A root-expressed magnesium transporter of the MRS2/MGT gene family in Arabidopsis thaliana allows for growth in low-Mg2+ environments

Mechanism of zinc stress on magnesium deficiency in rice plants (Oryza sativa L.): Insights from magnesium isotopes

Magnesium (Mg) and zinc (Zn) are essential nutrients for plants. Mg deficiency often occurs in rice plants grown in Zn-polluted soil. However, the mechanism for this correlation is unclear. Here, we performed culture experiments on rice plants (Oryza sativa L.) and used Mg isotopes to investigate mechanisms of Zn stress on plant Mg deficiency. Our results show that excess Zn can significantly reduce the uptake of Mg in rice tissues. The root displays positive Δ26Mgplant-nutrient values (δ26Mgplant-δ26Mgnutrient; 1.90 ‰ to 2.06 ‰), which suggests that Mg enters the root cells mainly via Mg-specific transporters rather than non-selective diffusion. The decreased Δ26Mgplant-nutrient values with increasing Zn supply can be explained by the competition between Zn and Mg, both of which combine with same transporters in roots. In contrast, the shoots (stem and leaf) display much lower δ26Mg values than roots, which suggests that the transport of Mg from roots to aerial biomass is mainly via free Mg ions, during which Zn cannot competitively inhibit the movement of Mg. Our study suggests that the Mg deficiency in rice plants can be caused by high Zn-levels in soils and highlights the necessity of soil Zn-remediation in solving Mg deficiency problems in rice plants.

Rare earth elements perturb root architecture and ion homeostasis in Arabidopsis thaliana

Rare earth elements (REEs) are crucial elements for current high-technology and renewable energy advances. In addition to their increasing usage and their low recyclability leading to their release into the environment, REEs are also used as crop fertilizers. However, little is known regarding the cellular and molecular effects of REEs in plants, which is crucial for better risk assessment, crop safety and phytoremediation. Here, we analysed the ionome and transcriptomic response of Arabidopsis thaliana exposed to a light (lanthanum, La) and a heavy (ytterbium, Yb) REE. At the transcriptome level, we observed the contribution of ROS and auxin redistribution to the modified root architecture following REE exposure. We found indications for the perturbation of Fe homeostasis by REEs in both roots and leaves of Arabidopsis suggesting competition between REEs and Fe. Furthermore, we propose putative ways of entry of REEs inside cells through transporters of microelements. Finally, similar to REE accumulating species, organic acid homeostasis (e.g. malate and citrate) appears critical as a tolerance mechanism in response to REEs. By combining ionomics and transcriptomics, we elucidated essential patterns of REE uptake and toxicity response of Arabidopsis and provide new hypotheses for a better evaluation of the impact of REEs on plant homeostasis.

Unraveling the genetics of heat tolerance in chickpea landraces (Cicer arietinum L.) using genome-wide association studies

Chickpea, being an important grain legume crop, is often confronted with the adverse effects of high temperatures at the reproductive stage of crop growth, drastically affecting yield and overall productivity. The current study deals with an extensive evaluation of chickpea genotypes, focusing on the traits associated with yield and their response to heat stress. Notably, we observed significant variations for these traits under both normal and high-temperature conditions, forming a robust basis for genetic research and breeding initiatives. Furthermore, the study revealed that yield-related traits exhibited high heritability, suggesting their potential suitability for marker-assisted selection. We carried out single-nucleotide polymorphism (SNP) genotyping using the genotyping-by-sequencing (GBS) method for a genome-wide association study (GWAS). Overall, 27 marker-trait associations (MTAs) linked to yield-related traits, among which we identified five common MTAs displaying pleiotropic effects after applying a stringent Bonferroni-corrected p-value threshold of <0.05 [-log10(p) > 4.95] using the BLINK (Bayesian-information and linkage-disequilibrium iteratively nested keyway) model. Through an in-depth in silico analysis of these markers against the CDC Frontier v1 reference genome, we discovered that the majority of the SNPs were located at or in proximity to gene-coding regions. We further explored candidate genes situated near these MTAs, shedding light on the molecular mechanisms governing heat stress tolerance and yield enhancement in chickpeas such as indole-3-acetic acid-amido synthetase GH3.1 with GH3 auxin-responsive promoter and pentatricopeptide repeat-containing protein, etc. The harvest index (HI) trait was associated with marker Ca3:37444451 encoding aspartic proteinase ortholog sequence of Oryza sativa subsp. japonica and Medicago truncatula, which is known for contributing to heat stress tolerance. These identified MTAs and associated candidate genes may serve as valuable assets for breeding programs dedicated to tailoring chickpea varieties resilient to heat stress and climate change.

Genetic analysis of the variation for mineral accumulation in the leaves and seeds of natural germplasm of Brassica rapa L. (AA) and the its derived forms extracted from an allotetraploid B.juncea L.(AABB)

Brassica rapa L. (2n = 20; AA) is a vegetable and oilseed crop that is grown all over the world. Its leaves, shoots, and seeds store significant amounts of minerals. We used inductively coupled plasma-optical emission spectroscopy (ICP-OES) to determine the concentrations of eleven minerals in the leaves and seeds of 195 advanced generation inbred lines, of which 92 represented natural (NR) B. rapa and the remaining 103 were derived (DR) from a set of mother genotypes originally extracted from an allotetraploid B. juncea (2n = 36; AABB). The inbred lines differed for the composition of leaf and seed minerals. Leaf concentrations of N, K, Zn, and Se were higher in the DR subpanel as compared to NR subpanel, along with high seed accumulations of K and Se. DArT genotyping and genome wide association mapping led to the identification of SNPs associated with leaf and seed mineral compositions. Chromosomes A03, A05, and A10 harboured the most associated loci. Annotations of the regions adjacent to respective GWAS peaks allowed prediction of genes known for acquisition, transport, and accumulation of minerals and heavy metal detoxification. Transcriptome analysis revealed differential expression patterns of the predicted candidates, with most genes either down-regulated in derived genotypes relative to natural forms or their expression being comparable between the two. General downregulation may be a consequence of extracting B. rapa from allotetraploid B. juncea through genome resection. Some of the identified SNPs may be used as DNA markers for breeding programmes designed to modify the leaf and seed mineral compositions.

