Polysaccharide root exudates and rhizosheaths in barley.

Casings of soil surrounding plant roots, ubiquitous in cereals, are known as rhizosheaths. They have been proposed as facilitators in root-soil relationships and water dynamics in periods of drought. Rhizosheaths are formed by the entanglement of soil particles in root hairs and the action of adhesive molecules exuded by roots, within which high molecular weight (HMW) polysaccharides have been indicated as key factors. But the glycobiology of cereal exudates is only beginning to be understood. This investigation utilised three techniques to obtain root exudates: hydroponics, short seedling incubations in water and soil-suction-lysimeters. Surveying with a panel of monoclonal antibodies directed to cell wall polysaccharides, identified arabinogalactan-protein (AGP), xyloglucan, and heteroxylan epitopes across the different exudate collection methods and a range of barley genotypes. This glycan profiling was used in conjunction with spatial analysis of polysaccharide exudates; seedling prints on nitrocellulose sheets characterised the patterns of polysaccharide release from along the root axes including a xylogalacturonan epitope (LM8) previously only associated with root apices. Immunofluorescence microscopy established that certain AGP epitopes and a novel beta-glucan epitope are abundant at root hair surfaces of barley plants. From these results it is proposed that groups of glycan epitopes have distinct functions; those presented on root hairs are associated with soil binding and other epitopes provide services for a developing root system. Further analysis into root exudate-soil interactions suggests the adhesive capacity of the HMW components of exudate, in a soil-binding assay, is linked to the rhizosheath weight of barley roots. But also that is likely there is a trade-off in carbon allocation by the plant between root growth and HMW exudation. Such work can hopefully build into the body of knowledge of root systems that can one day influence targeted crop breeding strategies to mitigate against drought.