Critical Issues in the Study of Magnesium Transport Systems and Magnesium Deficiency Symptoms in Plants

Structural and biochemical analysis of the unique interactions of the Campylobacter jejuni CorA channel protein with divalent cations

CorA is a Mg2+ channel that plays a key role in the homeostasis of intracellular Mg2+ in bacteria and archaea. CorA consists of a cytoplasmic domain and a transmembrane domain and generates a Mg2+ pathway by forming a pentamer in the cell membrane. CorA gating is regulated via negative feedback by Mg2+, which is accommodated by the pentamerization interface of the CorA cytoplasmic domain (CorACD). The Mg2+-binding sites of CorACD differ depending on the species, suggesting that the Mg2+-binding modes and Mg2+-mediated gating mechanisms of CorA vary across prokaryotes. To define the Mg2+-binding mechanism of CorA in the Campylobacter jejuni pathogen, we structurally and biochemically characterized C. jejuni CorACD (cjCorACD). cjCorACD adopts a three-layered α/β/α structure as observed in other CorA orthologs. Interestingly, cjCorACD exhibited enhanced thermostability in the presence of Ca2+, Ni2+, Zn2+, or Mn2+ in addition to Mg2+, indicating that cjCorACD interacts with diverse divalent cations. This cjCorACD stabilization is mediated by divalent cation accommodation by negatively charged residues located at the bottom of the cjCorACD structure away from the pentamerization interface. Consistently, cjCorACD exists as a monomer irrespective of the presence of divalent cations. We concluded that cjCorACD binds divalent cations in a unique pentamerization-independent manner.

Mg2+ limitation leads to a decrease in chlorophyll, resulting in an unbalanced photosynthetic apparatus in the cyanobacterium Synechocytis sp. PCC6803

Mg2+, the most abundant divalent cation in living cells, plays a pivotal role in numerous enzymatic reactions and is of particular importance for organisms performing oxygenic photosynthesis. Its significance extends beyond serving as the central ion of the chlorophyll molecule, as it also acts as a counterion during the light reaction to balance the proton gradient across the thylakoid membranes. In this study, we investigated the effects of Mg2+ limitation on the physiology of the well-known model microorganism Synechocystis sp. PCC6803. Our findings reveal that Mg2+ deficiency triggers both morphological and functional changes. As seen in other oxygenic photosynthetic organisms, Mg2+ deficiency led to a decrease in cellular chlorophyll concentration. Moreover, the PSI-to-PSII ratio decreased, impacting the photosynthetic efficiency of the cell. In line with this, Mg2+ deficiency led to a change in the proton gradient built up across the thylakoid membrane upon illumination.

Metabolome profiling and transcriptome analysis unveiling the crucial role of magnesium transport system for magnesium homeostasis in tea plants

Magnesium (Mg2+) is a crucial nutrient for the growth and development of Camellia sinensis and is closely related to the quality of tea. However, the underlying mechanisms responding to low-Mg 2+ stress in tea plants remain largely unknown. In this study, photosynthetic parameters, metabolomics, and transcriptomics were utilized to explore the potential effects of low Mg2+ on the growth and metabolism of C. sinensis. Low-Mg2+ treatment increased the ratio of shoot dry weight to root dry weight but decreased the photosynthesis of C. sinensis. Forty and thirty metabolites were impacted by Mg2+ shortage in C. sinensis shoots and roots, respectively. Integrated transcriptome and metabolome analyses revealed the possible reasons for the decreased contents of chlorophyll and catechins and the increased theanine content in C. sinensis roots. Weighted gene co-expression network analysis indicated that the Mg2+ transport system was essential in the regulation of Mg2+ homeostasis in C. sinensis, in which CsMGT5 was identified to be the key regulator according to CsMGT5-overexpressing and complementary assays in Arabidopsis thaliana. Moreover, silencing of CsMGT5 in vivo reduced the content of chlorophyll in C. sinensis shoots. In addition, CsMGT5 might collaborate with ammonium transporters to keep the amino acid content steady, suggesting its potential application for tea quality improvement. All these findings demonstrate the key roles of CsMGTs for Mg2+ homeostasis in C. sinensis, providing a theoretical basis for Mg2+ efficient utilization in plants.

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.

New insights into comprehensive analysis of magnesium transporter (MGT) gene family in rice (Oryza sativa L.)

Magnesium transporters (MGTs) regulate magnesium absorption, transport, and redistribution in higher plants. To investigate the role of the Oryza sativa MGTs gene family members under salt stress, this study analyzed the protein properties, gene structure, phylogenetic relationship, synteny patterns, expression, and co-expression networks of 23 non-redundant OsMGT. The evolutionary relationship of the OsMGT gene family was fully consistent with their functional domain, and were divided into three main classes based on the conserved domain: MMgT, CorA-like, and NIPA. The α/β patterns in the protein structures were highly similar in the CorA-like and NIPA members, with the conserved structures in the Mg2+-binding and catalytic regions. The CorA-like clade-related proteins demonstrated the highest numbers of protein channels with Pro, Ser, Lys, Gly, and Tyr, as the critical binding residues. The expression analysis of OsMGT genes in various tissues showed that MGTs' gene family may possess critical functions during rice development. Gene expression analysis of candidate OsMGT using reverse-transcription quantitative real-time PCR (RT-qPCR) found that four OsMGT genes exhibited different expression patterns in salt-sensitive and salt-tolerant rice genotypes. We hypothesize that the OsMGT gene family members may be involved in responses to salt stress. These findings could be useful for further functional investigation of MGTs as well as defining their involvement in abiotic stress studies.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03735-4.

