Involved in auxin biosynthesis (PubMed:24371168). Converts the indole-3-pyruvic acid (IPA) produced by the TAA family to indole-3-acetic acid (IAA) (By similarity). Seems not able to use tryptamine (TAM) as substrate (By similarity). Probably responsible for auxin biosynthesis in leaves and involved in the regulation of lateral leaf growth (Ref.3). Required for maintaining water homeostasis and an appropriate root to shoot ratio (By similarity). Required for the inhibition of root growth by ethylene in etiolated seedlings (By similarity). Functions downstream of the ethylene-response transcription factor EIL1 (By similarity). H(+) + indole-3-pyruvate + NADPH + O2 = (indol-3-yl)acetate + CO2 + H2O + NADP(+) Expressed in organs undergoing active growth and cell division. Narrow and rolled leaf phenotype. Calli overexpressing YUCCA8 contain increased levels of auxin and exhibit overgrowing roots and abnormal root morphology. Belongs to the FMO family.