Auxin promotes gibberellin biosynthesis in decapitated tobacco plants

Abstract.

Excision of the apical bud (decapitation) of tobacco (Nicotiana tabacum L.) plants reduced the endogenous levels of indole-3-acetic acid (IAA), gibberellin A₂₀ (GA₂₀), and GA₁ (the bioactive GA), in internode tissue below the excision site. Application of IAA to the stump of decapitated plants dramatically increased GA₂₀ content, to a level 3-fold greater than in intact plants. Gibberellin A₁ content was also increased by IAA. Decapitation reduced the conversion of [¹⁴C]GA₁₉ to [¹⁴C]GA₂₀ and of [¹⁴C]GA₂₀ to [¹⁴C]GA₁, and appeared to promote the deactivation pathway [¹⁴C]GA₂₀ to [¹⁴C]GA₂₉ to [¹⁴C]GA₂₉-catabolite. Application of auxin counteracted these effects, but did not restore the conversion of [¹⁴C]GA₂₀ to [¹⁴C]GA₁ to the level found in intact plants. The results indicate that auxin is necessary for normal GA biosynthesis in stems of tobacco.