Expression of KT/KUP genes in Arabidopsis and the role of root hairs in K+ uptake

A charged existence: A century of transmembrane ion transport in plants

If the past century marked the birth of membrane transport as a focus for research in plants, the past 50 years has seen the field mature from arcane interest to a central pillar of plant physiology. Ion transport across plant membranes accounts for roughly 30% of the metabolic energy consumed by a plant cell, and it underpins virtually every aspect of plant biology, from mineral nutrition, cell expansion, and development to auxin polarity, fertilization, plant pathogen defense, and senescence. The means to quantify ion flux through individual transporters, even single channel proteins, became widely available as voltage clamp methods expanded from giant algal cells to the fungus Neurospora crassa in the 1970s and the cells of angiosperms in the 1980s. Here, I touch briefly on some key aspects of the development of modern electrophysiology with a focus on the guard cells of stomata, now without dispute the premier plant cell model for ion transport and its regulation. Guard cells have proven to be a crucible for many technical and conceptual developments that have since emerged into the mainstream of plant science. Their study continues to provide fundamental insights and carries much importance for the global challenges that face us today.

Genome-wide identification and expression analysis of the HAK/KUP/KT gene family in Moso bamboo

The K+ uptake permease/high-affinity K+/K+ transporter (KUP/HAK/KT) family is the most prominent group of potassium (K+) transporters, playing a key role in K+ uptake, transport, plant growth and development, and stress tolerance. However, the presence and functions of the KUP/HAK/KT family in Moso bamboo (Phyllostachys edulis (Carriere) J. Houzeau), the fastest-growing plant, have not been studied. In this study, we identified 41 KUP/HAK/KT genes (PeHAKs) distributed across 18 chromosomal scaffolds of the Moso bamboo genome. PeHAK is a typical membrane protein with a conserved structural domain and motifs. Phylogenetic tree analysis classified PeHAKs into four distinct clusters, while collinearity analysis revealed gene duplications resulting from purifying selection, including both tandem and segmental duplications. Enrichment analysis of promoter cis-acting elements suggested their plausible role in abiotic stress response and hormone induction. Transcriptomic data and STEM analyses indicated that PeHAKs were involved in tissue and organ development, rapid growth, and responded to different abiotic stress conditions. Subcellular localization analysis demonstrated that PeHAKs are predominantly expressed at the cell membrane. In-situ PCR experiments confirmed that PeHAK was mainly expressed in the lateral root primordia. Furthermore, the involvement of PeHAKs in potassium ion transport was confirmed by studying the potassium ion transport properties of a yeast mutant. Additionally, through homology modeling, we revealed the structural properties of HAK as a transmembrane protein associated with potassium ion transport. This research provides a solid basis for understanding the classification, characterization, and functional analysis of the PeHAK family in Moso bamboo.

Positive Regulatory Roles of Manihot esculenta HAK5 under K+ Deficiency or High Salt Stress

HAK/KUP/KT family members have been identified as playing key roles in K+ uptake and salt tolerance in numerous higher plants. However, their functions in cassava (Manihot esculenta Cantz) remain unknown. In this study, a gene encoding for a high-affinity potassium transporter (MeHAK5) was isolated from cassava and its function was investigated. Subcellular localization analysis showed that MeHAK5 is a plasma membrane-localized transporter. RT-PCR and RT-qPCR indicated that MeHAK5 is predominantly expressed in cassava roots, where it is upregulated by low potassium or high salt; in particular, its highest expression levels separately increased by 2.2 and 2.9 times after 50 µM KCl and 150 mM NaCl treatments. When heterologously expressed in yeast, MeHAK5 mediated K+ uptake within the cells of the yeast strain CY162 and rescued the salt-sensitive phenotype of AXT3K yeast. MeHAK5 overexpression in transgenic Arabidopsis plants exhibited improved growth and increased shoot K+ content under low potassium conditions. Under salt stress, MeHAK5 transgenic Arabidopsis plants accumulated more K+ in the shoots and roots and had reduced Na+ content in the shoots. As a result, MeHAK5 transgenic Arabidopsis demonstrated a more salt-tolerant phenotype. These results suggest that MeHAK5 functions as a high-affinity K+ transporter under K+ starvation conditions, improving K+/Na+ homeostasis and thereby functioning as a positive regulator of salt stress tolerance in transgenic Arabidopsis. Therefore, MeHAK5 may be a suitable candidate gene for improving K+ utilization efficiency and salt tolerance.

Comparative Transcriptome Analysis Reveals the Genes and Pathways Related to Wheat Root Hair Length

Tube-like outgrowths from root epidermal cells, known as root hairs, enhance water and nutrient absorption, facilitate microbial interactions, and contribute to plant anchorage by expanding the root surface area. Genetically regulated and strongly influenced by environmental conditions, longer root hairs generally enhance water and nutrient absorption, correlating with increased stress resistance. Wheat, a globally predominant crop pivotal for human nutrition, necessitates the identification of long root hair genotypes and their regulatory genes to enhance nutrient capture and yield potential. This study focused on 261 wheat samples of diverse genotypes during germination, revealing noticeable disparities in the length of the root hair among the genotypes. Notably, two long root hair genotypes (W106 and W136) and two short root hair genotypes (W90 and W100) were identified. Transcriptome sequencing resulted in the development of 12 root cDNA libraries, unveiling 1180 shared differentially expressed genes (DEGs). Further analyses, including GO function annotation, KEGG enrichment, MapMan metabolic pathway analysis, and protein-protein interaction (PPI) network prediction, underscored the upregulation of root hair length regulatory genes in the long root hair genotypes. These included genes are associated with GA and BA hormone signaling pathways, FRS/FRF and bHLH transcription factors, phenylpropanoid, lignin, lignan secondary metabolic pathways, the peroxidase gene for maintaining ROS steady state, and the ankyrin gene with diverse biological functions. This study contributes valuable insights into modulating the length of wheat root hair and identifies candidate genes for the genetic improvement of wheat root traits.

Melatonin Modulates Tomato Root Morphology by Regulating Key Genes and Endogenous Hormones

Melatonin plays a vital role in plant growth and development. In this study, we treated hydroponically grown tomato roots with various concentrations of exogenous melatonin (0, 10, 30, and 50 μmol·L-1). We utilized root scanning and microscopy to examine alterations in root morphology and cell differentiation and elucidated the mechanism by which melatonin regulates these changes through the interplay with endogenous hormones and relevant genes. The results showed that for melatonin at concentrations ranging between 10 and 30 μmol·L-1, the development of lateral roots were significantly stimulated, the root hair growth was enhanced, and biomass accumulation and root activity were increased. Furthermore, we elucidated that melatonin acts as a mediator for the expression of genes, such as SlCDKA1, SlCYCA3;1, SlARF2, SlF3H, and SlKT1, which are involved in the regulation of root morphology changes. Additionally, we observed that melatonin influences the levels of endogenous hormones, including ZT, GA3, IAA, ABA, and BR, which subsequently impact the root morphology development of tomato roots. In summary, this study shows that tomato root morphology can be promoted by the optimal concentration of exogenous melatonin (10-30 μmol·L-1).

Arabidopsis transcriptomic analysis reveals cesium inhibition of root growth involves abscisic acid signaling

MAIN CONCLUSION

This is the first report on the involvement of abscisic acid signaling in regulating post-germination growth under Cs stress, not related to potassium deficiency. Cesium (Cs) is known to exert toxicity in plants by competition and interference with the transport of potassium (K). However, the precise mechanism of how Cs mediates its damaging effect is still unclear. This fact is mainly attributed to the large effects of lower K uptake in the presence of Cs that shadow other crucial effects by Cs that were not related to K. RNA-seq was conducted on Arabidopsis roots grown to identify putative genes that are functionally involved to investigate the difference between Cs stress and low K stress. Our transcriptome data demonstrated Cs-regulated genes only partially overlap to low K-regulated genes. In addition, the divergent expression trend of High-affinity K+ Transporter (HAK5) from D4 to D7 growth stage suggested participation of other molecular events besides low K uptake under Cs stress. Potassium deficiency triggers expression level change of the extracellular matrix, transfer/carrier, cell adhesion, calcium-binding, and DNA metabolism genes. Under Cs stress, genes encoding translational proteins, chromatin regulatory proteins, membrane trafficking proteins and defense immunity proteins were found to be primarily regulated. Pathway enrichment and protein network analyses of transcriptome data exhibit that Cs availability are associated with alteration of abscisic acid (ABA) signaling, photosynthesis activities and nitrogen metabolism. The phenotype response of ABA signaling mutants supported the observation and revealed Cs inhibition of root growth involved in ABA signaling pathway. The rather contrary response of loss-of-function mutant of Late Embryogenesis Abundant 7 (LEA7) and Translocator Protein (TSPO) further suggested low K stress and Cs stress may activate different salt tolerance responses. Further investigation on the crosstalk between K transport, signaling, and salt stress-responsive signal transduction will provide a deeper understanding of the mechanisms and molecular regulation underlying Cs toxicity.

