Catalyzes the reduction of 3'-oxosphinganine (3-ketodihydrosphingosine/KDS) to sphinganine (dihydrosphingosine/DHS), the second step of de novo sphingolipid biosynthesis. In plants, sphingolipids seems to play a critical role in mineral ion homeostasis, most likely through their involvement in the ion transport functionalities of membrane systems in the root. Lacks stereospecificity and can also produce L-threo-DHS in addition to D-erythro-DHS