All we see is a piece of bruised fruit on the supermarket shelf – but what are the mechanisms behind bruising and browning, and how does it protect damaged plants against both bacterial and fungal disease?

Making sure that food reaches the consumer in good condition is both big business and an environmental issue.

Catechol oxidase activity may be of huge economic importance. It has been estimated that half the world’s fruit and vegetable crop is lost due to post-harvest browning reactions due to the enzyme.

Every piece of food has an environmental impact, and the fertilisers and energy used to produce it are wasted if it is thrown away uneaten. Uneaten food is also a financial loss to the producer or retailer.

This project starter contains ideas for investigating the enzyme activity in plant tissues, suitable for Advanced Higher Biology investigations and EPQs.

Background information

Catechol oxidase (also called polyphenoloxidase) is an enzyme found in a wide variety of plants.  It is responsible for the darkening observed when many fruits or vegetables are cut or bruised.

Catechol oxidase catalyses the reaction of catechol, which is colourless, to the yellow compound, ortho-quinone (o-quinone).  o-quinone then reacts with oxygen in the air to form brown-black compounds called melanins.

Catechol is present in small quantities in the vacuoles of cells of many plant tissues.  Catechol oxidase is present in the cell cytoplasm.

If the plant tissues are damaged, the catechol is released and the enzyme converts the catechol to ortho-quinone, which is a natural antiseptic.

Catechol oxidase therefore has a role in plant defence mechanisms, helping to protect damaged plants against both bacterial and fungal disease.

It has been suggested that the quantity of catechol oxidase produced by a plant may be related to its susceptibility to fungal infection.

Catechol oxidase activity is also of economic importance.  It has been estimated that half the world’s fruit and vegetable crop is lost due to post-harvest browning reactions due to the enzyme.

 

Further reading

• WRAP website. WRAP is a UK charity that focuses on the sustainable use of resources. They have a particular focus on understanding the causes of food waste and hence reducing it.
• Genetically modified non-browning apples are approved in the U.S. This article from Discover Magazine describes a new transgenic variety of apple that ‘turns off’ the gene that controls the browning process seen when an apple is cut and exposed to the air (Discover Magazine, February 18, 2015).
• ‘Smarter’ plant breeding to cut lettuce waste. Researchers from Harper Adams University are identifying the genes that make certain varieties of lettuce more susceptible to browning than others. They hope this will enable them to breed new varieties that will take longer to turn brown, especially in bagged salad mixes.
• Writtle College has a well-respected centre for Post-Harvest Technology. Their research gives an insight into the key issues currently being addressed by UK businesses and research.

 

Practical Investigations – step-by-step protocols

We’ve put together step-by-step protocols for you to use for a practical investigation on this topic.