Cloning and characterizations of plant K+ transport systems aside from Arabidopsis have been increasing over the past decade, favored by the availability of more and more plant genome sequences. ...Information now available enables the comparison of some of these systems between species. In this review, we focus on three families of plant K+ transport systems that are active at the plasma membrane: the Shaker K+ channel family, comprised of voltage-gated channels that dominate the plasma membrane conductance to K+ in most environmental conditions, and two families of transporters, the HAK/KUP/KT K+ transporter family, which includes some high-affinity transporters, and the HKT K+ and/or Na+ transporter family, in which K+-permeable members seem to be present in monocots only. The three families are briefly described, giving insights into the structure of their members and on functional properties and their roles in Arabidopsis or rice. The structure of the three families is then compared between plant species through phylogenic analyses. Within clusters of ortologues/paralogues, similarities and differences in terms of expression pattern, functional properties and, when known, regulatory interacting partners, are highlighted. The question of the physiological significance of highlighted differences is also addressed.
Plant outward-rectifying K⁺ channels mediate K⁺ efflux from guard cells during stomatal closure and from root cells into the xylem for root-shoot allocation of potassium (K). Intriguingly, the gating ...of these channels depends on the extracellular K⁺ concentration, although the ions carrying the current are derived from inside the cell. This K⁺ dependence confers a sensitivity to the extracellular K⁺ concentration (K⁺) that ensures that the channels mediate K⁺ efflux only, regardless of the K⁺ prevailing outside. We investigated the mechanism of K⁺-dependent gating of the K⁺ channel SKOR of Arabidopsis by site-directed mutagenesis. Mutations affecting the intrinsic K⁺ dependence of gating were found to cluster in the pore and within the sixth transmembrane helix (S6), identifying an 'S6 gating domain' deep within the membrane. Mapping the SKOR sequence to the crystal structure of the voltage-dependent K⁺ channel KvAP from Aeropyrum pernix suggested interaction between the S6 gating domain and the base of the pore helix, a prediction supported by mutations at this site. These results offer a unique insight into the molecular basis for a physiologically important K⁺-sensory process in plants.
Potassium is a major inorganic constituent of the living cell and the most abundant cation in the cytosol. It plays a role in various functions at the cell level, such as electrical neutralization of ...anionic charges, protein synthesis, long- and short-term control of membrane polarization, and regulation of the osmotic potential. Through the latter function, K+ is involved at the whole-plant level in osmotically driven functions such as cell movements, regulation of stomatal aperture, or phloem transport. Thus, plant growth and development require that large amounts of K+ are taken up from the soil and translocated to the various organs. In most ecosystems, however, soil K+ availability is low and fluctuating, so plants have developed strategies to take up K+ more efficiently and preserve vital functions and growth when K+ availability is becoming limited. These strategies include increased capacity for high-affinity K+ uptake from the soil, K+ redistribution between the cytosolic and vacuolar pools, ensuring cytosolic homeostasis, and modification of root system development and architecture. Our knowledge about the mechanisms and signalling cascades involved in these different adaptive responses has been rapidly growing during the last decade, revealing a highly complex network of interacting processes. This review is focused on the different physiological responses induced by K+ deprivation, their underlying molecular events, and the present knowledge and hypotheses regarding the mechanisms responsible for K+ sensing and signalling.
