An underground tale: contribution of microbial activity to plant iron acquisition

Iron (Fe) is an essential nutrient for plants, and it serves as a cofactor for a wide variety of cellular processes, such as oxygen transport, cellular respiration, chlorophyll biosynthesis, thylakoid biogenesis and chloroplast development. However, the Fe bioavailability in well-aerated soils is often severely limited, particularly in calcareous soils, which occupy 30 % of the Earth’s surface. Hence, Fe-deficiency-induced chlorosis is a serious problem leading to yield loss and reduced quality in agricultural production. Fe deficiency in plants is also closely related to the prevalence of Fe-deficiency-induced anaemia in humans.

Plants have evolved at least two mechanisms favouring efficient acquisition of Fe. Strategy I, which occurs in non-graminaceous plants, relies on acidification of the rhizosphere to increase the solubility of ferric Fe compounds through proton extrusion, trans-plasma membrane electron transfer to reduce Fe to its more soluble ferrous form via ferric chelate reductase (FRO2) and transport of Fe into root cells by iron-regulated transporter 1. Strategy II, which is utilized by the Gramineae, relies on extrusion of mugineic acid family phytosiderophores (MAs) via efflux transporter of MAs (e.g. TOM1) to solubilize Fe in the rhizosphere, and subsequent transport of the Fe(III)–phytosiderophore complex across the plasma membrane of the root epidermal cell via yellow stripe1 transporter 1.

These two strategies have been thought to ensure normal growth for many so-called ‘Fe-efficient’ plants under Fe-limited conditions. In the last decade, however, several lines of evidence have shown that these strategies alone are insufficient to prevent plants from suffering Fe deficiency in Fe-limited soils. How soil micro-organisms promote plant Fe acquisition is still largely unknown. Nevertheless, researchers have made great efforts to uncover this interesting and important underground mechanism in recent decades and have obtained many valuable clues. Based on these clues, this review discusses the possible mechanisms for soil micro-organism promotion of plant Fe acquisition.

Jin, C.W., Ye, Y.Q., & Zheng, S.J. An underground tale: contribution of microbial activity to plant iron acquisition via ecological processes. (2014) Annals of Botany, 113(1), 7-18.
Iron (Fe) deficiency in crops is a worldwide agricultural problem. Plants have evolved several strategies to enhance Fe acquisition, but increasing evidence has shown that the intrinsic plant-based strategies alone are insufficient to avoid Fe deficiency in Fe-limited soils. Soil micro-organisms also play a critical role in plant Fe acquisition; however, the mechanisms behind their promotion of Fe acquisition remain largely unknown.
This review focuses on the possible mechanisms underlying the promotion of plant Fe acquisition by soil micro-organisms.