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(4) (GA(4)) to GA(58) (12-OH-GA(4)) in vitro. MdDOX-Co also metabolized its precursors GA(12) and GA(9) to GA(111) (12-OH-GA(12)) and GA(70) (12-OH-GA(9)), respectively; (ii) Of the three 12-OH-GAs, GA(58) was still active physiologically, but not GA(70) or GA(111); (iii) Arabidopsis MdDOX-Co OE transformants converted exogenously applied deuterium-labeled (d(2))-GA(12) to d(2)-GA(111) but not to d(2)-GA(58), whereas transformants converted applied d(2)-GA(9) to d(2)-GA(58); (iv) GA(111) is converted poorly to GA(70) by GA 20-oxidases in vitro when GA(12) is efficiently metabolized to GA(9); (v) no GA(58) was detected endogenously in MdDOX-Co OE transformants. Overall, we conclude that 12-hydroxylation of GA(12) by MdDOX-Co prevents the biosynthesis of biologically active GAs in planta, resulting in columnar phenotypes.