Molecular cloning and characterisation of banana fruit polyphenol oxidase

Polyphenol oxidase (PPO; EC 1.10.3.2) is the enzyme thought to be responsib le for browning in banana [Musa cavendishii (AAA group, Cavendish subgroup) cv. Williams] fruit. Banana flesh was high in PPO activity throughout grow th and ripening. Peel showed high levels of activity early in development b ut activity declined until ripening started and then remained constant. PPO activity in fruit was not substantially induced after wounding or treatmen t with 5-methyl jasmonate. Banana flowers and unexpanded leaf roll had high PPO activities with lower activities observed in mature leaves, roots and stem. Four different PPO cDNA clones were amplified from banana fruit (BPO1 , BPO11, BPO34 and BPO35). Full-length cDNA and genomic clones were isolate d for the most abundant sequence (BPO1) and the genomic clone was found to contain an 85-bp intron. Introns have not been previously found in PPO gene s. Northern analysis revealed the presence of BPO1 mRNA in banana flesh ear ly in development but little BPO1 mRNA was detected at the same stage in ba nana peel. BPO11 transcript was only detected in very young flesh and there was no detectable expression of BPO34 or BPO35 in developing fruit samples . PPO transcripts were also low throughout ripening in both flesh and peel. BPO1 transcripts were readily detected in flowers, stem, roots and leaf ro ll samples but were not detected in mature leaves. BPO11 showed a similar p attern of expression to BPO1 in these tissues but transcript levels were mu ch lower. BPO34 and BPO35 mRNAs were only detected at a low level in flower s and roots and BPO34 transcript was detected in mature leaves, the only cl one to do so. The results suggest that browning of banana fruit during ripe ning results from release of pre-existing PPO enzyme, which is synthesiscd very early in fruit development.