**5. Conclusions**

From its role as an antioxidant essential for photosynthesis and for detoxifying ROS from endogenous and exogenous sources, to its role in regulating cell division and flowering, to its function as a co-factor in multiple enzymatic reactions, ascorbic acid has fundamentally enabled the colonization of land by plant species. This is likely due to the challenge that the rise in atmospheric oxygen during Earth's past presented to multicellular organisms, which required limiting the harmful consequences of increased exposure to oxygen that a land-based existence entails. Vitamin C is critical to plants as it is unlikely they could tolerate a single day of exposure to sunlight without ascorbic acid detoxifying the ROS generated by photosynthetic activity. In contrast, animals unable to synthesize ascorbic acid, such as humans, can survive the absence of the vitamin for weeks or even months before succumbing to disease and death. Despite the importance of its role in detoxifying ROS, ascorbic acid's functions are now so integrated into plant growth and development that its importance cannot be underestimated. Because of the complexity of its many roles, any attempts to

engineer changes in ascorbic acid content in plants that improves one aspect, such as nutritional content, will require close examination of how such changes impact the overall health and performance of the plant under field conditions. In addition to the engineering approaches described above, genetic diversity within plants offers another means to increase Asc content through standard breeding approaches [109], although whether changes in Asc content through these means may limit any deleterious effects on plant growth and development is unknown at this time. The most successful strategies will undoubtedly involve highly targeted approaches to alter ascorbic acid content in specific cell types or tissues to achieve a desired end while limiting possible unintended consequences in other aspects of growth, development, and responses to biotic and abiotic stresses.
