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.