Response of Medical Cannabis to Magnesium (Mg) Supply at the Vegetative Growth Phase

Recent studies demonstrated a significant impact of some major macronutrients on function and production of medical cannabis plants, yet information on the effect of most nutrients, including Mg, is scarce. Magnesium is required for major physiological functions and metabolic processes in plants, and in the present study we studied the effects of five Mg treatments (2, 20, 35, 70, and 140 mg L^-1 Mg), on plant development and function, and distribution of minerals in drug-type (medical) cannabis plants, at the vegetative growth phase. The plants were cultivated in pots under controlled environment conditions. The results demonstrate that plant development is optimal under Mg supply of 35-70 mg L^-1 (ppm), and impaired under lower Mg input of 2-20 mg L^-1. Two mg L^-1 Mg resulted in visual deficiency symptoms, shorter plants, reduced photosynthesis rate, transpiration rate, photosynthetic pigments and stomatal conduction in young-mature leaves, and a 28% reduction of total plant biomass compared to the optimal supply of 35 mg L^-1 Mg. The highest supply level of 140 mg L^-1 Mg induced a small decrease in physiological function, which did not affect morphological development and biomass accumulation. The low-deficient Mg supply of 2 mg L^-1 Mg stimulated Mg uptake and accumulation of N, P, K, Ca, Mn, and Zn in the plant. Increased Mg supply impaired uptake of Ca and K and their root-to-shoot translocation, demonstrating competitive cation inhibition. Mg-deficiency symptoms developed first in old leaves (at 2 mg L^-1 Mg) and progressed towards young-mature leaves, demonstrating ability for Mg in-planta storage and remobilization. Mg toxicity symptoms appeared in old leaves from the bottom of the plants, under 140 mg L^-1 Mg. Taken together, the findings suggest 35-70 mg L^-1 Mg as the optimal concentration range for cannabis plant development and function at the vegetative growth phase.

Identification and Expression of the CorA/MRS2/ALR Type Magnesium Transporters in Tomato

Magnesium (Mg²⁺) is the most abundant divalent ion in plants, participating in numerous metabolic processes in growth and development. CorA/MRS2/ALR type Mg²⁺ transporters are essential for maintaining Mg²⁺ homeostasis in plants. However, the candidate protein and its potential functions in the tomato plant have not been fully understood. In this study, we identified seven MGT genes (SlMRS2) in tomato based on sequence similarity, domain analysis, conserved motif identification, and structure prediction. Two SlMRS2 genes were analyzed in the bacterial strain MM281, and a functional complementary assay demonstrated their high-affinity transport of Mg²⁺. Quantitative real-time PCR analysis revealed that the expressions of these Mg²⁺ transporters were down-regulated in leaves under Mg²⁺ limitation, with a greater impact on lower and middle leaves compared to young leaves. Conversely, under Mg²⁺ toxicity, several genes were up-regulated in leaves with a circadian rhythm. Our findings indicate that members of the SlMRS2 family function as Mg²⁺ transporters and lay the groundwork for further analysis of their distinct functions in tomato.

Physiological and transcriptomic responses to magnesium deficiency in Neolamarckia Cadamba

Magnesium (Mg²⁺) is a critical component of chlorophyll and enzymes involved in various physiological and biochemical processes essential for plant growth, biomass accumulation, and photosynthesis. Mg²⁺ deficiency (MgD) is common in hot and rainy subtropical areas due to its easy loss from soil. Neolamarckia cadamba, an important tropical tree in South Asia, faces severe effects of MgD, however, the responses of N. cadamba to MgD stress remain unclear. In here, effects of N. cadamba under MgD stress were investigated. The study revealed that MgD had lower plant biomass, fresh and dry weight, root length, root volume, and surface area compared to CK (normal Mg²⁺). As treatment time increased, the leaves began to yellow, and lesions appeared. Chlorophyll a, chlorophyll b, and total chlorophyll content, along with fluorescence-related parameters and leaf photosynthetic capacity, were significantly reduced in MgD stress compared to CK treatment. Transcriptome analysis showed that transporters as well as transcription factors (TFs) from MYC (v-myc avian myelocytomatosis viral oncogene homolog), MYB (v-myb avian myeloblastosis viral oncogene homolog), bHLH (basic helix-loop-helix) and WRKY families were upregulated in leaves at 10 d of MgD stress, indicating that magnesium signaling transduction might be activated to compensate MgD. In addition, genes including chlorophyll(ide) b reductase (NYC1/NOL) chlorophyll/bacteriochlorophyll synthase (G4) and 7-hydroxymethyl chlorophyll a reductase synthesizing (HCAR) chlorophyll a and chlorophyll b were down-regulated in leaves, while those scavenging reactive oxygen species (ROS) were mainly up-regulated at 10 d of MgD stress. These results shed light on underlying MgD in N. cadamba.

Uncovering the role of wheat magnesium transporter family genes in abiotic responses

BACKGROUND

The CorA / MGT / MRS2 family proteins are an important group of magnesium transporter proteins that maintain magnesium ion homeostasis in plant cells. However, little is known about the MGT functions in wheat.

METHODS

The known MGT sequences were used as queries to BlastP against wheat genome IWGSC RefSeq v2.1 assembly (E-value <10-5). Chromosome localization information for each TaMGT gene was obtained from the GFF3 file of the wheat genome data (IWGSCv2.1).The sequence of 1500 bp upstream of the TaMGT genes was extracted from the wheat genome data. The cis-elements were analyzed using PlantCARE online tool.

