Vitamin C or ascorbic acid is an antioxidant required for a range of essential metabolic reactions in all animals and plants. Plants synthesize ascorbic acid by a sequence of enzyme-driven steps, which convert sugars (monosaccharides) to vitamin C in a process called ascorbate biosynthesis. Professor Roger Hellens, working with Dr William Laing from New Zealand’s Plant and Food Research has unraveled a feedback regulation mechanism by which the plants regulate their levels of vitamin C.
Professor Hellens said plants responded to factors in the environment like extreme light or drought by producing vitamin C, a powerful antioxidant, to protect themselves from damage.
“Each cell assesses whether it should produce more of the antioxidant which would absorb the energy from the high levels of light or stop the damaging oxidative process in the dehydrated plant.”
GDP-L-galactose phosphorylase is a key enzyme that drives the ascorbate biosynthesis pathway. Scientists identified upstream cis-acting elements on the GDP-L-galactose phosphorylase RNA transcript that are regulated by ascorbic acid at high concentrations to inhibit the translation in to protein. Inhibition of GDP-L-galactose phosphorylase protein synthesis keeps a check on the ascorbate biosynthesis pathway thus controlling the levels of Vitamin C.
“In vitamin C regulation, it is the ascorbate molecules which interact with a critical enzyme in the biochemical pathways to make vitamin C. Plants can move the level of ascorbic acid between cells as needed.” Professor Hellens said plants had two ways to regulate cell processes. “One way is during transcription when DNA is turned into the messenger molecule RNA, the molecule that distinguishes cells into different types of tissue. The second way is to regulate while turning RNA into an enzyme that makes vitamin C. “So if a cell wants to increase its level of vitamin C it’s generally got two ways to do it – and we’ve discovered vitamins C uses the second method, and in an unexpected way. “We discovered it’s not whether the cell is making the RNA but whether the RNA is converted into a protein that is the deciding mechanism.”
“It’s very interesting because we found it was the level of vitamin C itself in each cell that decides whether RNA turns into the protein which makes vitamin C.”
“Understanding these mechanisms may help in plant breeding programmes to produce hardier plant crops and improve human health because iron deficiency anaemia is the most common form of malnutrition worldwide,” said Professor Hellens, from QUT’s Institute of Future Environments.
“This discovery will also help us to understand why some plants such as the Kakadu plum are able to accumulate super-high levels of vitamin C. Vitamin C is important in our diet because it enables more iron, which carries oxygen to our cells, to be taken up and absorbed. We humans gradually lost the ability to produce our own vitamin C thousands of years ago because it was so abundant in our hominid ancestors’ largely fruit diet. As we know, fruit can be higher in vitamin C than leafy vegetables so we can now study why fruit is so high and why some fruits make huge amounts.”
Original article can be accessed here.