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.
Summary
Damage can be signalled by extracellular ATP (eATP) using plasma membrane (PM) receptors to effect cytosolic free calcium ion (Ca2+cyt) increase as a second messenger. The downstream PM Ca2+ ...channels remain enigmatic. Here, the Arabidopsis thaliana Ca2+ channel subunit CYCLIC NUCLEOTIDE‐GATED CHANNEL2 (CNGC2) was identified as a critical component linking eATP receptors to downstream Ca2+cyt signalling in roots.
Extracellular ATP‐induced changes in single epidermal cell PM voltage and conductance were measured electrophysiologically, changes in root Ca2+cyt were measured with aequorin, and root transcriptional changes were determined by quantitative real‐time PCR. Two cngc2 loss‐of‐function mutants were used: cngc2‐3 and defence not death1 (which expresses cytosolic aequorin).
Extracellular ATP‐induced transient depolarization of Arabidopsis root elongation zone epidermal PM voltage was Ca2+ dependent, requiring CNGC2 but not CNGC4 (its channel co‐subunit in immunity signalling). Activation of PM Ca2+ influx currents also required CNGC2. The eATP‐induced Ca2+cyt increase and transcriptional response in cngc2 roots were significantly impaired.
CYCLIC NUCLEOTIDE‐GATED CHANNEL2 is required for eATP‐induced epidermal Ca2+ influx, causing depolarization leading to Ca2+cyt increase and damage‐related transcriptional response.
We have investigated OsHKT2;1 natural variation in a collection of 49 cultivars with different levels of salt tolerance and geographical origins. The effect of identified polymorphism on OsHKT2;1 ...activity was analysed through heterologous expression of variants in Xenopus oocytes. OsHKT2;1 appeared to be a highly conserved protein with only five possible amino acid substitutions that have no substantial effect on functional properties. Our study, however, also identified a new HKT isoform, No‐OsHKT2;2/1 in Nona Bokra, a highly salt‐tolerant cultivar. No‐OsHKT2;2/1 probably originated from a deletion in chromosome 6, producing a chimeric gene. Its 5′ region corresponds to that of OsHKT2;2, whose full‐length sequence is not present in Nipponbare but has been identified in Pokkali, a salt‐tolerant rice cultivar. Its 3′ region corresponds to that of OsHKT2;1. No‐OsHKT2;2/1 is essentially expressed in roots and displays a significant level of expression at high Na+ concentrations, in contrast to OsHKT2;1. Expressed in Xenopus oocytes or in Saccharomyces cerevisiae, No‐OsHKT2;2/1 exhibited a strong permeability to Na+ and K+, even at high external Na+ concentrations, like OsHKT2;2, and in contrast to OsHKT2;1. Our results suggest that No‐OsHKT2;2/1 can contribute to Nona Bokra salt tolerance by enabling root K+ uptake under saline conditions.
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.
A functional Shaker potassium channel requires assembly of four α-subunits encoded by a single gene or various genes from the Shaker family. In Arabidopsis thaliana, AtKC1, a Shaker α-subunit that is ...silent when expressed alone, has been shown to regulate the activity of AKT1 by forming heteromeric AtKC1-AKT1 channels. Here, we investigated whether AtKC1 is a general regulator of channel activity. Co-expression in Xenopus oocytes of a dominant negative (pore-mutated) AtKC1 subunit with the inward Shaker channel subunits KAT1, KAT2 or AKT2, or the outward subunits SKOR or GORK, revealed that the three inward subunits functionally interact with AtKC1 while the outward ones cannot. Localization experiments in plant protoplasts showed that KAT2 was able to re-locate AtKC1 fused to GFP from endomembranes to the plasma membrane, indicating that heteromeric AtKC1-KAT2 channels are efficiently targeted to the plasma membrane. Functional properties of heteromeric channels involving AtKC1 and KAT1, KAT2 or AKT2 were analysed by voltage clamp after co-expression of the respective subunits in Xenopus oocytes. AtKC1 behaved as a regulatory subunit within the heterotetrameric channel, reducing the macroscopic conductance and negatively shifting the channel activation potential. Expression studies showed that AtKC1 and its identified Shaker partners have overlapping expression patterns, supporting the hypothesis of a general regulation of inward channel activity by AtKC1 in planta. Lastly, AtKC1 disruption appeared to reduce plant biomass production, showing that AtKC1-mediated channel activity regulation is required for normal plant growth.
Durum wheat,
Triticum turgidum
subsp.
durum
(Desf.) Husn., is one of the most salt-sensitive cereal crops, but the physiological responses of different cultivars to salt stress vary. Cultivars that ...are suited to arid conditions like in Algeria may not necessarily be tolerant to increased salinity. When 10-day seedlings of Algerian durum wheat varieties Hedba 3 (HD3) and Mohamed Ben Bachir (MBB) were subjected to salt stress, they accumulated proline and expressed stress-related and proline metabolism genes in a classic salt-stress response. Expression of the selective sodium transporter genes
HKT1;4-1
and
-2
was found to be organ-specific and modulated by salt stress in both cultivars. Adding proline to the salt-containing growth medium alleviated some salt stress effects such as decrease in water content, ion leakage and expression oxidative stress markers while growth parameters were partially rescued to different extents in the two cultivars. Durum wheat seedlings accumulated sodium ions (Na
+
) at the expense of potassium ions (K
+
) under salt stress which lowered the in planta K
+
/Na
+
ratio. The two durum wheat cultivars studied here respond differently to salt stress in terms of responsiveness to proline,
HKT1;4
gene expression, and Na
+
and K
+
accumulation. Notably, salt stress can be partially alleviated by proline in the drought-resistant cultivar MBB, even though it is relatively salt sensitive.