RESULT

A total of 24 MGT genes were identified on 18 chromosomes of wheat. After functional domain analysis, only TaMGT1A, TaMGT1B, and TaMGT1D had GMN mutations to AMN, while all the other genes had conserved GMN tripeptide motifs. Expression profiling showed that the TaMGT genes were differentially expressed under different stresses and at different growth and development stages. The expression levels of TaMGT4B and TaMGT4A were significantly up-regulated in cold damage. In addition, qRT-PCR results also confirmed that these TaMGT genes are involved in the wheat abiotic stress responses.

CONCLUSION

In conclusion, The results of our research provide a theoretical basis for further research on the function of TaMGT gene family in wheat.

Ecological strategy of Phyllostachys heteroclada oliver in the riparian zone based on ecological stoichiometry

The abnormality of seasonal water level fluctuation in the riparian zone causes various ecological and environmental problems, such as vegetation degradation, biodiversity reduction, soil erosion, and landscape transformation, thereby critically modifying the ecosystem structure and functions. This necessitates the development of a dominant vegetation zone with competitive potential. In this study, we investigated the content and distribution pattern of nutrient elements in each organ of the dominant bamboo species, Phyllostachys heteroclada, in the riparian zone. We also analyzed the morphological characteristics, root aeration tissue structure, root oxygen exchange capacity, ATP supply situation, and leaf PSII photosynthetic mechanism of two bamboo species (P. heteroclada and P. nigra) in the riparian zone. Compared with P. nigra, the roots of P. heteroclada formed well-developed oxygen storage and transport structure, i.e., aeration tissue, and exhibited root oxygen secretion in the waterlogging environment of the riparian zone, whereas the roots maintained a high ATP content through energy metabolism, thus benefiting mineral absorption and transport. Moreover, the accumulation of N, P, Ca, Mg, and Fe in the leaves of P. heteroclada was greater under waterlogging conditions than under non-waterlogging conditions, which is the basis for the efficient operation of the photosynthetic mechanism of the leaves. Compared with waterlogged P. nigra, the PSII electron acceptor Q_A of P. heteroclada leaves had a vigorous reducing ability and showed higher efficiency of light uptake energy as well as higher quantum yield indexes ϕ(Eo) and ϕ(Po). This study demonstrates that the ecological adaptive regulation strategies of P. heteroclada in the riparian zone are intrinsic driving factors affecting their stoichiometric characteristics, including changes in the absorption and transport of minerals caused by root aeration structure and energy metabolism. Moreover, carbon production and allocation may be caused by the stable photosynthetic mechanism and source-sink relationship of leaves. Through the synergistic regulation of different organs realizing their roles and functions, P. heteroclada developed ecological stoichiometry characteristics adapted to the riparian zone.

Recent Advances in the Structural Biology of Mg2+ Channels and Transporters

Magnesium ions (Mg²⁺) are the most abundant divalent cations in living organisms and are essential for various physiological processes, including ATP utilization and the catalytic activity of numerous enzymes. Therefore, the homeostatic mechanisms associated with cellular Mg²⁺ are crucial for both eukaryotic and prokaryotic organisms and are thus strictly controlled by Mg²⁺ channels and transporters. Technological advances in structural biology, such as the expression screening of membrane proteins, in meso phase crystallization, and recent cryo-EM techniques, have enabled the structure determination of numerous Mg²⁺ channels and transporters. In this review article, we provide an overview of the families of Mg²⁺ channels and transporters (MgtE/SLC41, TRPM6/7, CorA/Mrs2, CorC/CNNM), and discuss the structural biology prospects based on the known structures of MgtE, TRPM7, CorA and CorC.

The protein kinase SlCIPK23 boosts K+ and Na+ uptake in tomato plants

Regulation of root transport systems is essential under fluctuating nutrient supply. In the case of potassium (K⁺ ), HAK/KUP/KT K⁺ transporters and voltage-gated K⁺ channels ensure root K⁺ uptake in a wide range of K⁺ concentrations. In Arabidopsis, the CIPK23/CBL1-9 complex regulates both transporter- and channel-mediated root K⁺ uptake. However, research about K⁺ homeostasis in crops is in demand due to species-specific mechanisms. In the present manuscript, we studied the contribution of the voltage-gated K⁺ channel LKT1 and the protein kinase SlCIPK23 to K⁺ uptake in tomato plants by analysing gene-edited knockout tomato mutant lines, together with two-electrode voltage-clamp experiments in Xenopus oocytes and protein-protein interaction analyses. It is shown that LKT1 is a crucial player in tomato K⁺ nutrition by contributing approximately 50% to root K⁺ uptake under K⁺ -sufficient conditions. Moreover, SlCIPK23 was responsible for approximately 100% of LKT1 and approximately 40% of the SlHAK5 K⁺ transporter activity in planta. Mg⁺² and Na⁺ compensated for K⁺ deficit in tomato roots to a large extent, and the accumulation of Na⁺ was strongly dependent on SlCIPK23 function. The role of CIPK23 in Na⁺ accumulation in tomato roots was not conserved in Arabidopsis, which expands the current set of CIPK23-like protein functions in plants.

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.