The aquaporin MePIP2;7 improves MeMGT9-mediated Mg2 + acquisition in cassava

Aquaporins are important transmembrane water transport proteins which transport water and several neutral molecules. However, how aquaporins are involved in the synergistic transport of Mg2+ and water remains poorly understood. Here, we found that the cassava aquaporin MePIP2;7 was involved in Mg2+ transport through interaction with MeMGT9, a lower affinity magnesium transporter protein. Knockdown of MePIP2;7 in cassava led to magnesium deficiency in basal mature leaves with chlorosis and necrotic spots on their edges and starch over-accumulation. Mg2+ content was significantly decreased in leaves and roots of MePIP2;7-RNA interference (PIP-Ri) plants grown in both field and Mg2+ -free hydroponic solution. Xenopus oocyte injection analysis verified that MePIP2;7 possessed the ability to transport water only and MeMGT9 was responsible for Mg2+ efflux. More importantly, MePIP2;7 improved the transportability of Mg2+ via MeMGT9 as verified using the CM66 mutant complementation assay and Xenopus oocytes expressing system. Yeast two-hybrid, bimolecular fluorescence complementation, co-localization, and co-immunoprecipitation assays demonstrated the direct protein-protein interaction between MePIP2;7 and MeMGT9 in vivo. Mg2+ flux was significantly elevated in MePIP2;7-overexpressing lines in hydroponic solution through non-invasive micro-test technique analysis. Under Mg2+ -free condition, the retarded growth of PIP-Ri transgenic plants could be recovered with Mg2+ supplementation. Taken together, our results demonstrated the synergistic effect of the MePIP2;7 and MeMGT9 interaction in regulating water and Mg2+ absorption and transport in cassava.

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.

Magnesium transporter CsMGT10 of tea plants plays a key role in chlorosis leaf vein greening

Magnesium (Mg2+), as the central atom of chlorophyll, is the most abundant divalent cation for plant growth and development in living cells. MRS2/MGT magnesium transporters play important roles in coping with magnesium stress, chloroplast development and photosynthesis. However, the molecular mechanism of MGT influencing tea plant leaf vein color remains unknown. Here, we demonstrate that CsMGT10 may be a potential transporter influencing leaf vein color. CsMGT10 belongs to Clade A member of MRS2/MGT family. CsMGT10 has the highest expression level in leaves of tea plants. And it is mainly expressed in aboveground parts, especially in vascular bundles. Moreover, CsMGT10 localizes to the chloroplast envelope of tea plants with a high affinity to Mg2+. And the GMN motif is required for its magnesium transport function. Ectopic expression of CsMGT10 in Arabidopsis leaf variegation mutant var5-1 can restore green color of chlorosis leaf veins, and the contents of chlorophyll and carotenoid change significantly, proving its essential role in leaf vein greening. Furthermore, the chlorophyll and carotenoid of tea leaves treated with CsMGT10 antisense oligonucleotides also decrease significantly. Our findings indicate that CsMGT10 mainly acts as Mg2+ transporter in chloroplast envelope of leaf veins, which may play a key role in leaf vein greening of tea plants.

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.

Mutations in RZF1, a zinc-finger protein, reduce magnesium uptake in roots and translocation to shoots in rice

Magnesium (Mg) homeostasis is critical for maintaining many biological processes, but little information is available to comprehend the molecular mechanisms regulating Mg concentration in rice (Oryza sativa). To make up for the lack of information, we aimed to identify mutants defective in Mg homeostasis through a forward genetic approach. As a result of the screening of 2,825 M2 seedlings mutated by ion-beam irradiation, we found a rice mutant that showed reduced Mg content in leaves and slightly increased Mg content in roots. Radiotracer 28Mg experiments showed that this mutant, named low-magnesium content 1 (LMGC1), has decreased Mg2+ influx in the root and Mg2+ translocation from root to shoot. Consequently, LMGC1 is sensitive to the low Mg condition and prone to develop chlorosis in the young mature leaf. The MutMap method identified a 7.4-kbp deletion in the LMGC1 genome leading to a loss of two genes. Genome editing using CRISPR-Cas9 further revealed that one of the two lost genes, a gene belonging to the RanBP2-type zinc-finger family that we named RanBP2-TYPE ZINC FINGER1 (OsRZF1), was the causal gene of the low Mg phenotype. OsRZF1 is a nuclear protein and may have a fundamental role in maintaining Mg homeostasis in rice plants.

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.

Mapping QTLs and gene validation studies for Mg2+ uptake and translocation using a MAGIC population in rice

Magnesium (Mg) is an essential element for plant growth and development. Rice is an important food crop in the world, but there are few studies on the uptake and translocation of Mg²⁺ in rice. We used a multi-parent advanced generation inter-cross (MAGIC) population constructed using four parental lines and genotyped by a 55 K rice SNP array for association analysis to locate QTLs related to Mg²⁺ uptake and translocation in rice at the seedling stage. Four QTLs (qRMg1, qRMg2, qRMg7 and qRMg8) were detected for the root Mg²⁺ concentration, which explained 11.45-13.08% of the phenotypic variation. The Mg²⁺ transporter gene, OsMGT1, was within the region of qRMg1. Three QTLs (qSMg3, qSMg7 and qSMg10) were detected for the shoot Mg²⁺ concentration, which explained 4.30-5.46% of the phenotypic variation. Two QTLs (qTrMg3 and qTrMg8) were found to affect the translocation of Mg²⁺ from the roots to the shoots, and explained 10.91% and 9.63% of phenotypic variation. qSMg3 and qTrMg3 might be the same, since they are very close to each other on chromosome 3. Analysis of candidate genes in the region of qSMg3 and qTrMg3 through qRT-PCR, complementation assay in the yeast Mg²⁺ transport-defective mutant CM66, and sequence analysis of the parental lines suggested that LOC_Os03g04360 may play important roles in Mg²⁺ uptake, translocation and accumulation in rice. Overexpression of LOC_Os03g04360 can significantly increase the Mg²⁺ concentration in rice seedlings, especially under the condition of low Mg²⁺ supply.

Genome-wide identification of Mg2+ transporters and functional characteristics of DlMGT1 in Dimocarpus longan

Longan (Dimocarpus Longan) is one of the most important fruit crops in Southern China. Lack of available Mg in acidic soil conditions is a limitation to further increasing longan yield. Magnesium transporter (MGT/MRS2) mediates the uptake, transport, and redistribution of Mg2+ in higher plants. To understand the role of MGTs family members in longan Mg deficiency. We identified and analyzed the protein characteristics, phylogeny, expression changes, subcellular localization, and transcriptional regulation of DlMGTs members. The results showed that, twelve DlMGTs are localized in the cell membrane, chloroplast, and nucleus. The evolutionary differences in MGTs between herbaceous and woody species in different plants. The DlMGTs promoters contained many cis-acting elements and transcription factor binding sites related to the hormone, environmental, and stress response. Subcellular localization assays showed that DlMGT1 localizes in the cell membrane of Arabidopsis protoplasts. The candidate transcription factor DlGATA16, which may regulate the expression of DlMGT1, was localized in the nucleus of tobacco leaves. Dual luciferase analysis demonstrated that DlGATA16 is a potential factor regulating the transcriptional activity of DlMGT1. In this study, we identified and analyzed DlMGTs on a genome-wide scale and the subcellular localization and interaction of DlMGT1 and DlGATA16, which has important implications for further functional analysis studies of MGTs and the use of MGT for longan genetic improvement.

