Abstract

Plant roots have a dynamic relationship with the surrounding soil, which forms a vital interface for the terrestrial biosphere. Without a strong interface with soil, plants could not extract the necessary resources needed for growth. As a part of a multifaceted strategy, plant roots release a variety of high and low molecular weight compounds into the soil. This exudate is believed to increase water and nutrient uptake, form the first barrier of defence, and aid in the symbiosis with fungi and bacteria. 

This investigation reports on the identity and biochemistry of the polysaccharides released from the roots of several crops and one basal land plant, and explores their possible functions. Crops were grown hydroponically in order to isolate the polysaccharides released by their roots. After growth, the hydroponic media were screened with a library of monoclonal antibodies (MAb). The MAbs revealed the presence of arabinogalactan-protein (AGP), extensin, xylan and xyloglucan. Signatures of these polysaccharides were also determined by monosaccharide linkage analysis. 

By using anion-exchange Epitope Detection Chromatography, polysaccharides released into the hydroponic medium of the crops were separated for further immunochemical analysis. This analysis demonstrated that the polysaccharides released by wheat were part of a multi-polysaccharide complex, Root Exudate Complex 1 (REC1). A similar polysaccharide complex, formed of AGP-xyloglucan (REC2) was also found to be released by liverworts, which were not previously known to secrete polysaccharides. Novel soil analytics were developed in this study to decipher the effects of polysaccharides released by roots on soil aggregate status. 

Tamarind seed xyloglucan, xylan from birchwood, and isolated REC1 from wheat were each demonstrated to increase the abundance of soil aggregates, with REC1 shown to be most effective. This increase in the abundance aggregates may help plants to bioengineer the rhizosphere resulting in increased uptake of resources required for growth.

This work is dedicated to my parents, Mrs Debra and Mr Glen Galloway, who without their continual support this work would not be possible.

Summary of research

•  Monoclonal antibodies are useful tools for exploring polysaccharides released from plant roots.

• Hydroponics is an effective method of growth for isolating polysaccharides released from plant roots.

• Profiles of the released polysaccharides from grasses and eudicotyledons differed.

• Major polysaccharides released from the roots of wheat and other cereals were AGP, extensin, xylan, xyloglucan and possibly mannan.

• Major polysaccharides released by cereals were a part of multi-polysaccharide complex, REC1.

• Liverwort rhizoids released an AGP-xyloglucan complex, REC2.

• Multi-polysaccharide complexes may be released across the plant kingdom.

• Commercial plant cell wall polysaccharides, xylan and xyloglucan, as well as REC1 isolated from Cadenza roots promote soil aggregation. REC1 was found to be the most effective at aggregating soil.