Chapter 13 - Azospirillum—a free-living nitrogen-fixing bacterium

Abstract

Azospirillum is a genus of plant growth promoting bacteria (PGPBs), which have been widely studied over the past few decades. In addition to fixing atmospheric nitrogen, Azospirillum inoculation also provides plants with phytohormones such as indole-3-acetic acid, which are essential for the growth of plants. According to some studies, Azospirillum seems to play an imperative role in conferring plant tolerance to abiotic and biotic stresses, perhaps through the release of phytohormones. As a result of an increase in phytohormone levels within the jasmonic acid/ethylene pathway, induced systemic resistance can control tolerance to biotic stresses in an independent manner from salicylic acid (SA). The mechanism by which systemic acquired resistance develops has been demonstrated previously to be the result of intermediate SA levels, studied previously about phytopathogenic organisms. As part of both mechanisms, the NPR1 protein plays a role in the induction of defense genes by acting as a coactivator. As a result of systemic tolerance, Azospirillum is capable of increasing plant growth under abiotic stresses by a variety of mechanisms including antioxidants, osmotic adjustment, phytohormones production, and defense strategies such as pathogen-related gene expression. If the mechanisms activated by Azospirillum were to be investigated in detail, it may be possible to use PGPB as a major strategy to mitigate the impacts of biotic and abiotic stresses on agricultural productivity.