by Richard Aiken MD PhD @rcaiken
The US Department of Agriculture (USDA) on February 13, 2016, approved the first genetically modified (GM) apple developed to resist browning. They will go into production in the Midwest in the next few weeks (February, 2017).
Browning is caused by polyphenol oxidases (PPOs) naturally present in fruit and vegetables. When fruit is cut or bruised, these enzymes catalyze the oxidation of polyphenols to quinones, causing oxidative browning. The damage is superficial but can affect the taste and texture of the apple as well as its cosmetic qualities. In the Arctic varieties, the GM apples were genetically engineered with a transgene that produces specific RNAs to silence the expression of at least four browning PPO genes.
Is this a good idea?
The enzymes in the class polyphenol oxidase (PPO) appear to reside in the plastids of all land plants and are released when the plastid cell membrane is disrupted. PPO is thought to play an important role in the resistance of plants to microbial and viral infections and to adverse climatic conditions such as drought as although all land plants have PPO content, no PPO-like sequences have been reported in marine plants such as algae.
As stated above, in the presence of oxygen from air, the enzyme catalyzes the first steps in the biochemical conversion of phenolics to produce quinones, which undergo further polymerization to yield dark, insoluble polymers referred to as melanin. This is the same melanin that determines darkness of human skin and hair. In plants, melanin forms barriers and has antimicrobial properties that prevent the spread of infection in plant tissues.
Phenolic compounds are responsible for the color of many plants and impart taste and flavor, but more importantly, they are important phytonutrients and antioxidants.
Alteration of polyphenol oxidase
Given the activity of PPO in the adaptation of plants to, for example, plant dehydration, what are the implications of altered PPO activity on plant development, phenotype, and yield? A clear effect of PPO silencing was observed, for example, in walnut plants which developed spontaneous necrotic lesions in the leaves suggesting increased susceptibility to oxidative stress.
A potential role for PPO in photosynthesis has been speculated.
Data suggest that PPO activity can confer both a productive advantage and be associated with an increased risk of oxidative damage. While PPO activity can be associated with non-enzymatic reactive oxygen species scavenging involving flavonoid and phenolic acid substrates, a role for PPO in plant function may also be associated with its pro-antioxidant activity through the generation of secondary reaction products.
So it is obvious that the role of PPO is extensive and not fully understood. It appears premature to genetically modify plants to remove this complex molecule.
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 Parveen I, Threadgill MD, Moorby JM, Winters A. 2010. Oxidative phenols in forage crops containing polyphenol oxidase enzymes. Journal of Agricultural and Food Chemistry 58, 1371–1382.
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