Multiple signaling networks of extracellular ATP, hydrogen peroxide, calcium, and nitric oxide in the mediation of root ion fluxes in secretor and non-secretor mangroves under salt stress

Using 3-month-old seedlings of Kandelia obovata and Aegiceras corniculatum, we compared multiple signaling networks controlling K⁺/Na⁺ homeostasis in salt-secreting and non-secreting mangroves. K. obovata roots exhibited a higher capacity to extrude Na⁺ and take up H⁺ after exposure to 100mM NaCl for 12h compared to A. corniculatum. However, the salt-induced Na⁺/H⁺ exchange was inhibited by amiloride, a Na⁺/H⁺ antiporter inhibitor, and sodium orthovanadate, a H⁺-ATPase inhibitor, indicating that Na⁺ efflux resulted from active Na⁺ exclusion. Ca²⁺, H₂O₂, NO, and extracellular ATP (eATP) enhanced Na⁺ extrusion by 27-152% in the salinized roots of K. obovata and A. corniculatum. Moreover, eATP, H₂O₂, and Ca²⁺ interacted with each other to increase Na⁺ extrusion in the two species. However, the interactions between NO and other signals differed in the secretor and non-secretor mangroves. Under a saline environment, NO enhanced the Na⁺ efflux elicited by Ca²⁺ in A. corniculatum. In contrast, NO reduced the Na⁺ efflux elicited by eATP, H₂O₂, and Ca²⁺ in K. obovata. In the two mangrove species, salt caused a net K⁺ efflux, but it was markedly restricted by the K⁺ channel blocker TEA, indicating that the K⁺ efflux is mediated by K⁺-permeable channels. In A. corniculatum, Ca²⁺ interacted with H₂O₂, NO, and eATP to reduce the K⁺ loss during salt treatment, whereas eATP enhanced the inhibitory effect of H₂O₂, Ca²⁺, and NO on K⁺ efflux in the non-secretor K. obovata. Thus, species-specific salt signaling networks regulating root K⁺ and Na⁺ flux were established in the secreting and non-secreting mangroves.