Some soil microorganisms are involved in the complex interactions with plants and earthworms, through the production of indole acetic acid (IAA) which modifies plant growth and development. In a factorial experiment testing the impact of the presence/absence of plants and earthworms on IAA production by cultivable bacteria, we observed that plants were decreasing IAA production of 43%, whereas earthworms were increasing it of 46%. In the presence of both plant and earthworms, IAA production was as low as in the presence of plant control, showing that plants influence on IAA production by microorganisms prevails on earthworm influence. We discuss functional reasons which could explain this result. 1. Introduction Soil microorganisms can be closely associated with soil animals such as earthworms for the delivery of ecosystem functions such as organic matter mineralization [1–3]. The release of nutrients due to this dual interaction has consequences on plant growth [4–6]. In addition to nutrients, interdependence between microorganisms, plants, and earthworms has led to the selection of diversified signal molecules, from low-molecular weight compounds such as auxin (e.g., IAA), amino acids, and secondary metabolites, to high-molecular weight compounds such as proteins and polysaccharides (for review see [7]). Emission of signal molecules by microorganisms can be deleterious for plants in the case of pathogenic bacteria and invertebrate herbivores, or positive with mycorrhizas, nitrogen-fixing symbiotic bacteria, or free living bacteria such as plant growth promoting rhizobacteria (PGPR) [7–9]. Auxin-like compounds have also been detected in earthworm dejections [10, 11], and could be responsible for the positive effect of earthworms [12] and vermicompost [10] on plant growth. The same molecule can have positive or negative effects according to its concentration and plant sensitivity [13]. There are very few studies which consider simultaneously the impact of plants and earthworms on microbial activities. However, it is of prior importance to establish a hierarchy in biotic factors influencing soil microorganisms. The purpose of our study was to determine the hierarchy of biotic factors on IAA-producing bacteria: do microorganisms obey to plants or earthworms? 2. Materials and Methods To answer this question, we set up an experiment with 4 treatments: soil without macroorganisms (S), with the plant Arabidopsis thaliana (P), and with the earthworm Aporrectodea caliginosa (E), with both plant and earthworms (PE). Nine cm diameter-10？cm high PVC experimental
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