Chromosaponin I specifically interacts with AUX1 protein in regulating thegravitropic response of arabidopsis roots

We have found that chromosaponin I (CSI), a gamma -pyronyl-triterpenoid sap onin isolated from pea (Pisum sativum L. cv Alaska), specifically interacts with AUX1 protein in regulating the gravitropic response of Arabidopsis ro ots. Application of 60 muM CSI disrupts the vertically oriented elongation of wild-type roots grown on agar plates but orients the elongation of agrav itropic mutant aux1-7 roots toward the gravity. The CSI-induced restoration of gravitropic response in nux1-7 roots was not observed in other agravitr opic mutants, axr2 and eir1-1. Because the aux1-7 mutant is reduced in sens itivity to auxin and ethylene, we examined the effects of CSI on another au xin-resistant mutant, axr1-3, and ethylene-insensitive mutant ein2-1. In au x1-7 roots, CSI stimulated the uptake of [H-3]indole-3-acetic acid (IAA) an d 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, aux122, the agravitropic nature of the roots and IAA uptake were not affected by CSI. This close correlation betw een auxin uptake and gravitropic bending suggests that CSI may regulate gra vitropic response by inhibiting or stimulating the uptake of endogenous aux in in root cells. CSI exhibits selective influence toward IAA versus l-naph thaleneacetic acid as to auxin-induced inhibition in root growth and auxin uptake. The selective action of CSI toward IAA along with the complete inse nsitivity of the null mutant aux122 toward CSI strongly suggest that CSI sp ecifically interacts with AUX1 protein.