Potassium and sodium transport in non-animal cells: the Trk/Ktr/HKT transporter family

Experimental evolution of gene essentiality in bacteria

Essential gene products carry out fundamental cellular activities in interaction with other components. However, the lack of essential gene mutants and appropriate methodologies to link essential gene functions with their partners poses significant challenges. Here, we have generated deletion mutants in 32 genes previously identified as essential, with 23 mutants showing extremely slow growth in the SK36 strain of Streptococcus sanguinis. The 23 genes corresponding to these mutants encode components of diverse pathways, are widely conserved among bacteria, and are essential in many other bacterial species. Whole-genome sequencing of 243 independently evolved populations of these mutants has identified >1000 spontaneous suppressor mutations in experimental evolution. Many of these mutations define new gene and pathway relationships, such as F1Fo-ATPase/V1Vo-ATPase/TrkA1-H1 that were demonstrated across multiple Streptococcus species. Patterns of spontaneous mutations occurring in essential gene mutants differed from those found in wildtype. While gene duplications occurred rarely and appeared most often at later stages of evolution, substitutions, deletions, and insertions were prevalent in evolved populations. These essential gene deletion mutants and spontaneous mutations fixed in the mutant populations during evolution establish a foundation for understanding gene essentiality and the interaction of essential genes in networks.

Bacterial cell volume regulation and the importance of cyclic di-AMP

SUMMARYNucleotide-derived second messengers are present in all domains of life. In prokaryotes, most of their functionality is associated with general lifestyle and metabolic adaptations, often in response to environmental fluctuations of physical parameters. In the last two decades, cyclic di-AMP has emerged as an important signaling nucleotide in many prokaryotic lineages, including Firmicutes, Actinobacteria, and Cyanobacteria. Its importance is highlighted by the fact that both the lack and overproduction of cyclic di-AMP affect viability of prokaryotes that utilize cyclic di-AMP, and that it generates a strong innate immune response in eukaryotes. In bacteria that produce the second messenger, most molecular targets of cyclic di-AMP are associated with cell volume control. Besides, other evidence links the second messenger to cell wall remodeling, DNA damage repair, sporulation, central metabolism, and the regulation of glycogen turnover. In this review, we take a biochemical, quantitative approach to address the main cellular processes that are directly regulated by cyclic di-AMP and show that these processes are very connected and require regulation of a similar set of proteins to which cyclic di-AMP binds. Altogether, we argue that cyclic di-AMP is a master regulator of cell volume and that other cellular processes can be connected with cyclic di-AMP through this core function. We further highlight important directions in which the cyclic di-AMP field has to develop to gain a full understanding of the cyclic di-AMP signaling network and why some processes are regulated, while others are not.

Metagenomic insights into microbial adaptation to the salinity gradient of a typical short residence-time estuary

BACKGROUND

Microbial adaptation to salinity has been a classic inquiry in the field of microbiology. It has been demonstrated that microorganisms can endure salinity stress via either the "salt-in" strategy, involving inorganic ion uptake, or the "salt-out" strategy, relying on compatible solutes. While these insights are mostly based on laboratory-cultured isolates, exploring the adaptive mechanisms of microorganisms within natural salinity gradient is crucial for gaining a deeper understanding of microbial adaptation in the estuarine ecosystem.

RESULTS

Here, we conducted metagenomic analyses on filtered surface water samples collected from a typical subtropical short residence-time estuary and categorized them by salinity into low-, intermediate-, and high-salinity metagenomes. Our findings highlighted salinity-driven variations in microbial community composition and function, as revealed through taxonomic and Clusters of Orthologous Group (COG) functional annotations. Through metagenomic binning, 127 bacterial and archaeal metagenome-assembled genomes (MAGs) were reconstructed. These MAGs were categorized as stenohaline-specific to low-, intermediate-, or high-salinity-based on the average relative abundance in one salinity category significantly exceeding those in the other two categories by an order of magnitude. Those that did not meet this criterion were classified as euryhaline, indicating a broader range of salinity tolerance. Applying the Boruta algorithm, a machine learning-based feature selection method, we discerned important genomic features from the stenohaline bacterial MAGs. Of the total 12,162 COGs obtained, 40 were identified as important features, with the "inorganic ion transport and metabolism" COG category emerging as the most prominent. Furthermore, eight COGs were implicated in microbial osmoregulation, of which four were related to the "salt-in" strategy, three to the "salt-out" strategy, and one to the regulation of water channel activity. COG0168, annotated as the Trk-type K+ transporter related to the "salt-in" strategy, was ranked as the most important feature. The relative abundance of COG0168 was observed to increase with rising salinity across metagenomes, the stenohaline strains, and the dominant Actinobacteriota and Proteobacteria phyla.

CONCLUSIONS

We demonstrated that salinity exerts influences on both the taxonomic and functional profiles of the microbial communities inhabiting the estuarine ecosystem. Our findings shed light on diverse salinity adaptation strategies employed by the estuarine microbial communities, highlighting the crucial role of the "salt-in" strategy mediated by Trk-type K+ transporters for microorganisms thriving under osmotic stress in the short residence-time estuary. Video Abstract.

Improving salt-tolerant artificial consortium of Bacillus amyloliquefaciens for bioconverting food waste to lipopeptides

High-salt content in food waste (FW) affects its resource utilization during biotransformation. In this study, adaptive laboratory evolution (ALE), gene editing, and artificial consortia were performed out to improve the salt-tolerance of Bacillus amyloliquefaciens for producing lipopeptide under FW and seawater. High-salt stress significantly decreased lipopeptide production in the B. amyloliquefaciens HM618 and ALE strains. The total lipopeptide production in the recombinant B. amyloliquefaciens HM-4KSMSO after overexpressing the ion transportor gene ktrA and proline transporter gene opuE and replacing the promoter of gene mrp was 1.34 times higher than that in the strain HM618 in medium containing 30 g/L NaCl. Lipopeptide production under salt-tolerant consortia containing two strains (HM-4KSMSO and Corynebacterium glutamicum) and three-strains (HM-4KSMSO, salt-tolerant C. glutamicum, and Yarrowia lipolytica) was 1.81- and 2.28-fold higher than that under pure culture in a medium containing FW or both FW and seawater, respectively. These findings provide a new strategy for using high-salt FW and seawater to produce value-added chemicals.

Structural basis and synergism of ATP and Na+ activation in bacterial K+ uptake system KtrAB

The K+ uptake system KtrAB is essential for bacterial survival in low K+ environments. The activity of KtrAB is regulated by nucleotides and Na+. Previous studies proposed a putative gating mechanism of KtrB regulated by KtrA upon binding to ATP or ADP. However, how Na+ activates KtrAB and the Na+ binding site remain unknown. Here we present the cryo-EM structures of ATP- and ADP-bound KtrAB from Bacillus subtilis (BsKtrAB) both solved at 2.8 Å. A cryo-EM density at the intra-dimer interface of ATP-KtrA was identified as Na+, as supported by X-ray crystallography and ICP-MS. Thermostability assays and functional studies demonstrated that Na+ binding stabilizes the ATP-bound BsKtrAB complex and enhances its K+ flux activity. Comparing ATP- and ADP-BsKtrAB structures suggests that BsKtrB Arg417 and Phe91 serve as a channel gate. The synergism of ATP and Na+ in activating BsKtrAB is likely applicable to Na+-activated K+ channels in central nervous system.

c-di-AMP determines the hierarchical organization of bacterial RCK proteins

In bacteria, intracellular K+ is involved in the regulation of membrane potential, cytosolic pH, and cell turgor as well as in spore germination, environmental adaptation, cell-to-cell communication in biofilms, antibiotic sensitivity, and infectivity. The second messenger cyclic-di-AMP (c-di-AMP) has a central role in modulating the intracellular K+ concentration in many bacterial species, controlling transcription and function of K+ channels and transporters. However, our understanding of how this regulatory network responds to c-di-AMP remains poor. We used the RCK (Regulator of Conductance of K+) proteins that control the activity of Ktr channels in Bacillus subtilis as a model system to analyze the regulatory function of c-di-AMP with a combination of in vivo and in vitro functional and structural characterization. We determined that the two RCK proteins (KtrA and KtrC) are neither physiologically redundant or functionally equivalent. KtrC is the physiologically dominant RCK protein in the regulation of Ktr channel activity. In explaining this hierarchical organization, we found that, unlike KtrA, KtrC is very sensitive to c-di-AMP inactivation and lack of c-di-AMP regulation results in RCK protein toxicity, most likely due to unregulated K+ flux. We also found that KtrC can assemble with KtrA, conferring c-di-AMP regulation to the functional KtrA/KtrC heteromers and potentially compensating KtrA toxicity. Altogether, we propose that the central role of c-di-AMP in the control of the K+ machinery, by modulating protein levels through gene transcription and by regulating protein activity, has determined the evolutionary selection of KtrC as the dominant RCK protein, shaping the hierarchical organization of regulatory components of the K+ machinery.