Building a proton gradient across a biological membrane and between different tissues is amatter of great importance for plant development and nutrition. To gain a better understanding of proton ...distribution in the plant root apoplast as well as across the plasma membrane, we generated Arabidopsis plants expressing stable membrane-anchored ratiometric fluorescent sensors based on pHluorin. These sensors enabled noninvasive pH-specific measurements in mature root cells from the medium–epidermis interface up to the inner cell layers that lie beyond the Casparian strip. The membrane-associated apoplastic pH was much more alkaline than the overall apoplastic space pH. Proton concentration associated with the plasma membrane was very stable, even when the growth medium pH was altered. This is in apparent contradiction with the direct connection between root intercellular space and the external medium. The plasma membrane-associated pH in the stele was the most preserved and displayed the lowest apoplastic pH (6.0 to 6.1) and the highest transmembrane delta pH (1.5 to 2.2). Both pH values also correlated well with optimal activities of channels and transporters involved in ion uptake and redistribution from the root to the aerial part. In growth medium where ionic content is minimized, the root plasma membrane-associated pH was more affected by environmental proton changes, especially for the most external cell layers. Calcium concentration appears to play a major role in apoplastic pH under these restrictive conditions, supporting a role for the cell wall in pH homeostasis of the unstirred surface layer of plasma membrane in mature roots.
In vivo analyses have identified different functional types of ion channels in various plant tissues and cells. The
Arabidopsis genome contains ∼70 genes for ion channels, of which 57 might be ...cation-selective channels (K
+, Ca
2+ or poorly discriminating channels). Here, we describe the different families of (putative) cation channels: the Shakers, the two-P-domain and Kir K
+ channels (encoded by the
KCO genes), the cyclic-nucleotide-gated channels, the glutamate receptors, and the Ca
2+ channel TPC1. We also compare molecular data with the data obtained
in planta, which should lead to a better understanding of the identity of these channels and provide clues about their roles in plant nutrition and cell signalling.
The various cation channel activities characterized in situ in the plasma membrane of Arabidopsis in light of the different ion channel gene families identified in the genome of this species.
The pH homeostasis of endomembranes is essential for cellular functions. In order to provide direct pH measurements in the endomembrane system lumen, we targeted genetically encoded ratiometric pH ...sensors to the cytosol, the endoplasmic reticulum, and the trans-Golgi, or the compartments labeled by the vacuolar sorting receptor (VSR), which includes the trans-Golgi network and prevacuoles. Using noninvasive live-cell imaging to measure pH, we show that a gradual acidification from the endoplasmic reticulum to the lytic vacuole exists, in both tobacco (Nicotiana tabacum) epidermal (ΔpH -1.5) and Arabidopsis thaliana root cells (ΔpH -2.1). The average pH in VSR compartments was intermediate between that of the trans-Golgi and the vacuole. Combining pH measurements with in vivo colocalization experiments, we found that the trans-Golgi network had an acidic pH of 6.1, while the prevacuole and late prevacuole were both more alkaline, with pH of 6.6 and 7.1, respectively. We also showed that endosomal pH, and subsequently vacuolar trafficking of soluble proteins, requires both vacuolar-type H + ATPase—dependent acidification as well as proton efflux mediated at least by the activity of endosomal sodium/proton NHX-type antiporters.
Potassium (K
+) is the most abundant cation in the cytosol, and plant growth requires that large amounts of K
+ are transported from the soil to the growing organs. K
+ uptake and fluxes within the ...plant are mediated by several families of transporters and channels. Here, we describe the different families of K
+-selective channels that have been identified in plants, the so-called Shaker, TPK and Kir-like channels, and what is known so far on their regulations and physiological functions in the plant.
Soil salinity is an increasing menace that affects agriculture across the globe. Plant adaptation to high salt concentrations involves integrated functions, including control of Na+ uptake, ...translocation and compartmentalization. Na+ transporters belonging to the HKT family have been shown to be involved in tolerance to mild salt stress in glycophytes such as Arabidopsis, wheat and rice by contributing to Na+ exclusion from aerial tissues. Here, we have analysed the role of the HKT transporter HKT2;1, which is permeable to K+ and Na+, in barley, a relatively salt‐tolerant crop that displays a salt‐including behaviour. In Xenopus oocytes, HvHKT2;1 co‐transports Na+ and K+ over a large range of concentrations, displaying low affinity for Na+, variable affinity for K+ depending on external Na+ concentration, and inhibition by K+ (Ki approximately 5 mm). HvHKT2;1 is predominantly expressed in the root cortex. Transcript levels are up‐regulated in both roots and shoots by low K+ growth conditions, and in shoots by high Na+ growth conditions. Over‐expression of HvHKT2;1 led to enhanced Na+ uptake, higher Na+ concentrations in the xylem sap, and enhanced translocation of Na+ to leaves when plants were grown in the presence of 50 or 100 mm NaCl. Interestingly, these responses were correlated with increased barley salt tolerance. This suggests that one of the factors that limits barley salt tolerance is the capacity to translocate Na+ to the shoot rather than accumulation or compartmentalization of this cation in leaf tissues. Thus, over‐expression of HvHKT2;1 leads to increased salt tolerance by reinforcing the salt‐including behaviour of barley.
