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 high molecular weight (HMW) fractions of root exudates have been characterized in the context of root tip mucilage but their roles in rhizospheres and rhizosheath formation are far from being fully elucidated and their exudation from regions of root bodies other than root tips has been 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 HMW polysaccharides collected hydroponically from roots and has led to simple MAb-based procedures to study the release of polysaccharides from root bodies and root hairs (Galloway et al. 2020). The paired images below show a bright-field image of soil particles attached to wheat root hairs and the corresponding immunofluorescence image of the LM11 heteroxylan epitope that is present in root exudates and in this case also at the root hair and soil particle surfaces.

Rhizosheat soil particles attached to wheat root hairs

Analysis of wild type and a root hairless mutant of barley has confirmed the structural complexity of glucan-rich exudate polysaccharides from cereal roots and provides evidence that polysaccharide profiles are altered in the absence of root hairs. A putative soil-binding polysaccharide complex is reduced in abundance in the absence of root hairs whereas root-tip released xyloglucan is increased in abundance (Galloway et al. 2022). Immunoprinting or imaging of root exudates on nitrocellulose sheets is a powerful way to gain insights into root exudate polysaccharide patterns of release as shown below with the paired images of the root system of a barley root hairless mutant and the corresponding immunoprint on a nitrocellulose sheet of the release of the LM25 xyloglucan epitope.

Paired images of the root system of a barley root hairless mutant and the immunoprint of release of the LM25 xyloglucan epitope on a nitrocellulose sheet