One of the origins of plasma membrane phosphatidylserine in plant cells is a local synthesis by a serine exchange activity

In plant cells, as in animal cells, the endoplasmic reticulum (ER) is considered to be the major site of phospholipid synthesis, and it has been shown that phosphatidylserine (PS) reaches the plasma membrane via the vesicular ER‐Golgi‐plasma membrane pathway in leek cells. However, it has never been determined whether the plasma membrane of leek cells is able to synthesize PS. We have analyzed the distribution of PS synthesizing enzymes along the vesicular pathway. In ER, Golgi and plasma membrane fractions isolated from leek cells, we have measured the activity of the two biosynthetic pathways leading to the synthesis of PS, i.e. serine exchange and CTP cytidylyltransferase plus PS synthase. We have found a high serine exchange activity in the plasma membrane fraction, and then determined that this membrane is able to synthesize both long chain fatty acid‐ and very long chain fatty acid‐containing PS. Therefore, the PS in the plasma membrane of leek cells has two different origins: the intracellular vesicular pathway from the ER and a local synthesis in the plasma membrane.