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The limited growth and nutrition of plants growing in semi-arid zones may be overcome by the inoculation with selected soil microorganisms [bacteria or arbuscular mycorrhizal fungi (AMF)]. In this article we investigate how autochthonous AM fungi and the two most abundant cultivable bacterial groups, isolated from dried degraded (B1) and non-degraded (B2) soil, affect Trifolium repens growth under water limiting conditions. On the other hand, and with attempt to characterize biochemically both isolates, we also analysed the indole acetic acid (IAA) and proline production as well as the antioxidant response of both isolates subjected to an increasing osmotic stress degree caused by Polyethylene glycol (PEG). When the bacteria were grown in axenic culture at increasing osmotic stress caused by PEG levels (from 0 to 30%) they show different osmotic response. B2 produced the highest IAA and proline amount under the strongest stress condition (30%). Similarly, under 30% PEG, B2 showed 6 times less CAT and two times more APX than B1 while SOD resulted similar in both strains. Bacterial CAT and APX activities were more sensitive than SOD to osmotic stress which is an indication of bacterial response to drought and reflect the diversity and intrinsic osmotic stress tolerance of these both bacteria. AMF or bacterial inoculated plants widely decreased stomatal conductance and increased the relative water content, both values are important for plants growing in soil with water limitation. (C) 2011 Elsevier Masson SAS. All rights reserved.