Plant Calcium Signaling in Response to Potassium Deficiency

Corchorus tridens L.: A Review of Its Botany, Phytochemistry, Nutritional Content and Pharmacological Properties

Phytotherapy is a cost-effective alternative that continues to evolve. This has sparked significant research interest in naturally occurring compounds found in edible plants that possess antibacterial, antioxidant, and anticancer properties. Corchorus tridens L. is a wild edible plant widely recognised for its edible leaves, which are used for vegetable and animal feed. The plant is widely distributed across the African continent and is utilised in numerous countries for treating fever, pain, inflammation, and sexually transmitted diseases. Extracts from various parts of this plant exhibit antimicrobial, antioxidant, and pesticidal properties. This plant is a rich source of amino acids, vitamins, essential fatty acids, proteins, and minerals, as well as secondary metabolites such as alkaloids, flavonoids, quinines, steroids, terpenoids, phenols, and tannins. Additional studies are still needed to determine other biological activities, such as anti-inflammatory activity, involvement in the treatment of measles, prevention of anaemia, and pain-relieving properties. The current review aims to provide information on the characteristics, distribution, nutritional content, bioactive compounds, traditional uses, and biological activities of the edible plant species C. tridens L. to stimulate further research interest to address the existing literature gaps concerning this plant.

Can nutrients act as signals under abiotic stress?

Plant cells are in constant communication to coordinate development processes and environmental reactions. Under stressful conditions, such communication allows the plant cells to adjust their activities and development. This is due to intercellular signaling events which involve several components. In plant development, cell-to-cell signaling is ensured by mobile signals hormones, hydrogen peroxide (H2O2), nitric oxide (NO), or hydrogen sulfide (H2S), as well as several transcription factors and small RNAs. Mineral nutrients, including macro and microelements, are determinant factors for plant growth and development and are, currently, recognized as potential signal molecules. This review aims to highlight the role of nutrients, particularly calcium, potassium, magnesium, nitrogen, phosphorus, and iron as signaling components with special attention to the mechanism of response against stress conditions.

The Roles of Calcineurin B-like Proteins in Plants under Salt Stress

Salinity stands as a significant environmental stressor, severely impacting crop productivity. Plants exposed to salt stress undergo physiological alterations that influence their growth and development. Meanwhile, plants have also evolved mechanisms to endure the detrimental effects of salinity-induced salt stress. Within plants, Calcineurin B-like (CBL) proteins act as vital Ca2+ sensors, binding to Ca2+ and subsequently transmitting signals to downstream response pathways. CBLs engage with CBL-interacting protein kinases (CIPKs), forming complexes that regulate a multitude of plant growth and developmental processes, notably ion homeostasis in response to salinity conditions. This review introduces the repercussions of salt stress, including osmotic stress, diminished photosynthesis, and oxidative damage. It also explores how CBLs modulate the response to salt stress in plants, outlining the functions of the CBL-CIPK modules involved. Comprehending the mechanisms through which CBL proteins mediate salt tolerance can accelerate the development of cultivars resistant to salinity.

Transcriptomic analysis reveals that methyl jasmonate confers salt tolerance in alfalfa by regulating antioxidant activity and ion homeostasis

Introduction

Alfalfa, a globally cultivated forage crop, faces significant challenges due to its vulnerability to salt stress. Jasmonates (JAs) play a pivotal role in modulating both plant growth and response to stressors.

Methods

In this study, alfalfa plants were subjected to 150 mM NaCl with or without methyl jasmonate (MeJA). The physiological parameters were detected and a transcriptomic analysis was performed to elucidate the mechanisms underlying MeJA-mediated salt tolerance in alfalfa.

