Auxin promotes gibberellin biosynthesis in decapitated tobacco plants

Excision of the apical bud (decapitation) of tobacco (Nicotiana tabacum L.) plants reduced the endogenous levels of indole-3-acetic acid (IAA), gibber ellin A(20) (GA(20)), and GA, (the bioactive GA), in internode tissue below the excision site. Application of IAA to the stump of decapitated plants d ramatically increased GA(20) content, to a level 3-fold greater than in int act plants. Gibberellin A, content was also increased by IAA. Decapitation reduced the conversion of [C-14]GA(19) to [C-14]GA(20) and of [C-14]GA(20) to [C-14]GA(1), and appeared to promote the deactivation pathway [C-14]GA(2 0) to [C-14]GA(29) to [C-14]GA(29)-catabolite. Application of auxin counter acted these effects, but did not restore the conversion of [C-14]GA(20) to [C-14]GA(1) to the level found in intact plants. The results indicate that auxin is necessary for normal GA biosynthesis in stems of tobacco.