Editing Vitamin C (section)

=== Plant synthesis ===
[[File:Vitamin C Biosynthesis in Plants.svg|thumb|upright=1.75|class=skin-invert-image|Vitamin C biosynthesis in plants]]
There are many different biosynthesis pathways to ascorbic acid in plants. Most proceed through products of [[glycolysis]] and other [[metabolic pathway]]s. For example, one pathway utilizes plant [[cell wall]] polymers.<ref name="valpuesta"/> The principal plant ascorbic acid biosynthesis pathway seems to be via {{sm|l}}-galactose. The enzyme [[L-galactose 1-dehydrogenase|{{sm|l}}-galactose dehydrogenase]] catalyzes the overall [[Organic redox reaction|oxidation]] to the [[lactone]] and isomerization of the lactone to the C4-hydroxyl group, resulting in {{sm|l}}-galactono-1,4-lactone.<ref name="West Sussex 2009"/> {{sm|l}}-Galactono-1,4-lactone then reacts with the mitochondrial flavoenzyme [[Galactonolactone dehydrogenase|{{sm|l}}-galactonolactone dehydrogenase]]<ref name="pmid18190525">{{cite journal | vauthors = Leferink NG, van den Berg WA, van Berkel WJ | title = l-Galactono-gamma-lactone dehydrogenase from Arabidopsis thaliana, a flavoprotein involved in vitamin C biosynthesis | journal = The FEBS Journal | volume = 275 | issue = 4 | pages = 713–26 | date = February 2008 | pmid = 18190525 | doi = 10.1111/j.1742-4658.2007.06233.x | s2cid = 25096297 | doi-access = free | title-link = doi }}</ref> to produce ascorbic acid.<ref name="West Sussex 2009"/> {{sm|l}}-Ascorbic acid has a negative feedback on {{sm|l}}-galactose dehydrogenase in spinach.<ref name="pmid15509850">{{cite journal | vauthors = Mieda T, Yabuta Y, Rapolu M, Motoki T, Takeda T, Yoshimura K, Ishikawa T, Shigeoka S | title = Feedback inhibition of spinach L-galactose dehydrogenase by L-ascorbate | journal = Plant & Cell Physiology | volume = 45 | issue = 9 | pages = 1271–9 | date = September 2004 | pmid = 15509850 | doi = 10.1093/pcp/pch152 | doi-access = free | title-link = doi }}</ref> Ascorbic acid efflux by embryos of dicot plants is a well-established mechanism of iron reduction and a step obligatory for iron uptake.{{efn| Dicot plants transport only [[ferrous iron]] (Fe<sup>2+</sup>), but if the iron circulates as [[ferric]] complexes (Fe<sup>3+</sup>), it has to undergo a reduction before it can be actively transported. Plant embryos efflux high amounts of ascorbate that chemically reduce iron(III) from ferric complexes.<ref name="pmid24347170">{{cite journal | vauthors = Grillet L, Ouerdane L, Flis P, Hoang MT, Isaure MP, Lobinski R, Curie C, Mari S | title = Ascorbate efflux as a new strategy for iron reduction and transport in plants | journal = The Journal of Biological Chemistry | volume = 289 | issue = 5 | pages = 2515–25 | date = January 2014 | pmid = 24347170 | pmc = 3908387 | doi = 10.1074/jbc.M113.514828 | doi-access = free | title-link = doi }}</ref>}}

All plants synthesize ascorbic acid. Ascorbic acid functions as a cofactor for enzymes involved in photosynthesis, synthesis of plant hormones, as an antioxidant and regenerator of other antioxidants.<ref name=Gallie2013>{{cite journal | vauthors = Gallie DR | title = L-ascorbic acid: a multifunctional molecule supporting plant growth and development | journal = Scientifica | volume = 2013 | pages = 1–24 | year = 2013 | pmid = 24278786 | pmc = 3820358 | doi = 10.1155/2013/795964 | doi-access = free | title-link = doi }}</ref> Plants use multiple pathways to synthesize vitamin C. The major pathway starts with glucose, [[fructose]] or [[mannose]] (all simple sugars) and proceeds to {{sm|l}}-[[galactose]], {{sm|l}}-galactonolactone and ascorbic acid.<ref name=Gallie2013 /><ref name=Mellidou2017 /> This biosynthesis is regulated following a [[diurnal rhythm]].<ref name="Mellidou2017" /> Enzyme expression peaks in the morning to supporting biosynthesis for when mid-day sunlight intensity demands high ascorbic acid concentrations.<ref name=Mellidou2017>{{cite journal | vauthors = Mellidou I, Kanellis AK | title = Genetic control of ascorbic acid biosynthesis and recycling in horticultural crops | journal = Frontiers in Chemistry | volume = 5 | pages = 50 | year = 2017 | pmid = 28744455 | pmc = 5504230 | doi = 10.3389/fchem.2017.00050 | bibcode = 2017FrCh....5...50M | doi-access = free | title-link = doi }}</ref><ref name="pmid27179323">{{cite journal | vauthors = Bulley S, Laing W | title = The regulation of ascorbate biosynthesis | journal = Current Opinion in Plant Biology | volume = 33 | pages = 15–22 | date = October 2016 | pmid = 27179323 | doi = 10.1016/j.pbi.2016.04.010 | series = SI: 33: Cell signalling and gene regulation 2016 | bibcode = 2016COPB...33...15B }}</ref> Minor pathways may be specific to certain parts of plants; these can be either identical to the vertebrate pathway (including the GLO enzyme), or start with inositol and get to ascorbic acid via {{sm|l}}-galactonic acid to {{sm|l}}-galactonolactone.<ref name=Gallie2013 />