Thermospermine is a structural isomer of spermine, which is one of the polyamines studied extensively in the past, and is produced from spermidine by the action of thermospermine synthase encoded by a gene named ACAULIS5 (ACL5) in plants. According to recent genome sequencing analyses, ACL5-like genes are widely distributed throughout the plant kingdom. In Arabidopsis, ACL5 is expressed specifically during xylem formation from procambial cells to differentiating xylem vessels. Loss-of-function mutants of ACL5 display overproliferation of xylem vessels along with severe dwarfism, suggesting that thermospermine plays a role in the repression of xylem differentiation. Studies of suppressor mutants of acl5 that recover the wild-type phenotype in the absence of thermospermine suggest that thermospermine acts on the translation of specific mRNAs containing upstream open reading frames (uORFs). Thermospermine is a novel type of plant growth regulator and may also serve in the control of wood biomass production.