Iron (Fe) is necessary for all living cells, but its bioavailability is often limited. Fe deficiency limits agriculture in many areas and affects over a billion human beings worldwide. In mammals, NRAMP2/DMT1/DCT1 was identified as a major pathway for Fe acquisition and recycling. In plants, AtNRAMP3 and AtNRAMP4 are induced under Fe deficiency. The similitude of AtNRAMP3 and AtNRAMP4 expression patterns and their common targeting to the vacuole, together with the lack of obvious phenotype in nramp3-1 and nramp4-1 single knockout mutants, suggested a functional redundancy. Indeed, the germination of nramp3 nramp4 double mutants is arrested under low Fe nutrition and fully rescued by high Fe supply. Mutant seeds have wild type Fe content, but fail to retrieve Fe from the vacuolar globoids. Our work thus identifies for the first time the vacuole as an essential compartment for Fe storage in seeds. Our data indicate that mobilization of vacuolar Fe stores by AtNRAMP3 and AtNRAMP4 is crucial to support Arabidopsis early development until efficient systems for Fe acquisition from the soil take over.

Taking into account the strong iron competition in the rhizosphere and the high affinity of pyoverdines for Fe(III), these molecules are expected to interfere with the iron nutrition of plants, as they do with rhizospheric microbes. The impact of Fe-pyoverdine on iron content of Arabidopsis thaliana was compared with that of Fe-EDTA. Iron chelated to pyoverdine was incorporated in a more efficient way than when chelated to EDTA, leading to increased plant growth of the wild type. A transgenic line of A. thaliana overexpressing ferritin showed a higher iron content than the wild type when supplemented with Fe-EDTA but a lower iron content when supplemented with Fe-pyoverdine despite its increased reductase activity, suggesting that this activity was not involved in the iron uptake from pyoverdine. A mutant knock-out iron transporter IRT1 showed lower iron and chlorophyll contents when supplemented with Fe-EDTA than the wild type but not when supplemented with Fe-pyoverdine, indicating that, in contrast to iron from EDTA, iron from pyoverdine was not incorporated through the IRT1 transporter. Altogether these data suggest that iron from Fe-pyoverdine was not incorporated in planta through the strategy I, which is based on reductase activity and IRT1 transporter. This is supported by the presence of pyoverdine in planta as shown by enzyme-linked immunosorbent assay and by tracing 15N of 15N-pyoverdine.

Plant ferritins are key iron-storage proteins that share important structural and functional similarities with animal ferritins. However, specific features characterize plant ferritins, among which are plastid cellular localization and transcriptional regulation by iron. Ferritin synthesis is developmentally and environmentally controlled, in part through the differential expression of the various members of a small gene family. Furthermore, a strict requirement for plant ferritin synthesis regulation is attested to by alterations of the photosynthetic apparatus and of iron homeostasis in transgenic tobaccos overexpressing these proteins. Plant ferritin gene regulation appears to consist of a complex interplay of transcriptional and posttranscriptional mechanisms, involving cellular relays such as plant hormones, oxidative steps and Ser/Thr phosphatase.

SummaryIntracellular iron concentration requires tight control and is regulated both at the uptake and storage levels. Our knowledge of the role that the iron-storage protein ferritins play in plants is still very limited. Overexpression of this protein, either in the cytoplasm or the plastids of transgenic tobacco, was obtained by placing soybean ferritin cDNA cassettes under the control of the CAMV 35S promoter. The protein accumulated in 4-and 6-dayold seedlings and in leaves of 3-week-old plants but not in dry seeds or in 2-day-old seedlings, which is consistent with previous reports describing a post-transcriptional control of ferritin amounts during the germination process. Overaccumulated ferritin in leaves was correctly assembled as 24-mers. Transformants were more resistant to methylviologen toxicity, indicating that the transgenic ferritins were functional in vivo. Ferritin overaccumulation in transgenic tobacco leaves leads to an illegitimate iron sequestration. As a consequence, these transgenic plants behave as iron deficient and activate iron transport systems as revealed by an increase in root ferric reductase activity and in leaf iron content.