Quantitative proteomics of Sesuvium portulacastrum leaves revealed that ion transportation by V-ATPase and sugar accumulation in chloroplast played crucial roles in halophyte salt tolerance.

UNLABELLED: Physiological and proteomic responses of Sesuvium portulacastrum leaves under salinity were investigated. Different from glycophytes, this halophyte had optimal growth at 200-300mM NaCl and accumulated more starch grains in chloroplasts under high salinity. Increased contents of soluble sugars, proline, and Na(+) were observed upon salinity. X-ray microanalysis revealed that Na(+) was mainly compartmentalized into cell vacuole. Quantitative proteomics produced 96 salt responsive proteins, and the majority was chloroplast-located proteins. Gene ontology analysis revealed that proteins involved in ion binding, proton transport, photosynthesis and ATP synthesis were overrepresented. The expressions of a Na(+)/H(+) antiporter and several ATP synthase subunits were activated upon high salinity. ATP hydrolysis assay demonstrated that V-ATPase activity at tonoplast was dramatically increased upon NaCl whereas vacuolar H(+)-pyrophosphatase and plasma membrane P-ATPase activities were not increased, which indicated that sodium compartmentalization was mainly performed by enhancing V-ATPase activity rather than P-ATPase and H(+)-pyrophosphatase. Accumulation of soluble sugars as well as sodium compartmentalization maintained the osmotic balance between vacuole and cytoplasm, which finally established ionic homeostasis in saline cells in true halophytes.

BIOLOGICAL SIGNIFICANCE: Physiological and proteomic analyses of S. portulacastrum leaves under different salinities were investigated. This true halophyte accumulated more soluble sugars, starch, proline and Na(+) under high salinity. Differential proteomics produced 96 salt responsive proteins and the majority was involved in ion binding, proton transport, photosynthesis, and ATP synthesis. A Na(+)/H(+) antiporter and several ATP synthase subunits were induced upon high salinity. ATP hydrolysis assay demonstrated that V-ATPase activity at tonoplast was dramatically increased whereas vacuolar H(+)-pyrophosphatase and plasma membrane ATPase activities were stable upon NaCl. These findings demonstrated that the increased Na(+) was compartmentalized into vacuole by enhancing V-ATPase activity rather than H(+)-ATPase.