Polyphenol oxidase is induced by chilling and wounding in pineapple

Abstract

Polyphenol oxidase (PPO) activity was found to be low in the leaves, roots, inflorescence tissues and developing and mature fruit of pineapple (Ananas comosus L.). In fruit affected by the chill-induced internal browning disorder known as Blackheart, PPO activity was 10-fold higher than in unaffected fruit, and there was a direct correlation between PPO activity and the severity of Blackheart symptoms. Degenerate oligonucleotide primers were designed to conserved regions of plant PPO genes, and used to amplify two distinct pineapple PPO cDNAs, designated PINPPO1 (2181 bp) and PINPPO2 (1319 bp), which share 81% sequence identity at the DNA level and show a high degree of homology to other plant PPO genes. PINPPO1 encodes a peptide of 604 amino acids, including a putative transit peptide of 95 amino acids and two copper-binding regions, CuA and CuB, which are highly conserved in plant PPOs. Southern analysis suggested the presence of at least four PPO genes in pineapple. Expression of PINPPO1 and PINPPO2 was low in roots, leaves, inflorescence tissues and developing fruit, but was strongly up-regulated in response to chilling and wounding. These results indicate that PPO is synthesised de novo in response to chilling of pineapple fruit, and implicate a role for the enzyme in the development of Blackheart disorder.