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), gibberellin 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 dramatically increased GA(20) content, to a level 3-fold greater than in intact 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(20) to [C-14]GA(29) to [C-14]GA(29)-catabolite. Application of auxin counteracted 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.