Selectable marker genes are widely used for the efficient transformation of crop plants. In most cases, antibiotic or herbicide resistance marker genes are preferred because they tend to be most efficient. Due mainly to consumer and grower concerns, considerable effort is being put into developing strategies (site-specific recombination, homologous recombination, transposition, and cotransformation) to eliminate the marker gene from the nuclear or chloroplast genome after selection. For the commercialization of genetically transformed plants, use of a completely marker-free technology would be desirable, since there would be no involvement of antibiotic resistance genes or other marker genes with negative connotations for the public. With this goal in mind, a technique for apple transformation was developed without use of any selectable marker. Transformation of the apple genotype "M.26" with the constructs pPin2Att35SGUSintron and pPin2MpNPR1 was achieved. In different experiments, 22.0-25.4% of regenerants showed integration of the gene of interest. Southern analysis in some transformed lines confirmed the integration of one copy of the gene. Some of these transformed lines have been propagated and used to determine the uniformity of transformed tissues in the plantlets. The majority of the lines are uniformly transformed plants, although some lines are chimeric, as also occurs with the conventional transformation procedure using a selectable marker gene. A second genotype of apple, "Galaxy," was also transformed with the same constructs, with a transformation efficiency of 13%.

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The first report of transformed apple plants in 1989 raised expectations for new apple cultivars that would be better tasting, healthier, and easier to grow. Although, many different traits have now been introduced successfully into apple, no transformed cultivars have yet made it to commercial production. Most early reports on transformed apple described “proof of concept” experiments involving the development of regeneration protocols, and the choice of appropriate promoters and selectable markers. More recently, the focus has moved onto functional testing of traits of potential commercial interest. These traits can be grouped into two categories: horticultural production traits and fruit‐focused traits. Production traits of interest include bacterial, fungal and pest resistance, dwarfing, propagation, stress resistance, precocity, storage life, and self‐fertility. Examples of fruit‐focused traits include novel health properties, flavor, reduced browning, color, and reduced allergenicity. This review will consider reports of characters introduced into apple that are useful to growers and consumers, and looks toward future trends, targets, and challenges.