Results

Results showed that exogenous MeJA regulated alfalfa seed germination and primary root growth in a dose-dependent manner, with 5µM MeJA exerting the most efficient in enhancing salt tolerance. MeJA at this concentration elavated the salt tolerance of young alfalfa seedlings by refining plant growth, enhancing antioxidant capacity and ameliorating Na+ overaccumulation. Subsequent transcriptomic analysis identified genes differentially regulated by MeJA+NaCl treatment and NaCl alone. PageMan analysis revealed several significantly enriched categories altered by MeJA+NaCl treatment, compared with NaCl treatment alone, including genes involved in secondary metabolism, glutathione-based redox regulation, cell cycle, transcription factors (TFs), and other signal transductions (such as calcium and ROS). Further weighted gene co-expression network analysis (WGCNA) uncovered that turquoise and yellow gene modules were tightly linked to antioxidant enzymes activity and ion content, respectively. Pyruvate decar-boxylase (PDC) and RNA demethylase (ALKBH10B) were identified as the most central hub genes in these two modules. Also, some TFs-hub genes were identified by WGCNA in these two modules highly positive-related to antioxidant enzymes activity and ion content.

Discussion

MeJA triggered a large-scale transcriptomic remodeling, which might be mediated by transcriptional regulation through TFs or post-transcriptional regulation through demethylation. Our findings contributed new perspectives for understanding the underneath mechanisms by which JA-mediated salt tolerance in alfalfa.

Genome-wide identification and expression analysis of the cyclic nucleotide-gated ion channel (CNGC) gene family in Saccharum spontaneum

BACKGROUND

Cyclic nucleotide-gated ion channels (CNGCs) are nonselective cation channels that are ubiquitous in eukaryotic organisms. As Ca²⁺ channels, some CNGCs have also proven to be K⁺-permeable and involved in plant development and responses to environmental stimuli. Sugarcane is an important sugar and energy crop worldwide. However, reports on CNGC genes in sugarcane are limited.

RESULTS

In this study, 16 CNGC genes and their alleles were identified from Saccharum spontaneum and classified into 5 groups based on phylogenetic analysis. Investigation of gene duplication and syntenic relationships between S. spontaneum and both rice and Arabidopsis demonstrated that the CNGC gene family in S. spontaneum expanded primarily by segmental duplication events. Many SsCNGCs showed variable expression during growth and development as well as in tissues, suggesting functional divergence. Light-responsive cis-acting elements were discovered in the promoters of all the identified SsCNGCs, and the expression of most of the SsCNGCs showed a diurnal rhythm. In sugarcane, the expression of some SsCNGCs was regulated by low-K⁺ treatment. Notably, SsCNGC13 may be involved in both sugarcane development and its response to environmental stimuli, including response to low-K⁺ stress.

CONCLUSION

This study identified the CNGC genes in S. spontaneum and provided insights into the transcriptional regulation of these SsCNGCs during development, circadian rhythm and under low-K⁺ stress. These findings lay a theoretical foundation for future investigations of the CNGC gene family in sugarcane.

The Roles of CDPKs as a Convergence Point of Different Signaling Pathways in Maize Adaptation to Abiotic Stress

The calcium ion (Ca²⁺), as a well-known second messenger, plays an important role in multiple processes of growth, development, and stress adaptation in plants. As central Ca²⁺ sensor proteins and a multifunctional kinase family, calcium-dependent protein kinases (CDPKs) are widely present in plants. In maize, the signal transduction processes involved in ZmCDPKs' responses to abiotic stresses have also been well elucidated. In addition to Ca²⁺ signaling, maize ZmCDPKs are also regulated by a variety of abiotic stresses, and they transmit signals to downstream target molecules, such as transport proteins, transcription factors, molecular chaperones, and other protein kinases, through protein interaction or phosphorylation, etc., thus changing their activity, triggering a series of cascade reactions, and being involved in hormone and reactive oxygen signaling regulation. As such, ZmCDPKs play an indispensable role in regulating maize growth, development, and stress responses. In this review, we summarize the roles of ZmCDPKs as a convergence point of different signaling pathways in regulating maize response to abiotic stress, which will promote an understanding of the molecular mechanisms of ZmCDPKs in maize tolerance to abiotic stress and open new opportunities for agricultural applications.

