The high-affinity metal Transporters NRAMP1 and IRT1 Team up to Take up Iron under Sufficient Metal Provision

Nutri-cereal tissue-specific transcriptome atlas during development: Functional integration of gene expression to identify mineral uptake pathways in little millet (Panicum sumatrense)

Little millet (Panicum sumatrense Roth ex Roem. & Schult.) is an essential minor millet of southeast Asia and Africa's temperate and subtropical regions. The plant is stress-tolerant, has a short life cycle, and has a mineral-rich nutritional profile associated with unique health benefits. We report the developmental gene expression atlas of little millet (genotype JK-8) from ten tissues representing different stages of its life cycle, starting from seed germination and vegetative growth to panicle maturation. The developmental transcriptome atlas led to the identification of 342 827 transcripts. The BUSCO analysis and comparison with the transcriptomes of related species confirm that this study presents high-quality, in-depth coverage of the little millet transcriptome. In addition, the eFP browser generated here has a user-friendly interface, allowing interactive visualizations of tissue-specific gene expression. Using these data, we identified transcripts, the orthologs of which in Arabidopsis and rice are involved in nutrient acquisition, transport, and response pathways. The comparative analysis of the expression levels of these transcripts holds great potential for enhancing the mineral content in crops, particularly zinc and iron, to address the issue of "hidden hunger" and to attain nutritional security, making it a valuable asset for translational research.

Metal Homeostasis in Land Plants: A Perpetual Balancing Act Beyond the Fulfilment of Metalloproteome Cofactor Demands

One of life's decisive innovations was to harness the catalytic power of metals for cellular chemistry. With life's expansion, global atmospheric and biogeochemical cycles underwent dramatic changes. Although initially harmful, they permitted the evolution of multicellularity and the colonization of land. In land plants as primary producers, metal homeostasis faces heightened demands, in part because soil is a challenging environment for nutrient balancing. To avoid both nutrient metal limitation and metal toxicity, plants must maintain the homeostasis of metals within tighter limits than the homeostasis of other minerals. This review describes the present model of protein metalation and sketches its transfer from unicellular organisms to land plants as complex multicellular organisms. The inseparable connection between metal and redox homeostasis increasingly draws our attention to more general regulatory roles of metals. Mineral co-option, the use of nutrient or other metals for functions other than nutrition, is an emerging concept beyond that of nutritional immunity.

Physiological and molecular mechanisms of plant-root responses to iron toxicity

The chemical form and physiological activity of iron (Fe) in soil are dependent on soil pH and redox potential (Eh), and Fe levels in soils are frequently elevated to the point of causing Fe toxicity in plants, with inhibition of normal physiological activities and of growth and development. In this review, we describe how iron toxicity triggers important physiological changes, including nitric-oxide (NO)-mediated potassium (K+) efflux at the tips of roots and accumulation of reactive oxygen species (ROS) and reactive nitrogen (RNS) in roots, resulting in physiological stress. We focus on the root system, as the first point of contact with Fe in soil, and describe the key processes engaged in Fe transport, distribution, binding, and other mechanisms that are drawn upon to defend against high-Fe stress. We describe the root-system regulation of key physiological processes and of morphological development through signaling substances such as ethylene, auxin, reactive oxygen species, and nitric oxide, and discuss gene-expression responses under high Fe. We especially focus on studies on the physiological and molecular mechanisms in rice and Arabidopsis under high Fe, hoping to provide a valuable theoretical basis for improving the ability of crop roots to adapt to soil Fe toxicity.

Plant iron status regulates ammonium-use efficiency through protein N-glycosylation

Improving nitrogen-use efficiency is an important path toward enhancing crop yield and alleviating the environmental impacts of fertilizer use. Ammonium (NH4+) is the energetically preferred inorganic N source for plants. The interaction of NH4+ with other nutrients is a chief determinant of ammonium-use efficiency (AUE) and of the tipping point toward ammonium toxicity, but these interactions have remained ill-defined. Here, we report that iron (Fe) accumulation is a critical factor determining AUE and have identified a substance that can enhance AUE by manipulating Fe availability. Fe accumulation under NH4+ nutrition induces NH4+ efflux in the root system, reducing both growth and AUE in Arabidopsis (Arabidopsis thaliana). Low external availability of Fe and a low plant Fe status substantially enhance protein N-glycosylation through a Vitamin C1-independent pathway, thereby reducing NH4+ efflux to increase AUE during the vegetative stage in Arabidopsis under elevated NH4+ supply. We confirm the validity of the iron-ammonium interaction in the important crop species lettuce (Lactuca sativa). We further show that dolomite can act as an effective substrate to subdue Fe accumulation under NH4+ nutrition by reducing the expression of Low Phosphate Root 2 and acidification of the rhizosphere. Our findings present a strategy to improve AUE and reveal the underlying molecular-physiological mechanism.

Soil and Mineral Nutrients in Plant Health: A Prospective Study of Iron and Phosphorus in the Growth and Development of Plants

Plants being sessile are exposed to different environmental challenges and consequent stresses associated with them. With the prerequisite of minerals for growth and development, they coordinate their mobilization from the soil through their roots. Phosphorus (P) and iron (Fe) are macro- and micronutrient; P serves as an important component of biological macromolecules, besides driving major cellular processes, including photosynthesis and respiration, and Fe performs the function as a cofactor for enzymes of vital metabolic pathways. These minerals help in maintaining plant vigor via alterations in the pH, nutrient content, release of exudates at the root surface, changing dynamics of root microbial population, and modulation of the activity of redox enzymes. Despite this, their low solubility and relative immobilization in soil make them inaccessible for utilization by plants. Moreover, plants have evolved distinct mechanisms to cope with these stresses and coregulate the levels of minerals (Fe, P, etc.) toward the maintenance of homeostasis. The present study aims at examining the uptake mechanisms of Fe and P, and their translocation, storage, and role in executing different cellular processes in plants. It also summarizes the toxicological aspects of these minerals in terms of their effects on germination, nutrient uptake, plant-water relationship, and overall yield. Considered as an important and indispensable component of sustainable agriculture, a separate section covers the current knowledge on the cross-talk between Fe and P and integrates complete and balanced information of their effect on plant hormone levels.

NRAMPs and manganese: Magic keys to reduce cadmium toxicity and accumulation in plants

Cadmium (Cd), a toxic heavy metal, poses significant threats to both crop production and human health worldwide. Manganese (Mn), an essential micronutrient, plays a crucial role in plant growth and development. NRAMPs (Natural Resistance-Associated Macrophage Proteins) function as common transporters for both Cd and Mn. Deep understanding of the regulatory mechanisms governing NRAMP-mediated Cd and Mn transport is imperative for developing the crop varieties with high tolerance and low accumulation of Cd. This review reported the advance in studies on the fundamental properties and classification of NRAMPs in plants, and structural characteristics, expression patterns, and diverse functions of NRAMP genes across different plant species. We highlighted the pivotal role of NRAMPs in Cd/Mn uptake and transport in plants as a common transporter. Finally, we also comprehensively discussed over the strategies for reducing Cd uptake and accumulation in plants through using antagonism of Mn over Cd and altering the expression of NRAMP genes.

miR397b-LAC2 module regulates cadmium stress response by coordinating root lignification and copper homeostasis in Arabidopsis thaliana

Non-essential heavy metal cadmium (Cd) is toxic to plants and animals. Cadmium affects plant photosynthesis, respiration, and causes water imbalance and may lead to plant death. Cadmium induces toxicity by interfering with the essential metal copper (Cu) homeostasis, which affects plant nutrition. Though root lignin biosynthesis is positively regulated by Cd stress, the underlying mechanisms promoting lignin accumulation and controlling Cd-induced Cu limitation responses are unclear. Here, we elucidated the role of Cu-responsive microRNA (miR397b) in Arabidopsis thaliana plants for Cd stress by targeting the LACCASE2 (LAC2) gene. This study demonstrated the fundamental mechanism of miR397b-mediated Cd stress response by enhancing the lignin content in root tissues. We developed miR397b over-expressing plants, which showed considerable Cd stress tolerance. Plants with knockdown function of LAC2 also showed significant tolerance to Cd stress. miR397b overexpressing and lac2 mutant plants showed root reduction, higher biomass and chlorophyll content, and significantly lower Reactive Oxygen Species (ROS). This study demonstrated the miR397b-mediated Cd stress response in Arabidopsis by enhancing the lignin content in root tissues. We conclude that modulation in miR397b can be potentially used for improving plants for Cd tolerance and Cu homeostasis.

