Metal accumulation in seeds is a prerequisite for germination and establishment of plants but also for micronutrient delivery to humans. To investigate metal transport processes and their interactions in seeds, we focused on METAL TOLERANCE PROTEIN8 (MTP8), a tonoplast transporter of the manganese (Mn) subclade of cation diffusion facilitators, which in Arabidopsis (Arabidopsis thaliana) is expressed in embryos of seeds. The x-ray fluorescence imaging showed that expression of MTP8 was responsible for Mn localization in subepidermal cells on the abaxial side of the cotyledons and in cortical cells of the hypocotyl. Accordingly, under low Mn availability, MTP8 increased seed stores of Mn, required for efficient seed germination. In mutant embryos lacking expression of VACUOLAR IRON TRANSPORTER1 (VIT1), MTP8 built up iron (Fe) hotspots in MTP8-expressing cells types, suggesting that MTP8 transports Fe in addition to Mn. In mtp8 vit1 double mutant seeds, Mn and Fe were distributed in all cell types of the embryo. An Fe transport function of MTP8 was confirmed by its ability to complement Fe hypersensitivity of a yeast mutant defective in vacuolar Fe transport. Imbibing mtp8-1 mutant seeds in the presence of Mn or subjecting seeds to wet-dry cycles showed that MTP8 conferred Mn tolerance. During germination, MTP8 promoted reallocation of Fe from the vasculature. These results indicate that cell type-specific accumulation of Mn and Fe in seeds depends on MTP8 and that this transporter plays an important role in the generation of seed metal stores as well as for metal homeostasis and germination efficiency under challenging environmental conditions.

Biology Centre of the Czech Academy of Sciences Institute of Plant Molecular Biology Department of Plant Biophysics and Biochemistry CZ 37005 České Budějovice Czech Republic

Department of Genetics and Bioengineering Izmir University of Economics 35330 Izmir Turkey

Diamond Light Source Harwell Science and Innovation Campus OX11 0DE Didcot United Kingdom

Laboratory of Plant Nutrition Department of Plant Science Faculty of Agriculture Utsunomiya University 321 8505 Utsunomiya Japan

Molecular Plant Nutrition Leibniz Institute for Plant Genetics and Crop Plant Research D 06466 Gatersleben Germany

Plant Nutrition Laboratory Institute of Agricultural and Nutritional Sciences Martin Luther University Halle Wittenberg D 06099 Halle Germany

Spring 8 Japan Synchrotron Radiation Research Institute 679 5198 Japan

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