Plant growth under low K⁺ availability or salt stress requires tight control of K⁺ and Na⁺ uptake, long-distance transport, and accumulation. The family of membrane transporters named HKT (for ...High-Affinity K⁺ Transporters), permeable either to K⁺ and Na⁺ or to Na⁺ only, is thought to play major roles in these functions. Whereas Arabidopsis (Arabidopsis thaliana) possesses a single HKT transporter, involved in Na⁺ transport in vascular tissues, a larger number of HKT transporters are present in rice (Oryza sativa) as well as in other monocots. Here, we report on the expression patterns and functional properties of three rice HKT transporters, OsHKT1;1, OsHKT1;3, and OsHKT2;1. In situ hybridization experiments revealed overlapping but distinctive and complex expression patterns, wider than expected for such a transporter type, including vascular tissues and root periphery but also new locations, such as osmocontractile leaf bulliform cells (involved in leaf folding). Functional analyses in Xenopus laevis oocytes revealed striking diversity. OsHKT1;1 and OsHKT1;3, shown to be permeable to Na⁺ only, are strongly different in terms of affinity for this cation and direction of transport (inward only or reversible). OsHKT2;1 displays diverse permeation modes, Na⁺-K⁺ symport, Na⁺ uniport, or inhibited states, depending on external Na⁺ and K⁺ concentrations within the physiological concentration range. The whole set of data indicates that HKT transporters fulfill distinctive roles at the whole plant level in rice, each system playing diverse roles in different cell types. Such a large diversity within the HKT transporter family might be central to the regulation of K⁺ and Na⁺ accumulation in monocots.
Large-scale identification of genes expressed in roots of the model plant Arabidopsis was performed by serial analysis of gene expression (SAGE), on a total of 144,083 sequenced tags, representing at ...least 15,964 different mRNAs. For tag to gene assignment, we developed a computational approach based on 26,620 genes annotated from the complete sequence of the genome. The procedure selected warrants the identification of the genes corresponding to the majority of the tags found experimentally, with a high level of reliability, and provides a reference database for SAGE studies in Arabidopsis. This new resource allowed us to characterize the expression of more than 3,000 genes, for which there is no expressed sequence tag (EST) or cDNA in the databases. Moreover, 85% of the tags were specific for one gene. To illustrate this advantage of SAGE for functional genomics, we show that our data allow an unambiguous analysis of most of the individual genes belonging to 12 different ion transporter multigene families. These results indicate that, compared with EST-based tag to gene assignment, the use of the annotated genome sequence greatly improves gene identification in SAGE studies. However, more than 6,000 different tags remained with no gene match, suggesting that a significant proportion of transcripts present in the roots originate from yet unknown or wrongly annotated genes. The root transcriptome characterized in this study markedly differs from those obtained in other organs, and provides a unique resource for investigating the functional specificities of the root system. As an example of the use of SAGE for transcript profiling in Arabidopsis, we report here the identification of 270 genes differentially expressed between roots of plants grown either with NO3
- or NH4NO3 as N source.
![External K[superscript [+]]...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0960-7412/small "External K[superscript [+]] modulates the activity of the Arabidopsis potassium channel SKOR via an unusual mechanism")
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