Physiological and Agronomical Aspects of Phytohormone Production by Model Plant-Growth-Promoting Rhizobacteria (PGPR) Belonging to the Genus Azospirillum

Abstract

The functional analysis of phytohormone production, interaction, and regulation in higher plants has re-emerged in the past 10 years due to spectacular advances in integrative study models. However, plants are not axenic in natural conditions and are usually colonized or influenced directly by different microorganisms such as rhizobacteria of which many have the ability to produce phytohormones. This review summarizes information related to the biosynthesis, metabolism, regulation, physiological role, and agronomical impact of phytohormones produced by the model plant-growth-promoting rhizobacteria (PGPR) belonging to the genus Azospirillum, considered to be one of the most representative PGPR. We include exhaustive information about the phytohormones auxins, gibberellins, cytokinins, ethylene, and abscisic acid, as well as the plant growth regulators polyamines and nitric oxide. We deal with their metabolism by Azospirillum sp. in chemically defined medium, in plant–microbe interactions, or in the context of the agronomical use of Azospirillum sp.