Cloning of PPO Genes

Scientific interest in molecular biology of PPO is that the molecular weight and the form of PPO have long been a matter of speculation because of the various forms of the PPO as described earlier. Identification of PPO genes offers a great potential to produce transgenic plants having regulated levels of PPO. Because PPO is responsible for significant decreases in postharvest quality of many crops due to enzymatic browning associated with injury or senescence, downregulation of PPO using antisense could significantly increase quality attributes of most fruits and vegetables. However, overexpression of PPO may potentially minimize pest damage to crop plants. This plant de fense system is one of many theories of PPO function in plant. Enzymatic browning generated by PPO in a multitude of plant—pest interactions and its wound inducibility have led some scientists to the view that the primary role of PPO is in plant defense (58). Therefore, the challenge here is to selectively modify PPO expression in specific tissue to minimize its effects on browning of fruits and vegetables, while maximizing PPO expression in other parts of the plant to maintain pathogen or herbivore resistance. Much progress has been made recent years. PPO genes have been characterized from broad bean, tomato, potato, and grape, and all PPOs cloned have been found to encode mature peptides of 57,000 to 62,000 Da with 8,000 to 11,000 Da putative transit peptides, which posttransla-tionally directs the protein to the chloroplast envelope (7,35,51-53). The catalytic centers of the copper binding regions for V. faba and tomato PPOs have been elucidated. The further characterization of PPO in various plants is being made, and there is a great potential in near future to manipulate the levels of PPO in plant foods to overcome the adverse effects of PPO.

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