The structure-activity relationships of the naturally occurring auxin-inhibiting substance, 6-methoxy-2-benzoxazolinone, isolated from maize shoots, and its artificial analogues with respect to auxin activity and membrane-bound auxin-binding protein, were studied. 6-Isobutoxy-2-benzoxazolinone strongly inhibited auxin (1-naphthylacetic acid, NAA)-induced growth of etiolated maize coleoptile segments. 6-Isopropoxy-2-benzoxazolinone, 6-ethoxy-2-benzoxazolinone and 6-methoxy-2-benzoxazolinone showed significant inhibitory activity. However, 2-benzoxazolin-one and 6-benzyloxy-2-benzoxazolinone did not inhibit auxin-induced growth. On the basis of these data, it seemed reasonable to assume that the auxin-inhibitory activity was enhanced by increasing the side chain length at position C-6 on the 2-benzoxazolinone, although the attachment of a ring at C-6 destroyed activity. Competition by benzoxazolinones with an alkoxy group at C-6 with 3H-NAA at auxin-binding protein(s) isolated from endoplasmic reticulum (ER) membrane of maize shoots showed a positive correlation with their physiological effects. However, since the inhibitory activity of the benzoxazolinones for auxin-receptor(s) binding was small compared with their physiological activity, the benzoxazolinones may contribute to inhibition of auxin-induced growth through interference with other auxin-receptors.
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