Catalyzes the synthesis of indole-3-acetic acid (IAA)-amino acid conjugates, providing a mechanism for the plant to cope with the presence of excessive free auxin (PubMed:18192436). Produces more IAA-Asp levels than IAA-Ala levels in vitro (PubMed:18192436). May participate in the activation of disease resistance by preventing the accumulation of free IAA, which reduces the expression of a group of auxin-responsive genes encoding expansins that control cell wall loosening and expansion (PubMed:18192436). Contributes to late events in stamen and carpel differentiation, and influences floret fertility (PubMed:22016342). Expressed in the inner floral organs (lodicules, stamens and carpels) and at lower levels in lemmas and paleas. Expressed at early stage of flower development in floral meristem and at later stage in lemma, palea and carpel primordia. Induced by auxin. Slight increase in disease susceptibility to Xanthomonas oryzae pv oryzae. Plants overexpressing GH3-8 exhibit enhanced disease resistance to Xanthomonas oryzae pv oryzae (PubMed:18192436). Plants overexpressing GH3-8 exhibit abnormal morphology and dwarf phenotype (PubMed:18192436, PubMed:22016342). Plants silencing GH3-8 produce mostly non-viable pollen grains (PubMed:22016342). Belongs to the IAA-amido conjugating enzyme family.