Phytohormones and Other Plant Growth Regulators Produced by PGPR: The Genus Azospirillum

One of the first mechanisms proposed to explain the plant growth promotion due to bacterial inoculation has been the nitrogen biological fixation; however, this mechanism has been of less agronomic significance than was initially expected. In counterpart, other mechanisms have been proposed to explain these inoculation effects, such as production of phytohormones and other plant growth regulators. Nowadays, we know that benefic effect of inoculation with rhizobacteria can be correlated with production and metabolism of chemically defined compounds such as auxins, cytokinins, gibberellins, and other plant growth regulators, such as abscisic acid and nitric oxide. In this chapter, we highlight the analytical and biological methods used to identify and quantify the most relevant phytohormones and plant growth regulators produced by Azospirillum sp. and other benefic rhizobacteria.