Polyamines interact with hydroxyl radicals in activating Ca²⁺ and K⁺ transport across the root epidermal plasma membranes

Reactive oxygen species (ROS) are integral components of the plant adaptive responses to environment. Importantly, ROS affect the intracellular Ca²⁺ dynamics by activating a range of nonselective Ca²⁺-permeable channels in plasma membrane (PM). Using patch-clamp and noninvasive microelectrode ion flux measuring techniques, we have characterized ionic currents and net K⁺ and Ca2⁺ fluxes induced by hydroxyl radicals (OH^•) in pea (Pisum sativum) roots. OH^•, but not hydrogen peroxide, activated a rapid Ca²⁺ efflux and a more slowly developing net Ca²⁺ influx concurrent with a net K⁺ efflux. In isolated protoplasts, OH^• evoked a nonselective current, with a time course and a steady-state magnitude similar to those for a K⁺ efflux in intact roots. This current displayed a low ionic selectivity and was permeable to Ca²⁺. Active OH^•-induced Ca²⁺ efflux in roots was suppressed by the PMCa²⁺ pump inhibitors eosine yellow and erythrosine B. The cation channel blockers gadolinium, nifedipine, and verapamil and the anionic channel blockers 5-nitro-2(3-phenylpropylamino)-benzoate and niflumate inhibited OH^•-induced ionic currents in root protoplasts and K⁺ efflux and Ca²⁺ influx in roots. Contrary to expectations, polyamines (PAs) did not inhibit the OH^•-induced cation fluxes. The net OH^•-induced Ca²⁺ efflux was largely prolonged in the presence of spermine, and all PAs tested (spermine, spermidine, and putrescine) accelerated and augmented the OH^•-induced net K⁺ efflux from roots. The latter effect was also observed in patch-clamp experiments on root protoplasts. We conclude that PAs interact with ROS to alter intracellular Ca²⁺ homeostasis by modulating both Ca²⁺ influx and efflux transport systems at the root cell PM.