In this study, The response characteristics of the primary root and lateral root to exogenous hormones in Arabidopsis wild type and amos2 mutant were investigated. The primary root elongation of mutant seedlings was less inhibited than wild-type root over a range of synthetic auxin 2,4-D concentrations; the auxin response of the amos2 mutant was also comparable to that of two auxin-related mutants aux1-7 and axr4-2, and the both known mutants, aux1-7 and axr4-2, were more insensitive than amos2. The amos2 mutant was also resistant to auxin transport inhibitor TIBA but hypersensitive to the ethylene precursor ACC. The amos2 mutant did not show signficant differences from wild type in root elongation responses to cytokinin benzyl adenine (6-BA) and abscisic acid (ABA). Exogenous auxin and ethylene antagonist Ag+ strongly induced lateral root formation in both wild-type and amos2 seedlings, but there were slightly but signi?cantly higher levels of lateral root induction in amos2 seedlings than that of wild-type plants. The amos2 mutant displayed low fertility rates in the early stage of development, but the amos2 mutant was not different from the wild type in gravitropism and hookless hypocotyl. All the results suggested that AMOS2 plays a role in the regulation of lateral root and flower development under normal growth conditions.