Origin of plant auxin biosynthesis

•Current phylogenetic data do not support the existence of IAA biosynthesis in algae.•Plant Trp-dependent IAA biosynthesis has been shaped by gene transfer.•IAA biosynthesis in plants evolved in response to microbe–plant interactions.

The recent finding of the tryptophan aminotransferase (TAA)/flavin monooxygenase (YUC) pathway as the principal route of auxin production in plants provides an opportunity to revisit the origin of plant auxin biosynthesis. Phylogenetic analyses of the TAA and YUC gene families provide very little evidence for the production of indole-3-acetic acid (IAA) in algae. Instead, horizontal gene transfer of YUCs from bacteria to the ancestral land plant suggests that the TAA/YUC pathway is a land plant innovation. In this Opinion article we postulate that the origin of tryptophan-dependent IAA biosynthesis in land plants might have evolved in response to interactions with microbes, particularly bacteria, allowing plants to counteract bacterial activities and control their own auxin signaling.