Magnesium Signaling in Plants

Plant transfer factors of naturally occurring radioactive decay series radionuclides, 7Be, and 137Cs in a semi-arid climate

Arid and semi-arid climates give rise to drought stress in plants, implying an increased uptake of radionuclides through both leaves and roots. This study was carried out in the Tabernas Desert (Almería, Spain), classified as an arid climate. Seventeen plants were analyzed, collected from four areas of the study site. The soils in the four areas were statistically comparable and showed a chemical composition and activity concentrations characteristic of uncontaminated soils, with activity concentrations corresponding to world averages, i.e., 31 ± 4 Bq·kg-1 for the uranium series, 37 ± 4 Bq·kg-1 for thorium one, and 344 ± 17 Bq·kg-1 for 40K. Conversely, the soils showed that 7Be was below the limit of detection, levels of 137Cs equivalent to fallout, but an average 210Pbex of 19 ± 9 Bq·kg-1. The radionuclides of the three naturally occurring radioactive decay series are mainly present in the insoluble fraction, with 226Ra being the most abundant. The transfer factors showed the usual behaviour of Cs > Ra > U > Th. The activity concentrations of 7Be and 210Pb showed a statistically significant linear relationship, indicating greater incorporation through the plant leaves.

Adenylate-driven equilibration of both ribo- and deoxyribonucleotides is under magnesium control: Quantification of the Mg2+-signal

Nucleoside mono-, di- and triphosphates (NMP, NDP, and NTP) and their deoxy-counterparts (dNMP, dNDP, dNTP) are involved in energy metabolism and are the building blocks of RNA and DNA, respectively. The production of NTP and dNTP is carried out by several NMP kinases (NMPK) and NDP kinases (NDPK). All NMPKs are fully reversible and use defined Mg-free and Mg-complexed nucleotides in both directions of their reactions, with Mg2+ controlling the ratios of Mg-free and Mg-complexed reactants. Their activities are driven by adenylates produced by adenylate kinase which controls the direction of NMPK and NDPK reactions, depending on the energy status of a cell. This enzymatic machinery is localized in the cytosol, mitochondria, and plastids, i.e. compartments with high energy budgets and where (except for cytosol) RNA and DNA synthesis occur. Apparent equilibrium constants of NMPKs, based on total nucleotide contents, are [Mg2+]-dependent. This allows for an indirect estimation of internal [Mg2+], which constitutes a signal of the energetic status of a given tissue/cell/compartment. Adenylates contribute the most to this Mg2+-signal, followed by uridylates, guanylates, and cytidylates, with deoxynucleotides' contribution deemed negligible. A method to quantify the Mg2+-signal, using nucleotide datasets, is discussed.

Multi-omics analysis uncovers the transcriptional regulatory mechanism of magnesium Ions in the synthesis of active ingredients in Sophora tonkinensis

Magnesium (Mg) plays a pivotal role as an essential component of plant chlorophyll and functions as a critical coenzyme. However, research exploring the regulatory mechanisms of magnesium ions on the synthesis of secondary metabolites is still in its early stages. Sophora tonkinensis is a widely utilized medicinal plant in China, recognized for its diverse secondary metabolites with active properties. This study investigates variations in these ingredients in tissue-cultured seedlings under varying magnesium concentrations. Simultaneously, an omics data analysis was conducted on tissue-cultured seedlings subjected to treatments with magnesium and low magnesium. These comprehensive omics analyses aimed to elucidate the mechanisms through which magnesium influences active components, growth, and development. Magnesium exerts a pervasive influence on various metabolic pathways, forming an intricate network. Research findings indicate that magnesium impacts diverse metabolic processes, including the absorption of potassium and calcium, as well as photosynthetic activity. Consequently, these influences lead to discernible changes in the levels of pharmacologically active compounds and the growth and developmental status.This study is the first to employ a multi-omics data analysis in S. tonkinensis. This methodology allows us to uncover the overarching impact of metabolic networks on the levels of various active ingredients and specific phenotypes.