The Molecular Mechanism of Potassium Absorption, Transport, and Utilization in Rice

Potassium is essential for plant growth and development and stress adaptation. The maintenance of potassium homeostasis involves a series of potassium channels and transporters, which promote the movement of potassium ions (K+) across cell membranes and exhibit complex expression patterns and regulatory mechanisms. Rice is a major food crop in China. The low utilization rate of potassium fertilizer limits the yield and quality of rice. Elucidating the molecular mechanisms of potassium absorption, transport, and utilization is critical in improving potassium utilization efficiency in rice. Although some K+ transporter genes have been identified from rice, research on the regulatory network is still in its infancy. Therefore, this review summarizes the relevant information on K+ channels and transporters in rice, covering the absorption of K+ in the roots, transport to the shoots, the regulation pathways, the relationship between K+ and the salt tolerance of rice, and the synergistic regulation of potassium, nitrogen, and phosphorus signals. The related research on rice potassium nutrition has been comprehensively reviewed, the existing research foundation and the bottleneck problems to be solved in this field have been clarified, and the follow-up key research directions have been pointed out to provide a theoretical framework for the cultivation of potassium-efficient rice.

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.

Role of Potassium-Dependent Alternative Splicing of MYB59 in the Maintenance of Potassium Concentration in Shoots of Arabidopsis thaliana

Potassium (K) is a major plant nutrient. K+ is taken up by channel and transporter proteins in roots and translocated from roots to shoots via the xylem. In Arabidopsis thaliana, the K+ transporter NPF7.3 mediates K+ loading into the xylem and the transcription factor MYB59 is responsible for NPF7.3 expression. Here, we demonstrate that MYB59 is regulated by alternative splicing in response to K availability. Three splicing isoforms of MYB59 are detected in roots: an isoform with the first intron spliced out encodes a protein with the full DNA-binding motif (MYB59α), and two isoforms with the first intron retained partially or completely encode a protein missing part of the DNA-binding motif (MYB59β). Functional analysis showed that only MYB59α is capable of inducing the expression of NPF7.3. The abundance of the MYB59α isoform increased under low K, but the total abundance of MYB59 transcripts did not change, indicating that MYB59α is increased by modification of the splicing pattern in response to low K. Although MYB59α is increased by low K, NPF7.3 expression remained constant independent of K. In addition, there was no significant difference in NPF7.3 expression between an MYB59 knockout mutant and the wild type under normal K. These results suggest that an unknown mechanism is involved in NPF7.3 expression under normal K and switches roles with MYB59 under low K. We propose that the regulation of MYB59 by alternative splicing is required for the maintenance of shoot K concentration in adaptation to low K.

Dynamic transcriptome analysis unravels key regulatory genes of maize root growth and development in response to potassium deficiency

MAIN CONCLUSION

Integrated root phenotypes and transcriptome analysis have revealed key candidate genes responsible for maize root growth and development in potassium deficiency. Potassium (K) is a vital macronutrient for plant growth, but our understanding of its regulatory mechanisms in maize root system architecture (RSA) and K+ uptake remains limited. To address this, we conducted hydroponic and field trials at different growth stages. K+ deficiency significantly inhibited maize root growth, with metrics like total root length, primary root length, width and maximum root number reduced by 50% to 80% during early seedling stages. In the field, RSA traits exhibited maximum values at the silking stage but continued to decline thereafter. Furthermore, K deprivation had a pronounced negative impact on root morphology and RSA growth and grain yield. RNA-Seq analysis identified 5972 differentially expressed genes (DEGs), including 17 associated with K+ signaling, transcription factors, and transporters. Weighted gene co-expression network analysis revealed 23 co-expressed modules, with enrichment of transcription factors at different developmental stages under K deficiency. Several DEGs and transcription factors were predicted as potential candidate genes responsible for maize root growth and development. Interestingly, some of these genes exhibited homology to well-known regulators of root architecture or development in Arabidopsis, such as Zm00001d014467 (AtRCI3), Zm00001d011237 (AtWRKY9), and Zm00001d030862 (AtAP2/ERF). Identifying these key genes helps to provide a deeper understanding of the molecular mechanisms governing maize root growth and development under nutrient deficient conditions offering potential benefits for enhancing maize production and improving stress resistance through targeted manipulation of RSA traits in modern breeding efforts.

Xylem K+ loading modulates K+ and Cs+ absorption and distribution in Arabidopsis under K+-limited conditions

Potassium (K+) is an essential macronutrient for plant growth. The transcriptional regulation of K+ transporter genes is one of the key mechanisms by which plants respond to K+ deficiency. Among the HAK/KUP/KT transporter family, HAK5, a high-affinity K+ transporter, is essential for root K+ uptake under low external K+ conditions. HAK5 expression in the root is highly induced by low external K+ concentration. While the molecular mechanisms of HAK5 regulation have been extensively studied, it remains unclear how plants sense and coordinates K+ uptake and translocation in response to changing environmental conditions. Using skor mutants, which have a defect in root-to-shoot K+ translocation, we have been able to determine how the internal K+ status affects the expression of HAK5. In skor mutant roots, under K+ deficiency, HAK5 expression was lower than in wild-type although the K+ concentration in roots was not significantly different. These results reveal that HAK5 is not only regulated by external K+ conditions but it is also regulated by internal K+ levels, which is in agreement with recent findings. Additionally, HAK5 plays a major role in the uptake of Cs+ in roots. Therefore, studying Cs+ in roots and having more detailed information about its uptake and translocation in the plant would be valuable. Radioactive tracing experiments revealed not only a reduction in the uptake of 137Cs+ and 42K+in skor mutants compared to wild-type but also a different distribution of 137Cs+ and 42K+ in tissues. In order to gain insight into the translocation, accumulation, and repartitioning of both K+ and Cs+ in plants, long-term treatment and split root experiments were conducted with the stable isotopes 133Cs+ and 85Rb+. Finally, our findings show that the K+ distribution in plant tissues regulates root uptake of K+ and Cs+ similarly, depending on HAK5; however, the translocation and accumulation of the two elements are different.

The Sweet Potato K+ Transporter IbHAK11 Regulates K+ Deficiency and High Salinity Stress Tolerance by Maintaining Positive Ion Homeostasis

The K⁺ transporter KT/HAK/KUP (K⁺ transporter/high-affinity K⁺/K⁺ uptake) family has a critical effect on K⁺ uptake and translocation in plants under different environmental conditions. However, the functional analysis of KT/HAK/KUP members in sweet potatoes is still limited. The present work reported the physiological activity of a new gene, IbHAK11, in the KT/HAK/KUP family in sweet potatoes. IbHAK11 expression increased significantly in the low K⁺-tolerant line compared with the low K⁺-sensitive line following treatment with low K⁺ concentrations. IbHAK11 upregulation promoted root growth in Arabidopsis under low K⁺ conditions. Under high saline stress, transgenic lines had superior growth and photosynthetic characteristics compared with the wild-type (WT). As for IbHAK11-overexpressing plants, activation of both the non-enzymatic and enzymatic reactive oxygen species (ROS) scavenging systems was observed. Therefore, IbHAK11-overexpressing plants had lower malondialdehyde (MDA) and ROS levels (including H₂O₂ and O^2-) compared with WT under salt-induced stress. We also found that under both low K⁺ and high salinity conditions, overexpression of IbHAK11 enhanced K⁺ translocation from the root to the shoot and decreased Na⁺ absorption in Arabidopsis. Consequently, IbHAK11 positively regulated K⁺ deficiency and high salinity stresses by regulating K⁺ translocation and Na⁺ uptake, thus maintaining K⁺/Na⁺ homeostasis in plants.

