Action of malformin A1 on gravitropic curvature in primary roots of maize (Zea mays L.)

Malformins, a small family of cyclic pentapeptides, are active plant growth regulators isolated from the fungusAspergillus niger. We purified malformin A₁ from the crude malformin A mixture, and studied its action in the gravitropic response of maize roots. Intact primary roots that had been pretreated vertically with malformin A₁ were placed in a humidified box in the horizontal position. Positive curvature (downward) was inhibited in the pretreated roots compared with the control. In addition, we measured the lateral transport of IAA in primary roots. Roots pretreated with malformin A, did not show asymmetric distribution of IAA between the upper and lower sides of the elongation zone. Malformin A, also stimulated ethylene production in maize root segments. Our results had suggested that malformin A₁ might inhibit the lateral transport of IAA across the roots from the upper to the lower side because of an increased level of ethylene. Therefore, we placed more IAA on the upper side at the initial phase of gravistimulation. These results were consistent with malformin A₁-pretreated roots showing inhibited positive gravitropic curvature.