Changes in polyamine titers associated with the proline response and osmotic adjustment of rape leaf discs submitted to osmotic stresses

The relationships between the changes in putrescine (Put) and polyamine (PA) contents and proline accumulation induced by osmotic stress were investigated using rape leaf discs (RLD) as an experimental model. The aliphatic PAs, Put, spermidine (Spd) and spermine (Spm), were detected in fresh cut RLD and the most abundant among them is Spd. In addition, the HPLC profile of the benzoylated PAs presented an unknown peak X23 which eluted after that of Spm and did not correspond to any commercially available PAs. X23 occurred also under conjugated forms which were not destroyed in acidic conditions. The study of the time courses of the changes in the levels of free PAs, X23 and proline demonstrated that in response to a moderate osmotic shock (-1.5 MPa) enhancement of PAs was detected after a 2-h lag phase, while the onset of proline accumulation started 2 h later. When RLD were submitted to wilting or to increased osmotic stress, using a range of osmotica, they exhibited osmotic adjustment which can be associated with an increase in putrescine and spermidine contents followed by a moderate rise of spermine. Both free and conjugated PA levels changed. The involvement of these compounds in metabolic adjustment, as well as their relationship with proline metabolism under stress conditions, are discussed.