The apple gene responsible for columnar tree shape reduces the abundance of biologically active gibberellin.

Ectopic expression of the apple 2-oxoglutarate-dependent dioxygenase (DOX, 2ODD) gene, designated MdDOX-Co, is thought to cause the columnar shape of apple trees. However, the mechanism underlying the formation of such a unique tree shape remains unclear. To solve this problem, we demonstrated that Arabidopsis thaliana overexpressing MdDOX-Co contained reduced levels of biologically active gibberellin (GA) compared with wild type. In summary: (i) with biochemical approaches, the gene product MdDOX-Co was shown to metabolize active GA A (GA ) to GA (12-OH-GA ) in vitro. MdDOX-Co also metabolized its precursors GA and GA to GA (12-OH-GA ) and GA (12-OH-GA ), respectively; (ii) Of the three 12-OH-GAs, GA was still active physiologically, but not GA or GA ; (iii) Arabidopsis MdDOX-Co OE transformants converted exogenously applied deuterium-labeled (d )-GA to d -GA but not to d -GA , whereas transformants converted applied d -GA to d -GA ; (iv) GA is converted poorly to GA by GA 20-oxidases in vitro when GA is efficiently metabolized to GA ; (v) no GA was detected endogenously in MdDOX-Co OE transformants. Overall, we conclude that 12-hydroxylation of GA by MdDOX-Co prevents the biosynthesis of biologically active GAs in planta, resulting in columnar phenotypes.