Involvement of brassinosteroids in the gravitropic response of primary root of maize

Abstract

Exogenously applied brassinolide (BL, 10-9-10-5 M) increased gravitropic in maize (Zea mays) primary roots. The BL-enhanced gravitropic curvature was clearly promoted in the presence of indole-3-acetic acid (IAA, 10-10-10-8 M), indicating that BL is interactive with IAA during the gravitropic response. The interactive effect between. BL and IAA was completely diminished by treatment of p-chlorophenoxy isobutric acid, an auxin action antagonist. The activation of the gravitropic response by BL in the absence and in the presence of IAA was nullified by application of 2,3,5-triiodobenzoic acid, a polar auxin transport inhibitor. The data indicate that brassinosteroids (BRs) might be involved in auxin-mediated processes for the gravitropic response. Gas chromotography-selected ion-monitoring analysis revealed that maize primary roots contained approximately 0.3 ng g-1 fresh weight castasterone as an endogenous BR. Exogenously applied castasterone also increased the gravitropic response of maize roots in an IAA-dependent manner. This study provides the first evidence, to our knowledge, for occurrence and gravitropic activity of BRs in plant roots.