Structural insights into ion selectivity and transport mechanisms of Oryza sativa HKT2;1 and HKT2;2/1 transporters

Plant high-affinity K+ transporters (HKTs) play a pivotal role in maintaining the balance of Na+ and K+ ions in plants, thereby influencing plant growth under K+-depleted conditions and enhancing tolerance to salinity stress. Here we report the cryo-electron microscopy structures of Oryza sativa HKT2;1 and HKT2;2/1 at overall resolutions of 2.5 Å and 2.3 Å, respectively. Both transporters adopt a dimeric assembly, with each protomer enclosing an ion permeation pathway. Comparison between the selectivity filters of the two transporters reveals the critical roles of Ser88/Gly88 and Val243/Gly243 in determining ion selectivity. A constriction site along the ion permeation pathway is identified, consisting of Glu114, Asn273, Pro392, Pro393, Arg525, Lys517 and the carboxy-terminal Trp530 from the neighbouring protomer. The linker between domains II and III adopts a stable loop structure oriented towards the constriction site, potentially participating in the gating process. Electrophysiological recordings, yeast complementation assays and molecular dynamics simulations corroborate the functional importance of these structural features. Our findings provide crucial insights into the ion selectivity and transport mechanisms of plant HKTs, offering valuable structural templates for developing new salinity-tolerant cultivars and strategies to increase crop yields.

Inverse regulation of SOS1 and HKT1 protein localization and stability by SOS3/CBL4 in Arabidopsis thaliana

To control net sodium (Na+) uptake, Arabidopsis plants utilize the plasma membrane (PM) Na+/H+ antiporter SOS1 to achieve Na+ efflux at the root and Na+ loading into the xylem, and the channel-like HKT1;1 protein that mediates the reverse flux of Na+ unloading off the xylem. Together, these opposing transport systems govern the partition of Na+ within the plant yet they must be finely co-regulated to prevent a futile cycle of xylem loading and unloading. Here, we show that the Arabidopsis SOS3 protein acts as the molecular switch governing these Na+ fluxes by favoring the recruitment of SOS1 to the PM and its subsequent activation by the SOS2/SOS3 kinase complex under salt stress, while commanding HKT1;1 protein degradation upon acute sodic stress. SOS3 achieves this role by direct and SOS2-independent binding to previously unrecognized functional domains of SOS1 and HKT1;1. These results indicate that roots first retain moderate amounts of salts to facilitate osmoregulation, yet when sodicity exceeds a set point, SOS3-dependent HKT1;1 degradation switches the balance toward Na+ export out of the root. Thus, SOS3 functionally links and co-regulates the two major Na+ transport systems operating in vascular plants controlling plant tolerance to salinity.

Maternal Embryo Effect Arrest 31 (MEE31) is a moonlighting protein involved in GDP-D-mannose biosynthesis and KAT1 potassium channel regulation

Due to anthropogenic global warming, droughts are expected to increase and water availability to decrease in the coming decades. For this reason, research is increasingly focused on developing plant varieties and crop cultivars with reduced water consumption. Transpiration occurs through stomatal pores, resulting in water loss. Potassium plays a significant role in stomatal regulation. KAT1 is an inward-rectifying potassium channel that contributes to stomatal opening. Using a yeast high-throughput screening of an Arabidopsis cDNA library, MEE31 was found to physically interact with KAT1. MEE31 was initially identified in a screen for mutants with delayed embryonic development. The gene encodes a conserved phosphomannose isomerase (PMI). We report here that MEE31 interacts with and increases KAT1 activity in yeast and this interaction was also confirmed in plants. In addition, MEE31 complements the function of the yeast homologue, whereas the truncated version recovered in the screening does not, thus uncoupling the enzymatic activity from KAT1 regulation. We show that MEE31 overexpression leads to increased stomatal opening in Arabidopsis transgenic lines. Our data suggest that MEE31 is a moonlighting protein involved in both GDP-D-mannose biosynthesis and KAT1 regulation.

Ion Changes and Signaling under Salt Stress in Wheat and Other Important Crops

High concentrations of sodium (Na+), chloride (Cl-), calcium (Ca2+), and sulphate (SO42-) are frequently found in saline soils. Crop plants cannot successfully develop and produce because salt stress impairs the uptake of Ca2+, potassium (K+), and water into plant cells. Different intracellular and extracellular ionic concentrations change with salinity, including those of Ca2+, K+, and protons. These cations serve as stress signaling molecules in addition to being essential for ionic homeostasis and nutrition. Maintaining an appropriate K+:Na+ ratio is one crucial plant mechanism for salt tolerance, which is a complicated trait. Another important mechanism is the ability for fast extrusion of Na+ from the cytosol. Ca2+ is established as a ubiquitous secondary messenger, which transmits various stress signals into metabolic alterations that cause adaptive responses. When plants are under stress, the cytosolic-free Ca2+ concentration can rise to 10 times or more from its resting level of 50-100 nanomolar. Reactive oxygen species (ROS) are linked to the Ca2+ alterations and are produced by stress. Depending on the type, frequency, and intensity of the stress, the cytosolic Ca2+ signals oscillate, are transient, or persist for a longer period and exhibit specific "signatures". Both the influx and efflux of Ca2+ affect the length and amplitude of the signal. According to several reports, under stress Ca2+ alterations can occur not only in the cytoplasm of the cell but also in the cell walls, nucleus, and other cell organelles and the Ca2+ waves propagate through the whole plant. Here, we will focus on how wheat and other important crops absorb Na+, K+, and Cl- when plants are under salt stress, as well as how Ca2+, K+, and pH cause intracellular signaling and homeostasis. Similar mechanisms in the model plant Arabidopsis will also be considered. Knowledge of these processes is important for understanding how plants react to salinity stress and for the development of tolerant crops.

Genome-Wide Identification and Characterization of the HAK Gene Family in Quinoa (Chenopodium quinoa Willd.) and Their Expression Profiles under Saline and Alkaline Conditions

The high-affinity K+ transporter (HAK) family, the most prominent potassium transporter family in plants, which involves K+ transport, plays crucial roles in plant responses to abiotic stresses. However, the HAK gene family remains to be characterized in quinoa (Chenopodium quinoa Willd.). We explored HAKs in quinoa, identifying 30 members (CqHAK1-CqHAK30) in four clusters phylogenetically. Uneven distribution was observed across 18 chromosomes. Furthermore, we investigated the proteins' evolutionary relationships, physicochemical properties, conserved domains and motifs, gene structure, and cis-regulatory elements of the CqHAKs family members. Transcription data analysis showed that CqHAKs have diverse expression patterns among different tissues and in response to abiotic stresses, including drought, heat, low phosphorus, and salt. The expressional changes of CqHAKs in roots were more sensitive in response to abiotic stress than that in shoot apices. Quantitative RT-PCR analysis revealed that under high saline condition, CqHAK1, CqHAK13, CqHAK19, and CqHAK20 were dramatically induced in leaves; under alkaline condition, CqHAK1, CqHAK13, CqHAK19, and CqHAK20 were dramatically induced in leaves, and CqHAK6, CqHAK9, CqHAK13, CqHAK23, and CqHAK29 were significantly induced in roots. Our results establish a foundation for further investigation of the functions of HAKs in quinoa. It is the first study to identify the HAK gene family in quinoa, which provides potential targets for further functional study and contributes to improving the salt and alkali tolerance in quinoa.

