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.
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