Throughout evolution, a number of animals including humans have lost the ability to
synthesize ascorbic acid (ascorbate, vitamin C), an essential molecule in the physiology
of animals and plants. In addition to its main role as an antioxidant and cofactor in redox
reactions, recent reports have shown an important role of ascorbate in the activation of
epigenetic mechanisms controlling cell differentiation, dysregulation of which can lead
to the development of certain types of cancer. Although fruits and vegetables constitute
the main source of ascorbate in the human diet, rising its content has not been a
major breeding goal, despite the large inter- and intraspecific variation in ascorbate
content in fruit crops. Nowadays, there is an increasing interest to boost ascorbate
content, not only to improve fruit quality but also to generate crops with elevated
stress tolerance. Several attempts to increase ascorbate in fruits have achieved fairly
good results but, in some cases, detrimental effects in fruit development also occur,
likely due to the interaction between the biosynthesis of ascorbate and components
of the cell wall. Plants synthesize ascorbate de novo mainly through the Smirnoff-
Wheeler pathway, the dominant pathway in photosynthetic tissues. Two intermediates
of the Smirnoff-Wheeler pathway, GDP-D-mannose and GDP-L-galactose, are also
precursors of the non-cellulosic components of the plant cell wall. Therefore, a better
understanding of ascorbate biosynthesis and regulation is essential for generation of
improved fruits without developmental side effects. This is likely to involve a yet unknown
tight regulation enabling plant growth and development, without impairing the cell redox
state modulated by ascorbate pool. In certain fruits and developmental conditions, an
alternative pathway from D-galacturonate might be also relevant.