Transcriptional and metabolic responses of apple to different potassium environments

Potassium (K) is one of the most important macronutrients for plant development and growth. The influence mechanism of different potassium stresses on the molecular regulation and metabolites of apple remains largely unknown. In this research, physiological, transcriptome, and metabolite analyses were compared under different K conditions in apple seedlings. The results showed that K deficiency and excess conditions influenced apple phenotypic characteristics, soil plant analytical development (SPAD) values, and photosynthesis. Hydrogen peroxide (H₂O₂) content, peroxidase (POD) activity, catalase (CAT) activity, abscisic acid (ABA) content, and indoleacetic acid (IAA) content were regulated by different K stresses. Transcriptome analysis indicated that there were 2,409 and 778 differentially expressed genes (DEGs) in apple leaves and roots under K deficiency conditions in addition to 1,393 and 1,205 DEGs in apple leaves and roots under potassium excess conditions, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment showed that the DEGs were involved in flavonoid biosynthesis, photosynthesis, and plant hormone signal transduction metabolite biosynthetic processes in response to different K conditions. There were 527 and 166 differential metabolites (DMAs) in leaves and roots under low-K stress as well as 228 and 150 DMAs in apple leaves and roots under high-K stress, respectively. Apple plants regulate carbon metabolism and the flavonoid pathway to respond to low-K and high-K stresses. This study provides a basis for understanding the metabolic processes underlying different K responses and provides a foundation to improve the utilization efficiency of K in apples.

Characterization of HAK protein family in Casuarina equisetifolia and the positive regulatory role of CeqHAK6 and CeqHAK11 genes in response to salt tolerance

The potassium transporter group of the HAK/KUP/KT (high-affinity K⁺)/KUP (K⁺ uptake)/KT (K⁺ transporter) family plays a crucial role in plant growth and development as well as in environmental adaptation such as tolerance to salt stress. HAK/KUP/KT genes and their functions have been characterized for a number of plant species, but they remain unknown for Casuarina equisetifolia, an important tree species for coastal protection in southern China and many other countries. In this study, 25 HAK genes were identified in the C. equisetifolia genome. Their gene structure, conserved motif, phylogeny, and expression were comprehensively and systematically analyzed to understand their functions. All HAK genes were relatively conserved and could be divided into four clusters. The expression level of two particular genes, CeqHAK11 and CeqHAK6, increased significantly with the duration of salt treatment. To further elucidated their function in response to salt stress, subcellular localization, and their functional analysis were developed. Results revealed that CeqHAK11 and CeqHAK6 were localized on the plasma membrane, which mainly mediated high-affinity K⁺ uptake. Overexpression of CeqHAK6 or CeqHAK11 in Arabidopsis showed higher germination and survival rates and longer root length than wild-type (WT) under salt stress, suggesting that both genes improve tolerance to salt stress. Moreover, CeqHAK6 and CeqHAK11 improved their ability to tolerate salt stress by increasing the K⁺/Na⁺ ratio and antioxidant enzyme activities (CAT, POD, and SOD), and decreasing reactive oxygen species (ROS) accumulation. Consequently, CeqHAK6 and CeqHAK11 were verified as potassium transport proteins and could be applied for further molecular breeding for salt tolerance in C. equisetifolia or other crops to increasing salt tolerance.

Potassium nutrient status drives posttranslational regulation of a low-K response network in Arabidopsis

Under low-potassium (K+) stress, a Ca2+ signaling network consisting of calcineurin B-like proteins (CBLs) and CBL-interacting kinases (CIPKs) play essential roles. Specifically, the plasma membrane CBL1/9-CIPK pathway and the tonoplast CBL2/3-CIPK pathway promotes K+ uptake and remobilization, respectively, by activating a series of K+ channels. While the dual CBL-CIPK pathways enable plants to cope with low-K+ stress, little is known about the early events that link external K+ levels to the CBL-CIPK proteins. Here we show that K+ status regulates the protein abundance and phosphorylation of the CBL-CIPK-channel modules. Further analysis revealed low K+-induced activation of VM-CBL2/3 happened earlier and was required for full activation of PM-CBL1/9 pathway. Moreover, we identified CIPK9/23 kinases to be responsible for phosphorylation of CBL1/9/2/3 in plant response to low-K+ stress and the HAB1/ABI1/ABI2/PP2CA phosphatases to be responsible for CBL2/3-CIPK9 dephosphorylation upon K+-repletion. Further genetic analysis showed that HAB1/ABI1/ABI2/PP2CA phosphatases are negative regulators for plant growth under low-K+, countering the CBL-CIPK network in plant response and adaptation to low-K+ stress.

Wheat potassium transporter TaHAK13 mediates K+ absorption and maintains potassium homeostasis under low potassium stress

Potassium (K) is an essential nutrient for plant physiological processes. Members of the HAK/KUP/KT gene family act as potassium transporters, and the family plays an important role in potassium uptake and utilization in plants. In this study, the TaHAK13 gene was cloned from wheat and its function characterized. Real-time quantitative PCR (RT-qPCR) revealed that TaHAK13 expression was induced by environmental stress and up-regulated under drought (PEG6000), low potassium (LK), and salt (NaCl) stress. GUS staining indicated that TaHAK13 was mainly expressed in the leaf veins, stems, and root tips in Arabidopsis thaliana, and expression varied with developmental stage. TaHAK13 mediated K⁺ absorption when heterologously expressed in yeast CY162 strains, and its activity was slightly stronger than that of a TaHAK1 positive control. Subcellular localization analysis illustrated that TaHAK13 was located to the plasma membrane. When c(K⁺) ≤0.01 mM, the root length and fresh weight of TaHAK13 transgenic lines (athak5/TaHAK13, Col/TaHAK13) were significantly higher than those of non-transgenic lines (athak5, Col). Non-invasive micro-test technology (NMT) indicated that the net K influx of the transgenic lines was also higher than that of the non-transgenic lines. This suggests that TaHAK13 promotes K⁺ absorption, especially in low potassium media. Membrane-based yeast two-hybrid (MbY2H) and luciferase complementation assays (LCA) showed that TaHAK13 interacted with TaNPF5.10 and TaNPF6.3. Our findings have helped to clarify the biological functions of TaHAK13 and established a theoretical framework to dissect its function in wheat.

Potassium transporter KUP9 participates in K+ distribution in roots and leaves under low K+ stress

Potassium (K) is a major essential element in plant cells, and KUP/HAK/KT-type K+ transporters participate in the absorption of K+ into roots and in the long-distance transport to above-ground parts. In Arabidopsis thaliana, KUP9 is involved in the transport of K+ and Cs+ in roots. In this study, we investigated KUP9 function in relation to the K+ status of the plant. The expression of KUP9 was upregulated in older leaves on K+-depleted medium, compared to the expression of the other 12 KUP genes in the KUP/HAK/KT family in Arabidopsis. When grown on low K+ medium, the kup9 mutant had reduced chlorophyll content in seedlings and chlorosis in older rosette leaves. Tissue-specific expression of KUP9 determined by KUP9 promoter:GUS assay depended on the K+ status of the plants: In K+ sufficient medium, KUP9 was expressed in the leaf blade towards the leaf tip, whereas in K+ depleted medium expression was mainly found in the petioles. In accordance with this, K+ accumulated in the roots of kup9 plants. The short-term 43K+ tracer measurement showed that 43K was transferred at a lower rate in roots and shoots of kup9, compared to the wild type. These data show that KUP9 participates in the distribution of K+ in leaves and K+ absorption in roots under low K+ conditions.

