Improved sample preparation for fruits allowed histochemistry and X-ray microscopy to reveal conserved iron hotspots

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 physiological significance remained largely unexplored. In this study, we investigated metal accumulation sites in fruits of 28 major crops by using X-ray and histochemical-based techniques. We found that calcium accumulated in the outermost hardened tissues, potassium in sugar accumulating fleshy tissues, and iron (Fe) in vascular tissues in a conserved manner. Vascular Fe pattern traced to the seed revealed an Fe reservoir at the fruit-seed juncture in tomato, which persisted in the seeds chalazal region upon dispersal. To determine the physiological function of these stored reserves, we manipulated Fe bioavailability. Two opposely acting chelators, desferoxamine, an Fe immobilizer, delayed germination, while nicotianamine, a mobilizer, accelerated it in wild-type plants but not in mutants with low chalazal Fe. Additionally, external Fe supplementation also increased germination speed in a dose-dependent manner. Collectively, these findings demonstrate that fruit vasculature serves as a critical delivery system to establish seed Fe pools, which are determinants of seed germination speed. This study provides the first comprehensive atlas of metal hotspots in fleshy fruits and links anatomical distribution to a defined physiological mechanism in seed biology.