Plant High-Affinity Potassium (HKT) Transporters Involved in Salinity Tolerance: Structural Insights to Probe Differences in Ion Selectivity

Abstract

High-affinity Potassium Transporters (HKTs) belong to an important class of integral membrane proteins (IMPs) that facilitate cation transport across the plasma membranes of plant cells. Some members of the HKT protein family have been shown to be critical for salinity tolerance in commercially important crop species, particularly in grains, through exclusion of Na+ ions from sensitive shoot tissues in plants. However, given the number of different HKT proteins expressed in plants, it is likely that different members of this protein family perform in a range of functions. Plant breeders and biotechnologists have attempted to manipulate HKT gene expression through genetic engineering and more conventional plant breeding methods to improve the salinity tolerance of commercially important crop plants. Successful manipulation of a biological trait is more likely to be effective after a thorough understanding of how the trait, genes and proteins are interconnected at the whole plant level. This article examines the current structural and functional knowledge relating to plant HKTs and how their structural features may explain their transport selectivity. We also highlight specific areas where new knowledge of plant HKT transporters is needed. Our goal is to present how knowledge of the structure of HKT proteins is helpful in understanding their function and how this understanding can be an invaluable experimental tool. As such, we assert that accurate structural information of plant IMPs will greatly inform functional studies and will lead to a deeper understanding of plant nutrition, signalling and stress tolerance, all of which represent factors that can be manipulated to improve agricultural productivity.

Amelioration of Soybean Plant from Saline-Induced Condition by Exopolysaccharide Producing <i>Pseudomonas</i>-Mediated Expression of High Affinity K⁺-transporter (HKT1) Gene

“…This is the main gene regulating Na + and K + levels under salinity stress. It has been reported that the HKT1 transporter belongs to a class of integral proteins (inositol monophosphate) that facilitate cation transport across the plasma membrane of plant cells 34 . Further, HKT1 was segregated into two subgroups based on their transport selectivity: group 1 was described as Na + uniporters, while group 2 allowed both Na + and K + symport transport under specific conditions, and Na + uniport transport at high Na + concentration 35 .…”

Comparative expression of candidate genes involved in sodium transport and compartmentation in citrus

“…HKTs are segregated into two subgroups based on their transport selectivity. Group 1 are described as Na + uniporters, while group 2 are thought to allow Na + and K + transport as well as Na + uniport at high Na + concentrations (see review by Waters et al, 2013). HKT genes from dicot species fall within the first major subfamily (HKT1), while graminaceous species often present HKT genes in both (HKT1, HKT2) families (Platten et al, 2006).…”

Identification of a Stelar-Localized Transport Protein That Facilitates Root-to-Shoot Transfer of Chloride in Arabidopsis

“…Significant progress has been made in characterizing the genes encoding proteins that are central to regulating shoot Na + accumulation during salt stress (Roy et al, 2014;Munns and Gilliham, 2015). In particular, the functions of the high affinity K + transporter HKT gene family (Uozumi et al, 2000;Mäser et al, 2002;Berthomieu et al, 2003;Davenport et al, 2007;Waters et al, 2013;Byrt et al, 2014) and the Na + /H + antiporter and salt overly sensitive (SOS) gene families (Wu et al, 1996;Shi et al, 2000Shi et al, , 2002Qiu et al, 2002;Bassil and Blumwald, 2014) have been areas of intensive research. Manipulation of these pathways has led to improvements in the salt tolerance of the model plant Arabidopsis (Arabidopsis thaliana; Sunarpi et al, 2005;Møller et al, 2009) and crops (Ren et al, 2005;James et al, 2006;Byrt et al, 2007;Qiu et al, 2011;Munns et al, 2012).…”