Apple breeding has made significant progress through genetic engineering, with the development of pest-resistant, dwarfing, easy-to-root, and storage-enhanced varieties. One of the key approaches is the creation of insect-resistant transgenic apple plants. The Bt (Bacillus thuringiensis) gene, which encodes an insecticidal crystal protein (ICP), has been widely used in this context. Since 1991, researchers in the U.S. have successfully introduced ICP genes into apple plants, and by 1992, field trials were approved. Domestically, Cheng Jiasheng and others also utilized Bt genes to produce transgenic apple lines that showed good insect resistance. Another important gene is CpTI, which offers broad-spectrum protection against various pests. In 1992, foreign scientists used CpTI to transform green-sleeved apples, while domestic research teams applied it to Royal Gala, confirming the presence of the target gene through Southern hybridization.
Disease resistance in apples has focused on fire blight, caused by *Erwinia amylovora*. Researchers have employed genes such as attacinE, cecropinSB-37, and Shiva-1. Norelli et al. developed 250 transformation systems using Agrobacterium-mediated methods, testing them in both greenhouse and field conditions. Plants carrying attacinE showed strong resistance, with some reducing disease incidence by up to 50%. However, other combinations, like Shiva-1 and attacinE in Royal Gala, did not show consistent results in controlled environments.
Dwarfing varieties are another area of interest. Genes like ipt and rolA/B/C/D, involved in plant hormone regulation, have been used to induce dwarfing traits. Trifonova introduced the ipt gene into Granny Smith, resulting in mostly normal plants except for a few showing moderate dwarfism. Holefors used rolA to transform M26 rootstock, producing plants with shorter stature and reduced biomass. Further studies combined rolA, rolABC, phyB, and phyA genes to enhance dwarfing in M26, with varying degrees of success.
Rooting ability is crucial for propagation. M26 rootstock is notoriously difficult to root, but introducing T-DNA from the Ri plasmid of *Agrobacterium rhizogenes* significantly improved rooting. Additionally, the roIB gene, known for inducing hairy roots, was used to enhance auxin sensitivity and rooting capacity in M26.
In terms of storage, apples face challenges due to ripening during post-harvest. Genetic modification targeting genes like ACC oxidase, ACC synthase, and PG has shown promise. Transgenic Royal Gala apples expressing antisense versions of these genes are being tested to improve shelf life and reduce spoilage.
Herbicide tolerance has also been achieved. The als gene from *Arabidopsis* was introduced into Royal Gala via Agrobacterium, resulting in transgenic plants that exhibited stable resistance. Seed tests confirmed that the gene was inherited at a 1:1 ratio when exposed to herbicides like Glean.
These advancements highlight the potential of genetic engineering in addressing major challenges in apple cultivation, from pest and disease control to improved storage and propagation.
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