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.

Aims The aim of this research was to analyse the quorum‐sensing (QS) and quorum‐quenching (QQ) mechanisms based on N‐acyl‐l‐homoserine lactones (AHLs) in Azospirillum brasilense Az39, a strain with remarkable capacity to benefit a wide range of crops under agronomic conditions. Methods and results We performed an in silico and in vitro analysis of the quorum mechanisms in A. brasilense Az39. The results obtained in vitro using the reporter strains Chromobacterium violaceum and Agrobacterium tumefaciens and liquid chromatography coupled with mass‐mass spectrometry analysis showed that although Az39 does not produce AHL molecules, it is capable of degrading them by at least two hypothetical enzymes identified by bioinformatics approach, associated with the bacterial cell. In Az39 cultures supplemented with 500 nmol l−1 of the C3 unsubstituted AHLs (C4, C6, C8, C10, C12, C14), AHL levels were lower than in noninoculated LB media controls. Similar results were observed upon the addition of AHLs with hydroxy (OH‐) and keto (oxo‐) substitutions in C3. These results not only demonstrate the ability of Az39 to degrade AHLs. They also show the wide spectrum of molecules that can be degraded by this bacterium. Conclusions Although A. brasilense Az39 is a silent bacterium unable to produce AHL signals, it is able to interrupt the communications between other bacteria and/or plants by a QQ activity. Significance and Impact of the Study This is the first report confirming by unequivocal methodology the ability of A. brasilense, one of the most agriculturally used benefic bacteria around the world, to degrade AHLs by a QQ mechanism.

The plant-growth-promoting bacterium Azospirillum brasilense is able to associate with the microalgae Chlorella sorokiniana. Attachment of A. brasilense increases the metabolic performances of the microalgae. Recent genome analyses have revealed that the A. brasilense Az39 genome contains two complete sets of genes encoding type VI secretion systems (T6SS), including the T6SS1 that is induced by the indole-3-acetic acid (IAA) phytohormone. The T6SS is a multiprotein machine, widespread in Gramnegative bacteria, that delivers protein effectors in both prokaryotic and eukaryotic cells. Here we show that the A. brasilense T6SS is required for Chlorella-Azospirillum synthetic mutualism. Our data demonstrate that the T6SS is an important determinant to promote production of lipids, carbohydrates and photosynthetic pigments by the microalgae. We further show that this is likely due to the role of the T6SS during the attachment stage and for the production of IAA phytohormones. Finally, we demonstrate that the A. brasilense T6SS provides antagonistic activities against a number of plant pathogens such as Agrobacterium, Pectobacterium, Dickeya and Ralstonia species in vitro, suggesting that, in addition to promoting growth, A. brasilense might confer T6SS-dependent bio-control protection to microalgae and plants against bacterial pathogens.

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Strain Az39T of Azospirillum is a diazotrophic plant growth-promoting bacterium isolated in 1982 from the roots of wheat plants growing in Marcos Juárez, Córdoba, Argentina. It produces indole-3-acetic acid in the presence of l-tryptophan as a precursor, grows at 20–38 °C (optimal 38 °C), and the cells are curved or spiral-shaped, with diameters ranging from 0.5–0.9 to 1.8–2.2 µm. They contain C16 : 0, C18 : 0 and C18 : 1  ω7c/ω6c as the main fatty acids. Phylogenetic analysis of its 16S rRNA gene sequence confirmed that this strain belongs to the genus Azospirillum , showing a close relationship with Azospirillum baldaniorum Sp245T, Azospirillum brasilense Sp7T and Azospirillum formosense CC-Nfb-7T. Housekeeping gene analysis revealed that Az39T, together with five strains of the genus (Az19, REC3, BR 11975, MTCC4035 and MTCC4036), form a cluster apart from A. baldaniorum Sp245T, A. brasilense Sp7T and A. formosense CC-Nfb-7T. Average nucleotide identity (ANI) and digital DNA–DNA hybridization (dDDH) between Az39T and the aforementioned type strains revealed values below 96 %, the circumscription limit for the species delineation (ANI: 95.3, 94.1 and 94.0 %; dDDH: 62.9, 56.3 and 55.6 %). Furthermore, a phylogeny evaluation of the core proteome, including 809 common shared proteins, showed an independent grouping of Az39T, Az19, REC3, BR 11975, MTCC4035 and MTCC4036. The G+C content in the genomic DNA of these six strains varied from 68.3 to 68.5 %. Based on the combined phylogenetic, genomic and phenotypic characterization presented here, we consider that strain Az39T, along with strains Az19, REC3, BR 11975, MTCC4035 and MTCC4036, are members of a new Azospirillum species, for which the name Azospirillum argentinense sp. nov. is proposed. The type strain is Az39T (=LBPCV39T=BR 148428T=CCCT 22.01T).