Multi-Environment Quantitative Trait Loci Mapping for Grain Iron and Zinc Content Using Bi-parental Recombinant Inbred Line Mapping Population in Pearl Millet

Pearl millet is a climate-resilient, nutritious crop with low input requirements that could provide economic returns in marginal agro-ecologies. In this study, we report quantitative trait loci (QTLs) for iron (Fe) and zinc (Zn) content from three distinct production environments. We generated a genetic linkage map using 210 F₆ recombinant inbred line (RIL) population derived from the (PPMI 683 × PPMI 627) cross using genome-wide simple sequence repeats (SSRs). The molecular linkage map (seven linkage groups) of 151 loci was 3,273.1 cM length (Kosambi). The content of grain Fe in the RIL population ranged between 36 and 114 mg/Kg, and that of Zn from 20 to 106 mg/Kg across the 3 years (2014-2016) at over the three locations (Delhi, Dharwad, and Jodhpur). QTL analysis revealed a total of 22 QTLs for grain Fe and Zn, of which 14 were for Fe and eight were for Zn on three consecutive years at all locations. The observed phenotypic variance (R ²) explained by different QTLs for grain Fe and Zn content ranged from 2.85 (QGFe.E3.2014-2016_Q3) to 19.66% (QGFe.E1.2014-2016_Q3) and from 2.93 (QGZn.E3.2014-2016_Q3) to 25. 95% (QGZn.E1.2014-2016_Q1), respectively. Two constitutive expressing QTLs for both Fe and Zn co-mapped in this population, one on LG 2 and second one on LG 3. Inside the QTLs candidate genes such as Ferritin gene, Al³⁺ Transporter, K⁺ Transporters, Zn²⁺ transporters and Mg²⁺ transporters were identified using bioinformatics approaches. The identified QTLs and candidate genes could be useful in pearl millet population improvement programs, seed, restorer parents, and marker-assisted selection programs.

Magnesium Fertilization Affected Rice Yields in Magnesium Sufficient Soil in Heilongjiang Province, Northeast China

Magnesium (Mg) plays a crucial role in rice yield. Heilongjiang Province is the main rice-producing region of China, playing an important role in guaranteeing China's and the world's grain security. However, rarely Mg fertilization is applied in this province. Soil Mg status of main rice-producing areas in Heilongjiang Province was investigated and Mg fertilizer experiments were conducted aiming to provide fertilizer recommendation in this region. A total of 358 soil samples from the 0-20 cm and 20-40 cm soil layer from the main rice-producing areas of Heilongjiang Province were collected to analyze soil exchangeable Mg (ex-Mg) and relative chemical properties. Meanwhile, field experiments of soil and foliar Mg application were performed in 2017-2019 to identify the effect of this nutrient on rice yield. The results showed that the ex-Mg concentration in the 0-20 cm and 20-40 cm soil layer was 282 mg kg^-1 and 243 mg kg^-1, respectively. Moreover, ex-Mg ranged on the abundant and exceptionally abundant level accounted for 75% in 0-20 cm and 55.3% in 20-40 cm. The ex-Mg concentration in the upper soil layer was higher than in the lower soil layer and varied depending on regions, which the west part of Heilongjiang Province showed the highest concentration in both soil layers. Correlation analysis showed that there had a significant (P < 0.05) linear relationship between ex-Mg and pH, CEC, ex-K, Ca, K/Mg, and Ca/Mg. Meanwhile, the results of path coefficients demonstrated that pH, CEC, and Ca/Mg had the most direct effect on ex-Mg concentration among these above factors. Soil Mg application had little effect on rice yield, which might be related to the soil Mg concentration and availability, and root uptake activity. Foliar Mg application increased rice yield by 8.45% (P < 0.05) compared to without Mg treatment, increased 1,000-grain weight by 2.62% (P < 0.05), and spikelet number per panicle by 4.19% (P < 0.05). In general, the paddy soil ex-Mg concentration in Heilongjiang Province was abundant. Soil-applied Mg played little role in rice yield in ex-Mg abundant regions, while foliar application increased rice yields significantly via increasing 1,000-grain weight and spikelet number per panicle.

Genome-wide comparative analysis of Mg transporter gene family between Triticum turgidum and Camelina sativa

Magnesium (Mg) as a bimetal plays critical roles in biochemical processes, membrane stability, and enzyme activity. Mg transporters (MGTs) are involving in maintaining Mg homeostasis in cells. Although the MGT family members have been identified in different plant species, there is no comprehensive analysis of the other plants' MGT genes. In the current study, 62 and 41 non-redundant putative MGT proteins were recognized into the genome of Camelina sativa, and Triticum turgidum and they were compared based on physicochemical properties, protein structure, expression, and interaction. All identified MGTs were classified into three subgroups, NIPA, CorA, and MRS2/MGT, based on conserved-motifs distribution. The results showed that the secondary structure pattern in NIPA and MRS2 subfamily members in both studied plant species were highly similar. Furthermore, MGTs encompass the conserved structures and the critical sites mainly in the metal ion and Mg²⁺ binding centers as well as the catalytic sites were observed. The highest numbers of protein channels were predicted in CorA proteins in both C. sativa and T. turgidum with 24 and 17 channel numbers, respectively. The Ser, Pro, Gly, Lys, Tyr, and Arg amino acids were predicted as the binding residues in MGTs channel regions. The expression pattern of identified genes demonstrated that MGT genes have diverse tissue-specific expression and stress response expression patterns. Besides, 147 co-expressed genes with MGTs were clustered into the eight co-expression nodes involved in N-glycan biosynthesis, protein processing in the endoplasmic reticulum, carbon metabolism, biosynthesis of amino acids, and endocytosis. In the present study, all interpretations are based on in silico predictions, which can be used in further studies related to functional genomics of MGT genes.

