Vacuolar iron stores gated by NRAMP3 and NRAMP4 are the primary source of iron in germinating seeds

During seed germination, iron (Fe) stored in vacuoles is exported by the redundant NRAMP3 and NRAMP4 transporter proteins. A double nramp3 nramp4 mutant is unable to mobilize Fe stores and does not develop in the absence of external Fe. We used RNA sequencing to compare gene expression in nramp3 nramp4 and wild type during germination and early seedling development. Even though sufficient Fe was supplied, the Fe-responsive transcription factors bHLH38, 39, 100, and 101 and their downstream targets FRO2 and IRT1 mediating Fe uptake were strongly upregulated in the nramp3 nramp4 mutant. Activation of the Fe deficiency response was confirmed by increased ferric chelate reductase activity in the mutant. At early stages, genes important for chloroplast redox control (FSD1 and SAPX), Fe homeostasis (FER1 and SUFB), and chlorophyll metabolism (HEMA1 and NYC1) were downregulated, indicating limited Fe availability in plastids. In contrast, expression of FRO3, encoding a ferric reductase involved in Fe import into the mitochondria, was maintained, and Fe-dependent enzymes in the mitochondria were unaffected in nramp3 nramp4. Together, these data show that a failure to mobilize Fe stores during germination triggered Fe deficiency responses and strongly affected plastids, but not mitochondria

Bypassing Iron Storage in Endodermal Vacuoles Rescues the Iron Mobilization Defect in the Natural Resistance Associated-Macrophage Protein3natural Resistance Associated-Macrophage Protein4 Double Mutant

To improve seed iron (Fe) content and bioavailability, it is crucial to decipher the mechanisms that control Fe storage during seed development. In Arabidopsis (Arabidopsis thaliana) seeds, most Fe is concentrated in insoluble precipitates, with phytate in the vacuoles of cells surrounding the vasculature of the embryo. NATURAL RESISTANCE ASSOCIATED-MACROPHAGE PROTEIN3 (AtNRAMP3) and AtNRAMP4 function redundantly in Fe retrieval from vacuoles during germination. When germinated under Fe-deficient conditions, development of the nramp3nramp4 double mutant is arrested as a consequence of impaired Fe mobilization. To identify novel genes involved in seed Fe homeostasis, we screened an ethyl methanesulfonate-mutagenized population of nramp3nramp4 seedlings for mutations suppressing their phenotypes on low Fe. Here, we report that, among the suppressors, two independent mutations in the VACUOLAR IRON TRANSPORTER1 (AtVIT1) gene caused the suppressor phenotype. The AtVIT1 transporter is involved in Fe influx into vacuoles of endodermal and bundle sheath cells. This result establishes a functional link between Fe loading in vacuoles by AtVIT1 and its remobilization by AtNRAMP3 and AtNRAMP4. Moreover, analysis of subcellular Fe localization indicates that simultaneous disruption of AtVIT1, AtNRAMP3, and AtNRAMP4 limits Fe accumulation in vacuolar globoids

Editorial: Iron Nutrition and Interactions in Plants

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Manganese matters: feeding manganese into the secretory system for cell wall synthesis

International audienc

Autophagy is essential for optimal translocation for iron to seeds in Arabidopsis

Micronutrient deficiencies affect a large part of the world population. They are mostly due to the consumption of grains with insufficient content of Fe or Zn. It is therefore important to improve our knowledge of the mechanisms of micronutrient loading to seeds. Nutrient loaded in seeds originate either from de novo uptake by roots or recycling from leaves. Autophagy is a conserved mechanism for nutrient recycling in eukaryotes and was shown to be involved in nitrogen remobilization to seeds. Measuring the distribution of metal nutrients at the end of the life in Arabidopsis thaliana plants impaired in autophagy, we have investigated the role of autophagy in metal micronutrient translocation to seeds. We found that several Arabidopsis genotypes impaired in autophagy display defects in nutrient remobilization to seeds. In atg5-1, which is completely defective in autophagy, the efficiency of Fe translocation from vegetative organs to seeds was severely decreased even when Fe was provided during seed formation. Combining atg5-1 with sid2 mutation that counteracts premature senescence associated to autophagy deficiency and using 57Fe pulse labelling, we could propose a two step mechanism in which iron taken up de novo during seed formation is first accumulated in vegetative organs and subsequently remobilized to seeds. Finally, we showed that translocations of zinc and manganese to seeds are also dependent on autophagy. Our results highlight the importance of autophagy for optimal micronutrient remobilization to seeds. Fine tuning autophagy during seed formation opens new possibilities to improve this trait

Autophagy as a possible mechanism for micronutrient remobilization from leaves to seeds.

Seed formation is an important step of plant development which depends on nutrient allocation. Uptake from soil is an obvious source of nutrients which mainly occurs during vegetative stage. Because seed filling and leaf senescence are synchronized, subsequent mobilization of nutrients from vegetative organs also play an essential role in nutrient use efficiency, providing source-sink relationships. However, nutrient accumulation during the formation of seeds may be limited by their availability in source tissues. While several mechanisms contributing to make leaf macronutrients available were already described, little is known regarding micronutrients such as metals. Autophagy, which is involved in nutrient recycling, was already shown to play a critical role in nitrogen remobilization to seeds during leaf senescence. Because it is a non-specific mechanism, it could also control remobilization of metals. This article reviews actors and processes involved in metal remobilization with emphasis on autophagy and methodology to study metal fluxes inside the plant. A better understanding of metal remobilization is needed to improve metal use efficiency in the context of biofortification

Importing Manganese into the Chloroplast: Many Membranes to Cross

International audienc

The iron will of the research community: advances in iron nutrition and interactions in lockdown times

International audienc

Mobilization of iron from intracellular stores

International audiencePlants rely on iron (Fe) for a wealth of biochemical reactions including photosynthesis, respiration and reactive oxygen detoxification. They use two main strategies to mobilize Fe from the soil. Strategy I relies on reduction of FeIII by a membrane ferric chelate reductase and the secretion of coumarins to mobilize soil Fe, making it available for uptake by the divalent metal transporter IRT1. Strategy II, which is used only by graminaceous species, relies on the secretion of phytosiderophores. At some stages, intracellular stores play a critical role in Fe supply. In a plant cell, Fe may be stored in vacuoles or in plastids. We have shown that AtNRAMP3 and 4 are required for Fe mobilisation from embryo vacuoles during germination (Lanquar et al., 2005, EMBO J., 24, 4041-4051). When germinated under Fe deficiency, nramp3nramp4 mutants are strongly chlorotic and their development is arrested. To identify new players in intracellular Fe homeostasis, we have looked for mutations that suppress nramp3nramp4 chlorotic phenotype. In one of the suppressors, the causal mutation affected VIT1, the gene encoding the transporter for Fe uptake into the vacuole (Mary et al., 2015, Plant Physiol. 169(1):748-59). In another one, a mutation in the gene encoding the Pleckstrin Homology protein 1 shifted the localization of AtNRAMP1 from the Trans Golgi Network to the vacuolar membrane where it could compensate for the lack of AtNRAMP3 and 4 (Agorio et al., 2017, PNAS, 114(16):E3354-E3363)

Drought Tolerant Transgenic Plants Expressing Antisense Transcript of a Plant Homolog of Mammalian Glutamate Receptors

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