Potassium transport and signaling in higher plants

GORK K+ channel structure and gating vital to informing stomatal engineering

The Arabidopsis GORK channel is a major pathway for guard cell K+ efflux that facilitates stomatal closure. GORK is an outwardly-rectifying member of the cyclic-nucleotide binding-homology domain (CNBHD) family of K+ channels with close homologues in all other angiosperms known to date. Its bioengineering has demonstrated the potential for enhanced carbon assimilation and water use efficiency. Here we identify critical domains through structural and functional analysis, highlighting conformations that reflect long-lived closed and pre-open states of GORK. These conformations are marked by interactions at the cytosolic face of the membrane between so-called voltage-sensor, C-linker and CNBHD domains, the latter relocating across 10 Å below the voltage sensor. The interactions center around two coupling sites that functional analysis establish are critical for channel gating. The channel is also subject to putative, ligand-like interactions within the CNBHD, which leads to its gating independence of cyclic nucleotides such as cAMP or cGMP. These findings implicate a multi-step mechanism of semi-independent conformational transitions that underlie channel activity and offer promising new sites for optimizing GORK to engineer stomata.

Co-inoculation of Potassium Solubilizing Bacteria and Rhizophagus irregularis Promotes the Growth and Potassium Accumulation of Robinia pseudoacacia L. Seedlings

Potassium (K) in plants participates in a variety of physiological processes and is kept at a higher concentration than its soluble form in soil. Potassium solubilizing bacteria (KSB) release K from minerals. Arbuscular mycorrhizal (AM) fungi facilitate bacterial movement along their extraradical hyphae and improve plant K status. However, the interaction of KSB and AM fungi is rarely reported. This study aimed to isolate KSB and evaluate their interaction with AM fungi in promoting plant K uptake and growth. Soil was sampled from the rhizosphere of Robinia pseudoacacia in the southern area of the Loess Plateau, where soil available K is lower than plant demand. KSB from soil was isolated using a select medium in which K-feldspar is the only K source. KSB that showed an obvious dissolving circle and relatively high K solubilizing efficiency (over 20%) were isolated and identified. A pot experiment was conducted in a randomized design to evaluate the effect of KSB and AM fungi and their interaction. Four out of 12 isolated strains that showed high potassium solubilizing efficiency were from the genus Pseudomonas. Inoculation of KSB promoted the growth and K content of R. pseudoacacia. KSB showed a variety of (increase, decrease, and not-obvious) influences on the colonization status of R. irregularis. Co-inoculation of R. irregularis and KSB promoted plant growth, K content of the plant, and the available K in the growth substrate. This study provided a basis for the utilization of KSB and AM fungi as biofertilizers in the Loess Plateau.

Manifold roles of potassium in mediating drought tolerance in plants and its underlying mechanisms

Drought stress (DS) is a major devastating factor affecting plant growth and development worldwide. Potassium (K) is considered a vigorous moiety and stress alleviator, which crop cultivars need for better yield. It is also helpful in alleviating the DS-induced negative consequences by regulating various morphological, physiological, biochemical, and molecular mechanisms in plants. Particularly, the K application improves plant tolerance against DS by improving plant growth parameters, photosynthetic pigments, cell turgor pressure, osmotic pressure, nutritional balance, compatible solutes, and the plant's antioxidant defense system. Apart from its role as a constituent of the plant structure, biochemical processes such as protein synthesis, carbohydrate metabolism, and enzyme activation are also regulated by K. However, the exact K-mediated molecular mechanisms of DS tolerance are still unclear and require more investigation. The present review aims to provide insight into the role of K in regulating various morphological and physico-chemical aspects under DS. It also emphasizes the crosstalk of K with other nutrients and phytohormones, as well as molecular mechanisms for K homeostasis under DS. We have also shed light on genomics analysis to discover K transporter's novel genes in different plant species.

Genome-wide identification, characterization and expression pattern analysis of HAK/KUP/KT potassium transporter gene family in potato

The HAK/KUP/KT (High-affinity K+ transporters/K+ uptake permeases/K+ transporters) is the largest and most dominant potassium transporter family in plants, playing a crucial role in various biological processes. However, our understanding of HAK/KUP/KT gene family in potato (Solanum tuberosum L.) remains limited and unclear. In this study, 24 HAK/KUP/KT genes (StHAKs) were identified through a genome-wide analysis and were found to be unevenly distributed across ten chromosomes. Based on phylogenetic analysis, these StHAK gene family members were classified into four distinct clusters. All StHAK protein sequences contained the conserved motifs and domains. Promoter cis-acting elements analysis revealed that most StHAK gene family members in potatoes were associated with responses to light and hormones such as abscisic acid or methyl jasmonate, however, many motifs responsive to hormones and stress conditions have not been clearly studied or reported in plants. Synteny analysis suggested that 33, 19, 8, 1 StHAK genes were orthologous to those in soybean, cassava, Arabidopsis and rice, respectively. The previously published RNA-seq results, transcriptomic data and qRT-PCR experiments indicated that the expression profiles of these StHAK genes were tissue-specific and were influenced by multiple factors, including biotic and abiotic stress, hormone, potassium fertilizer. To provide a clear and convenient view of StHAK gene expression across different tissues in potato, we generated a cartoon heatmap to vividly illustrate the tissue-specific expression of StHAK genes, which is unprecedented in the gene family analysis of potato. At last, we identified genes such as StHAK8, StHAK14, and StHAK22 with high expression in potato tubers using qRT-PCR, suggesting their potential involvement in tuber growth and development. This can contribute to a deeper understanding of the mechanism of potassium absorption and transportation in potatoes. It has laid a solid theoretical foundation for the genetic regulation of potassium nutritional efficiency in potatoes and the breeding of potato varieties with high potassium efficiency.

Metal ion transport in maize: survival in a variable stress environment

Maize (Zea mays) is the most widely cultivated crop in the world. Maize production is closely linked to the extensive uptake and utilization of various mineral nutrients. Potassium (K), calcium (Ca), and magnesium (Mg) are essential metallic macronutrients for plant growth and development. Sodium (Na) is an essential micronutrient for some C4 and CAM plants. Several metallic micronutrients like iron (Fe), manganese (Mn), and zinc (Zn) serve as enzyme components or co-factors in plant cells. Maize has to face the combined ion stress conditions in the natural environment. The limited availability of these nutrients in soils restricts maize production. In saline land, excessive Na could inhibit the uptake of mineral elements. Additionally, aluminum (Al) and heavy metal cadmium (Cd) and lead (Pb) in soils are toxic to maize and pose a threat to food security. Thus, plants must evolve complex mechanisms to increase nutrient uptake and utilization while restraining harmful elements. This review summarizes the research progress on the uptake and transport of metal ions in maize, highlights the regulation mechanism of metal ion transporters under stress conditions, and discusses the future challenges for the improvement of maize with high nutrient utilization efficiency (NUE).

HDC1 Promotes Primary Root Elongation by Regulating Auxin and K+ Homeostasis in Response to Low-K+ Stress

Plants frequently encounter relatively low and fluctuating potassium (K+) concentrations in soil, with roots serving as primary responders to this stress. Histone modifications, such as de-/acetylation, can function as epigenetic markers of stress-inducible genes. However, the signaling network between histone modifications and low-K+ (LK) response pathways remains unclear. This study investigated the regulatory role of Histone Deacetylase Complex 1 (HDC1) in primary root growth of Arabidopsis thaliana under K+ deficiency stress. Using a hdc1-2 mutant line, we observed that HDC1 positively regulated root growth under LK conditions. Compared to wild-type (WT) plants, the hdc1-2 mutant exhibited significantly inhibited primary root growth under LK conditions, whereas HDC1-overexpression lines displayed opposite phenotypes. No significant differences were observed under HK conditions. Further analysis revealed that the inhibition of hdc1-2 on root growth was due to reduced apical meristem cell proliferation rather than cell elongation. Notably, the root growth of hdc1-2 showed reduced sensitivity compared to WT after auxin treatment under LK conditions. HDC1 may regulate root growth by affecting auxin polar transport and subsequent auxin signaling, as evidenced by the altered expression of auxin transport genes. Moreover, the organ-specific RT-qPCR analyses unraveled that HDC1 negatively regulates the expression of CBL-CIPK-K+ channel-related genes such as CBL1, CBL2, CBL3, AKT1, and TPK1, thereby establishing a molecular link between histone deacetylation, auxin signaling, and CBLs-CIPKs pathway in response to K+ deficiency.