Functional and biotechnological cues of potassium homeostasis for stress tolerance and plant development

Potassium (K+) is indispensable for the regulation of a plethora of functions like plant metabolism, growth, development, and abiotic stress responses. K+ is associated with protein synthesis and entangled in the activation of scores of enzymes, stomatal regulation, and photosynthesis. It has multiple transporters and channels that assist in the uptake, efflux, transport within the cell as well as from soil to different tissues, and the grain filling sites. While it is implicated in ion homeostasis during salt stress, it acts as a modulator of stomatal movements during water deficit conditions. K+ is reported to abate the effects of chilling and photooxidative stresses. K+ has been found to ameliorate effectively the co-occurrence of drought and high-temperature stresses. Nutrient deficiency of K+ makes leaves necrotic, leads to diminished photosynthesis, and decreased assimilate utilization highlighting the role it plays in photosynthesis. Notably, K+ is associated with the detoxification of reactive oxygen species (ROS) when plants are exposed to diverse abiotic stress conditions. It is irrefutable now that K+ reduces the activity of NADPH oxidases and at the same time maintains electron transport activity, which helps in mitigating the oxidative stress. K+ as a macronutrient in plant growth, the role of K+ during abiotic stress and the protein phosphatases involved in K+ transport have been reviewed. This review presents a holistic view of the biological functions of K+, its uptake, translocation, signaling, and the critical roles it plays under abiotic stress conditions, plant growth, and development that are being unraveled in recent times.

The calcium sensor CBL7 is required for Serendipita indica-induced growth stimulation in Arabidopsis thaliana, controlling defense against the endophyte and K+ homoeostasis in the symbiosis

Calcium is an important second messenger in plants. The activation of Ca²⁺ signalling cascades is critical in the activation of adaptive processes in response to environmental stimuli. Root colonization by the growth promoting endophyte Serendipita indica involves the increase of cytosolic Ca²⁺ levels in Arabidopsis thaliana. Here, we investigated transcriptional changes in Arabidopsis roots during symbiosis with S. indica. RNA-seq profiling disclosed the induction of Calcineurin B-like 7 (CBL7) during early and later phases of the interaction. Consistently, reverse genetic evidence highlighted the functional relevance of CBL7 and tested the involvement of a CBL7-CBL-interacting protein kinase 13 signalling pathway. The loss-of-function of CBL7 abolished the growth promoting effect and affected root colonization. The transcriptomics analysis of cbl7 revealed the involvement of this Ca²⁺ sensor in activating plant defense responses. Furthermore, we report on the contribution of CBL7 to potassium transport in Arabidopsis. We analysed K⁺ contents in wild-type and cbl7 plants and observed a significant increase of K⁺ in roots of cbl7 plants, while shoot tissues demonstrated K⁺ depletion. Taken together, our work associates CBL7 with an important role in the mutual interaction between Arabidopsis and S. indica and links CBL7 to K⁺ transport.

Transcriptional network underpinning ploidy-related elevated leaf potassium in neo-tetraploids

Whole-genome duplication generates a tetraploid from a diploid. Newly created tetraploids (neo-tetraploids) of Arabidopsis (Arabidopsis thaliana) have elevated leaf potassium (K), compared to their diploid progenitor. Micro-grafting has previously established that this elevated leaf K is driven by processes within the root. Here, mutational analysis revealed that the K+-uptake transporters K+ TRANSPORTER 1 (AKT1) and HIGH AFFINITY K+ TRANSPORTER 5 (HAK5) are not necessary for the difference in leaf K caused by whole-genome duplication. However, the endodermis and salt overly sensitive and abscisic acid-related signaling were necessary for the elevated leaf K in neo-tetraploids. Contrasting the root transcriptomes of neo-tetraploid and diploid wild-type and mutants that suppress the neo-tetraploid elevated leaf K phenotype allowed us to identify a core set of 92 differentially expressed genes associated with the difference in leaf K between neo-tetraploids and their diploid progenitor. This core set of genes connected whole-genome duplication with the difference in leaf K between neo-tetraploids and their diploid progenitors. The set of genes is enriched in functions such as cell wall and Casparian strip development and ion transport in the endodermis, root hairs, and procambium. This gene set provides tools to test the intriguing idea of recreating the physiological effects of whole-genome duplication within a diploid genome.

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.

Genome-wide identification and multiple abiotic stress transcript profiling of potassium transport gene homologs in Sorghum bicolor

Potassium (K⁺) is the most abundant cation that plays a crucial role in various cellular processes in plants. Plants have developed an efficient mechanism for the acquisition of K⁺ when grown in K⁺ deficient or saline soils. A total of 47 K⁺ transport gene homologs (27 HAKs, 4 HKTs, 2 KEAs, 9 AKTs, 2 KATs, 2 TPCs, and 1 VDPC) have been identified in Sorghum bicolor. Of 47 homologs, 33 were identified as K⁺ transporters and the remaining 14 as K⁺ channels. Chromosome 2 has been found as the hotspot of K⁺ transporters with 9 genes. Phylogenetic analysis revealed the conservation of sorghum K⁺ transport genes akin to Oryza sativa. Analysis of regulatory elements indicates the key roles that K⁺ transport genes play under different biotic and abiotic stress conditions. Digital expression data of different developmental stages disclosed that expressions were higher in milk, flowering, and tillering stages. Expression levels of the genes SbHAK27 and SbKEA2 were higher during milk, SbHAK17, SbHAK11, SbHAK18, and SbHAK7 during flowering, SbHAK18, SbHAK10, and 23 other gene expressions were elevated during tillering inferring the important role that K⁺ transport genes play during plant growth and development. Differential transcript expression was observed in different tissues like root, stem, and leaf under abiotic stresses such as salt, drought, heat, and cold stresses. Collectively, the in-depth genome-wide analysis and differential transcript profiling of K⁺ transport genes elucidate their role in ion homeostasis and stress tolerance mechanisms.

Comparative transcriptome analysis between inbred lines and hybrids provides molecular insights into K+ content heterosis of tobacco (Nicotiana tabacum L.)

Potassium (K⁺) is essential for crop growth. Increasing the K⁺ content can often directly promote the improvement of crop yield and quality. Heterosis plays an important role in genetic improvement and leads to genetic gains. We found that the K⁺ content of tobacco showed significant heterosis, which is highly significant for cultivating tobacco varieties with high K⁺ content. However, the mechanism by which K⁺ content heterosis occurs in tobacco leaves is not clear. In this study, a comprehensive comparative transcriptome sequencing analysis of root samples from the hybrid G70 × GDH11 and its parental inbred lines G70 and GDH11 was performed to elucidate the importance of the root uptake capacity of K⁺ in the formation of heterosis. The results showed that 29.53% and 60.49% of the differentially expressed genes (DEGs) exhibited dominant and over-dominant expression patterns, respectively. These non-additive upregulated DEGs were significantly enriched in GO terms, such as metal ion transport and reaction, ion balance and homeostasis, ion channel activity, root meristem growth, and regulation of root hairs. The KEGG annotation results indicated that these genes were mainly involved in the pathways such as energy metabolism, carbohydrate formation, amino acid metabolism, and signal transduction. Further analysis showed that probable potassium transporter 17 (NtKT17) and potassium transporter 5-like (NtKT5), associated with potassium ion absorption, glutamate receptor 2.2-like and glutamate receptor 2.8-like, associated with ion channel activity, LOC107782957, protein detoxification 42-like, and probable glutamate carboxypeptidase 2, associated with root configuration, showed a significantly higher expression in the hybrids. These results indicated that the over-dominant expression pattern of DEGs played a key role in the heterosis of K⁺ content in tobacco leaves, and the overexpression of the genes related to K⁺ uptake, transport, and root development in hybrids helped to improve the K⁺ content of plants, thus showing the phenomenon of heterosis.

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.

Genome-wide identification and expression analysis of HAK/KUP/KT potassium transporter provides insights into genes involved in responding to potassium deficiency and salt stress in pepper (Capsicum annuum L.)

