Knowing that kiwifruit were very rich in vitamin C, researchers at Plant & Food Research were prompted in 2002 to find out why. Initial investigations revealed that a biosynthetic pathway for vitamin C production in plants was not well understood, so they set about confirming a pathway first proposed by British scientists Glen Wheeler and Nickolas Smirnoff in 1998.
The pathway consists of a number of steps, each one catalysed by a unique enzyme. Several of these enzymes had been postulated but not identified. Using kiwifruit as an enzyme source, in 2004, the team were able to confirm the existence of an enzyme capable of converting L-galactose-1-phosphate into L-galactose, which was one of the steps in the process.
Further research followed, and working with various kiwifruit varieties, coupled with the world’s largest kiwifruit DNA database, the team isolated the last undiscovered enzyme in the vitamin C production pathway. This enzyme is an important control point of vitamin C production in plants catalysing the step GDP-galactose to L-galactose-1-phosphate. This discovery in 2007 gained worldwide recognition and completed a scientific quest that began in Europe in the 1930s.
Plant & Food Research scientists have now turned their attention to the numerous varieties of kiwifruit and why it is that vitamin C levels vary markedly from one variety to another. Work is under way to try to identify the genes controlling the production of vitamin C in these different varieties.
Dr Sean Bulley is a member of the team of scientists working on this project. He has identified a number of candidate genes from the kiwifruit database and is conducting experiments to validate their involvement.
Once this has been done, a plant breeding programme can be initiated that utilises these genes in such a way that the plant vitamin C production process is switched on to a higher than normal level. The knowledge gained from this work with kiwifruit can then be used to develop cultivars of other plants, such as apples, that will produce fruit with higher levels of vitamin C. Work is also being done to identify molecular markers that will enable the quick recognition of seedlings capable of high levels of vitamin C biosynthesis. The fruit produced from these seedlings, on maturity, can be used by plant breeders to develop new varieties with high vitamin C content.
Ultimately the aim is to help breeders identify good parents – breeders then make the cross. Seeds from the cross are collected and planted out, and seedlings are screened for the presence of correct marker, greatly cutting down the numbers of seedlings to grow up to see what kind of fruit they bear, saving time and space. A limited number of selections are taken forward to advanced trials resulting in a new variety with desirable characteristics including high vitamin C. The new selections have to meet a range of criteria as only the best fruit will do.
Importance of vitamin C
Vitamin C is a crucial component in the human diet and although only small amounts are required on a daily basis (45mg per day) its long-term absence can result in death. Most of our daily requirement of vitamin C is obtained through the consumption of fresh fruit and vegetables. The outcomes of Plant & Food’s research programme could well be a fruit variety such as an apple or kiwifruit with a recommended daily dose of vitamin C in one piece of fruit.