Polyethylene terephthalate nanoplastics affect potassium accumulation in foxtail millet (Setaria italica) seedlings

BACKGROUND

As modern industrial activities have advanced, the prevalence of microplastics and nanoplastics in the environment has increased, thereby impacting plant growth. Potassium is one of the most crucial nutrient cations for plant biology. Understanding how polyethylene terephthalate (PET) treatment affects potassium uptake will deepen our understanding of plant response mechanisms to plastic pollution.

RESULTS

In this study, we examined the impact of PET micro- and nanoplastics on foxtail millet seedling growth and potassium accumulation. Additionally, we measured reactive oxygen species (ROS) production, antioxidant enzyme activities, and the expression levels of the corresponding enzyme-encoding genes. Our findings indicated that the germination and seedling growth of foxtail millet were not significantly affected by exposure to PET plastics. However, the ROS levels in foxtail millet increased under these conditions. This increase in ROS led to the upregulation of several genes involved in K+ uptake and transport (SiHAK1, SiHAK2, SiAKT2/3, SiHKT2;2, SiHKT1;1, SiGORK, and SiSKOR), thereby increasing K+ accumulation in foxtail millet leaves. Further research revealed that higher K+ concentrations in plant leaves were correlated with increased expression of the antioxidant-related genes SiCAT1, SiPOD1, and SiSOD3, as well as increased activities of the corresponding antioxidant enzymes. This response helps mitigate the excessive accumulation and damage caused by ROS in plant cells after PET nanoplastic treatment, suggesting a potential stress response mechanism in foxtail millet against nanoplastic pollution.

CONCLUSIONS

Our research indicates that PET nanoplastic treatment induces the expression of genes related to K+ uptake in foxtail millet through ROS signaling, leading to increased K+ accumulation in the leaves. This process mitigates the ROS damage caused by PET nanoplastic treatment by increasing the expression and activity of genes encoding antioxidant enzymes. The present research has unveiled the K+ accumulation-related response mechanism of foxtail millet to PET nanoplastic treatment, contributing significantly to our understanding of both the potassium absorption regulation mechanism in plants and the broader impact of plastic pollution on agricultural crops. This discovery not only highlights the complexity of plant responses to environmental stressors but also underscores the importance of considering such responses when evaluating the ecological and agricultural implications of plastic pollution.

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.

A tonoplast-localized TPK-type K+ transporter (TPKa) regulates potassium accumulation in tobacco

Potassium ion (K+) is one of the most essential nutrients for the growth and development of tobacco (Nicotiana tabacum L.), however, the molecular regulation of K+ concentration in tobacco remains unclear. In this study, a two-pore K (TPK) channel gene NtTPKa was cloned from tobacco, and NtTPKa protein contains the unique K+ selection motif GYGD and its transmembrane region primarily locates in the tonoplast membrane. The expression of NtTPKa gene was significantly increased under low-potassium stress conditions. The concentrations of K+ in tobacco were significantly increased in the NtTPKa RNA interference lines and CRISPR/Cas9 knockout mutants. In addition, the transport of K+ by NtTPKa was validated using patch clamp technique, and the results showed that NtTPKa channel protein exclusively transported K+ in a concentration-dependent manner. Together, our results strongly suggested that NtTPKa is a key gene in maintaining K+ homeostasis in tobacco, and it could provide a new genetic resource for increasing the concentration of K+ in tobacco.

Comparative Insights into Photosynthetic, Biochemical, and Ultrastructural Mechanisms in Hibiscus and Pelargonium Plants

Understanding photosynthetic mechanisms in different plant species is crucial for advancing agricultural productivity and ecological restoration. This study presents a detailed physiological and ultrastructural comparison of photosynthetic mechanisms between Hibiscus (Hibiscus rosa-sinensis L.) and Pelargonium (Pelargonium zonale (L.) L'Hér. Ex Aiton) plants. The data collection encompassed daily photosynthetic profiles, responses to light and CO2, leaf optical properties, fluorescence data (OJIP transients), biochemical analyses, and anatomical observations. The findings reveal distinct morphological, optical, and biochemical adaptations between the two species. These adaptations were associated with differences in photochemical (AMAX, E, Ci, iWUE, and α) and carboxylative parameters (VCMAX, ΓCO2, gs, gm, Cc, and AJMAX), along with variations in fluorescence and concentrations of chlorophylls and carotenoids. Such factors modulate the efficiency of photosynthesis. Energy dissipation mechanisms, including thermal and fluorescence pathways (ΦPSII, ETR, NPQ), and JIP test-derived metrics highlighted differences in electron transport, particularly between PSII and PSI. At the ultrastructural level, Hibiscus exhibited optimised cellular and chloroplast architecture, characterised by increased chloroplast density and robust grana structures. In contrast, Pelargonium displayed suboptimal photosynthetic parameters, possibly due to reduced thylakoid counts and a higher proportion of mitochondria. In conclusion, while Hibiscus appears primed for efficient photosynthesis and energy storage, Pelargonium may prioritise alternative cellular functions, engaging in a metabolic trade-off.

Arabidopsis HAK5 under low K+ availability operates as PMF powered high-affinity K+ transporter

Plants can survive in soils of low micromolar potassium (K+) concentrations. Root K+ intake is accomplished by the K+ channel AKT1 and KUP/HAK/KT type high-affinity K+ transporters. Arabidopsis HAK5 mutants impaired in low K+ acquisition have been identified already more than two decades ago, the molecular mechanism, however, is still a matter of debate also because of lack of direct measurements of HAK5-mediated K+ currents. When we expressed AtHAK5 in Xenopus oocytes together with CBL1/CIPK23, no inward currents were elicited in sufficient K+ media. Under low K+ and inward-directed proton motive force (PMF), the inward K+ current increased indicating that HAK5 energetically couples the uphill transport of K+ to the downhill flux of H+. At extracellular K+ concentrations above 25 μM, the initial rise in current was followed by a concentration-graded inactivation. When we replaced Tyr450 in AtHAK5 to Ala the K+ affinity strongly decreased, indicating that AtHAK5 position Y450 holds a key for K+ sensing and transport. When the soil K+ concentration drops toward the range that thermodynamically cannot be covered by AKT1, the AtHAK5 K+/H+ symporter progressively takes over K+ nutrition. Therefore, optimizing K+ use efficiency of crops, HAK5 could be key for low K+ tolerant agriculture.

X-ray fluorescence and XANES spectroscopy revealed diverse potassium chemistries and colocalization with phosphorus in the ectomycorrhizal fungus Paxillus ammoniavirescens

Ectomycorrhizal (ECM) fungi play a major role in forest ecosystems and managed tree plantations. Particularly, they facilitate mineral weathering and nutrient transfer towards colonized roots. Among nutrients provided by these fungi, potassium (K) has been understudied compared to phosphorus (P) or nitrogen (N). The ECM fungus Paxillus ammoniavirescens is a generalist species that interacts with the root of many trees and can directly transfer K to them, including loblolly pine. However, the forms of K that ECM fungi can store is still unknown. Here, we used synchrotron potassium X-ray fluorescence (XRF) and K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy on P. ammoniavirescens growing in axenic conditions to investigate the K chemistries accumulating in the center and the edge of the mycelium. We observed that various K forms accumulated in different part of the mycelium, including K-nitrate (KNO3), K-C-O compounds (such as K-tartrate K2(C4H4O6) and K-oxalate (K2C2O4)), K-S and K-P compounds. Saprotrophic fungi have been shown to excrete carboxylic acids, which in turn play a role in soil mineral weathering. Our finding of several K counter-ions to carboxylic acids may suggest that, besides their direct transfer to colonized roots, K ions can also be involved in the production of compounds necessary for sourcing nutrients from their surrounding environment by ECM fungi. Additionally, this work reveals that XANES spectroscopy can be used to identify the various forms of K accumulating in biological systems.