Unveiling Potassium and Sodium Ion Dynamics in Living Plants with an In-Planta Potentiometric Microneedle Sensor

Potassium and sodium ions (K+ and Na+) play crucial roles in influencing plant growth and health status. Unfortunately, current strategies to determine the concentrations of such ions are destructive for the plants because it is necessary to collect/extract the sap for further analysis and produce either scattered or delayed results. Here, we introduce a new potentiometric dual microneedle sensor for nondestructive, real-time, and continuous monitoring of K+ and Na+ concentrations in living plants. The developed sensors show a response time <5 s, close-to-Nernstian slope (∼55 mV dec-1), resiliency to five insertions on the stem, good repeatability (max. %RSD = 0.3%) and reversibility (max. %RSD = 3%), appropriate continuous operation for 24 h, and linear range of responses that cover expected plant physiological levels (5-50 mM for Na+ and 50-120 mM for K+). Moreover, the accuracy was successfully investigated by comparing the results provided by the microneedle sensors to those obtained by a standard reference method (e.g., ion chromatography). Finally, we demonstrate that the developed analytical device is capable of tracking K+ and Na+ transportation from the hydroponic solution to the stem within 5-10 min. This research will contribute to establishing a new generation of analytical platforms for smart agriculture offering real-time information.

Exploring the interplay between angiosperm chlorophyll metabolism and environmental factors

MAIN CONCLUSION

In this review, we summarize how chlorophyll metabolism in angiosperm is affected by the environmental factors: light, temperature, metal ions, water, oxygen, and altitude. The significance of chlorophyll (Chl) in plant leaf morphogenesis and photosynthesis cannot be overstated. Over time, researchers have made significant advancements in comprehending the biosynthetic pathway of Chl in angiosperms, along with the pivotal enzymes and genes involved in this process, particularly those related to heme synthesis and light-responsive mechanisms. Various environmental factors influence the stability of Chl content in angiosperms by modulating Chl metabolic pathways. Understanding the interplay between plants Chl metabolism and environmental factors has been a prominent research topic. This review mainly focuses on angiosperms, provides an overview of the regulatory mechanisms governing Chl metabolism, and the impact of environmental factors such as light, temperature, metal ions (iron and magnesium), water, oxygen, and altitude on Chl metabolism. Understanding these effects is crucial for comprehending and preserving the homeostasis of Chl metabolism.

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 power of magnesium: unlocking the potential for increased yield, quality, and stress tolerance of horticultural crops

Magnesium (Mg2+) is pivotal for the vitality, yield, and quality of horticultural crops. Central to plant physiology, Mg2+ powers photosynthesis as an integral component of chlorophyll, bolstering growth and biomass accumulation. Beyond basic growth, it critically affects crop quality factors, from chlorophyll synthesis to taste, texture, and shelf life. However, Mg2 + deficiency can cripple yields and impede plant development. Magnesium Transporters (MGTs) orchestrate Mg2+ dynamics, with notable variations observed in horticultural species such as Cucumis sativus, Citrullus lanatus, and Citrus sinensis. Furthermore, Mg2+ is key in fortifying plants against environmental stressors and diseases by reinforcing cell walls and spurring the synthesis of defense substances. A burgeoning area of research is the application of magnesium oxide nanoparticles (MgO-NPs), which, owing to their nanoscale size and high reactivity, optimize nutrient uptake, and enhance plant growth and stress resilience. Concurrently, modern breeding techniques provide insights into Mg2+ dynamics to develop crops with improved Mg2+ efficiency and resilience to deficiency. Effective Mg2+ management through soil tests, balanced fertilization, and pH adjustments holds promise for maximizing crop health, productivity, and sustainability. This review unravels the nuanced intricacies of Mg2+ in plant physiology and genetics, and its interplay with external factors, serving as a cornerstone for those keen on harnessing its potential for horticultural excellence.

