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Choline is a fundamental metabolite in plants because of its contribution to the synthesis of the membrane phospholipid phosphatidylcholine, which accounts for 40 to 60% of lipids in non-plastid plant membranes. Choline is also a precursor for the formation of glycine betaine in certain plants such as spinach, where this osmoprotectant is accumulated and confers also tolerance to salinity, drought, and other environmental stresses. In addition choline has been recognized as an essential nutrient for humans. The choline biosynthetic pathway enables plants to decouple choline synthesis from lipid metabolism and provides them with the metabolic flexibility to adapt to environmental conditions where large and variable amounts of choline are beneficial for survival. The first step in choline biosynthesis is the direct decarboxylation of serine to ethanolamine, which is catalyzed by a serine decarboxylase unique to plants. Ethanolamine is widely recognized as the entrance compound to choline biosynthesis. The synthesis of choline from ethanolamine may take place at three parallel pathways, where three consecutive N-methylation steps are carried out either on free-bases, phospho-bases, phosphatidyl-bases or a mixture of the latter. The biosynthesis of choline appears to be regulated by a feedback response of phosphocholine inhibiting its own synthesis by decreasing N-methyltransferase (PEAMT) activities involved in the pathway. The activity of PEAMT was considerably increased when plants were exposed to salt stress, which is consistent with the high demand for choline as osmoprotectant precursor in such plants. The release of choline from the different pathway levels is species-specific. Phosphocholine can either be directly dephosphorylated to release choline as observed in spinach or incorporated into phosphatidylcholine with the subsequent release of choline, as in tobacco.
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