Interactions between Brassinosteroids and Strigolactones in Alleviating Salt Stress in Maize

Exogenous brassinolide (BR) and strigolactones (SLs) play an important role in alleviating salt stress in maize. We studied the morphological and physiological responses of the salt-sensitive genotype PH4CV and salt-tolerant genotype Zheng58 to BR (1.65 nM), SL (1 µM), and BS (1.65 nM BR + 1 µM SL) under salt stress. Phenotypic analysis showed that salt stress significantly inhibited the growth of maize seedlings and significantly increased the content of Na+ in the roots. Exogenous hormones increased oxidase activity and decreased Na+ content in the roots and mitigated salt stress. Transcriptome analysis showed that the interaction of BR and SL is involved in photosynthesis-antenna proteins, the TCA cycle, and plant hormone signal transduction pathways. This interaction influences the expression of chlorophyll a/b-binding protein and glucose-6-phosphate isomerase 1 chloroplastic, and aconitase genes are affected. Furthermore, the application of exogenous hormones regulates the expression of genes associated with the signaling pathways of cytokinin (CK), gibberellins (GA), auxin (IAA), brassinosteroid (BR), abscisic acid (ABA), and jasmonic acid (JA). Additionally, exogenous hormones inhibit the expression of the AKT2/3 genes, which are responsible for regulating ion transduction and potassium ion influx. Four candidate genes that may regulate the seedling length of maize were screened out through WGCNA. Respective KOG notes concerned inorganic ion transport and metabolism, signal transduction mechanisms, energy production and conversion, and amino acid transport and metabolism. The findings of this study provide a foundation for the proposition that BR and SL can be employed to regulate salt stress alleviation in maize.

Genome-wide transcriptome and gene family analysis reveal candidate genes associated with potassium uptake of maize colonized by arbuscular mycorrhizal fungi

BACKGROUND

Potassium (K) is an essential nutrient for plant growth and development. Maize (Zea mays) is a widely planted crops in the world and requires a huge amount of K fertilizer. Arbuscular mycorrhizal fungi (AMF) are closely related to the K uptake of maize. Genetic improvement of maize K utilization efficiency will require elucidating the molecular mechanisms of maize K uptake through the mycorrhizal pathway. Here, we employed transcriptome and gene family analysis to elucidate the mechanism influencing the K uptake and utilization efficiency of mycorrhizal maize.

METHODS AND RESULTS

The transcriptomes of maize were studied with and without AMF inoculation and under different K conditions. AM symbiosis increased the K concentration and dry weight of maize plants. RNA sequencing revealed that genes associated with the activity of the apoplast and nutrient reservoir were significantly enriched in mycorrhizal roots under low-K conditions but not under high-K conditions. Weighted gene correlation network analysis revealed that three modules were strongly correlated with K content. Twenty-one hub genes enriched in pathways associated with glycerophospholipid metabolism, glycerolipid metabolism, starch and sucrose metabolism, and anthocyanin biosynthesis were further identified. In general, these hub genes were upregulated in AMF-colonized roots under low-K conditions. Additionally, the members of 14 gene families associated with K obtain were identified (ARF: 38, ILK: 4, RBOH: 12, RUPO: 20, MAPKK: 89, CBL: 14, CIPK: 44, CPK: 40, PIN: 10, MYB: 174, NPF: 79, KT: 19, HAK/HKT/KUP: 38, and CPA: 8) from maize. The transcript levels of these genes showed that 92 genes (ARF:6, CBL:5, CIPK:13, CPK:2, HAK/HKT/KUP:7, PIN:2, MYB:26, NPF:16, RBOH:1, MAPKK:12 and RUPO:2) were upregulated with AM symbiosis under low-K conditions.

CONCLUSIONS

This study indicated that AMF increase the resistance of maize to low-K stress by regulating K uptake at the gene transcription level. Our findings provide a genome-level resource for the functional assignment of genes regulated by K treatment and AM symbiosis in K uptake-related gene families in maize. This may contribute to elucidate the molecular mechanisms of maize response to low K stress with AMF inoculation, and provided a theoretical basis for AMF application in the crop field.

Evaluating the effects of azelaic acid in the metabolism of Arabidopsis thaliana seedlings through untargeted metabolomics and ionomics approaches

The present study demonstrates that low concentrations of azelaic acid (AZA) significantly impact the metabolism of Arabidopsis thaliana seedlings, leading to imbalances in numerous minerals and metabolites due to AZA-induced stress. Untargeted metabolomic analyses were conducted on untreated and AZA-treated seedlings at two time points: 7 and 14 days after treatment initiation. The results revealed a general accumulation of sugars (e.g., glucose, mannose, xylose), amino acids (e.g., lysine, GABA, threonine, glutamine), and organic acids (e.g., glutaric acid, shikimic acid, succinic acid) in AZA treated-seedlings, suggesting that AZA triggers stress responses in Arabidopsis. Ionomic analysis revealed that AZA induces phosphorus deficiency, which plants compensate by increasing malate content in the roots. Additionally, AZA treatment induced putrescine accumulation within the root, a metabolic biomarker of potassium deficiency and plant stress. The metabolomic profile showed elevated levels of different specialized metabolites, such as nitrogen- and sulphur-containing compounds, and altered levels of various phytohormones, including jasmonates and brassinosteroids, implicated in plant protection under biotic and/or abiotic stresses. These findings support the hypothesis that AZA's mode of action is associated with an auxin imbalance, suggesting its function as an auxinic herbicide. The observed increases in starch and jasmonates, coupled with the disruptions in potassium homeostasis, are linked to the previously reported alterations in the auxin transport, root architecture and gravitropic root response. Statistical analyses were applied, including Kruskal-Wallis tests for ionomic data, as well as multifactor analysis, Principal Component Analysis, Orthogonal Partial Least Squares-Discriminant Analysis, and enrichment pathway analysis for metabolomic data, ensuring the robustness and validity of these findings.

Dynamics of Actin Filaments Play an Important Role in Root Hair Growth under Low Potassium Stress in Arabidopsis thaliana

Potassium (K) is an essential nutrient for the growth and development of plants. Root hairs are the main parts of plants that absorb K+. The regulation of plant root hair growth in response to a wide range of environmental stresses is crucially associated with the dynamics of actin filaments, and the thick actin bundles at the apical and sub-apical regions are essential for terminating the rapid elongation of root hair cells. However, the dynamics and roles of actin filaments in root hair growth in plants' response to low K+ stress are not fully understood. Here, we revealed that root hairs grow faster and longer under low K+ stress than the control conditions. Compared to control conditions, the actin filaments in the sub-apex of fast-growing wild-type root hairs were longer and more parallel under low K+ stress, which correlates with an increased root hair growth rate under low K+ stress; the finer actin filaments in the sub-apex of the early fully grown Col-0 root hairs under low K+ stress, which is associated with low K+ stress-induced root hair growth time. Further, Arabidopsis thaliana actin bundling protein Villin1 (VLN1) and Villin4 (VLN4) was inhibited and induced under low K+ stress, respectively. Low K+ stress-inhibited VLN1 led to decreased bundling rate and thick bundle formation in the early fully grown phase. Low K+ stress-induced VLN4 functioned in keeping long filaments in the fast-growing phase. Furthermore, the analysis of genetics pointed out the involvement of VLN1 and VLN4 in the growth of root hairs under the stress of low potassium levels in plants. Our results provide a basis for the dynamics of actin filaments and their molecular regulation mechanisms in root hair growth in response to low K+ stress.

