Non-Starch Polysaccharides in Durum Wheat: A Review

Abstract

Durum wheat is one of most important cereal crops that serves as a staple dietary component for humans and domestic animals. It provides antioxidants, proteins, minerals and dietary fibre, which have beneficial properties for humans, especially as related to the health of gut microbiota. Dietary fibre is defined as carbohydrate polymers that are non-digestible in the small intestine. However, this dietary component can be digested by microorganisms in the large intestine and imparts physiological benefits at daily intake levels of 30-35 g. Dietary fibre in cereal grains largely comprises cell wall polymers and includes insoluble (cellulose, part of the hemicellulose component and lignin) and soluble (arabinoxylans and (1,3;1,4)-β-glucans) fibre. More specifically, certain components provide immunomodulatory and cholesterol lowering activity, faecal bulking effects, enhanced absorption of certain minerals, prebiotic effects and, through these effects, reduce the risk of type II diabetes, cardiovascular disease and colorectal cancer. Thus, dietary fibre is attracting increasing interest from cereal processors, producers and consumers. Compared with other components of the durum wheat grain, fibre components have not been studied extensively. Here, we have summarised the current status of knowledge on the genetic control of arabinoxylan and (1,3;1,4)-β-glucan synthesis and accumulation in durum wheat grain. Indeed, the recent results obtained in durum wheat open the way for the improvement of these important cereal quality parameters.

Keywords: (1,3; 1,4)-β-glucans; arabinoxylan; durum wheat; fibre content.

Figure 1

Structures of non-starch polysaccharides in plants. The backbone of (1,3;1,4)-β-glucan is based on (1,4)-β-linked monosaccharides, whereas the backbone of arabinoxylan shows L-arabinose units attached to O-2 or O-3 of D-xylose residues. Some arabinose residues are esterified at O-5 by a hydroxycinnamic acid. D-Glucuronic residues were reported as substituents at O-2 of the xylose residues such as acetyl groups. Figure redrawn from Burton et al., 2010 [22].

Figure 2

Schematic representation of durum wheat chromosomes (A and B genomes) from the consensus map [57] and chromosomes 3D, 4D and 6D from bread wheat with quantitative trait loci (QTL) summary for (1,3;1,4)-β-glucan and arabinoxylan trait detected in references from Table 2 and Table 3. Markers, on the right chromosome side, are reported every 20 cM approximately. cM distances are indicated on the left side of the bar. Red solid bars indicate the QTL confidence interval regions for (1,3;1,4)-β-glucan, while blue bars designated the arabinoxylan QTL.

Figure 3

Cellulose synthase superfamily gene trees constructed using RAxML. Red external nodes indicate monocots, blue nodes indicate eudicots, and green nodes indicates the basal angiosperm Amborella trichopoda. Figure from Little et al., 2018 [54].

Figure 4

Schematic representation of CslF6 and CslH gene structures in durum wheat (A and B genomes). Intron and exon sizes are shown as well as the whole gene (in brackets the total length). Figure redrawn from Marcotuli et al., 2018 [69].

Figure 5

Predicted roles of glycosyltransferase (GT) gene families in arabinoxylan biosynthesis.

Figure 6

¹H NMR spectra for oligosaccharides DP5 (a), DP6 (b) and DP7 (c) in durum lines. Figure redrawn from Marcotuli et al., 2016 [95].