Rising from the sea: correlations between sulfated polysaccharides and salinity in plants

Abstract

High salinity soils inhibit crop production worldwide and represent a serious agricultural problem. To meet our ever-increasing demand for food, it is essential to understand and engineer salt-resistant crops. In this study, we evaluated the occurrence and function of sulfated polysaccharides in plants. Although ubiquitously present in marine algae, the presence of sulfated polysaccharides among the species tested was restricted to halophytes, suggesting a possible correlation with salt stress or resistance. To test this hypothesis, sulfated polysaccharides from plants artificially and naturally exposed to different salinities were analyzed. Our results revealed that the sulfated polysaccharide concentration, as well as the degree to which these compounds were sulfated in halophytic species, were positively correlated with salinity. We found that sulfated polysaccharides produced by Ruppia maritima Loisel disappeared when the plant was cultivated in the absence of salt. However, subjecting the glycophyte Oryza sativa Linnaeus to salt stress did not induce the biosynthesis of sulfated polysaccharides but increased the concentration of the carboxylated polysaccharides; this finding suggests that negatively charged cell wall polysaccharides might play a role in coping with salt stress. These data suggest that the presence of sulfated polysaccharides in plants is an adaptation to high salt environments, which may have been conserved during plant evolution from marine green algae. Our results address a practical biological concept; additionally, we suggest future strategies that may be beneficial when engineering salt-resistant crops.

Figure 1. The biosynthesis of sulfated polysaccharides by R. maritima is correlated with salinity levels.

Sulfated and carboxylated polysaccharides extracted from R. maritima were quantified (A) and analyzed by size exclusion chromatography (B–D). (A) Sulfated polysaccharides were quantified using a metachromatic reaction (solid circle), and carboxylated polysaccharides were quantified based on their hexuronic acid concentration using the carbazol reaction (open circle). Values are expressed based on their concentration at 35 ppt salinity (100%). Polysaccharides extracted from plants cultivated at 35 ppt salinity (B), after salt removal (C), and after salt was re-introduced (D) were analyzed by size exclusion chromatography on a Superose 12 column coupled to a FPLC system. Fractions were tested using the metachromasy reaction (solid circle), carbazol reaction (open circles), and phenol-sulfuric acid (continuous line) to quantify total hexose.

Figure 2. The occurrence of sulfated polysaccharides in plants and algae.

Phylogenetic relationships, including the divergence times of algae, pteridophytes, angiosperms, and a summary of the monosaccharide composition of their sulfated polysaccharide are listed. Sulfated polysaccharides extracted from different plant species were purified using anion-exchange chromatography and their monosaccharide composition was evaluated by paper chromatography after acid hydrolysis. The sulfated polysaccharides from the seagrasses species R. maritima, H. decipiens and H. wrightii contain galactose (A), those from the mangrove species R. mangle and A. schaueriana contain arabinose and galactose (B), while pteridophytes, which were represented by the species A. aureum, contain glucose (D). No sulfated polysaccharides were detected in terrestrial plants (Z. maize, P. vulgaris and O. sativa). Marine green algae (Chlorophyceae) contained a complex mixture of sulfated polysaccharides, with fractions containing arabinose, galactose and glucose (E). Red algae (Rhodophyceae) (F) and brown algae (Phaeophyta) (G) sulfated polysaccharides are composed mainly by galactose and fucose units, respectively.

Figure 3. Salinity affects the degree to which sulfated polysaccharides are sulfated.

Sulfated polysaccharides extracted from H. wrightii collected from an environment with 35 ppt salinity (A, closed symbol) and from a 38 ppt salinity environment (A, open symbol) were analyzed using anion-exchange chromatography (Mono-Q column coupled with a FPLC system). Sulfated polysaccharides from R. maritima collected from environments with 15 ppt (B, closed symbol) and 35 ppt (B, open symbol) salinity were also compared by the same procedure. Fractions were assayed using metachromasy. Some fractions were pooled, as indicated (1–5), and analyzed by agarose gel electrophoresis (graph inset), and sulfate concentration was determined (localized above fractions number, under brackets, and expressed as sulfate/monosaccharide units).

Figure 4. An increase in carboxylated polysaccharides is observed when rice is grown under salt stress.

O. sativa specimens were grown for two weeks in freshwater (control) or in the presence of 200 mM NaCl. After polysaccharide extraction, the concentration of sulfated polysaccharides was determined by metachromasy (negative result, data now shown), and the presence of carboxylated polysaccharides was determined using the carbazol reaction.