REGULAR ARTICLE Selection of mixed inoculants exhibiting growth-promoting activity in rice plants from undefined consortia obtained by continuous enrichment Javier Vanegas & Daniel Uribe-Vélez Received: 6 June 2013 /Accepted: 22 October 2013 /Published online: 6 November 2013 # Springer Science+Business Media Dordrecht 2013 Abstract Aims The main objective of this study was to test the hypothesis that isolating and characterizing bacterial sus- pensions (undefined consortia) from the root/rhizosphere of rice will contribute to the selection of mixtures of rhizobacteria with better growth-promoting activity of rice plants. Methods Bacterial consortia were obtained from roots/rhizosphere soil samples of rice plants grown un- der upland and irrigated production systems. Those undefined consortia were subjected to five consecutive passes every 7 days in NFb (N-free broth) semisolid medium. Thereafter, strains of each growth-promoting consortia were isolated by plating on three different culture media. Then, undefined consortia, as well as mix and single bacterial strains, were characterized in terms of indoleacetic acid production, nitrogen fixation capacity, and growth promotion of rice plants. Results Of the 72 consortia analyzed, 41.7 % and 50.0 % increased nitrogenase activity and the produc- tion of indolic compounds, respectively, after 5 contin- uous passes in NFB medium. Three undefined consor- tia, 11 single strains and 5 strain mixtures, exhibited plant growth promotion in rice plants under greenhouse conditions. Conclusions Continuous enrichment in Nfb medium of undefined consortia from root/rhizosphere soil is a good strategy for the selection of plant growth-promoting bacteria for rice plants. Keywords Rice . Diazotrophs . Plant growth-promoting rhizobacteria . Microbial consortia Introduction Plant growth-promoting rhizobacteria (PGPR), such as Azotobacter , Clostridium, Azospirillum, Herbaspirillum, Burkholderia, Rhizobium (Choudhury and Kennedy 2004), and a variety of Enterobacteriaceas (Kennedy et al. 2004), have been previously isolated from the rhi- zosphere of rice based on their capacity to promote plant growth by N fixation, increase root surface area by the secretion of plant growth regulators such as indoleacetic acid (IAA), increase the availability of nutrients, or pro- mote growth by combined modes of action (Vessey 2003; Rodrigues et al. 2008). The activities of individual PGPR strains have been demonstrated through the development of mixed inoculants, thereby allowing fertilizer applica- tions to be decreased by up to 25 and 50 % for N and P, respectively, compared to uninoculated controls (Bashan et al. 2004). Mixed inoculants,defined as mixtures of two or more strains, have been produced with endophytic (Govindarajan et al. 2008) or rhizospheric PGPR with high nitrogenase activity (Alam et al. 2001), phosphate solubilization capacity, production of growth-regulating Plant Soil (2014) 375:215–227 DOI 10.1007/s11104-013-1960-1 Responsible Editor: Jesus Mercado-Blanco. J. Vanegas : D. Uribe-Vélez (*) Biotechnology Institute, Universidad Nacional de Colombia, Cra 30 45-03, A.A 14-490, Bogotá, Colombia e-mail: duribev@unal.edu.co