Root exudates & plant-soil interactions

Cell walls can be viewed as extended cell surfaces. Secretions and exudates, often rich in polysaccharides, act outside and beyond coherent cell walls and can be viewed as extending the phenotypes of plants even further and in a diversity of ways (Galloway et al. 2020). Root exudates are a major phenotypic extension of roots within soils, lead to the modification of soils & their biota and result in zones of influence in soils around roots known as rhizospheres. In some instances, cylinders of soil remain tightly adhered to root axes, even after excavation, and these are known as rhizosheaths. Root exudates are thought to account for a significant proportion of photosynthate and include a vast array of molecules from small metabolites to large macromolecules including proteins and polysaccharides. Polysaccharides in exudate high molecular mass (HMM) fractions have been characterized in the context of root tip mucilage but their roles in rhizospheres are far from being fully elucidated and their exudation from other regions of root bodies little studied.

Deployment of our sets of polysaccharide MAbs led to the discovery that xylogucan, a major matrix polysaccharide of many flowering plant cell walls, is released from plant roots and rhizoids and moreover has soil-binding properties (Galloway et al. 2018). Soil-binding and aggregation of soil particles are important factors in rhizosphere and rhizosheath formation and are likely to be one facet of the functions and activities of root exudate polysaccharides. To enable the characterization of root-released soil-binding factors we have developed a simple soil-binding assay that quantifies soil adhesion to polymers applied to nitrocellulose substrates (Akhtar et al. 2018) and its protocol is shown in a video.  Work with wheat and maize polysaccharide root exudates has indicated that xyloglucan motifs are just a minor component of the HMM polysaccharides collected hydroponically from roots and has led to simple MAb-based assays for the release of polysaccharides from root bodies and root hairs (Galloway et al. 2020).