Iron histolocalization in Amazon canga plants reveals phylogeny-driven adaptation strategies

Abstract

Iron (Fe) is an essential nutrient for plant growth and development; however, it can become toxic when present in excess in the plant tissues. This study examines the iron-regulating mechanisms in six species (three monocots and three eudicots) from three distinct canga physiognomies – open canga, grasslands, and shrublands – located in the iron-rich ecosystems of Serra dos Carajás, eastern Amazon (Brazil). Using histochemical analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy, we examined iron absorption, translocation, and accumulation in root and leaf tissues. Our findings revealed phylogenetically driven strategies, with distinct responses between monocots and eudicots. In monocots, such as Axonopus carajasensis, iron exclusion was facilitated by iron plaque formation and a robust endodermis, while iron was accumulated in bundle sheath cells of leaves to prevent toxicity. Conversely, eudicots like Mimosa acutistipula accumulated iron in mesophyll cells, with phenolic compounds aiding in chelation and oxidative stress mitigation. Trichomes on the adaxial surfaces of Paspalum cangarum and Brasilianthus carajensis appeared to trap iron, potentially shielding the stomata from clogging. Furthermore, the formation of aerenchyma in monocot roots improved oxygen transport and facilitated iron plaque formation in waterlogged conditions. These distinct phylogenetic responses may contribute to the coexistence of plant species from different evolutionary lineages across the environmental gradients of canga. These adaptive strategies highlight the ecological success of canga species in iron-rich environments, allowing them to thrive in extreme conditions.