In plants, the HAK/KUP/KT family is the largest group of potassium transporters, and it plays an important role in mineral element absorption, plant growth, environmental stress adaptation, and symbiosis. Although these important genes have been investigated in many plant species, limited information is currently available on the HAK/KUP/KT genes for Pepper (Capsicum annuum L.). In the present study, a total of 20 CaHAK genes were identified from the pepper genome and the CaHAK genes were numbered 1 - 20 based on phylogenetic analysis. For the genes and their corresponding proteins, the physicochemical properties, phylogenetic relationship, chromosomal distribution, gene structure, conserved motifs, gene duplication events, and expression patterns were analyzed. Phylogenetic analysis divided CaHAK genes into four cluster (I-IV) based on their structural features and the topology of the phylogenetic tree. Purifying selection played a crucial role in the evolution of CaHAK genes, while whole-genome triplication contributed to the expansion of the CaHAK gene family. The expression patterns showed that CaHAK proteins exhibited functional divergence in terms of plant K⁺ uptake and salt stress response. In particular, transcript abundance of CaHAK3 and CaHAK7 was strongly and specifically up-regulated in pepper roots under low K⁺ or high salinity conditions, suggesting that these genes are candidates for high-affinity K⁺ uptake transporters and may play crucial roles in the maintenance of the Na⁺/K⁺ balance during salt stress in pepper. In summary, the results not only provided the important information on the characteristics and evolutionary relationships of CaHAKs, but also provided potential genes responding to potassium deficiency and salt stress.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-022-03136-z.

Morphological, physiological, biochemical, and transcriptome studies reveal the importance of transporters and stress signaling pathways during salinity stress in Prunus

The almond crop has high economic importance on a global scale, but its sensitivity to salinity stress can cause severe yield losses. Salt-tolerant rootstocks are vital for crop economic feasibility under saline conditions. Two commercial rootstocks submitted to salinity, and evaluated through different parameters, had contrasting results with the survival rates of 90.6% for 'Rootpac 40' (tolerant) and 38.9% for 'Nemaguard' (sensitive) under salinity (Electrical conductivity of water = 3 dS m^-1). Under salinity, 'Rootpac 40' accumulated less Na and Cl and more K in leaves than 'Nemaguard'. Increased proline accumulation in 'Nemaguard' indicated that it was highly stressed by salinity compared to 'Rootpac 40'. RNA-Seq analysis revealed that a higher degree of differential gene expression was controlled by genotype rather than by treatment. Differentially expressed genes (DEGs) provided insight into the regulation of salinity tolerance in Prunus. DEGs associated with stress signaling pathways and transporters may play essential roles in the salinity tolerance of Prunus. Some additional vital players involved in salinity stress in Prunus include CBL10, AKT1, KUP8, Prupe.3G053200 (chloride channel), and Prupe.7G202700 (mechanosensitive ion channel). Genetic components of salinity stress identified in this study may be explored to develop new rootstocks suitable for salinity-affected regions.

Morphological, physiological, biochemical, and transcriptome studies reveal the importance of transporters and stress signaling pathways during salinity stress in Prunus

The almond crop has high economic importance on a global scale, but its sensitivity to salinity stress can cause severe yield losses. Salt-tolerant rootstocks are vital for crop economic feasibility under saline conditions. Two commercial rootstocks submitted to salinity, and evaluated through different parameters, had contrasting results with the survival rates of 90.6% for ‘Rootpac 40’ (tolerant) and 38.9% for ‘Nemaguard’ (sensitive) under salinity (Electrical conductivity of water = 3 dS m⁻¹). Under salinity, ‘Rootpac 40’ accumulated less Na and Cl and more K in leaves than ‘Nemaguard’. Increased proline accumulation in ‘Nemaguard’ indicated that it was highly stressed by salinity compared to ‘Rootpac 40’. RNA-Seq analysis revealed that a higher degree of differential gene expression was controlled by genotype rather than by treatment. Differentially expressed genes (DEGs) provided insight into the regulation of salinity tolerance in Prunus. DEGs associated with stress signaling pathways and transporters may play essential roles in the salinity tolerance of Prunus. Some additional vital players involved in salinity stress in Prunus include CBL10, AKT1, KUP8, Prupe.3G053200 (chloride channel), and Prupe.7G202700 (mechanosensitive ion channel). Genetic components of salinity stress identified in this study may be explored to develop new rootstocks suitable for salinity-affected regions.

Deciphering Molecular Mechanisms Involved in Salinity Tolerance in Guar (Cyamopsis tetragonoloba (L.) Taub.) Using Transcriptome Analyses

Guar is a commercially important legume crop known for guar gum. Guar is tolerant to various abiotic stresses, but the mechanisms involved in its salinity tolerance are not well established. This study aimed to understand molecular mechanisms of salinity tolerance in guar. RNA sequencing (RNA-Seq) was employed to study the leaf and root transcriptomes of salt-tolerant (Matador) and salt-sensitive (PI 340261) guar genotypes under control and salinity. Our analyses identified a total of 296,114 unigenes assembled from 527 million clean reads. Transcriptome analysis revealed that the gene expression differences were more pronounced between salinity treatments than between genotypes. Differentially expressed genes associated with stress-signaling pathways, transporters, chromatin remodeling, microRNA biogenesis, and translational machinery play critical roles in guar salinity tolerance. Genes associated with several transporter families that were differentially expressed during salinity included ABC, MFS, GPH, and P-ATPase. Furthermore, genes encoding transcription factors/regulators belonging to several families, including SNF2, C₂H₂, bHLH, C3H, and MYB were differentially expressed in response to salinity. This study revealed the importance of various biological pathways during salinity stress and identified several candidate genes that may be used to develop salt-tolerant guar genotypes that might be suitable for cultivation in marginal soils with moderate to high salinity or using degraded water.

PERK13 modulates phosphate deficiency-induced root hair elongation in Arabidopsis

Phosphate starvation (-Pi)-induced root hair is crucial for enhancing plants' Pi absorption. Proline-rich extensin-like receptor kinase 13 (PERK13) is transcriptionally induced by -Pi and co-expressed with genes associated with root hair growth. However, how PERK13 participates in -Pi-induced root hair growth remains unclear. Here, we found that PERK13 was transcriptionally responsive to Pi, nitrogen, and iron deficiencies. Loss of PERK13 function (perk13) enhanced root hair growth under Pi/nitrogen limitation. Similar phenotype was also observed in transgenic lines overexpressing PERK13 (PERK13ox). Under -Pi, both perk13 and PERK13ox showed prolonged root hair elongation and increased reactive oxygen species (ROS). Deletion analysis showed, in PERK13ox, the extracellular domain was indispensable for PERK13 in -Pi-induced root hair growth. Different transcription profiles were observed under -Pi between perk13 and PERK13ox with the jasmonate zim-domain genes being repressed in perk13 and genes involved in cell wall remodeling being increased in PERK13ox. Taken together, we demonstrated that PERK13 participates in -Pi-induced root hair growth probably via regulating root hair elongation and the generation of ROS. Our study also suggested PERK13 probably being a vital hub coupling the environmental cues and root hair growth, and might play dual roles in -Pi-induced root hair growth via different processes.

Cesium tolerance is enhanced by a chemical which binds to BETA-GLUCOSIDASE 23 in Arabidopsis thaliana

Cesium (Cs) is found at low levels in nature but does not confer any known benefit to plants. Cs and K compete in cells due to the chemical similarity of Cs to potassium (K), and can induce K deficiency in cells. In previous studies, we identified chemicals that increase Cs tolerance in plants. Among them, a small chemical compound (C₁₇H₁₉F₃N₂O₂), named CsToAcE1, was confirmed to enhance Cs tolerance while increasing Cs accumulation in plants. Treatment of plants with CsToAcE1 resulted in greater Cs and K accumulation and also alleviated Cs-induced growth retardation in Arabidopsis. In the present study, potential target proteins of CsToAcE1 were isolated from Arabidopsis to determine the mechanism by which CsToAcE1 alleviates Cs stress, while enhancing Cs accumulation. Our analysis identified one of the interacting target proteins of CsToAcE1 to be BETA-GLUCOSIDASE 23 (AtβGLU23). Interestingly, Arabidopsis atβglu23 mutants exhibited enhanced tolerance to Cs stress but did not respond to the application of CsToAcE1. Notably, application of CsToAcE1 resulted in a reduction of Cs-induced AtβGLU23 expression in wild-type plants, while this was not observed in a high affinity transporter mutant, athak5. Our data indicate that AtβGLU23 regulates plant response to Cs stress and that CsToAcE1 enhances Cs tolerance by repressing AtβGLU23. In addition, AtHAK5 also appears to be involved in this response.

