Extracellular ATP, hydrogen peroxide, calcium and nitric oxide mediate root ion fluxes in Bruguiera gymnorrhiza subjected to salt stress

Using the non-invasive micro-test technique (NMT), the effects of extracellular ATP (eATP), hydrogen peroxide (H₂O₂), calcium (Ca²⁺) and nitric oxide (NO) on root K⁺/Na⁺ homeostasis were investigated in a non-secretor mangrove species Bruguiera gymnorrhiza under short-term (ST, 24 hours) salt stress (100 mmol/L NaCl). The experiments of NaCl and Mannitol in B. gymnorrhiza roots indicated that there were mainly high ion-specific effects on NaCl-induced ion fluxes in roots. Salt stress resulted in an increased Na⁺ efflux along the root axis of B. gymnorrhiza. However, the salt-enhanced Na⁺ efflux was inhibited by amiloride, a Na⁺/H⁺ antiporter inhibitor or sodium orthovanadate, a plasma membrane (PM) H⁺-ATPase inhibitor obviously, suggesting that the Na⁺ efflux was resulted from active Na⁺ exclusion. NaCl treatment caused an evident K⁺ efflux in B. gymnorrhiza, but it was significantly restricted by the K⁺ channel blocker, TEA, suggesting that the K⁺ efflux was mediated by K⁺ permeable channels. Exogenously applied eATP (300 μmol/L), H₂O₂ (10 mmol/L), Ca²⁺ (10 mmol/L), or SNP (NO donor, 100 μmol/L) enhanced the salt-elicited Na⁺ efflux but reduced K⁺ efflux under ST treatment. Under salt stress, H₂O₂ and Ca²⁺ showed a high capacity to exclude Na⁺, while the inhibition of K⁺ efflux was more pronounced by Ca²⁺ or NO application. In conclusion, our data revealed that eATP, H₂O₂, Ca²⁺, and NO upward regulated Na⁺ /H⁺ antiporter and H⁺ pump, an activity of Na⁺/H⁺ antiport system, which compelled the Na⁺ /H⁺ exchange across the plasma membrane and restricted the K⁺ loss via depolarization-activated K⁺ channels simultaneously.