Proteomic Analysis of Roots Response to Potassium Deficiency and the Effect of TaHAK1-4A on K+ Uptake in Wheat

Potassium (K+) is essential for plant growth and stress responses. A deficiency in soil K+ contents can result in decreased wheat quality and productivity. Thus, clarifying the molecular mechanism underlying wheat responses to low-K+ (LK) stress is critical. In this study, a tandem mass tag (TMT)-based quantitative proteomic analysis was performed to investigate the differentially abundant proteins (DAPs) in roots of the LK-tolerant wheat cultivar “KN9204” at the seedling stage after exposure to LK stress. A total of 104 DAPs were identified in the LK-treated roots. The DAPs related to carbohydrate and energy metabolism, transport, stress responses and defense, and post-translational modifications under LK conditions were highlighted. We identified a high-affinity potassium transporter (TaHAK1-4A) that was significantly up-regulated after the LK treatment. Additionally, TaHAK1-4A was mainly expressed in roots, and the encoded protein was localized in the plasma membrane. The complementation assay in yeast suggested that TaHAK1-4A mediates K+ uptake under extreme LK conditions. The overexpression of TaHAK1-4A increased the fresh weight and root length of Arabidopsis under LK conditions and improved the growth of Arabidopsis athak5 mutant seedlings, which grow poorly under LK conditions. Moreover, silencing of TaHAK1-4A in wheat roots treated with LK stress decreased the root length, dry weight, K+ concentration, and K+ influx. Accordingly, TaHAK1-4A is important for the uptake of K+ by roots exposed to LK stress. Our results reveal the protein metabolic changes in wheat induced by LK stress. Furthermore, we identified a candidate gene potentially relevant for developing wheat lines with increased K+ use efficiency.

Reducing potassium deficiency by using sodium fertilisation

Potassium (K) is the most abundant cation in the vast majority of plants. It is required in large quantities which, in an agronomic context, typically necessitates application of K in the form of potash or other K fertilisers. Recently, the price of K fertiliser has risen dramatically, a situation that is paralleled by increasing K deficiency of soils around the globe. A potential solution to this problem is to reduce crop K fertiliser dependency by replacing it with sodium (Na) fertiliser which carries a much smaller price tag. In this paper we discuss the physiological roles of K and Na and the implications of Na fertilisation for crop cultivation and soil management. By using greenhouse growth assays we show distinct growth promotion after Na fertilisation in wheat, tomato, oilseed and sorghum. Our results also show that up to 60% of tissue K can be substituted by Na without growth penalty. Based on these data, simple economic models suggest that (part) replacement of K fertiliser with Na fertiliser leads to considerable savings.

The Calcium-Dependent Protein Kinase TaCDPK27 Positively Regulates Salt Tolerance in Wheat

As essential calcium ion (Ca2+) sensors in plants, calcium-dependent protein kinases (CDPKs) function in regulating the environmental adaptation of plants. However, the response mechanism of CDPKs to salt stress is not well understood. In the current study, the wheat salt-responsive gene TaCDPK27 was identified. The open reading frame (ORF) of TaCDPK27 was 1875 bp, coding 624 amino acids. The predicted molecular weight and isoelectric point were 68.905 kDa and 5.6, respectively. TaCDPK27 has the closest relationship with subgroup III members of the CDPK family of rice. Increased expression of TaCDPK27 in wheat seedling roots and leaves was triggered by 150 mM NaCl treatment. TaCDPK27 was mainly located in the cytoplasm. After NaCl treatment, some of this protein was transferred to the membrane. The inhibitory effect of TaCDPK27 silencing on the growth of wheat seedlings was slight. After exposure to 150 mM NaCl for 6 days, the NaCl stress tolerance of TaCDPK27-silenced wheat seedlings was reduced, with shorter lengths of both roots and leaves compared with those of the control seedlings. Moreover, silencing of TaCDPK27 further promoted the generation of reactive oxygen species (ROS); reduced the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT); aggravated the injury to photosystem II (PS II); and increased programmed cell death (PCD) in wheat leaves under NaCl treatment, confirming that the TaCDPK27-silenced seedlings exhibited more NaCl injury than control seedlings. Taken together, the decrease in NaCl tolerance in TaCDPK27-silenced seedlings was due to excessive ROS accumulation and subsequent aggravation of the NaCl-induced PCD. TaCDPK27 may be essential for positively regulating salt tolerance in wheat seedlings.

