P5CS plays a key role in proline biosynthesis, leading to osmoregulation in plants. Involved in abiotic stress tolerance. ATP + L-glutamate = ADP + L-glutamyl 5-phosphate L-glutamate 5-semialdehyde + NADP(+) + phosphate = H(+) + L-glutamyl 5-phosphate + NADPH Feedback regulated by proline. Amino-acid biosynthesis; L-proline biosynthesis; L-glutamate 5-semialdehyde from L-glutamate: step 1/2. Amino-acid biosynthesis; L-proline biosynthesis; L-glutamate 5-semialdehyde from L-glutamate: step 2/2. Tobacco plants over-expressing P5CS1 and P5CS2 have elevated proline levels and display enhanced abiotic stress tolerance. In the N-terminal section; belongs to the glutamate 5-kinase family. In the C-terminal section; belongs to the gamma-glutamyl phosphate reductase family.