Plant-microbe interactions in the rhizosphere for smarter and more sustainable crop fertilization: the case of PGPR-based biofertilizers

Biofertilizers based on plant growth promoting rhizobacteria (PGPR) are nowadays gaining increasingly attention as a modern tool for a more sustainable agriculture due to their ability in ameliorating root nutrient acquisition. For many years, most research was focused on the screening and characterization of PGPR functioning as nitrogen (N) or phosphorus (P) biofertilizers. However, with the increasing demand for food using far fewer chemical inputs, new investigations have been carried out to explore the potential use of such bacteria also as potassium (K), sulfur (S), zinc (Zn), or iron (Fe) biofertilizers. In this review, we update the use of PGPR as biofertilizers for a smarter and more sustainable crop production and deliberate the prospects of using microbiome engineering-based methods as potential tools to shed new light on the improvement of plant mineral nutrition. The current era of omics revolution has enabled the design of synthetic microbial communities (named SynComs), which are emerging as a promising tool that can allow the formulation of biofertilizers based on PGPR strains displaying multifarious and synergistic traits, thus leading to an increasingly efficient root acquisition of more than a single essential nutrient at the same time. Additionally, host-mediated microbiome engineering (HMME) leverages advanced omics techniques to reintroduce alleles coding for beneficial compounds, reinforcing positive plant-microbiome interactions and creating plants capable of producing their own biofertilizers. We also discusses the current use of PGPR-based biofertilizers and point out possible avenues of research for the future development of more efficient biofertilizers for a smarter and more precise crop fertilization. Furthermore, concerns have been raised about the effectiveness of PGPR-based biofertilizers in real field conditions, as their success in controlled experiments often contrasts with inconsistent field results. This discrepancy highlights the need for standardized protocols to ensure consistent application and reliable outcomes.

A viral protein competitively bound to rice CIPK23 inhibits potassium absorption and facilitates virus systemic infection in rice

Potassium (K+) plays a crucial role as a macronutrient in the growth and development of plants. Studies have definitely determined the vital roles of K+ in response to pathogen invasion. Our previous investigations revealed that rice plants infected with rice grassy stunt virus (RGSV) displayed a reduction in K+ content, but the mechanism by which RGSV infection subverts K+ uptake remains unknown. In this study, we found that overexpression of RGSV P1, a specific viral protein encoded by viral RNA1, results in enhanced sensitivity to low K+ stress and exhibits a significantly lower rate of K+ influx compared to wild-type rice plants. Further investigation revealed that RGSV P1 interacts with OsCIPK23, an upstream regulator of Shaker K+ channel OsAKT1. Moreover, we found that the P1 protein recruits the OsCIPK23 to the Cajal bodies (CBs). In vivo assays demonstrated that the P1 protein competitively binds to OsCIPK23 with both OsCBL1 and OsAKT1. In the nucleus, the P1 protein enhances the binding of OsCIPK23 to OsCoilin, a homologue of the signature protein of CBs in Arabidopsis, and facilitates their trafficking through these CB structures. Genetic analysis indicates that mutant in oscipk23 suppresses RGSV systemic infection. Conversely, osakt1 mutants exhibited increased sensitivity to RGSV infection. These findings suggest that RGSV P1 hinders the absorption of K+ in rice plants by recruiting the OsCIPK23 to the CB structures. This process potentially promotes virus systemic infection but comes at the expense of inhibiting OsAKT1 activity.

Mitogen-activated protein kinase MxMPK3-2 mediated phosphorylation of MxZR3.1 participates in regulating iron homoeostasis in apple rootstocks

The micronutrient iron plays a crucial role in the growth and development of plants, necessitating meticulous regulation for its absorption by plants. Prior research has demonstrated that the transcription factor MxZR3.1 restricts iron absorption in apple rootstocks; however, the precise mechanism by which MxZR3.1 contributes to the regulation of iron homoeostasis in apple rootstocks remains unexplored. Here, MxMPK3-2, a protein kinase, was discovered to interact with MxZR3.1. Y2H, bimolecular fluorescence complementation and pull down experiments were used to confirm the interaction. Phosphorylation and cell semi-degradation tests have shown that MxZR3.1 can be used as a substrate of MxMPK3-2, which leads to the MxZR3.1 protein being more stable. In addition, through tobacco transient transformation (LUC and GUS) experiments, it was confirmed that MxZR3.1 significantly inhibited the activity of the MxHA2 promoter, while MxMPK3-2 mediated phosphorylation at the Ser94 site of MxZR3.1 further inhibited the activity of the MxHA2 promoter. It is tightly controlled to absorb iron during normal growth and development of apple rootstocks due to the regulatory effect of the MxMPK3-2-MxZR3.1 module on MxHA2 transcription level. Consequently, this research has revealed the molecular basis of how the MxMPK3-2-MxZR3.1 module in apple rootstocks controls iron homoeostasis by regulating the MxHA2 promoter's activity.

Chloroplast ion homeostasis - what do we know and where should we go?

Plant yields heavily depend on proper macro- and micronutrient supply from the soil. In the leaf cells, nutrient ions fulfill specific roles in biochemical reactions, especially photosynthesis housed in the chloroplast. Here, a well-balanced ion homeostasis is maintained by a number of ion transport proteins embedded in the envelope and thylakoid membranes. Ten years ago, the first alkali metal transporters from the K+ EFFLUX ANTIPORTER family were discovered in the model plant Arabidopsis. Since then, our knowledge about the physiological importance of these carriers and their substrates has greatly expanded. New insights into the role of alkali ions in plastid gene expression and photoprotective mechanisms, both prerequisites for plant productivity in natural environments, were gained. The discovery of a Cl- channel in the thylakoid and several additional plastid alkali and alkali metal transport proteins have advanced the field further. Nevertheless, scientists still have long ways to go before a complete systemic understanding of the chloroplast's ion transportome will emerge. In this Tansley review, we highlight and discuss the achievements of the last decade. More importantly, we make recommendations on what areas to prioritize, so the field can reach the next milestones. One area, laid bare by our similarity-based comparisons among phototrophs is our lack of knowledge what ion transporters are used by cyanobacteria to buffer photosynthesis fluctuations.

WRKY6 transcription factor modulates root potassium acquisition through promoting expression of AKT1 in Arabidopsis

Potassium (K+), being an essential macronutrient in plants, plays a central role in many aspects. Root growth is highly plastic and is affected by many different abiotic stresses including nutrient deficiency. The Shaker-type K+ channel Arabidopsis (Arabidopsis thaliana) K+ Transporter 1 (AKT1) is responsible for K+ uptake under both low and high external K+ conditions. However, the upstream transcription factor of AKT1 is not clear. Here, we demonstrated that the WRKY6 transcription factor modulates root growth to low potassium (LK) stress in Arabidopsis. WRKY6 showed a quick response to LK stress and also to many other abiotic stress treatments. The two wrky6 T-DNA insertion mutants were highly sensitive to LK treatment, whose primary root lengths were much shorter, less biomass and lower K+ content in roots than those of wild-type plants, while WRKY6-overexpression lines showed opposite phenotypes. A further investigation showed that WRKY6 regulated the expression of the AKT1 gene via directly binding to the W-box elements in its promoter through EMSA and ChIP-qPCR assays. A dual luciferase reporter analysis further demonstrated that WRKY6 enhanced the transcription of AKT1. Genetic analysis further revealed that the overexpression of AKT1 greatly rescued the short root phenotype of the wrky6 mutant under LK stress, suggesting AKT1 is epistatic to WRKY6 in the control of LK response. Further transcriptome profiling suggested that WRKY6 modulates LK response through a complex regulatory network. Thus, this study unveils a transcription factor that modulates root growth under potassium deficiency conditions by affecting the potassium channel gene AKT1 expression.

Research Progress on Plant Shaker K+ Channels

Plant growth and development are driven by intricate processes, with the cell membrane serving as a crucial interface between cells and their external environment. Maintaining balance and signal transduction across the cell membrane is essential for cellular stability and a host of life processes. Ion channels play a critical role in regulating intracellular ion concentrations and potentials. Among these, K+ channels on plant cell membranes are of paramount importance. The research of Shaker K+ channels has become a paradigm in the study of plant ion channels. This study offers a comprehensive overview of advancements in Shaker K+ channels, including insights into protein structure, function, regulatory mechanisms, and research techniques. Investigating Shaker K+ channels has enhanced our understanding of the regulatory mechanisms governing ion absorption and transport in plant cells. This knowledge offers invaluable guidance for enhancing crop yields and improving resistance to environmental stressors. Moreover, an extensive review of research methodologies in Shaker K+ channel studies provides essential reference solutions for researchers, promoting further advancements in ion channel research.

