Metal accumulation sites in fruits

Global fruit production suffers from pre- and post- harvest losses, part of which are related to metal deficiencies. Despite fruits being one of the most widely consumed plant parts, the spatial distribution of metals and their possible physiological impact remained largely unexplored. In this study, we searched for conserved metal accumulation sites in fruits of various crops and investigated their physiological function. By using X-ray and histochemical-based techniques, we found that calcium accumulated in the outermost hardened tissues, potassium in fleshy tissues, and iron (Fe) in vascular tissues. Furthermore, seeds accumulated Fe in the fruit-seed juncture, which remained in the chalazal part of the seed upon dispersal. In Arabidopsis thaliana, seed chalazal Fe formed a distinct circular shape, which appeared diffuse in metal tolerance protein 8 mutants. Seed chalazal Fe appeared also in tomato seeds, albeit more diffused and at higher concentrations. In tomato, treatment with the Fe immobilizer desferoxamine delayed germination in wild-type plants but not in mutants with low chalazal Fe, indicating that chalazal Fe may contribute to germination. This study marks the first systematic application of X-ray fluorescence and histochemistry to investigate conserved metal localization sites in mature fleshy fruits. The discovery of metal hotspots in fruits opens new research avenues to understand fruit physiology, which may, in the long run, contribute to food security.