ACTINOMYCIN-D INHIBITS THE GA(3)-INDUCED ELONGATION OF AZUKI BEAM EPICOTYLS AND THE REORIENTATION OF CORTICAL MICROTUBULES

Actinomycin D inhibited the elongation of epicotyl segments from azuki bean seedlings that was induced by simultaneous treatment with IAA an d GA(3). The drug also inhibited the elongation of the segments that w as caused by IAA alone when it was applied together with IAA. When the segments were pretreated with GA(3) and then incubated with IAA, GA(3 ) promoted the elongation caused by IAA and brought about a predominan ce of transverse cortical microtubules (MTs) in the epidermal cells of the segments. The change in the arrangement of MTs caused by pretreat ment with GA(3) was evident 1 h after the start of subsequent incubati on with IAA when the effect of pretreatment with GA(3) on the elongati on had not yet become apparent. Pretreatment with GA(3) did not cause any change in the arrangement of MTs when GA(3)-pretreated segments we re not incubated subsequently with IAA. Although actinomycin D applied before treatment with IAA did not inhibit the IAA-induced elongation, the drug diminished the promotion of the elongation caused by pretrea tment with GA(3) and prevented GA(3) from bringing about a predominanc e of transverse MTs when the drug was applied during the pretreatment with GA(3). GA(3)-induced synthesis of mRNA seems to be involved in th e promotion by GA(3) of IAA-induced elongation and in the GA(3)-induce d rearrangement of cortical MTs.