Iron-Ferritin accumulation in fruits of two <i>Tristerix</i> epiphytes plants: ecological relevance as nutritional source for frugivores

Tristerix corymbosus and Tristerix Aphyllus are keystone mistletoes in southern South American ecosystems, providing critical nutritional resources to frugivores during winter, a period of severe food limitation. However, despite their ecological relevance, how iron is stored in Tristerix fruits and embryos, and the developmental mechanisms underlying this process, remain unknown. Here, we investigated iron accumulation, storage forms, and hormonal sensitivity in the fruits and embryos of two closely related epiphytic mistletoes, Tristerix corymbosus (hemiparasitic) and T. aphyllus (holoparasitic). Using histological analyses, immunodetection, and elemental quantification, we show that iron in both species predominantly accumulates as iron-FERRITIN complexes within the embryo, a storage form of iron associated with high bioavailability for animal consumers. Total iron concentrations in mature fruits indicate that both species represent a nutritionally relevant iron source in the trophic chain. Notably, embryos of both species remain green at maturity, suggesting altered regulation of late embryogenesis. We further demonstrate that T. corymbosus embryos are highly insensitive to abscisic acid (ABA) during germination, in contrast to Arabidopsis thaliana, despite exhibiting elevated endogenous ABA levels. This reduced ABA sensitivity correlates with chlorophyll retention, early xylem differentiation, and strong accumulation of FERRITIN proteins in the embryo, linking hormonal regulation to iron storage strategy. Our findings reveal a previously unrecognized connection between ABA sensitivity, embryo developmental state, and iron storage form in epiphytic mistletoes. These traits likely enhance seedling establishment and increase the nutritional value of fruits for frugivores, underscoring the central ecological role of T. corymbosus and T. aphyllus in winter food webs and nutrient flow in terrestrial ecosystems.