The Gene Conferring Columnar-Type Growth in Apple Encodes a Putative 2OG-Fe(II) Oxygenase

The ‘Wijcik’ mutant of the apple cultivar ‘McIntosh’ shows columnar-type growth, characterized by altered branching and a thick stem. Instead of normal branches, spur-like, short branches are formed on Wijcik trees, leading to apples produced close to the stem. Columnar-type growth is a desirable trait, as columnar apple trees need considerably less pruning compared to standard trees. Because of their compact nature, more trees can be planted on the same area, leading to a potential yield increase.

The genetic basis for columnar growth is a somatic mutation that results in dominant inheritance of the columnar growth trait. The columnar gene (Co) has previously been mapped on apple chromosome 10. We reduced the genetic window for Co to 0.56 cm using a total of 1500 progeny trees resulting from crosses of Wijcik with other apple cultivars. This region corresponds to 393 kb in the Golden delicious genome.

BAC libraries prepared from genomic DNA of Wijcik and McIntosh were screened in order to identify the clones covering the complete Co region of both cultivars. The BAC inserts were sequenced and the Co region sequences of McIntosh and Wijcik were compared. Only one sequence difference was found: a 1956 bp insertion in Wijcik.

Gene expression analyses of predicted genes around the Wijcik insertion revealed the specific upregulation of a putative 2OG-Fe(II) oxygenase in buds of Wijcik and columnar progeny trees. Arabidopsis plants constitutively expressing this gene showed a phenotype resembling that of Wijcik, suggesting the successful identification of Co.