Polyphenol oxidases (PPOs) catalyse the oxidation of monophenols and/or o-diphenols to highly reactive o-quinones, which in turn interact with oxygen and proteins to form reactive oxygen species (ROS) and typical brown-pigmented complexes. Hence PPOs can affect local levels of oxygen and ROS. Although the currently known substrates are located in the vacuole, the enzyme is targeted to the thylakoid lumen, suggesting a role for PPOs in photosynthesis.
Boeckx et al. subject wild-type red clover (Trifolium pratense) and a low-PPO mutant to either environmental stress (cold and high light) or oxidative stress generated via methyl viologen, and find that PPO activity in leaves does not correspond with a direct role for the enzyme in the regulation or protection of photosynthesis under cold stress. However, more extensive oxidative damage to protein in mutants than wild-types is observed after treatment of attached leaves with methyl viologen, and PPO activity could be associated with an increased capacity to dissipate excess energy, but only at relatively low methyl viologen doses.
This article appears in the special issue ROS and NO Reactions in Plants.