Study on the effect of magnesium on leaf metabolites, growth and quality of tea tree

Magnesium (Mg) is one of the essential elements for the growth of tea trees. In this study, we investigated changes in metabolites, photosynthetic fluorescence parameters and quality indexes of tea leaves under different concentrations of magnesium treatment, and the results showed that there were no significant differences in the quantity and total content of metabolites in tea leaves under different Mg concentrations. The results of volcano map analysis showed that the content of 235 metabolites in tea leaves showed an increasing trend and the content of 243 metabolites showed a decreasing trend with the increase of Mg concentration. The results of the combined analysis of the OPLS-DA model and bubble map showed that 45 characteristic metabolites were screened at different concentrations of Mg. Among these, the content of 24 characteristic metabolites showed an increasing trend and 21 characteristic metabolites showed a decreasing trend with the increase of Mg concentrations. The results of KEEG pathway enrichment showed that 24 characteristic metabolites with a upward trend were significantly enriched in saccharides metabolism, nucleic acid metabolism and vitamin metabolism, while the 21 characteristic metabolites with a downward trend were enriched in the synthesis of plant secondary metabolites, phenylpropanoid biosynthesis, biosynthesis of terpenoids, synthesis and metabolism of alkaloids, and synthesis and metabolism of amino acids. It can be inferred that Mg regulation was beneficial to enhance the photosynthetic capacity of tea trees, improve the accumulation and metabolism of carbohydrate substances in tea trees, and thus promoted the growth of tea trees, but was not conducive to the synthesis of secondary metabolites and amino acids related to tea quality. The results of photosynthetic fluorescence parameters and quality indexes of the tea tree confirmed the conclusion predicted by metabolomics. This study provided a reference for regulating of the growth and quality of tea trees with Mg fertilizer in tea plantations.

Transcriptome sequencing and functional verification revealed the roles of exogenous magnesium in tobacco anti-PVY infection

Potato virus Y (PVY) infection causes necrosis and curling of leaves, which seriously affect the yield and quality of Solanaceous crops. The roles of nutrient elements in the regulation of plant resistance to virus infection has been widely reported, while the mechanisms are poorly studied. Previous studies in our laboratory have demonstrated that foliar spraying of MgSO4 could induce Nicotiana tabacum resistance to PVY by increasing the activity of defense-related enzymes. Consistent with the results, we found that exogenous magnesium (Mg) had a certain effect on N. tabacum anti-PVY infection. Meanwhile, Illumina RNA sequencing revealed that Mg induced resistance to PVY infection was mainly by regulating carbohydrate metabolism and transportation, nitrogen metabolism, Ca2+ signal transduction and oxidative phosphorylation. Moreover, we used virus-induced gene silencing assays to verify the function of homologs of five N. tabacum genes involved in above pathways in N. benthamiana. The results showed that NbTPS and NbGBE were conducive to PVY infection, while NbPPases and NbNR were related to resistance to PVY infection. These results suggested a novel strategy for resistance to PVY infection and provided a theoretical basis for virus-resistance breeding.

Exploring Redox Modulation of Plant UDP-Glucose Pyrophosphorylase

UDP-glucose (UDPG) pyrophosphorylase (UGPase) catalyzes a reversible reaction, producing UDPG, which serves as an essential precursor for hundreds of glycosyltransferases in all organisms. In this study, activities of purified UGPases from sugarcane and barley were found to be reversibly redox modulated in vitro through oxidation by hydrogen peroxide or oxidized glutathione (GSSG) and through reduction by dithiothreitol or glutathione. Generally, while oxidative treatment decreased UGPase activity, a subsequent reduction restored the activity. The oxidized enzyme had increased K_m values with substrates, especially pyrophosphate. The increased K_m values were also observed, regardless of redox status, for UGPase cysteine mutants (Cys102Ser and Cys99Ser for sugarcane and barley UGPases, respectively). However, activities and substrate affinities (K_ms) of sugarcane Cys102Ser mutant, but not barley Cys99Ser, were still prone to redox modulation. The data suggest that plant UGPase is subject to redox control primarily via changes in the redox status of a single cysteine. Other cysteines may also, to some extent, contribute to UGPase redox status, as seen for sugarcane enzymes. The results are discussed with respect to earlier reported details of redox modulation of eukaryotic UGPases and regarding the structure/function properties of these proteins.

