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Mixed inoculation of the grain legume faba bean (Vicia faba L.) with Rhizobium leguminosarum bv. viceae strain Z25 and ®ve different Azotobacter chroococcum or A. vinelandii strains, under gnotobiotic conditions of culture, resulted in signi®cant effects on nodulation, plant growth and nitrogenase activity (acetylene-reduction activity, ARA) of nodulated roots at thē owering stage. A. chroococcum strain H23 and A. vinelandii strains ATCC 12837 and Dv42 promoted plant growth, signi®cantly increasing dry matter accumulation in all plant parts including nodules, as well as total N content. A. chroococcum strain DR26 increased ARA of nodulated roots over 100% when compared with control plants inoculated with Rhizobium alone. Only A. chroococcum strain DR25 decreased plant growth. The effects of each Azotobacter strain are signi®cantly related to viable cell numbers applied as inocula. #

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Large amounts of amino acids are produced by nitrogen-fixing bacteria such as Azotobacter, Azospirillum, Rhizobium, Mesorhizobium and Sinorhizobium when growing in culture media amended with different carbon and nitrogen sources. This kind of bacteria live in close association with plant roots enhanced plant growth mainly as a result of their ability to fix nitrogen, improving shoot and root development suppression of pathogenic bacteria and fungi, and increase of available P concentration. Also, it has been strongly evidenced that production of biologically substances such as amino acids by these rhizobacteria are involved in many of the processes that explain plant-grown promotion. This paper reviews literature concerning amino acids production by nitrogen-fixing bacteria. The role of amino acids in microbial interactions in the rhizosphere and establishment of plant bacterial association is also discussed.