Extracellular ATP mediates cellular K⁺/Na⁺ homeostasis in two contrasting poplar species under NaCl stress

Abstract

Key message

eATP mediates cellular K ⁺ and Na ⁺ homeostasis in two contrasting poplar species differing in salt tolerance.

Abstract

Using the non-invasive micro-test technology (NMT), the effects of extracellular ATP (eATP) on salt-altered flux profiles of K⁺, Na⁺, and H⁺ were investigated in salt-tolerant poplar species, Populus euphratica and salt-sensitive P. popularis. A short-term NaCl (100 mM NaCl, 12 h) resulted in a Na⁺ efflux and a correspondingly increased H⁺ influx in P. euphratica cells, but the effect was not seen in P. popularis. ATP (50 μM) enhanced exchange of Na⁺ with H⁺ in salt-stressed cells of two species (6, 12 h), especially in P. popularis. However, the ATP-stimulated Na⁺ efflux and H⁺ influx were significantly inhibited by amiloride (a Na⁺/H⁺ antiporter inhibitor) or sodium orthovanadate (a plasma membrane H⁺-ATPase inhibitor), indicating that the ATP induction of Na⁺ extrusion resulted from an active Na⁺/H⁺ antiport across the plasma membrane (PM). NaCl accelerated K⁺ efflux in the two species, with a more pronounced effect in the salt-sensitive poplar. The salt-induced K⁺ efflux was markedly restricted by the K⁺ channel blocker, tetraethylammonium chloride, indicating that the K⁺ efflux is mediated by depolarization-activated outward rectifying K⁺ channels and non-selective cation channels. ATP benifited poplar cells, especially the salt-sensitive P. popularis, in maintaining K⁺ homeostasis under external salinity. This was likely the result of activated H⁺ pump in the PM, which restricted the K⁺ efflux through the inhibition of depolarization-activated K⁺ channels in both species. Na⁺ and K⁺ flux recordings revealed that non-hydrolysing analogues of ATP, αβ-meATP (50 μM), and ATPγS (50 μM) produced an effect similar to that of the hydrolysable form but with a more pronounced effect. However, ADP- and AMP-stimulated cells (50 μM) exhibited behaviors different from those invoked by ATP, αβ-meATP, and ATPγS treatments. eATP signalling in K⁺ and Na⁺ homeostasis was blocked by the antagonists of animal P2 receptors, PPADS, and suramin. Moreover, ATP-stimulated Na⁺ extrusion and reduction of K⁺ loss in NaCl-stressed cells were inhibited by LaCl₃ (an inhibitor of Ca²⁺-permeable channels) and DPI (an inhibitor of PM NADPH oxidase), indicating that ATP signalling was mediated via second messengers, H₂O₂ and Ca²⁺, in the two poplars differing in salt tolerance.