Coding, Expression, Targeting, Import and Processing of Distinct Polyphenoloxidases in Tissues of Higher Plants

Abstract

Polyphenol oxidase (PPO) catalyzes the oxidation of phenols to quinones at the expense of O2. PPOs are ubiquitous in higer plants, and their role in oxidative browning of plant tissues causes large annual losses to food production. Despite the importance of PPOs to agriculture, the function(s) of PPOs in higher plants are not understood. Among other roles, PPOs have been proposed to participate in aspects of chloroplast metabolism, based on their occurrence in plastids and high Km for O2. Due to the ability of PPO to catalyze formation of highly reactive quinones, PPOs have also been proposed to be involved in a wide array of defensive interactions with insect, bacterial, and fungal pests. Physiological and biochemical studies of PPO have provided few answers to the major problems of PPO function, subcellular localization, and biochemical properties. This proposal achieved the following major objectives: cloning of PPO cDNAs in potato and tomato; characterization of the tomato PPO gene family; antisense downregulation of the tomato PPO gene family; and reduction in post-harvest enzymic browning of potato through expression of antisense PPO genes under the control of tuber-specific promoters. In addition, we established the lumenal localization of PPO, characterized and clarified the means by which PPOs are imported and processed by chloroplasts, and provided insight into the factors which control localization of PPOs. This proposal has thereby provided fundamental advances in the understanding of this enzyme and the control of its expression.