Modulation of potassium transport to increase abiotic stress tolerance in plants

Potassium is the major cation responsible for the maintenance of the ionic environment in plant cells. Stable potassium homeostasis is indispensable for virtually all cellular functions, and, concomitantly, viability. Plants must cope with environmental changes such as salt or drought that can alter ionic homeostasis. Potassium fluxes are required to regulate the essential process of transpiration, so a constraint on potassium transport may also affect the plant's response to heat, cold, or oxidative stress. Sequencing data and functional analyses have defined the potassium channels and transporters present in the genomes of different species, so we know most of the proteins directly participating in potassium homeostasis. The still unanswered questions are how these proteins are regulated and the nature of potential cross-talk with other signaling pathways controlling growth, development, and stress responses. As we gain knowledge regarding the molecular mechanisms underlying regulation of potassium homeostasis in plants, we can take advantage of this information to increase the efficiency of potassium transport and generate plants with enhanced tolerance to abiotic stress through genetic engineering or new breeding techniques. Here, we review current knowledge of how modifying genes related to potassium homeostasis in plants affect abiotic stress tolerance at the whole plant level.

Halophytes as new model plant species for salt tolerance strategies

Soil salinity is becoming a growing issue nowadays, severely affecting the world's most productive agricultural landscapes. With intersecting and competitive challenges of shrinking agricultural lands and increasing demand for food, there is an emerging need to build resilience for adaptation to anticipated climate change and land degradation. This necessitates the deep decoding of a gene pool of crop plant wild relatives which can be accomplished through salt-tolerant species, such as halophytes, in order to reveal the underlying regulatory mechanisms. Halophytes are generally defined as plants able to survive and complete their life cycle in highly saline environments of at least 200-500 mM of salt solution. The primary criterion for identifying salt-tolerant grasses (STGs) includes the presence of salt glands on the leaf surface and the Na⁺ exclusion mechanism since the interaction and replacement of Na⁺ and K⁺ greatly determines the survivability of STGs in saline environments. During the last decades or so, various salt-tolerant grasses/halophytes have been explored for the mining of salt-tolerant genes and testing their efficacy to improve the limit of salt tolerance in crop plants. Still, the utility of halophytes is limited due to the non-availability of any model halophytic plant system as well as the lack of complete genomic information. To date, although Arabidopsis (Arabidopsis thaliana) and salt cress (Thellungiella halophila) are being used as model plants in most salt tolerance studies, these plants are short-lived and can tolerate salinity for a shorter duration only. Thus, identifying the unique genes for salt tolerance pathways in halophytes and their introgression in a related cereal genome for better tolerance to salinity is the need of the hour. Modern technologies including RNA sequencing and genome-wide mapping along with advanced bioinformatics programs have advanced the decoding of the whole genetic information of plants and the development of probable algorithms to correlate stress tolerance limit and yield potential. Hence, this article has been compiled to explore the naturally occurring halophytes as potential model plant species for abiotic stress tolerance and to further breed crop plants to enhance salt tolerance through genomic and molecular tools.

Transcriptomic and functional characterization reveals CsHAK5;3 as a key player in K+ homeostasis in grafted cucumbers under saline conditions

Grafting can improve the salt tolerance of many crops. However, critical genes in scions responsive to rootstock under salt stress remain a mystery. We found that pumpkin rootstock decreased the content of Na⁺ by 70.24 %, increased the content of K⁺ by 25.9 %, and increased the K⁺/Na⁺ ratio by 366.0 % in cucumber scion leaves. RNA-seq analysis showed that ion transport-related genes were the key genes involved in salt stress tolerance in grafted cucumber. The identification and analysis of the expression of K⁺ transporter proteins in cucumber and pumpkin revealed six and five HAK5 members, respectively. The expression of CsHAK5;3 in cucumber was elevated in different graft combinations under salt stress and most notably in cucumber scion/pumpkin rootstock. CsHAK5;3 was localized to the plasma membrane, and a yeast complementation assay revealed that it can transport K⁺. CsHAK5;3 knockout in hairy root mutants decreased the K⁺ content of leaves (45.6 %) and roots (50.3 %), increased the Na⁺ content of leaves (29.3 %) and roots (34.8 %), and decreased the K⁺/Na⁺ ratio of the leaves (57.9 %) and roots (62.9 %) in cucumber. However, CsHAK5;3 overexpression in hairy roots increased the K⁺ content of the leaves (31.2 %) and roots (38.3 %), decreased the Na⁺ content of leaves (17.2 %) and roots (14.3 %), and increased the K⁺/Na⁺ ratio of leaves (58.9 %) and roots (61.6 %) in cucumber. In conclusion, CsHAK5;3 in cucumber can mediate K⁺ transport and is one of the key target pumpkin genes that enhance salt tolerance of cucumber grafted.

Two Trk/Ktr/HKT-type potassium transporters, TrkG and TrkH, perform distinct functions in Escherichia coli K-12

Escherichia coli K-12 possesses two versions of Trk/Ktr/HKT-type potassium ion (K⁺) transporters, TrkG and TrkH. The current paradigm is that TrkG and TrkH have largely identical characteristics, and little information is available regarding their functional differences. Here, we show using cation uptake experiments with K⁺ transporter knockout mutants that TrkG and TrkH have distinct ion transport activities and physiological roles. K⁺-transport by TrkG required Na⁺, whereas TrkH-mediated K⁺ uptake was not affected by Na⁺. An aspartic acid located five residues away from a critical glycine in the third pore-forming region might be involved in regulation of Na⁺-dependent activation of TrkG. In addition, we found that TrkG but not TrkH had Na⁺ uptake activity. Our analysis of K⁺ transport mutants revealed that TrkH supported cell growth more than TrkG; however, TrkG was able to complement loss of TrkH-mediated K⁺ uptake in E. coli. Furthermore, we determined that transcription of trkG in E. coli was downregulated but not completely silenced by the xenogeneic silencing factor H-NS (histone-like nucleoid structuring protein or heat-stable nucleoid-structuring protein). Taken together, the transport function of TrkG is clearly distinct from that of TrkH, and TrkG seems to have been accepted by E. coli during evolution as a K⁺ uptake system that coexists with TrkH.

Genome-wide identification, characterization and expression analysis of HAK genes and decoding their role in responding to potassium deficiency and abiotic stress in Medicago truncatula

Background

The HAK family is the largest potassium (K⁺) transporter family, vital in K⁺ uptake, plant growth, and both plant biotic and abiotic stress responses. Although HAK family members have been characterized and functionally investigated in many species, these genes are still not studied in detail in Medicago truncatula, a good model system for studying legume genetics.

Methods

In this study, we screened the M. truncatula HAK family members (MtHAKs). Furthermore, we also conducted the identification, phylogenetic analysis, and prediction of conserved motifs of MtHAKs. Moreover, we studied the expression levels of MtHAKs under K⁺ deficiency, drought, and salt stresses using quantitative real-time PCR (qRT-PCR).

Results

We identified 20 MtHAK family members and classified them into three clusters based on phylogenetic relationships. Conserved motif analyses showed that all MtHAK proteins besides MtHAK10 contained the highly conserved K⁺ transport domain (GVVYGDLGTSPLY). qRT-PCR analysis showed that several MtHAK genes in roots were induced by abiotic stress. In particular, MtHAK15, MtHAK17, and MtHAK18 were strongly up-regulated in the M. truncatula roots under K⁺ deficiency, drought, and salt stress conditions, thereby implying that these genes are good candidates for high-affinity K⁺ uptake and therefore have essential roles in drought and salt tolerance.

Discussions

Our results not only provided the first genetic description and evolutionary relationships of the K⁺ transporter family in M. truncatula, but also the potential information responding to K⁺ deficiency and abiotic stresses, thereby laying the foundation for molecular breeding of stress-resistant legume crops in the future.