Noncoding-RNA-Mediated Regulation in Response to Macronutrient Stress in Plants

Macronutrient elements including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) are required in relatively large and steady amounts for plant growth and development. Deficient or excessive supply of macronutrients from external environments may trigger a series of plant responses at phenotypic and molecular levels during the entire life cycle. Among the intertwined molecular networks underlying plant responses to macronutrient stress, noncoding RNAs (ncRNAs), mainly microRNAs (miRNAs) and long ncRNAs (lncRNAs), may serve as pivotal regulators for the coordination between nutrient supply and plant demand, while the responsive ncRNA-target module and the interactive mechanism vary among elements and species. Towards a comprehensive identification and functional characterization of nutrient-responsive ncRNAs and their downstream molecules, high-throughput sequencing has produced massive omics data for comparative expression profiling as a first step. In this review, we highlight the recent findings of ncRNA-mediated regulation in response to macronutrient stress, with special emphasis on the large-scale sequencing efforts for screening out candidate nutrient-responsive ncRNAs in plants, and discuss potential improvements in theoretical study to provide better guidance for crop breeding practices.

Recent Advances in Genome-wide Analyses of Plant Potassium Transporter Families

Plants require potassium (K⁺) as a macronutrient to support numerous physiological processes. Understanding how this nutrient is transported, stored, and utilized within plants is crucial for breeding crops with high K⁺ use efficiency. As K⁺ is not metabolized, cross-membrane transport becomes a rate-limiting step for efficient distribution and utilization in plants. Several K⁺ transporter families, such as KUP/HAK/KT and KEA transporters and Shaker-like and TPK channels, play dominant roles in plant K⁺ transport processes. In this review, we provide a comprehensive contemporary overview of our knowledge about these K⁺ transporter families in angiosperms, with a major focus on the genome-wide identification of K⁺ transporter families, subcellular localization, spatial expression, function and regulation. We also expanded the genome-wide search for the K⁺ transporter genes and examined their tissue-specific expression in Camelina sativa, a polyploid oil-seed crop with a potential to adapt to marginal lands for biofuel purposes and contribution to sustainable agriculture. In addition, we present new insights and emphasis on the study of K⁺ transporters in polyploids in an effort to generate crops with high K⁺ Utilization Efficiency (KUE).

Comparative physiological and transcriptomic analysis reveals salinity tolerance mechanisms in Sorghum bicolor (L.) Moench

Mota Maradi is a sorghum line that exhibits holistic salinity tolerance mechanisms, making it a viable potential donor in breeding efforts for improved sorghum lines. High soil salinity is one of the global challenges for crop growth and productivity. Understanding the salinity tolerance mechanisms in crops is necessary for genetic breeding of salinity-tolerant crops. In this study, physiological and molecular mechanisms in sorghum were identified through a comparative analysis between a Nigerien salinity-tolerant sorghum landrace, Mota Maradi, and the reference sorghum line, BTx623. Significant differences on physiological performances were observed, particularly on growth and biomass gain, photosynthetic rate, and the accumulation of Na⁺, K⁺, proline, and sucrose. Transcriptome profiling of the leaves, leaf sheaths, stems, and roots revealed contrasting differentially expressed genes (DEGs) in Mota Maradi and BTx623 which supports the physiological observations from both lines. Among the DEGs, ion transporters such as HKT, NHX, AKT, HAK5, and KUP3 were likely responsible for the differences in Na⁺ and K⁺ accumulation. Meanwhile, DEGs involved in photosynthesis, cellular growth, signaling, and ROS scavenging were also identified between these two genotypes. Functional and pathway analysis of the DEGs has revealed that these processes work in concert and are crucial in elevated salinity tolerance in Mota Maradi. Our findings indicate how different complex processes work synergistically for salinity stress tolerance in sorghum. This study also highlights the unique adaptation of landraces toward their respective ecosystems, and their strong potential as genetic resources for future plant breeding endeavors.

MYB77 regulates high-affinity potassium uptake by promoting expression of HAK5

In Arabidopsis, the high-affinity K⁺ transporter HAK5 is the major pathway for root K⁺ uptake when below 100 µM; HAK5 responds to Low-K⁺ (LK) stress by strongly and rapidly increasing its expression during K⁺ -deficiency. Therefore, positive regulators of HAK5 expression have the potential to improve K⁺ uptake under LK. Here, we show that mutants of the transcription factor MYB77 share a LK-induced leaf chlorosis phenotype, lower K⁺ content, and lower Rb⁺ uptake of the hak5 mutant, but not the shorter root growth, and that overexpression of MYB77 enhanced K⁺ uptake and improved tolerance to LK stress. Furthermore, we demonstrated that MYB77 positively regulates the expression of HAK5, by binding to the HAK5 promoter and enhances high-affinity K⁺ uptake of roots. As such, our results reveal a novel pathway for enhancing HAK5 expression under LK stress, and provides a candidate for increasing the tolerance of plants to LK.

Rhizosphere Bacteria in Plant Growth Promotion, Biocontrol, and Bioremediation of Contaminated Sites: A Comprehensive Review of Effects and Mechanisms

Agriculture in the 21st century is facing multiple challenges, such as those related to soil fertility, climatic fluctuations, environmental degradation, urbanization, and the increase in food demand for the increasing world population. In the meanwhile, the scientific community is facing key challenges in increasing crop production from the existing land base. In this regard, traditional farming has witnessed enhanced per acre crop yields due to irregular and injudicious use of agrochemicals, including pesticides and synthetic fertilizers, but at a substantial environmental cost. Another major concern in modern agriculture is that crop pests are developing pesticide resistance. Therefore, the future of sustainable crop production requires the use of alternative strategies that can enhance crop yields in an environmentally sound manner. The application of rhizobacteria, specifically, plant growth-promoting rhizobacteria (PGPR), as an alternative to chemical pesticides has gained much attention from the scientific community. These rhizobacteria harbor a number of mechanisms through which they promote plant growth, control plant pests, and induce resistance to various abiotic stresses. This review presents a comprehensive overview of the mechanisms of rhizobacteria involved in plant growth promotion, biocontrol of pests, and bioremediation of contaminated soils. It also focuses on the effects of PGPR inoculation on plant growth survival under environmental stress. Furthermore, the pros and cons of rhizobacterial application along with future directions for the sustainable use of rhizobacteria in agriculture are discussed in depth.

Role of potassium transporter KUP8 in plant responses to heavy metals

Heavy metal concentrations, which have been increasing over the last 200 years, affect soil quality and crop yields. These elements are difficult to eliminate from soils and may constitute a human health hazard by entering the food chain. Recently, we obtained a selection of mutants with different degrees of tolerance to a mixture of heavy metals (HMmix) in order to gain a deeper insight into the underlying mechanism regulating plant responses to these elements. In this study, we characterized the mutant obtained Atkup8 (in this work, Atkup8-2), which showed one of the most resistant phenotypes, as determined by seedling root length. Atkup8-2 is affected in the potassium transporter KUP8, a member of the high-affinity K⁺ uptake family KUP/HAK/KT. Atkup8-2 mutants, which are less affected as measured by seedling root length under HMmix conditions, showed a resistant phenotype with respect to WT seedlings which, despite their delayed growth, are able to develop true leaves at levels similar to those under control conditions. Adult Atkup8-2 plants had a higher fresh weight than WT plants, a resistant phenotype under HMmix stress conditions and lower levels of oxidative damage. KUP8 did not appear to be involved in heavy metal or macro- and micro-nutrient uptake and translocation from roots to leaves, as total concentrations of these elements were similar in both Atkup8-2 and WT plants. However, alterations in cellular K⁺ homeostasis in this mutant cannot be ruled out.

Identification and characterization of HAK/KUP/KT potassium transporter gene family in barley and their expression under abiotic stress

BACKGROUND

HAK/KUP/KT (High-affinity K⁺ transporters/K⁺ uptake permeases/K⁺ transporters) is the largest potassium transporter family in plants, and plays pivotal roles in K⁺ uptake and transport, as well as biotic and abiotic stress responses. However, our understanding of the gene family in barley (Hordeum vulgare L.) is quite limited.

