Résumé
Iron is an essential micronutrient for plants and associated microorganisms. Iron nutrition of these organisms relies on the soil supply. However, bioavailability of iron in cultivated soils is low. Plants and microorganisms have thus evolved active strategies of iron uptake based on acidification, chelation, and/or reduction processes. Iron acquisition by these organisms leads to complex interactions ranging from mutualism to competition. In the rhizosphere, plants support abundant and active microbial communities through the release of rhizodeposits. Iron uptake by these microorganisms and by the host plant decrease even more the concentration of iron in solution. Therefore, there is an intense competition for iron among rhizosphere microorganisms, favoring those with the most efficient iron uptake strategy. This is the case for fluorescent Pseudomonas bacteria that synthesize siderophores, called pyoverdines or pseudobactines, which have a high affinity for iron and suppress fungal phytopathogens and deleterious microorganisms. Pyoverdines also elicit plant defense reactions and contribute to plant iron acquisition. Taken together, these mutual effects promote plant growth and health. However, competition for iron may also occur between plants and microbes during pathogenesis. Siderophores contribute to the iron uptake of the host plant and to the virulence of pathogens; conversely, host plants activate mechanisms aimed at depriving pathogens of nutritional iron. The iron-withholding mechanisms of the host plant rely on controlling its iron homeostasis. In this chapter, we describe the strategies of iron uptake of plants and microorganisms, the resulting complex interactions between them, and the challenges represented by their monitoring in agroecology.