Interspecific hybridization and inoculation with Pantoea eucalypti improve forage performance of Lotus crop species under alkaline stress

This study was designed to elucidate the physiological responses of three Lotus forage accessions to alkaline stress, and the influence of inoculating with Pantoea eucalypti endophyte strain on alkaline stress mitigation. A diploid L. corniculatus (Lc) accession, L. tenuis (Lt), and the interspecific hybrid Lt × Lc obtained from these two parental lines were exposed to alkaline stress (pH 8.2). Both Lt and the Lt × Lc hybrid are alkaline-tolerant compared to Lc, based on observations that dry mass was not reduced under stress, and there were no chlorosis symptoms on leaf blades. In all three Lotus accessions, Fe2+ concentration under stress decreased in aerial parts and simultaneously increased in roots. Inoculation with P. eucalypti considerably increased Fe2+ content in shoots of all three Lotus forage species under alkaline treatment. Photochemical efficiency of PSII was affected in Lc accession only when exposed to alkaline treatment. However, when cultivated under alkalinity with inoculation, plants recovered and had photosynthetic parameters equivalent to those in the control treatment. Together, the results highlight the importance of inoculation with P. eucalypti, which contributes significantly to mitigating alkaline stress. All results provide useful information for improving alkaline tolerance traits of Lotus forage species and their interspecific hybrids.

Local distribution of manganese to leaf sheath is mediated by OsNramp5 in rice

To play essential roles of manganese (Mn) in plant growth and development, it needs to be transported to different organs and tissues after uptake. However, the molecular mechanisms underlying Mn distribution between different tissues are poorly understood. We functionally characterized a member of rice natural resistance-associated macrophage protein (NRAMP) family, OsNramp5 in terms of its tissue specificity of gene expression, cell-specificity of protein localization, phenotypic analysis of leaf growth and response to Mn fluctuations. OsNramp5 is highly expressed in the leaf sheath. Immunostaining revealed that OsNramp5 is polarly localized at the proximal side of xylem parenchyma cells of the leaf sheath. Both the gene expression and protein abundance of OsNramp5 are unaffected by different Mn concentrations. Knockout of OsNramp5 decreased the distribution of Mn to the leaf sheath, but increased the distribution to the leaf blade at both low and high Mn supplies, resulting in reduced growth of leaf sheath. Furthermore, expression of OsNramp5 under the control of root-specific promoter in osnramp5 mutant complemented Mn uptake, but could not complement Mn distribution to the leaf sheath. These results indicate that OsNramp5 expressed in the leaf sheath plays an important role in unloading Mn from the xylem for the local distribution in rice.

Mitigating cadmium accumulation in dicotyledonous vegetables by iron fertilizer through inhibiting Fe transporter IRT1-mediated Cd uptake

The solubility of cadmium (Cd) in soil and its transfer to plants are influenced by soil pH. While increasing soil pH reduces Cd solubility and accumulation in rice plants grown in acidic soils, its effect on Cd accumulation in vegetables remains inconclusive. Here, we investigated the impact of soil pH on Cd accumulation in dicotyledonous vegetables and elucidated the underlying molecular mechanisms. Soils collected from various locations were supplemented with varying quantities of lime to achieve soil pH values of around 5.0, 6.0, 7.0, and 8.0. Raising soil pH from around 5.0 to 8.0 markedly decreased extractable Cd. However, increasing soil pH tended to promote shoot Cd accumulation in dicotyledonous vegetable species including lettuce, pakchoi, and Chinese cabbage, and the model dicotyledonous plant Arabidopsis thaliana. Conversely, soil pH increase resulted in a monotonic decrease in rice Cd accumulation. In our hydroponic experiments, we discovered that iron (Fe) deficiency substantially increased Cd uptake and accumulation in dicotyledonous plants but not in rice. Increasing soil pH reduced soil Fe availability and induced the Fe transporter gene IRT1 expression in dicotyledonous vegetables roots, which led to an increase in IRT1-mediated Cd uptake and subsequently increased Cd accumulation as soil pH increases. A comprehensive model incorporating extractable Cd and root IRT1 expression better explained Cd accumulation in vegetable shoots. The application of 50 mg/kg of Fe fertilizer in neutral or alkaline soils resulted in a significant reduction in Cd accumulation by 34-58% in dicotyledonous vegetables. These findings reveal that increasing soil pH has two opposite effects, decreasing soil Cd availability while promoting Cd uptake through IRT1 upregulation, reconciling the inconsistency in its effect on Cd accumulation in dicotyledonous plants. Our findings provide important insights for understanding the factors affecting Cd uptake in plants and offer a practical solution to mitigate Cd contamination in vegetables.

Dissecting the promotional effect of zinc on cadmium translocation from roots to shoots in rice

It is often expected that Zn decreases Cd accumulation in plants due to competition for the same transporters. Here, we found that increasing Zn supply markedly increased the root-to-shoot translocation of Cd in rice. RNA sequencing showed that high Zn up-regulated expression of genes involved in glutathione biosynthesis and metabolism and the Zn/Cd transporter gene OsHMA2, but down-regulated expression of genes related to Zn uptake. Knockout of the iron or Zn transporter genes OsIRT1, OsIRT2, or OsZIP9 did not affect the Zn promotional effect on Cd translocation. Knockout of the manganese/Cd transporter gene OsNRAMP5 greatly reduced Cd uptake but did not affect the Zn promotional effect. Variation in the tonoplast transporter gene OsHMA3 affected Cd translocation but did not change the Zn promotional effect. Knockout of the Zn/Cd transporter gene OsHMA2 not only decreased Cd and Zn translocation, but also abolished the Zn promotional effect. Increased expression of OsHMA2 under high Zn conditions supports the hypothesis that this transporter participates in the promotional effect of Zn on Cd translocation. The results also show that OsIRT1, OsIRT2, and OsZIP9 made only small contributions to Cd uptake under low Zn conditions but not under high Zn conditions, whereas the dominant role of OsNRAMP5 in Cd uptake diminished under low Zn conditions.

An intracellular transporter OsNRAMP7 is required for distribution and accumulation of iron into rice grains

Iron (Fe) is an essential micronutrient for plant growth and human health. Plants have evolved an efficient transport system for absorbing and redistributing Fe from the soil to other organs; however, the molecular mechanisms underlying Fe loading into grains are poorly understood. Our study shows that OsNRAMP7, a member of the natural resistance-associated macrophage protein (NRAMP) family, is a rice Fe transporter that localizes to the Golgi and trans-Golgi network (TGN). OsNRAMP7 was highly expressed in leaf blade, node I, pollen, and vascular tissues of almost tissues at the rice flowering stage. OsNRAMP7 knockdown by RNA interference (RNAi) increased Fe accumulation in the flag leaf blade, but decreased the Fe concentration in node I and rice grains. In addition, the knockdown of OsNRAMP7 also reduced grain fertility, pollen viability, and grain Fe concentration in the paddy fields; OsNRAMP7 overexpression significantly promoted Fe accumulation in the grains. Thus, our results suggest that OsNRAMP7 is required for the distribution and accumulation of Fe in rice grains and its overexpression could be a novel strategy for Fe biofortification in staple food crops.

Overexpression of ApHIPP26 from the Hyperaccumulator Arabis paniculata Confers Enhanced Cadmium Tolerance and Accumulation to Arabidopsis thaliana

Cadmium (Cd) is a toxic heavy metal that seriously affects metabolism after accumulation in plants, and it also causes adverse effects on humans through the food chain. The HIPP gene family has been shown to be highly tolerant to Cd stress due to its special domain and molecular structure. This study described the Cd-induced gene ApHIPP26 from the hyperaccumulator Arabis paniculata. Its subcellular localization showed that ApHIPP26 was located in the nucleus. Transgenic Arabidopsis overexpressing ApHIPP26 exhibited a significant increase in main root length and fresh weight under Cd stress. Compared with wild-type lines, Cd accumulated much more in transgenic Arabidopsis both aboveground and underground. Under Cd stress, the expression of genes related to the absorption and transport of heavy metals underwent different changes in parallel, which were involved in the accumulation and distribution of Cd in plants, such as AtNRAMP6 and AtNRAMP3. Under Cd stress, the activities of antioxidant enzymes (superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase) in the transgenic lines were higher than those in the wild type. The physiological and biochemical indices showed that the proline and chlorophyll contents in the transgenic lines increased significantly after Cd treatment, while the malondialdehyde (MDA) content decreased. In addition, the gene expression profile analysis showed that ApHIPP26 improved the tolerance of Arabidopsis to Cd by regulating the changes of related genes in plant hormone signal transduction pathway. In conclusion, ApHIPP26 plays an important role in cadmium tolerance by alleviating oxidative stress and regulating plant hormones, which provides a basis for understanding the molecular mechanism of cadmium tolerance in plants and provides new insights for phytoremediation in Cd-contaminated areas.

