmHKT1;5-A, encodes a Na+-selective transporter located on the plasma membrane of root cells surrounding xylem vessels, which is therefore ideally localized to withdraw Na+ from the xylem and reduce transport of Na+ to leaves. Field trials on saline soils demonstrate that the presence of TmHKT1;5-A significantly reduces leaf [Na+] and increases durum wheat grain yield by 25% compared to near-isogenic lines without the Nax2 locus.Rana Munns, Richard A. James, Bo Xu, Asmini Athman, Simon J. Conn, Charlotte Jordans, Caitlin S. Byrt, Ray A. Hare, Stephen D. Tyerman, Mark Tester, Darren Plett and Matthew Gilliha