Chloride homeostasis via enhanced salt ion secretion selectivity contributes to salt tolerance of recretohalophytic mangrove Avicennia marina

Avicennia marina, a pioneer recretohalophyte mangrove species living in intertidal wetlands, has high salinity tolerance. To understand the mechanism of salt adaptability, the seedlings of A. marina were treated with different salinities. We hypothesized that A. marina would demonstrate adaptive mechanisms at both physiological and molecular levels to tolerate tidal salinity through ion selectivity and chloride homeostasis.

In this study, the combination of growth, chlorophyll fluorescence analysis, electron microscopy, electrophysiological and molecular approaches were employed to investigate the mechanism of salt adaptability of A. marina seedlings.

400 mM NaCl promoted chloroplast development, and chlorophyll fluorescence Fv/Fm, Fv/Fo, and qP decreased with increasing NaCl concentration, while ETR remained stable. NaCl contributed to increased Na⁺ and Cl⁻ content in roots, leaves and exudates of salt glands, while relative high Cl⁻ was stored in the roots and more Na⁺ in the leaves. Flux measurements of Na⁺, Cl⁻, K⁺, H⁺, and NO₃⁻ in the leaf salt glands and the root tips treated with 0 and 400 mM NaCl indicated the presence of an ion selectivity that was suppressed by specific Na⁺/H⁺ antiporter inhibitor amiloride, PM H⁺-ATPase inhibitor vanadate, and Na⁺:K⁺:2Cl⁻ cotransporter inhibitor bumetanide. Further analysis showed that the expressions of chloride transport-related homolog genes CLCc, CLCf, and CCC1 were significantly up-regulated, whereas CLCd was down-regulated under salt treatment.

In conclusion, the chloride homeostasis regulated by salt ion secretion selectivity and transport-associated genes in leaf and root serves a crucial role in the salt tolerance of A. marina.