Multilayered regulation of iron homeostasis in Arabidopsis

Iron (Fe) is an essential micronutrient for plant growth and development due to its role in crucial processes such as photosynthesis and modulation of the redox state as an electron donor. While Fe is one of the five most abundant metals in the Earth's crust, it is poorly accessible to plants in alkaline soils due to the formation of insoluble complexes. To limit Fe deficiency symptoms, plant have developed a highly sophisticated regulation network including Fe sensing, transcriptional regulation of Fe-deficiency responsive genes, and post-translational modifications of Fe transporters. In this mini-review, we detail how plants perceive intracellular Fe status and how they regulate transporters involved in Fe uptake through a complex cascade of transcription factors. We also describe the current knowledge about intracellular trafficking, including secretion to the plasma membrane, endocytosis, recycling, and degradation of the two main Fe transporters, IRON-REGULATED TRANSPORTER 1 (IRT1) and NATURAL RESISTANCE ASSOCIATED MACROPHAGE PROTEIN 1 (NRAMP1). Regulation of these transporters by their non-Fe substrates is discussed in relation to their functional role to avoid accumulation of these toxic metals during Fe limitation.

TaNRAMP3 is essential for manganese transport in Triticum aestivum

Manganese (Mn) is an essential trace element for almost all living organisms. In plants, Mn deficiency, which is occurs in calcareous soils or alkaline soils, severely limiting crop yields. However, the potential mechanism of Mn transport in Triticum aestivum is still obscure. Here, we found that TaNRAMP3, a member of the naturally resistant macrophage protein (NRAMP) family in Triticum aestivum, is located in the plasma membrane of protoplasts and functions as an influx transporter for Mn in yeast (Δsmf1). The expression of TaNRAMP3 was induced under Mn-deficiency conditions. Furthermore, TaNRAMP3-RNAi plants exhibited a sensitive phenotype, while transgenic plants overexpressing TaNRAMP3 showed a tolerant phenotype. In addition, TaNRAMP3 rescued the sensitive phenotype of Arabidopsis nramp1 mutant under Mn deficiency condition. In summary, our study reveals the key role of TaNRAMP3 in Mn transport in Triticum aestivum, allowing it to adapt to Mn-deficiency stress. These findings provide new insights for the cultivation of Mn-deficiency tolerant wheat varieties.

Natural variation of nutrient homeostasis among laboratory and field strains of Chlamydomonas reinhardtii

Natural variation among individuals and populations exists in all species, playing key roles in response to environmental stress and adaptation. Micro- and macronutrients have a wide range of functions in photosynthetic organisms, and mineral nutrition thus plays a sizable role in biomass production. To maintain nutrient concentrations inside the cell within physiological limits and prevent the detrimental effects of deficiency or excess, complex homeostatic networks have evolved in photosynthetic cells. The microalga Chlamydomonas reinhardtii (Chlamydomonas) is a unicellular eukaryotic model for studying such mechanisms. In this work, 24 Chlamydomonas strains, comprising field isolates and laboratory strains, were examined for intraspecific differences in nutrient homeostasis. Growth and mineral content were quantified in mixotrophy, as full nutrition control, and compared with autotrophy and nine deficiency conditions for macronutrients (-Ca, -Mg, -N, -P, and -S) and micronutrients (-Cu, -Fe, -Mn, and -Zn). Growth differences among strains were relatively limited. However, similar growth was accompanied by highly divergent mineral accumulation among strains. The expression of nutrient status marker genes and photosynthesis were scored in pairs of contrasting field strains, revealing distinct transcriptional regulation and nutrient requirements. Leveraging this natural variation should enable a better understanding of nutrient homeostasis in Chlamydomonas.

Forging a symbiosis: transition metal delivery in symbiotic nitrogen fixation

Symbiotic nitrogen fixation carried out by the interaction between legumes and rhizobia is the main source of nitrogen in natural ecosystems and in sustainable agriculture. For the symbiosis to be viable, nutrient exchange between the partners is essential. Transition metals are among the nutrients delivered to the nitrogen-fixing bacteria within the legume root nodule cells. These elements are used as cofactors for many of the enzymes controlling nodule development and function, including nitrogenase, the only known enzyme able to convert N2 into NH3 . In this review, we discuss the current knowledge on how iron, zinc, copper, and molybdenum reach the nodules, how they are delivered to nodule cells, and how they are transferred to nitrogen-fixing bacteria within.

Phosphorylation by CIPK23 regulates the high-affinity Mn transporter NRAMP1 in Arabidopsis

Manganese (Mn) is essential for plants but is toxic when taken up in excess. To maintain Mn homeostasis, the root Mn transporter natural resistance associated macrophage protein 1 (NRAMP1) cycles from the plasma membrane to endosomes upon phosphorylation. To identify the kinase involved, a split-luciferase screening was carried out between NRAMP1 and kinases of the CIPK family and identified CIPK23 as a partner of NRAMP1. The interaction was confirmed by split-mCitrine bimolecular fluorescence complementation and co-immunoprecipitation assays. In vitro phosphorylation assays pinpointed two CIPK23 target residues in NRAMP1, among which serine 20, important for endocytosis. Interestingly, Mn-induced internalization of NRAMP1 was unaffected by cipk23 mutation suggesting a potential redundancy between CIPK23 and other kinase(s). How CIPK23 could regulate NRAMP1 in response to Mn availability is discussed.

MsNRAMP2 Enhances Tolerance to Iron Excess Stress in Nicotiana tabacum and MsMYB Binds to Its Promoter

Iron(Fe) is a trace metal element necessary for plant growth, but excess iron is harmful to plants. Natural resistance-associated macrophage proteins (NRAMPs) are important for divalent metal transport in plants. In this study, we isolated the MsNRAMP2 (MN_547960) gene from alfalfa, the perennial legume forage. The expression of MsNRAMP2 is specifically induced by iron excess. Overexpression of MsNRAMP2 conferred transgenic tobacco tolerance to iron excess, while it conferred yeast sensitivity to excess iron. Together with the MsNRAMP2 gene, MsMYB (MN_547959) expression is induced by excess iron. Y1H indicated that the MsMYB protein could bind to the "CTGTTG" cis element of the MsNRAMP2 promoter. The results indicated that MsNRAMP2 has a function in iron transport and its expression might be regulated by MsMYB. The excess iron tolerance ability enhancement of MsNRAMP2 may be involved in iron transport, sequestration, or redistribution.

Cell type-specific mapping of ion distribution in Arabidopsis thaliana roots

Cell type-specific mapping of element distribution is critical to fully understand how roots partition nutrients and toxic elements with aboveground parts. In this study, we developed a method that combines fluorescence-activated cell sorting (FACS) with inductively coupled plasma mass spectrometry (ICP-MS) to assess the ionome of different cell populations within Arabidopsis thaliana roots. The method reveals that most elements exhibit a radial concentration gradient increasing from the rhizodermis to inner cell layers, and detected previously unknown ionomic changes resulting from perturbed xylem loading processes. With this approach, we also identify a strong accumulation of manganese in trichoblasts of iron-deficient roots. We demonstrate that confining manganese sequestration in trichoblasts but not in endodermal cells efficiently retains manganese in roots, therefore preventing toxicity in shoots. These results indicate the existence of cell type-specific constraints for efficient metal sequestration in roots. Thus, our approach opens an avenue to investigate element compartmentation and transport pathways in plants.

