Specificity of polyamine effects on NaCl-induced ion flux kinetics and salt stress amelioration in plants

Polyamine (PA) levels in plants increase considerably under saline conditions. Because such an increase is believed to be beneficial for stress resistance, exogenous application of PAs has often been advocated as a means of ameliorating the detrimental effects of salinity. Results, however, are rather controversial, ranging from a significant amelioration to being ineffective or even toxic. The reasons for this controversy remain elusive. The ability of a root to retain K+ in the presence of NaCl was used as a physiological indicator to evaluate the ameliorative effects of PA. Pre-treatment with 1 mM Spm4+ (spermine), Spd3+ (spermidine) or Put2+ (putrescine) prevented salt-induced K+ leak only in the mature root zone of hydroponically grown maize and Arabidopsis. In contrast, in the distal elongation root zone, PA pre-treatment resulted in an even larger NaCl-induced K+ efflux, with the effect ranging from Spm4+ >Spd3+ = Put2+. A similar sequence has been also reported for H+ pump inhibition, measured for both root zones. It appears that PAs affect cell membrane transporters in a highly specific way, with a relatively narrow ‘window’ in which amelioration is observed. We suggest that the ameliorative affect of PAs is the result of a complex combination of factors which might potentially include PA transport and accumulation in the cell cytosol, their metabolization and the functional expression of the specific target proteins or signaling elements.