Response: high-affinity potassium uptake in plants

Gene expression analysis of rice seedling under potassium deprivation reveals major changes in metabolism and signaling components

Plant nutrition is one of the important areas for improving the yield and quality in crops as well as non-crop plants. Potassium is an essential plant nutrient and is required in abundance for their proper growth and development. Potassium deficiency directly affects the plant growth and hence crop yield and production. Recently, potassium-dependent transcriptomic analysis has been performed in the model plant Arabidopsis, however in cereals and crop plants; such a transcriptome analysis has not been undertaken till date. In rice, the molecular mechanism for the regulation of potassium starvation responses has not been investigated in detail. Here, we present a combined physiological and whole genome transcriptomic study of rice seedlings exposed to a brief period of potassium deficiency then replenished with potassium. Our results reveal that the expressions of a diverse set of genes annotated with many distinct functions were altered under potassium deprivation. Our findings highlight altered expression patterns of potassium-responsive genes majorly involved in metabolic processes, stress responses, signaling pathways, transcriptional regulation, and transport of multiple molecules including K⁺. Interestingly, several genes responsive to low-potassium conditions show a reversal in expression upon resupply of potassium. The results of this study indicate that potassium deprivation leads to activation of multiple genes and gene networks, which may be acting in concert to sense the external potassium and mediate uptake, distribution and ultimately adaptation to low potassium conditions. The interplay of both upregulated and downregulated genes globally in response to potassium deprivation determines how plants cope with the stress of nutrient deficiency at different physiological as well as developmental stages of plants.

Identification of SNP mutations in DREB1, HKT1, and WRKY1 genes involved in drought and salt stress tolerance in durum wheat (Triticum turgidum L. var durum)

Tolerance mechanisms to salinity and drought stress are quite complex. Plants have developed a complex and elaborate signaling network that ensures their adaptation to this stress. For example, salinity tolerance is thought to be due to three main factors: Na(+) exclusion, tolerance to Na(+) in the tissues and osmotic tolerance. Recently, many transcription factors for tolerance to salt and drought stresses have been identified. In this study, multialignments of conserved domains in DREB1, WRKY1 transcription factors (TFs), and HKT-1 have been utilized to design specific primers in order to identify functional single nucleotide polymorphisms (SNPs). These primers have been used to probe on several genotypes of durum wheat that are differentially tolerant to salt and drought stress; they were grown in increasing concentrations of NaCl. The selected portions have been analyzed using high-resolution melting curve (HRM) technology that currently represents one of the most recent and powerful tools for detecting SNP and INDEL mutations. Analyzing the amplification profiles, observed in the resulting melting curves, samples corresponding to different treatment conditions were selected, sequenced, and aligned with the homolog sequences present in gene databases to identify and characterize potential SNP and INDEL mutations. The PCR amplicons, containing single and double SNPs, produced distinctive HRM profiles. By sequencing the polymerase chain reaction (PCR) products, several SNPs have been identified and validated. All the discovered mutations were able to generate changes in amino acid sequences of the corresponding proteins. Most of the identified SNPs were found in salt and drought tolerant durum wheat genotypes. These varieties are of great value for durum wheat breeding works.

The effect of a genetically reduced plasma membrane protonmotive force on vegetative growth of Arabidopsis

The plasma membrane proton gradient is an essential feature of plant cells. In Arabidopsis (Arabidopsis thaliana), this gradient is generated by the plasma membrane proton pump encoded by a family of 11 genes (abbreviated as AHA, for Arabidopsis H(+)-ATPase), of which AHA1 and AHA2 are the two most predominantly expressed in seedlings and adult plants. Although double knockdown mutant plants containing T-DNA insertions in both genes are embryonic lethal, under ideal laboratory growth conditions, single knockdown mutant plants with a 50% reduction in proton pump concentration complete their life cycle without any observable growth alteration. However, when grown under conditions that induce stress on the plasma membrane protonmotive force (PMF), such as high external potassium to reduce the electrical gradient or high external pH to reduce the proton chemical gradient, aha2 mutant plants show a growth retardation compared with wild-type plants. In this report, we describe the results of studies that examine in greater detail AHA2's specific role in maintaining the PMF during seedling growth. By comparing the wild type and aha2 mutants, we have measured the effects of a reduced PMF on root and hypocotyl growth, ATP-induced skewed root growth, and rapid cytoplasmic calcium spiking. In addition, genome-wide gene expression profiling revealed the up-regulation of potassium transporters in aha2 mutants, indicating, as predicted, a close link between the PMF and potassium uptake at the plasma membrane. Overall, this characterization of aha2 mutants provides an experimental and theoretical framework for investigating growth and signaling processes that are mediated by PMF-coupled energetics at the cell membrane.

