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

Global identification of metal ion transporters in chickpea and delineating the role of CaYSL4 in orchestrating iron content

Metal ion transporters (MITs) are vital to maintain proper metal homeostasis during growth and development of plants thereby necessitating their identification and characterization. Considering the economic importance of chickpea in human nutrition, the molecular behaviour and biological functions of the metal ion transporters (MIT) encoding gene families remains highly relevant in recent times. Global identification of MITs revealed a total of 12 CAXs, 6 CTRs, 11 MGTs, 15 MTPs, 9 NRAMPs, 16 OPTs, and 14 ZIPs responsible for metal ion transport. Assessment of phylogenetic relationships, chromosomal distribution, gene structure and motif analysis of MITs suggested their diverse functions. The yellow stripe-like (YSL) family of transporters is an important family whose members have been suggested to have a role in metal ion translocation and assimilation. Expression analysis of key YSLs including CaYSL1, CaYSL4, CaYSL6 and CaYSL16 indicated their significant involvement in conferring tolerance to Fe starvation. Notable was the expression of CaYSL4 that showed specific expression in flower, leaf, shoot, seed at 30 DAA and 40DAA after 7 and 10 day of Fe-deficiency treatment. It was found to be localized in the plasma membrane. RNAi-mediated silencing of CaYSL4 demonstrated its critical role in orchestrating Fe, Zn, Cu and Mn translocation in chickpea seeds. Collectively, the comprehensive analysis of MITs coupled with the functional role of CaYSL4 provides critical insight into the complex regulation of Fe ion transport and distribution that will enable breeding of nutritionally enhanced chickpea varieties.

NnMTP10 from Nelumbo nucifera acts as a transporter mediating manganese and iron efflux

Deficiency or excess of mineral elements in the environment is a primary factor limiting crop yields and nutritional quality. Lotus (Nelumbo nucifera) is an important aquatic crop in Asia, but the mechanism for accumulating mineral nutrients and coping with nutrient deficiency/excess is still largely unknown. Here, we identified NnMTP10, a member of the cation diffusion facilitator family, by screening the cDNA library of lotus. Subcellular localization to the plasma membrane, increased manganese (Mn) and iron (Fe) tolerance and reduced metal accumulation in yeast transformants demonstrated that the protein functions as an exporter to mediate the efflux of Mn and Fe. Arabidopsis overexpressing NnMTP10 exhibited less Mn accumulation in roots, increased long-distance transport to shoots, and increased tolerance to Mn stress under high-Mn conditions. However, the accumulation and tolerance of Fe in Arabidopsis transformants are opposite to those of Mn. Further analysis revealed that excessive Fe in the root apoplast exported by NnMTP10 was sequestrated by the cell wall, thereby reducing the transport of Fe to the shoot. Correspondingly, the expression of NnMTP10 in the roots of lotus was increased under the high-Mn treatment but decreased under the high-Fe treatment. These results suggest that NnMTP10 is involved in the long-distance transport of Mn and Fe in lotus and may play a role in coordinating the adaptation to stresses caused by excessive Mn and Fe.

The Role of Metal Tolerance Proteins (MTPs) Associated with the Homeostasis of Divalent Mineral Elements in Ga-Treated Rice Plants

Mineral elements typically act as transported substrates for metal tolerance proteins (MTPs). The chelation of MTPs with heavy metal ions is a suggestive detoxification pathway in plants; therefore, the trade-off between transporting mineral elements and chelating excess toxic metal ions is inevitable. Gallium (Ga) is an emerging pollutant associated with high-tech industries. This study investigated the impact of Ga stress on MTPs, subsequently altering the transport and distribution of mineral elements. Gallium exposure reduced rice seedling biomass, with roots accumulating more Ga than shoots. Ga stress also changed the rice plants' subcellular mineral element distribution. PCR assays showed that Ga stress negatively affected all genes belonging to the Mn group, except OsMTP9. While Mn accumulation in the rice cellular compartments did not respond positively to Ga stress, OsMTP8, OsMTP8.1, OsMTP11, and OsMTP11.1 were found to be intimately connected to Mn transport and repressed by increased Ga accumulation in roots. Mg and Cu accumulated in the cytosol and organelles of Ga-treated rice plants, while OsMTP9 expression increased, demonstrating its importance in transporting Mg and Cu. A positive link between Ga stress and Zn accumulation in the cytosol and organelles was found, and OsMTP7 and OsMTP12 expression was positive, suggesting that Ga stress did not impair their Zn transport. Notably, Ga exposure down-regulated Fe-transporting OsMTP1 and OsMTP6, wherein the subcellular concentrations of Fe showed negative responses to Ga accumulation. These findings provide valuable insights into elucidating the roles of OsMTPs in Ga tolerance and the transport of these mineral elements.