RESULTS

In the present study, we identified 27 barley HAK/KUP/KT genes (hereafter called HvHAKs) through a genome-wide analysis. These HvHAKs were unevenly distributed on seven chromosomes, and could be phylogenetically classified into four clusters. All HvHAK protein sequences possessed the conserved motifs and domains. However, the substantial difference existed among HAK members in cis-acting elements and tissue expression patterns. Wheat had the most orthologous genes to barley HAKs, followed by Brachypodium distachyon, rice and maize. In addition, six barley HAK genes were selected to investigate their expression profiling in response to three abiotic stresses by qRT-PCR, and their expression levels were all up-regulated under salt, hyperosmotic and potassium deficiency treatments.

CONCLUSION

Twenty seven HAK genes (HvHAKs) were identified in barley, and they differ in tissue expression patterns and responses to salt stress, drought stress and potassium deficiency.

Membrane voltage as a dynamic platform for spatiotemporal signaling, physiological, and developmental regulation

Membrane voltage arises from the transport of ions through ion-translocating ATPases, ion-coupled transport of solutes, and ion channels, and is an integral part of the bioenergetic "currency" of the membrane. The dynamics of membrane voltage-so-called action, systemic, and variation potentials-have also led to a recognition of their contributions to signal transduction, both within cells and across tissues. Here, we review the origins of our understanding of membrane voltage and its place as a central element in regulating transport and signal transmission. We stress the importance of understanding voltage as a common intermediate that acts both as a driving force for transport-an electrical "substrate"-and as a product of charge flux across the membrane, thereby interconnecting all charge-carrying transport across the membrane. The voltage interconnection is vital to signaling via second messengers that rely on ion flux, including cytosolic free Ca2+, H+, and the synthesis of reactive oxygen species generated by integral membrane, respiratory burst oxidases. These characteristics inform on the ways in which long-distance voltage signals and voltage oscillations give rise to unique gene expression patterns and influence physiological, developmental, and adaptive responses such as systemic acquired resistance to pathogens and to insect herbivory.

Interrelations of nutrient and water transporters in plants under abiotic stress

Environmental changes cause abiotic stress in plants, primarily through alterations in the uptake of the nutrients and water they require for their metabolism and growth and to maintain their cellular homeostasis. The plasma membranes of cells contain transporter proteins, encoded by their specific genes, responsible for the uptake of nutrients and water (aquaporins). However, their interregulation has rarely been taken into account. Therefore, in this review we identify how the plant genome responds to abiotic stresses such as nutrient deficiency, drought, salinity and low temperature, in relation to both nutrient transporters and aquaporins. Some general responses or regulation mechanisms can be observed under each abiotic stress such as the induction of plasma membrane transporter expression during macronutrient deficiency, the induction of tonoplast transporters and reduction of aquaporins during micronutrients deficiency. However, drought, salinity and low temperatures generally cause an increase in expression of nutrient transporters and aquaporins in tolerant plants. We propose that both types of transporters (nutrients and water) should be considered jointly in order to better understand plant tolerance of stresses.

Arabidopsis leaf hydraulic conductance is regulated by xylem sap pH, controlled, in turn, by a P-type H+ -ATPase of vascular bundle sheath cells

The leaf vascular bundle sheath cells (BSCs) that tightly envelop the leaf veins, are a selective and dynamic barrier to xylem sap water and solutes radially entering the mesophyll cells. Under normal conditions, xylem sap pH below 6 is presumably important for driving and regulating the transmembranal solute transport. Having discovered recently a differentially high expression of a BSC proton pump, AHA2, we now test the hypothesis that it regulates the xylem sap pH and leaf radial water fluxes. We monitored the xylem sap pH in the veins of detached leaves of wild-type Arabidopsis, AHA mutants and aha2 mutants complemented with AHA2 gene solely in BSCs. We tested an AHA inhibitor (vanadate) and stimulator (fusicoccin), and different pH buffers. We monitored their impact on the xylem sap pH and the leaf hydraulic conductance (K_leaf ), and the effect of pH on the water osmotic permeability (P_f ) of isolated BSCs protoplasts. We found that AHA2 is necessary for xylem sap acidification, and in turn, for elevating K_leaf . Conversely, AHA2 knockdown, which alkalinized the xylem sap, or, buffering its pH to 7.5, reduced K_leaf , and elevating external pH to 7.5 decreased the BSCs P_f . All these showed a causative link between AHA2 activity in BSCs and leaf radial hydraulic water conductance.

Transcriptome Analysis Unravels Key Factors Involved in Response to Potassium Deficiency and Feedback Regulation of K+ Uptake in Cotton Roots

To properly understand cotton responses to potassium (K+) deficiency and how its shoot feedback regulates K+ uptake and root growth, we analyzed the changes in root transcriptome induced by low K+ (0.03 mM K+, lasting three days) in self-grafts of a K+ inefficient cotton variety (CCRI41/CCRI41, scion/rootstock) and its reciprocal grafts with a K+ efficient variety (SCRC22/CCRI41). Compared with CCRI41/CCRI41, the SCRC22 scion enhanced the K+ uptake and root growth of CCRI41 rootstock. A total of 1968 and 2539 differently expressed genes (DEGs) were identified in the roots of CCRI41/CCRI41 and SCRC22/CCRI41 in response to K+ deficiency, respectively. The overlapped and similarly (both up- or both down-) regulated DEGs in the two grafts were considered the basic response to K+ deficiency in cotton roots, whereas the DEGs only found in SCRC22/CCRI41 (1954) and those oppositely (one up- and the other down-) regulated in the two grafts might be the key factors involved in the feedback regulation of K+ uptake and root growth. The expression level of four putative K+ transporter genes (three GhHAK5s and one GhKUP3) increased in both grafts under low K+, which could enable plants to cope with K+ deficiency. In addition, two ethylene response factors (ERFs), GhERF15 and GhESE3, both down-regulated in the roots of CCRI41/CCRI41 and SCRC22/CCRI41, may negatively regulate K+ uptake in cotton roots due to higher net K+ uptake rate in their virus-induced gene silencing (VIGS) plants. In terms of feedback regulation of K+ uptake and root growth, several up-regulated DEGs related to Ca2+ binding and CIPK (CBL-interacting protein kinases), one up-regulated GhKUP3 and several up-regulated GhNRT2.1s probably play important roles. In conclusion, these results provide a deeper insight into the molecular mechanisms involved in basic response to low K+ stress in cotton roots and feedback regulation of K+ uptake, and present several low K+ tolerance-associated genes that need to be further identified and characterized.

Effects of Supplemental Lighting on Potassium Transport and Fruit Coloring of Tomatoes Grown in Hydroponics

Supplemental blue/red lighting accelerated fruit coloring and promoted lycopene synthesis in tomato fruits. Potassium (K) is the most enriched cation in tomato fruits, and its fertigation improved tomato yield and fruit color. However, the effects of supplemental lighting on K uptake and transport by tomatoes and whether supplemental lighting accelerates fruit coloring through enhancing K uptake and transport are still unclear. We investigated the effects of supplemental light-emitting diode (LED) lighting (SL; 100% red, 100% blue; 75% red combined 25% blue) on K uptake in roots and transport in the fruits as well as the fruit coloring of tomatoes (Micro-Tom) grown in an experimental greenhouse in hydroponics. The use of red SL or red combined blue SL enhanced K uptake and K accumulation as well as carotenoid (phytoene, lycopene, γ-carotene, and β-carotene) content in fruits by increasing photosynthesis, plant growth, and fruit weight. The genes related to ethylene signaling were upregulated by red SL. Quantitative real-time PCR (qRT-PCR) results showed that K transporter genes (SlHAKs) are differentially expressed during fruit development and ripening. The highest-expressed gene was SlHAK10 when fruit reached breaker and ripening. SlHAK3 and SlHAK19 were highly expressed at breaker, and SlHAK18 was highly expressed at ripening. These might be related to the formation of tomato fruit ripening and quality. SlHAK4, SlHAK6, SlHAK8, and SlHAK9 were significantly downregulated with fruit ripening and induced by low K. The expression level of SlHAK6, SlHAK10, SlHAK15, and SlHAK19 were significantly increased by blue SL or red combined blue SL during breaker and ripening. Blue SL or red combined blue SL increased content of phytoene, β-carotene, α-carotene, and γ-carotene and accelerated fruit coloring by enhancing K uptake in roots and transport in fruits during fruit ripening. This was consistent with the expression level of SlHAK6, SlHAK10, SlHAK15, and SlHAK19 during fruit development and ripening. The key genes of photoreceptors, light signaling transcript factors as well as abscisic acid (ABA) transduction induced by blue SL or red combined blue SL were consistent with the upregulated genes of SlHAK6, SlHAK10, SlHAK15, and SlHAK19 under blue SL and red combined blue SL. The K transport in tomato fruits might be mediated by light signaling and ABA signaling transduction. These results provide valuable information for fruit quality control and the light regulating mechanism of K transport and fruit coloring in tomatoes.

