Polysaccharides of cereal root exudates

Plant roots have important roles in the uptake of water and nutrient from soil. To do this efficiently in a constantly changing environment roots release exudates to manipulate the rhizosphere. The exudates help with nutrient acquisition, defence, forming microbial symbiosis and soil structure. Using a high-throughput soil binding assay, polysaccharides of both microbial and plant origin have shown to be efficient soil binders. Specifically, β-glucans, complex gums and xyloglucan bound the most soil. Using techniques previously used for cell wall polysaccharide analyses, epitopes of polysaccharides similar to the strongest soil binders were found in high molecular weight exudates collected from cereal roots. Xyloglucan, heteroxylan and arabinogalactan proteins were present in the high molecular weight exudates using monoclonal antibodies but a more complex polysaccharide with linkages not usually present in the cell wall was also found by the method of monosaccharide linkage analysis. β-glucans and mannans were also present in the high molecular weight exudates. The chemical degradation of the polysaccharides decreased the soil binding of the high molecular weight exudates. Changes in nutrient levels did not affect the soil binding capacity of the exudates released but xyloglucan decreased. Removal of major macronutrients did not affect soil binding or polysaccharide release. The exudates released by the root hairless mutant of barley (brb) did show differences in soil binding and the polysaccharide epitopes released compared to the WT barley. The root exudate of brb barley contained more xyloglucan than the WT barley but further analyses using anion exchange chromatography discovered that a xyloglucan fraction that may be part of a complex with acidic epitopes containing arabinogalactan proteinss and xylan was decreased in the root exudate of brb barley. There was an increase of a neutral fraction of xyloglucan in the root exudate of brb barley. The acidic polysaccharides released from the roots of WT barley bound the most soil overall indicating a role in rhizosheath formation alongside root hairs. The roles of the neutral polymers are not yet known. The novel methods to dissect high molecular weight root exudates have led to a deeper understanding of the structure and possible role of the exudates. It has also highlighted the heterogeneous nature of the structure of the polysaccharides across cereal crops, cultivars and mutants may be used in future work to understand the genetic control of these exudates.