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This study aimed to examine the effects of inoculation with wild type (Sp7) and mutant strains of Azospirillum brasilense on the Vicia sativa spp. nigra (vetch)- Rhizobium leguminosarum bv. viciae ( Rlv) symbiosis. The A. brasilense mutants were ipdC and napA, impaired in indole pyruvate decarboxylase and periplasmic nitrate reductase, respectively; and acdS, carrying the ACC deaminase gene. Inoculations were done in pots, pouches and hydroponics and we measured shoot and root weight parameters as well as effects on root morphology and nod gene induction ability by roots. In pots, wild type Sp7 and the acdS strain, but ipdC and napA mutants, lead to an increase in root hair density, 3-4 cm above the root tip. In pouches, combined inoculation with Rlv and strains Sp7, acdS or ipdC, but napA, increased shoot dry matter and nodulation relative to Rlv alone. In a hydroponic system, co-inoculation with strains Sp7 or acdS, but with ipdC and napA mutants, enhanced root secretion of nod gene-inducing flavonoids in comparison with Rlv-inoculated plants. These results support that auxin production by A. brasilense has a positive effect on root secretion of nod gene-inducing flavonoids and auxin absorption activity by the plant. [ABSTRACT FROM AUTHOR]