Foliar Spraying of Solanum tuberosum L. with CaCl2 and Ca(NO3)2: Interactions with Nutrients Accumulation in Tubers

Calcium is essential for plants, yet as its mobility is limited, the understanding of the rate of Ca²⁺ accumulation and deposition in tissues of tubers, as well as the interactions with other critical nutrients prompted this study. To assess the interactions and differential accumulation of micro and macronutrients in the tissues of tubers, Solanum tuberosum L. varieties Agria and Rossi were cultivated and, after the beginning of tuberization, four foliar sprayings (at 8–10 day intervals) with CaCl₂ (3 and 6 kg ha⁻¹) or Ca(NO₃)₂ (2 and 4 kg ha⁻¹) solutions were performed. It was found that both fertilizers increased Ca accumulation in tubers (mostly in the parenchyma tissues located in the center of the equatorial region). The functioning of the photosynthetic apparatus was not affected until the 3rd application but was somewhat affected when approaching the end of the crop cycle (after the 4th application), although the lower dose of CaCl₂ seemed to improve the photochemical use of energy, particularly when compared with the greater dose of Ca(NO₃)₂. Still, none of these impacts modified tuber height and diameter. Following the increased accumulation of Ca, in the tubers of both varieties, the mean contents of P, K, Na, Fe, and Zn revealed different accumulation patterns. Moreover, accumulation of K, Fe, Mn, and Zn prevailed in the epidermis, displaying a contrasting pattern relative to Ca. Therefore, Ca accumulation revealed a heterogeneous trend in the different regions analyzed, and Ca enrichment of tubers altered the accumulation of other nutrients.

Plant Responses to Herbivory, Wounding, and Infection

Plants have various self-defense mechanisms against biotic attacks, involving both physical and chemical barriers. Physical barriers include spines, trichomes, and cuticle layers, whereas chemical barriers include secondary metabolites (SMs) and volatile organic compounds (VOCs). Complex interactions between plants and herbivores occur. Plant responses to insect herbivory begin with the perception of physical stimuli, chemical compounds (orally secreted by insects and herbivore-induced VOCs) during feeding. Plant cell membranes then generate ion fluxes that create differences in plasma membrane potential (Vm), which provokes the initiation of signal transduction, the activation of various hormones (e.g., jasmonic acid, salicylic acid, and ethylene), and the release of VOCs and SMs. This review of recent studies of plant–herbivore–infection interactions focuses on early and late plant responses, including physical barriers, signal transduction, SM production as well as epigenetic regulation, and phytohormone responses.

Hyperlipidemia patients carrying LDLR splicing mutation c.1187-2A>G respond favorably to rosuvastatin and PCSK9 inhibitor evolocumab

Mutations in the LDL receptor gene LDLR cause familial hypercholesterolemia (FH); however, the pharmacogenomics of specific LDLR mutations remains poorly understood. The goals of this study were to identify the genetic cause of a three-generation Chinese family affected with autosomal dominant FH, and to investigate the response of FH patients in the family to statin and evolocumab. Whole exome sequencing of the FH family with four patients and six unaffected members identified a heterozygous splicing mutation (c.1187-2A>G) in LDLR. The mutation co-segregated with FH in the family, providing strong genetic evidence to support its pathogenicity. The proband was a 48-year-old male FH patient who had an acute myocardial infarction (MI) and ventricular fibrillation (VF), and showed LDL-C of 5.23 mmol/L. A combination of life style modifications on food and exercise and treatment with rosuvastatin reduced his LDL-C to 2.05-2.80 mmol/L. Addition of ezetimibe did not improve rosuvastatin therapy, but addition of evolocumab further reduced LDL-C by 70% to 0.7 mmol/L at the first time and by 67% to 1.31 mmol/L at the second time. Rosuvastatin also reduced LDL-C for proband's father and sister by 40% and 43-63%, respectively. Lovastatin alone or addition to rosuvastatin treatment did not have any effect on LDL-C for the proband and his son. Both patients carry ApoE 3/4 genotype and SLCO1B1 rs4149056 TT genotype. These results suggest that combined treatment with rosuvastatin (but not lovastatin or ezetimibe) and evolocumab can control LDL-C to meet the LDL-C treatment goal for patients with LDLR splicing mutation c.1187-2A>G.

