A Grain of Salt

Abstract

We quantified grain sodium (Na + ) content across a barley GWAS panel grown under optimal conditions. We identified a strong association with a region containing two low and one high Na + accumulating haplotypes of a Class 1 HIGH-AFFINITY POTASSIUM TRANSPORTER (HKT1;5) known to be involved in regulating plant Na + homeostasis. The haplotypes exhibited an average 1.8-fold difference in grain Na + content. We show that an L189P substitution disrupts Na + transport in the high Na + lines, disturbs the plasma membrane localisation typical of HKT1;5 and induces a conformational change in the protein predicted to compromise function. Under NaCl stress, lines containing P189 accumulate high levels of Na + , but show no significant difference in biomass. P189 increases in frequency from wildspecies to elite cultivars leading us to speculate that the compromised haplotype is undergoing directional selection possibly due to the value of Na + as a functional nutrient in non-saline environments.

Dissecting new genetic components of salinity tolerance in two-row spring barley at the vegetative and reproductive stages

“…Using the classical model, we observed a significant association on chromosome 4H (112.65, 114.45 cM) for sodium and potassium contents (ionic phase) as measured by flame photometry in leaf samples collected from plants under saline conditions (S5 Table). The same locus was detected by Hazzouri et al [12] and more recently by Houston et al [13], the latter identifying HKT1;5 as the gene underlying the association. This observation verifies that the genetic material and stress level applied at TPA were appropriate for the purpose of the experiment.…”

“…In terms of the ionic component of salinity tolerance, Hazzouri et al [12] performed an association mapping study, where a USDA mini-core barley collection was screened for salinity tolerance under field conditions. A locus associated with leaf sodium content was identified on chromosome 4HL; the suggested candidate gene was HKT1;5, a high-affinity potassium transporter [12][13][14]. The increased salinity tolerance may be achieved by HKT1;5 retrieving Na + from the xylem sap at the root level, which would prevent Na + from accumulating in the shoot [12,[15][16][17].…”

A single residue deletion in the barley HKT1;5 P189 variant restores plasma membrane localisation but not Na+ conductance

“…High soil sodium chloride or salinity (elevated Na + and Clconcentrations) interfere with cell expansion, photosynthetic activity, and can cause leaf senescence and reductions in reproductive capacity when accumulated in cells [4][5][6][7]. Na + is J o u r n a l P r e -p r o o f considered to be a non-essential element for the growth and development of most plants, although some plants benefit from high intracellular Na + concentrations when the essential K + level in the soil is low [8].…”

“…Natural sequence variations of transport proteins at single or multiple residues are useful tools for examining the functional relationships bestowed by specific residues, where a transporter can lose function, change selectivity, and have modified transport rates or other properties [8,19,23,24]. However, it is impossible to prove the exact molecular mechanism that underlies functional changes of transporters by examining a sequence alone, unless conducting multidisciplinary analyses.…”

“…However, it is impossible to prove the exact molecular mechanism that underlies functional changes of transporters by examining a sequence alone, unless conducting multidisciplinary analyses. Examples of such approaches include research on HKT1;5 genes and corresponding gene products in wheat [24][25][26] and barley (Hordeum vulgare, Hv) [8]. In the barley study [8], grain Na + across a genome-wide association panel was quantified and variants of a class 1 HKT1;5 responsible for Na + content variation under non-saline conditions were identified.…”