We have found that chromosaponin I (CSI), a gamma-pyronyl-triterpenoid saponin isolated from pea (Pisum sativum L. cv Alaska), specifically interacts with AUX1 protein in regulating the gravitropic response of Arabidopsis roots. Application of 60 microM CSI disrupts the vertically oriented elongation of wild-type roots grown on agar plates but orients the elongation of agravitropic mutant aux1-7 roots toward the gravity. The CSI-induced restoration of gravitropic response in aux1-7 roots was not observed in other agravitropic mutants, axr2 and eir1-1. Because the aux1-7 mutant is reduced in sensitivity to auxin and ethylene, we examined the effects of CSI on another auxin-resistant mutant, axr1-3, and ethylene-insensitive mutant ein2-1. In aux1-7 roots, CSI stimulated the uptake of [(3)H]indole-3-acetic acid (IAA) and induced gravitropic bending. In contrast, in wild-type, axr1-3, and ein2-1 roots, CSI slowed down the rates of gravitropic bending and inhibited IAA uptake. In the null allele of aux1, aux1-22, the agravitropic nature of the roots and IAA uptake were not affected by CSI. This close correlation between auxin uptake and gravitropic bending suggests that CSI may regulate gravitropic response by inhibiting or stimulating the uptake of endogenous auxin in root cells. CSI exhibits selective influence toward IAA versus 1-naphthaleneacetic acid as to auxin-induced inhibition in root growth and auxin uptake. The selective action of CSI toward IAA along with the complete insensitivity of the null mutant aux1-22 toward CSI strongly suggest that CSI specifically interacts with AUX1 protein.