VIT1-dependent Fe distribution in seeds is conserved in dicots

Abstract

One third of the people suffer from iron (Fe) Fe deficiency. An underlying factor for this malnutrition is insufficient Fe intake from the diet. A major part of the human diet includes seeds of staple crops, which contain Fe that is poorly bioavailable. One reason for the low bioavailability is these seeds store Fe in cellular compartments that also contain antinutrients, such as phytate. Thus, several studies focused on decreasing phytate concentrations. As an alternative approach to increase bioavailable Fe, Fe reserves might be directed to cellular compartments such as plastids that are free of phytate. Previous studies indicated that Fe reserves can be relocalized inside the seed to the desired compartment by genetic modification, provided that a suitable iron transporter protein is used. However, to the best of our knowledge, a Fe transporter localizing to plastids have not been identified in seeds to date. To discover novel Fe transporters, we screened Fe patterns in seeds of distinct plant lineages, hypothesizing Fe hyperaccumulating sites would indicate Fe transporter presence. To this end, metal localizations in seeds of more than twenty species were investigated using histochemical or X-ray based techniques. Results showed that in Rosids, the largest clade of eudicots, Fe reserves were primarily confined in the embryo part of the seeds. Furthermore, inside the embryos, Fe was enriched in the endodermal cell layer, a well-known feature that is mediated by vacuolar Fe transporter, VIT1 in model plant Arabidopsis thaliana. This enrichment was well conserved in and beyond Rosid species. Finally, a few seeds showed novel Fe patterns, including Carica papaya which concentrated large Fe reserves exclusively in plastids called amyloplasts. Generally, Fe stored in amyloplast is considered bioavailable. Taken together, this study suggests dicot seeds store Fe mainly in the embryo, with a VIT1-dependent enrichment in its endodermal cell layer and indicate Carica papaya possess a strong Fe transporter at the plastid membrane. Once it is identified that might be useful in biofortification, as a novel tool to shift Fe to compartments where it is more bioavailable.