Effects of the auxin-inhibiting substances raphanusanin and benzoxazolinone on apical dominance of pea seedlings

The effects of the auxin-inhibiting substances raphanusanin ((3R*, 6S*)-3-[methoxy (methylthio) methyl]-2-pyrrolidinethione, raphanusanin B) and benzoxazolinone (6-methoxy-2-bezoxazolinone, MBOA) (Figure 1) on apical dominance of pea (Pisum sativum L. cv. Alaska) seedlings were studied. Application of raphanusanin B or MBOA to the apical bud, internode, or lateral bud of pea seedlings released apical dominance in either intact or indole-3-acetic acid (IAA)-treated, decapitated plants. These results suggest that the auxin-inhibiting substances raphanusanin B and MBOA have activity in releasing apical dominance. Conversely, the auxin transport inhibitors 2,3,4-triiodobenzoic acid (TIBA) and 1-naphthylphthalamic acid (NPA) did not stimulate lateral bud growth when they were applied directly to the lateral bud, although application to the apical bud or internode released apical dominance. Therefore, the mode of action of raphanusanin B and MBOA in apical dominance is clearly different from that of auxin transport inhibitors. Raphanusanin B and MBOA may suppress the synthesis of growth-inhibiting factor(s) of the lateral bud induced by endogenous auxin transported from the apical bud or exogenously applied auxin, and/or the action of the factor(s).