Effects of Magnesium on Transcriptome and Physicochemical Index of Tea Leaves

Magnesium (Mg) is one of the essential elements for the growth of tea tree and is extremely important for its development. In this study, we investigated the effect of Mg on the transcriptome and physicochemical indexes of tea leaves, and the results showed that Mg could significantly affect the gene expression of tea leaves. The results of Orthogonal Partial Least-Squares Discriminant Analysis (OPLS-DA) model analysis showed that a total of 300 key genes (Variable Importance for the Projection, VIP > 1) were screened under different concentrations of Mg treatment, among which 140 genes were up-regulated and 160 genes were down-regulated. The bubble map was used to screen the characteristic genes from the above key genes, and a total of 121 representative characteristic genes were obtained, mainly involving 9 metabolic pathways. Among them, gene expression of three metabolic pathways, including porphyrin metabolism, alpha-linolenic acid metabolism and photosynthesis, showed an increasing trend with the increase of Mg concentration, while gene expression of four metabolic pathways, including biosynthesis of secondary metabolites, anthocyanin biosynthesis, ABC transporters, pentose and glucuronate interconversions, showed a decreasing trend. The results of physiological index analysis showed that with the increase of Mg concentration, the photosynthetic physiological index, theanine and soluble sugar content of tea leaves showed an increasing trend, while the content of tea polyphenol, flavone and caffeine showed a decreasing trend. The results of TOPSIS analysis showed that the physiological indexes of tea trees most affected by Mg were chlorophyll, tea polyphenols and flavonoids, while the metabolic pathways most affected by Mg on gene expression were the metabolic pathways and biosynthesis of secondary metabolites. It can be seen that the effects of Mg on tea tree were mainly related to photosynthesis and synthesis of secondary metabolites, and Mg was beneficial for improving the photosynthetic capacity of tea tree, enhancing the accumulation of primary metabolites, and thus increasing tea yield. However, Mg was not conducive to the synthesis of secondary metabolites of tea tree and the accumulation of main quality indexes of tea leaves.

Toward understanding the emergence of life: A dual function of the system of nucleotides in the metabolically closed autopoietic organization

General structure of metabolism includes the reproduction of catalysts that govern metabolism. In this structure, the system becomes autopoietic in the sense of Maturana and Varela, and it is closed to efficient causation as defined by Robert Rosen. The autopoietic maintenance and operation of the catalysts takes place via the set of free nucleotides while the synthesis of catalysts occurs via the information encoded by the set of nucleotides arranged in polymers of RNA and DNA. Both energy charge and genetic information use the components of the same pool of nucleoside triphosphates, which is equilibrated by thermodynamic buffering enzymes such as nucleoside diphosphate kinase and adenylate kinase. This occurs in a way that the system becomes internally stable and metabolically closed, which initially could be realized at the level of ribozymes catalyzing basic metabolic reactions as well as own reproduction. The function of ATP, GTP, UTP, and CTP is dual, as these species participate both in the general metabolism as free nucleotides and in the transfer of genetic information via covalent polymerization to nucleic acids. The changes in their pools directly impact both bioenergetic pathways and nucleic acid turnover. Here we outline the concept of metabolic closure of biosystems grounded in the dual function of nucleotide coenzymes that serve both as energetic and informational molecules and through this duality generate the autopoietic performance and the ability for codepoietic evolutionary transformations of living systems starting from the emergence of prebiotic systems.

