From root to leaf: Cell walls of the halophytic grass Distichlis laxiflora comprise highly substituted glucuronoarabinoxylans

Abstract

Halophytic plants develop their life cycle in high salinity conditions. Several physiological responses explain this, but little was explored regarding the role of their cell walls (CW). In grasses, which have small amounts of pectins, hemicelluloses should show the greatest effects of salt stress. In this work, CW from roots, shoots, and leaves from Distichlis laxiflora collected in spring (the regrowth period) in halophytic (H) and mesophytic (M) meadows were studied. This is a first approach to the CW of halophytic grasses, with an evaluation of differences between organs, and between sites with distinct salinity. The main alkaline fractions contain glucuronoarabinoxylans (GAX) as major component, followed by small amounts of pectins and β-glucans. Analysis of their composition showed that polysaccharides from leaves of both sites differ little, while greater differences (involving xylose, glucuronic acid, glucose, and arabinose) are found within roots. If the alkaline extracts and the final residue are considered together, both leaves and shoots have ≥50% of GAX and 20–30% cellulose in their CW, while roots show similar amounts of GAX and cellulose (35–40% of each). Additionally, GAX from roots show a higher degree of substitution with glucuronic acid, regarding the other organs and GAX reported for other grasses, and considerable amounts of 4-linked xylopyranose residues disubstituted with single ramifications of α-L-arabinofuranose. As the first barrier in contact with soil salinity, roots respond through the modification of the classical structure of GAX, which would be more suitable to entrap Na+ ions, by the presence of more negative charges.