The Endophytic Microbiome as a Hotspot of Synergistic Interactions, with Prospects of Plant Growth Promotion

The plant root is the primary site of interaction between plants and associated microorganisms and constitutes the main components of plant microbiomes that impact crop production. The endophytic bacteria in the root zone have an important role in plant growth promotion. Diverse microbial communities inhabit plant root tissues, and they directly or indirectly promote plant growth by inhibiting the growth of plant pathogens, producing various secondary metabolites. Mechanisms of plant growth promotion and response of root endophytic microorganisms for their survival and colonization in the host plants are the result of complex plant-microbe interactions. Endophytic microorganisms also assist the host to sustain different biotic and abiotic stresses. Better insights are emerging for the endophyte, such as host plant interactions due to advancements in 'omic' technologies, which facilitate the exploration of genes that are responsible for plant tissue colonization. Consequently, this is informative to envisage putative functions and metabolic processes crucial for endophytic adaptations. Detection of cell signaling molecules between host plants and identification of compounds synthesized by root endophytes are effective means for their utilization in the agriculture sector as biofertilizers. In addition, it is interesting that the endophytic microorganism colonization impacts the relative abundance of indigenous microbial communities and suppresses the deleterious microorganisms in plant tissues. Natural products released by endophytes act as biocontrol agents and inhibit pathogen growth. The symbiosis of endophytic bacteria and arbuscular mycorrhizal fungi (AMF) affects plant symbiotic signaling pathways and root colonization patterns and phytohormone synthesis. In this review, the potential of the root endophytic community, colonization, and role in the improvement of plant growth has been explained in the light of intricate plant-microbe interactions.

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 Complex Story of Plant Cyclic Nucleotide-Gated Channels

Plant cyclic nucleotide-gated channels (CNGCs) are tetrameric cation channels which may be activated by the cyclic nucleotides (cNMPs) adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP). The genome of Arabidopsis thaliana encodes 20 CNGC subunits associated with aspects of development, stress response and immunity. Recently, it has been demonstrated that CNGC subunits form heterotetrameric complexes which behave differently from the homotetramers produced by their constituent subunits. These findings have widespread implications for future signalling research and may help explain how specificity can be achieved by CNGCs that are known to act in disparate pathways. Regulation of complex formation may involve cyclic nucleotide-gated channel-like proteins.

Looking for Root Hairs to Overcome Poor Soils

Breeding new cultivars allowing reduced fertilization and irrigation is a major challenge. International efforts towards this goal focus on noninvasive methodologies, platforms for high-throughput phenotyping of large plant populations, and quantitative description of root traits as predictors of crop performance in environments with limited water and nutrient availability. However, these high-throughput analyses ignore one crucial component of the root system: root hairs (RHs). Here, we review current knowledge on RH functions, mainly in the context of plant hydromineral nutrition, and take stock of quantitative genetics data pointing at correlations between RH traits and plant biomass production and yield components.

Genome-wide characterization and expression analysis of HAK K+ transport family in Ipomoea

The potassium transporter high-affinity K⁺ transporter/K⁺ uptake permease/K⁺ transporter (HAK/KUP/KT) family plays a vital role in potassium uptake, and potassium ion (K⁺)-mediated environmental stress. In the present study, we identified 22 IbHAK/KUP/KT (HAK) genes in sweet potato [Ipomoea batata (L.) Lam] and the same number of HAK genes from sweet potato wild relative Ipomoea trifida. Phylogeny analysis indicated that the HAKs can be divided into five clades. Chromosomal distribution and genome synteny analyses revealed two tandem-duplicated gene pairs IbHAK16/17 and IbHAK17/18 on chromosomes 13 and eight segmental-duplicated gene pairs on chromosomes 1, 3, 5, 8, 10, 12, 14 among the IbHAK gene family. Eleven orthologous HAK gene pairs between I. batata and I. trifida were involved in the duplication of genomic blocks based on comparative genomic analysis. The Ka/Ks ratios of these IbHAK genes ranged from 0.02 to 0.55(< 1), further indicated that purifying selection was the primary force driving the evolution of HAKs in Ipomoea. A heat map based on RNA-seq data showed that 13 HAKs in Xushu32 (a K⁺-tolerant sweet potato genotype) and 10 HAKs in Ningzi1 (a K⁺-sensitive sweet potato genotype) in response to K⁺ deficiency stress. Quantitative real-time PCR (qRT-PCR) analysis revealed IbHAK2, -3, -8, -10, -11, -18, -19, and -21 were induced in both Xushu32 and Ningzi1 under low K⁺ stress. Compared with other IbHAK genes, IbHAK8 showed more strongly upregulation after exposure to drought and salt stress. Furthermore, co-expression analysis showed that only IbHAK8 of 22 IbHAK genes involved in network interactions with 30 genes related to abiotic and biotic stresses. Taken together, these results are helpful for further functional studies on IbHAK and molecular breeding of sweet potato.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-020-02552-3.

Genome-Wide Identification and Expression Pattern Analysis of the HAK/KUP/KT Gene Family of Cotton in Fiber Development and Under Stresses

The potassium transporter family HAK/KUP/KT is a large group of proteins that are important in plant potassium transport and plays a crucial role in plant growth and development, especially in economic crops. Although HAK/KUP/KT genes have been identified in many species, research on these genes in cotton is still quite rare. In this study, in total, 21, 24, 45, and 44 HAK/KUP/KT genes were identified in Gossypium arboreum, Gossypium raimondii, Gossypium hirsutum, and Gossypium barbadense, respectively. Phylogenetic analysis showed that these genes were divided into four clusters. The G. hirsutum gene promoters contained diverse cis-regulatory elements, such as drought-responsive elements, low temperature-responsive elements, and other elements. The RNA-seq data and qRT-PCR results showed that HAK/KUP/KT genes had different expression patterns in fiber development. The qRT-PCR results of drought and NaCl treatment indicated that HAK/KUP/KT genes might play important roles in abiotic stress responses. These results will provide molecular insights into potassium transporter research in cotton.

Genome-Wide Survey and Expression Analysis of the KT/HAK/KUP Family in Brassica napus and Its Potential Roles in the Response to K+ Deficiency

The KT/HAK/KUP (HAK) family is the largest potassium (K⁺) transporter family in plants, which plays key roles in K⁺ uptake and homeostasis, stress resistance, and root and embryo development. However, the HAK family has not yet been characterized in Brassica napus. In this study, 40 putative B. napus HAK genes (BnaHAKs) are identified and divided into four groups (Groups I–III and V) on the basis of phylogenetic analysis. Gene structure analysis revealed 10 conserved intron insertion sites across different groups. Collinearity analysis demonstrated that both allopolyploidization and small-scale duplication events contributed to the large expansion of BnaHAKs. Transcription factor (TF)-binding network construction, cis-element analysis, and microRNA prediction revealed that the expression of BnaHAKs is regulated by multiple factors. Analysis of RNA-sequencing data further revealed extensive expression profiles of the BnaHAKs in groups II, III, and V, with limited expression in group I. Compared with group I, most of the BnaHAKs in groups II, III, and V were more upregulated by hormone induction based on RNA-sequencing data. Reverse transcription-quantitative polymerase reaction analysis revealed that the expression of eight BnaHAKs of groups I and V was markedly upregulated under K⁺-deficiency treatment. Collectively, our results provide valuable information and key candidate genes for further functional studies of BnaHAKs.