Effects of Chromosaponin I and Brassinolide on the Growth of Roots in Etiolated Arabidopsis Seedlings

The stimulatory effects of chromosaponin I (CSI) on the growth of roots in etiolated Arabidopsis thaliana (ecotype Columbia) seedlings were compared with those of brassinolide (BR) bearing a structural similarity to CSI. The optimum concentrations of CSI and BR were 100 μmol/L and 1 nmol/L, respectively. The roots grew curved on wetted filter paper in the absence of CSI, but elongated straight in the presence of CSI. CSI increased the length of root cells and decreased the diameter of roots. However, BR did not increase the cell length although it decreased the root diameter. Neither CSI- nor BR-induced growth in roots was detected in ethylene-insensitive mutants etrl-1 and ein2-1, supporting the idea that CSI and BR stimulate the growth of roots by interfering with the action of ethylene. However, the mechanism of CSI action is not the same as that of BR.