Potassium deficiency enhances imbalances in rice water relations under water deficit by decreasing leaf hydraulic conductance

Potassium (K+) is an essential macronutrient for appropriate plant development and physiology. However, little is known about the mechanisms involved in the regulation of leaf water relations by K under water deficit. A pot experiment with two K supplies of 0.45 and 0 g K2O per pot (3 kg soil per pot) and two watering conditions (well-watered and water-deficit) was conducted to explore the effects of K deficiency on canopy transpiration characteristics, leaf water status, photosynthesis, and hydraulic traits in two rice genotypes with contrasting resistance to drought. The results showed that K deficiency reduced canopy transpiration rate by decreasing stomatal conductance, which led to higher canopy temperatures, resulting in limited water deficit tolerance in rice. In addition, K deficiency led to further substantial reductions in leaf relative water content and water potential under water deficit, which increased the imbalance in leaf water relations under water deficit. Notably, K deficiency limited leaf gas exchange by reducing leaf hydraulic conductance, but decreased the intrinsic water use efficiency under water deficit, especially for the drought-resistant cultivar. Further analysis of the underlying process of leaf hydraulic resistance revealed that the key limiting factor of leaf hydraulic conductance under K deficiency was the outside-xylem hydraulic conductance rather than the xylem hydraulic conductance. Overall, our results provide a comprehensive perspective for assessing leaf water relations under K deficiency, water deficit, and their combined stresses, which will be useful for optimal rice fertilization strategies.

Arbuscular Mycorrhizal Fungi Improve Lycium barbarum Potassium Uptake by Activating the Expression of LbHAK

Arbuscular mycorrhizal (AM) fungi can establish a mutualistic relationship with the roots of most terrestrial plants to increase plant nutrient uptake. The effects of potassium uptake and transport by AM symbiosis are much less reported compared to other nutrients. In this research, a heterologous yeast system was used to verify that the LbHAK has capacity for potassium uptake. The split-roots system implemented using seedlings of Lycium barbarum confirmed that R. irregularis locally induced LbHAK expression, which means that LbHAK is only expressed in mycorrhizal roots. Furthermore, the impacts of overexpression of LbHAK on the growth, nutrients and water uptake, and transport of mycorrhizal tobacco (inoculation with Rhizophagus irregularis) at 0.2 mM and 2 mM K conditions were assessed. The mycorrhizal tobacco growth and potassium accumulation were significantly enhanced through LbHAK overexpression in tobacco. In addition, overexpression of LbHAK substantially enhanced phosphorus content, while stimulating the expression of NtPT4, Rir-AQP1, and Rir-AQP2 in mycorrhizal tobacco. Moreover, LbHAK overexpression greatly promoted AM colonization. LbHAK has a potential role in facilitating potassium absorption through the mycorrhizal pathway, and overexpression of LbHAK in tobacco may promote the transport of potassium, phosphorus, and water from AM fungi to tobacco. These data imply the important roles played by the LbHAK in AM-fungi-induced potassium uptake in L. barbarum and in improving plant nutrients and AM colonization.

Genomic analysis reveals phylogeny of Zygophyllales and mechanism for water retention of a succulent xerophyte

Revealing the genetic basis for stress-resistant traits in extremophile plants will yield important information for crop improvement. Zygophyllum xanthoxylum, an extant species of the ancient Mediterranean, is a succulent xerophyte that can maintain a favorable water status under desert habitats; however, the genetic basis of this adaptive trait is poorly understood. Furthermore, the phylogenetic position of Zygophyllales, to which Z. xanthoxylum belongs, remains controversial. In this study, we sequenced and assembled the chromosome-level genome of Z. xanthoxylum. Phylogenetic analysis showed that Zygophyllales and Myrtales form a separated taxon as a sister to the clade comprising fabids and malvids, clarifying the phylogenetic position of Zygophyllales at whole-genome scale. Analysis of genomic and transcriptomic data revealed multiple critical mechanisms underlying the efficient osmotic adjustment using Na+ and K+ as "cheap" osmolytes that Z. xanthoxylum has evolved through the expansion and synchronized expression of genes encoding key transporters/channels and their regulators involved in Na+/K+ uptake, transport, and compartmentation. It is worth noting that ZxCNGC1;1 (cyclic nucleotide-gated channels) and ZxCNGC1;2 constituted a previously undiscovered energy-saving pathway for Na+ uptake. Meanwhile, the core genes involved in biosynthesis of cuticular wax also featured an expansion and upregulated expression, contributing to the water retention capacity of Z. xanthoxylum under desert environments. Overall, these findings boost the understanding of evolutionary relationships of eudicots, illustrate the unique water retention mechanism in the succulent xerophyte that is distinct from glycophyte, and thus provide valuable genetic resources for the improvement of stress tolerance in crops and insights into the remediation of sodic lands.

Potassium rhythms couple the circadian clock to the cell cycle

Circadian (~24 h) rhythms are a fundamental feature of life, and their disruption increases the risk of infectious diseases, metabolic disorders, and cancer1-6. Circadian rhythms couple to the cell cycle across eukaryotes7,8 but the underlying mechanism is unknown. We previously identified an evolutionarily conserved circadian oscillation in intracellular potassium concentration, [K+]i9,10. As critical events in the cell cycle are regulated by intracellular potassium11,12, an enticing hypothesis is that circadian rhythms in [K+]i form the basis of this coupling. We used a minimal model cell, the alga Ostreococcus tauri, to uncover the role of potassium in linking these two cycles. We found direct reciprocal feedback between [K+]i and circadian gene expression. Inhibition of proliferation by manipulating potassium rhythms was dependent on the phase of the circadian cycle. Furthermore, we observed a total inhibition of cell proliferation when circadian gene expression is inhibited. Strikingly, under these conditions a sudden enforced gradient of extracellular potassium was sufficient to induce a round of cell division. Finally, we provide evidence that interactions between potassium and circadian rhythms also influence proliferation in mammalian cells. These results establish circadian regulation of intracellular potassium levels as a primary factor coupling the cell- and circadian cycles across diverse organisms.

Seeds exhibit the most stable elemental composition with nitrogen addition in an Inner Mongolian grassland

Variation in biomass elemental composition of grassland plants may have important implications for ecosystem functioning in response to global change. However, relevant studies have mostly focused on variation of nitrogen (N) and phosphorus (P) concentrations in plant leaves, while few studies have evaluated other elements and plant organs of grassland species. Here, we examined the effects of N addition on multi-element concentrations, and analyzed their patterns across different organs (leaf, stem, root and seed) of five plant species in a steppe community of the Inner Mongolian grassland. Our results showed that seeds exhibited the most stable elemental composition with N addition, and that manganese (Mn) and iron (Fe) concentrations were substantially more variable than macro-elements in response to N addition. In particular, we identified a set of significant negative relationships between elemental concentrations and their corresponding CVs (coefficients of variation) for all plant organs as a whole and for each individual organ. We further found that changes in soil pH and the availability of soil nutrients contributed mostly to variation in the biomass elemental composition of major plants in this community. These findings are important for accurately assessing the effects of N deposition on the biochemical cycling of nutrient elements in grassland ecosystems, and provide critical clues for developing effective approaches to adaptively managing grassland resources as well as mitigating the impact of global change on the dryland ecosystems in the Mongolia Plateau.

The reduced growth due to elevated CO2 concentration hinders the sexual reproduction of mature Northern pipevine (Aristolochia contorta Bunge)

The phenology has gained considerably more attention in recent times of climate change. The transition from vegetative to reproductive phases is a critical process in the life history of plants, closely tied to phenology. In an era of climate change, understanding how environmental factors affect this transition is of paramount importance. This study consisted of field surveys and a greenhouse experiment on the reproductive biology of Northern pipevine (Aristolochia contorta Bunge). During field surveys, we investigated the environmental factors and growth characteristics of mature A. contorta, with a focus on both its vegetative and reproductive phases. In its successful flowering during the reproductive phase, A. contorta grew under the conditions of 40% relative light intensity and 24% soil moisture content, and had a vertical rhizome. In the greenhouse experiments, we examined the impact of increased CO2 concentration on the growth and development of 10-year-old A. contorta, considering the effect of rhizome direction. Planted with a vertical rhizome direction, A. contorta exhibited sufficient growth for flowering under ambient CO2 concentrations. In contrast, when planted with a horizontal rhizome direction, it was noted to significantly impede successful growth and flowering under elevated CO2 concentrations. This hindered the process of flowering, highlighting the pivotal role of substantial vegetative growth in achieving successful flowering. Furthermore, we observed a higher number of underground buds and shoots under the conditions of elevated CO2 concentration and a horizontal rhizome direction instead of flowering. Elevated CO2 concentrations also exhibited diverse effects on mature A. contorta's flower traits, resulting in smaller flower size, shorter longevity, and reduced stigma receptivity, and pollen viability. The study shed light on elevated CO2 concentrations can hinder growth, potentially obstructing sexual reproduction and diminishing genetic diversity.

