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 A₄ (GA₄) to GA₅₈ (12-OH-GA₄) in vitro. MdDOX-Co also metabolized its precursors GA₁₂ and GA₉ to GA₁₁₁ (12-OH-GA₁₂) and GA₇₀ (12-OH-GA₉), respectively; (ii) Of the three 12-OH-GAs, GA₅₈ was still active physiologically, but not GA₇₀ or GA₁₁₁; (iii) Arabidopsis MdDOX-Co OE transformants converted exogenously applied deuterium-labeled (d₂)-GA₁₂ to d₂-GA₁₁₁ but not to d₂-GA₅₈, whereas transformants converted applied d₂-GA₉ to d₂-GA₅₈; (iv) GA₁₁₁ is converted poorly to GA₇₀ by GA 20-oxidases in vitro when GA₁₂ is efficiently metabolized to GA₉; (v) no GA₅₈ was detected endogenously in MdDOX-Co OE transformants. Overall, we conclude that 12-hydroxylation of GA₁₂ by MdDOX-Co prevents the biosynthesis of biologically active GAs in planta, resulting in columnar phenotypes.