Azospirillum is one of the most studied plant growth-promoting bacteria (PGPB); it represents a common model for plantbacterial interactions. While Azospirillum brasilense is the species that is most widely known, at least 22 species, including 17 firmly validated species, have been identified, isolated from agricultural soils as well as habitats as diverse as contaminated soils, fermented products, sulfide springs, and microbial fuel cells. Over the last 40 years, studies on Azospirillum-plant interactions have introduced a wide array of mechanisms to demonstrate the beneficial impacts of this bacterium on plant growth. Multiple phytohormones, plant regulators, nitrogen fixation, phosphate solubilization, a variety of small-sized molecules and enzymes, enhanced membrane activity, proliferation of the root system, enhanced water and mineral uptake, mitigation of environmental stressors, and competition against pathogens have been studied, leading to the concept of the Multiple Mechanisms Hypothesis. This hypothesis is based on the assumption that no single mechanism is involved in the promotion of plant growth; it posits that each case of inoculation entails a combination of a few or many mechanisms. Looking specifically at the vast amount of information about the stimulatory effect of phytohormones on root development and biological nitrogen fixation, the Efficient Nutrients Acquisition Hypothesis model is proposed. Due to the existence of extensive agriculture that covers an area of more than 60 million hectares of crops, such as soybeans, corn, and wheat, for which the bacterium has proven to have some agronomic efficiency, the commercial use of Azospirillum is widespread in South America, with over 100 products already in the market in Argentina, Brazil, and Uruguay. Studies on Azospirillum inoculation in several crops have shown positive and variable results, due in part to crop management practices and environmental conditions. The combined inoculation of legumes with rhizobia and Azospirillum (co-inoculation) has become an emerging agriculture practice in the last several years, mainly for soybeans, showing high reproducibility and efficiency under field conditions. This review also addresses the use of Azospirillum for purposes other than agriculture, such as the recovery of eroded soils or the bioremediation of contaminated soils. Furthermore, the synthetic mutualistic interaction of Azospirillum with green microalgae has been developed as a new and promising biotechnological application, extending its use beyond agriculture.

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Indole-3-acetic acid (IAA) is one of the most important molecules produced by Azospirillum sp., given that it affects plant growth and development. Azospirillum brasilense strains Sp245 and Az39 (pFAJ64) were pre-incubated in MMAB medium plus 100 mg/mL L-tryptophan and treated with or exposed to the following (a) abiotic and (b) biotic stress effectors: (a) 100 mM NaCl or NaSO 4.0% (w/v) PEG 0.5 mM HO 0.1 mM abscisic acid, 0.1 mM 1-aminocyclopropane 1-carboxylic acid, 45 °C or daylight, and (b) 4.0% (v/v) filtered supernatant of Pseudomonas savastanoi (Ps) or Fusarium oxysporum (Fo), 0.1 mM salicylic acid (SA), 0.1 mM methyl jasmonic acid (MeJA), and 0.01% (w/v) chitosan (CH). After 30 and 120 min of incubation, biomass production, cell viability, IAA concentration (µg/mL), and ipdC gene expression were measured. Our results show that IAA production increases with daylight or in the presence of PEG, ABA, SA, CH, and Fo. On the contrary, exposure to 45 °C or treatment with HO NaCl, NaSO ACC, MeJA, and Ps decrease IAA biosynthesis. In this report, growth and IAA biosynthesis in A. brasilense under biotic and abiotic stress conditions are discussed from the point of view of their role in bacterial lifestyle and their potential application as bioproducts.

Azospirillum sp. is one of the most studied genera of plant growth-promoting rhizobacteria (PGPR). The ability of Azospirillum sp. to promote plant growth has been associated with its ability to produce several phytohormones, such as auxins, gibberellins and cytokinins, but mainly indole-3-acetic acid (IAA). It has been propoosed that the production of IAA explains the positive effects of co-inoculation with Azospirillum sp. on the rhizobia-legume symbiosis. In this study, we constructed an IAA-deficient mutant of A. brasilense Az39 (ipdC − ) by using a restriction-free cloning method. We inoculated soybean seeds with 1•10 6 cfu•seed −1 of Bradyrhizobium japonicum E109 and co-inoculating leaves at the V3 stage with 1•10 8 cfu.plant −1 of A. brasilense Az39 wt or ipdC − or inoculated leaves with 20 μg.plant −1 synthetic IAA. The results confirmed soybean growth promotion as there was increased total plant and root length, aerial and root dry weight, number of nodules on the primary root, and an increase in the symbiosis established with B. japonicum E109. Nodule weight also increased after foliar co-inoculation with the IAA-producer A. brasilense Az39. The exogenous application of IAA decreased aerial and root length, as well as the number of nodules on primary roots in comparison with the Az39 wt strain. These results allow us to propose a biological model of response to foliar co-inoculation of soybean with IAA-producing rhizobacteria. This model clearly shows that both the presence of microorganism as part of the colonization process and the production of IAA in situ are co-responsible, via plant signaling molecules, for the positive effects on plant growth and symbiosis establishment.

We present the complete genome sequence of Azospirillum brasilense Az39, isolated from wheat roots in the central region of Argentina and used as inoculant in extensive and intensive agriculture during the last four decades. The genome consists of 7.39 Mb, distributed in six replicons: one chromosome, three chromids, and two plasmids.