Magnesium and cell energetics: At the junction of metabolism of adenylate and non-adenylate nucleotides

Free magnesium (Mg²⁺) represents a powerful signal arising from interconversions of adenylates (ATP, ADP and AMP). This is a consequence of the involvement of adenylate kinase (AK) which equilibrates adenylates and uses defined species of Mg-complexed and Mg-free adenylates in both directions of its reaction. However, cells contain also other reversible Mg²⁺-dependent enzymes that equilibrate non-adenylate nucleotides (uridylates, cytidylates and guanylates), i.e. nucleoside monophosphate kinases (NMPKs) and nucleoside diphosphate kinase (NDPK). Here, we propose that AK activity is tightly coupled to activities of NMPK and NDPK, linking adenylate equilibrium to equilibria of other nucleotides, and with [Mg²⁺] controlling the ratios of Mg-chelated and Mg-free nucleotides. This coupling establishes main hubs for adenylate-driven equilibration of non-adenylate nucleotides, with [Mg²⁺] acting as signal arising from all nucleotides rather than adenylates only. Further consequences involve an overall adenylate control of UTP-, GTP- and CTP-dependent pathways and the availability of substrates for RNA and DNA synthesis.

The Role of Taraxacum mongolicum in a Puccinellia tenuiflora Community under Saline-Alkali Stress

Coexisting salt and alkaline stresses seriously threaten plant survival. Most studies have focused on halophytes; however, knowledge on how plants defend against saline-alkali stress is limited. This study investigated the role of Taraxacum mongolicum in a Puccinellia tenuiflora community under environmental saline-alkali stress to analyse the response of elements and metabolites in T. mongolicum, using P. tenuiflora as a control. The results show that the macroelements Ca and Mg are significantly accumulated in the aboveground parts (particularly in the stem) of T. mongolicum. Microelements B and Mo are also accumulated in T. mongolicum. Microelement B can adjust the transformation of sugars, and Mo contributes to the improvement in nitrogen metabolism. Furthermore, the metabolomic results demonstrate that T. mongolicum leads to decreased sugar accumulation and increased amounts of amino acids and organic acids to help plants resist saline-alkali stress. The resource allocation of carbon (sugar) and nitrogen (amino acids) results in the accumulation of only a few phenolic metabolites (i.e., petunidin, chlorogenic acid, and quercetin-3-O-rhamnoside) in T. mongolicum. These phenolic metabolites help to scavenge excess reactive oxygen species. Our study primarily helps in understanding the contribution of T. mongolicum in P. tenuiflora communities on coping with saline-alkali stress.

The Role of Transmembrane Proteins in Plant Growth, Development, and Stress Responses

Transmembrane proteins participate in various physiological activities in plants, including signal transduction, substance transport, and energy conversion. Although more than 20% of gene products are predicted to be transmembrane proteins in the genome era, due to the complexity of transmembrane domains they are difficult to reliably identify in the predicted protein, and they may have different overall three-dimensional structures. Therefore, it is challenging to study their biological function. In this review, we describe the typical structures of transmembrane proteins and their roles in plant growth, development, and stress responses. We propose a model illustrating the roles of transmembrane proteins during plant growth and response to various stresses, which will provide important references for crop breeding.

Chlorophyll decomposition is accelerated in banana leaves after the long-term magnesium deficiency according to transcriptome analysis

Magnesium (Mg) is an essential macronutrient for plant growth and development. Physiological and transcriptome analyses were conducted to elucidate the adaptive mechanisms to long-term Mg deficiency (MD) in banana seedlings at the 6-leaf stage. Banana seedlings were irrigated with a Mg-free nutrient solution for 42 days, and a mock control was treated with an optimum Mg supply. Leaf edge chlorosis was observed on the 9th leaf, which gradually turned yellow from the edge to the interior region. Accordingly, the total chlorophyll content was reduced by 47.1%, 47.4%, and 53.8% in the interior, center and edge regions, respectively, and the net photosynthetic rate was significantly decreased in the 9th leaf. Transcriptome analysis revealed that MD induced 9,314, 7,425 and 5,716 differentially expressed genes (DEGs) in the interior, center and edge regions, respectively. Of these, the chlorophyll metabolism pathway was preferentially enriched according to Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. The expression levels of the five candidate genes in leaves were consistent with what is expected during chlorophyll metabolism. Our results suggest that changes in the expression of genes related to chlorophyll synthesis and decomposition result in the yellowing of banana seedling leaves, and these results are helpful for understanding the banana response mechanism to long-term MD.

