A Barley Efflux Transporter Operates in a Na+-Dependent Manner, as Revealed by a Multidisciplinary Platform.

Plant growth and survival depend upon the activity of membrane transporters that control the movement and distribution of solutes into, around, and out of plants. Although many plant transporters are known, their intrinsic properties make them difficult to study. In barley (Hordeum vulgare), the root anion-permeable transporter Bot1 plays a key role in tolerance to high soil boron, facilitating the efflux of borate from cells. However, its three-dimensional structure is unavailable and the molecular basis of its permeation function is unknown. Using an integrative platform of computational, biophysical, and biochemical tools as well as molecular biology, electrophysiology, and bioinformatics, we provide insight into the origin of transport function of Bot1. An atomistic model, supported by atomic force microscopy measurements, reveals that the protein folds into 13 transmembrane-spanning and five cytoplasmic α-helices. We predict a trimeric assembly of Bot1 and the presence of a Na(+) ion binding site, located in the proximity of a pore that conducts anions. Patch-clamp electrophysiology of Bot1 detects Na(+)-dependent polyvalent anion transport in a Nernstian manner with channel-like characteristics. Using alanine scanning, molecular dynamics simulations, and transport measurements, we show that conductance by Bot1 is abolished by removal of the Na(+) ion binding site. Our data enhance the understanding of the permeation functions of Bot1.

Root cell wall solutions for crop plants in saline soils.

The root growth of most crop plants is inhibited by soil salinity. Roots respond by modulating metabolism, gene expression and protein activity, which results in changes in cell wall composition, transport processes, cell size and shape, and root architecture. Here, we focus on the effects of salt stress on cell wall modifying enzymes, cellulose microfibril orientation and non-cellulosic polysaccharide deposition in root elongation zones, as important determinants of inhibition of root elongation, and highlight cell wall changes linked to tolerance to salt stressed and water limited roots. Salt stress induces changes in the wall composition of specific root cell types, including the increased deposition of lignin and suberin in endodermal and exodermal cells. These changes can benefit the plant by preventing water loss and altering ion transport pathways. We suggest that binding of Na+ ions to cell wall components might influence the passage of Na+ and that Na+ can influence the binding of other ions and hinder the function of pectin during cell growth. Naturally occurring differences in cell wall structure may provide new resources for breeding crops that are more salt tolerant.