Two magnesium transporters in the chloroplast inner envelope essential for thylakoid biogenesis in Arabidopsis

Magnesium (Mg²⁺ ) serves as a cofactor for a number of photosynthetic enzymes in the chloroplast, and is the central atom of the Chl molecule. However, little is known about the molecular mechanism of Mg²⁺ transport across the chloroplast envelope. Here, we report the functional characterization of two transport proteins in Arabidopsis: Magnesium Release 8 (MGR8) and MGR9, of the ACDP/CNNM family, which is evolutionarily conserved across all lineages of living organisms. Both MGR8 and MGR9 genes were expressed ubiquitously, and their encoded proteins were localized in the inner envelope of chloroplasts. Mutations of MGR8 and MGR9 together, but neither of them alone, resulted in albino ovules and chlorotic seedlings. Further analysis revealed severe defects in thylakoid biogenesis and assembly of photosynthetic complexes in the double mutant. Both MGR8 and MGR9 functionally complemented the growth of the Salmonella typhimurium mutant strain MM281, which lacks Mg²⁺ uptake capacity. The embryonic and early seedling defects of the mgr8/mgr9 double mutant were rescued by the expression of MGR9 under the embryo-specific ABI3 promoter. The partially rescued mutant plants were hypersensitive to Mg²⁺ deficient conditions and contained less Mg²⁺ in their chloroplasts than wild-type plants. Taken together, we conclude that MGR8 and MGR9 serve as Mg²⁺ transporters and are responsible for chloroplast Mg²⁺ uptake.

Two transporters mobilize magnesium from vacuolar stores to enable plant acclimation to magnesium deficiency

Magnesium (Mg) is an essential metal for chlorophyll biosynthesis and other metabolic processes in plant cells. Mg is largely stored in the vacuole of various cell types and remobilized to meet cytoplasmic demand. However, the transport proteins responsible for mobilizing vacuolar Mg2+ remain unknown. Here, we identified two Arabidopsis (Arabidopsis thaliana) Mg2+ transporters (MAGNESIUM TRANSPORTER 1 and 2; MGT1 and MGT2) that facilitate Mg2+ mobilization from the vacuole, especially when external Mg supply is limited. In addition to a high degree of sequence similarity, MGT1 and MGT2 exhibited overlapping expression patterns in Arabidopsis tissues, implying functional redundancy. Indeed, the mgt1 mgt2 double mutant, but not mgt1 and mgt2 single mutants, showed exaggerated growth defects as compared to the wild type under low-Mg conditions, in accord with higher expression levels of Mg-starvation gene markers in the double mutant. However, overall Mg level was also higher in mgt1 mgt2, suggesting a defect in Mg2+ remobilization in response to Mg deficiency. Consistently, MGT1 and MGT2 localized to the tonoplast and rescued the yeast (Saccharomyces cerevisiae) mnr2Δ (manganese resistance 2) mutant strain lacking the vacuolar Mg2+ efflux transporter. In addition, disruption of MGT1 and MGT2 suppressed high-Mg sensitivity of calcineurin B-like 2 and 3 (cbl2 cbl3), a mutant defective in vacuolar Mg2+ sequestration, suggesting that vacuolar Mg2+ influx and efflux processes are antagonistic in a physiological context. We further crossed mgt1 mgt2 with mgt6, which lacks a plasma membrane MGT member involved in Mg2+ uptake, and found that the triple mutant was more sensitive to low-Mg conditions than either mgt1 mgt2 or mgt6. Hence, Mg2+ uptake (via MGT6) and vacuolar remobilization (through MGT1 and MGT2) work synergistically to achieve Mg2+ homeostasis in plants, especially under low-Mg supply in the environment.

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.

Structural and functional comparison of magnesium transporters throughout evolution

Magnesium (Mg²⁺) is the most prevalent divalent intracellular cation. As co-factor in many enzymatic reactions, Mg²⁺ is essential for protein synthesis, energy production, and DNA stability. Disturbances in intracellular Mg²⁺ concentrations, therefore, unequivocally result in delayed cell growth and metabolic defects. To maintain physiological Mg²⁺ levels, all organisms rely on balanced Mg²⁺ influx and efflux via Mg²⁺ channels and transporters. This review compares the structure and the function of prokaryotic Mg²⁺ transporters and their eukaryotic counterparts. In prokaryotes, cellular Mg²⁺ homeostasis is orchestrated via the CorA, MgtA/B, MgtE, and CorB/C Mg²⁺ transporters. For CorA, MgtE, and CorB/C, the motifs that form the selectivity pore are conserved during evolution. These findings suggest that CNNM proteins, the vertebrate orthologues of CorB/C, also have Mg²⁺ transport capacity. Whereas CorA and CorB/C proteins share the gross quaternary structure and functional properties with their respective orthologues, the MgtE channel only shares the selectivity pore with SLC41 Na⁺/Mg²⁺ transporters. In eukaryotes, TRPM6 and TRPM7 Mg²⁺ channels provide an additional Mg²⁺ transport mechanism, consisting of a fusion of channel with a kinase. The unique features these TRP channels allow the integration of hormonal, cellular, and transcriptional regulatory pathways that determine their Mg²⁺ transport capacity. Our review demonstrates that understanding the structure and function of prokaryotic magnesiotropic proteins aids in our basic understanding of Mg²⁺ transport.

Four plasma membrane-localized MGR transporters mediate xylem Mg2+ loading for root-to-shoot Mg2+ translocation in Arabidopsis

Magnesium (Mg²⁺), an essential structural component of chlorophyll, is absorbed from the soil by roots and transported to shoots to support photosynthesis in plants. However, the molecular mechanisms underlying root-to-shoot Mg²⁺ translocation remain largely unknown. We describe here the identification of four plasma membrane (PM)-localized transporters, named Mg²⁺ release transporters (MGRs), that are critical for root-to-shoot Mg transport in Arabidopsis. Functional complementation assays in a Mg²⁺-uptake-deficient bacterial strain confirmed that these MGRs conduct Mg²⁺ transport. PM-localized MGRs (MGR4, MGR5, MGR6, and MGR7) were expressed primarily in root stellar cells and participated in the xylem loading step of the long-distance Mg²⁺ transport process. In particular, MGR4 and MGR6 played a major role in shoot Mg homeostasis, as their loss-of-function mutants were hypersensitive to low Mg²⁺ but tolerant to high Mg²⁺ conditions. Reciprocal grafting analysis further demonstrated that MGR4 functions in the root to determine shoot Mg²⁺ accumulation and physiological phenotypes caused by both low- and high-Mg²⁺ stress. Taken together, our study has identified the long-sought transporters responsible for root-to-shoot Mg²⁺ translocation in plants.