Effects of Magnesium, Pyrophosphate and Phosphonates on Pyrophosphorolytic Reaction of UDP-Glucose Pyrophosphorylase

UDP-glucose pyrophosphorylase (UGPase) carries a freely reversible reaction, using glucose-1-P and UTP to produce UDP-glucose (UDPG) and pyrophosphate (PP_i), with UDPG being essential for glycosylation reactions in all organisms including, e.g., synthesis of sucrose, cellulose and glycoproteins. In the present study, we found that free magnesium (Mg²⁺) had profound effects on the reverse reaction of purified barley UGPase, and was absolutely required for its activity, with an apparent K_m of 0.13 mM. More detailed analyses with varied concentrations of MgPP_i allowed us to conclude that it is the MgPP_i complex which serves as true substrate for UGPase in its reverse reaction, with an apparent K_m of 0.06 mM. Free PP_i was an inhibitor in this reaction. Given the key role of PPi in the UGPase reaction, we have also tested possible effects of phosphonates, which are analogs of PPi and phosphate (P_i). Clodronate and etidronate (PP_i analogs) had little or no effect on UGPase activity, whereas fosetyl-Al (P_i analog), a known fungicide, acted as effective near-competitive inhibitor versus PP_i, with K_i of 0.15 mM. The data are discussed with respect to the role of magnesium in the UGPase reaction and elucidating the use of inhibitors in studies on cellular function of UGPase and related enzymes.

Ion transporters and their regulatory signal transduction mechanisms for salinity tolerance in plants

Soil salinity is one of the most serious threats to plant growth and productivity. Due to global climate change, burgeoning population and shrinking arable land, there is an urgent need to develop crops with minimum reduction in yield when cultivated in salt-affected areas. Salinity stress imposes osmotic stress as well as ion toxicity, which impairs major plant processes such as photosynthesis, cellular metabolism, and plant nutrition. One of the major effects of salinity stress in plants includes the disturbance of ion homeostasis in various tissues. In the present study, we aimed to review the regulation of uptake, transport, storage, efflux, influx, and accumulation of various ions in plants under salinity stress. We have summarized major research advancements towards understanding the ion homeostasis at both cellular and whole-plant level under salinity stress. We have also discussed various factors regulating the function of ion transporters and channels in maintaining ion homeostasis and ionic interactions under salt stress, including plant antioxidative defense, osmo-protection, and osmoregulation. We further elaborated on stress perception at extracellular and intracellular levels, which triggers downstream intracellular-signaling cascade, including secondary messenger molecules generation. Various signaling and signal transduction mechanisms under salinity stress and their role in improving ion homeostasis in plants are also discussed. Taken together, the present review focuses on recent advancements in understanding the regulation and function of different ion channels and transporters under salt stress, which may pave the way for crop improvement.