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.

The EIN3/EIL-ERF9-HAK5 transcriptional cascade positively regulates high-affinity K+ uptake in Gossypium hirsutum

High-affinity K+ (HAK) transporters play essential roles in facilitating root K+ uptake in higher plants. Our previous studies revealed that GhHAK5a, a member of the HAK family, is crucial for K+ uptake in upland cotton. Nevertheless, the precise regulatory mechanism governing the expression of GhHAK5a remains unclear. The yeast one-hybrid screening was performed to identify the transcription factors responsible for regulating GhHAK5a, and ethylene response factor 9 (GhERF9) was identified as a potential candidate. Subsequent dual-luciferase and electrophoretic mobility shift assays confirmed that GhERF9 binds directly to the GhHAK5a promoter, thereby activating its expression. Silencing of GhERF9 decreased the expression of GhHAK5a and exacerbated K+ deficiency symptoms in leaves, also decreased K+ uptake rate and K+ content in roots. Additionally, it was observed that the application of ethephon (an ethylene-releasing reagent) resulted in a significant upregulation of GhERF9 and GhHAK5a, accompanied by an increased rate of K+ uptake. Expectedly, GhEIN3b and GhEIL3c, the two key components involved in ethylene signaling, bind directly to the GhERF9 promoter. These findings provide valuable insights into the molecular mechanisms underlying the expression of GhHAK5a and ethylene-mediated K+ uptake and suggest a potential strategy to genetically enhance cotton K+ uptake by exploiting the EIN3/EILs-ERF9-HAK5 module.

Calcium regulation of the Arabidopsis Na+/K+ transporter HKT1;1 improves seed germination under salt stress

Calcium is known to improve seed-germination rates under salt stress. We investigated the involvement of calcium ions (Ca2+) in regulating HIGH-AFFINITY K+ TRANSPORTER 1 (HKT1; 1), which encodes a Na+/K+ transporter, and its post-translational regulator TYPE 2C PROTEIN PHOSPHATASE 49 (PP2C49), in germinating Arabidopsis (Arabidopsis thaliana) seedlings. Germination rates of hkt1 mutant seeds under salt stress remained unchanged by CaCl2 treatment in wild-type Arabidopsis, whereas pp2c49 mutant seeds displayed improved salt-stress tolerance in the absence of CaCl2 supplementation. Analysis of HKT1;1 and PP2C49 promoter activity revealed that CaCl2 treatment results in radicle-focused expression of HKT1;1 and reduction of the native radicle-exclusive expression of PP2C49. Ion-content analysis indicated that CaCl2 treatment improves K+ retention in germinating wild-type seedlings under salt stress, but not in hkt1 seedlings. Transgenic seedlings designed to exclusively express HKT1;1 in the radicle during germination displayed higher germination rates under salt stress than the wild type in the absence of CaCl2 treatment. Transcriptome analysis of germinating seedlings treated with CaCl2, NaCl, or both revealed 118 upregulated and 94 downregulated genes as responsive to the combined treatment. Bioinformatics analysis of the upstream sequences of CaCl2-NaCl-treatment-responsive upregulated genes revealed the abscisic acid response element CACGTGTC, a potential CaM-binding transcription activator-binding motif, as most prominent. Our findings suggest a key role for Ca2+ in mediating salt-stress responses during germination by regulating genes that function to maintain Na+ and K+ homeostasis, which is vital for seed germination under salt stress.

X-ray absorption spectroscopy and theoretical investigations of the effect of extended ligands in potassium organic matter interaction

Potassium (K) is an essential nutrient for plant growth, and despite its abundance in soil, most of the K is structurally bound in minerals, limiting its bioavailability and making this soil K reservoir largely inaccessible to plants. Microbial biochemical weathering has been shown to be a promising pathway to sustainably increase plant available K. However, the mechanisms underpinning microbial K uptake, transformation, storage, and sharing are poorly resolved. To better understand the controls on microbial K transformations, we performed K K-edge x-ray absorption near-edge structure (XANES) spectroscopy on K-organic salts, including acetate, citrate, nitrate, oxalate, and tartrate, which are frequently observed as low molecular weight organic acids secreted by soil microbes, as well as humic acid, which acts as a proxy for higher molecular weight organic acids. The organic salts display feature-rich K XANES spectra, each demonstrating numerous unique features spanning ∼13 eV range across the absorption edge. In contrast, the spectra for humic acid have one broad, wide feature across the same energy range. We used a combination of time-dependent density functional theory and the Bethe-Salpeter equation based approach within the OCEAN code to simulate the experimental spectra for K-nitrate (KNO3) and K-citrate [K3(C6H5O7)·H2O] to identify the electronic transitions that give rise to some of the outlying and unique spectral features in the organic salts. KNO3 has both the lowest and highest lying energy features, and K3(C6H5O7)·H2O is produced by several soil microbes and is effective at mineral weathering. Our results analyze the K-organic salt bonding in detail to elucidate why the spectral shapes differ and indicate that the K K-edge XANES spectra are associated with the entire ligand despite similar first-shell bonding environments around the K center. The improved understanding of K bonding environments with organic ligands and their use for interpretation of the K-XANES spectra provides an important toolkit to understand how K is transformed by microbial processes and made bioavailable for plant uptake.

Transcriptome analysis and identification of the low potassium stress-responsive gene SiSnRK2.6 in foxtail millet (Setaria italica L.)

KEY MESSAGE

The transcriptome is beneficial for dissecting the mechanism of millet in response to low potassium stress and SiSnRK2.6 was identified as a potential target for improving low potassium stress tolerance. Foxtail millet (Setaria italica L.), which originated in China, has high nutrient utilization character. Nevertheless, the molecular mechanism of its tolerance to low potassium stress is largely unclear. In this research, the low potassium tolerant variety "Yugu28" was screened out by low potassium stress treatment, and the transcriptome of "Yugu28" under low potassium stress was comprehensively analyzed. A total of 4254 differentially expressed genes (DEGs) were identified, including 1618 up-regulated and 2636 down-regulated genes, respectively. In addition, there were 302 transcription factor (TF) genes in the DEGs and MYB TFs accounted for the highest proportion, which was 14.9%. After functional analysis of all DEGs, a total of 7 genes involved in potassium transport and potassium ion channels and 50 genes corresponding to hormones were screened. The expression levels of randomly selected 17 DEGs were verified by qRT-PCR and the results coincided well with the RNA-seq analysis, indicating the reliability of our transcriptome data. Moreover, one of the ABA signaling pathway genes, SiSnRK2.6, was identified and selected for further functional verification. Compared with the wild type, transgenic rice with ecotopic expression of SiSnRK2.6 showed remarkably increased root length and root number, indicating that overexpression of SiSnRK2.6 can enhance the resistance of transgenic plants to low potassium stress.

Deciphering the effect of salinity and boron stress on broccoli plants reveals that membranes phytosterols and PIP aquaporins facilitate stress adaptation

Abiotic stresses, such as salinity and boron toxicity/deficiency, are prevalent in arid and semi-arid regions where broccoli is largely cultivated. This study aimed to investigate the physiological response of broccoli leaves to these stresses, focusing on parameters such as growth, relative water content, stomatal conductance, and mineral concentration after 15 days of treatment application. The effects of individual and combined stresses of salinity and boron (deficiency and toxicity) were examined. Additionally, the study explored the molecular aspects of PIP aquaporins in relation to their presence in the plasma membrane and their interaction with the lipid environment. The results showed that the combined stress of salinity and boron deficiency resulted in a significant reduction in plant biomass, suggesting a specific adaptation to this stress combination. Changes in stomatal conductance and mineral nutrient levels indicated that the adaptation mechanisms were associated with water and boron concentration in the leaves. The expression patterns of PIP aquaporins varied among the different stress treatments, either individually or in combination. Furthermore, the presence of aquaporins in the plasma membrane and microsomal fraction highlighted the potential regulatory roles of trafficking along with the membrane composition, particularly the concentration of phytosterols. The results underscore the importance of water transport by aquaporins and their interaction with the sterol composition in the membranes, in facilitating salinity-boron stress adaptation mechanisms.

