Ascorbic acid is of vital importance to the health of both animals and plants. In the human diet, ascorbic acid is known as vitamin C. Vitamins are defined as organic compounds required by an organism in a small amount that cannot be made by the organism. Humans cannot synthesize vitamin C and therefore must obtain it via their diet. As plants can synthesize their own ascorbic acid, this organic compound is not called a vitamin in plants. Plants synthesize ascorbic acid in very high concentrations and within the plant cells it acts as an antioxidant, providing plants with protection against oxygen free radicals generated by a wide variety of environmental assaults and also day-to-day conditions.  Ascorbic acid also is used in plants (and in animals) by a large number of enzymes. These enzymes are not able to do their jobs within the cell without ascorbic acid. Given these critical roles of ascorbic acid in both plants and humans, it is important that we have a full understanding of how plants synthesize this small molecule. Such information could be utilized in the future to aid in improving both plant and animal health, especially under adverse conditions.

Plants are able to control how much ascorbic acid they produce. For example, a plant grown in the shade synthesizes less ascorbic acid then the same plant growing under high light.  The main pathway by which plants synthesize ascorbic acid is called the Smirnoff/Wheeler pathway or alternatively, the D-mannose/L-galactose pathway. The latter name refers to the key intermediate sugars in this pathway during the enzymatic conversion of the simple sugar D-glucose to ascorbic acid.  The enzymes that catalyze each step in this pathway have been identified. What is not well understood is how this pathway is regulated. That is, how the players in this pathway are manipulated such that the plant can generate high amounts of ascorbic acid under certain conditions and then dial down the pathway at other times.  

Our lab at SUNY Cortland is studying the Vtc3 gene in the model plant Arabidopsis thaliana. This gene is present in all green plants and is likely to be of fundamental importance. Previously we isolated mutant Arabidopsis plants that have a defective vtc3 gene and are deficient in ascorbic acid. These mutants are not able to up-regulate ascorbic acid levels under high light or elevated temperatures and we believe that the VTC3 protein is therefore involved in the regulation of ascorbic acid synthesis in plants. 

 The VTC3 protein is highly unusual. The first half of the protein is predicted to act as a protein kinase while the second half of the protein is predicted to be a protein phosphatase. In the vast majority of cases, two separate genes encode these two types of proteins.  Protein kinases place phosphate groups on other proteins (or on themselves) and this leads to a change in the activity of the protein. Protein phosphatases remove phosphate groups from other proteins and also therefore alter the activity of this other protein(s).  Since the VTC3 protein contains both these, it is likely that the two proteins are working together.

In the research supported by this grant, we have discovered that both parts of the VTC3 protein likely have a positive role in ascorbic acid synthesis. That is, both parts of the protein are needed in order for the plant to make elevated levels of this small molecule. We have also shown that VTC3 is able to transmit a molecular signal to the enzymatic pathway that is needed to synthesize ascorbic acid, and is therefore indeed a player in the regulation of this pathway. However, increasing the level of VTC3 protein by itself is not enough to raise the levels of ascorbic acid above that of a normal plant. Therefore, it is clear that VTC3 alone is not the only important regulator of this pa...

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