Systematic Analysis of NRAMP Family Genes in Areca catechu and Its Response to Zn/Fe Deficiency Stress

Areca catechu is a commercially important medicinal plant widely cultivated in tropical regions. The natural resistance-associated macrophage protein (NRAMP) is widespread in plants and plays critical roles in transporting metal ions, plant growth, and development. However, the information on NRAMPs in A. catechu is quite limited. In this study, we identified 12 NRAMPs genes in the areca genome, which were classified into five groups by phylogenetic analysis. Subcellular localization analysis reveals that, except for NRAMP2, NRAMP3, and NRAMP11, which are localized in chloroplasts, all other NRAMPs are localized on the plasma membrane. Genomic distribution analysis shows that 12 NRAMPs genes are unevenly spread on seven chromosomes. Sequence analysis shows that motif 1 and motif 6 are highly conserved motifs in 12 NRAMPs. Synteny analysis provided deep insight into the evolutionary characteristics of AcNRAMP genes. Among the A. catechu and the other three representative species, we identified a total of 19 syntenic gene pairs. Analysis of Ka/Ks values indicates that AcNRAMP genes are subjected to purifying selection in the evolutionary process. Analysis of cis-acting elements reveals that AcNRAMP genes promoter sequences contain light-responsive elements, defense- and stress-responsive elements, and plant growth/development-responsive elements. Expression profiling confirms distinct expression patterns of AcNRAMP genes in different organs and responses to Zn/Fe deficiency stress in leaves and roots. Taken together, our results lay a foundation for further exploration of the AcNRAMPs regulatory function in areca response to Fe and Zn deficiency.

Plasma membrane-associated calcium signaling modulates cadmium transport

Cadmium (Cd) is a toxic heavy element for plant growth and development, and plants have evolved many strategies to cope with Cd stress. However, the mechanisms how plants sense Cd stress and regulate the function of transporters remain very rudimentary. Here, we found that Cd stress induces obvious Ca²⁺ signals in Arabidopsis roots. Furthermore, we identified the calcium-dependent protein kinases CPK21 and CPK23 that interacted with the Cd transporter NRAMP6 through a variety of protein interaction techniques. Then, we confirmed that the cpk21 23 double mutants significantly enhanced the sensitive phenotype of cpk23 single mutant under Cd stress, while the overexpression and continuous activation of CPK21 and CPK23 enhanced plants tolerance to Cd stress. Multiple biochemical and physiological analyses in yeast and plants demonstrated that CPK21/23 phosphorylate NRAMP6 primarily at Ser489 and Thr505 to inhibit the Cd transport activity of NRAMP6, thereby improving the Cd tolerance of plants. Taken together, we found a plasma membrane-associated calcium signaling that modulates Cd tolerance. These results provide new insights into the molecular breeding of crop tolerance to Cd stress.

The overexpression of LOW PHOSPHATE ROOT 1 (LPR1) negatively regulates Arabidopsis growth in response to Cadmium (Cd) stress

Cadmium (Cd) is a highly toxic element that is easily absorbed by plant, and the mechanisms of the plant response to Cd toxicity are very complex. In this study, the role of LPR1 (LOW Phosphate Root 1) encoding a cell-wall-targeted ferroxidase in Cd stress was investigated. The results showed that the overexpression of LPR1 caused an average reduction of 23%-40% in the primary root lengths, 67%-73% in the fresh weights, 32%-46% in the lengths of the non-root hair zone (NRHZ) and 70%-71% in the chlorophyll contents in both LPR1-OX lines when compared with the wild type (WT), while there were no significant changes in these traits between the WT and mutant lpr1 lines under Cd stress (7.5 μmol/L CdSO₄). Further investigation showed that the overexpression of LPR1 triggered reactive oxygen species (ROS) bursts and reduced the entry of available iron (Fe²⁺) into the cell, which induced the expression of iron-regulated transporter 1 (IRT1). The up-regulation of IRT1 contributed to the increase of Cd accumulation and growth retardation under Cd stress. Exogenous Fe and ROS scavengers down-regulated the IRT1's expression and alleviated the growth inhibition in LPR1-OX lines, indicating that LPR1-dependent ROS up-regulated IRT1, which subsequently exacerbated the Cd influx into plants. Our findings highlight a pathway of LPR1-mediated plant responding to Cd toxicity stress through the regulation of ROS and Fe homeostasis.

Promotive Role of 5-Aminolevulinic Acid or Salicylic Acid Combined with Citric Acid on Sunflower Growth by Regulating Manganese Absorption

Manganese (Mn) is an essential nutrient in most organisms. Establishing an effective regulatory system of Mn absorption is important for sustainable crop development. In this study, we selected sunflower as the model plant to explore the effects of 5-aminolevulinic acid (ALA) or salicylic acid (SA) combined with citric acid (CA) on Mn absorption. Six-leaf-old sunflower plants were exposed to 0.8 g kg−1 Mn for one week and then treated with chelating agents, i.e., CA (10 mmol kg−1), and different concentrations of ALA and SA for one week. The results showed that Mn-treated plants had significantly increased H2O2, O2− and MDA contents in leaves compared with the control. Under the Mn + CA treatment, ALA or SA2 significantly activated the antioxidant defense system by increasing SOD, POD and CAT activities in leaves. Moreover, the application of CA significantly increased the Mn uptake in sunflower roots compared with Mn treatment alone; however, did not accelerate the translocation efficiency of Mn from sunflower roots to shoots. Moreover, ultrastructural and RT-qPCR results further demonstrated that ALA/SA could recover the adverse impact of excessive Mn accumulation in sunflowers. Like a pump, ALA/SA regulated the translocation efficiency and promoted the transportation of Mn from roots to shoots. This study provides insights into the promotive role of ALA/SA combined with CA on sunflower growth by regulating Mn absorption, which would be beneficial for regulating Mn absorption in soil with an Mn deficit.

Genome-Wide Identification and Expression Analysis Reveals Roles of the NRAMP Gene Family in Iron/Cadmium Interactions in Peanut

The natural resistance-associated macrophage protein (NRAMP) family plays crucial roles in metal uptake and transport in plants. However, little is known about their functions in peanut. To understand the roles of AhNRAMP genes in iron/cadmium interactions in peanut, genome-wide identification and bioinformatics analysis was performed. A total of 15 AhNRAMP genes were identified from the peanut genome, including seven gene pairs derived from whole-genome duplication and a segmental duplicated gene. AhNRAMP proteins were divided into two distinct subfamilies. Subfamily I contains eight acid proteins with a specific conserved motif 7, which were predicted to localize in the vacuole membrane, while subfamily II includes seven basic proteins sharing specific conserved motif 10, which were localized to the plasma membrane. Subfamily I genes contained four exons, while subfamily II had 13 exons. AhNRAMP proteins are perfectly modeled on the 5m94.1.A template, suggesting a role in metal transport. Most AhNRAMP genes are preferentially expressed in roots, stamens, or developing seeds. In roots, the expression of most AhNRAMPs is induced by iron deficiency and positively correlated with cadmium accumulation, indicating crucial roles in iron/cadmium interactions. The findings provide essential information to understand the functions of AhNRAMPs in the iron/cadmium interactions in peanuts.

Iron Nutrition in Plants: Towards a New Paradigm?

Iron (Fe) is an essential micronutrient for plant growth and development. Fe availability affects crops' productivity and the quality of their derived products and thus human nutrition. Fe is poorly available for plant use since it is mostly present in soils in the form of insoluble oxides/hydroxides, especially at neutral to alkaline pH. How plants cope with low-Fe conditions and acquire Fe from soil has been investigated for decades. Pioneering work highlighted that plants have evolved two different strategies to mine Fe from soils, the so-called Strategy I (Fe reduction strategy) and Strategy II (Fe chelation strategy). Strategy I is employed by non-grass species whereas graminaceous plants utilize Strategy II. Recently, it has emerged that these two strategies are not fully exclusive and that the mechanism used by plants for Fe uptake is directly shaped by the characteristics of the soil on which they grow (e.g., pH, oxygen concentration). In this review, recent findings on plant Fe uptake and the regulation of this process will be summarized and their impact on our understanding of plant Fe nutrition will be discussed.

