Toward Iron-Biofortified Rice: Integrating BRUTUS-Family Regulation and Iron Chaperone Biology

Abstract

Abstract Rice is both a leading model for cereal molecular genetics and one of the world’s most important staple foods, which makes it a decisive crop for iron (Fe) biofortification. However, the breeding goal is not simply to increase total plant Fe or even total grain Fe. For populations consuming polished rice, breeders must maximize Fe retained in the edible fraction, deposited in chemically useful forms, and maintained as nutritionally accessible after processing and digestion. This requirement exposes the limitations of older strategies focused only on stronger uptake or bulk transport. In this review, we argue that rice Fe biofortification should now be viewed as an integrated breeding problem spanning Fe acquisition, internal mobility, Fe sensing and prioritization, and intracellular Fe storage. We first summarize the classical rice framework that underpins biofortification, including Strategy II-type uptake, nicotianamine and deoxymugineic acid chemistry, long-distance transport, grain loading, ferritin buffering, and vacuolar sequestration. We then review the main breeding routes already used in rice, from natural variation, QTL analysis, and genome-wide association studies to transgenic gene stacking and genome editing. Particular emphasis is placed on BRUTUS-family/OsHRZ regulators, because they function as upstream Fe sensors and negative regulatory nodes whose perturbation can enhance Fe accumulation in shoots and grains. We also discuss the emerging concept of Fe chaperones, introduced by mammalian PCBP proteins and the recently proposed Medicago ICHAP pathway, as a potential next-generation precision layer for directing Fe to ferritin-rich or otherwise favorable grain sinks. We conclude that future high-Fe rice is most likely to emerge from phenotype-guided pyramiding of complementary alleles around a suitable OsHRZ allele, rather than from optimization of any single component or from a fixed sequence of trait layers.