Genome-wide identification and expression analysis of magnesium transporter gene family in grape (Vitis vinifera)

BACKGROUND

Magnesium ion is one of the essential mineral elements for plant growth and development, which participates in a variety of physiological and biochemical processes. Since there is no report on the research of magnesium ion transporter in grape, the study of the structure and function of magnesium ion transporters (MGT) is helpful to understand the dynamic balance mechanism of intracellular magnesium ions and their inter- or intra-cellular activities.

RESULT

In this study, we identified the members of MGT protein family in grape and performed the phylogenetic and expression analysis. We have identified nine VvMGT genes in grape genome, which are distributed on eight different chromosomes. Phylogenetic analysis showed that MGT family members of grapes were divided into five subfamilies and had obvious homology with Arabidopsis, maize, and pear. Based on transcriptome data from the web databases, we analyzed the expression patterns of VvMGTs at different development stages and in response to abiotic stresses including waterlogging, drought, salinity, and copper. Using qRT-PCR method, we tested the expression of grape VvMGTs under magnesium and aluminum treatments and found significant changes in VvMGTs expression. In addition, four of the MGT proteins in grape were located in the nucleus.

CONCLUSION

Overall, in this study we investigated the structural characteristics, evolution pattern, and expression analysis of VvMGTs in depth, which laid the foundation for further revealing the function of VvMGT genes in grape.

Physiological Essence of Magnesium in Plants and Its Widespread Deficiency in the Farming System of China

Magnesium (Mg) is an essential nutrient for a wide array of fundamental physiological and biochemical processes in plants. It largely involves chlorophyll synthesis, production, transportation, and utilization of photoassimilates, enzyme activation, and protein synthesis. As a multifaceted result of the introduction of high-yielding fertilizer-responsive cultivars, intensive cropping without replenishment of Mg, soil acidification, and exchangeable Mg (Ex-Mg) leaching, Mg has become a limiting nutrient for optimum crop production. However, little literature is available to better understand distinct responses of plants to Mg deficiency, the geographical distribution of soil Ex-Mg, and the degree of Mg deficiency. Here, we summarize the current state of knowledge of key plant responses to Mg availability and, as far as possible, highlight spatial Mg distribution and the magnitude of Mg deficiency in different cultivated regions of the world with a special focus on China. In particular, ~55% of arable lands in China are revealed Mg-deficient (< 120 mg kg-1 soil Ex-Mg), and Mg deficiency literally becomes increasingly severe from northern (227-488 mg kg-1) to southern (32-89 mg kg-1) China. Mg deficiency primarily traced back to higher depletion of soil Ex-Mg by fruits, vegetables, sugarcane, tubers, tea, and tobacco cultivated in tropical and subtropical climate zones. Further, each unit decline in soil pH from neutral reduced ~2-fold soil Ex-Mg. This article underscores the physiological importance of Mg, potential risks associated with Mg deficiency, and accordingly, to optimize fertilization strategies for higher crop productivity and better quality.

An Arabidopsis vasculature distributed metal tolerance protein facilitates xylem magnesium diffusion to shoots under high-magnesium environments

Magnesium (Mg²⁺ ) is an essential metal for plant growth; however, its over-accumulation in cells can be cytotoxic. The metal tolerance protein family (MTP) belongs to an ubiquitous family of cation diffusion facilitator (CDF) proteins that export divalent metal cations for metal homeostasis and tolerance in all organisms. We describe here the identification of MTP10 to be critical for xylem Mg homeostasis in Arabidopsis under high Mg²⁺ conditions. The Arabidopsis plant contains 12 MTP genes, and only knockout of MTP10 decreased the tolerance of high-Mg stress. The functional complementation assays in a Mg²⁺ -uptake-deficient bacterial strain MM281 confirmed that MTP10 conducted Mg²⁺ transport. MTP10 is localized to the plasma membrane of parenchyma cells around the xylem. Reciprocal grafting analysis further demonstrated that MTP10 functions in the shoot to determine the shoot growth phenotypes under high Mg²⁺ conditions. Moreover, compared to the wild type, the mtp10 mutant accumulated more Mg²⁺ in xylem sap under high-Mg stress. This study reveals that MTP10 facilitates Mg²⁺ diffusion from the xylem to shoots and thus determines Mg homeostasis in shoot vascular tissues during high-Mg stress.

Differences in mineral accumulation and gene expression profiles between two metal hyperaccumulators, Noccaea japonica and Noccaea caerulescens ecotype Ganges, under excess nickel condition

Here we compare mineral accumulation and global gene expression patterns between two metal hyperaccumulator plants - Noccaea japonica, originating from Ni-rich serpentine soils, and Noccaea caerulescens (ecotype Ganges), originating from Zn/Pb-mine soils - under excess Ni conditions. Significant differences in the accumulation of K, P, Mg, B, and Mo were explained by the expression levels of specific transporters for each mineral. We previously showed that total Ni accumulation in the whole plant is higher in N. caerulescens than in N. japonica. Here we found a similar tendency for Fe under excess Ni; however, the expression of iron-regulated transporter 1 (IRT1), which encodes the primary Fe uptake transporter and causes excess Ni uptake in Arabidopsis thaliana, was higher in N. japonica. NjIRT1 has a point mutation at Asp100, which is essential for Fe transport, and so might lack its Fe and possibly Ni transport function. Noccaea japonica might have lost its IRT1 function, which would prevent excess Ni uptake via IRT1 in Ni-rich soils, and come to rely on other transporters.

The transcription factor gene RDD4 contributes to the control of nutrient ion accumulation in rice

In this study, we investigated the expression and functions of the transcription factor gene RDD4 (rice Dof daily fluctuations 4), which has sequence similarity to RDD1 that controls nutrient ion accumulation in rice. RDD4 protein was highly accumulated in leaf sheaths and localized to vascular bundles. RDD4-overexpressing plants (RDD4-OX) improved the accumulation of various nutrient ions, irrespective of nutrient concentration in a hydroponic solution. K⁺ and Cl^- deficiencies induced the accumulation of other cations and anions, respectively. Interestingly, in RDD4-OX plants K⁺ and Cl^- deficiencies increased PO₄ ^3- and Mg²⁺ contents, respectively, despite opposite electric charges. Furthermore, PO₄ ^3- deficiency induced NO₃ ^- and Mg²⁺ accumulation in RDD4-OX plants. These data show that RDD4 is associated with the control of nutrient ion contents within plants. Also, photosynthetic CO₂ assimilation in RDD4-OX plants was higher than in wild-type (WT) plants, although the sizes of shoots and panicles decreased in RDD4-OX plants. Subsequent microarray analysis indicated that OsFWL7, similar to maize CNR1 that negatively regulates plant size, showed the most significant difference in its expression levels between WT and RDD4-OX plants. Based on these results, it is hypothesized that a prominent increase in the OsFWL7 expression reduces plant size in RDD4-OX plants.

