Involvement of Brassinosteroids in the Gravitropic Response of Primary Root of Maize

Abstract

Exogenously applied brassinolide (BL, 10Ϫ9 -10 Ϫ5 m) increased gravitropic curvature in maize (Zea mays) primary roots. The BL-enhanced gravitropic curvature was clearly promoted in the presence of indole-3-acetic acid (IAA, 10 Ϫ10 -10 Ϫ8 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 Ϫ1 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.Since brassinolide (BL) has been identified as a plant growth promoting substance in rape pollen (Grove et al., 1979), over 40 members of related steroids, collectively named as brassinosteroids (BRs), have been characterized in the entire plant kingdom (Kim, 1991; Fujioka, 1999). Early studies investigated possible physiological roles of BRs by exogenous application. The results of those studies suggested that BRs might be involved in the regulation of cell elongation and division, leaf bending, reproductive and vascular development, membrane polarization and proton pump, source/sink definition, and modulation of stress (for review, see Yokota and Takahashi,

Inhibition of Brassinosteroid Biosynthesis by Either a dwarf4 Mutation or a Brassinosteroid Biosynthesis Inhibitor Rescues Defects in Tropic Responses of Hypocotyls in the Arabidopsis Mutant nonphototropic hypocotyl 4

“…BR promotes gravitropism of light-grown tomato (Lycopersicon esculentum) hypocotyl cuttings in the light condition (Park, 1998). A synergistic positive interaction between BR and auxin was reported for gravitropism of partially deetiolated bean (Phaseolus vulgaris) hypocotyl sections (Meudt, 1987) and dark-grown maize (Zea mays) primary roots (Kim et al, 2000). Recently, Li et al (2005) have also shown promotive effects of BR on root and hypocotyl gravitropism in Arabidopsis.…”

The Fast and Transient Transcriptional Network of Gravity and Mechanical Stimulation in the Arabidopsis Root Apex

“…4). AtPEN1 is known to be involved in sterol biosynthesis, and its fast up-regulation during gravitropism might be important for the synthesis of brassinosteroids, which are involved in gravitropic signal transduction in maize roots (Kim et al, 2000). SAMT enzymes methylate secondary metabolites, and some of them, such as methylsalicylate and methyljasmonate, are known to be in several signal transduction pathways that lead to systemic and local defense responses (Shulaev et al, 1997;Zubieta et al, 2003;Hendricks et al, 2004).…”

“…Lateral redistribution of radiolabeled indole-3-acetic acid (IAA) has been measured in both shoots (Parker and Briggs, 1990) and roots (Young et al, 1990;Young and Evans, 1996), and the redistribution of IAA has been shown to precede differential growth (Parker and Briggs, 1990). Other plant hormones, specifically brassinosteroids (Kim et al, 2000), ethylene (Lee et al, 1990;PhilosophHadas et al, 1996;Madlung et al, 1999), cytokinin (Golan et al, 1996), and gibberellic acid (Moore and Dickey, 1985;Rood et al, 1987;Brock and Kaufman, 1988;Chaban et al, 1999) have been shown to also play a role in gravitropism. Reactive oxygen species may function as a downstream component in auxinmediated signal transduction (Joo et al, 2001).…”

Effect of 22(S), 23(S)-Homobrassinolide on Adventitious Root Formation in Grape Rootstocks

“…An interaction between brassinosteroid and auxin at the rooting site might have resulted in the promotion of root formation around the base of the cuttings. Auxin response has been found to be connected to brassinosteroid, acting in concert with auxin to induce root growth in maize (Kim et al, 2000). Brassinosteroid and auxin are significant growth regulators that have effects on cell division and expansion, and on root growth (Bao et al, 2004).…”