Nitrogen-potassium balance improves leaf photosynthetic capacity by regulating leaf nitrogen allocation in apple

Nitrogen (N) and potassium (K) are two important mineral nutrients in regulating leaf photosynthesis. However, the influence of N and K interaction on photosynthesis is still not fully understood. Using a hydroponics approach, we studied the effects of different N and K conditions on the physiological characteristics, N allocation and photosynthetic capacity of apple rootstock M9T337. The results showed that high N and low K conditions significantly reduced K content in roots and leaves, resulting in N/K imbalance, and allocated more N in leaves to non-photosynthetic N. Low K conditions increased biochemical limitation (BL), mesophyll limitation (MCL), and stomatal limitation (SL). By setting different N supplies, lowering N levels under low K conditions increased the proportion of water-soluble protein N (Nw) and sodium dodecyl sulfate-soluble proteins (Ns) by balancing N/K and increased the proportion of carboxylation N and electron transfer N. This increased the maximum carboxylation rate and mesophyll conductance, which reduced MCL and BL and alleviated the low K limitation of photosynthesis in apple rootstocks. In general, our results provide new insights into the regulation of photosynthetic capacity by N/K balance, which is conducive to the coordinated supply of N and K nutrients.

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.

TORC pathway intersects with a calcium sensor kinase network to regulate potassium sensing in Arabidopsis

Potassium (K) is an essential macronutrient for plant growth, and its availability in the soil varies widely, requiring plants to respond and adapt to the changing K nutrient status. We show here that plant growth rate is closely correlated with K status in the medium, and this K-dependent growth is mediated by the highly conserved nutrient sensor, target of rapamycin (TOR). Further study connected the TOR complex (TORC) pathway with a low-K response signaling network consisting of calcineurin B-like proteins (CBL) and CBL-interacting kinases (CIPK). Under high K conditions, TORC is rapidly activated and shut down the CBL-CIPK low-K response pathway through regulatory-associated protein of TOR (RAPTOR)-CIPK interaction. In contrast, low-K status activates CBL-CIPK modules that in turn inhibit TORC by phosphorylating RAPTOR, leading to dissociation and thus inactivation of the TORC. The reciprocal regulation of the TORC and CBL-CIPK modules orchestrates plant response and adaptation to K nutrient status in the environment.

Rice potassium transporter OsHAK18 mediates phloem K+ loading and redistribution

High-affinity K+ transporters/K+ uptake permeases/K+ transporters (HAK/KUP/KT) are important pathways mediating K+ transport across cell membranes, which function in maintaining K+ homeostasis during plant growth and stress response. An increasing number of studies have shown that HAK/KUP/KT transporters play crucial roles in root K+ uptake and root-to-shoot translocation. However, whether HAK/KUP/KT transporters also function in phloem K+ translocation remain unclear. In this study, we revealed that a phloem-localized rice HAK/KUP/KT transporter, OsHAK18, mediated cell K+ uptake when expressed in yeast, Escherichia coli and Arabidopsis. It was localized at the plasma membrane. Disruption of OsHAK18 rendered rice seedlings insensitive to low-K+ (LK) stress. After LK stress, some WT leaves showed severe wilting and chlorosis, whereas the corresponding leaves of oshak18 mutant lines (a Tos17 insertion line and two CRISPR lines) remained green and unwilted. Compared with WT, the oshak18 mutants accumulated more K+ in shoots but less K+ in roots after LK stress, leading to a higher shoot/root ratio of K+ per plant. Disruption of OsHAK18 does not affect root K+ uptake and K+ level in xylem sap, but it significantly decreases phloem K+ concentration and inhibits root-to-shoot-to-root K+ (Rb+ ) translocation in split-root assay. These results reveal that OsHAK18 mediates phloem K+ loading and redistribution, whose disruption is in favor of shoot K+ retention under LK stress. Our findings expand the understanding of HAK/KUP/KT transporters' functions and provide a promising strategy for improving rice tolerance to K+ deficiency.

Emerging Roles of Receptor-like Protein Kinases in Plant Response to Abiotic Stresses

The productivity of plants is hindered by unfavorable conditions. To perceive stress signals and to transduce these signals to intracellular responses, plants rely on membrane-bound receptor-like kinases (RLKs). These play a pivotal role in signaling events governing growth, reproduction, hormone perception, and defense responses against biotic stresses; however, their involvement in abiotic stress responses is poorly documented. Plant RLKs harbor an N-terminal extracellular domain, a transmembrane domain, and a C-terminal intracellular kinase domain. The ectodomains of these RLKs are quite diverse, aiding their responses to various stimuli. We summarize here the sub-classes of RLKs based on their domain structure and discuss the available information on their specific role in abiotic stress adaptation. Furthermore, the current state of knowledge on RLKs and their significance in abiotic stress responses is highlighted in this review, shedding light on their role in influencing plant-environment interactions and opening up possibilities for novel approaches to engineer stress-tolerant crop varieties.

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.

Calcium signaling-mediated transcriptional reprogramming during abiotic stress response in plants

Calcium (Ca2+) is a second messenger in plants growth and development, as well as in stress responses. The transient elevation in cytosolic Ca2+ concentration have been reported to be involved in plants response to abiotic and biotic stresses. In plants, Ca2+-induced transcriptional changes trigger molecular mechanisms by which plants adapt and respond to environment stresses. The mechanism for transcription regulation by Ca2+ could be either rapid in which Ca2+ signals directly cause the related response through the gene transcript and protein activities, or involved amplification of Ca2+ signals by up-regulation the expression of Ca2+ responsive genes, and then increase the transmission of Ca2+ signals. Ca2+ regulates the expression of genes by directly binding to the transcription factors (TFs), or indirectly through its sensors like calmodulin, calcium-dependent protein kinases (CDPK) and calcineurin B-like protein (CBL). In recent years, significant progress has been made in understanding the role of Ca2+-mediated transcriptional regulation in different processes in plants. In this review, we have provided a comprehensive overview of Ca2+-mediated transcriptional regulation in plants in response to abiotic stresses including nutrition deficiency, temperature stresses (like heat and cold), dehydration stress, osmotic stress, hypoxic, salt stress, acid rain, and heavy metal stress.

Monitoring nutrients in plants with genetically encoded sensors: achievements and perspectives

Understanding mechanisms of nutrient allocation in organisms requires precise knowledge of the spatiotemporal dynamics of small molecules in vivo. Genetically encoded sensors are powerful tools for studying nutrient distribution and dynamics, as they enable minimally invasive monitoring of nutrient steady-state levels in situ. Numerous types of genetically encoded sensors for nutrients have been designed and applied in mammalian cells and fungi. However, to date, their application for visualizing changing nutrient levels in planta remains limited. Systematic sensor-based approaches could provide the quantitative, kinetic information on tissue-specific, cellular, and subcellular distributions and dynamics of nutrients in situ that is needed for the development of theoretical nutrient flux models that form the basis for future crop engineering. Here, we review various approaches that can be used to measure nutrients in planta with an overview over conventional techniques, as well as genetically encoded sensors currently available for nutrient monitoring, and discuss their strengths and limitations. We provide a list of currently available sensors and summarize approaches for their application at the level of cellular compartments and organelles. When used in combination with bioassays on intact organisms and precise, yet destructive analytical methods, the spatiotemporal resolution of sensors offers the prospect of a holistic understanding of nutrient flux in plants.

