Polyamines are small metabolites present in all living cells and play fundamental roles in numerous physiological events in plants. The aminopropyltransferases (APTs), spermidine synthase (SPDS), spermine synthase (SPMS) and thermospermine synthase (ACL5), are essential enzymes in the polyamine biosynthesis pathway. In angiosperms, SPMS has evolved from SPDS via gene duplication, whereas in gymnosperms APTs are mostly unexplored and no SPMS gene has been reported. Vuosku and colleagues aimed to investigate the functional properties of the SPDS and ACL5 proteins of Scots pine (Pinus sylvestris L.) to examine the role and evolution of APTs in higher plants.
The team analysed Scots pine seeds and seedlings for polyamines by high-performance liquid chromatography (HPLC) and the expression of PsSPDS and PsACL5 genes by in situ hybridization. They produced recombinant proteins of PsSPDS and PsACL5 and investigated the proteins for functional properties. Also gene structures, promoter regions and phylogenetic relationships of PsSPDS and PsACL5 genes were analysed.
Vuosku et al. found Scots pine tissues contained spermidine, spermine and thermospermine. PsSPDS enzyme catalysed synthesis of both spermidine and spermine. PsACL5 was found to produce thermospermine, and PsACL5 gene expression was localized in the developing procambium in embryos and tracheary elements in seedlings.
Contrary to previous views, Vuosku et al.’s results demonstrate that SPMS activity is not a novel feature developed solely in the angiosperm lineage of seed plants but also exists as a secondary property in the Scots pine SPDS enzyme. The discovery of bifunctional SPDS from an evolutionarily old conifer reveals the missing link in the evolution of the polyamine biosynthesis pathway. The finding emphasizes the importance of pre-existing secondary functions in the evolution of new enzyme activities via gene duplication. The results also associate PsACL5 with the development of vascular structures in Scots pine.