Polyamines are small polycationic organic molecules present in all living beings. They regulate plant development processes such as flowering time, stem elongation, early fruit development, and senescence. Thermospermine is a tetraamine, unique to plants and prokaryotes, whose deficiency in Arabidopsis thaliana –caused by a knockout mutation in the single thermospermine synthase gene, ACL5¬– provokes a dwarf phenotype, as well as severe defects in the metaxylem differentiation, and an altered deposition of cell-wall material in these vessels. 
Aminopropyl transferases catalyze the synthesis of tri-and tetraamine, by adding an aminopropyl group to a di-or triamine, respectively. They show a high sequence similarity to each other, shared also with putrescine-N-methyltransferases, another group of enzymes characteristic of Solanaceae. Phylogenetic studies have shown that aminopropyl transferases and putrescine-N-methyltransferases are evolutionarily related.
The goals that we considered in this thesis were:
• to establish the mechanism by which thermospermine controls xylem differentation; and
• to determine the structural differences between the various aminopropyl- and N-methyltransferases that underlie their specificity.
The first goal was approached by a search for extragenic suppressors of acl5. We identified 40 suppressors, 11 of which present mutation in the 5’-UTR of three bHLH genes named AJAX. These mutations are located in a sequence that encodes a small peptide whose role is to repress the translation of the main ORF. Our in vitro studies reveal that the suppression is achieved by interfering with the synthesis or activity of this small peptide, therefore allowing the synthesis of AJAX proteins. The role of these transcription factors in the regulation of gene expression during xylem development partially overlaps with that of thermospermine, although not completely, as indicated by transcriptomic analyses of AJAX2 overexpressing plants. 
The second objective was approached by the comparison between active centers of aminopropyl transferases and putrescine-N-methyltransferases, and subsequent identification of aminoacids with a potential key role in the establishment of differential activity. The corresponding aminopropyl transferases from Arabidopsis were produced in their native version and with the key residues mutated as in putrescine-N-methyltransferases, and their kinetic parameters were determined. In none of the cases we observed conversion of one specific activity into another, but we confirmed the importance of those residues for a normal level of enzyme activity.