Root hairs: the villi of plants

Strikingly, evolution shaped similar tubular structures at the µm to mm scale in roots of sessile plants and in small intestines of mobile mammals to ensure an efficient transfer of essential nutrients from 'dead matter' into biota. These structures, named root hairs (RHs) in plants and villi in mammals, numerously stretch into the environment, and extremely enlarge root and intestine surfaces. They are believed to forage for nutrients, and mediate their uptake. While the conceptional understanding of plant RH function in hydromineral nutrition seems clear, experimental evidence presented in textbooks is restricted to a very limited number of reference-nutrients. Here, we make an element-by-element journey through the periodic table and link individual nutrient availabilities to the development, structure/shape and function of RHs. Based on recent developments in molecular biology and the identification of mutants differing in number, length or other shape-related characteristics of RHs in various plant species, we present comprehensive advances in (i) the physiological role of RHs for the uptake of specific nutrients, (ii) the developmental and morphological responses of RHs to element availability and (iii) RH-localized nutrient transport proteins. Our update identifies crucial roles of RHs for hydromineral nutrition, mostly under nutrient and/or water limiting conditions, and highlights the influence of certain mineral availabilities on early stages of RH development, suggesting that nutritional stimuli, as deficiencies in P, Mn or B, can even dominate over intrinsic developmental programs underlying RH differentiation.

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.

Current Understandings on Magnesium Deficiency and Future Outlooks for Sustainable Agriculture

The productivity of agricultural produce is fairly dependent on the availability of nutrients and efficient use. Magnesium (Mg2+) is an essential macronutrient of living cells and is the second most prevalent free divalent cation in plants. Mg2+ plays a role in several physiological processes that support plant growth and development. However, it has been largely forgotten in fertilization management strategies to increase crop production, which leads to severe reductions in plant growth and yield. In this review, we discuss how the Mg2+ shortage induces several responses in plants at different levels: morphological, physiological, biochemical and molecular. Additionally, the Mg2+ uptake and transport mechanisms in different cellular organelles and the role of Mg2+ transporters in regulating Mg2+ homeostasis are also discussed. Overall, in this review, we critically summarize the available information about the responses of Mg deficiency on plant growth and development, which would facilitate plant scientists to create Mg2+-deficiency-resilient crops through agronomic and genetic biofortification.

Slowing Development Facilitates Arabidopsis mgt Mutants to Accumulate Enough Magnesium for Pollen Formation and Fertility Restoration

Magnesium (Mg) is an abundant and important cation in cells. Plants rely on Mg transporters to take up Mg from the soil, and then Mg is transported to anthers and other organs. Here, we showed that MGT6+/- plants display reduced fertility, while mgt6 plants are fertile. MGT6 is expressed in the anther at the early stages. Pollen mitosis and intine formation are impaired in aborted pollen grains (PGs) of MGT6+/- plants, which is similar to the defective pollen observed in mgt5 and mgt9 mutants. These results suggest that Mg deficiency leads to pollen abortion in MGT6+/- plants. Our data showed that mgt6 organs including buds develop significantly slower and mgt6 stamens accumulate a higher level of Mg, compared with wild-type (WT) and MGT6+/- plants. These results indicate that slower bud development allows mgt6 to accumulate sufficient amounts of Mg in the pollen, explaining why mgt6 is fertile. Furthermore, we found that mgt6 can restore fertility of mgt5, which has been reported to be male sterile due to defects in Mg transport from the tapetum to microspores and that an additional Mg supply can restore its fertility. Interestingly, mgt5 fertility is recovered when grown under short photoperiod conditions, which is a well-known factor regulating plant fertility. Taken together, these results demonstrate that slow development is a general mechanism to restore mgts fertility, which allows other redundant magnesium transporter (MGT) members to transport sufficient Mg for pollen formation.

Functional analysis of whether the glycine residue of the GMN motif of the Arabidopsis MRS2/MGT/CorA-type Mg2+ channel protein AtMRS2-11 is critical for Mg2+ transport activity

Magnesium (Mg²⁺) plays a critical role in many physiological processes. The AtMRS2/MGT family, which contains nine Arabidopsis genes (and two pseudogenes), belongs to a eukaryotic subset of the CorA superfamily of divalent cation transporters. AtMRS2-11/MGT10 possesses the signature GlyMetAsn sequence (the GMN motif) conserved in the CorA superfamily; however, little is known about the role of the GMN motif in AtMRS2. Direct measurement using the fluorescent dye mag-fura-2 revealed that reconstituted AtMRS2-11 mediated rapid Mg²⁺ uptake into proteoliposomes at extraliposomal Mg²⁺ concentrations of 10 and 20 mM. Mutations in the GMN motif, G417 to A, S or V, did not show a significant change in Mg²⁺ uptake relative to the wild-type protein. The G417W mutant exhibited a significant increase in Mg²⁺ uptake. The functional complementation assay in Escherichia coli strain TM2 showed that E. coli cells expressing AtMRS2-11 with mutations in G of the GMN motif did not grow in LB medium without Mg²⁺ supplementation, while growth was observed in LB medium supplemented with 0.5 mM Mg²⁺; no difference was observed between the growth of TM2 cells expressing the AtMRS2-11 G417W mutant and that of cells expressing wild-type AtMRS2-11. These results suggested that the Mg²⁺ transport activity of the AtMRS2-11 GMN-motif mutants was low at low physiological Mg²⁺ concentrations; thus, the Gly residue is critical for Mg²⁺ transport, and the Mg²⁺ transport activity of the GMN-motif mutants was increased at high Mg²⁺ concentrations.

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.

The Genome of Microthlaspi erraticum (Brassicaceae) Provides Insights Into the Adaptation to Highly Calcareous Soils

Microthlaspi erraticum is widely distributed in temperate Eurasia, but restricted to Ca²⁺-rich habitats, predominantly on white Jurassic limestone, which is made up by calcium carbonate, with little other minerals. Thus, naturally occurring Microthlaspi erraticum individuals are confronted with a high concentration of Ca²⁺ ions while Mg²⁺ ion concentration is relatively low. As there is a competitive uptake between these two ions, adaptation to the soil condition can be expected. In this study, it was the aim to explore the genomic consequences of this adaptation by sequencing and analysing the genome of Microthlaspi erraticum. Its genome size is comparable with other diploid Brassicaceae, while more genes were predicted. Two Mg²⁺ transporters known to be expressed in roots were duplicated and one showed a significant degree of positive selection. It is speculated that this evolved due to the pressure to take up Mg²⁺ ions efficiently in the presence of an overwhelming amount of Ca²⁺ ions. Future studies on plants specialized on similar soils and affinity tests of the transporters are needed to provide unequivocal evidence for this hypothesis. If verified, the transporters found in this study might be useful for breeding Brassicaceae crops for higher yield on Ca²⁺-rich and Mg²⁺ -poor soils.

