Polyamine metabolism and activation of lipid signalling pathways in Arabidopsis thaliana

Polyamine metabolism has emerged during the last 40 years as an important stress-related pathway and its accumulation is often a metabolic hallmark of enhanced plant stress tolerance. These small and versatile polycationic molecules are not only involved in stress responses, but also in many aspects of cellular biology and -physiology, affecting processes such as ion transport, protein- and lipid-kinase activity, transcription, protein synthesis, or stabilization of membrane- and nuclear components. Nonetheless, the molecular mechanism by which polyamines exert such a wide range of effects is still not well understood. To extend these studies, in this thesis we investigate the polyamine back-conversion pathway during salt stress in Arabidopsis thaliana using loss-of-function mutants of the AtPAO5 gene, the most-induced PAO member in salt conditions. In a different approach, we investigate the effect of polyamines on plasma membrane-associated phospholipid signalling, using an optimized ³²P_i-labelling procedure that analyses phospholipid synthesis and turnover in vivo. In those studies, we demonstrate that polyamines trigger an increase of the lipid-signalling molecules phosphatidylinositol 4,5-bisphosphate and phosphatidic acid, in root cells of Arabidopsis seedlings. The detailed characterization of the phospholipid responses carried out in this thesis, provides novel and interesting tools to further unravel the role of polyamines in eukaryotic cell biology in general and in plant biology in particular.