A root-localized gene in normal apples is ectopically expressed in aerial parts of columnar apples

Abstract

The columnar apple ‘McIntosh Wijcik’, which is a mutation of ‘McIntosh’ shoots, has short internodes, thick stems, upright growth, poor lateral branches, and increased spur density. These columnar traits are controlled by a single dominant gene known as Co. We previously identified a putative dioxygenase gene (designated as 91071) as a promising Co candidate (expressed in the shoot apices of ‘McIntosh Wijcik’). However, tissue expression and function of the 91071 gene in noncolumnar apples is still not clear. In this study, we used reverse transcription polymerase chain reaction to demonstrate that the 91071 gene is mainly expressed in the roots of noncolumnar apples, whereas it is also expressed in shoot apices and leaves of columnar apples. In situ hybridization revealed that the 91071 gene is expressed at the primordium of lateral roots and root tips of both noncolumnar and columnar apple trees and in the shoot meristem and leaf primordium in the columnar apple ‘McIntosh Wijcik’. Grafting experiments of noncolumnar scion onto columnar rootstocks revealed that the columnar growth phenotype is not transmissible from rootstock to scion. These results indicated that ectopic expression of the 91071 gene in aerial parts causes columnar growth, whereas the expression of the 91071 gene in roots does not produce columnar growth. Furthermore, transgenic apples overexpressing the 91071 gene showed larger median adventitious root length and higher median number of lateral roots than control apples. Our result suggests that the 91071 gene may be related to root development.