Agronomic biofortification in crops with Zn: the zinc-iron interaction dilemma

Agronomic biofortification of staple food crops with external supply of micronutrients offers a viable solution to alleviate the plant diet-based micronutrient deficiency, without any yield trade-off. Zinc (Zn) has been one of the key targeted micronutrients in the biofortification program of Sub-Saharan Africa, Middle East and South-East Asia. Agronomic Zn biofortification, while promising for addressing Zn deficiencies in humans, raises valid concerns regarding its potential impact on grain iron (Fe) content. This is because Zn and Fe can interact at multiple levels, within soil, plant root, shoot, leaves and grains during transport and translocation. Emerging evidences suggest that biofortifying crops with Zn may trigger systemic responses that alter Fe homeostasis, potentially lowering Fe accumulation (antagonistic) in edible tissues of staples. Nevertheless, there is a lack of clarity in the field studies on the effects of Zn biofortification on Fe levels. This review article aims to synthesize a robust narrative on Zn-Fe interactions in biofortified crops and possible causes of negative interaction within plant body. Though, the additive or synergistic interaction between these two essential micronutrients is advantageous in a Zn biofortification program; but such interaction may be highly local stress specific. On the contrary, any loss of Fe during the process is undesirable, as the nutritional significance of Fe is comparable to that of Zn in human health. Yet, unlike other nutrient interactions (e.g., phosphorus (P) × Zn, nitrogen (N) × Zn), their interaction (Zn-Fe) is much less examined. But now, amid reports on unwitting depleting trend in grain micronutrients content in cereals, observed across the globe over the past 70 years, Zn - Fe interaction should apparently be given importance. In this review, we also highlighted on some possible interventions, agronomic or genetic or integrated approaches, to discard such negative interactions between Zn and Fe for future biofortification program. Addressing this trade-off (Zn-Fe interactions in crops) is the key to ensure that biofortification strategies do not inadvertently raise other micronutrient deficiencies. A focus on feasible management practices to curb such negative interaction between Zn and Fe will determine the success of agronomic Zn biofortification.