Research Advances on Molecular Mechanism of Salt Tolerance in Suaeda

Plant growth and development are inevitably affected by various environmental factors. High salinity is the main factor leading to the reduction of cultivated land area, which seriously affects the growth and yield of plants. The genus Suaeda is a kind of euhalophyte herb, with seedlings that grow rapidly in moderately saline environments and can even survive in conditions of extreme salinity. Its fresh branches can be used as vegetables and the seed oil is rich in unsaturated fatty acids, which has important economic value and usually grows in a saline environment. This paper reviews the progress of research in recent years into the salt tolerance of several Suaeda species (for example, S. salsa, S. japonica, S. glauca, S. corniculata), focusing on ion regulation and compartmentation, osmotic regulation of organic solutes, antioxidant regulation, plant hormones, photosynthetic systems, and omics (transcriptomics, proteomics, and metabolomics). It helps us to understand the salt tolerance mechanism of the genus Suaeda, and provides a theoretical foundation for effectively improving crop resistance to salt stress environments.

Integrated Analysis of Single-Molecule Real-Time Sequencing and Next-Generation Sequencing Eveals Insights into Drought Tolerance Mechanism of Lolium multiflorum

Lolium multiflorum is widely planted in temperate and subtropical regions globally, and it has high economic value owing to its use as forage grass for a wide variety of livestock and poultry. However, drought seriously restricts its yield and quality. At present, owing to the lack of available genomic resources, many types of basic research cannot be conducted, which severely limits the in-depth functional analysis of genes in L. multiflorum. Therefore, we used single-molecule real-time (SMRT) and next-generation sequencing (NGS) to sequence the complex transcriptome of L. multiflorum under drought. We identified 41,141 DEGs in leaves, 35,559 DEGs in roots, respectively. Moreover, we identified 1243 alternative splicing events under drought. LmPIP5K9 produced two different transcripts with opposite expression patterns, possibly through the phospholipid signaling pathway or the negatively regulated sugar-mediated root growth response to drought stress, respectively. Additionally, 13,079 transcription factors in 90 families were obtained. An in-depth analysis of R2R3-MYB gene family members was performed to preliminarily demonstrate their functions by utilizing subcellular localization and overexpression in yeast. Our data make a significant contribution to the genetics of L. multiflorum, offering a current understanding of plant adaptation to drought stress.

Distinct Functions of the Atypical Terminal Hydrophilic Domain of the HKT Transporter in the Liverwort Marchantia polymorpha

K+/Na+ homeostasis is important for land plants, particularly under salt stress. In this study, the structure and ion transport properties of the high-affinity K+ transporter (HKT) of the liverwort Marchantia polymorpha were investigated. Only one HKT gene, MpHKT1, was identified in the genome of M. polymorpha. Phylogenetic analysis of HKT proteins revealed that non-seed plants possess HKTs grouped into a clade independent of the other two clades including HKTs of angiosperms. A distinct long hydrophilic domain was found in the C-terminus of MpHKT1. Complementary DNA (cDNA) of truncated MpHKT1 (t-MpHKT1) encoding the MpHKT_Δ596-812 protein was used to examine the functions of the C-terminal domain. Both MpHKT1 transporters fused with enhanced green fluorescent protein at the N-terminus were localized to the plasma membrane when expressed in rice protoplasts. Two-electrode voltage clamp experiments using Xenopus laevis oocytes indicated that MpHKT1 mediated the transport of monovalent alkali cations with higher selectivity for Na+ and K+, but truncation of the C-terminal domain significantly reduced the transport activity with a decrease in the Na+ permeability. Overexpression of MpHKT1 or t-MpHKT1 in M. polymorpha conferred accumulation of higher Na+ levels and showed higher Na+ uptake rates, compared to those of wild-type plants; however, phenotypes with t-MpHKT1 were consistently weaker than those with MpHKT1. Together, these findings suggest that the hydrophilic C-terminal domain plays a unique role in the regulation of transport activity and ion selectivity of MpHKT1.

The PTS Ntr -KdpDE-KdpFABC Pathway Contributes to Low Potassium Stress Adaptation and Competitive Nodulation of Sinorhizobium fredii

In all ecological niches, potassium is actively consumed by diverse prokaryotes and their interacting eukaryote hosts. It is only just emerging that potassium is a key player in host-pathogen interactions, and the role of potassium in mutualistic interactions remains largely unknown.

Ethylene-induced potassium transporter AcKUP2 gene is involved in kiwifruit postharvest ripening

BACKGROUND

Potassium (K) is important in the regulation of plant growth and development. It is the most abundant mineral element in kiwifruit, and its content increases during fruit ripening. However, how K⁺ transporter works in kiwifruit postharvest maturation is not yet clear.

RESULTS

Here, 12 K⁺ transporter KT/HAK/KUP genes, AcKUP1 ~ AcKUP12, were isolated from kiwifruit, and their phylogeny, genomic structure, chromosomal location, protein properties, conserved motifs and cis-acting elements were analysed. Transcription analysis revealed that AcKUP2 expression increased rapidly and was maintained at a high level during postharvest maturation, consistent with the trend of K content; AcKUP2 expression was induced by ethylene, suggesting that AcKUP2 might play a role in ripening. Fluorescence microscopy showed that AcKUP2 is localised in the plasma membrane. Cis-elements, including DER or ethylene response element (ERE) responsive to ethylene, were found in the AcKUP2 promoter sequence, and ethylene significantly enhanced the AcKUP2 promoter activity. Furthermore, we verified that AcERF15, an ethylene response factor, directly binds to the AcKUP2 promoter to promote its expression. Thus, AcKUP2 may be an important potassium transporter gene which involved in ethylene-regulated kiwifruit postharvest ripening.

CONCLUSIONS

Therefore, our study establishes the first genome-wide analysis of the kiwifruit KT/HAK/KUP gene family and provides valuable information for understanding the function of the KT/HAK/KUP genes in kiwifruit postharvest ripening.

Transcriptome-wide identification and expression analysis of the KT/HAK/KUP family in Salicornia europaea L. under varied NaCl and KCl treatments

Background

The KT/HAK/KUP (KUP) transporters play important roles in potassium (K⁺) uptake and translocation, regulation of osmotic potential, salt tolerance, root morphogenesis and plant development. However, the KUP family has not been systematically studied in the typical halophyte Salicornia europaea L., and the specific expression patterns of SeKUPs under NaCl condition and K⁺ deficiency are unknown.

Methods

In this study, SeKUPs were screened from PacBio transcriptome data of Salicornia europaea L. using bioinformatics. The identification, phylogenetic analysis and prediction of conserved motifs of SeKUPs were extensively explored. Moreover, the expression levels of 24 selected SeKUPs were assayed by real-time quantitative polymerase chain reaction (RT-qPCR).

Results

In this study, a total of 24 putative SeKUPs were identified in S. europaea. Nineteen SeKUPs with the fixed domain EA[ML]FADL were used to construct the phylogenetic tree, and they were divided into four clusters (clusters I-IV). MEME analysis identified 10 motifs in S. europaea, and the motif analysis suggested that 19 of the identified SeKUPs had at least four K⁺ transporter motifs existed in all SeKUPs (with the exception of SeKUP-2). The RT-qPCR analysis showed that the expression levels of most SeKUPs were significantly up-regulated in S. europaea when they were exposed to K⁺ deficiency and high salinity, implying that these SeKUPs may play a key role in the absorption and transport of K⁺ and Na⁺ in S. europaea.

Discussions

Our results laid the foundation for revealing the salt tolerance mechanism of SeKUPs, and provided key candidate genes for further studies on the function of KUP family in S. europaea.