Magnesium Signaling in Plants

Free magnesium (Mg2+) is a signal of the adenylate (ATP+ADP+AMP) status in the cells. It results from the equilibrium of adenylate kinase (AK), which uses Mg-chelated and Mg-free adenylates as substrates in both directions of its reaction. The AK-mediated primary control of intracellular [Mg2+] is finely interwoven with the operation of membrane-bound adenylate- and Mg2+-translocators, which in a given compartment control the supply of free adenylates and Mg2+ for the AK-mediated equilibration. As a result, [Mg2+] itself varies both between and within the compartments, depending on their energetic status and environmental clues. Other key nucleotide-utilizing/producing enzymes (e.g., nucleoside diphosphate kinase) may also be involved in fine-tuning of the intracellular [Mg2+]. Changes in [Mg2+] regulate activities of myriads of Mg-utilizing/requiring enzymes, affecting metabolism under both normal and stress conditions, and impacting photosynthetic performance, respiration, phloem loading and other processes. In compartments controlled by AK equilibrium (cytosol, chloroplasts, mitochondria, nucleus), the intracellular [Mg2+] can be calculated from total adenylate contents, based on the dependence of the apparent equilibrium constant of AK on [Mg2+]. Magnesium signaling, reflecting cellular adenylate status, is likely widespread in all eukaryotic and prokaryotic organisms, due simply to the omnipresent nature of AK and to its involvement in adenylate equilibration.

Mineral Biofortification of Vegetables as a Tool to Improve Human Diet

Vegetables represent pillars of good nutrition since they provide important phytochemicals such as fiber, vitamins, antioxidants, as well as minerals. Biofortification proposes a promising strategy to increase the content of specific compounds. As minerals have important functionalities in the human metabolism, the possibility of enriching fresh consumed products, such as many vegetables, adopting specific agronomic approaches, has been considered. This review discusses the most recent findings on agronomic biofortification of vegetables, aimed at increasing in the edible portions the content of important minerals, such as calcium (Ca), magnesium (Mg), iodine (I), zinc (Zn), selenium (Se), iron (Fe), copper (Cu), and silicon (Si). The focus was on selenium and iodine biofortification thus far, while for the other mineral elements, aspects related to vegetable typology, genotypes, chemical form, and application protocols are far from being well defined. Even if agronomic fortification is considered an easy to apply technique, the approach is complex considering several interactions occurring at crop level, as well as the bioavailability of different minerals for the consumer. Considering the latter, only few studies examined in a broad approach both the definition of biofortification protocols and the quantification of bioavailable fraction of the element.

Leaf chlorosis in Arabidopsis thaliana hybrids is associated with transgenerational decline and imbalanced ribosome number

The interaction of two parental genomes can result in negative outcomes in offspring, also known as hybrid incompatibility. We have previously reported a case in which two recessively interacting alleles result in hybrid chlorosis in Arabidopsis thaliana. A DEAD-box RNA helicase 18 (AtRH18) was identified to be necessary for chlorosis. In this study, we use a sophisticated genetic approach to investigate genes underlying hybrid chlorosis. Sequence comparisons, DNA methylation inhibitor drug treatment and segregation analysis were used to investigate the epigenetic regulation of hybrid chlorosis. Relative rRNA numbers were quantified using real-time quantitative PCR. We confirmed the causality of AtRH18 and provided evidence for the involvement of the promoter region of AtRH18 in the hybrid chlorosis. Furthermore, AtMOM1 from the second parent was identified as the likely candidate gene on chromosome 1. Chlorotic hybrids displayed transgenerational decline in chlorosis, and DNA demethylation experiment restored chlorophyll levels in chlorotic hybrids. Quantification of rRNA indicated that hybrid chlorosis was associated with an imbalance in the ratio of cytosolic and plastid ribosomes. Our findings highlight that the epigenetic regulation of AtRH18 causes hybrid breakdown and provide novel information about the role of AtRH18 in plant development.

Identification and functional analysis of the CorA/MGT/MRS2-type magnesium transporter in banana

Magnesium (Mg) plays an irreplaceable role in plant growth and development. Mg transporters, especially CorA/MGT/MRS2 family proteins, played a vital role in regulating Mg content in plant cells. Although extensive work has been conducted in model crops, such as Arabidopsis, rice, and maize, the relevant information is scarce in tropical crops. In this study, 10 MaMRS2 genes in banana (Musa acuminata) were isolated from its genome and classified into five distinct clades. The putative physiochemical properties, chromosome location, gene structure, cis-acting elements, and duplication relationships in between these members were analyzed. Complementary experiments revealed that three MaMRS2 gene members (MaMRS2-1, MaMRS2-4, MaMRS2-7), from three distinct phylogenetic branches, were capable of restoring the function of Mg transport in Salmonella typhimurium mutants. Semi-quantitative RT-PCR showed that MaMRS2 genes were differentially expressed in banana cultivar ‘Baxijiao’ (Musa spp. AAA Cavendish) seedlings. The result was confirmed by real-time PCR analysis, in addition to tissue specific expression, expression differences among MaMRS2 members were also observed under Mg deficiency conditions. These results showed that Mg transporters may play a versatile role in banana growth and development, and our work will shed light on the functional analysis of Mg transporters in banana.

An SMU Splicing Factor Complex Within Nuclear Speckles Contributes to Magnesium Homeostasis in Arabidopsis

Mg2+ is among the most abundant divalent cations in living cells. In plants, investigations on magnesium (Mg) homeostasis are restricted to the functional characterization of Mg2+ transporters. Here, we demonstrate that the splicing factors SUPPRESSORS OF MEC-8 AND UNC-52 1 (SMU1) and SMU2 mediate Mg homeostasis in Arabidopsis (Arabidopsis thaliana). A low-Mg sensitive Arabidopsis mutant was isolated, and the causal gene was identified as SMU1 Disruption of SMU2, a protein that can form a complex with SMU1, resulted in a similar low-Mg sensitive phenotype. In both mutants, an Mg2+ transporter gene, Mitochondrial RNA Splicing 2 (MRS2-7), showed altered splicing patterns. Genetic evidence indicated that MRS2-7 functions in the same pathway as SMU1 and SMU2 for low-Mg adaptation. In contrast with previous results showing that the SMU1-SMU2 complex is the active form in RNA splicing, MRS2-7 splicing was promoted in the smu2 mutant overexpressing SMU1, indicating that complex formation is not a prerequisite for the splicing. We found here that formation of the SMU1-SMU2 complex is an essential step for their compartmentation in the nuclear speckles, a type of nuclear body enriched with proteins that participate in various aspects of RNA metabolism. Taken together, our study reveals the involvement of the SMU splicing factors in plant Mg homeostasis and provides evidence that complex formation is required for their intranuclear compartmentation.

