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Plant Physiol, February 2001, Vol. 125, pp. 990-1000
Chromosaponin I Specifically Interacts with AUX1 Protein in
Regulating the Gravitropic Response of Arabidopsis Roots
Abidur
Rahman,
Arifa
Ahamed,
Taisaku
Amakawa,
Nobuharu
Goto, and
Seiji
Tsurumi*
Graduate School of Science and Technology (A.R., A.A.,
T.A.) and Radioisotope Research Center (S.T.), Kobe University,
Rokkodai, Nadaku, Kobe 657-8501, Japan; and Department of Biology,
Miyagi University of Education, Aoba-Ku, Sendai 980-0845, Japan (N.G.)
We have found that chromosaponin I (CSI), a
 -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 µM 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 [3H]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.
*
Corresponding author; e-mail tsurumis{at}scitec.kobe-u.ac.jp;
fax 81-78-803-5989.
© 2001 American Society of Plant Physiologists
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