Faced with declining soil‐water potential, plants synthesize abscisic acid (ABA), which then triggers stomatal closure to conserve tissue moisture. Closed stomates, however, also create several ...physiological dilemmas. Among these, the large CO₂ influx required for net photosynthesis will be disrupted. Depleting CO₂ in the plant will in turn bias stomatal opening by suppressing ABA sensitivity, which then aggravates transpiration further. We have investigated the molecular basis of how C3 plants resolve this H₂O–CO₂ conflicting priority created by stomatal closure. Here, we have identified in Arabidopsis thaliana an early drought‐induced spermidine spermine‐N¹‐acetyltransferase homolog, which can slow ABA‐mediated stomatal closure. Evidence from genetic, biochemical and physiological analyses has revealed that this protein does so by acetylating the metabolite 1,3‐diaminopropane (DAP), thereby turning on the latter's intrinsic activity. Acetylated DAP triggers plasma membrane electrical and ion transport properties in an opposite way to those by ABA. Thus in adapting to low soil‐water availability, acetyl‐DAP could refrain stomates from complete closure to sustain CO₂ diffusion to photosynthetic tissues.
Potassium (K+) plays a number of important roles in plant growth and development. Over the past few years, molecular approaches associated with electrophysiological analyses have greatly advanced our ...understanding of K+ transport in plants. A large number of genes encoding K+ transport systems have been identified, revealing a high level of complexity. Characterization of some transport systems is providing exciting information at the molecular level on functions such as root K+ uptake and secretion into the xylem sap, K+ transport in guard cells, or K+ influx into growing pollen tubes. In this review, we take stock of this recent molecular information. The main families of plant K+ transport systems (Shaker and KCO channels, KUP/HAK/KT and HKT transporters) are described, along with molecular data on how these systems are regulated. Finally, we discuss a few physiological questions on which molecular studies have shed new light.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Summary
The occurrence of radiocesium in food has raised sharp health concerns after nuclear accidents. Despite being present at low concentrations in contaminated soils (below μm), cesium (Cs+) can ...be taken up by crops and transported to their edible parts. This plant capacity to take up Cs+ from low concentrations has notably affected the production of rice (Oryza sativa L.) in Japan after the nuclear accident at Fukushima in 2011. Several strategies have been put into practice to reduce Cs+ content in this crop species such as contaminated soil removal or adaptation of agricultural practices, including dedicated fertilizer management, with limited impact or pernicious side‐effects. Conversely, the development of biotechnological approaches aimed at reducing Cs+ accumulation in rice remain challenging. Here, we show that inactivation of the Cs+‐permeable K+ transporter OsHAK1 with the CRISPR‐Cas system dramatically reduced Cs+ uptake by rice plants. Cs+ uptake in rice roots and in transformed yeast cells that expressed OsHAK1 displayed very similar kinetics parameters. In rice, Cs+ uptake is dependent on two functional properties of OsHAK1: (i) a poor capacity of this system to discriminate between Cs+ and K+; and (ii) a high capacity to transport Cs+ from very low external concentrations that is likely to involve an active transport mechanism. In an experiment with a Fukushima soil highly contaminated with 137Cs+, plants lacking OsHAK1 function displayed strikingly reduced levels of 137Cs+ in roots and shoots. These results open stimulating perspectives to smartly produce safe food in regions contaminated by nuclear accidents.
Significance Statement
After nuclear accidents, production of safe food is challenged by the presence of radiocesium in the environment, since crops take up cesium (Cs+) from contaminated soils and transport it to edible parts. Here we report that inactivation with the CRISPR‐Cas system of the main transporter that takes up Cs+ from the soil in rice, OsHAK1, leads to strikingly reduced levels of 137Cs+ in shoots when grown in highly contaminated soil from Fukushima.
Microscopic pores present in the epidermis of plant aerial organs, called stomata, allow gas exchanges between the inner photosynthetic tissue and the atmosphere. Regulation of stomatal aperture, ...preventing excess transpirational vapor loss, relies on turgor changes of two highly differentiated epidermal cells surrounding the pore, the guard cells. Increased guard cell turgor due to increased solute accumulation results in stomatal opening, whereas decreased guard cell turgor due to decreased solute accumulation results in stomatal closing. Here we provide direct evidence, based on reverse genetics approaches, that the Arabidopsis GORK Shaker gene encodes the major voltage-gated outwardly rectifying K+channel of the guard cell membrane. Expression of GORK dominant negative mutant polypeptides in transgenic Arabidopsis was found to strongly reduce outwardly rectifying K+channel activity in the guard cell membrane, and disruption of the GORK gene (T-DNA insertion knockout mutant) fully suppressed this activity. Bioassays on epidermal peels revealed that disruption of GORK activity resulted in impaired stomatal closure in response to darkness or the stress hormone azobenzenearsonate. Transpiration measurements on excised rosettes and intact plants (grown in hydroponic conditions or submitted to water stress) revealed that absence of GORK activity resulted in increased water consumption. The whole set of data indicates that GORK is likely to play a crucial role in adaptation to drought in fluctuating environments.
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