What Is Expected from the Genus Azospirillum as a Plant Growth-Promoting Bacteria?

Bacteria capable of stimulating plant growth are generally known as plant growth-promoting bacteria (PGPB). Among them are Azospirillum species that influence plant growth through different mechanisms. Azospirillum is a Gram-negative bacterium that belongs to the alphaproteobacteria phylum. On the basis of the newly discovered species (at present 15), it is present not only in a wide diversity of plants, including those of agronomic importance such as cereals, sugarcane; and forage grasses, but also in other non-Poaceae plant species. Due to the capacity for improving plant yield in agronomically important crops, Azospirillum possesses biotechnological application as inoculant or biofertilizer. Among the mechanisms involved in promoting plant growth are N2 fixation, P solubilization, phytohormone production (auxins, cytokinins, and gibberellins), increased nutrient uptake, enhanced stress resistance, vitamin production, siderophores, and biocontrol activity. Some of them, as well as their agricultural application, are discussed in this chapter.