Feruloylation and structure of arabinoxylan in wheat endosperm cell walls from RNAi lines with suppression of genes responsible for backbone synthesis and decoration.

Jackie Freeman,Peter R Shewry,Rowan A.C Mitchell,Jane L Ward,Alison Lovegrove,Ondrej Kosik,Mark D Wilkinson

doi:10.1111/pbi.12727

Abstract

SummaryArabinoxylan (AX) is the major component of the cell walls of wheat grain (70% in starchy endosperm), is an important determinant of end‐use qualities affecting food processing, use for animal feed and distilling and is a major source of dietary fibre in the human diet. AX is a heterogeneous polysaccharide composed of fractions which can be sequentially extracted by water (WE‐AX), then xylanase action (XE‐AX) leaving an unextractable (XU‐AX) fraction. We determined arabinosylation and feruloylation of AX in these fractions in both wild‐type wheat and RNAi lines with decreased AX content (TaGT43_2 RNAi, TaGT47_2 RNAi) or decreased arabinose 3‐linked to mono‐substituted xylose (TaXAT1 RNAi). We show that these fractions are characterized by the degree of feruloylation of AX, <5, 5–7 and 13–19 mg bound ferulate (g−1 AX), and their content of diferulates (diFA), <0.3, 1–1.7 and 4–5 mg (g−1 AX), for the WE, XE and XU fractions, respectively, in all RNAi lines and their control lines. The amount of AX and its degree of arabinosylation and feruloylation were less affected by RNAi transgenes in the XE‐AX fraction than in the WE‐AX fraction and largely unaffected in the XU‐AX fraction. As the majority of diFA is associated with the XU‐AX fraction, there was only a small effect (TaGT43_2 RNAi, TaGT47_2 RNAi) or no effect (TaXAT1 RNAi) on total diFA content. Our results are compatible with a model where, to maintain cell wall function, diFA is maintained at stable levels when other AX properties are altered.

Highlights

The composition of the cell walls of wheat grain is unusual in that they are largely comprised of arabinoxylan (AX), with no lignin and low amounts of cellulose in starchy endosperm and aleurone tissues
The abundance of AX oligosaccharides (AXOS) containing xylose mono- and di-substituted by Araf was measured in solubilized fractions and mono- and di-substituted AXOS were shown to be decreased in TaGT43_2 and TaGT47_2 RNAi lines (Lovegrove et al, 2013), whereas in TaXAT1 RNAi lines, there was no effect on di-substituted but a 70-80% decrease in mono-substituted AXOS (Anders et al, 2012)
It is assumed that all bound ferulate in wheat endosperm is ester-linked to AX (Saulnier et al, 2007) so an overall value of ferulic acid (FA) and diFA per unit AX where AX is estimated from monosaccharide is given

Summary

Introduction

The composition of the cell walls of wheat grain is unusual in that they are largely comprised of arabinoxylan (AX), with no lignin and low amounts of cellulose in starchy endosperm and aleurone tissues. Starchy endosperm cell walls are primary cell walls containing 70% AX, 20% b-(1,3;1,4) glucan, 2%–7% glucomannan and 2%–4% cellulose. The composition of aleurone cell walls is similar to that of starchy endosperm tissue comprising 65% AX, 30% b-(1,3;1,4) glucan and 2% glucomannan and cellulose, whereas, whilst AX is still the major component of pericarp cell walls (60% AX), they contain 30% cellulose and 12% lignin (Shewry et al, 2014). Ester-linked pCA is not detected in pure starchy endosperm tissue dissected from wheat grain but is concentrated in the aleurone layer of wheat grain (Barron et al, 2007). Esterlinked ferulic acid can undergo oxidative dimerization forming ether or C–C bonds linking chains of AX or glucuronoarabinoxylan (GAX) (Ishii, 1991), or of (G)AX to lignin in lignified tissues (Ralph et al, 1995)

Results

Discussion

Conclusion