To exclude or to accumulate? Revealing the role of the sodium HKT1;5 transporter in plant adaptive responses to varying soil salinity

Arid/semi-arid and coastal agricultural areas of the world are especially vulnerable to climate change-driven soil salinity. Salinity tolerance in plants is a complex trait, with salinity negatively affecting crop yield. Plants adopt a range of mechanisms to combat salinity, with many transporter genes being implicated in Na⁺-partitioning processes. Within these, the high-affinity K⁺ (HKT) family of transporters play a critical role in K⁺ and Na⁺ homeostasis in plants. Among HKT transporters, Type I transporters are Na⁺-specific. While Arabidopsis has only one Na ⁺ -specific HKT (AtHKT1;1), cereal crops have a multiplicity of Type I and II HKT transporters. AtHKT1; 1 (Arabidopsis thaliana) and HKT1; 5 (cereal crops) 'exclude' Na⁺ from the xylem into xylem parenchyma in the root, reducing shoot Na⁺ and hence, confer sodium tolerance. However, more recent data from Arabidopsis and crop species show that AtHKT1;1/HKT1;5 alleles have a strong genetic association with 'shoot sodium accumulation' and concomitant salt tolerance. The review tries to resolve these two seemingly contradictory effects of AtHKT1;1/HKT1;5 operation (shoot exclusion vs shoot accumulation), both conferring salinity tolerance and suggests that contrasting phenotypes are attributable to either hyper-functional or weak AtHKT1;1/HKT1;5 alleles/haplotypes and are under strong selection by soil salinity levels. It also suggests that opposite balancing mechanisms involving xylem ion loading in these contrasting phenotypes exist that require transporters such as SOS1 and CCC. While HKT1; 5 is a crucial but not sole determinant of salinity tolerance, investigation of the adaptive benefit(s) conferred by naturally occurring intermediate HKT1;5 alleles will be important under a climate change scenario.

Identification and Characterization of Rice OsHKT1;3 Variants

In rice, the high-affinity K⁺ transporter, OsHKT1;3, functions as a Na⁺-selective transporter. mRNA variants of OsHKT1;3 have been reported previously, but their functions remain unknown. In this study, five OsHKT1;3 variants (V1-V5) were identified from japonica rice (Nipponbare) in addition to OsHKT1;3_FL. Absolute quantification qPCR analyses revealed that the transcript level of OsHKT1;3_FL was significantly higher than other variants in both the roots and shoots. Expression levels of OsHKT1;3_FL, and some variants, increased after 24 h of salt stress. Two electrode voltage clamp experiments in a heterologous expression system using Xenopus laevis oocytes revealed that oocytes expressing OsHKT1;3_FL and all of its variants exhibited smaller Na⁺ currents. The presented data, together with previous data, provide insights to understanding how OsHKT family members are involved in the mechanisms of ion homeostasis and salt tolerance in rice.

Functional Characterization of Multiple Ehrlichia chaffeensis Sodium (Cation)/Proton Antiporter Genes Involved in the Bacterial pH Homeostasis

Ehrlichia chaffeensis causes human monocytic ehrlichiosis. Little is known about how this and other related tick-borne rickettsia pathogens maintain pH homeostasis in acidified phagosomes and the extracellular milieu. The membrane-bound sodium (cation)/proton antiporters are found in a wide range of organisms aiding pH homeostasis. We recently reported a mutation in an antiporter gene of E. chaffeensis (ECH_0379) which causes bacterial in vivo attenuation. The E. chaffeensis genome contains 10 protein coding sequences encoding for predicted antiporters. We report here that nine of these genes are transcribed during the bacterial growth in macrophages and tick cells. All E. chaffeensis antiporter genes functionally complemented antiporter deficient Escherichia coli. Antiporter activity for all predicted E. chaffeensis genes was observed at pH 5.5, while gene products of ECH_0179 and ECH_0379 were also active at pH 8.0, and ECH_0179 protein was complemented at pH 7.0. The antiporter activity was independently verified for the ECH_0379 protein by proteoliposome diffusion analysis. This is the first description of antiporters in E. chaffeensis and demonstrates that the pathogen contains multiple antiporters with varying biological functions, which are likely important for the pH homeostasis of the pathogen’s replicating and infectious forms.

Differential regulation of drought stress by biological membrane transporters and channels

Stress arising due to abiotic factors affects the plant's growth and productivity. Among several existing abiotic stressors like cold, drought, heat, salinity, heavy metal, etc., drought condition tends to affect the plant's growth by inducing two-point effect, i.e., it disturbs the water balance as well as induces toxicity by disturbing the ion homeostasis, thus hindering the growth and productivity of plants, and to survive under this condition, plants have evolved several transportation systems that are involved in regulating the drought stress. The role of membrane transporters has gained interest since genetic engineering came into existence, and they were found to be the important modulators for tolerance, avoidance, ion movements, stomatal movements, etc. Here in this comprehensive review, we have discussed the role of transporters (ABA, protein, carbohydrates, etc.) and channels that aids in withstanding the drought stress as well as the regulatory role of transporters involved in osmotic adjustments arising due to drought stress. This review also provides a gist of hydraulic conductivity by roots that are involved in regulating the drought stress.

Potassium physiology from Archean to Holocene: A higher-plant perspective

In this paper, we discuss biological potassium acquisition and utilization processes over an evolutionary timescale, with emphasis on modern vascular plants. The quintessential osmotic and electrical functions of the K⁺ ion are shown to be intimately tied to K⁺-transport systems and membrane energization. Several prominent themes in plant K⁺-transport physiology are explored in greater detail, including: (1) channel mediated K⁺ acquisition by roots at low external [K⁺]; (2) K⁺ loading of root xylem elements by active transport; (3) variations on the theme of K⁺ efflux from root cells to the extracellular environment; (4) the veracity and utility of the "affinity" concept in relation to transport systems. We close with a discussion of the importance of plant-potassium relations to our human world, and current trends in potassium nutrition from farm to table.

Improvement of the yield-related response of mycorrhized Lallemantia iberica to salinity through sulfur-oxidizing bacteria

BACKGROUND

To investigate the effects of salinity as a serious environmental limiter of productivity on the yield-related traits of Lallemantia iberica, a split-plot experiment was performed for 2 years (2017-2018) based on a randomized complete block design with three replications at Urmia University (37°33'09″N, 45°05'53″E). The main plots included salinity stress at two levels (6.72 dS m^-1 , and 0.91 dS m^-1 as control), and subplots were inoculants at four levels (Funneliformis mosseae, Thiobacillus sp., F. mosseae + Thiobacillus sp., and no inoculation).

RESULTS

In the saline condition, serious reductions in yield and yield components (numbers of capsules per plant, seeds per capsule, and seeds per plant, 1000-seed weight, seed and biological yields), concentrations of leaf phosphorus and potassium, and relative mycorrhizal dependency were observed, but against the harvest index the leaf sulfur and sodium contents were increased. Moreover, all morphological traits (plant height, number of branches and leaves, leaf weight, stem weight, and ratio of leaf weight to stem weight) were decreased under salinity conditions. Mycorrhizal inoculation enhanced the salinity-induced reduction of yield and morphological traits to some extent. Inoculation with Thiobacillus had superiority in some of the yield and morphological characteristics compared with those in the non-inoculated plants.

CONCLUSION

Salinity stress can significantly affect the yield, morphological characteristics, nutrients content, and mycorrhizal dependency of L. iberica plants. This study exhibited the significant effects of single and simultaneous applications of F. mosseae and Thiobacillus on plant growth and yield in saline soils. © 2020 Society of Chemical Industry.

Bactericidal fully human single-chain fragment variable antibodies protect mice against methicillin-resistant Staphylococcus aureus bacteraemia

OBJECTIVES

The increasing prevalence of antibiotic-resistant Staphylococcus aureus, besides the inadequate numbers of effective antibiotics, emphasises the need to find new therapeutic agents against this lethal pathogen.

METHODS

In this study, to obtain antibody fragments against S. aureus, a human single-chain fragment variable (scFv) library was enriched against living methicillin-resistant S. aureus (MRSA) cells, grown in three different conditions, that is human peripheral blood mononuclear cells with plasma, whole blood and biofilm. The antibacterial activity of scFvs was evaluated by the growth inhibition assay in vitro. Furthermore, the therapeutic efficacy of anti-S. aureus scFvs was appraised in a mouse model of bacteraemia.