Iron Availability and Homeostasis in Plants: A Review of Responses, Adaptive Mechanisms, and Signaling

Iron is an essential element for all living organisms, playing a major role in plant biochemistry as a redox catalyst based on iron redox properties. Iron is the fourth most abundant element of the Earth's crust, but its uptake by plants is complex because it is often in insoluble forms that are not easily accessible for plants to use. The physical and chemical speciation of iron, as well as rhizosphere activity, are key factors controlling the bioavailability of Fe. Iron can be under reduced (Fe²⁺) or oxidized (Fe³⁺) ionic forms, adsorbed onto mineral surfaces, forming complexes with organic molecules, precipitated to form poorly crystalline hydroxides to highly crystalline iron oxides, or included in crystalline Fe-rich mineral phases. Plants must thus adapt to a complex and changing iron environment, and their response is finely regulated by multiple signaling pathways initiated by a diversity of stimulus perceptions. Higher plants possess two separate strategies to uptake iron from rhizosphere soil: the chelation strategy and the reduction strategy in grass and non-grass plants, respectively. Molecular actors involved in iron uptake and mobilization through the plant have been characterized for both strategies. All these processes that contribute to iron homeostasis in plants are highly regulated in response to iron availability by downstream signaling responses, some of which are characteristic signaling signatures of iron dynamics, while others are shared with other environmental stimuli. Recent research has thus revealed key transcription factors, cis-acting elements, post-translational regulators, and other molecular mechanisms controlling these genes or their encoded proteins in response to iron availability. In addition, the most recent research is increasingly highlighting the crosstalk between iron homeostasis and nutrient response regulation. These regulatory processes help to avoid plant iron concentrations building up to potential cell functioning disruptions that could adversely affect plant fitness. Indeed, when iron is in excess in the plant, it can lead to the production and accumulation of dangerous reactive oxygen species and free radicals (H₂O₂, HO^•, O₂^•-, HO^•₂) that can cause considerable damages to most cellular components. To cope with iron oxidative stress, plants have developed defense systems involving the complementary action of antioxidant enzymes and molecular antioxidants, safe iron-storage mechanisms, and appropriate morphological adaptations.

The transcription factor WRKY12 negatively regulates iron entry into seeds in Arabidopsis

Yellow Stripe 1-Like 1 (YSL1) and Yellow Stripe 1-Like 3 (YSL3) transport metal-nicotianamine (NA) complexes to leaves, pollen, and developing seeds and play an important role in regulating iron (Fe) accumulation during the seed development and maturation stages; however, how their gene transcript levels are regulated remains unknown. In this study, we used yeast one-hybrid screening to identify a transcription factor, WRKY12, in Arabidopsis that directly regulates the transcription levels of YSL1 and YSL3 genes. WRKY12 has opposite expression patterns to YSL1 and YSL3. wrky12 mutants are tolerant to Fe deficiency, whereas WRKY12 overexpression lines are sensitive to Fe deficiency. During the development and maturation of seeds, WRKY12 can directly bind to the promoters of YSL1 and YSL3 and inhibit their expression. Genetic analysis showed that WRKY12 functions upstream of YSL1 and YSL3 in Fe intake during the seed development and maturation stages. Together, our results suggest that WRKY12 negatively regulates the iron intake in plant seeds by inhibiting the expression of YSL1 and YSL3.

Similarities and differences in iron homeostasis strategies between graminaceous and nongraminaceous plants

Iron (Fe) homeostasis is essential for both plant development and human nutrition. The maintenance of Fe homeostasis involves a complex network in which Fe signaling nodes and circuits coordinate tightly Fe transporters, ferric reductases, H⁺ -ATPases, low-molecular-mass metal chelators, and transporters of chelators and Fe-chelate complexes. Early-stage studies have revealed different strategies for Fe homeostasis between graminaceous and nongraminaceous plants. Recent progress has refreshed our understanding of previous knowledge, especially on the uptake, phloem transport and systemic signaling of Fe. This review attempts to summarize recent exciting and potentially influential studies on the various routes of Fe uptake and distribution in plants, focusing on breakthroughs that have changed our understanding of plant Fe nutrition.

The vacuolar transporter LaMTP8.1 detoxifies manganese in leaves of Lupinus albus

Manganese (Mn) is an essential microelement, but overaccumulation is harmful to many plant species. Most plants have similar minimal Mn requirements, but the tolerance to elevated Mn varies considerably. Mobilization of phosphate (P) by plant roots leads to increased Mn uptake, and shoot Mn levels have been reported to serve as an indicator for P mobilization efficiency in the presence of P deficiency. White lupin (Lupinus albus L.) mobilizes P and Mn with outstanding efficiency due to the formation of determinate cluster roots that release carboxylates. The high Mn tolerance of L. albus goes along with shoot Mn accumulation, but the molecular basis of this detoxification mechanism has been unknown. In this study, we identify LaMTP8.1 as the transporter mediating vacuolar sequestration of Mn in the shoot of white lupin. The function of Mn transport was demonstrated by yeast complementation analysis, in which LaMTP8.1 detoxified Mn in pmr1∆ mutant cells upon elevated Mn supply. In addition, LaMTP8.1 also functioned as an iron (Fe) transporter in yeast assays. The expression of LaMTP8.1 was particularly high in old leaves under high Mn stress. However, low P availability per se did not result in transcriptional upregulation of LaMTP8.1. Moreover, LaMTP8.1 expression was strongly upregulated under Fe deficiency, where it was accompanied by Mn accumulation, indicating a role in the interaction of these micronutrients in L. albus. In conclusion, the tonoplast-localized Mn transporter LaMTP8.1 mediates Mn detoxification in leaf vacuoles, providing a mechanistic explanation for the high Mn accumulation and Mn tolerance in this species.

POPEYE intercellular localization mediates cell-specific iron deficiency responses

Plants must tightly regulate iron (Fe) sensing, acquisition, transport, mobilization, and storage to ensure sufficient levels of this essential micronutrient. POPEYE (PYE) is an iron responsive transcription factor that positively regulates the iron deficiency response, while also repressing genes essential for maintaining iron homeostasis. However, little is known about how PYE plays such contradictory roles. Under iron-deficient conditions, pPYE:GFP accumulates in the root pericycle while pPYE:PYE-GFP is localized to the nucleus in all Arabidopsis (Arabidopsis thaliana) root cells, suggesting that PYE may have cell-specific dynamics and functions. Using scanning fluorescence correlation spectroscopy and cell-specific promoters, we found that PYE-GFP moves between different cells and that the tendency for movement corresponds with transcript abundance. While localization to the cortex, endodermis, and vasculature is required to manage changes in iron availability, vasculature and endodermis localization of PYE-GFP protein exacerbated pye-1 defects and elicited a host of transcriptional changes that are detrimental to iron mobilization. Our findings indicate that PYE acts as a positive regulator of iron deficiency response by regulating iron bioavailability differentially across cells, which may trigger iron uptake from the surrounding rhizosphere and impact root energy metabolism.

Ca2+-dependent phosphorylation of NRAMP1 by CPK21 and CPK23 facilitates manganese uptake and homeostasis in Arabidopsis

Homeostasis of the essential micronutrient manganese (Mn) is crucially determined through availability and uptake efficiency in all organisms. Mn deficiency of plants especially occurs in alkaline and calcareous soils, seriously restricting crop yield. However, the mechanisms underlying the sensing and signaling of Mn availability and conferring regulation of Mn uptake await elucidation. Here, we uncover that Mn depletion triggers spatiotemporally defined long-lasting Ca²⁺ oscillations in Arabidopsis roots. These Ca²⁺ signals initiate in individual cells, expand, and intensify intercellularly to transform into higher-order multicellular oscillations. Furthermore, through an interaction screen we identified the Ca²⁺-dependent protein kinases CPK21 and CPK23 as Ca²⁺ signal-decoding components that bring about translation of these signals into regulation of uptake activity of the high-affinity Mn transporter natural resistance associated macrophage proteins 1 (NRAMP1). Accordingly, a cpk21/23 double mutant displays impaired growth and root development under Mn-limiting conditions, while kinase overexpression confers enhanced tolerance to low Mn supply to plants. In addition, we define Thr498 phosphorylation within NRAMP1 as a pivot mechanistically determining NRAMP1 activity, as revealed by biochemical assays and complementation of yeast Mn uptake and Arabidopsis nramp1 mutants. Collectively, these findings delineate the Ca²⁺-CPK21/23-NRAMP1 axis as key for mounting plant Mn homeostasis.

