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

SUMMARY 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. Significance Statement The ectopic expression of the gene MdDOX‐Co is thought to result in apple trees with a columnar shape, but the mechanism is unknown. We found that Arabidopsis overexpressing MdDOX‐Co becomes dwarf and the MdDOX‐Co‐hydroxylated biosynthetic precursor GA111 was very poorly metabolized by the gibberellin biosynthetic enzyme. We concluded that the ectopic metabolism of the gibberellin precursor by MdDOX‐Co led to the depletion of bioactive gibberellin.