Proline metabolism in Arabidopsis. Schematic illustration of the current knowledge on Pro metabolism and its intracellular distribution. Black arrows indicate metabolic fluxes, enzymes are given in bold blue letters, for abbreviations see the main text. The molecular identity of mitochondrial and plastidic Pro and glutamate transporters is currently not known. Alternating localisation between cytosol and plastids was demonstrated for P5CS1 and P5CS2, the localisation of P5CR requires further detailed analysis. Export or leakage of GSA/P5C from mitochondria was postulated but experimental evidence is lacking.