Mapping Pectic-Polysaccharide Epitopes in Cell Walls of Forage Chicory (Cichorium intybus) Leaves.

Xuezhao Sun,Ian G Andrew,Yuhua He,Philip J Harris,Simone O Hoskin,Keith N Joblin

doi:10.3389/fpls.2021.762121

Abstract

The cell walls of forage chicory (Cichorium intybus) leaves are known to contain high proportions of pectic polysaccharides. However, little is known about the distribution of pectic polysaacharides among walls of different cell types/tissues and within walls. In this study, immunolabelling with four monoclonal antibodies was used to map the distribution of pectic polysaccharides in the cell walls of the laminae and midribs of these leaves. The antibodies JIM5 and JIM7 are specific for partially methyl-esterified homogalacturonans; LM5 and LM6 are specific for (1→4)-β-galactan and (1→5)-α-arabinan side chains, respectively, of rhamnogalacturonan I. All four antibodies labelled the walls of the epidermal cells with different intensities. JIM5 and JIM7, but not LM5 or LM6, labelled the middle lamella, tricellular junctions, and the corners of intercellular spaces of ground, xylem and phloem parenchyma. LM5, but not LM6, strongly labelled the walls of the few sclerenchyma fibres in the phloem of the midrib and lamina vascular bundles. The LM5 epitope was absent from some phloem parenchyma cells. LM6, but not LM5, strongly labelled the walls of the stomatal guard cells. The differential distribution of pectic epitopes among walls of different cell types and within walls may reflect the deposition and modification of these polysaccharides which are involved in cell wall properties and cell development.

Highlights

The cell walls of flowering plants contain polysaccharides together with other substances such as phenolic components, proteins, and glycoproteins
In collenchyma and epidermal cells, the thick walls are important in holding the leaf rigid and are possibly strengthened by the pectin hydrogel, with calcium bridging of low methyl HG contributing to wall strength (Markov et al, 2017)
Our study showed the JIM5 epitope was more abundant in the walls of the epidermis, hypodermis, and vascular bundles in the midrib and the epidermis of the lamina

Summary

Introduction

The cell walls of flowering plants contain polysaccharides together with other substances such as phenolic components, proteins, and glycoproteins. In the primary cell walls of eudicotyledons and non-commelinid monocotyledons, pectic polysaccharides are major components (Harris, 2005). HG is primarily a (1→4)-α-linked polymer of galacturonic acid residues, which may be methyl esterified (Hocq et al, 2017). RG-I is a branched polymer with a backbone of alternating galacturonic acid and rhamnose residues, the latter bearing (1→5)-α-arabinan and/or (1→4)-β-galactan side-chains (Silva et al, 2016). Specific pectic polysaccharides can be localised within cell walls using monoclonal antibodies that recognise specific structures (epitopes) within them. JIM7 binds to structures where every second GalA residue is methyl esterified and there is no preference for the esterification state of the GalA residue between (Clausen et al, 2003). LM5 binds to an epitope with a minimum of three residues at the non-reducing end of a linear (1→4)-β-galactan (Jones et al, 1997; Andersen et al, 2016), and LM6 binds to an epitope with about five residues within a linear (1→5)-α-arabinan (Willats et al, 1998)

Methods

Results

Conclusion