Potassium and phosphorus content ratio in hydroponic culture affects tomato plant growth and nutrient uptake

Mineral nutrient deficiencies induce a cascade of physiological, morphological, and biochemical changes in plants which reduce vegetative growth. In this work, the impact of P and K concentration levels on tomato plant development grown in hydroponic culture was investigated. Root morphology, chlorophyll a fluorescence, phosphorus (P) and potassium (K) content, and shoot and root biomass were analyzed. Root morphology showed significant differences among the plants grown in hydroponic culture with different concentrations of P and K. Plant root/shoot dry biomass ratio decreased by 22 and 35% for P₁₅K₀ and P₃₀K_0, respectively, compared to the control (P₃₀K₂₃₂). The deficiency of P and K (individually or both) reduced significantly the root mass density parameter. For example, root mass density decreased by 38% at P₁₅K₀ treatment compared to control. Correlation analysis showed that the P and K content ratio in shoot and root was significantly and positively correlated with root volume. Deficiencies in K and P decreased the relative size of the PSI final electron acceptor pool and the electron flow on the acceptor side of PSI. Tomato growth response depend on the availability of P and K, however, interactions between these two nutrients could influence their uptake and utilization.

Potassium signaling in plant abiotic responses: Crosstalk with calcium and reactive oxygen species/reactive nitrogen species

Potassium ion (K⁺) has been regarded as an essential signaling in plant growth and development. K⁺ transporters and channels at transcription and protein levels have been made great progress. K⁺ can enhance plant abiotic stress resistance. Meanwhile, it is now clear that calcium (Ca²⁺), reactive oxygen species (ROS), and reactive nitrogen species (RNS) act as signaling molecules in plants. They regulate plant growth and development and mediate K⁺ transport. However, the interaction of K⁺ with these signaling molecules remains unclear. K⁺ may crosstalk with Ca²⁺ and ROS/RNS in abiotic stress responses in plants. Also, there are interactions among K⁺, Ca²⁺, and ROS/RNS signaling pathways in plant growth, development, and abiotic stress responses. They regulate ion homeostasis, antioxidant system, and stress resistance-related gene expression in plants. Future work needs to focus on the deeper understanding of molecular mechanism of crosstalk among K⁺, Ca²⁺, and ROS/RNS under abiotic stress.

Integrated Analysis of miRNAs Associated With Sugarcane Responses to Low-Potassium Stress

Sugarcane is among the most important global crops and a key bioenergy source. Sugarcane production is restricted by limited levels of available soil potassium (K⁺). The ability of plants to respond to stressors can be regulated by a range of microRNAs (miRNAs). However, there have been few studies regarding the roles of miRNAs in the regulation of sugarcane responses to K⁺-deficiency. To understand how these non-coding RNAs may influence sugarcane responses to low-K⁺ stress, we conducted expression profiling of miRNAs in sugarcane roots under low-K⁺ conditions via high-throughput sequencing. This approach led to the identification of 324 and 42 known and novel miRNAs, respectively, of which 36 were found to be differentially expressed miRNAs (DEMs) under low-K⁺ conditions. These results also suggested that miR156-x/z and miR171-x are involved in these responses as potential regulators of lateral root formation and the ethylene signaling pathway, respectively. A total of 705 putative targets of these DEMs were further identified through bioinformatics predictions and degradome analyses, and GO and KEGG enrichment analyses revealed these target mRNAs to be enriched for catalytic activity, binding functions, metabolic processes, plant hormone signal transduction, and mitogen-activated protein kinase (MAPK) signaling. In summary, these data provide an overview of the roles of miRNAs in the regulation of sugarcane response to low-K⁺ conditions.