Stable Isotope Fractionation of Metals and Metalloids in Plants: A Review

This work critically reviews stable isotope fractionation of essential (B, Mg, K, Ca, Fe, Ni, Cu, Zn, Mo), beneficial (Si), and non-essential (Cd, Tl) metals and metalloids in plants. The review (i) provides basic principles and methodologies for non-traditional isotope analyses, (ii) compiles isotope fractionation for uptake and translocation for each element and connects them to physiological processes, and (iii) interlinks knowledge from different elements to identify common and contrasting drivers of isotope fractionation. Different biological and physico-chemical processes drive isotope fractionation in plants. During uptake, Ca and Mg fractionate through root apoplast adsorption, Si through diffusion during membrane passage, Fe and Cu through reduction prior to membrane transport in strategy I plants, and Zn, Cu, and Cd through membrane transport. During translocation and utilization, isotopes fractionate through precipitation into insoluble forms, such as phytoliths (Si) or oxalate (Ca), structural binding to cell walls (Ca), and membrane transport and binding to soluble organic ligands (Zn, Cd). These processes can lead to similar (Cu, Fe) and opposing (Ca vs. Mg, Zn vs. Cd) isotope fractionation patterns of chemically similar elements in plants. Isotope fractionation in plants is influenced by biotic factors, such as phenological stages and plant genetics, as well as abiotic factors. Different nutrient supply induced shifts in isotope fractionation patterns for Mg, Cu, and Zn, suggesting that isotope process tracing can be used as a tool to detect and quantify different uptake pathways in response to abiotic stresses. However, the interpretation of isotope fractionation in plants is challenging because many isotope fractionation factors associated with specific processes are unknown and experiments are often exploratory. To overcome these limitations, fundamental geochemical research should expand the database of isotope fractionation factors and disentangle kinetic and equilibrium fractionation. In addition, plant growth studies should further shift toward hypothesis-driven experiments, for example, by integrating contrasting nutrient supplies, using established model plants, genetic approaches, and by combining isotope analyses with complementary speciation techniques. To fully exploit the potential of isotope process tracing in plants, the interdisciplinary expertise of plant and isotope geochemical scientists is required.

Autophagy and the Energy Status of Plant Cells

Abstract

In plant cells the homeostatic control of energy balance involves the production and recycling of adenylates with macroergic bonds, ATP and ADP. The maintenance of anabolic processes requires the relative saturation of the adenylate pool with high energy phosphoanhydride bonds. The bulk of ATP synthesis is carried out both in mitochondria and in chloroplasts while optimal ATP levels within other cell compartments are maintained by adenylate kinases (AK). AK activity was recently found in cytosol, mitochondria, plastids and the nucleus. ATP synthesis in energy-producing organelles, as well as redistribution of nutrients among cellular compartments, requires fine-tuned regulation of ion homeostasis. A special role in energy metabolism is played by autophagy, a process of active degradation of unwanted and/or damaged cell components and macromolecules within the central lytic vacuole. So-called constitutive autophagy controls the quality of cellular contents under favorable conditions, i.e., when the cellular energy status is high. Energy depletion can lead to the activation of the pro-survival process of autophagic removal and utilization of damaged structures; the breakdown products are then used for ATP regeneration and de novo synthesis of macromolecules. Mitophagy and chlorophagy maintain the populations of healthy and functional energy-producing “stations”, preventing accumulation of defective mitochondria and chloroplasts as potential sources of dangerous reactive oxygen species. However, the increase of autophagic flux above a threshold level can lead to the execution of the vacuolar type of programmed cell death (PCD). In this case autophagy also contributes to preservation of energy through support of the outflow of nutrients from dying cells to healthy neighboring tissues. In plants, two central protein kinases, SnRK1 (Snf1-related protein kinase 1) and TOR (target of rapamycin), are responsible for the regulation of the metabolic switch between anabolic and catabolic pathways. TOR promotes the energy-demanding metabolic reactions in response to nutrient availability and simultaneously suppresses catabolism including autophagy. SnRK1, the antagonist of TOR, senses a decline in cellular energy supply and reacts by inducing autophagy through several independent pathways. Here, we provide an overview of the recent knowledge about the interplay between SnRK1 and TOR, autophagy and PCD in course of the regulation of energy balance in plants.