Magnesium Deficiency Reduced the Yield and Seed Germination in Wax Gourd by Affecting the Carbohydrate Translocation

Magnesium (Mg) is a particular mineral nutrient greatly affecting the size and activity of sink organs. Wax gourd crop with its fruits having fresh weight up to 20-50 kg per single fruit serves as an excellent experimental plant species for better understanding the role of varied Mg nutrition in sink strength and yield formation. This study aimed to investigate the effects of Mg deficiency on fruit yield and seed vigor in wax gourd grown under field conditions. Plants were grown under field conditions until maturity with increasing soil Mg applications. At the beginning of fruit formation, leaves were used to analyze concentrations of sucrose, starch and Mg as well as phloem export of sucrose. At maturity, fruit yield was determined and the seeds collected were used in germination studies and starch analysis. Low Mg supply resulted in a significant impairment in fruit fresh yield, which was closely associated with higher accumulation of starch and sucrose in source leaves and lower amount of sucrose in phloem exudate. Seeds obtained from Mg deficiency plants exhibited lower amount of starch and substantial reduction in both germination capacity and seedling establishment when compared to the seeds from the Mg adequate plants. Our study revealed that magnesium deficiency significantly diminished fruit yield of field-grown wax gourd, most probably by limiting the carbohydrate transport from source organs to developing fruit. Ensuring sufficient Mg supply to plant species with high sink size such as wax gourd, during the reproductive growth stage, is a critical factor for achieving higher fruit yield formation and also better vigor of next-generation seeds.

Mg2+ homeostasis and transport in cyanobacteria - at the crossroads of bacterial and chloroplast Mg2+ import

Magnesium cation (Mg2+) is the most abundant divalent cation in living cells, where it is required for various intracellular functions. In chloroplasts and cyanobacteria, established photosynthetic model systems, Mg2+ is the central ion in chlorophylls, and Mg2+ flux across the thylakoid membrane is required for counterbalancing the light-induced generation of a ΔpH across the thylakoid membrane. Yet, not much is known about Mg2+ homoeostasis, transport and distribution within cyanobacteria. However, Mg2+ transport across membranes has been studied in non-photosynthetic bacteria, and first observations and findings are reported for chloroplasts. Cyanobacterial cytoplasmic membranes appear to contain the well-characterized Mg2+ channels CorA and/or MgtE, which both facilitate transmembrane Mg2+ flux down the electrochemical gradient. Both Mg2+ channels are typical for non-photosynthetic bacteria. Furthermore, Mg2+ transporters of the MgtA/B family are also present in the cytoplasmic membrane to mediate active Mg2+ import into the bacterial cell. While the cytoplasmic membrane of cyanobacteria resembles a 'classical' bacterial membrane, essentially nothing is known about Mg2+ channels and/or transporters in thylakoid membranes of cyanobacteria or chloroplasts. As discussed here, at least one Mg2+ channelling protein must be localized within thylakoid membranes. Thus, either one of the 'typical' bacterial Mg2+ channels has a dual localization in the cytoplasmic plus the thylakoid membrane, or another, yet unidentified channel is present in cyanobacterial thylakoid membranes.

Growth response of the ichthyotoxic haptophyte, Prymnesium parvum Carter, to changes in sulfate and fluoride concentrations

Golden alga Prymnesium parvum Carter is a euryhaline, ichthyotoxic haptophyte (Chromista). Because of its presumed coastal/marine origin where SO₄^2- levels are high, the relatively high SO₄^2- concentration of its brackish inland habitats, and the sensitivity of marine chromists to sulfur deficiency, this study examined whether golden alga growth is sensitive to SO₄^2- concentration. Fluoride is a ubiquitous ion that has been reported at higher levels in golden alga habitat; thus, the influence of F^- on growth also was examined. In low-salinity (5 psu) artificial seawater medium, overall growth was SO₄^2—dependent up to 1000 mg l^-1 using MgSO₄ or Na₂SO₄ as source; the influence on growth rate, however, was more evident with MgSO₄. Transfer from 5 to 30 psu inhibited growth when salinity was raised with NaCl but in the presence of seawater levels of SO₄^2-, these effects were fully reversed with MgSO₄ as source and only partially reversed with Na₂SO₄. Growth inhibition was not observed after acute transfer to 30 psu in a commercial sea salt mixture. In 5-psu medium, F^- inhibited growth at all concentrations tested. These observations support the hypothesis that spatial differences in SO₄^2- –but not F^-–concentration help drive the inland distribution and growth of golden alga and also provide physiological relevance to reports of relatively high Mg²⁺ concentrations in golden alga habitat. At high salinity, however, the ability of sulfate to maintain growth under osmotic stress was weak and overshadowed by the importance of Mg²⁺. A mechanistic understanding of growth responses of golden alga to SO₄^2-, Mg²⁺ and other ions at environmentally relevant levels and under different salinity scenarios will be necessary to clarify their ecophysiological and evolutionary relevance.

