Molecular Markers for Ion Compartmentation in Cells of Higher Plants

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Abstract
Vacuoles of high purity were isolated from the leaves of the halophyte Suaeda maritima (L.) Dum. The relative compositions of phospholipids, phytosterols, and fatty acids in the tonoplast membrane were determined and membrane fluidity was assessed by electron spin resonance. The characteristics of the tonoplast were consistent with minimizing passive permeability to NaCI. The phospholipid: protein ratio (1.1: 1.0) was higher than that recorded in other membrane preparations, including vacuoles from beetroot storage material, commensurate with the low density (1.05 g cm−3) of the S. maritima tonoplast. The tonoplast fatty acids were highly saturated and dominated by n-hexadecanoic acid and n-octadecanoic acid. Phytosterols identified by gas chromatography were cholesterol, campesterol, stigmasterol, and β-sitosterol. Cholesterol was a trace percentage in protoplasts, but comprised 30% of the tonoplast sterols. Semi-quantitative analysis by chromatography on silica gel revealed an enrichment in the tonoplast of glycolipid which was not accounted for as chloroplast contamination. The fluidity of the tonoplast, determined by electron spin resonance, was lower than the protoplasts, consistent with the high degree of saturation of the fatty acid chains. The relevance of the lipid composition of the tonoplast to its central role in ion compartmentation within the halophyte cell is discussed.