Abstract
Understanding how seeds obtain and store nutrients is key to developing crops with higher agronomic and nutritional value. We have uncovered unique patterns of micronutrient localization in seeds using synchrotron X-ray fluorescence (SXRF). Although all four members of the Arabidopsis thaliana Mn-CDF family can transport Mn, here we show that only mtp8-2 has an altered Mn distribution pattern in seeds. In an mtp8-2 mutant, Mn no longer accumulates in hypocotyl cortex cells and sub-epidermal cells of the embryonic cotyledons, but rather accumulates with Fe in the cells surrounding the vasculature, a pattern previously shown to be determined by the vacuolar transporter VIT1. We also show that MTP8, unlike the other three Mn-CDF family members, can transport Fe and is responsible for localization of Fe to the same cells that store Mn. When both the VIT1 and MTP8 transporters are non-functional, there is no accumulation of Fe or Mn in specific cell types; rather these elements are distributed amongst all cell types in the seed. Disruption of the putative Fe binding sites in MTP8 resulted in loss of ability to transport Fe but did not affect the ability to transport Mn.
Highlights
We are interested in seeds as sources of micronutrients due to the high prevalence of micronutrient deficiencies in many human populations that consume primarily plant-based diets[1, 2]
Examination of publically available gene expression data revealed that MTP8, MTP9, MTP10 and MTP11 are all expressed in seeds (Supplementary Fig. S2)
Of the four A. thaliana Mn-CDFs, MTP8 had the highest expression during embryogenesis, with expression increasing over the course of embryo development whereas MTP11 expression decreases during this same time period
Summary
We first asked if all of the Arabidopsis Mn-CDF proteins could functionally complement the Mn-hypersensitive yeast mutant Δpmr[1]. Mtp[] mutants have a phenotype when grown in alkaline soil (Fig. 2a) They are smaller, have lower chlorophyll content (Fig. 2b) and have significantly higher concentrations of Mn in their leaves than wild type plants (Fig. 2c). When grown on low Fe medium, both mtp[] and vit[1] exhibit a seedling lethal phenotype while the mtp8vit[1] double mutant shows growth similar to that of wild type (Fig. 2g). Proper localization of Mn in the embryo is essential for seed germination and seedling growth under high Mn stress or Fe deficiency conditions It is clear from the localization results that MTP8 must be able to transport Fe in addition to Mn. To confirm this, we tested whether MTP8 could complement the Fe hypersensitivity phenotype of the yeast Δccc[1] mutant. Our characterization of the MTP8 transporter has contributed to understanding how seeds store nutrients and how plants protect themselves from excess metals, both of which are key to developing crops with higher agronomic and nutritional value
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