Amélie Semblat

Abstract

Iron is an essential micronutrient for plants, indispensable for fundamental processes such as photosynthesis
and respiration. However, its low bioavailability in many soils, particularly calcareous or high-pH soils, limits
plant growth and causes iron chlorosis. Although plants have developed mechanisms to mobilize iron, these
strategies remain insufficient in soils poor in bioavailable iron. Bacteria of the genus Pseudomonas spp. are
known for their ability to colonize the rhizosphere, produce siderophores assimilable by plants, and thus
promote their growth and health. However, gaps remain concerning the role of rhizospheric microbial
communities in iron dynamics, as well as the impact of agroecological practices, such as cereal-legume
associations, on plant iron nutrition. In this context, this thesis focused on the influence of plants (species,
genotype, variety) and cereal-legume associations on the rhizospheric microbiota and plant iron content. Two
pea varieties (one tolerant and the other sensitive to iron chlorosis) and one wheat variety were studied, as well
as their associations on different soil types. In addition, several pea varieties, representative of two distinct
genotypes, were included in the study to broaden the scope of the results. Finally, the impact of recruited
microbiota on plant growth and nutrition was analyzed using Synthetic Communities (SynComs). The results
identified eleven key bacterial groups, notably Pseudomonadales and Enterobacterales, for their role in iron
mobilization. Pseudomonadales, present in all studied conditions, constitute a stable element of the wheat and
pea microbiota. In contrast, Enterobacterales showed interesting capabilities, such as iron phosphate
solubilization, and were preferentially associated with the iron chlorosis-sensitive pea variety in a specific soil,
suggesting targeted recruitment. A field study confirmed this preferential recruitment, which was correlated
with improved plant nutrition. The interactions between Pseudomonas and Enterobacter, studied in vitro,
revealed promising synergies in iron mobilization and phosphate solubilization. The constructed SynComs
demonstrated a beneficial effect on plant growth, although no significant effect on iron nutrition was detected.
In pea-wheat intercropping systems, wheat benefited from improved iron assimilation due to a better
acquisition strategy, without compromising the iron status of peas. A notable increase in the abundance of
Pseudomonas spp. in the rhizosphere of pea plants was also observed in intercropped cultures, suggesting that
the microbiota directly contributes to these benefits. In conclusion, this research highlights the crucial role of
plant-microbiota and plant-plant interactions in iron mobilization. Pseudomonadales, ubiquitous components of
microbiota associated with legumes and cereals, occupy a central place in this dynamic, while Enterobacterales
provide specific benefits depending on the context.