Paclobutrazol (PB), a triazole growth retardant and an inhibitor of gibberellin biosynthesis, reduced at 17 ~tM concentration the adventitious root formation of bean primary leaf cuttings. Treatments with 5 ~tM ABA or 4 lxM Ethrel, an ethylenereleasing compound, restored the roofing of PB-treated cuttings. Ethylene production and the content of the precursor 1-aminocyclopropane-1-carboxylic acid (ACC) were enhanced in root-forming tissues of PB-treated petioles 48 h after ABA application. The effect of ABA could be abolished by 10 ~tM COC12, an inhibitor of ACC oxidase. Thus, ABA might stimulate roofing through its effect on ethylene release. 2 mM silver thiosulphate, an inhibitor of ethylene action, decreased the roofing of PB-treated cuttings similarly to Co 2+, but failed to negate the ABA effect. These data indicate that the effect of PB on rhizogenesis is not associated directly with the inhibition of the biosynthesis of gibberellins.
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