A triplicated wheat-rye chromosome segment including several 12-OXOPHYTODIENOATE REDUCTASE III genes influences magnesium partitioning and impacts wheat performance at low magnesium supply

We previously reported a structural rearrangement between wheat (Triticum aestivum) and rye (Secale cereale) chromosomes 1BS/1RS that increased the dosage of 12-OXOPHYTODIENOATE REDUCTASE III (OPRIII) genes involved in jasmonate biosynthesis (henceforth, 1RW line), and that drastically reduced primary root growth relative to a control line with the intact 1RS chromosome (henceforth, 1RS). In this study, we show that the increased gene-dosage of this region is associated with increases in the shoot-root partitioning of magnesium (Mg). Moreover, both a CRISPR-edited 1RW line with reduced OPRIII dosage and the 1RW line treated with the jasmonate biosynthesis inhibitor ibuprofen showed reduced differences in shoot-root Mg partitioning than 1RW. The observed differences in Mg partitioning between 1RS and 1RW plants occur over a wide range of external Mg supplies and imply opposite trends of Mg accumulation in roots and shoots. Furthermore, we show an association between the increase of shoot-root Mg partitioning and increased tolerance of the 1RW line to low levels of Mg supply. In summary, our results provide evidence of the role of the jasmonate pathway on the dynamics of Mg accumulation in roots and shoots, which correlates with the performance of wheat plants under conditions of Mg scarcity.

Genome-wide identification of metal tolerance protein genes in Quercus dentata and their roles in response to various heavy metal stresses

Metal tolerance protein (MTP) is a cation transporter that plays an important role in tolerance to heavy metal stress. However, thus far, there has been no genome-wide investigation of the MTP gene family in Quercus plants. Quercus dentata is one of the main constructive species of forest in northern China. It has strong tolerance to a variety of heavy metal stresses. In this study, 25 MTPs were identified from the Q. dentata genome and classified into three subfamilies and seven groups according to their sequence characteristics and phylogenetic relationships. Both tandem and segmental duplication events contributed to the expansion of the QdMTP gene family. Interestingly, all 10 tandem duplication events contributed to the expansion of the Mn-CDF subfamily. The expression of Mn-CDF subfamily members in different organs and tissues of Q. dentata was different, and they responded differently to manganese, iron, zinc and cadmium stress treatments. QdMTP10.7, a member of the Mn-CDF subfamily, enhanced yeast growth under manganese, zinc and iron stresses. The subcellular localization in tobacco leaf epidermis cells showed that QdMTP10.7 was located in vacuoles. These data generated from this study provide an important foundation to elucidate the biological roles of QdMTP genes related to heavy metal tolerance in Q. dentata.

Metal tolerance protein CsMTP4 has dual functions in maintaining zinc homeostasis in tea plant

Plants have evolved a series of zinc (Zn) homeostasis mechanisms to cope with the fluctuating Zn in the environment. How Zn is taken up, translocated and tolerate by tea plant remains unknown. In this study, on the basis of RNA-Sequencing, we isolated a plasma membrane-localized Metal Tolerance Protein (MTP) family member CsMTP4 from Zn-deficient tea plant roots and investigated its role in regulation of Zn homeostasis in tea plant. Heterologous expression of CsMTP4 specifically enhanced the tolerance of transgenic yeast to Zn excess. Moreover, overexpression of CsMTP4 in tea plant hairy roots stimulated Zn uptake under Zn deficiency. In addition, CsMTP4 promoted the growth of transgenic Arabidopsis plants by translocating Zn from roots to shoots under Zn deficiency and conferred the tolerance to Zn excess by enhancing the efflux of Zn from root cells. Transcriptome analysis of the CsMTP4 transgenic Arabidopsis found that the expression of Zn metabolism-related genes were differentially regulated compared with wild-type plants when exposed to Zn deficiency and excess conditions. This study provides a mechanistic understanding of Zn uptake and translocation in plants and a new strategy to improve phytoremediation efficiency.

