Apple breeding has advanced significantly through genetic engineering, with a focus on developing pest-resistant, dwarfing, easy-to-root, and storage-enhanced varieties. Currently, insect-resistant transgenic apple plants have been successfully developed using genes such as Bt (Bacillus thuringiensis) and CpTI. The Bt gene encodes an insecticidal crystal protein (ICP), which was first used to transform apples in the U.S. as early as 1991. In 1992, field trials of ICP-transgenic apple plants were approved. Domestically, researchers like Cheng Jiasheng also used Bt genes to produce transgenic apple plants and conducted resistance tests. The CpTI gene, known for its broad-spectrum insect resistance, was introduced into green-sleeved apples in 1992. Chinese scientists also applied CpTI to Royal Gala apples, confirming the presence of the target gene through Southern hybridization.
In disease-resistant research, fire blight (caused by *Erwinia amylovora*) is a major concern. Genes like attacinE, cecropinSB-37, and cecropin Shiva-1 have been widely studied. Norelli et al. transformed rootstock M7 and Royal Jura with four bacterial cell-lyzing genes, including attacinE, SB-37, Shiva-1, and hen white lysozyme. The attacinE-transformed plants showed strong resistance, with some reducing disease incidence by up to 50%. However, when Shiva-1 and attacinE were introduced into Royal Gala, greenhouse results were inconsistent.
Dwarfing varieties have been achieved by introducing genes related to plant hormone synthesis, such as ipt and rolA, rolB, rolC, and rolD. Trifonova introduced the ipt gene into Granny Smith, resulting in mostly normal plants except for two showing intermediate growth. Holefors used rolA to transform M26 rootstock, producing shorter trees with reduced biomass. To enhance dwarfing, rolA, rolABC, Arabidopsis phyB, and oat phyA were introduced into M26, leading to varying degrees of dwarfism.
Easy-to-root varieties are crucial for propagation. M26 rootstock, typically hard to root, was improved by Lamber introducing T-DNA from *Agrobacterium rhizogenes*. Transformed plants exhibited better rooting ability. The roIB gene, known for inducing hairy roots, was used by Welander to enhance auxin sensitivity and rooting capacity in M26.
For fruit storage, genetic modification targets genes like ACC oxidase, ACC synthase, and PG. These genes have been cloned and used to create antisense transgenic apples, aiming to improve post-harvest stability. Initial results show promising storage performance.
Herbicide tolerance has also been achieved. The als gene from *Arabidopsis* was introduced into Royal Gala via Agrobacterium-mediated transformation. Seeds were tested with Glean herbicide, showing stable inheritance of the gene at a 1:1 ratio. This advancement supports more sustainable orchard management.
Overall, genetic engineering continues to play a vital role in enhancing apple traits, offering solutions to pests, diseases, and agricultural challenges.
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