Azospirillum: Diversity, Distribution, and Biotechnology Applications

Abstract

Plant growth-promoting bacteria (PGPB) are a well-studied group that can be found in the genus
Azospirillum. Plants benefit from Azospirillum inoculation because of the bacterium's ability to fix
atmospheric nitrogen and produce phytohormones like indole-3-acetic acid. Tolerance of abiotic and biotic
stressors in plants may be mediated by phytohormones serving as signaling molecules, and recent research
has assigned a significant role to Azospirillum in this process. In contrast to the systemic acquired
resistance, which has been studied in relation to phytopathogens, the tolerance of biotic stresses is regulated
by mechanisms of induced systemic resistance, mediated by elevated levels of phytohormones in the
jasmonic acid/ethylene pathway. The NPR1 protein, which functions as a co-activator in the activation of
defense genes, is involved in both of these processes. Induced systemic tolerance, which is mediated by
antioxidants, osmotic adjustment, generation of phytohormones, and defensive tactics such the expression
of pathogenesis-related genes, is another way by which Azospirillum may boost plant development. Insight
into the Azospirillum-induced plant processes may lead to the discovery of PGPB's role as a significant
method for reducing the negative impact of biotic and abiotic stressors on agricultural output. One of the
primary initiatives for reducing fertilizers consumption is the creation of cultivars with increased nitrogen
use efficiency (NUE), in combination with the application of plant growth-promoting bacteria. However, it
has been reported that the production of phytohormones by Azospirillum strains used in commercial
inoculants formulations plays a crucial role in plant growth promotion, so their use is generally
recommended in conjunction with regular N-fertilizer doses. In addition, there is still reported high
variability in the effectiveness of Azospirillum inoculants under field conditions, which means that the
inoculation technology is being adopted more as an additional management practice than as a replacement
for chemical inputs.