Sodium transport in plants: a critical review

Sodium (Na) toxicity is one of the most formidable challenges for crop production world-wide. Nevertheless, despite decades of intensive research, the pathways of Na(+) entry into the roots of plants under high salinity are still not definitively known. Here, we review critically the current paradigms in this field. In particular, we explore the evidence supporting the role of nonselective cation channels, potassium transporters, and transporters from the HKT family in primary sodium influx into plant roots, and their possible roles elsewhere. We furthermore discuss the evidence for the roles of transporters from the NHX and SOS families in intracellular Na(+) partitioning and removal from the cytosol of root cells. We also review the literature on the physiology of Na(+) fluxes and cytosolic Na(+) concentrations in roots and invite critical interpretation of seminal published data in these areas. The main focus of the review is Na(+) transport in glycophytes, but reference is made to literature on halophytes where it is essential to the analysis.

Insights into the salt tolerance mechanism in barley (Hordeum vulgare) from comparisons of cultivars that differ in salt sensitivity

Although barley (Hordeum vulgare L.) is a salt-tolerant crop, the underlying physiological and molecular mechanisms of salt tolerance remain to be elucidated. Therefore, we investigated the response of salt-tolerant (K305) and salt-sensitive (I743) cultivars to salt stress at both physiological and molecular levels. Salt treatment increased xylem sap osmolarity, which was attributed primarily to a rise in Na(+) and Cl(-) concentration; enhanced accumulation of the ions in shoots; and reduced plant growth more severely in I743 than K305. The concentration of K(+) in roots and shoots decreased during 8 h of salt treatment in both cultivars but with no marked difference between cultivars. Hence, the severe growth reduction in I743 is attributed to the elevated levels of (mainly) Na(+) in shoots. Analysis of gene expression using quantitative RT-PCR showed that transcripts of K(+)-transporters (HvHAK1 and HvAKT1), vacuolar H(+)-ATPase and inorganic pyrophosphatase (HvHVA/68 and HvHVP1) were more abundant in shoots of K305 than in shoots of I743. Expression of HvHAK1 and Na(+)/H(+) antiporters (HvNHX1, HvNHX3 and HvNHX4) was higher in roots of K305 than in I743 with prolonged exposure to salt. Taken together, these results suggest that the better performance of K305 compared to I743 during salt stress may be related to its greater ability to sequester Na(+) into sub-cellular compartments and/or maintain K(+) homeostasis.

HKT1 mediates sodium uniport in roots. Pitfalls in the expression of HKT1 in yeast

The function of HKT1 in roots is controversial. We tackled this controversy by studying Na+ uptake in barley (Hordeum vulgare) roots, cloning the HvHKT1 gene, and expressing the HvHKT1 cDNA in yeast (Saccharomyces cerevisiae) cells. High-affinity Na+ uptake was not detected in plants growing at high K+ but appeared soon after exposing the plants to a K(+)-free medium. It was a uniport, insensitive to external K+ at the beginning of K+ starvation and inhibitable by K+ several hours later. The expression of HvHKT1 in yeast was Na+ (or K+) uniport, Na(+)-K+ symport, or a mix of both, depending on the construct from which the transporter was expressed. The Na+ uniport function was insensitive to external K+ and mimicked the Na+ uptake carried out by the roots at the beginning of K+ starvation. The K+ uniport function only took place in yeast cells that were completely K+ starved and disappeared when internal K+ increased, which makes it unlikely that HvHKT1 mediates K+ uptake in roots. Mutation of the first in-frame AUG codon of HvHKT1 to CUC changed the uniport function into symport. The expression of the symport from either mutants or constructs keeping the first in-frame AUG took place only in K(+)-starved cells, while the uniport was expressed in all conditions. We discuss here that the symport occurs only in heterologous expression. It is most likely related to the K+ inhibitable Na+ uptake process of roots that heterologous systems fail to reproduce.

