Plants like Arabidopsis use arginine for storing nitrogen and proline for stress defense. A specific enzyme, ornithine-δ-aminotransferase, is crucial for breaking down arginine but not for making proline. This enzyme helps convert ornithine to a compound called P5C, which is important for arginine breakdown. Plants without this enzyme couldn't use arginine or ornithine as nitrogen sources. However, they could still make proline in response to stress. This shows that proline production in Arabidopsis doesn't rely on the same process as breaking down arginine and ornithine.