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.
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Acknowledgments We thank B Glick, J Vanderleyden, U Priefer and A van Brussel for kindly supplying bacterial strains and F Temprano for kindly supplying vetch seeds for this study. This research was supported by grants numbers 261070509 and 277032209 from the Israeli Ministry of Agriculture. MS Dardanelli is member of the research career of Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina.
- Nitric oxide
- nod gene expression