STRUCTURE-ACTIVITY-RELATIONSHIPS OF BENZOXAZOLINONES WITH RESPECT TO AUXIN-INDUCED GROWTH AND AUXIN-BINDING PROTEIN

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 a nd membrane-bound auxin-binding protein, were studied. 6-Isobutoxy-2-b enzoxazolinone strongly inhibited auxin (1-naphthylacetic acid, NAA)-i nduced growth of etiolated maize coleoptile segments. 6-Isopropoxy-2-b enzoxazolinone, 6-ethoxy-2-benzoxazolinone and 6-methoxy-2-benzoxazoli none showed significant inhibitory activity. However, 2-benzoxazolinon e and 6-benzyloxy-2-benzoxazolinone did not inhibit auxin-induced grow th. On the basis of these data, it seemed reasonable to assume that th e auxin-inhibitory activity was enhanced by increasing the side chain length at position C-6 on the 2-benzoxazolinone, although the attachme nt of a ring at C-6 destroyed activity. Competition by benzoxazolinone s with an alkoxy group at C-6 with H-3-NAA at auxin-binding protein(s) isolated from endoplasmic reticulum (ER) membrane of maize shoots sho wed a positive correlation with their physiological effects. However, since the inhibitory activity of the benzoxazolinones for auxin-recept or(s) binding was small compared with their physiological activity, th e benzoxazolinones may contribute to inhibition of auxin-induced growt h through interference with other auxin-receptors.