Dual functions of thermospermine in plant growth and stress responses.

In seed plants, putrescine, spermidine, and spermine are ubiquitously present, whereas a structural isomer of spermine, thermospermine (TSpm), is synthesized mainly in the vascular tissue. Initially identified in the bacterium Thermus thermophilus, TSpm was later shown to be synthesized in Arabidopsis thaliana by ACAULIS5 (ACL5). ACL5 gene homologs may have been acquired early in plant evolution via endosymbiotic gene transfer from a cyanobacterial ancestor. Loss-of-function acl5 mutants exhibit a dwarf phenotype and excessive vascular xylem formation. Subsequent studies, including analysis of suppressor-of-acl5 (sac) mutants, revealed that TSpm exerts a critical role in the repression of vascular xylem proliferation by acting in upstream open-reading-frame (uORF)-dependent translational regulation of specific mRNAs. A recent study revealed functional TSpm binding to the peptidyl transferase center of 25 S rRNA promoted by methylation of residue U2952. Like other polyamines, TSpm has also been shown to participate in stress responses, enhancing tolerance to salt, drought, heat, and pathogen challenges in multiple species. Collectively, TSpm represents a unique polyamine with dual roles in xylem development and stress adaptation, whose evolutionary origin and molecular mechanisms provide insights into the specialization of polyamine biology. Further studies in nonvascular plants and algae are needed to elucidate the ancestral functions of TSpm.