Genetic Diversity and Genome Wide Association Study of β-Glucan Content in Tetraploid Wheat Grains

Non-starch polysaccharides (NSPs) have many health benefits, including immunomodulatory activity, lowering serum cholesterol, a faecal bulking effect, enhanced absorption of certain minerals, prebiotic effects and the amelioration of type II diabetes. The principal components of the NSP in cereal grains are (1,3;1,4)-β-glucans and arabinoxylans. Although (1,3;1,4)-β-glucan (hereafter called β-glucan) is not the most representative component of wheat cell walls, it is one of the most important types of soluble fibre in terms of its proven beneficial effects on human health. In the present work we explored the genetic variability of β-glucan content in grains from a tetraploid wheat collection that had been genotyped with a 90k-iSelect array, and combined this data to carry out an association analysis. The β-glucan content, expressed as a percentage w/w of grain dry weight, ranged from 0.18% to 0.89% across the collection. Our analysis identified seven genomic regions associated with β-glucan, located on chromosomes 1A, 2A (two), 2B, 5B and 7A (two), confirming the quantitative nature of this trait. Analysis of marker trait associations (MTAs) in syntenic regions of several grass species revealed putative candidate genes that might influence β-glucan levels in the endosperm, possibly via their participation in carbon partitioning. These include the glycosyl hydrolases endo-β-(1,4)-glucanase (cellulase), β-amylase, (1,4)-β-xylan endohydrolase, xylanase inhibitor protein I, isoamylase and the glycosyl transferase starch synthase II.     

Water-soluble (1→3), (1→4)-β-d-glucans from barley (Hordeum vulgare) endosperm. I. Physicochemical properties

Physicochemical methods have been used to define molecular weight, molecular weight distribution, solution behaviour and shape of (1→3), (1→4)-β-d-glucans purified from the 40°C water-extract of barley endosperm by precipitation with 30% saturated ammonium sulphate. The molecular weight and solution properties of a (1→3), (1→4)-β-d-glucan from Australian grown barley (cv. Clipper) are compared with a commercially available preparation. Weight and number average molecular weights are 290 000 and 210 000 respectively for the Clipper (1→3), (1→4)-β-d-glucan and 160 000 and 150 000 respectively for the commercial preparation. The degree of polydispersity is small, but this probably results from the selection of a specific population of (1→3), (1→4)-β-d-glucan molecules during isolation. The higher molecular weight of the Clipper (1→3), (1→4)-β-d-glucan is reflected in higher sedimentation coefficient and intrinsic viscosity values. Viscosity and sedimentation data indicate that the molecules are highly asymmetric, with axial ratios of approximately 100 and 80 for the Clipper (1→3), (1→4)-β-d-glucan and the commercial preparation, respectively. Both polysaccharides appear to exist in solution as extended, worm-like chains.     

Isolation of plasma membrane from protoplasts of Lolium multiflorum (ryegrass) endosperm cells.

Plasma membranes have been isolated from protoplasts of suspension-cultured ryegrass (Lolium multiflorum) endosperm cells. The protoplast membrane is coated before cell disruption with murine myeloma protein J539, a galactose-binding immunoglobulin A. The plasma membrane is labelled with 125I by using chemically or enzymically catalysed iodination techniques, or, more conveniently, by using 125I-labelled myeloma protein J539, which enables the membrane to be simultaneously coated and labelled. Protoplast lysis is effected by gentle mechanical means after swelling in hypo-osmotic medium. The plasma-membrane fraction is recovered at low centrifugal forces by fractionation of cell lysates on a discontinuous sucrose/sorbitol gradient. The plasma-membrane fraction is enriched 96-fold on a protein basis with respect to the specific radioactivity of 125I-labeled myeloma protein J539 in the homogenate. Electron microscopy showed long membrane profiles often associated with one another.     

Three-dimensional structure of the barley beta-D-glucan glucohydrolase in complex with a transition state mimic

Glucophenylimidazole (PheGlcIm), a tetrahydroimidazopyridine-type inhibitor and 4H3 conformer mimic of a glucoside, binds very tightly to a barley beta-d-glucan glucohydrolase, with a Ki constant of 2 x 10(-9) m and a DeltaG of 51 kJ mol(-1). PheGlcIm binds to the barley beta-d-glucan glucohydrolase approximately 2 x 10(5) times tighter than laminarin, which is the best non-synthetic ground-state substrate found so far for this enzyme, 10(6) times tighter than 4-nitrophenyl beta-d-glucopyranoside, and 2 x 10(7) tighter than glucose. The three-dimensional structure of the beta-d-glucan glucohydrolase with bound PheGlcIm indicates that the complex resembles a hypothetical transition state during the hydrolytic cycle, that the enzyme derives substrate binding energy from the "aglycone" portion of the ligand, and that it also reveals an anti-protonation trajectory for hydrolysis. Continuous electron densities at the 1.6 sigma level form between the three active site residues Asp95, His207, and Asp285, and the C6OH, C7OH, C8OH, and C9OH groups of PheGlcIm. These electron densities correspond to the most favorable interactions in the three-dimensional structure of the beta-d-glucan glucohydrolase-PheGlcIm complex and indicate atomic distances equal to or less than 2.55 A. The crystallographic data were corroborated with ab initio molecular orbital calculations. The data indicate that the 4E conformation of the glucose part of PheGlcIm is critical for tight binding and provide the first evidence for probable substrate distortion during catalysis by this enzyme.