Polyamines cause plasma membrane depolarization, activate Ca2+-, and modulate H+-ATPase pump activity in pea roots

Polyamines regulate a variety of cation and K⁺ channels, but their potential effects on cation-transporting ATPases are underexplored. In this work, noninvasive microelectrode ion flux estimation and conventional microelectrode techniques were applied to study the effects of polyamines on Ca²⁺ and H⁺ transport and membrane potential in pea roots. Externally applied spermine or putrescine (1mM) equally activated eosin yellow (EY)-sensitive Ca²⁺ pumping across the root epidermis and caused net H⁺ influx or efflux. Proton influx induced by spermine was suppressed by EY, supporting the mechanism in which Ca²⁺ pump imports 2 H⁺ per each exported Ca²⁺. Suppression of the Ca²⁺ pump by EY diminished putrescine-induced net H⁺ efflux instead of increasing it. Thus, activities of Ca²⁺ and H⁺ pumps were coupled, likely due to the H⁺-pump inhibition by intracellular Ca²⁺. Additionally, spermine but not putrescine caused a direct inhibition of H⁺ pumping in isolated plasma membrane vesicles. Spermine, spermidine, and putrescine (1mM) induced membrane depolarization by 70, 50, and 35 mV, respectively. Spermine-induced depolarization was abolished by cation transport blocker Gd³⁺, was insensitive to anion channels’ blocker niflumate, and was dependent on external Ca²⁺. Further analysis showed that uptake of polyamines but not polyamine-induced cationic (K⁺+Ca²⁺+H⁺) fluxes were a main cause of membrane depolarization. Polyamine increase is a common component of plant stress responses. Activation of Ca²⁺ efflux by polyamines and contrasting effects of polyamines on net H⁺ fluxes and membrane potential can contribute to Ca²⁺ signalling and modulate a variety of transport processes across the plasma membrane under stress.