The diamine putrescine, the triamine spermidine and the tetramine spermine occur as free cations and as conjugates with phenolic acids and macromolecules in plants. Their levels of concentration increase greatly when the environment suffers changes; specially when there is potassium deficiency, water deficits, salinity stress, anaerobiosis and acid stress. Because polyamines are synthesized by amino acid decarboxylation reactions, where H+ is consumed, polyamine accumulation may function as a way for plants to keep their pH at a constant value. Polyamines also serve as precursors of various alkaloids that play important roles in plants defense.

The first polyamine formed is putrescine. This may happen by decarboxylation of ornithine or arginine.  These reactions are catalyzed by the enzymes ornithine decarboxylase (ODC) or arginine decarboxylase (ADC), respectively. The polyamines spermidine and spermine are formed by adding an aminopropyl moeity to putrescine and spermidine, respectively.

Polyamines can occur as free molecular bases in plant cells, but they also occur as conjugates, associated with small molecules like phenolic acids and macromolecules such as proteins.  Polyamines are usually conjugated to cinnamic acids, very often to p-coumaric, ferulic and caffeic acids. 

Adequate levels of polyamines are necessary for optimal growth and replication of plants, bacteria and fungi. 

End of Characterization