The potassium transporter AtHAK5 functions in K(+) deprivation-induced high-affinity K(+) uptake and AKT1 K(+) channel contribution to K(+) uptake kinetics in Arabidopsis roots

Potassium is an important macronutrient and the most abundant cation in plants. Because soil mineral conditions can vary, plants must be able to adjust to different nutrient availabilities. Here, we used Affymetrix Genechip microarrays to identify genes responsive to potassium (K(+)) deprivation in roots of mature Arabidopsis (Arabidopsis thaliana) plants. Unexpectedly, only a few genes were changed in their expression level after 6, 48, and 96 h of K(+) starvation even though root K(+) content was reduced by approximately 60%. AtHAK5, a potassium transporter gene from the KUP/HAK/KT family, was most consistently and strongly up-regulated in its expression level across 48-h, 96-h, and 7-d K(+) deprivation experiments. AtHAK5 promoter-beta-glucuronidase and -green fluorescent protein fusions showed AtHAK5 promoter activity in the epidermis and vasculature of K(+) deprived roots. Rb(+) uptake kinetics in roots of athak5 T-DNA insertion mutants and wild-type plants demonstrated the absence of a major part of an inducible high-affinity Rb(+)/K(+) (K(m) approximately 15-24 microm) transport system in athak5 plants. In comparative analyses, uptake kinetics of the K(+) channel mutant akt1-1 showed that akt1-1 roots are mainly impaired in a major transport mechanism, with an apparent affinity of approximately 0.9 mm K(+)(Rb(+)). Data show adaptation of apparent K(+) affinities of Arabidopsis roots when individual K(+) transporter genes are disrupted. In addition, the limited transcriptome-wide response to K(+) starvation indicates that posttranscriptional mechanisms may play important roles in root adaptation to K(+) availability in Arabidopsis. The results demonstrate an in vivo function for AtHAK5 in the inducible high-affinity K(+) uptake system in Arabidopsis roots.

Sodium transport and HKT transporters: the rice model

Na+ uptake in the roots of K+-starved seedlings of barley, rice, and wheat was found to exhibit fast rate, low Km, and high sensitivity to K+. Sunflower plants responded in a similar manner but the uptake was not K+ sensitive. Ba2+ inhibited Na+ uptake, but not K+ uptake in rice roots. This demonstrated that Na+ and K+ uptake are mediated by different transporters, and that K+ blocked but was not transported by the Na+ transporter. The genome of rice cv. Nipponbare contains seven HKT genes, which may encode Na+ transporters, plus two HKT pseudogenes. Yeast expressions of OsHKT1 and OsHKT4 proved that they are Na+ transporters of high and low affinity, respectively, which are sensitive to K+ and Ba2+. Parallel experiments of K+ and Na+ uptake in yeast expressing the wheat or rice HKT1 transporters proved that they were very different; TaHKT1 transported K+ and Na+, and OsHKT1 only Na+. Transcript expressions in shoots of the OsHKT genes were fairly constant and insensitive to changes in the K+ and Na+ concentrations of the nutrient solution. In roots, the expressions were much lower than in shoots, except for OsHKT4 and OsHKT1 in K+-starved plants. We propose that OsHKT transporters are involved in Na+ movements in rice, and that OsHKT1 specifically mediates Na+ uptake in rice roots when the plants are K+ deficient. The incidence of HKT ESTs in several plant species suggests that the rice model with many HKT genes applies to other plants.

High-affinity potassium transport in barley roots. Ammonium-sensitive and -insensitive pathways

In an attempt to understand the process mediating K(+) transport into roots, we examined the contribution of the NH(4)(+)-sensitive and NH(4)(+)-insensitive components of Rb(+) transport to the uptake of Rb(+) in barley (Hordeum vulgare L.) plants grown in different ionic environments. We found that at low external Rb(+) concentrations, an NH(4)(+)-sensitive component dominates Rb(+) uptake in plants grown in the absence of NH(4)(+), while Rb(+) uptake preferentially occurs through an NH(4)(+)-insensitive pathway in plants grown at high external NH(4)(+) concentrations. A comparison of the Rb(+)-uptake properties observed in roots with those found in heterologous studies with yeast cells indicated that the recently cloned HvHAK1 K(+) transporter may provide a major route for the NH(4)(+)-sensitive component. HvHAK1 failed to complement the growth of a yeast strain defective in NH(4)(+) transport, suggesting that it could not act as an NH(4)(+) transporter. Heterologous studies also showed that the HKT1 K(+)/Na(+)-cotransporter may act as a pathway for high-affinity Rb(+) transport sensitive to NH(4)(+). However, we found no evidence of an enhancement of Rb(+) uptake into roots due to Na(+) addition. The possible identity of the systems contributing to the NH(4)(+)-insensitive component in barley plants is discussed.