Short-Term Magnesium Deficiency Triggers Nutrient Retranslocation in Arabidopsis thaliana

Magnesium (Mg) is essential for many biological processes in plant cells, and its deficiency causes yield reduction in crop systems. Low Mg status reportedly affects photosynthesis, sucrose partitioning and biomass allocation. However, earlier physiological responses to Mg deficiency are scarcely described. Here, we report that Mg deficiency in Arabidopsis thaliana first modified the mineral profile in mature leaves within 1 or 2 days, then affected sucrose partitioning after 4 days, and net photosynthesis and biomass production after 6 days. The short-term Mg deficiency reduced the contents of phosphorus (P), potassium, manganese, zinc and molybdenum in mature but not in expanding (young) leaves. While P content decreased in mature leaves, P transport from roots to mature leaves was not affected, indicating that Mg deficiency triggered retranslocation of the mineral nutrients from mature leaves. A global transcriptome analysis revealed that Mg deficiency triggered the expression of genes involved in defence response in young leaves.

Identification of the magnesium transport (MGT) family in Poncirus trifoliata and functional characterization of PtrMGT5 in magnesium deficiency stress

At least eight MGT genes were identified in citrus and PtrMGT5 plays important role in maintaining Mg homeostasis in citrus by getting involved in the Mg absorption and transport. Magnesium (Mg) is an essential macronutrient for plant growth and development, and the magnesium transporter (MGT) genes participate in mediate Mg²⁺ uptake, translocation and sequestration into cellular storage compartments. Although several MGT genes have been characterized in various plant species, a comprehensive analysis of the MGT gene family in citrus is still uncharacterized. In this study, eight PtrMGT genes were identified through genome-wide analyses. Phylogenetic analyses revealed that PtrMGT genes were classified into five distinct subfamilies. A quantitative RT-PCR analysis showed that eight PtrMGT genes were expressed in all of the detected tissues and they mainly expressed in the vegetative organs. Expression analyses revealed the PtrMGT genes responded to various Mg deficiency stresses, including absolute Mg deficiency and antagonistic Mg deficiency which caused by low pH or Al toxicity. PtrMGT5, which localizes to the plasma membrane and was transcriptionally active, was functionally characterized. PtrMGT5 overexpression considerably enhanced absolute Mg deficiency and antagonistic Mg deficiency tolerance in transgenic Arabidopsis plants, which was accompanied by increased fresh weight and Mg content, whereas opposite changes were observed when PtrMGT5 homolog in Valencia Orange callus was knocked down. Taken together, PtrMGT5 plays important role in maintaining Mg homeostasis in citrus by getting involved in the Mg absorption and transport.

Identification of a Dominant Chlorosis Phenotype Through a Forward Screen of the Triticum turgidum cv. Kronos TILLING Population

Durum wheat (Triticum turgidum) derives from a hybridization event approximately 400,000 years ago which led to the creation of an allotetraploid genome. The evolutionary recent origin of durum wheat means that its genome has not yet been fully diploidised. As a result, many of the genes present in the durum genome act in a redundant fashion, where loss-of-function mutations must be present in both gene copies to observe a phenotypic effect. Here, we use a novel set of induced variation within the cv. Kronos TILLING population to identify a locus controlling a dominant, environmentally dependent chlorosis phenotype. We carried out a forward screen of the sequenced cv. Kronos TILLING lines for senescence phenotypes and identified a line with a dominant early senescence and chlorosis phenotype. Mutant plants contained less chlorophyll throughout their development and displayed premature flag leaf senescence. A segregating population was classified into discrete phenotypic groups and subjected to bulked-segregant analysis using exome capture followed by next-generation sequencing. This allowed the identification of a single region on chromosome 3A, Yellow Early Senescence 1 (YES-1), which was associated with the mutant phenotype. While this phenotype was consistent across 4 years of field trials in the United Kingdom, the mutant phenotype was not observed when grown in Davis, CA (United States). To obtain further SNPs for fine-mapping, we isolated chromosome 3A using flow sorting and sequenced the entire chromosome. By mapping these reads against both the cv. Chinese Spring reference sequence and the cv. Kronos assembly, we could identify high-quality, novel EMS-induced SNPs in non-coding regions within YES-1 that were previously missed in the exome capture data. This allowed us to fine-map YES-1 to 4.3 Mb, containing 59 genes. Our study shows that populations containing induced variation can be sources of novel dominant variation in polyploid crop species, highlighting their importance in future genetic screens. We also demonstrate the value of using cultivar-specific genome assemblies alongside the gold-standard reference genomes particularly when working with non-coding regions of the genome. Further fine-mapping of the YES-1 locus will be pursued to identify the causal SNP underpinning this dominant, environmentally dependent phenotype.

The different tolerance to magnesium deficiency of two grapevine rootstocks relies on the ability to cope with oxidative stress

BACKGROUND

Magnesium (Mg) deficiency causes physiological and molecular responses, already dissected in several plant species. The study of these responses among genotypes showing a different tolerance to the Mg shortage can allow identifying the mechanisms underlying the resistance to this nutritional disorder. To this aim, we compared the physiological and molecular responses (e.g. changes in root metabolome and transcriptome) of two grapevine rootstocks exhibiting, in field, different behaviors with respect to Mg shortage (1103P, tolerant and SO4 susceptible).

RESULTS

The two grapevine rootstocks confirmed, in a controlled growing system, their behavior in relation to the tolerance to Mg deficiency. Differences in metabolite and transcriptional profiles between the roots of the two genotypes were mainly linked to antioxidative compounds and the cell wall constituents. In addition, differences in secondary metabolism, in term of both metabolites (e.g. alkaloids, terpenoids and phenylpropanoids) and transcripts, assessed between 1103P and SO4 suggest a different behavior in relation to stress responses particularly at early stages of Mg deficiency.

CONCLUSIONS

Our results suggested that the higher ability of 1103P to tolerate Mg shortage is mainly linked to its capability of coping, faster and more efficiently, with the oxidative stress condition caused by the nutritional disorder.

