Iron dynamics in the rhizosphere as a case study for analyzing interactions between soils, plants and microbes

Abstract

International audienceIron is an essential element for plants and microbes. However, in most cultivated soils, the concentration of iron available for these living organisms is very low because its solubility is controlled by stable hydroxides, oxyhydroxides and oxides. In the rhizosphere, there is a high demand of iron because of the iron uptake by plants, and microorganisms which density and activity are promoted by the release of root exudates. Plants and microbes have evolved active strategies of iron uptake. Iron incorporation by these organisms lead to complex interactions ranging from competition to mutualism. These complex interactions are under the control of physico-chemical properties of the soils in which they occur, and reciprocally iron uptake strategies of plants and microbes impact these soil properties. These iron-mediated nteractions between soils, plants and microbes impact the plant growth and health and their analysis, together with that of the resulting iron dynamics, is of a major agronomic interest. Analysis of the complex interactions soils, plants and microbes represent also a unique opportunity to progress in our knowledge of the rhizosphere ecology. This progression requires merging complementary expertises and study strategies in soil science, plant biology and microbiology. This review provides information on (i) iron status in soil and rhizosphere, iron uptake by plants and microbes, and on (ii) the corresponding study strategies. Finally, illustrations of how integration of these approaches allows gaining knowledge in the complex interactions occurring in the rhizosphere are given

Improved iron acquisition ofAstragalus sinicusunder low iron-availability conditions by soil-borne bacteriaBurkholderia cepacia

“…Although Fe is highly abundant in the earth's crust, it is exceedingly insoluble in alkaline soils. Fe often forms scarce solubility of Fe 3+ oxyhydroxides that are not easily assimilated by most plant species (Lemanceau et al 2009). Many crop plants such as barely and peanut often exhibit Fe deficiency-induced leaf chlorosis under alkaline conditions (Takahashi et al 2001;Xiong et al 2013).…”

Feruloyl-CoA 6'-Hydroxylase1-Dependent Coumarins Mediate Iron Acquisition from Alkaline Substrates in Arabidopsis

“…Despite its high abundance in soils, Fe precipitates with phosphates or hydroxyl ions in well-aerated soils already at slightly acidic to alkaline pH levels (Lemanceau et al, 2009;Marschner, 2012). These reactions make Fe sparingly soluble for microorganisms and plants, which are ultimately responsible for entering Fe into the food chain.…”

Ecophysiology of iron homeostasis in plants

“…In low-oxygen conditions, the cells will express a low-affinity ferrous ion transporter Fet4p, which functions as an independent oxygen system for iron transport into the cell. This low-affinity mechanism (which is not regulated by the presence of iron) also transports cobalt, cadmium and nickel (Lemanceau et al 2009;Marschner et al 2011;Shi et al 2011;Nagata et al 2013). Mycorrhizal fungi have different mechanisms to tolerate Fe toxicity, such as the production of melanin, release of siderophores to the external environment, Fe complexation in the cytosol and Fe storage in vacuoles.…”