Cell wall extensins in root–microbe interactions and root secretions

Abstract

Extensins are cell wall glycoproteins, belonging to the hydroxyproline-rich glycoprotein (HRGP) family, which are involved in many biological functions, including plant growth and defence. Several reviews have described the involvement of HRGPs in plant immunity but little focus has been given specifically to cell wall extensins. Yet, a large set of recently published data indicates that extensins play an important role in plant protection, especially in root-microbe interactions. Here, we summarise the current knowledge on this topic and discuss the importance of extensins in root defence. We first provide an overview of the distribution of extensin epitopes recognised by different monoclonal antibodies among plants and discuss the relevance of some of these epitopes as markers of the root defence response. We also highlight the implication of extensins in different types of plant interactions elicited by either pathogenic or beneficial micro-organisms. We then present and discuss the specific importance of extensins in root secretions, as these glycoproteins are not only found in the cell walls but are also released into the root mucilage. Finally, we propose a model to illustrate the impact of cell wall extensin on root secretions.

Role of Protein Glycosylation in Host-Pathogen Interaction

“…This is because the attack of most pathogens is blocked by glycosylated proteins covering the host cell surface as barriers. Such barriers may act in two manners in host defense, (I) physically separate pathogens and host cells [75,76]; (II) chemically inhibit or kill pathogens [16,[77][78][79].…”

Root extracellular traps versus neutrophil extracellular traps in host defence, a case of functional convergence?

“…Furthermore, it has recently been shown that antimicrobial peptides called defensins are also present in the RETs of an Arabidopsis thaliana relative, the Brassicaceae species Heliophila coronopifolia (Weiller et al , ). The hemicellulosic polysaccharide xyloglucan and the cell wall glycoprotein extensin, two components of plant cell walls, were also detected in pea Pisum sativum and Arabidopsis thaliana RETs using specific antibodies and immuno‐microscopy (Figs and C; see also Castilleux et al , ). An extensin class 1 protein was also found in the pea root secretome – as revealed by proteomic analysis (Wen et al , ) – and xyloglucan was observed to form a web‐like fibrous structure, coating root border cells and holding them together (Fig.…”

“…(C, D) Immunofluorescence labelling with the anti‐extensin monoclonal antibody JIM11 in either Flg22‐elicited (C) or non‐elicited (D) A. thaliana root mucilage. Extensin is a cell wall glycoprotein that is involved in cell wall strengthening in response to biotic stress (Castilleux et al , ). Note the very strong staining of secreted mucilage with JIM11 (white arrowheads) in the Flg22‐elicited root tip (C).…”

“…However, these glycoproteins are likely to fulfill a structural function that contributes to RET stabilization and correct functioning. Extensin macromolecules could possibly interact through isodityrosine bonds – catalysed by peroxidases present in the RET secretions – thus forming a sort of physical barrier or scaffold template that reinforces the whole network (Castilleux et al , ). The scaffold could include other molecules such as xyloglucan and exDNA on which antimicrobial compounds could assemble in a rapid‐response system to pathogen infection (Wen et al , ; Ropitaux et al , ).…”

“…The release of NETs is a hallmark of their inflammatory and anti-microbial action (Brinkmann & Zychlinsky, 2012;Mayadas, Cullere & Lowell, 2014;Stoiber et al, 2015). NETs consist of exDNA of either nuclear or mitochondrial origin, histones and diverse antimicrobial proteins including defensins, cathepsin-G, lactoferrin, calprotectin, cathelicidin Extensin is a cell wall glycoprotein that is involved in cell wall strengthening in response to biotic stress (Castilleux et al, 2018). Note the very strong staining of secreted mucilage with JIM11 (white arrowheads) in the Flg22-elicited root tip (C).…”