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 transcriptome analysis of response of soybean (Glycine max) to cadmium stress under elevated CO2 concentration

The continuous accumulation of Cd has long-lasting detrimental effects on plant growth and food safety. Although elevated CO₂ concentration (EC) has been reported to reduce Cd accumulation and toxicity in plants, evidence on the functions of elevated CO₂ concentration and its mechanisms in the possible alleviation of Cd toxicity in soybean are limited. Here, we used physiological and biochemical methods together with transcriptomic comparison to explore the effects of EC on Cd-stressed soybean. Under Cd stress, EC significantly increased the weight of roots and leaves, promoted the accumulations of proline, soluble sugars, and flavonoid. In addition, the enhancement of GSH activity and GST gene expressions promoted Cd detoxification. These defensive mechanisms reduced the contents of Cd²⁺, MDA, and H₂O₂ in soybean leaves. The up-regulation of genes encoding phytochelatin synthase, MTPs, NRAMP, and vacuoles protein storage might play vital roles in the transportation and compartmentalization process of Cd. The MAPK and some transcription factors such as bHLH, AP2/ERF, and WRKY showed changed expressions and might be engaged in mediation of stress response. These findings provide a boarder view on the regulatory mechanism of EC on Cd stress and provide numerous potential target genes for future engineering of Cd-tolerant cultivars in soybean breeding programs under climate changes scenarios.

A metal tolerance protein, MTP10, is required for the calcium and magnesium homeostasis in Arabidopsis

Nutrient antagonism typically refers to the fact that too high a concentration of one nutrient inhibits the absorption of another nutrient. In plants, Ca²⁺ (Calcium) and Mg²⁺ (Magnesium) are the two most abundant divalent cations, which are known to have antagonistic interactions. Hence, maintaining their homeostasis is crucial for plant growth and development. In this study, we showed that MTP10 (Metal Tolerance Protein 10) is an important regulator for maintaining homeostasis of Mg and Ca in Arabidopsis. The mtp10 mutant displayed severe growth retardation in the presence of excess Mg²⁺, to which the addition of Ca²⁺ was able to rescue the phenotype of mtp10 mutant. Additionally, the deficiency of Ca²⁺ in the culture medium accelerated the high-Mg sensitivity of the mtp10 mutant. The yeast complementation assay suggested that AtMTP10 had no Ca²⁺ transport activity. And the ICP-MS data further confirmed the antagonistic relationship between Ca²⁺ and Mg²⁺, with the addition of Ca²⁺ reducing the excessive accumulation of Mg²⁺ and high-Mg inhibiting the uptake of Ca²⁺. We conclude that the Arabidopsis MTP10 is essential for the regulation of Mg and Ca homeostasis.

The ferroxidases are critical for Fe(II) oxidation in xylem to ensure a healthy Fe allocation in Arabidopsis thaliana

The long-distance transport of iron (Fe) in the xylem is critical for maintaining systemic Fe homeostasis in plants. The loading form of Fe(II) into the xylem and the long-distance translocation form of Fe(III)-citrate have been identified, but how Fe(II) is oxidized to Fe(III) in the xylem remains unknown. Here, we showed that the cell wall-resided ferroxidases LPR1 and LPR2 (LPRs) were both specifically expressed in the vascular tissues of Arabidopsis thaliana, while disruption of both of them increased Fe(II) in the xylem sap and caused excessive Fe deposition in the xylem vessel wall under Fe-sufficient conditions. As a result, a large amount of Fe accumulated in both roots and shoots, hindering plant growth. Moreover, under low-Fe conditions, LPRs were preferentially induced in old leaves, but the loss of LPRs increased Fe deposition in the vasculature of older leaves and impeded Fe allocation to younger leaves. Therefore, disruption of both LPRs resulted in severer chlorosis in young leaves under Fe-deficient conditions. Taken together, the oxidation of Fe(II) to Fe(III) by LPRs in the cell wall of vasculature plays an important role in xylem Fe allocation, ensuring healthy Fe homeostasis for normal plant growth.