The Electrostatic Basis of Diacylglycerol Pyrophosphate—Protein Interaction

Diacylglycerol pyrophosphate (DGPP) is an anionic phospholipid formed in plants, yeast, and parasites under multiple stress stimuli. It is synthesized by the phosphorylation action of phosphatidic acid (PA) kinase on phosphatidic acid, a signaling lipid with multifunctional properties. PA functions in the membrane through the interaction of its negatively charged phosphomonoester headgroup with positively charged proteins and ions. DGPP, like PA, can interact electrostatically via the electrostatic-hydrogen bond switch mechanism but differs from PA in its overall charge and shape. The formation of DGPP from PA alters the physicochemical properties as well as the structural dynamics of the membrane. This potentially impacts the molecular and ionic binding of cationic proteins and ions with the DGPP enriched membrane. However, the results of these important interactions in the stress response and in DGPP’s overall intracellular function is unknown. Here, using ³¹P MAS NMR, we analyze the effect of the interaction of low DGPP concentrations in model membranes with the peptides KALP23 and WALP23, which are flanked by positively charged Lysine and neutral Tryptophan residues, respectively. Our results show a significant effect of KALP23 on the charge of DGPP as compared to WALP23. There was, however, no significant effect on the charge of the phosphomonoester of DGPP due to the interaction with positively charged lipids, dioleoyl trimethylammonium propane (DOTAP) and dioleoyl ethyl-phosphatidylcholine (EtPC). Divalent calcium and magnesium cations induce deprotonation of the DGPP headgroup but showed no noticeable differences on DGPP’s charge. Our results lead to a novel model for DGPP—protein interaction.

Transcriptome analysis reveals MYB and WRKY transcription factors involved in banana (Musa paradisiaca AA) magnesium deficiency

The banana development was inhibited under the long-term magnesium deficiency (MD) stress, resulting in the leaf chlorosis. MYB108 and WRKY75 are involved in regulating the growth and development of banana leaves and roots under long-term MD. Magnesium deficiency (MD) causes plant growth inhibition, ageing acceleration, yield reduction and quality decline of banana (Musa paradisiaca AA), but the molecular regulatory mechanisms underlying the changes in response to long-term MD conditions remain unknown. In this study, a long-term MD experiment was performed with banana seedlings at the four-leaf stage. Compared to those in the control group, the growth of leaves and roots of seedlings in the long-term MD treatment experimental groups was inhibited, and the Mg content and chlorophyll contents were decreased. Leaves and roots of seedlings from the control and experimental groups were subsequently collected for RNA sequencing to identify the genes that respond to long-term MD. More than 50 million reads were identified from each sample, resulting in the detection of 3500 and 948 differentially expressed genes (DEGs) in the leaves and roots, respectively. MYB and WRKY transcription factors (TFs) involved in plant stress responses were selected for further analysis, and 102 MYB and 149 WRKY TFs were differentially expressed. Furthermore, two highly differentially expressed candidate genes, MYB108 and WRKY75, were functionally analyzed using Arabidopsis mutants grown under long-term MD conditions. The results showed that the density of root hairs on the wild type (WT) was than that on the myb108 and wrky75 mutants under MD, implying that the mutants were more sensitive to MD than the WT. This research broadens our understanding the underlying molecular mechanism of banana seedlings adapted to the long-term MD condition.

Pyrophosphate as an alternative energy currency in plants

In the conditions of [Mg2+] elevation that occur, in particular, under low oxygen stress and are the consequence of the decrease in [ATP] and increase in [ADP] and [AMP], pyrophosphate (PPi) can function as an alternative energy currency in plant cells. In addition to its production by various metabolic pathways, PPi can be synthesized in the combined reactions of pyruvate, phosphate dikinase (PPDK) and pyruvate kinase (PK) by so-called PK/PPDK substrate cycle, and in the reverse reaction of membrane-bound H+-pyrophosphatase, which uses the energy of electrochemical gradients generated on tonoplast and plasma membrane. The PPi can then be consumed in its active forms of MgPPi and Mg2PPi by PPi-utilizing enzymes, which require an elevated [Mg2+]. This ensures a continuous operation of glycolysis in the conditions of suppressed ATP synthesis, keeping metabolism energy efficient and less dependent on ATP.