Overexpression of the Arabidopsis gai gene in apple significantly reduces plant size

Genetic engineering is an attractive method to obtain dwarf plants in order to eliminate the extensive use of growth retardants in horticultural crop production. In this study, we evaluated the potential of using the Arabidopsis gai (gibberellic acid insensitive) gene to dwarf apple trees. The gai gene under 35S promoter was introduced in the apple rootstock A2 and the cultivars Gravenstein and McIntosh through Agrobacterium-mediated transformation. One transgenic clone was recovered for Gravenstein and McIntosh, and several transgenic clones for A2, confirmed by Southern blot analysis. Two weak bands were detected by Southern blot analysis in all the untransformed controls, possibly indicating the existence of the internal GAI gene in apple. Most of the transgenic plants showed reduced growth in vitro. Growth analyses in the greenhouse showed a clear reduction in stem length, internode length and node number for the dwarf clones. The normal phenotype of some transgenic clones appears to be associated with silencing of the introduced gai gene, confirmed by RT-PCR analysis. In general, transgenic clones showed reduced rooting ability, especially for the extremely compact ones.