Abstract
Rhizosheaths function in plant-soil interactions, and are proposed to form due to a mix of soil particle entanglement in root hairs and the action of adhesive root exudates. The soil-binding factors released into rhizospheres to form rhizosheaths have not been characterised. Analysis of the high-molecular-weight (HMW) root exudates of both wheat and maize plants indicate the presence of complex, highly branched polysaccharide components with a wide range of galactosyl, glucosyl and mannosyl linkages that do not directly reflect cereal root cell wall polysaccharide structures. Periodate oxidation indicates that it is the carbohydrate components of the HMW exudates that have soil-binding properties. The root exudates contain xyloglucan (LM25), heteroxylan (LM11/LM27) and arabinogalactan-protein (LM2) epitopes, and sandwich-ELISA evidence indicates that, in wheat particularly, these can be interlinked in multi-polysaccharide complexes. Using wheat as a model, exudate-binding monoclonal antibodies have enabled the tracking of polysaccharide release along root axes of young seedlings, and their presence at root hair surfaces and in rhizosheaths. The observations indicate that specific root exudate polysaccharides, distinct from cell wall polysaccharides, are adhesive factors secreted by root axes, and that they contribute to the formation and stabilisation of cereal rhizosheaths.
Highlights
The interface between plant roots and soils is highly complex
To study the polysaccharides released from wheat and maize roots, seedlings were grown in hydroponic systems in half-strength Hoagland nutrient solution
Cereal roots release structurally complex polysaccharides, distinct from cell wall polysaccharides. This analysis of polysaccharides released from wheat and maize roots, and collected in a hydroponic system, indicate a wide range of structural linkages in comparison to cell walls
Summary
Roots extend into soils that are highly heterogeneous substrates in terms of mechanical properties, chemistry and water/nutrient availability. It is well established that roots release exudates that includes a range of low and high molecular weight (HMW) compounds which are likely to influence a wide range of soil properties (Walker et al 2003; Naveed et al 2017). At root apices, roots exude polysaccharide-rich mucilage that is proposed to act as a lubricant to aid root penetration and to act in defence processes. In some instances, this is associated with the complete release of root cap cells, known as border cells, which remain physiologically active (Koroney et al 2016)
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