Polyphenol oxidase gene family: Differential expression during vegetative and reproductive development, and in response to injuries, and defensive functional analysis

Plant polyphenol oxidases (PPOs) catalyze the oxidation of phenols to quinones. The secondary reactions of quinones lead to formation of polymeric brown or black pigments which are responsible for significant post-harvest losses of fruits and vegetables. A defensive role for PPO has frequently been suggested due to its conspicuous reaction products upon wounding or pathogenesis, and its wound inducibility. However, no conclusive evidence has yet been provided. In order to obtain additional insight into the biological function(s) of PPOs, I investigated the expression of PPOs during vegetative and reproductive development, and in response to various injuries, and analyzed the possible defensive function of PPO.; I showed the systemic induction of potato PPO activity, and increased steady-state levels of PPOs and PPO mRNA in respond to wounding. Furthermore, I documented the differential expression of tomato PPO A/C, B, D, and E/F genes during vegetative and reproductive development. The most abundant PPO transcripts are detected in young tomato leaves and flowers. PPO B is the most abundant PPO transcript in young leaves, while PPO expression in the inflorescence is dominated by both PPO B and E/F transcripts. Additionally, I provided evidence for cell type-specific, developmentally programmed PPO turnover, and accumulation of PPO in specific idioblast cells of tomato.; I demonstrated that antisense suppression of PPO renders tomato plants more susceptible to a virulent bacterial pathogen Pseudomonas syringae pv. tomato, and therefore provided the first direct evidence for PPO's role in mediating disease resistance. The hypersusceptibility of antisense plants may result from decreased (40-fold) constitutive PPO levels, and/or inability of these plants to induce PPO upon infection. To understand the importance of PPO inducibility to plant defense, I identified PPO F as an inducible gene member and documented its differentially induced expression patterns in response to various injuries and wound signals. The systemic and localized induction patterns of PPO F and its responsiveness to salicylic acid, jasmonates, and ethylene implicate PPO F in general resistance. These findings must be considered in any attempt to modify PPO expression for post-harvest quality.