Magnesium Deficiency Triggers SGR-Mediated Chlorophyll Degradation for Magnesium Remobilization

Magnesium (Mg) is a relatively mobile element that is remobilized in plants under Mg-limited conditions through transport from old to young tissues. However, the physiological and molecular mechanisms underlying Mg remobilization in plants remain poorly understood. In this study, we investigated Mg remobilization in rice (Oryza sativa) as facilitated through a Mg dechelatase gene involved in chlorophyll degradation, STAY-GREEN (OsSGR). We first observed that mid-aged leaves of rice are more susceptible to Mg deficiency. Expression of OsSGR was specifically upregulated by Mg deficiency, and the response was more pronounced in mid-aged leaves. Knockout of OsSGR exhibited the stay-green phenotype, which hindered the mobility of Mg from mid-aged leaves to young developing leaves. This decline in Mg mobility was associated with inhibited growth of developing leaves in mutants under Mg-limited conditions. Furthermore, Mg deficiency enhanced reactive oxygen species (ROS) generation in mid-aged leaves. ROS levels, particularly hydrogen peroxide, in turn, positively regulated OsSGR expression, probably through chloroplast-to-nucleus signaling, which triggers chlorophyll degradation to protect mid-aged leaves from photodamage. Taken together, these results show that OsSGR-mediated chlorophyll degradation contributes to not only internal remobilization of Mg from mid-aged leaves to developing leaves, but also photooxidative protection of mid-aged leaves under Mg-limited conditions. ROS appear to act as feedback regulators of OsSGR expression to precisely govern chlorophyll degradation in mid-aged leaves where Mg and photosynthetic capacities are relatively high.

Magnesium uptake characteristics in Arabidopsis revealed by 28Mg tracer studies

The Mg²⁺ uptake system in Arabidopsis roots is Gd³⁺- and Fe²⁺-sensitive, and responds to a changing Mg²⁺ concentration within 1 h with the participation of AtMRS2 transporters. Magnesium (Mg²⁺) absorption and the mechanism regulating its activity have not been clarified yet. To address these issues, it is necessary to reveal the characteristics of Mg²⁺ uptake in roots. Therefore, we first investigated the Mg²⁺ uptake characteristics in roots of 1-week-old Arabidopsis plants using ²⁸Mg. The Mg²⁺ uptake system in roots was up-regulated within 1 h in response to the low Mg²⁺ condition. This induction was inhibited in Arabidopsis "mitochondrial RNA splicing 2/magnesium transport" mutants atmrs2-4/atmgt6 and atmrs2-7/atmgt7, while the expression of AtMRS2-4/AtMGT6 and AtMRS2-7/AtMGT7 genes in the Arabidopsis wild-type was not responsive to Mg²⁺ conditions. In addition, the Mg deficiency-induced Mg²⁺ uptake system was shut-down within 5 min when Mg²⁺ was resupplied to the environment. An inhibition study showed that the constitutive mechanism functioning in Mg²⁺ uptake under Mg²⁺ sufficient conditions was sensitive to a number of divalent and trivalent cations, particularly Gd³⁺ and Fe²⁺, but not to K⁺.

Symbiotic characteristics of Bradyrhizobium diazoefficiens USDA 110 mutants associated with shrubby sophora (Sophora flavescens) and soybean (Glycine max)

Site-specific insertion plasmid pVO155 was used to knockout the genes involved in the alternation of host range of strain Bradyrhizobium diazoefficiens USDA 110 from its original determinate-nodule-forming host soybean (Glycine max), to promiscuous and indeterminate-nodule-forming shrubby legume sophora (Sophora flavescens). Symbiotic phenotypes of these mutants inoculated to these two legumes, were compared to those infected by wild-type strain USDA 110. Six genes of the total fourteen Tn5 transposon mutated genes were broken using the pVO155 plasmid. Both Tn5 and pVO155-inserted mutants could nodulate S. flavescens with different morphologies of low-efficient indeterminate nodules. One to several rod or irregular bacteroids, containing different contents of poly-β-hydroxybutyrate or polyphosphate were found within the symbiosomes in nodulated cells of S. flavescens infected by the pVO155-inserted mutants. Moreover, none of bacteroids were observed in the pseudonodules of S. flavescens, infected by wild-type strain USDA 110. These mutants had the nodulation ability with soybean but the symbiotic efficiency reduced to diverse extents. These findings enlighten the complicated interactions between rhizobia and legumes, i. e., mutation of genes involved in metabolic pathways, transporters, chemotaxis and mobility could alter the rhizobial entry and development of the bacteroid inside the nodules of a new host legume.

Identification of Novel Genomic Loci Associated with Soybean Shoot Tissue Macro- and Micronutrient Concentrations

The mineral composition of crop shoot tissues is important for yield formation and nutrient remobilization to seeds. The natural diversity that exists within crop species can be used to investigate mechanisms that define plant mineral composition and to identify important genomic loci for these processes. The objective of this study was to determine shoot mineral nutrient concentrations in genetically diverse soybean [ (L.) Merr.] genotypes and to identify genomic regions associated with concentrations of different nutrients in shoot tissue. The genotypes were grown at two locations in 2 yr and characterized for macronutrient (Ca, Mg, P, K, and S) and micronutrient (B, Cu, Fe, Mn, and Zn) concentrations in shoot tissues. Genome-wide association studies were conducted with 31,748 single nucleotide polymorphisms (SNPs) via a unified mixed model to identify SNPs associated with macro- and micronutrient concentrations. The number of putative loci identified for the macronutrients ranged from 11 for Ca to 20 for K. For the micronutrients, the number ranged from 10 for Mn to 24 for Fe. In addition to colocated loci for multiple nutrients, 22 individual SNPs were associated with more than one nutrient such that 11 different nutrient combinations were encompassed by these SNPs. Ultimately, the putative loci identified in this study will need to be confirmed and are expected to aid in the identification of new sources of variation for use in soybean breeding programs as well as for mechanistic studies aimed at understanding the regulation of mineral nutrient uptake, translocation, and shoot tissue concentrations.