The TaWRKY22–TaCOPT3D Pathway Governs Cadmium Uptake in Wheat

Cadmium (Cd) is a heavy metal nonessential for plants; this toxic metal accumulation in crops has significant adverse effects on human health. The crosstalk between copper (Cu) and Cd has been reported; however, the molecular mechanisms remain unknown. The present study investigated the function of wheat Cu transporter 3D (TaCOPT3D) in Cd tolerance. The TaCOPT3D transcripts significantly accumulated in wheat roots under Cd stress. Furthermore, TaCOPT3D-overexpressing lines were compared to wildtype (WT) plants to test the role of TaCOPT3D in Cd stress response. Under 20 mM Cd treatment, TaCOPT3D-overexpressing lines exhibited more biomass and lower root, shoot, and grain Cd accumulation than the WT plants. In addition, overexpression of TaCOPT3D decreased the reactive oxygen species (ROS) levels and increased the active antioxidant enzymes under Cd conditions. Moreover, the transcription factor (TF) TaWRKY22, which targeted the TaCOPT3D promoter, was identified in the regulatory pathway of TaCOPT3D under Cd stress. Taken together, these results show that TaCOPT3D plays an important role in regulating plant adaptation to cadmium stress through bound by TaWRKY22. These findings suggest that TaCOPT3D is a potential candidate for decreasing Cd accumulation in wheat through genetic engineering.

Quantitative Trait Locus Mapping of Marsh Spot Disease Resistance in Cranberry Common Bean (Phaseolus vulgaris L.)

Common bean (Phaseolus vulgaris L.) is a food crop that is an important source of dietary proteins and carbohydrates. Marsh spot is a physiological disorder that diminishes seed quality in beans. Prior research suggested that this disease is likely caused by manganese (Mn) deficiency during seed development and that marsh spot resistance is controlled by at least four genes. In this study, genetic mapping was performed to identify quantitative trait loci (QTL) and the potential candidate genes associated with marsh spot resistance. All 138 recombinant inbred lines (RILs) from a bi-parental population were evaluated for marsh spot resistance during five years from 2015 to 2019 in sandy and heavy clay soils in Morden, Manitoba, Canada. The RILs were sequenced using a genotyping by sequencing approach. A total of 52,676 single nucleotide polymorphisms (SNPs) were identified and filtered to generate a high-quality set of 2066 SNPs for QTL mapping. A genetic map based on 1273 SNP markers distributed on 11 chromosomes and covering 1599 cm was constructed. A total of 12 stable and 4 environment-specific QTL were identified using additive effect models, and an additional two epistatic QTL interacting with two of the 16 QTL were identified using an epistasis model. Genome-wide scans of the candidate genes identified 13 metal transport-related candidate genes co-locating within six QTL regions. In particular, two QTL (QTL.3.1 and QTL.3.2) with the highest R2 values (21.8% and 24.5%, respectively) harbored several metal transport genes Phvul.003G086300, Phvul.003G092500, Phvul.003G104900, Phvul.003G099700, and Phvul.003G108900 in a large genomic region of 16.8–27.5 Mb on chromosome 3. These results advance the current understanding of the genetic mechanisms of marsh spot resistance in cranberry common bean and provide new genomic resources for use in genomics-assisted breeding and for candidate gene isolation and functional characterization.

Azospirillum brasilense Bacteria Promotes Mn2+ Uptake in Maize with Benefits to Leaf Photosynthesis

Azospirillum brasilense is a prolific grass-root colonizing bacteria well-known for its ability to promote plant growth in several cereal crops. Here we show that one of the mechanisms of action in boosting plant performance is through increased assimilation of the micronutrient manganese by the host. Using radioactive ⁵²Mn²⁺ (t_½ 5.59 d), we examined the uptake kinetics of this micronutrient in young maize plants, comparing the performance of three functional mutants of A. brasilense, including HM053, a high auxin-producing and high N₂-fixing strain; ipdC, a strain with a reduced auxin biosynthesis capacity; and FP10, a strain deficient in N₂-fixation that still produces auxin. HM053 had the greatest effect on host ⁵²Mn²⁺ uptake, with a significant increase seen in shoot radioactivity relative to non-inoculated controls. LA-ICP-MS analysis of root sections revealed higher manganese distributions in the endodermis of HM053-inoculated plants and overall higher manganese concentrations in leaves. Finally, increased leaf manganese concentration stimulated photosynthesis as determined by measuring leaf fixation of radioactive ¹¹CO₂ with commensurate increases in chlorophyll concentration.

Duplication of NRAMP3 gene in poplars generated two homologous transporters with distinct functions

Transition metals are essential for a wealth of metabolic reactions, but their concentrations need to be tightly controlled across cells and cell compartments, as metal excess or imbalance has deleterious effects. Metal homeostasis is achieved by a combination of metal transport across membranes and metal binding to a variety of molecules. Gene duplication is a key process in evolution, as emergence of advantageous mutations on one of the copies can confer a new function. Here, we report that the poplar genome contains two paralogues encoding NRAMP3 metal transporters localized in tandem. All Populus species analyzed had two copies of NRAMP3, whereas only one could be identified in Salix species indicating that duplication occurred when the two genera separated. Both copies are under purifying selection and encode functional transporters, as shown by expression in the yeast heterologous expression system. However, genetic complementation revealed that only one of the paralogues has retained the original function in release of metals stored in the vacuole previously characterized in A. thaliana. Confocal imaging showed that the other copy has acquired a distinct localization to the Trans Golgi Network (TGN). Expression in poplar suggested that the copy of NRAMP3 localized on the TGN has a novel function in the control of cell-to-cell transport of manganese. This work provides a clear case of neo-functionalization through change in the subcellular localization of a metal transporter as well as evidence for the involvement of the secretory pathway in cell-to-cell transport of manganese.

Altered metal distribution in the sr45-1 Arabidopsis mutant causes developmental defects

The plant serine/arginine-rich (SR) splicing factor SR45 plays important roles in several biological processes, such as splicing, DNA methylation, innate immunity, glucose regulation, and abscisic acid signaling. A homozygous Arabidopsis sr45-1 null mutant is viable, but exhibits diverse phenotypic alterations, including delayed root development, late flowering, shorter siliques with fewer seeds, narrower leaves and petals, and unusual numbers of floral organs. Here, we report that the sr45-1 mutant presents an unexpected constitutive iron deficiency phenotype characterized by altered metal distribution in the plant. RNA-Sequencing highlighted severe perturbations in metal homeostasis, the phenylpropanoid pathway, oxidative stress responses, and reproductive development. Ionomic quantification and histochemical staining revealed strong iron accumulation in the sr45-1 root tissues accompanied by iron starvation in aerial parts. Mis-splicing of several key iron homeostasis genes, including BTS, bHLH104, PYE, FRD3, and ZIF1, was observed in sr45-1 roots. We showed that some sr45-1 developmental abnormalities can be complemented by exogenous iron supply. Our findings provide new insight into the molecular mechanisms governing the phenotypes of the sr45-1 mutant.

NRAMP6 and NRAMP1 cooperatively regulate root growth and manganese translocation under manganese deficiency in Arabidopsis

The essential micronutrient manganese (Mn) in plants regulates multiple biological processes including photosynthesis and oxidative stress. Some Natural Resistance-Associated Macrophage Proteins (NRAMPs) have been reported to play critical roles in Mn uptake and reutilization in low Mn conditions. NRAMP6 was demonstrated to regulate cadmium tolerance and iron utilization in Arabidopsis. Nevertheless, it is unclear whether NRAMP6 plays a role in Mn nutrition. Here, we report that NRAMP6 cooperates with NRAMP1 in Mn utilization. Mutation of NRAMP6 in nramp1 but not in a wild-type background reduces root growth and Mn translocation from the roots to shoots under Mn deficient conditions. Grafting experiments revealed that NRAMP6 expression in both the roots and shoots is required for root growth and Mn translocation under Mn deficiency. We also showed that NRAMP1 could replace NRAMP6 to sustain root growth under Mn deficiency, but not vice versa. Mn deficiency does not affect the transcript level of NRAMP6, but is able to increase and decrease the protein accumulation of NRAMP6 in roots and shoots, respectively. Furthermore, NRAMP6 can be localized to both the plasma membrane and endomembranes including the endoplasmic reticulum, and Mn deficiency enhances the localization of NRAMP6 to the plasma membrane in Arabidopsis plants. NRAMP6 could rescue the defective growth of the yeast mutant Δsmf2, which is deficient in endomembrane Mn transport. Our results reveal the important role of NRAMP6 in Mn nutrition and in the long-distance signaling between the roots and shoots under Mn deficient conditions.

