The article discusses the role of auxin, a phytohormone, in microbial interactions with plants. It highlights that some microorganisms, particularly bacteria, can synthesize auxin, which can interfere with plant developmental processes regulated by auxin. The biosynthesis of auxin in bacteria follows multiple pathways, similar to those in plants, and indole-3-acetic acid (IAA), the major naturally occurring auxin, can act as a signaling molecule in microorganisms. IAA affects gene expression in certain microorganisms, suggesting a reciprocal signaling role in microbe-plant interactions. The article also explores the recent finding that auxin signaling in *Arabidopsis* is part of the plant defense system against phytopathogenic bacteria, where down-regulation of auxin signaling enhances susceptibility to bacterial pathogens. Additionally, the article reviews the regulation of bacterial IAA biosynthesis, including the role of environmental factors and genetic elements. It discusses the regulatory mechanisms of IAA biosynthesis in various bacteria, such as the positive feedback regulation in *Azospirillum brasilense* and the involvement of environmental cues in *Pseudomonas putida*. The article further examines the role of IAA as a microbial signal molecule, both in phytopathogens and plant beneficial bacteria, and its impact on gene expression and microbial behavior. Finally, it highlights the importance of auxin signaling in plant defense responses and the potential for IAA to influence bacterial-plant interactions.The article discusses the role of auxin, a phytohormone, in microbial interactions with plants. It highlights that some microorganisms, particularly bacteria, can synthesize auxin, which can interfere with plant developmental processes regulated by auxin. The biosynthesis of auxin in bacteria follows multiple pathways, similar to those in plants, and indole-3-acetic acid (IAA), the major naturally occurring auxin, can act as a signaling molecule in microorganisms. IAA affects gene expression in certain microorganisms, suggesting a reciprocal signaling role in microbe-plant interactions. The article also explores the recent finding that auxin signaling in *Arabidopsis* is part of the plant defense system against phytopathogenic bacteria, where down-regulation of auxin signaling enhances susceptibility to bacterial pathogens. Additionally, the article reviews the regulation of bacterial IAA biosynthesis, including the role of environmental factors and genetic elements. It discusses the regulatory mechanisms of IAA biosynthesis in various bacteria, such as the positive feedback regulation in *Azospirillum brasilense* and the involvement of environmental cues in *Pseudomonas putida*. The article further examines the role of IAA as a microbial signal molecule, both in phytopathogens and plant beneficial bacteria, and its impact on gene expression and microbial behavior. Finally, it highlights the importance of auxin signaling in plant defense responses and the potential for IAA to influence bacterial-plant interactions.