Molecular identification of the magnesium transport gene family in Brassica napus

Magnesium (Mg²⁺) is an essential element for plant growth. Its transport and homeostasis in plants is mainly maintained by the MRS2/MGT of Mg²⁺ transporters. Little is known about the MRS2/MGT gene family in Brassica napus L. (B. napus), one of the most important oil grains. In our present study, we identified 36 putative MRS2/MGT genes (BnMGTs) from B. napus and investigated their phylogeny, expression pattern and function. These BnMGT genes were sorted into five distinguished groups by the phylogenetic analysis, and they were clearly homologous with the MRS2/MGT genes in Arabidopsis and rice. Complementation assays using the Salmonella typhimurium mutant MM281 demonstrated that the BnMGT genes were capable of mediating Mg²⁺ uptake and transport, with varied affinities to Mg²⁺. The expression pattern analysis showed that the expression of BnMGTs were tissue-specific and varied in different tissues. This work provides the molecular basis to discover the function of BnMGT gene family in plant growth and development.

OsMGT1 Confers Resistance to Magnesium Deficiency By Enhancing the Import of Mg in Rice

Magnesium (Mg) is an essential nutrient element for plant growth and plays an important role in numerous physiological and biochemical processes. Mg deficiency inhibits plant growth and has become a growing problem for crop productions in agriculture. However, the molecular mechanisms for the resistance to Mg deficiency in plants were not well understood. In this study, we identified a Mg transporter gene OsMGT1 that confers resistance to Mg deficiency in rice (Oryza sativa). The expression of OsMGT1 was highly induced by Mg deficiency in shoots. Investigation of tissue expression patterns revealed that OsMGT1 was mainly expressed in the phloem region; however, Mg deficiency remarkably enhanced its expression in xylem parenchyma and mesophyll cells in shoots. Knockout of OsMGT1 resulted in a significant reduction in Mg content and biomass when grown at Mg-limited conditions. Furthermore, the sensitivity to low-Mg in mutants was intensified by excessive calcium supply. In addition, overexpression of OsMGT1 increased Mg content and biomass under low-Mg supply. In conclusion, our results indicate that OsMGT1 plays an important role in rice Mg import and is required for the resistance to Mg deficiency, which can be utilized for molecular breeding of low-Mg tolerant plants.

Vacuolar Proton Pyrophosphatase Is Required for High Magnesium Tolerance in Arabidopsis

Magnesium (Mg²⁺) is an essential nutrient in all organisms. However, high levels of Mg²⁺ in the environment are toxic to plants. In this study, we identified the vacuolar-type H⁺-pyrophosphatase, AVP1, as a critical enzyme for optimal plant growth under high-Mg conditions. The Arabidopsis avp1 mutants displayed severe growth retardation, as compared to the wild-type plants upon excessive Mg²⁺. Unexpectedly, the avp1 mutant plants retained similar Mg content to wild-type plants under either normal or high Mg conditions, suggesting that AVP1 may not directly contribute to Mg²⁺ homeostasis in plant cells. Further analyses confirmed that the avp1 mutant plants contained a higher pyrophosphate (PPi) content than wild type, coupled with impaired vacuolar H⁺-pyrophosphatase activity. Interestingly, expression of the Saccharomyces cerevisiae cytosolic inorganic pyrophosphatase1 gene IPP1, which facilitates PPi hydrolysis but not proton translocation into vacuole, rescued the growth defects of avp1 mutants under high-Mg conditions. These results provide evidence that high-Mg sensitivity in avp1 mutants possibly resulted from elevated level of cytosolic PPi. Moreover, genetic analysis indicated that mutation of AVP1 was additive to the defects in mgt6 and cbl2 cbl3 mutants that are previously known to be impaired in Mg²⁺ homeostasis. Taken together, our results suggest AVP1 is required for cellular PPi homeostasis that in turn contributes to high-Mg tolerance in plant cells.

Reduction of geomagnetic field (GMF) to near null magnetic field (NNMF) affects Arabidopsis thaliana root mineral nutrition

The Earth magnetic field (or geomagnetic field, GMF) is a natural component of our planet and variations of the GMF are perceived by plants with a still uncharacterized magnetoreceptor. The purpose of this work was to assess the effect of near null magnetic field (NNMF, ∼40 nT) on Arabidopsis thaliana Col0 root ion modulation. A time-course (from 10 min to 96 h) exposure of Arabidopsis to NNMF was compared to GMF and the content of some cations (NH₄⁺, K⁺, Ca²⁺ and Mg²⁺) and anions (Cl^-, SO₄⁼, NO₃^- and PO₄⁼) was evaluated by capillary electrophoresis. The expression of several cation and anion channel- and transporter-related genes was assessed by gene microarray. A few minutes after exposure to NNMF, Arabidopsis roots responded with a significant change in the content and gene expression of all nutrient ions under study, indicating the presence of a plant magnetoreceptor that responds immediately to MF variations by modulating channels, transporters and genes involved in mineral nutrition. The response of Arabidopsis to reduced MF was a general reduction of plant ion uptake and transport. Our data suggest the importance to understand the nature and function of the plant magnetoreceptor for future space programs involving plant growth in environments with a reduced MF.

Heterologous Expression of the Transcription Factor EsNAC1 in Arabidopsis Enhances Abiotic Stress Resistance and Retards Growth by Regulating the Expression of Different Target Genes

Heterologous expression of a transcription factor (TF) gene in a related species is a useful method for crop breeding and the identification of gene function. The differences in phenotype and target gene expression between HE lines (with the heterologous expression of an ortholog) and OX lines (with an overexpressed native gene) must be understood. EsNAC1, encoding a NAC protein and the ortholog of RD26 in Arabidopsis, was cloned from Eutrema salsugineum and introduced into Arabidopsis. The heterologous expression of EsNAC1 retarded the vegetative growth of Arabidopsis, and the transgenic plants (HE lines) showed much greater resistance to salt and oxidative stress than the wild type, Col-0. The HE lines accumulated 2.8-fold (8-h light) of starch, 1.42-fold of Chlorophyll a and 1.31-fold of Chlorophyll b than Col-0 during the light period, with obvious differences compared to the RD26OX line. A genome-wide ChIP (chromatin immunoprecipitation analysis)-on-chip assay revealed that EsNAC1 targeted promoters of different genes compared to RD26. In HE lines, EsNAC1 could specifically upregulate the expression level of TF genes NAC DOMAIN CONTAINING PROTEIN 62 (ANAC062), INTEGRASE-TYPE DNA-BINDING PROTEIN (TINY2), and MYB HYPOCOTYL ELONGATION-RELATED (MYBH) to show more effective abiotic stress resistance than RD26OX lines. Moreover, DELTA1-PYRROLINE-5-CARBOXYLATE SYNTHASE 1 (P5CS1), TRYPTOPHAN BIOSYNTHESIS 2 (TRP2) or GALACTINOL SYNTHASE 2 (GOLS2), was also specifically regulated by EsNAC1 to retard the vegetative growth of HE lines, but not the brassinosteroid singling pathway in RD26OX lines. These differences in phenotypes and metabolism between the HE lines and the RD26OX line implied that the differential features could be produced from the diversity of target genes in the transgenic plants when the ortholog was introduced.

