Apple (Malus pumila Mill.) is one of the most common fruit trees and has attracted many researchers in genetic transformation. Since the first trans-genic apple was reported in 1989 (James et al. 1989), more than 20 cultivars have been transformed successfully (Table 3.1). In most cases, Agrobacterium tumefaciens-mediated leaf disc transformation were adopted. Protoplast-mediated transformation seemed not to be available, since no integrated transgenic apple plant was regenerated with this method. Among these transformation cases, five agronomic traits were targeted: (1) genes including scab resistant genes like wheat puroindoline B (pinB), homologues of Cladosporium fulvum resistance genes of Vf (HcrVf2) from wild Malus species, endochitinase or exochitinase of the biocontrol fungus Trichoderma atroviride; (2) fire blight resistant genes like antimicrobial peptides (AMPs), endogenous alleles of the S-gene, an inducible antibacterial protein from Hyalophora cecropia pupae (attacin E), a modified cecropin SB37 gene (MB39); (3) rooting ability increasing genes like rol A, B, C; (4) insect resistant genes like the biotin-binding proteins avidin or strepavidin; and (5) fruit traits modifying genes like PPO (polyphenol oxidase), ACO and ACS (Table 3.1).
The most destructive disease of commercial apple orchards is scab caused by Venturia inaequalis, which attacks both the foliage and the fruit, thus resulting in reduced yield. Several apple cultivars highly susceptible to this disease such as 'Gala', 'Macintosh', 'Galaxy', and the hybrid 'Ariane' had been the subject of attempts to improve scab resistance by genetic engineering. Therefore, transgenic plant production represents the basis for further investigation of resistance mechanism and a step toward gene therapy of scab-susceptible cultivars that currently dominate the apple industry. As far as rootstock breeding is concerned, Agrobacterium rhizogenes and its root loci of rol A, B, C are considered helpful and can be transferred into plants. Several apple rootstocks such as M26, A2, Jorkj, M7 and Marubakaidou had been transformed successfully, which helped to resolve the difficulty of rooting and thus be attractive both for increasing productivity and for dwarfness. The antisense PPO gene was introduced into apple callus and shoot to reduce browning, which was not only useful for the food industry but also for the studies of the metabolism of polyphenols and the function of PPO (Murata et al. 2001). Transgenic 'Greensleeves' apple with reduced ethylene synthesis was obtained by expressing antisense apple ACO or ACS, which resulted in improved firmness and low ester accumulation in fruit for elongating the storage and shelf life (Dandekar et al. 2004).
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