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

Abstract

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.