Characterization and role of polysaccharides in root extracellular trap structuring in two Fabaceae species

The root extracellular trap (RET) is a network formed by root-associated, cap-derived cells (AC-DCs) and their mucilaginous secretions. The RET is located at the root tip where it establishes a protective interface against abiotic and biotic stresses. The RET mucilage is mainly composed of complex cell wall polysaccharides (including pectin and xyloglucan) and glycoproteins, that are entangled to each other within the network. Despite our knowledge of the composition of RET mucilage, the structural organization of the RET remains poorly understood. Here, we have investigated the role of pectin and xyloglucan polysaccharides in RET structuring in two Fabaceae species, namely Pisum sativum and Glycine max. We first characterized the composition of the RET using immunocytochemistry and gas chromatography techniques and then assessed the effect of treatments with hydrolytic enzymes targeting pectin and xyloglucan on the RET and its isolated mucilage. The effects of the enzyme treatments were determined by microscopical observations of RET surface and analysis of its physico-chemical properties using the high-pressure size-exclusion chromatography. The data show that RET of both species is enriched in pectin and xyloglucan, and it is quite recalcitrant to enzymatic hydrolysis as compared to other root tissues. In contrast, we found that enzymatic treatments were much more effective on isolated mucilage only when used at low concentrations. Taken together, the results indicate that the RET exhibits marked resistance to hydrolytic enzymes, which may be associated with its complex structural organization and potential macromolecular arrangements.