Azospirillum: benefits that go far beyond biological nitrogen fixation

Abstract

The genus Azospirillum comprises plant-growth-promoting bacteria (PGPB), which have been broadly studied. The benefits to plants by inoculation with Azospirillum have been primarily attributed to its capacity to fix atmospheric nitrogen, but also to its capacity to synthesize phytohormones, in particular indole-3-acetic acid. Recently, an increasing number of studies has attributed an important role of Azospirillum in conferring to plants tolerance of abiotic and biotic stresses, which may be mediated by phytohormones acting as signaling molecules. Tolerance of biotic stresses is controlled by mechanisms of induced systemic resistance, mediated by increased levels of phytohormones in the jasmonic acid/ethylene pathway, independent of salicylic acid (SA), whereas in the systemic acquired resistance—a mechanism previously studied with phytopathogens—it is controlled by intermediate levels of SA. Both mechanisms are related to the NPR1 protein, acting as a co-activator in the induction of defense genes. Azospirillum can also promote plant growth by mechanisms of tolerance of abiotic stresses, named as induced systemic tolerance, mediated by antioxidants, osmotic adjustment, production of phytohormones, and defense strategies such as the expression of pathogenesis-related genes. The study of the mechanisms triggered by Azospirillum in plants can help in the search for more-sustainable agricultural practices and possibly reveal the use of PGPB as a major strategy to mitigate the effects of biotic and abiotic stresses on agricultural productivity.

Assessing Forms of Application of Azospirillum brasilense Associated with Silicon Use on Wheat

“…One important feature of Ab-V5 and Ab-V6 is their capacity to induce genes related to the tolerance of biotic and abiotic stresses in plants [13,53], and the strains also carry several stress response genes, the majority of which are related to oxidative stresses. This growth promotion mechanisms might have improved the ability of the plants to more efficiently explore the soil, as indicated in previous studies using A. brasilense [4,[51][52][53][54][55].…”

“…Specifically, it was previously demonstrated in draft genome sequences that the strains Ab-V5 and Ab-V6 of A. brasilense, which were tested in the present study, carry similar nif and fix genes that confer their ability to fix atmospheric N [50]. Although the strains differ in their capacity to synthesize phytohormones [13,51,52], both share the same genes related to the synthesis of auxins. One important feature of Ab-V5 and Ab-V6 is their capacity to induce genes related to the tolerance of biotic and abiotic stresses in plants [13,53], and the strains also carry several stress response genes, the majority of which are related to oxidative stresses.…”

Humic Substances and Efficient Microorganisms: Elicitation of Medicinal Plants—A Review

“…The role of microorganisms applied to agriculture are diverse, for example, mycorrhizal arbuscular fungi have the ability to solubilize phosphorum (P) by secretion of phosphatases which degrade the P organic or the acids that dissolve inorganic compounds of P (Richardson et al, 2009;Yousefi et al, 2011). The free-living diazotrophic microorganisms (Azospirillum, Psudomonas and Cyanobacterium ) can fix nitrogen from the atmosphere to the soil to be mineralized and made it available to the plants in the form of ammonia they can also synthesize phytohormones (Fukami et al, 2018).…”

“…Facultative endophytic bacterium have the ability to colonize the interior and surface of the roots as a function of the chemotaxis of rhizosphere exudates (Ahemad & Kibret, 2014). They can convert atmospheric N to ammonium, produce phytohormones such as cytokinins, gibberilins and auxins (El-Hadi et al, 2009), increase water and mineral absorption, tolerance to abiotic stresses (Shameer & Prasad, 2018;Fukami et al, 2018;Silva & Pires, 2017), and are able to colonize the endorizosphere and xylem vessels (Fatma et al, 2008). Thus, there is a growing interest in inoculating microorganisms in vivo as they not only increase the productivity and phytochemical profile of medicinal plants but also reduce the use of chemical fertilizers (Adesemoye et al, 2009).…”

Inoculation of Wheat With Azospirillum spp.: A Comparison Between Foliar and In-furrow Applications

“…However, over the last 40 years, a wide array of investigations have confirmed the potential of the free-living bacteria Azospirillum spp. as fertilizer that, besides N fixation (Dobereiner & Day, 1976;Hungria et al, 2010) is also able to promote plant growth trough the production of phythormones (Dobbelaer et al,1999;Fukami et al, 2018). In fact, wheat and corn management based on partial mineral N fertilization coppled with Azospirillum inoculation has shown to be able meet the entire N requirement of these crops (Hungria et al, 2010;Cunha & Caierão, 2014;Marks et al, 2015).…”