Thermospermine synthase (, TSPMS, ACL5 (ACAULIS5), SAC51) is an enzyme with systematic name S-adenosylmethioninamine:spermidine 3-aminopropyltransferase (thermospermine synthesizing). This enzyme catalyses the following chemical reaction
This enzyme is required for correct xylem specification through regulation of the lifetime of the xylem elements.