Magnesium promotes root nodulation through facilitation of carbohydrate allocation in soybean

Magnesium (Mg) is an essential element for the growth of both plants and bacteria. Low availability of Mg in agriculture can limit crop productivity and quality. In addition to direct effects on plant growth, limited Mg supply may also impact biological dinitrogen (N2 ) fixation in nodules formed from symbiotic interactions between legumes and rhizobial bacteria. To date, the physiological mechanisms involved in Mg-dependent nodulation remains largely unknown. The objectives of this work were to assess how Mg supply affects nodule growth and development in symbiotic systems, and to test if any observed changes in nodule and soybean are correlated with Mg supply. Here, we found that external Mg supply enhanced nodule growth under nitrogen (N) limited conditions, and subsequently improved N2 fixation and soybean growth. Mg supply altered neither nodule structure nor Mg homeostasis, but remarkably promoted nodule enlargement, resulting in an increase in the number of big nodules. In addition, high Mg supply decreased starch and sucrose accumulation in leaves, and increased their concentrations in roots, which consequently enhanced carbohydrate import into the rhizobia infection zone of nodules. In this study, Mg was shown to promote nodule growth in soybean. This Mg-promoted nodule growth is derived from Mg-facilitated alteration of carbohydrate partitioning and transport into nodules.

Metabolomics Reveals Distinct Carbon and Nitrogen Metabolic Responses to Magnesium Deficiency in Leaves and Roots of Soybean [Glycine max (Linn.) Merr.]

Magnesium (Mg) deficiency, a widespread yet overlooked problem in agriculture, has been reported to retard plant growth and development, through affecting key metabolic pathways. However, the metabolic responses of plant to Mg deficiency is still not fully understood. Here we report a metabolomic study to evaluate the metabolic responses to Mg deficiency in soybean leaves and roots. Hydroponic grown soybean were exposed to Mg starvation for 4 and 8 days, respectively. Metabolic changes in the first mature trifoliolate leaves and roots were quantified by conducting GC-TOF-MS based metabolomic analysis. Principal component analysis (PCA) showed that Mg deficient plants became distinguishable from controls at 4 days after stress (DAS) at metabolic level, and were clearly discriminated at 8 DAS. Mg deficiency could cause large metabolite alterations on carbon and nitrogen metabolism. At 8 DAS, carbon allocation from shoot to root is decreased by Mg deficiency. Remarkably, most amino acids (such as phenylalanine, asparagine, leucine, isoleucine, glycine, glutamine, and serine) showed pronounced accumulation in the leaves, while most organic acids (including pyruvic acid, citric acid, 2-keto-glutaric acid, succinic acid, fumaric acid, and malic acid) were significantly decreased in the roots. Our study shows that the carbon and nitrogen metabolic responses are distinct in leaves and roots under Mg deficiency.

Metals induce transient folding and activation of the twister ribozyme

Twister is a small ribozyme present in almost all kingdoms of life that rapidly self-cleaves in variety of divalent metal ions. We used activity assays, bulk FRET and single-molecule FRET (smFRET) to understand how different metal ions promote folding and self-cleavage of the Oryza sativa Twister ribozyme. Although most ribozymes require additional Mg²⁺ for catalysis, Twister inverts this expectation, requiring 20–30 times less Mg²⁺ to self-cleave than to fold. Transition metals such as Co²⁺, Ni²⁺ and Zn²⁺ activate Twister more efficiently than Mg²⁺ ions. Although Twister is fully active in ≤ 0.5 mM MgCl₂, smFRET experiments showed that the ribozyme visits the folded state infrequently under these conditions. Comparison of folding and self-cleavage rates indicates that most folding events lead to catalysis, which correlates with metal bond strength. Thus, the robust activity of Twister reports on transient metal ion binding under physiological conditions.

Preparation and characterization of laser-melted Mg-Sn-Zn alloys for biomedical application

The rapid degradation rate of Magnesium (Mg) alloy limits its biomedical application even though it possesses outstanding biological performance and biomechanical compatibility. In this study, a combined method of laser rapid melting and alloying Zinc (Zn) was proposed to decrease the degradation rate of Mg-Sn alloy. The microstructure, degradation behaviors and mechanical properties of the laser-melted Mg-5Sn-xZn (x = 0, 2, 4, 6 and 8 wt.%) alloys were investigated. The results indicated that the grain size of the alloys decreased with increasing Zn content, due to the increased number of nucleation particles formed in the process of solidification. Moreover, the laser-melted Mg-Sn alloys possessed finer grains compared with traditional as-cast and as-rolled Mg-Sn alloys. The degradation rate of the alloys decreased with increasing Zn content (0-4 wt.%), which was ascribed to the grain refinement and the formation of Zn(OH)₂ protective layer. However, the degradation rate increased as the Zn content further increased (4-8 wt.%), which was caused by the galvanic corrosion between the Mg matrix and the generated Mg₇Zn₃ phase. Besides, Zn also increased the hardness of the alloys owing to the grain refinement strengthening and solid solution strengthening.

Regulation of Plant Mineral Nutrition: Transport, Sensing and Signaling

Limitation in crop yield productivity significantly contributes to the pressing problem of food security and malnutrition worldwide. [...].