| Summary: | <i>Phaseolus vulgaris</i> L. (Fabaceae) forms symbioses with arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing rhizobia (NFB). The tripartite relationship uses molecular singals to establish intracellular symbioses in roots. The goal of this study was to determine if <i>Rhizobium tropici</i> CIAT 899 and exogenous riboflavin (vitamin B2) have an effect on AMF species selection and root colonization of <i>P. vulgaris</i>. Using SSU rRNA fragment amplification of DNA extracted from <i>P. vulgaris</i> roots, we found that the presence of <i>R. tropici</i> altered the relative distribution of AMF species. <i>Dominikia bernensis</i> (Ohel) was the most abundant AMF species in <i>P. vulgaris</i> roots but when <i>R. tropici</i> was co-inoculated, <i>Glomus</i> species dominated. Rhizobacteria such as <i>R. tropici</i>, secrete riboflavin and could affect AMF symbiosis. Addition of 50 μM riboflavin to <i>P. vulgaris,</i> increased plant growth (28%), dry nodule weight (18%), AMF colonization (248%) and mycorrhizal vesicle frequency (56%) in bean roots. 3.12 and 12.5 µM riboflavin favored the presence of <i>Glomus macrocarpum</i> in <i>P. vulgaris</i> roots. This work provides the basis to further study of rhizobial and mycorrhizal co-inoculation of <i>Phaseolus vulgaris</i> bean.
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