Critical Reviews in Plant Sciences vol:14 pages:445-466
Recent advances in four aspects of the Azospririllum-plant root association, namely, the biosynthesis of phytohormones, nitrogen fixation, the mechanisms of plant root attachment, and genetic analysis of Azospirillum brasilense megaplasmids, are presented. At least three biosynthetic pathways are involved in indole-3-acetic acid (IAA) production in A. brasilense, that is, two Trp-dependent pathways identified as the indole-3-acetamide (IAM) pathway, the indole-3-pyruvic acid (IPyA) pathway, and a third as-yet unidentified pathway not using Trp as a precursor. So far, Azospirillum is the only bacterium for which a non-Trp-dependent route for IAA biosynthesis has been reported. The gene encoding the key enzyme in the IPyA pathway, indole-3-pyruvate decarboxylase, has been characterized recently. Azospirillum fixes atmospheric dinitrogen only at low oxygen tensions in media devoid of a combined nitrogen source. Both the synthesis and the activity of the nitrogenase enzyme are regulated according to these environmental conditions. The A. brasilense PII protein, encoded by the glnB gene, has been shown to play a central role in the regulation of nifHDK expression in response to the cellular nitrogen status by controlling the activity of the transcriptional activator protein, NifA. Posttranslational control of nitrogenase activity in response to ammonia and anaerobiosis involves the reversible ADP ribosylation of dinitrogenase reductase, mediated by the enzymes DraT and DraG, similar to what has been described for the photosynthetic bacterium Rhodospirillum rubrum. The structural genes draT and draC of A. brasilense and A. lipoferum have been cloned and sequenced. Mutational analysis of draT and draG has confirmed their roles in the regulation of nitrogenase activity. Attachment of Azospirillum to plant roots occurs in two distinct phases. The first, the adsorption phase, consists of a weak binding of the bacteria to the roots. In the second, or anchoring phase, the bacteria become irreversibly bound to the root surface. An as yet uncharacterized calcofluor-binding surface polysaccharide was found to be responsible for the anchoring of the bacteria to the roots, whereas the adsorption step is thought to be mediated by the polar flagellum. Detailed genetic analysis of the p90 megaplasmid of A. brasilense revealed, among other genes, the presence of numerous genes involved in the flagellation of Azospirillum.