A two-step adaptive walk rewires nutrient transport in a challenging edaphic environment

Most well-characterized cases of adaptation involve single genetic loci. Theory suggests that multilocus adaptive walks should be common, but these are challenging to identify in natural populations. Here, we combine trait mapping with population genetic modeling to show that a two-step process rewired nutrient homeostasis in a population of Arabidopsis as it colonized the base of an active stratovolcano characterized by extremely low soil manganese (Mn). First, a variant that disrupted the primary iron (Fe) uptake transporter gene (IRT1) swept quickly to fixation in a hard selective sweep, increasing Mn but limiting Fe in the leaves. Second, multiple independent tandem duplications occurred at NRAMP1 and together rose to near fixation in the island population, compensating the loss of IRT1 by improving Fe homeostasis. This study provides a clear case of a multilocus adaptive walk and reveals how genetic variants reshaped a phenotype and spread over space and time.

MTP8 from Triticum urartu Is Primarily Responsible for Manganese Tolerance

Mineral nutrients, such as manganese (Mn) and iron (Fe), play essential roles in many biological processes in plants but their over-enrichment is harmful for the metabolism. Metal tolerance proteins (MTPs) are involved in cellular Mn and Fe homeostasis. However, the transporter responsible for the transport of Mn in wheat is unknown. In our study, TuMTP8, a Mn-CDF transporter from diploid wheat (Triticum urartu), was identified. Expression of TuMTP8 in yeast strains of Δccc1 and Δsmf1 and Arabidopsis conferred tolerance to elevated Mn and Fe, but not to other metals (zinc, cobalt, copper, nickel, or cadmium). Compared with TuVIT1 (vacuole Fe transporter), TuMTP8 shows a significantly higher proportion in Mn transport and a smaller proportion in Fe transport. The transient analysis in tobacco epidermal cells revealed that TuMTP8 localizes to vacuolar membrane. The highest transcript levels of TuMTP8 were in the sheath of the oldest leaf and the awn, suggesting that TuMTP8 sequesters excess Mn into the vacuole in these organs, away from more sensitive tissues. These findings indicate that TuMTP8, a tonoplast-localized Mn/Fe transporter, functions as a primary balancer to regulate Mn distribution in T. urartu under elevated Mn conditions and participates in the intracellular transport and storage of excess Mn as a detoxification mechanism, thereby conferring Mn tolerance.

Infection by phloem-limited phytoplasma affects mineral nutrient homeostasis in tomato leaf tissues

Phytoplasmas are sieve-elements restricted wall-less, pleomorphic pathogenic microorganisms causing devastating damage to over 700 plant species worldwide. The invasion of sieve elements by phytoplasmas has several consequences on nutrient transport and metabolism, anyway studies about changes of the mineral-nutrient profile following phytoplasma infections are scarce and offer contrasting results. Here, we examined changes in macro- and micronutrient concentration in tomato plant upon 'Candidatus Phytoplasma solani' infection. To investigate possible effects of 'Ca. P. solani' infection on mineral element allocation, the mineral elements were separately analysed in leaf midrib, leaf lamina and root. Moreover, we focused our analysis on the transcriptional regulation of genes encoding trans-membrane transporters of mineral nutrients. To this aim, a manually curated inventory of differentially expressed genes encoding transporters in tomato leaf midribs was mined from the transcriptional profile of healthy and infected tomato leaf midribs. Results highlighted changes in ion homeostasis in the host plant, and significant modulations at transcriptional level of genes encoding ion transporters and channels.

Ca2+-dependent successive phosphorylation of vacuolar transporter MTP8 by CBL2/3-CIPK3/9/26 and CPK5 is critical for manganese homeostasis in Arabidopsis

Manganese (Mn) is an essential micronutrient for all living organisms. However, excess Mn supply that can occur in acid or waterlogged soils has toxic effects on plant physiology and development. Although a variety of Mn transporter families have been characterized, we have only a rudimentary understanding of how these transporters are regulated to uphold and adjust Mn homeostasis in plants. Here, we demonstrate that two calcineurin-B-like proteins, CBL2/3, and their interacting kinases, CIPK3/9/26, are key regulators of plant Mn homeostasis. Arabidopsis mutants lacking CBL2 and 3 or their interacting protein kinases CIPK3/9/26 exhibit remarkably high Mn tolerance. Intriguingly, CIPK3/9/26 interact with and phosphorylate the tonoplast-localized Mn and iron (Fe) transporter MTP8 primarily at Ser35, which is conserved among MTP8 proteins from various species. Mn transport complementation assays in yeast combined with multiple physiological assays indicate that CBL-CIPK-mediated phosphorylation of MTP8 negatively regulates its transport activity from the cytoplasm to the vacuole. Moreover, we show that sequential phosphorylation of MTP8, initially at Ser31/32 by the calcium-dependent protein kinase CPK5 and subsequently at Ser35 by CIPK26, provides an activation/deactivation fine-tuning mechanism for differential regulation of Mn transport. Collectively, our findings define a two-tiered calcium-controlled mechanism for dynamic regulation of Mn homeostasis under conditions of fluctuating Mn supply.

Calcium-permeable cation channels are involved in uranium uptake in Arabidopsis thaliana

Uranium (U) is a non-essential and toxic element that is taken up by plants from the environment. The assimilation pathway of U is still unknown in plants. In this study, we provide several evidences that U is taken up by the roots of Arabidopsis thaliana through Ca²⁺-permeable cation channels. First, we showed that deprivation of Arabidopsis plants with calcium induces a 1.5-fold increase in the capacity of roots to accumulate U, suggesting that calcium deficiency promotes the radionuclide import pathway. Second, we showed that external calcium inhibits U accumulation in roots, suggesting a common route for the uptake of both cations. Third, we found that gadolinium, nifedipine and verapamil inhibit the absorption of U, suggesting that different types of Ca²⁺-permeable channels serve as a route for U uptake. Last, we showed that U bioaccumulation in Arabidopsis mutants deficient for the Ca²⁺-permeable channels MCA1 and ANN1 is decreased by 40%. This suggests that MCA1 and ANN1 contribute to the absorption of U in different zones and cell layers of the root. Together, our results describe for the first time the involvement of Ca²⁺-permeable cation channels in the cellular uptake of U.

Root-to-shoot iron partitioning in Arabidopsis requires IRON-REGULATED TRANSPORTER1 (IRT1) protein but not its iron(II) transport function

IRON-REGULATED TRANSPORTER1 (IRT1) is the root high-affinity ferrous iron (Fe) uptake system and indispensable for the completion of the life cycle of Arabidopsis thaliana without vigorous Fe supplementation. Here we provide evidence supporting a second role of IRT1 in root-to-shoot partitioning of Fe. We show that irt1 mutants overaccumulate Fe in roots, most prominently in the cortex of the differentiation zone in irt1-2, compared to the wild type. Shoots of irt1-2 are severely Fe-deficient according to Fe content and marker transcripts, as expected. We generated irt1-2 lines producing IRT1 mutant variants carrying single amino-acid substitutions of key residues in transmembrane helices IV and V, Ser206 and His232, which are required for transport activity in yeast. Root short-term ⁵⁵ Fe uptake rates were uninformative concerning IRT1-mediated transport. Overall irt1-like concentrations of the secondary substrate Mn suggested that the transgenic Arabidopsis lines also remain incapable of IRT1-mediated root Fe uptake. Yet, IRT1_S206A partially complements rosette dwarfing and leaf chlorosis of irt1-2, as well as root-to-shoot Fe partitioning and gene expression defects of irt1-2, all of which are fully complemented by wild-type IRT1. Taken together, these results suggest a regulatory function for IRT1 in root-to-shoot Fe partitioning that does not require Fe transport activity of IRT1. Among the genes of which transcript levels are partially dependent on IRT1, we identify MYB DOMAIN PROTEIN10, MYB DOMAIN PROTEIN72 and NICOTIANAMINE SYNTHASE4 as candidates for effecting IRT1-dependent Fe mobilization in roots. Understanding the biological functions of IRT1 will help to improve Fe nutrition and the nutritional quality of agricultural crops.