Hippuris vulgaris could replace Myriophyllum aquaticum for efficiently removing water phosphorus under low temperature conditions in China

Phytoremediation is widely used for the restoration of aquatic environments. However, the phytoremediation effects and mechanisms of special submerged species of native aquatic plants, especially under low-temperature conditions, are not yet clear. In this study, two typical submerged plants, Myriophyllum aquaticum (M. aquaticum; an exotic species) and Hippuris vulgaris (H. vulgaris; a native species), in China were investigated for their phosphorus (P) removal efficiencies (RE_p) and the related mechanisms of phytophysiology and microorganisms in a low-temperature incubator (10 °C during the day and 2 °C at night). At an initial P level of 0.5 mg L^-1, the two plants exhibited similar RE_p, with the highest values (73.5%-92.1%) observed on days 3-6. After 18 days, the residual P concentration in the water was less than the Grade III limit value (0.2 mg L^-1; GB 3838-2002). However, M. aquaticum had a faster RE_p velocity than H. vulgaris at an initial P level of 3.0 mg L^-1, which was attributed to the mechanisms of plant and its interactions with microorganisms. Compared to the control group, the superoxide dismutase activity of H. vulgaris was significantly increased and its catalase activity was decreased, whereas for that of M. aquaticum was the opposite. Micro region X-ray fluorescence analysis revealed that there may be synergic absorption effects between P, S, and K, and antagonistic absorption action between P and Mn in H. vulgaris. In addition, Acinetobacter, Novosphingobium and Pseudomonas were enriched at 3.0 mg L^-1 P level with these two plants, but Chlorophyta only accumulated with H. vulgaris, respectively. Overall, the native species, H. vulgaris, could replace the exotic M. aquaticum to efficiently remove P from polluted water at low temperatures. These findings provide a theoretical foundation for submerged plants P removal capabilities, and the protection of local ecosystem diversity at low temperatures.

Calcium signalling components underlying NPK homeostasis: potential avenues for exploration

Plants require the major macronutrients, nitrogen (N), phosphorus (P) and potassium (K) for normal growth and development. Their deficiency in soil directly affects vital cellular processes, particularly root growth and architecture. Their perception, uptake and assimilation are regulated by complex signalling pathways. To overcome nutrient deficiencies, plants have developed certain response mechanisms that determine developmental and physiological adaptations. The signal transduction pathways underlying these responses involve a complex interplay of components such as nutrient transporters, transcription factors and others. In addition to their involvement in cross-talk with intracellular calcium signalling pathways, these components are also engaged in NPK sensing and homeostasis. The NPK sensing and homeostatic mechanisms hold the key to identify and understand the crucial players in nutrient regulatory networks in plants under both abiotic and biotic stresses. In this review, we discuss calcium signalling components/pathways underlying plant responses to NPK sensing, with a focus on the sensors, transporters and transcription factors involved in their respective signalling and homeostasis.

Tillage, green manuring and crop residue management impacts on crop productivity, potassium use efficiency and potassium fractions under rice-wheat system

The conventional crop production practices including intensive tillage and open field crop residue burning in world' largest rice-wheat system (RWS) are adversely affecting crop productivity besides deteriorating natural resources and ecosystems' sustainability. In order to improve system productivity, potassium (K) use efficiency and apparent K balance, adoption of conservation tillage in a RWS with residue management is considered highly effective. We therefore, studied the effect of wheat straw retention and green manure (GM) in rice (main plot treatment), and tillage and rice residue management in subsequent wheat (sub-plot treatments) on crop productivity, K use efficiency and its transformation amongst different fractions of variable solubility. These results revealed that rice straw retention along with GM significantly (p < 0.05) increased the rice yields by ∼5.3-6.7% and wheat yields by ∼10.2-16.9%, compared to the conventional tillage (CT) without GM. Green manuring during the intervening period (CTR_W0+GM) significantly increased the rice grain K uptake by ∼36.2% than in plots with no-GM (CTR_W0). However, it increased by ∼29.8% under CTR_W25+GM, compared with CTR_W25-GM treatment. As compared with CTR_W0, CTR_W0+GM significantly increased the reciprocal internal use efficiency of K of rice by 3.8 kg Mg^-1 grain yield (∼29.5%). However, CTR_W25+GM increased the RIUE_K of rice by 3.3 kg Mg^-1 grain yield (∼22.4%), compared with no-GM (CTR_W25). Although, apparent K balance was net negative for CTR_W25, ZTW_R100 treatments, yet there was decreased K mining of 56-262 kg K ha^-1 (∼11.9-61.2%) for CTR_W25 and ZTW_R100 over CTR_W0 and ZTW_R0. The increased crop yield, K uptake and K use efficiency were significantly related to K enrichment in water soluble K, exchangeable K, non-exchangeable-K, hydrochloric acid extractable-K, lattice-K and total K fractions by ∼1.3, 3.4, 18.6, 11.0 and 34.1%, respectively due to residue retention, compared with no residue. Therefore, conventional tillage with puddled transplanted rice (CTR) with wheat residue and green manure during intervening period (CTR_W25+GM), and zero tillage wheat with rice residue retention (ZTW_R100) were emerged as highly valuable technological options for mitigating soil degradation effects under intensive RWS for food grains in north-western India.

Single-Slice XRF Mapping of Light Elements in Frozen-Hydrated Allium schoenoprasum via a Self-Absorption-Corrected Hyperspectral Tomographic Reconstruction Approach

3D and 2D-cross-sectional X-ray fluorescence analysis of biological material is a powerful tool to image the distribution of elements and to understand and quantify metal homeostasis and the distribution of anthropogenic metals and nanoparticles with minimal preparation artifacts. Using tomograms recorded on cryogenically prepared leaves of Allium schoenoprasum, the cross-sectional distribution of physiologically relevant elements like calcium, potassium, manganese, and zinc could be tomographically reconstructed by peak fitting followed by a conventional maximum-likelihood algorithm with self-absorption correction to reveal the quantitative cross-sectional element distribution. If light elements such as S and P are located deep in the sample compared to the escape depth of their characteristic X-ray fluorescence lines, the quantitative reconstruction becomes inaccurate. As a consequence, noise is amplified to a magnitude where it might be misinterpreted as actual concentration. We show that a tomographic MCA hyperspectral reconstruction in combination with a self-absorption correction allows for fitting of the XRF spectra directly in real space, which significantly improves the qualitative and quantitative analysis of the light elements compared to the conventional method as noise and artifacts in the tomographic reconstruction are reduced. This reconstruction approach can substantially improve the quantitative analysis of trace elements as it allows the fitting of summed voxel spectra in anatomical regions of interest. The presented method can be applied to XRF 2D single-slice tomography data and 3D tomograms and is particularly relevant for, but not limited to, biological material in order to help retrieve self-absorption corrected quantitative reconstructions of the spatial distribution of light elements and ultra-trace-elements.

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.

LbKAT3 may assist in mycorrhizal potassium uptake, and overexpression of LbKAT3 may promote potassium, phosphorus, and water transport from arbuscular mycorrhizal fungi to the host plant

Potassium plays important roles in most plant physiological processes. Arbuscular mycorrhizal (AM) fungi promote plant water and mineral nutrient acquisition to promote plant growth. However, few studies have focused on the effect of AM colonization on potassium uptake by the host plant. In this study, the effects of an AM fungus (Rhizophagus irregularis) and potassium concentration (0, 3, or 10 mM K⁺) on Lycium barbarum were evaluated. A split-root test with L. barbarum seedlings was conducted, and the potassium uptake capacity of LbKAT3 was verified in yeast. A tobacco line overexpressing LbKAT3 was generated and mycorrhizal functions under two potassium concentrations (0.2 and 2 mM K⁺) were studied. Inoculation of R. irregularis and application of potassium increased the dry weight, and potassium and phosphorus contents of L. barbarum, and increased the colonization rate and arbuscule abundance of R. irregularis. In addition, the expression of LbKAT3 and AQP genes in L. barbarum was upregulated. Inoculation of R. irregularis induced LbPT4, Rir-AQP1, and Rir-AQP2 expression, and application of potassium upregulated the expression of these genes. Inoculation with the AM fungus locally regulated the expression of LbKAT3. Inoculation of R. irregularis improved the growth, and potassium and phosphorus contents, and induced NtPT4, Rir-AQP1, and Rir-AQP2 expression in tobacco overexpressing LbKAT3 under both potassium concentrations. Overexpression of LbKAT3 in tobacco improved the growth, potassium accumulation, and AM colonization, and upregulated the expression of NtPT4 and Rir-AQP1 in mycorrhizal tobacco. The results suggest that LbKAT3 may assist in mycorrhizal potassium uptake, and overexpression of LbKAT3 may promote potassium, phosphorus, and water transport from the AM fungus to tobacco.