Polyamines induce membrane depolarization and cationic fluxes in roots

Polyamines (PAs) are unique polycationic metabolites in living cells. Their levels change in a specific manner during different stresses. PAs may influence membrane transport and potential difference (PD) by blocking and modulating some K+ and cation channels, and H+ pumps. In barley and pea roots resting MP values are -110 and -140 mV; our data suggest that PD values are controlled by oppositely acting pump and non-selective cation channel (NSCC) activities, whereas K+ channels are less important. PAs (1 mM) provoked irreversible depolarizations in pea roots, with a relative potency spermine (Spm4+) > spermidine (Spd3+) > putrescine (Put2+). Spm4+- induced depolarization was: i) independent on the ROS-generation, ii) insensitive to anion channel blockers, iii) completely abolished by NSCC blocker Gd3+ and iv) slowed down by chelating of external Ca2+. At lower (0.1 mM) concentration Spm4+ caused a small but significant transient hyperpolarization. Ion flux measurements show that 0.1 mM and 1 mM Spm4+ induce a fast transient H+ efflux and a stable H+ influx, respectively. The latter was roughly equivalent to a concomitantly induced Ca2+ efflux. Spm4+ (1 mM)-induced cation fluxes were insensitive to the Ca2+/H+ exchanger inhibitor amiloride, but strongly suppressed by P-type pump blocker vanadate and Ca2+- pump blocker eosine yellow. Put2+ (1 mM) evoked a Ca2+ efflux equal to that caused by Spm4+, but in contrast to Spm4+ evoked a stable H+ efflux. Thus, Spm4+ and Put2+ activate the Ca2+-pump, whereas Put2+ >> Spm4+ activates also the H+ pump. Consequently, import of PAs, most likely via NSCCs, has to be considered to explain the PAs- induced depolarization. Supported by CONACYT (CB-2007 82913) and ARC grants.