RESULTS

Three scFv antibodies, that is MEH63, MEH158 and MEH183, with unique sequences, were found, which exhibited significant binding to S. aureus and reduced the viability of S. aureus in in vitro inhibition assays. Based on the results, MEH63, MEH158 and MEH183, in addition to their combination, could prolong the survival rate, reduce the bacterial burden in the blood and prevent inflammation and tissue destruction in the kidneys and spleen of mice with MRSA bacteraemia compared with the vehicle group (treated with normal saline).

CONCLUSION

The combination therapy with anti-S. aureus scFvs and conventional antibiotics might shed light on the treatment of patients with S. aureus infections.

Beyond Homeostasis: Potassium and Pathogenesis during Bacterial Infections

Potassium is an essential mineral nutrient required by all living cells for normal physiological function. Therefore, maintaining intracellular potassium homeostasis during bacterial infection is a requirement for the survival of both host and pathogen. However, pathogenic bacteria require potassium transport to fulfill nutritional and chemiosmotic requirements, and potassium has been shown to directly modulate virulence gene expression, antimicrobial resistance, and biofilm formation. Host cells also require potassium to maintain fundamental biological processes, such as renal function, muscle contraction, and neuronal transmission; however, potassium flux also contributes to critical immunological and antimicrobial processes, such as cytokine production and inflammasome activation. Here, we review the role and regulation of potassium transport and signaling during infection in both mammalian and bacterial cells and highlight the importance of potassium to the success and survival of each organism.

A single residue deletion in the barley HKT1;5 P189 variant restores plasma membrane localisation but not Na+ conductance

Leaf Na⁺ exclusion, mediated by plasma membrane-localised Class 1 High-affinity potassium (K⁺) Transporters (HKTs), is a key mechanism contributing to salinity tolerance of several major crop plants. We determined previously that the leucine to proline residue substitution at position 189 (L189P) in barley HvHKT1;5 disrupts its characteristic plasma membrane localisation and Na⁺ conductance. Here, we focus on a surprising observation that a single residue deletion of methionine at position 372 (M372del) within the conserved VMMYL motif in plant HKTs, restores plasma membrane localisation but not Na⁺ conductance in HvHKT1;5 P189. To clarify why the singular M372 deletion regains plasma membrane localisation, we built 3D models and defined α-helical assembly pathways of the P189 M372del mutant, and compared these findings to the wild-type protein, and the HvHKT1;5 L189 variant and its M372del mutant. We find that α-helical association and assembly pathways in HvHKT1;5 proteins fall in two contrasting categories. Inspections of structural flexibility through molecular dynamics simulations revealed that the conformational states of HvHKT1;5 P189 diverge from those of the L189 variant and M372del mutants. We propose that M372del in HvHKT1;5 P189 instigates structural rearrangements allowing routing to the plasma membrane, while the restoration of conductance would require further interventions. We integrate the microscopy, electrophysiology, and biocomputational data and discuss how a profound structural change in HvHKT1;5 P189 M372del impacts its α-helical protein association pathway and flexibility, and how these features underlie a delicate balance leading to restoring plasma membrane localisation but not Na⁺ conductance.

The response mechanism to salt stress in Arabidopsis transgenic lines over-expressing of GmG6PD

Glucose-6-phosphate dehydrogenase (G6PD or G6PDH) plays an important role in response to salt stress in plants. However, much less is known about G6PD proteins in soybean (Glycine max L.). Here, we found that a soybean cytosolic G6PD gene, GmG6PD7, was induced by NaCl. We generated Arabidopsis transgenic lines overexpressing GmG6PD7. The seed germination rate and primary root length of Arabidopsis thaliana over-expressing GmG6PD7 under NaCl treatment were enhanced. Salt stress induced an obvious increase of the total and cytosolic G6PD activity and the marked decrease of ROS levels in the transgenic plants. At the same time, over-expressing GmG6PD7 in Arabidopsis affected the glutathione and NADPH level and activated ROS scavengers, suggesting that GmG6PD7 contributes to increase salinity tolerance by decreasing ROS accumulation. What's more, we found GmG6PD7 overexpression led to the up-regulation of abscisic acid (ABA) degradation gene and the down-regulation of ABA synthesis and ABA-responsive genes, which finally reduced ABA content to improve seed germination rate under salinity stress. It was noteworthy that GmG6PD7 can rescue the seed and root phenotype of Arabidopsis cytosolic G6PD mutant (Atg6pd5 and Atg6pd6) under salt stress, suggesting cytosolic G6PD may have a conserved function in soybean and Arabidopsis.

The cross-kingdom roles of mineral nutrient transporters in plant-microbe relations

The regulation of plant physiology by plant mineral nutrient transporter (MNT) is well understood. Recently, the extensive characterization of beneficial and pathogenic plant-microbe interactions has defined the roles for MNTs in such relationships. In this review, we summarize the roles of diverse nutrient transporters in the symbiotic or pathogenic relationships between plants and microorganisms. In doing so, we highlight how MNTs of plants and microbes can act in a coordinated manner. In symbiotic relationships, MNTs play key roles in the establishment of the interaction between the host plant and rhizobium or mycorrhizae as well in the subsequent coordinated transport of nutrients. Additionally, MNTs may also regulate the colonization or degeneration of symbiotic microorganisms by reflecting the nutrient status of the plant and soil. This allows the host plant obtain nutrients from the soil in the most optimal manner. With pathogenic-interactions, MNTs influence pathogen proliferation, the efficacy of the host's biochemical defense and related signal transduction mechanisms. We classify the MNT effects in plant-pathogen interactions as either indirect by influencing the nutrient status and fitness of the pathogen, or direct by initiating host defense mechanisms. While such observations indicate the fundamental importance of MNTs in governing the interactions with a range of microorganisms, further work is needed to develop an integrative understanding of their functions.

Molecular Mechanisms for Bacterial Potassium Homeostasis

Potassium ion homeostasis is essential for bacterial survival, playing roles in osmoregulation, pH homeostasis, regulation of protein synthesis, enzyme activation, membrane potential adjustment and electrical signaling. To accomplish such diverse physiological tasks, it is not surprising that a single bacterium typically encodes several potassium uptake and release systems. To understand the role each individual protein fulfills and how these proteins work in concert, it is important to identify the molecular details of their function. One needs to understand whether the systems transport ions actively or passively, and what mechanisms or ligands lead to the activation or inactivation of individual systems. Combining mechanistic information with knowledge about the physiology under different stress situations, such as osmostress, pH stress or nutrient limitation, one can identify the task of each system and deduce how they are coordinated with each other. By reviewing the general principles of bacterial membrane physiology and describing the molecular architecture and function of several bacterial K⁺-transporting systems, we aim to provide a framework for microbiologists studying bacterial potassium homeostasis and the many K⁺-translocating systems that are still poorly understood.

Fluorometric Liposome Screen for Inhibitors of a Physiologically Important Bacterial Ion Channel

The bacterial K⁺ homeostasis machinery is widely conserved across bacterial species, and different from that in animals. Dysfunction in components of the machinery has an impact on intracellular turgor, membrane potential, adaptation to changes in both extracellular pH and osmolarity, and in virulence. Using a fluorescence-based liposome flux assay, we have performed a high-throughput screen to identify novel inhibitors of the KtrAB ion channel complex from Bacillus subtilis, a component of the K⁺ homeostasis machinery that is also present in many bacterial pathogens. The screen identified 41 compounds that inhibited K⁺ flux and that clustered into eight chemical groups. Many of the identified inhibitors were found to target KtrAB with an in vitro potency in the low μM range. We investigated the mechanisms of inhibition and found that most molecules affected either the membrane component of the channel, KtrB alone or the full KtrAB complex without a preference for the functional conformation of the channel, thus broadening their inhibitory action. A urea derivative molecule that inhibited the membrane component of KtrAB affected cell viability in conditions in which KtrAB activity is essential. With this proof-of-concept study, we demonstrate that targeting components of the K⁺ homeostasis machinery has the potential as a new antibacterial strategy and that the fluorescence-based flux assay is a robust tool for screening chemical libraries.

