Indole-3-Acetic Acid Biosynthesis by Endophytic Fungi and Its Association with Plant-Fungus Interactions

Indole-3-acetic acid (IAA) is a central auxin regulating plant growth and developmental plasticity, including cell division, elongation, vascular differentiation, and root architecture. Beyond plants, many fungi-including endophytic fungi that reside within plant tissues without visible disease symptoms-can synthesize IAA and thereby modulate plant performance and plant-fungus interactions. However, fungal IAA biosynthesis remains less resolved than bacterial or plant pathways, and mechanistic evidence is uneven across taxa. This review summarizes recent (2020-2025) advances in IAA-producing endophytic fungi, the main proposed biosynthetic routes and intermediates, and methodological considerations for attributing IAA production to the fungal partner. The reported associations of fungal IAA with beneficial interactions are then discussed, including growth promotion, stress tolerance, and interaction-mediated changes in the root system's architecture, with emphasis on the concentration- and context-dependent nature of plant responses. Overall, fungal IAA is associated with diverse plant-fungus interactions, as discussed in this review.