Abstract: In our previous study，a Mdsolo-LTR1 insertion was identified as a spur-specific solo LTR （long terminal repeat）located at position 3 768 578 bp on apple chromosome 4 in‘Red Delicious’apple spur mutants. The 1-aminocyclopropane-1-carboxylate oxidase（ACO1）gene，which encoded the rate-limiting enzyme synthesizing ethylene，was localized at approximately 50 kb downstrem of Mdsolo-LTR1 insertion site in spur mutants. In the present study，activity and function analysis of MdRDACO1 flanking the retrotransposon Mdsolo-LTR1 insertion site was further confirmed. The real-time PCR analysis showed that the expression levels of MdRDACO1 in apical bud（L9–L12）and stem（before shoot half lignification）of‘Red Delicious’were higher than those of in‘Oregon spur Delicious’. The function of MdRDACO1 gene was investigated by Agrobacterium-mediated genetic transformation into Arabidopsis. Overexpression of MdRDACO1 gene significantly increased hypocotyl elongation，plant height and the expression level of AtCPS1 gene in transgenic lines. Our results suggested that spur mutants could partly be attributed to Mdsolo-LTR1 insertion，which represses the expression activity of nearby MdRDACO1 gene，down-regulating the expression activity of MdCPS to synthesize gibberellins in spur mutants.

Key words: apple, spur mutation, retrotransposon, gene expression