In his studies on the response of plants to light and their roots to gravity, Charles Darwin proposed the existence of a substance produced in the apical regions of the stem and root, capable of moving long distances to drive cell elongation and differential growth [1]. This substance, which was named auxin, from the Greek word ‘auxin’ meaning ‘to grow’ is indole-3-acetic acid (IAA), one of the first plant hormones discovered [2].

Not only do plants produce IAA, but many microbes, particularly those that establish intimate relationships with eukaryotes, including bacteria and fungi, secrete auxin [3], IAA precursors [4], or compounds that mimic auxin action [5]. Bacteria producing auxin can live as endophytes inside plant tissues and contribute to drought and salt stress mitigation [6]. Human gut bacteria also produce IAA, which helps cancer patients react positively to chemotherapy [7], while engineered Escherichia coli to produce IAA inhibits tumour growth, improves survival, and reinforces immunity [8]. Thus, IAA is a universal molecule responsible for essential cellular functions in all major kingdoms of life (Figure 1).

We humans have a diet based on plants and food products from microbes that contribute to daily IAA intake, such as fresh salads, microgreens, vegetables, fruits, and yoghurt [9]. The possibility that bacteria living endophytically in plants may pass directly into our intestine and become part of our microbiota is high [10]. On the other hand, agriculture often uses animal and human manures, and then the excreted bacteria could eventually become components of the root microbiota [10]. Thus, microbial exchange is integral to eukaryotic fitness, where IAA, as a common bacterial, plant, and human metabolite, seemingly contributes to one's health. This review focuses on the universal role of IAA, its occurrence in organisms from different kingdoms, and its contribution to human health and wellness.