The aim of the study recently published in the journal Environmental Chemistry was to describe the mechanisms behind increased zinc (Zn) absorption by wheat roots due to the presence of citrate and histidine.

The various hypothetical mechanisms of Zn absorption in the presence of metal ligands were formalised in numerical models. The simulated absorption of Zn and ligands (citrate or histidine) was compared with that measured in three hydroponic experiments with two bread wheat cultivars.

The apoplasmic flux induced by transpiration and the dissociation of complexes resulting from limited diffusion of Zn²⁺ cannot explain the sharp increase in Zn absorption by wheat roots in the presence of citrate or histidine. The transpiration-induced flux is also insufficient to explain the high absorption of histidine, whereas it provides an adequate explanation for the low absorption of citrate. 

The authors deduce from the models that wheat roots probably absorb histidine and the undissociated Zn-histidine complex via membrane transporters, according to a chemical potential gradient generated by the plant's energy conversion. Histidine and Zn-histidine could use different transport systems, or a common system with a lower affinity for the complex.

Interestingly, Zn-citrate complexes do not appear to be absorbed directly, but dissociate rapidly after forming a ternary Zn-citrate complex with a biotic ligand at the root cell membrane. The released metal ion is internalised and the ligand recycled into the rhizosphere solution. According to the Michaelis-Menten parameters of root absorption kinetics, the affinity of the transport systems was ranked in the following order: histidine > Zn²⁺ > Zn-histidine > Zn-citrate.

These results encourage research aimed at identifying histidine transporters and Zn complexes with histidine and citrate. It should be noted that zinc deficiency affects one third of the world's population, mainly due to insufficient zinc content in crops, particularly cereals. These results may help in the development of wheat varieties and cultivation practices that increase the zinc content in crops.

Contact: Thibault Sterckeman, Research Engineer, UMR LSE (UL-INRAE)

Référence :

Citrate and histidine increase root zinc uptake by different mechanisms in wheat. Sterckeman et al, Environmental chemistry 23 (2026), EN25092.doi:10.1071/EN25092

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Laboratoire Énergies & Mécanique Théorique et Appliquée (LEMTA)

Laboratoire Interdisciplinaire des Environnements Continentaux (LIEC)