Benefits provided by four ectomycorrhizal fungi to Pinus taeda under different external potassium availabilities

Ectomycorrhizal fungi contribute to the nutrition of many woody plants, including those in the Pinaceae family. Loblolly pine (Pinus taeda L.), a native species of the Southeastern USA, can be colonized by multiple species of ectomycorrhizal fungi. The role of these symbionts in P. taeda potassium (K⁺) nutrition has not been previously investigated. Here, we assessed the contribution of four ectomycorrhizal fungi, Hebeloma cylindrosporum, Paxillus ammoniavirescens, Laccaria bicolor, and Suillus cothurnatus, in P. taeda K⁺ acquisition under different external K⁺ availabilities. Using a custom-made two-compartment system, P. taeda seedlings were inoculated with one of the four fungi, or kept non-colonized, and grown under K⁺-limited or -sufficient conditions for 8 weeks. Only the fungi had access to separate compartments in which rubidium, an analog tracer for K⁺, was supplied before harvest. Resulting effects of the fungi were recorded, including root colonization, biomass, and nutrient concentrations. We also analyzed the fungal performance in axenic conditions under varying supply of K⁺ and sodium. Our study revealed that these four ectomycorrhizal fungi are differentially affected by external K⁺ and sodium variations, that they are not able to provide similar benefits to the host P. taeda in our growing conditions, and that rubidium may be used with some limitations to estimate K⁺ transport from ectomycorrhizal fungi to colonized plants.

The CBL1/9-CIPK23-AKT1 complex is essential for low potassium response in cassava

Cassava is a food crop and an important energy crop worldwide. However, its yield and quality are easily affected by low K+ stress, and the molecular mechanism of potassium channel is unknown in cassava. Herein, we revealed that calcineurin B-like 1/9 (MeCBL1/9)-CBL-interacting protein kinase 23 (MeCIPK23)-K+ TRANSPORTER1 (MeAKT1) complex plays an important role in low potassium response in cassava. Firstly, this study verified the in vivo role of MeAKT1 in K+ uptake in yeast. Secondly, we found that MeCBL1, MeCBL9, MeCIPK23 and MeAKT1 are involved in the absorption of K+ in cassava, and MeCBL1/9-CIPK23 complex is essential for MeAKT1-mediated K+ uptake. Moreover, MeCBL1/9-MeCIPK23-MeAKT1 showed different expression in different cassava varieties contrasting in the resistance to low K+ stress. Taken together, this study provides new insights into further improvement of K+ uptake in cassava.

K Deprivation Modulates the Primary Metabolites and Increases Putrescine Concentration in Brassica napus

Potassium (K) plays a crucial role in plant growth and development and is involved in different physiological and biochemical functions in plants. Brassica napus needs higher amount of nutrients like nitrogen (N), K, phosphorus (P), sulfur (S), and boron (B) than cereal crops. Previous studies in B. napus are mainly focused on the role of N and S or combined deficiencies. Hence, little is known about the response of B. napus to K deficiency. Here, a physiological, biochemical, and molecular analysis led us to investigate the response of hydroponically grown B. napus plants to K deficiency. The results showed that B. napus was highly sensitive to the lack of K. The lower uptake and translocation of K induced BnaHAK5 expression and significantly declined the growth of B. napus after 14 days of K starvation. The lower availability of K was associated with a decrease in the concentration of both S and N and modulated the genes involved in their uptake and transport. In addition, the lack of K induced an increase in Ca2+ and Mg2+ concentration which led partially to the accumulation of positive charge. Moreover, a decrease in the level of arginine as a positively charged amino acid was observed which was correlated with a substantial increase in the polyamine, putrescine (Put). Furthermore, K deficiency induced the expression of BnaNCED3 as a key gene in abscisic acid (ABA) biosynthetic pathway which was associated with an increase in the levels of ABA. Our findings provided a better understanding of the response of B. napus to K starvation and will be useful for considering the importance of K nutrition in this crop.