GmAKT1 is involved in K+ uptake and Na+/K+ homeostasis in Arabidopsis and soybean plants

Plant roots absorb K+ from soil via K+ channels and transporters, which are important for stress responses. In this research, GmAKT1, an AKT1-type K+ channel, was isolated and characterized. The expression of GmAKT1 was induced by K+-starvation and salinity stresses, and it was preferentially expressed in the soybean roots. And GmAKT1 was located in the plasma membrane. As an inward K+ channel, GmAKT1 participated in K+ uptake, as well as rescued the low-K+-sensitive phenotype of the yeast mutant and Arabidopsis akt1 mutant. Overexpression of GmAKT1 significantly improved the growth of plants and increased K+ concentration, leading to lower Na+/K+ ratios in transgenic Arabidopsis and chimeric soybean plants with transgenic hairy roots. In addition, GmAKT1 overexpression resulted in significant upregulation of these ion uptake-related genes, including GmSKOR, GmsSOS1, GmHKT1, and GmNHX1. Our findings suggested that GmAKT1 plays an important part in K+ uptake under low-K+ condition, and could maintain Na+/K+ homeostasis under salt stress in Arabidopsis and soybean plants.

Plant HKT Channels: An Updated View on Structure, Function and Gene Regulation

HKT channels are a plant protein family involved in sodium (Na⁺) and potassium (K⁺) uptake and Na⁺-K⁺ homeostasis. Some HKTs underlie salt tolerance responses in plants, while others provide a mechanism to cope with short-term K⁺ shortage by allowing increased Na⁺ uptake under K⁺ starvation conditions. HKT channels present a functionally versatile family divided into two classes, mainly based on a sequence polymorphism found in the sequences underlying the selectivity filter of the first pore loop. Physiologically, most class I members function as sodium uniporters, and class II members as Na⁺/K⁺ symporters. Nevertheless, even within these two classes, there is a high functional diversity that, to date, cannot be explained at the molecular level. The high complexity is also reflected at the regulatory level. HKT expression is modulated at the level of transcription, translation, and functionality of the protein. Here, we summarize and discuss the structure and conservation of the HKT channel family from algae to angiosperms. We also outline the latest findings on gene expression and the regulation of HKT channels.

Plant mineral transport systems and the potential for crop improvement

Nutrient transporter genes could be a potential candidate for improving crop plants, with enhanced nutrient uptake leading to increased crop yield by providing tolerance against different biotic and abiotic stresses. The world's food supply is nearing a crisis in meeting the demands of an ever-growing global population, and an increase in both yield and nutrient value of major crops is vitally necessary to meet the increased population demand. Nutrients play an important role in plant metabolism as well as growth and development, and nutrient deficiency results in retarded plant growth and leads to reduced crop yield. A variety of cellular processes govern crop plant nutrient absorption from the soil. Among these, nutrient membrane transporters play an important role in the acquisition of nutrients from soil and transport of these nutrients to their target sites. In addition, as excess nutrient delivery has toxic effects on plant growth, these membrane transporters also play a significant role in the removal of excess nutrients in the crop plant. The key function provided by membrane transporters is the ability to supply the crop plant with an adequate level of tolerance against environmental stresses, such as soil acidity, alkalinity, salinity, drought, and pathogen attack. Membrane transporter genes have been utilized for the improvement of crop plants, with enhanced nutrient uptake leading to increased crop yield by providing tolerance against different biotic and abiotic stresses. Further understanding of the basic mechanisms of nutrient transport in crop plants could facilitate the advanced design of engineered plant crops to achieve increased yield and improve nutrient quality through the use of genetic technologies as well as molecular breeding. This review is focused on nutrient toxicity and tolerance mechanisms in crop plants to aid in understanding and addressing the anticipated global food demand.

The Many Roles of the Bacterial Second Messenger Cyclic di-AMP in Adapting to Stress Cues

Bacteria respond to changes in environmental conditions through adaptation to external cues. Frequently, bacteria employ nucleotide signaling molecules to mediate a specific, rapid response. Cyclic di-AMP (c-di-AMP) was recently discovered to be a bacterial second messenger that is essential for viability in many species. In this review, we highlight recent work that has described the roles of c-di-AMP in bacterial responses to various stress conditions. These studies show that depending on the lifestyle and environmental niche of the bacterial species, the c-di-AMP signaling network results in diverse outcomes, such as regulating osmolyte transport, controlling plant attachment, or providing a checkpoint for spore formation. c-di-AMP achieves this signaling specificity through expression of different classes of synthesis and catabolic enzymes as well as receptor proteins and RNAs, which will be summarized.

Fungal Shaker-like channels beyond cellular K+ homeostasis: A role in ectomycorrhizal symbiosis between Hebeloma cylindrosporum and Pinus pinaster

Potassium (K+) acquisition, translocation and cellular homeostasis are mediated by various membrane transport systems in all organisms. We identified and described an ion channel in the ectomycorrhizal fungus Hebeloma cylindrosporum (HcSKC) that harbors features of animal voltage-dependent Shaker-like K+ channels, and investigated its role in both free-living hyphae and symbiotic conditions. RNAi lines affected in the expression of HcSKC were produced and used for in vitro mycorrhizal assays with the maritime pine as host plant, under standard or low K+ conditions. The adaptation of H. cylindrosporum to the downregulation of HcSKC was analyzed by qRT-PCR analyses for other K+-related transport proteins: the transporters HcTrk1, HcTrk2, and HcHAK, and the ion channels HcTOK1, HcTOK2.1, and HcTOK2.2. Downregulated HcSKC transformants displayed greater K+ contents at standard K+ only. In such conditions, plants inoculated with these transgenic lines were impaired in K+ nutrition. Taken together, these results support the hypothesis that the reduced expression of HcSKC modifies the pool of fungal K+ available for the plant and/or affects its symbiotic transfer to the roots. Our study reveals that the maintenance of K+ transport in H. cylindrosporum, through the regulation of HcSKC expression, is required for the K+ nutrition of the host plant.

High affinity Na+ transport by wheat HKT1;5 is blocked by K. PLANT DIRECT 2020;4:e00275. [PMID: 33103046 PMCID: PMC7576878 DOI: 10.1002/pld3.275]

The wheat sodium transporters TmHKT1;5-A and TaHKT1;5-D are encoded by genes underlying the major shoot Na+ exclusion loci Nax2 and Kna1 from Triticum monococcum (Tm) and Triticum aestivum (Ta), respectively. In contrast to HKT2 transporters that have been shown to exhibit high affinity K+-dependent Na+ transport, HKT1 proteins have, with one exception, only been shown to catalyze low affinity Na+ transport and no K+ transport. Here, using heterologous expression in Xenopus laevis oocytes we uncover a novel property of HKT1 proteins, that both TmHKT1;5-A and TaHKT1;5-D encode dual (high and low) affinity Na+-transporters with the high-affinity component being abolished when external K+ is in excess of external Na+. Three-dimensional structural modeling suggested that, compared to Na+, K+ is bound more tightly in the selectivity filter region by means of additional van der Waals forces, which is likely to explain the K+ block at the molecular level. The low-affinity component for Na+ transport of TmHKT1;5-A had a lower K m than that of TaHKT1;5-D and was less sensitive to external K+. We propose that these properties contribute towards the improvements in shoot Na+-exclusion and crop plant salt tolerance following the introgression of TmHKT1;5-A into diverse wheat backgrounds.

The bacterial potassium transporter gene MbtrkH improves K+ uptake in yeast and tobacco

K+ is an essential nutrient for plant growth and is responsible for many important physiological processes. K+ deficiency leads to crop yield losses, and overexpression of K+ transporter genes has been proven to be an effective way to resolve this problem. However, current research on the overexpression of K+ transporter genes is limited to plant sources. TrkH is a bacterial K+ transporter whose function generally depends on the regulation of TrkA. To date, whether TrkH can improve K+ uptake in eukaryotic organisms is still unknown. In this study, a novel MbtrkH gene was cloned from marine microbial metagenomic DNA. Functional complementation and K+-depletion analyses revealed that MbTrkH functions in K+ uptake in the K+-deficient yeast strain CY162. Moreover, K+-depletion assays revealed that MbtrkH overexpression improves plant K+ uptake. K+ hydroponic culture experiments showed that, compared with WT tobacco lines, MbtrkH transgenic tobacco lines had significantly greater fresh weights, dry weights and K+ contents. These results indicate that MbTrkH promotes K+ uptake independently of TrkA in eukaryotes and provide a new strategy for improving K+-use efficiency in plants.