Natural variation among Arabidopsis thaliana accessions in tolerance to high magnesium supply

High magnesium (Mg2+) in some extreme serpentine soils or semi-arid regions is an important factor affecting crop growth and development. Specific loci that form the genetic framework underlying high Mg2+ homeostasis, however, are not well understood. By using GWA mapping on 388 accessions of Arabidopsis thaliana selected from a worldwide collection and genotyped at approximately 250,00 SNPs, we successfully identified 109 and 74 putative genetic regions associated in nutrient traits under normal (1,000 µM) and high Mg2+ (10,000 µM), respectively. Above 90% SNPs associated with nutrients including Mg2+ and only two SNPs shared between normal and high Mg2+. A single strong peak of SNPs associated with Ca concentration corresponding to candidate gene At1g60420 ARABIDOPSIS NUCLEOREDOXIN (AtNRX1) under high Mg2+ was further determined. Compared with wildtype, mutants of Atnrx1-1 and Atnrx1-2 supplied with high Mg2+ had higher Ca concentrations in the plant, and higher cytosolic Ca2+ concentrations during root elongation, as well as higher fresh weight and lateral-root number. This suggests that AtNRX1 was a critical gene negatively regulating Ca uptake under high Mg2+ conditions. The discovery could help to breed/select crops that can adapt to high-Mg2+ soils such as serpentine soils (high ratio of Mg2+: Ca2+) or Mars soil with high levels of magnesium sulfate.

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⁺.

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.

Molecular and functional characterization of the magnesium transporter gene ZmMGT12 in maize

Magnesium (Mg) is an essential mineral element for normal plant growth and development, and the CorA/MRS2/MGT-type Mg transporters play a significant role in maintaining Mg homeostasis in plants. In total, 12 maize CorA-like Mg²⁺ transporters have been identified, but none of them had been functionally characterized. Accordingly, we cloned and functionally characterized ZmMGT12 from the maize CorA-like gene family. ZmMGT12 exhibited the structure typical of Mg²⁺ transporters, i.e., two conserved TM domains and a GMN tripeptide motif. ZmMGT12, Arabidopsis AtMGT6, and rice OsMRS2-6 shared high protein sequence identity and thus clustered in the same phylogenetic branch, suggesting that they could be homologs. A functional complementation assay in the Salmonella typhimurium MM281 mutant indicated that ZmMGT12 possessed Mg²⁺ transport ability. ZmMGT12 was expressed in roots, stems, and leaves, with the highest expression in leaves. Moreover, ZmMGT12 expression was induced by light and exhibited a circadian expression pattern. In addition, the expression level of ZmMGT12 in leaf tissue was related to chlorophyll synthesis. Overexpression of ZmMGT12 in Arabidopsis caused no phenotypic change in transgenic plants, including in fresh shoot weight, chlorophyll content, shoot Mg²⁺ content, and chloroplast Mg²⁺ content. Together, these results suggest that ZmMGT12 is a Mg²⁺ transporter and may play a role in Mg transport into chloroplasts.

Magnesium Transporter MGT6 Plays an Essential Role in Maintaining Magnesium Homeostasis and Regulating High Magnesium Tolerance in Arabidopsis

Magnesium (Mg) is one of the essential nutrients for all living organisms. Plants acquire Mg from the environment and distribute within their bodies in the ionic form via Mg2+-permeable transporters. In Arabidopsis, the plasma membrane-localized magnesium transporter MGT6 mediates Mg2+ uptake under Mg-limited conditions, and therefore is important for the plant adaptation to low-Mg environment. In this study, we further assessed the physiological function of MGT6 using a knockout T-DNA insertional mutant allele. We found that MGT6 was required for normal plant growth during various developmental stages when the environmental Mg2+ was low. Interestingly, in addition to the hypersensitivity to Mg2+ limitation, mgt6 mutants displayed dramatic growth defects when external Mg2+ was in excess. Compared with wild-type plants, mgt6 mutants generally contained less Mg2+ under both low and high external Mg2+ conditions. Reciprocal grafting experiments further underpinned a role of MGT6 in a shoot-based mechanism for detoxifying excessive Mg2+ in the environment. Moreover, we found that mgt6 mgt7 double mutant showed more severe phenotypes compared with single mutants under both low- and high-Mg2+ stress conditions, suggesting that these two MGT-type transporters play an additive role in controlling plant Mg2+ homeostasis under a wide range of external Mg2+ concentrations.

The maize CorA/MRS2/MGT-type Mg transporter, ZmMGT10, responses to magnesium deficiency and confers low magnesium tolerance in transgenic Arabidopsis

ZmMGT10 was specifically expressed in maize roots and induced by a deficiency of magnesium. Overexpression of ZmMGT10 restored growth deficiency of the Salmonella typhimurium MM281 strain and enhanced the tolerance in Arabidopsis to stress induced by low magnesium levels by increasing uptake of Mg²⁺ via roots. CorA/MRS2/MGT-type Mg²⁺ transporters play a significant role in maintaining magnesium (Mg) homeostasis in plants. Although the maize CorA/MRS2/MGT family comprises of 12 members, currently no member has been functionally characterized. Here, we report the isolation and functional characterization of ZmMGT10 from the maize MRS2/MGT gene family. ZmMGT10 has a typical structure feature which includes two conserved TMs near the C-terminal end and an altered AMN tripeptide motif. The high sequence similarity and close phylogenetic relationship indicates that ZmMGT10 is probably the counterpart of Arabidopsis AtMGT6. The complementation of the Salmonella typhimurium mutated MM281 strain indicates that ZmMGT10 possesses the ability to transport Mg²⁺. ZmMGT10 was specifically expressed in the plant roots and it can be stimulated by a deficiency of Mg. Transgenic Arabidopsis plants which overexpressed ZmMGT10 grew more vigorously than wild-type plants under low Mg conditions, exhibited by longer root length, higher plant fresh weight and chlorophyll content, suggesting ZmMGT10 was essential for plant growth and development under low Mg conditions. Further investigations found that high accumulation of Mg²⁺ occurred in transgenic plants attributed to improved Mg²⁺ uptake and thereby enhanced tolerance to Mg deficiency. Results from this investigation illustrate that ZmMGT10 is a Mg transporter of maize which can enhance the tolerance to Mg deficient conditions by improving Mg²⁺ uptake in the transgenic plants of Arabidopsis.