The Ca2+-CaM Signaling Pathway Mediates Potassium Uptake by Regulating Reactive Oxygen Species Homeostasis in Tobacco Roots Under Low-K+ Stress

Potassium (K⁺) deficiency severely threatens crop growth and productivity. Calcium (Ca²⁺) signaling and its sensors play a central role in the response to low-K⁺ stress. Calmodulin (CaM) is an important Ca²⁺ sensor. However, the mechanism by which Ca²⁺ signaling and CaM mediate the response of roots to low-K⁺ stress remains unclear. In this study, we found that the K⁺ concentration significantly decreased in both shoots and roots treated with Ca²⁺ channel blockers, a Ca²⁺ chelator, and CaM antagonists. Under low-K⁺ stress, reactive oxygen species (ROS) accumulated, and the activity of antioxidant enzymes, NAD kinase (NADK), and NADP phosphatase (NADPase) decreased. This indicates that antioxidant enzymes, NADK, and NADPase might be downstream target proteins in the Ca²⁺-CaM signaling pathway, which facilitates K⁺ uptake in plant roots by mediating ROS homeostasis under low-K⁺ stress. Moreover, the expression of NtCNGC3, NtCNGC10, K⁺ channel genes, and transporter genes was significantly downregulated in blocker-treated, chelator-treated, and antagonist-treated plant roots in the low K⁺ treatment, suggesting that the Ca²⁺-CaM signaling pathway may mediate K⁺ uptake by regulating the expression of these genes. Overall, this study shows that the Ca²⁺-CaM signaling pathway promotes K⁺ absorption by regulating ROS homeostasis and the expression of K⁺ uptake-related genes in plant roots under low-K⁺ stress.

The molecular-physiological functions of mineral macronutrients and their consequences for deficiency symptoms in plants

The visual deficiency symptoms developing on plants constitute the ultimate manifestation of suboptimal nutrient supply. In classical plant nutrition, these symptoms have been extensively used as a tool to characterise the nutritional status of plants and to optimise fertilisation. Here we expand this concept by bridging the typical deficiency symptoms for each of the six essential macronutrients to their molecular and physiological functionalities in higher plants. We focus on the most recent insights obtained during the last decade, which now allow us to better understand the links between symptom and function for each element. A deep understanding of the mechanisms underlying the visual deficiency symptoms enables us to thoroughly understand how plants react to nutrient limitations and how these disturbances may affect the productivity and biodiversity of terrestrial ecosystems. A proper interpretation of visual deficiency symptoms will support the potential for sustainable crop intensification through the development of new technologies that facilitate automatised management practices based on imaging technologies, remote sensing and in-field sensors, thereby providing the basis for timely application of nutrients via smart and more efficient fertilisation.

GABA signaling in plants: targeting the missing pieces of the puzzle

The adaptation of plants to unstable environments relies on their ability to sense their surroundings and to generate and transmit corresponding signals to different parts of the plant to evoke changes necessary for optimizing growth and defense. Plants, like animals, contain a huge repertoire of intra- and intercellular signals, including organic and inorganic molecules. The occurrence of neurotransmitter-like signaling molecules in plants has been an intriguing field of research. Among these, γ-aminobutyric acid (GABA) was discovered in plants over half a century ago, and studies of its roles as a primary metabolite have been well documented, particularly in the context of stress responses. In contrast, evidence of the potential mechanism by which GABA acts as a signaling molecule in plants has only recently been reported. In spite of this breakthrough, the roles of GABA as a signaling molecule in plants have yet to be established and several aspects of the complexity of the GABA signaling system remain obscure. This review summarizes the uncertainties in GABA signaling in plants and suggests research directions and technologies that would help in answering unsolved questions.

