Phytohormone-Producing Rhizobacteria and Their Role in Plant Growth

Phytohormone-producing rhizobacteria are a group of beneficial bacteria residing in the rhizosphere that have the unique ability to produce, release, and also modulate phytohormones such as auxins, cytokinins, gibberellins, ethylene, and jasmonic acid (JA). This work explores a diverse group of rhizobacteria that possess the ability to synthesize and secrete phytohormones and their effects on the growth of different plants. Indole-3-acetic acid (IAA) is a commonly produced hormone by many rhizobacteria that include Azospirillum brasilense, Pseudomonas putida, and Pseudomonas fluorescens. IAA producers promote plant growth through multiple mechanisms. Gibberellic acid (GA3) produced by certain species of rhizobacteria, which include Serratia marcescens and Bacillus licheniformis, enhances plant height and biomass in different crops. Cytokinins are produced by rhizobacteria, including Bacillus, Pseudomonas, and Azospirillum. Few rhizobacteria strains also produce abscisic acid (ABA). For example, A. brasilense produces abscisic acid, which can regulate the plant water status and enhance drought tolerance in different crops. Several rhizobacteria, including P. fluorescens, P. putida, and Pseudomonas aeruginosa, have been reported to induce JA production in plants, promoting defense responses against pathogens. Overall, this work indicates that rhizobacteria produce key phytohormones, enabling them to promote plant growth through multifarious ways, and hence phytohormone-producing rhizobacteria are potential input in agricultural production.