Developmental pattern of aquaporin expression in barley (Hordeum vulgare L.) leaves

Aquaporins are multifunctional membrane channels which belong to the family of major intrinsic proteins (MIPs) and are best known for their ability to facilitate the movement of water. In the present study, earlier results from microarray experiments were followed up. These experiments had suggested that, in barley (Hordeum vulgare L.), aquaporin family members are expressed in distinct patterns during leaf development. Real-time PCR and in situ hybridization were used to analyse the level and tissue-distribution of expression of candidate aquaporins, focusing on plasma membrane and tonoplast intrinsic proteins (PIPs, TIPs). Water channel function of seven aquaporins, whose transcripts were the most abundant and the most variable, was tested through expression in yeast and, in part, through expression in oocytes. All PIP1 and PIP2 subfamily members changed in expression during leaf development, with expression being much higher or lower in growing compared with mature tissue. The same applied to those TIPs which were expressed at detectable levels. Specific roles during leaf development are proposed for particular aquaporins

Plant High-Affinity Potassium (HKT) transporters involved in salinity tolerance: structural insights to probe differences in ion selectivity

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.Shane Waters, Matthew Gilliham and Maria Hrmov

Genetic Variation of HvCBF Genes and Their Association with Salinity Tolerance in Tibetan Annual Wild Barley

The evaluation of both the genetic variation and the identification of salinity tolerant accessions of Tibetan annual wild barley (hereafter referred to as Tibetan barley) (Hordeum vulgare L. ssp. Spontaneum and H. vulgare L. ssp. agriocrithum) are essential for discovering and exploiting novel alleles involved in salinity tolerance. In this study, we examined tissue dry biomass and the Na+ and K+ contents of 188 Tibetan barley accessions in response to salt stress. We investigated the genetic variation of transcription factors HvCBF1, HvCBF3 and HvCBF4 within these accessions, conducting association analysis between these three genes and the respective genotypic salt tolerance. Salt stress significantly reduced shoot and root dry weight by 27.6% to 73.1% in the Tibetan barley lines. HvCBF1, HvCBF3 and HvCBF4 showed diverse sequence variation in amplicon as evident by the identification of single nucleotide polymorphisms (SNPs) and 3, 8 and 13 haplotypes, respectively. Furthermore, the decay of Linkage disequilibrium (LD) of chromosome 5 was 8.9 cM (r2<0.1). Marker bpb-4891 and haplotype 13 (Ps 610) of the HvCBF4 gene were significantly (P<0.05) and highly significantly (P<0.001) associated with salt tolerance. However, HvCBF1 and HvCBF3 genes were not associated with salinity tolerance. The accessions from haplotype 13 of the HvCBF4 gene showed high salinity tolerance, maintaining significantly lower Na+/K+ ratios and higher dry weight. It is thus proposed that these Tibetan barley accessions could be of value for enhancing salinity tolerance in cultivated barley

AtHKT1;1 Mediates Nernstian Sodium Channel Transport Properties in Arabidopsis Root Stelar Cells

The Arabidopsis AtHKT1;1 protein was identified as a sodium (Na+) transporter by heterologous expression in Xenopus laevis oocytes and Saccharomyces cerevisiae. However, direct comparative in vivo electrophysiological analyses of a plant HKT transporter in wild-type and hkt loss-of-function mutants has not yet been reported and it has been recently argued that heterologous expression systems may alter properties of plant transporters, including HKT transporters. In this report, we analyze several key functions of AtHKT1;1-mediated ion currents in their native root stelar cells, including Na+ and K+ conductances, AtHKT1;1-mediated outward currents, and shifts in reversal potentials in the presence of defined intracellular and extracellular salt concentrations. Enhancer trap Arabidopsis plants with GFP-labeled root stelar cells were used to investigate AtHKT1;1-dependent ion transport properties using patch clamp electrophysiology in wild-type and athkt1;1 mutant plants. AtHKT1;1-dependent currents were carried by sodium ions and these currents were not observed in athkt1;1 mutant stelar cells. However, K+ currents in wild-type and athkt1;1 root stelar cell protoplasts were indistinguishable correlating with the Na+ over K+ selectivity of AtHKT1;1-mediated transport. Moreover, AtHKT1;1-mediated currents did not show a strong voltage dependence in vivo. Unexpectedly, removal of extracellular Na+ caused a reduction in AtHKT1;1-mediated outward currents in Columbia root stelar cells and Xenopus oocytes, indicating a role for external Na+ in regulation of AtHKT1;1 activity. Shifting the NaCl gradient in root stelar cells showed a Nernstian shift in the reversal potential providing biophysical evidence for the model that AtHKT1;1 mediates passive Na+ channel transport properties

Adaptation des plantes au stress salin: caractérisation de transporteurs de sodium et de potassium de la famille HKT chez le riz

Le transport et la compartimentation de K+ et Na+ sont des fonctions essentielles chez tous les organismes vivants, animaux et végétaux, même si les mécanismes mis en place pour les réaliser sont différents dans les 2 règnes. Chez les plantes, au niveau agronomique, ces fonctions conditionnent par exemple la capacité de croissance sur un sol pauvre en K+ ou la tolérance à un sol salé. Dans l'objectif de mieux comprendre les mécanismes responsables du transport de K+ et Na+ chez les plantes, nous nous sommes intéressés à la famille de transporteurs HKT en choisissant le riz comme plante modèle. Alors que la famille HKT compte 1 seul membre (perméable à Na+ uniquement et impliqué dans le transport phloémien de cet ion) chez Arabidopsis, elle en compte 7 à 9 chez le riz, selon le cultivar. Notre stratégie associe des approches (1) de biologie cellulaire (hybridation in situ) pour préciser les patrons d'expression in planta de ces transporteurs, (2) d'électrophysiologie (voltage-clamp après expression en ovocyte de xénope) pour analyser leurs caractéristiques fonctionnelles, et (3) de génétique inverse (caractérisation phénotypique de lignées mutantes perte de fonction) pour étudier leurs rôles dans la plante entière. L'étude a concerné plus spécifiquement 3 transporteurs, OsHKT1, OsHKT4 et OsHKT6. Les patrons d'expression de ces 3 systèmes sont relativement larges, intégrant les tissus conducteurs, et se chevauchent souvent, par exemple dans les tissus périphériques de la racine ou les cellules bulliformes de l'épiderme foliaire. Au niveau fonctionnel, les 3 systèmes se distinguent en termes de perméabilité (OsHKT1 est perméable à la fois à K+ et Na+ alors que les 2 autres transporteurs ne le sont qu'à Na+), d'affinité pour Na+ et de rectification. Nous montrons qu'OsHKT1 présente la caractéristique de pouvoir fonctionner comme un symport Na+?K+ dans le domaine des faibles concentrations. L'analyse de 2 lignées mutantes oshkt1 perte de fonction suggère que cette capacité de symport s'exprime également in planta, dans la racine en présence de milieux dilués. Ces données fournissent les premiers indices de l'intervention de Na+ dans une activité de symport à haute affinité de K+ chez les plante