Close Temporal Relationship between Oscillating Cytosolic K+ and Growth in Root Hairs of Arabidopsis

Root hair elongation relies on polarized cell expansion at the growing tip. As a major osmotically active ion, potassium is expected to be continuously assimilated to maintain cell turgor during hair tip growth. However, due to the lack of practicable detection methods, the dynamics and physiological role of K⁺ in hair growth are still unclear. In this report, we apply the small-molecule fluorescent K⁺ sensor NK3 in Arabidopsis root hairs for the first time. By employing NK3, oscillating cytoplasmic K⁺ dynamics can be resolved at the tip of growing root hairs, similar to the growth oscillation pattern. Cross-correlation analysis indicates that K⁺ oscillation leads the growth oscillations by approximately 1.5 s. Artificially increasing cytoplasmic K⁺ level showed no significant influence on hair growth rate, but led to the formation of swelling structures at the tip, an increase of cytosolic Ca²⁺ level and microfilament depolymerization, implying the involvement of antagonistic regulatory factors (e.g., Ca²⁺ signaling) in the causality between cytoplasmic K⁺ and hair growth. These results suggest that, in each round of oscillating root hair elongation, the oscillatory cell expansion accelerates on the heels of cytosolic K⁺ increment, and decelerates with the activation of antagonistic regulators, thus forming a negative feedback loop which ensures the normal growth of root hairs.

Plasma Membrane Ca2+ Permeable Mechanosensitive Channel OsDMT1 Is Involved in Regulation of Plant Architecture and Ion Homeostasis in Rice

Plant architecture is an important factor for crop production. Plant height, tiller pattern, and panicle morphology are decisive factors for high grain yield in rice. Here, we isolated and characterized a T-DNA insertion rice mutant Osdmt1 (Oryza sativa dwarf and multi-tillering1) that exhibited a severe dwarf phenotype and multi-tillering. Molecular cloning revealed that DMT1 encodes a plasma membrane protein that was identified as a putative Ca²⁺ permeable mechanosensitive channel. The transcript expression level was significantly higher in the dmt1 mutant compared to wild type (WT). Additionally, the dmt1 homozygous mutant displayed a stronger phenotype than that of the WT and heterozygous seedlings after gibberellic acid (GA) treatment. RNA-seq and iTRAQ-based proteome analyses were performed between the dmt1 mutant and WT. The transcriptome profile revealed that several genes involved in GA and strigolactone (SL) biosyntheses were altered in the dmt1 mutant. Ca²⁺ and other ion concentrations were significantly enhanced in the dmt1 mutant, suggesting that DMT1 contributes to the accumulation of several ions in rice. Moreover, several EF-hand Ca²⁺ sensors, including CMLs (CaM-like proteins) and CDPKs (calcium-dependent protein kinases), displayed markedly altered transcript expression and protein levels in the dmt1 mutant. Overall, these findings aid in the elucidation of the multiply regulatory roles of OsDMT1/OsMCA1 in rice.

Actin Cytoskeleton as Actor in Upstream and Downstream of Calcium Signaling in Plant Cells

In plant cells, calcium (Ca²⁺) serves as a versatile intracellular messenger, participating in several fundamental and important biological processes. Recent studies have shown that the actin cytoskeleton is not only an upstream regulator of Ca²⁺ signaling, but also a downstream regulator. Ca²⁺ has been shown to regulates actin dynamics and rearrangements via different mechanisms in plants, and on this basis, the upstream signaling encoded within the Ca²⁺ transient can be decoded. Moreover, actin dynamics have also been proposed to act as an upstream of Ca²⁺, adjust Ca²⁺ oscillations, and establish cytosolic Ca²⁺ ([Ca²⁺]_cyt) gradients in plant cells. In the current review, we focus on the advances in uncovering the relationship between the actin cytoskeleton and calcium in plant cells and summarize our current understanding of this relationship.