Polyphenol Oxidase (PPO), responsible for apple browning, is
encoded by a diverse, multi-gene family whose members are expressed
both in early fruit development and in response to wounding. The
suppression of PPO, which is expected to lead to a nonbrowning
phenotype, requires the use of a chimeric suppression transgene
containing chosen sequences from the PPO gene family.
The PPO suppression transgene (PGAS) consists of 394, 457, 457 and
453 bp regions of apple PPO genes (PPO2, GPO3, APO5, pSR7,
respectively), placed in the sense orientation. Suppression is
achieved through the formation of dsRNA between the suppression
transcript and native PPO mRNA. This is subsequently processed into
small interfering RNAs (siRNAs) that direct the cleavage of the
The transgene is designed to reduce overall expression of the
entire apple PPO gene family, and to induce a reduced browning or
nonbrowning phenotype in apple.