Halophytic Hordeum brevisubulatum HbHAK1 Facilitates Potassium Retention and Contributes to Salt Tolerance

Potassium retention under saline conditions has emerged as an important determinant for salt tolerance in plants. Halophytic Hordeum brevisubulatum evolves better strategies to retain K⁺ to improve high-salt tolerance. Hence, uncovering K⁺-efficient uptake under salt stress is vital for understanding K⁺ homeostasis. HAK/KUP/KT transporters play important roles in promoting K⁺ uptake during multiple stresses. Here, we obtained nine salt-induced HAK/KUP/KT members in H. brevisubulatum with different expression patterns compared with H. vulgare through transcriptomic analysis. One member HbHAK1 showed high-affinity K⁺ transporter activity in athak5 to cope with low-K⁺ or salt stresses. The expression of HbHAK1 in yeast Cy162 strains exhibited strong activities in K⁺ uptake under extremely low external K⁺ conditions and reducing Na⁺ toxicity to maintain the survival of yeast cells under high-salt-stress. Comparing with the sequence of barley HvHAK1, we found that C170 and R342 in a conserved domain played pivotal roles in K⁺ selectivity under extremely low-K⁺ conditions (10 μM) and that A13 was responsible for the salt tolerance. Our findings revealed the mechanism of HbHAK1 for K⁺ accumulation and the significant natural adaptive sites for HAK1 activity, highlighting the potential value for crops to promote K⁺-uptake under stresses.

Changes in Expression Level of OsHKT1;5 Alters Activity of Membrane Transporters Involved in K+ and Ca2+ Acquisition and Homeostasis in Salinized Rice Roots

In rice, the OsHKT1;5 gene has been reported to be a critical determinant of salt tolerance. This gene is harbored by the SKC1 locus, and its role was attributed to Na+ unloading from the xylem. No direct evidence, however, was provided in previous studies. Also, the reported function of SKC1 on the loading and delivery of K+ to the shoot remains to be explained. In this work, we used an electrophysiological approach to compare the kinetics of Na+ uptake by root xylem parenchyma cells using wild type (WT) and NIL(SKC1) plants. Our data showed that Na+ reabsorption was observed in WT, but not NIL(SKC1) plants, thus questioning the functional role of HKT1;5 as a transporter operating in the direct Na+ removal from the xylem. Instead, changes in the expression level of HKT1;5 altered the activity of membrane transporters involved in K+ and Ca2+ acquisition and homeostasis in the rice epidermis and stele, explaining the observed phenotype. We conclude that the role of HKT1;5 in plant salinity tolerance cannot be attributed to merely reducing Na+ concentration in the xylem sap but triggers a complex feedback regulation of activities of other transporters involved in the maintenance of plant ionic homeostasis and signaling under stress conditions.

Supportive role of the Na+ transporter CmHKT1;1 from Cucumis melo in transgenic Arabidopsis salt tolerance through improved K+/Na+ balance

KEY MESSAGE

CmHKT1;1 selectively exports Na+ from plant cells. Upon NaCl stress, its expression increased in a salt-tolerant melon cultivar. Overexpression of CmHKT1;1 increased transgenic Arabidopsis salt tolerance through improved K+/Na+ balance. High-affinity K+ transporters (HKTs) are thought to be involved in reducing Na+ in plant shoots under salt stress and modulating salt tolerance, but their function in a moderately salt-tolerant species of melon (Cucumis melo L.) remains unclear. In this study, a Na+ transporter gene, CmHKT1;1 (GenBank accession number: MK986658), was isolated from melons based on genome data. The transcript of CmHKT1;1 was relatively more abundant in roots than in stems or leaves from melon seedlings. The tobacco transient expression system showed that CmHKT1;1 was plasma-membrane localized. Upon salt stress, CmHKT1;1 expression was more strongly upregulated in a salt-tolerant melon cultivar, 'Bingxuecui' (BXC) compared with a salt-sensitive cultivar, 'Yulu' (YL). Electrophysiological evidence demonstrated that CmHKT1;1 only transported Na+, rather than K+, when expressed in Xenopus laevis oocytes. Overexpression of CmHKT1;1 increased salt sensitivity in Saccharomyces cerevisiae and salt tolerance in Arabidopsis thaliana. Under NaCl treatments, transgenic Arabidopsis plants accumulated significantly lower concentrations of Na+ in shoots than wild type plants and showed a better K+/Na+ balance, leading to better Fv/Fm, root length, biomass, and enhanced plant growth. The CmHKT1;1 gene may serve as a useful candidate for improving crop salt tolerance.

Everybody loves cheese: crosslink between persistence and virulence of Shiga-toxin Escherichia coli

General cheese manufacturing involves high temperatures, fermentation and ripening steps that function as hurdles to microbial growth. On the other hand, the application of several different formulations and manufacturing techniques may create a bacterial protective environment. In cheese, the persistent behavior of Shiga toxin-producing Escherichia coli (STEC) relies on complex mechanisms that enable bacteria to respond to stressful conditions found in cheese matrix. In this review, we discuss how STEC manages to survive to high and low temperatures, hyperosmotic conditions, exposure to weak organic acids, and pH decreasing related to cheese manufacturing, the cheese matrix itself and storage. Moreover, we discuss how these stress responses interact with each other by enhancing adaptation and consequently, the persistence of STEC in cheese. Further, we show how virulence genes eae and tir are affected by stress response mechanisms, increasing either cell adherence or virulence factors production, which leads to a selection of more resistant and virulent pathogens in the cheese industry, leading to a public health issue.

The RNA-seq transcriptomic analysis reveals genes mediating salt tolerance through rapid triggering of ion transporters in a mutant barley

Considering the complex nature of salinity tolerance mechanisms, the use of isogenic lines or mutants possessing the same genetic background albeit different tolerance to salinity is a suitable method for reduction of analytical complexity to study these mechanisms. In the present study, whole transcriptome analysis was evaluated using RNA-seq method between a salt-tolerant mutant line "M4-73-30" and its wild-type "Zarjou" cultivar at seedling stage after six hours of exposure to salt stress (300 mM NaCl). Transcriptome sequencing yielded 20 million reads for each genotype. A total number of 7116 transcripts with differential expression were identified, 1586 and 1479 of which were obtained with significantly increased expression in the mutant and the wild-type, respectively. In addition, the families of WRKY, ERF, AP2/EREBP, NAC, CTR/DRE, AP2/ERF, MAD, MIKC, HSF, and bZIP were identified as the important transcription factors with specific expression in the mutant genotype. The RNA-seq results were confirmed at several time points using qRT-PCR for some important salt-responsive genes. In general, the results revealed that the mutant accumulated higher levels of sodium ion in the root and decreased its transfer to the shoot. Also, the mutant increased the amount of potassium ion leading to the maintenance a high ratio [K+]/[Na+] in the shoot compared to its wild-type via fast stomata closure and consequently transpiration reduction under the salt stress. Moreover, a reduction in photosynthesis and respiration was observed in the mutant, resulting in utilization of the stored energy and the carbon for maintaining the plant tissues, which is considered as a mechanism of salt tolerance in plants. Up-regulation of catalase, peroxidase, and ascorbate peroxidase genes has resulted in higher accumulation of H2O2 in the wild-type compared to the mutant. Therefore, the wild-type initiated rapid ROS signals which led to less oxidative scavenging in comparison with the mutant. The mutant increased expression in the ion transporters and the channels related to the salinity to maintain the ion homeostasis. In overall, the results demonstrated that the mutant responded better to the salt stress under both osmotic and ionic stress phases and lower damage was observed in the mutant compared to its wild-type under the salt stress.