The Arabidopsis HKT1 gene homolog mediates inward Na(+) currents in xenopus laevis oocytes and Na(+) uptake in Saccharomyces cerevisiae

The Na(+)-K(+) co-transporter HKT1, first isolated from wheat, mediates high-affinity K(+) uptake. The function of HKT1 in plants, however, remains to be elucidated, and the isolation of HKT1 homologs from Arabidopsis would further studies of the roles of HKT1 genes in plants. We report here the isolation of a cDNA homologous to HKT1 from Arabidopsis (AtHKT1) and the characterization of its mode of ion transport in heterologous systems. The deduced amino acid sequence of AtHKT1 is 41% identical to that of HKT1, and the hydropathy profiles are very similar. AtHKT1 is expressed in roots and, to a lesser extent, in other tissues. Interestingly, we found that the ion transport properties of AtHKT1 are significantly different from the wheat counterpart. As detected by electrophysiological measurements, AtHKT1 functioned as a selective Na(+) uptake transporter in Xenopus laevis oocytes, and the presence of external K(+) did not affect the AtHKT1-mediated ion conductance (unlike that of HKT1). When expressed in Saccharomyces cerevisiae, AtHKT1 inhibited growth of the yeast in a medium containing high levels of Na(+), which correlates to the large inward Na(+) currents found in the oocytes. Furthermore, in contrast to HKT1, AtHKT1 did not complement the growth of yeast cells deficient in K(+) uptake when cultured in K(+)-limiting medium. However, expression of AtHKT1 did rescue Escherichia coli mutants carrying deletions in K(+) transporters. The rescue was associated with a less than 2-fold stimulation of K(+) uptake into K(+)-depleted cells. These data demonstrate that AtHKT1 differs in its transport properties from the wheat HKT1, and that AtHKT1 can mediate Na(+) and, to a small degree, K(+) transport in heterologous expression systems.

Potassium uptake supporting plant growth in the absence of AKT1 channel activity: Inhibition by ammonium and stimulation by sodium

A transferred-DNA insertion mutant of Arabidopsis that lacks AKT1 inward-rectifying K+ channel activity in root cells was obtained previously by a reverse-genetic strategy, enabling a dissection of the K+-uptake apparatus of the root into AKT1 and non-AKT1 components. Membrane potential measurements in root cells demonstrated that the AKT1 component of the wild-type K+ permeability was between 55 and 63% when external [K+] was between 10 and 1,000 microM, and NH4+ was absent. NH4+ specifically inhibited the non-AKT1 component, apparently by competing for K+ binding sites on the transporter(s). This inhibition by NH4+ had significant consequences for akt1 plants: K+ permeability, 86Rb+ fluxes into roots, seed germination, and seedling growth rate of the mutant were each similarly inhibited by NH4+. Wild-type plants were much more resistant to NH4+. Thus, AKT1 channels conduct the K+ influx necessary for the growth of Arabidopsis embryos and seedlings in conditions that block the non-AKT1 mechanism. In contrast to the effects of NH4+, Na+ and H+ significantly stimulated the non-AKT1 portion of the K+ permeability. Stimulation of akt1 growth rate by Na+, a predicted consequence of the previous result, was observed when external [K+] was 10 microM. Collectively, these results indicate that the AKT1 channel is an important component of the K+ uptake apparatus supporting growth, even in the "high-affinity" range of K+ concentrations. In the absence of AKT1 channel activity, an NH4+-sensitive, Na+/H+-stimulated mechanism can suffice.

Rapid Up-regulation of HKT1, a high-affinity potassium transporter gene, in roots of barley and wheat following withdrawal of potassium

High-affinity K+ uptake in plant roots is rapidly up-regulated when K+ is withheld and down-regulated when K+ is resupplied. These processes make important contributions to plant K+ homeostasis. A cDNA coding for a high-affinity K+ transporter, HKT1, was earlier cloned from wheat (Triticum aestivum L.) roots and functionally characterized. We demonstrate here that in both barley (Hordeum vulgare L.) and wheat roots, a rapid and large up-regulation of HKT1 mRNA levels resulted when K+ was withdrawn from growth media. This effect was specific for K+; withholding N caused a modest reduction of HKT1 mRNA levels. Up-regulation of HKT1 transcript levels in barley roots occurred within 4 h of removing K+, which corresponds to the documented increase of high-affinity K+ uptake in roots following removal of K+. Increased expression of HKT1 mRNA was evident before a decline in total root K+ concentration could be detected. Resupply of 1 mM K+ was sufficient to strongly reduce HKT1 transcript levels. In wheat root cortical cells, both membrane depolarizations in response to 100 &mgr;M K+, Cs+, and Rb+, and high-affinity K+ uptake were enhanced by K+ deprivation. Thus, in both plant systems the observed physiological changes associated with manipulating external K+ supply were correlated with levels of HKT1 mRNA expression. Implications of these findings for K+ sensing and regulation of the HKT1 mRNA levels in plant roots are discussed.