Integrating microbial siderophores into concepts of plant iron nutrition.

Iron is a crucial micronutrient for plants, but its availability in soil is often limited. Iron deficiency compromises plant growth, and low iron content in crops contributes substantially to the 'hidden hunger' that affects human health globally. The elucidation of Strategy I (reduction-based) and Strategy II (phytosiderophore-based) for iron acquisition was a milestone in plant biology and enabled the development of biofortification concepts. However, recent genetic evidence reveals that the boundary between the two strategies is blurred, with many plants possessing elements of both. Here we show that plant iron uptake mechanisms are more complex and diverse than the classical dichotomy suggests. We review evidence for this integrative view and highlight the critical role of microbial siderophores. We explain how plants access iron from microbial siderophores not only indirectly through Strategy I and II pathways but also via the direct uptake of iron-siderophore complexes, an overlooked mechanism that we introduce as Strategy III. We propose three potential routes for this direct uptake and conclude that harnessing Strategy III holds great potential for novel agricultural interventions to enhance iron biofortification and improve human health.