Root-to-shoot iron partitioning in Arabidopsis requires IRON-REGULATED TRANSPORTER1 (IRT1) protein but not its iron(II) transport function

IRON-REGULATED TRANSPORTER1 (IRT1) is the root high-affinity ferrous iron (Fe) uptake system and indispensable for the completion of the life cycle of Arabidopsis thaliana without vigorous Fe supplementation. Here we provide evidence supporting a second role of IRT1 in root-to-shoot partitioning of Fe. We show that irt1 mutants overaccumulate Fe in roots, most prominently in the cortex of the differentiation zone in irt1-2, compared to the wild type. Shoots of irt1-2 are severely Fe-deficient according to Fe content and marker transcripts, as expected. We generated irt1-2 lines producing IRT1 mutant variants carrying single amino-acid substitutions of key residues in transmembrane helices IV and V, Ser206 and His232, which are required for transport activity in yeast. Root short-term ⁵⁵ Fe uptake rates were uninformative concerning IRT1-mediated transport. Overall irt1-like concentrations of the secondary substrate Mn suggested that the transgenic Arabidopsis lines also remain incapable of IRT1-mediated root Fe uptake. Yet, IRT1_S206A partially complements rosette dwarfing and leaf chlorosis of irt1-2, as well as root-to-shoot Fe partitioning and gene expression defects of irt1-2, all of which are fully complemented by wild-type IRT1. Taken together, these results suggest a regulatory function for IRT1 in root-to-shoot Fe partitioning that does not require Fe transport activity of IRT1. Among the genes of which transcript levels are partially dependent on IRT1, we identify MYB DOMAIN PROTEIN10, MYB DOMAIN PROTEIN72 and NICOTIANAMINE SYNTHASE4 as candidates for effecting IRT1-dependent Fe mobilization in roots. Understanding the biological functions of IRT1 will help to improve Fe nutrition and the nutritional quality of agricultural crops.

Knockout of FER decreases cadmium concentration in roots of Arabidopsis thaliana by inhibiting the pathway related to iron uptake

Identifying the genes that affect cadmium (Cd) accumulation in plants is a prerequisite for minimizing dietary Cd uptake from contaminated edible parts of plants by genetic engineering. This study showed that Cd stress inhibited the expression of FERONIA (FER) gene in the roots of wild-type Arabidopsis. Knockout of FER in fer-4 mutants downregulated the Cd-induced expression of several genes related to iron (Fe) uptake, including IRT1, bHLH38, NRAMP1, NRAMP3, FRO2 andFIT. In addition, the Cd concentration in fer-4 mutant roots reduced to approximately half of that in the wild-type seedlings. As a result, the Cd tolerance of fer-4 was higher. Furthermore, increased Fe supplementation had little effect on the Cd tolerance of fer-4 mutants, but clearly improved the Cd tolerance of wild-type seedlings, showing that the alleviation of Cd toxicity by Fe depends on the action of FER. Taken together, the findings demonstrate that the knockout of FER might provide a strategy to reduce Cd contamination and improve the Cd tolerance in plants by regulating the pathways related to Fe uptake.

A quick journey into the diversity of iron uptake strategies in photosynthetic organisms

Iron (Fe) is involved in multiple processes that contribute to the maintenance of the cellular homeostasis of all living beings. In photosynthetic organisms, Fe is notably required for photosynthesis. Although iron is generally abundant in the environment, it is frequently poorly bioavailable. This review focuses on the molecular strategies that photosynthetic organisms have evolved to optimize iron acquisition, using Arabidopsis thaliana, rice (Oryza sativa), and some unicellular algae as models. Non-graminaceous plants, including Arabidopsis, take up iron from the soil by an acidification-reduction-transport process (strategy I) requiring specific proteins that were recently shown to associate in a dedicated complex. On the other hand, graminaceous plants, such as rice, use the so-called strategy II to acquire iron, which relies on the uptake of Fe³⁺ chelated by phytosiderophores that are secreted by the plant into the rhizosphere. However, apart these main strategies, accessory mechanisms contribute to robust iron uptake in both Arabidopsis and rice. Unicellular algae combine reductive and non-reductive mechanisms for iron uptake and present important specificities compared to land plants. Since the majority of the molecular actors required for iron acquisition in algae are not conserved in land plants, questions arise about the evolution of the Fe uptake processes upon land colonization.

Simultaneous hyperaccumulation of rare earth elements, manganese and aluminum in Phytolacca americana in response to soil properties

The plant Phytolacca americana L. simultaneously hyperaccumulates manganese (Mn) and rare earth elements (REEs), but the underlying mechanisms are largely unknown. In this study, P. americana and the corresponding rhizosphere soil samples were collected from an ion-adsorption REE mine area in China, and the elemental composition and soil properties were analyzed in order to explore the relationship between metal accumulation and soil properties. The results show that P. americana accumulates high concentrations of REEs (up to 1040 mg kg^-1), Mn (up to 10400 mg kg^-1) and aluminum (Al) (up to 5960 mg kg^-1) in leaves. The REE concentrations in leaves were positively correlated with those of Al, Fe and Zn, while light REE concentrations were negatively correlated with P concentrations (p < 0.05). The soil properties explained 81.7%, 72.9% and 67.1% of REEs, Mn and Al accumulated in P. americana, respectively. The variation of REE accumulation in P. americana was primarily explained by plant available P (24.4%), pH (12.9%), TOC (9.4%) and total P (7.7%). The accumulation of Mn was primarily explained by plant available REEs (42.9%) and available Al (13.1%) while Al in P. americana was primarily explained by soil pH (14.4%). This study suggests the potential by regulation of soil properties in improving the efficiency of phytoextraction for REEs by hyperaccumulators.

Zinc uptake and HMA4 activity are required for micro- and macroelement balance in tobacco (Nicotiana tabacum)

The pleiotropic effects of zinc deficiency on ion homeostasis have already been described in several plants. Tobacco (Nicotiana tabacum) heavy metal ATPases HMA4.1 and HMA4.2 are involved in zinc and cadmium root-to-shoot translocation. In previous research, we have shown that N. tabacum HMA4 RNAi plants and HMA4 double-nonsense mutants exhibit strongly reduced zinc and cadmium levels in leaves as well as stunted growth. In this study, the ionome and transcriptome of these lines were investigated to better characterize the effect of reduced zinc levels and to understand the impaired growth phenotype. We found that, under standard greenhouse fertilization rates, these lines accumulated up to 4- to 6-fold more phosphorus, iron, manganese, and copper than their respective controls. Under field conditions, HMA4 double-mutant plants also exhibited similar accumulation phenotypes, albeit to a lower extent. In both HMA4 RNAi plants and HMA4 mutants, transcription analysis showed a local zinc-deficiency response in leaves as well as an FIT1-mediated iron-deficiency response in roots, likely contributing to iron and manganese uptake at the root level. A phosphate-starvation response involving HHO2 was also observed in HMA4-impaired plant leaves. The high level of phosphorus observed in HMA4-impaired plants is correlated with leaf swelling and necrosis. The upregulation of aquaporin genes is in line with cellular water influx and the observed leaf swelling phenotype. These results highlight the involvement of HMA4 in zinc homeostasis and related regulatory processes that balance the micro- and macroelements in above-ground organs.

Restriction of iron loading into developing seeds by a YABBY transcription factor safeguards successful reproduction in Arabidopsis

Iron (Fe) storage in plant seeds is not only necessary for seedling establishment following germination but is also a major source of dietary Fe for humans and other animals. Accumulation of Fe in seeds is known to be low during early seed development. However, the underlying mechanism and biological significance remain elusive. Here, we show that reduced expression of Arabidopsis YABBY transcription factor INNER NO OUTER (INO) increases embryonic Fe accumulation, while transgenic overexpression of INO results in the opposite effect. INO is highly expressed during early seed development, and decreased INO expression increases the expression of NATURAL RESISTANCE-ASSOCIATED MACROPHAGE PROTEIN 1 (NRAMP1), which encodes a transporter that contributes to seed Fe loading. The relatively high embryonic Fe accumulation conferred by decreased INO expression is rescued by the nramp1 loss-of-function mutation. We further demonstrated that INO represses NRAMP1 expression by binding to NRAMP1-specific promoter region. Interestingly, we found that excessive Fe loading into developing seeds of ino mutants results in greater oxidative damage, leading to increased cell death and seed abortion, a phenotype that can be rescued by the nramp1 mutation. Taken together, these results indicate that INO plays an important role in safeguarding reproduction by reducing Fe loading into developing seeds by repressing NRAMP1 expression.