A century of Azospirillum : plant growth promotion and agricultural promise

The genus Azospirillum celebrates 100 y since its discovery in 1925 by Martinus Willem Beijerinck, who worked with Spirillum lipoferum as a starting species. Decades of work involving laboratory and field research endorse their various beneficial properties, such as plant rooting, mineral nutrition, hormonal strengthening, and the activation of cellular and molecular responses, which lead to better growth, development, and productivity. Some hormones, such as auxins and cytokinins, potentiate root branching through their effects on mitosis, and via signal transduction mediated by the Target Of Rapamycin (TOR) kinase. Although initial efforts were aimed at clarifying the importance of biological nitrogen fixation in plant growth in the face of root colonization with Azospirillum, recent advances show that these bacteria also activate the mechanisms of acquisition of phosphorus and iron, two essential nutrients for fulfilling the plant's life cycle. In recent years, Azospirillum structural elements such as flagellin and lipopolysaccharides emerged as elicitors, influencing the development and defense of the host. Goals have also been achieved in formulating biotechnological products, whose application has increased in countries such as Argentina and Brazil, showing relevant and promising results toward saving fertilizer, optimizing management, and ultimately, making agriculture more sustainable.