ABSTRACTExogenously applied brassinolide (BL) increased both gravitropic curvature and length of primary roots of Arabidopsis at low concentration (10−10 M), whereas at higher concentration, BL further increased gravitropic curvature while it inhibited primary root growth. BRI1‐GFP plants possessing a high steady‐state expression level of a brassinosteroid (BR) receptor kinase rendered the plant's responses to gravity and root growth more sensitive, while BR‐insensitive mutants, bri1‐301 and bak1, delayed root growth and reduced their response to the gravitropic stimulus. The stimulatory effect of BL on the root gravitropic curvature was also enhanced in auxin transport mutants, aux1‐7 and pin2, relative to wild‐type plants, and increasing concentration of auxin attenuated BL‐induced root sensitivity to gravity. Interestingly, IAA treatment to the roots of bri1‐301 and bak1 plants or of plants pretreated with a BL biosynthetic inhibitor, brassinazole, increased their sensitivity to gravity, while these treatments for the BL‐hypersensitive transgenic plants, BRI1‐GFP and 35S‐BAK1, were less effective. Expression of a CYP79B2 gene, encoding an IAA biosynthetic enzyme, was suppressed in BL‐hypersensitive plant types and enhanced in BL‐insensitive or ‐deficient plants. In conclusion, our results indicate that BL interacts negatively with IAA in the regulation of plant gravitropic response and root growth, and its regulation is achieved partly by modulating biosynthetic pathways of the counterpart hormone.