However, for the convenience of the reader, these aspects and corresponding references are summarized in Tables 2 and 3, respectively. Auxins are a major class of phytohormones that are involved in the coordination of plant selleck screening library growth and development. The effects of azospirilla on plant root morphology (e.g.
elongation of primary roots and increase of the number and length of lateral roots) have been shown to correlate with exogenous levels of the auxin indole-3-acetate (IAA), evidencing that positive effects on roots upon inoculation with azospirilla are mainly owing to the production and secretion of IAA by these bacteria (Dobbelaere & Okon, 2007; Spaepen et al., 2007, 2009). In A. brasilense, 90% of IAA is produced by the indole-3-pyruvate (IPA) pathway in the presence of tryptophan (Vande Broek et al., 1999; Spaepen et al., 2007, 2009). In this bacterium, the rate-limiting step in IAA synthesis is catalyzed by the enzyme IPA decarboxylase, which catalyzes the conversion of IPA
into IAA, and is encoded by the ipdC gene. Transcription of the ipdC gene is positively regulated by its end product IAA, which constitutes a positive feedback loop regulation (Spaepen et al., 2007). Dual inoculation of several legumes with rhizobia and azospirilla significantly AC220 increases nodulation, nitrogen fixation, accumulation of macro- and microelements, and biomass as compared to inoculation with rhizobia alone Liothyronine Sodium (Helman et al., 2011; Table 2). An A. brasilense ipdC mutant was partially defective in nodulation and nitrogen fixation of common bean roots co-inoculated with rhizobia, in comparison with co-inoculation with the parental type Sp245. This indicates that there is a differential response of the plant roots to the auxin produced by bacteria (Remans et al., 2008). In agreement, recent experiments with vetch showed that the ipdC mutant induced
less root hair formation and induction of secretion of nod gene inducers by roots, relative to the wild type (Star et al., 2011). Moreover, comparison between the ipdC mutant and the wild type in inoculation experiments with wheat plants demonstrated a direct link between IAA production and effects on root morphology (Spaepen et al., 2008). When the native ipdC promoter was replaced by a constitutive or a plant-inducible promoter in strain Sp245, effects on root morphology were similar as those observed with the wild type, but at lower inoculum concentrations (Spaepen et al., 2008). The transcriptome of the ipdC mutant and the wild type were recently compared in absence or presence of exogenously added IAA by microarrays (Van Puyvelde et al., 2011). Inactivation of ipdC or addition of IAA resulted in broad transcriptional changes, leading to the conclusion that IAA is a signaling molecule in A. brasilense.