Overexpression of a Novel Class of Gibberellin 2-Oxidases Decreases Gibberellin Levels and Creates Dwarf Plants

Degradation of active C(19)-gibberellins (GAs) by dioxygenases through 2β-hydroxylation yields inactive GA products. We identified two genes in Arabidopsis (AtGA2ox7 and AtGA2ox8), using an activation-tagging mutant screen, that encode 2β-hydroxylases. GA levels in both activation-tagged lines were reduced significantly, and the lines displayed dwarf phenotypes typical of mutants with a GA deficiency. Increased expression of either AtGA2ox7 or AtGA2ox8 also caused a dwarf phenotype in tobacco, indicating that the substrates for these enzymes are conserved. AtGA2ox7 and AtGA2ox8 are more similar to each other than to other proteins encoded in the Arabidopsis genome, indicating that they may constitute a separate class of GA-modifying enzymes. Indeed, enzymatic assays demonstrated that AtGA2ox7 and AtGA2ox8 both perform the same GA modification: 2β-hydroxylation of C(20)-GAs but not of C(19)-GAs. Lines containing increased expression of AtGA2ox8 exhibited a GA dose–response curve for stem elongation similar to that of the biosynthetic mutant ga1-11. Double loss-of-function Atga2ox7 Atga2ox8 mutants had twofold to fourfold higher levels of active GAs and displayed phenotypes associated with excess GAs, such as early bolting in short days, resistance to the GA biosynthesis inhibitor ancymidol, and decreased mRNA levels of AtGA20ox1, a gene in the GA biosynthetic pathway.