Regulation of (1,3;1,4)-beta-glucan synthesis in the grasses
The overall aim of this project is to define the regulation and functional diversity of gene families that mediate the synthesis of plant cell wall polysaccharides, in particular the (1,3;1,4)-β-glucans, that are found in walls of commercially important grasses and cereals of the Poaceae.
Why is this aim important? A major part of human daily caloric intake is derived from a diverse range of foods prepared from grains of the grass family, including wheat, rice, sorghum, the millets and barley. Non-cellulosic polysaccharides from cereal grain cell walls are not digested by enzymes in the human small intestine and so contribute to total dietary fibre intake. Dietary fibre reduces the risk and adverse social and personal impacts of serious human health conditions such as colorectal cancer, cardiovascular disease and type II diabetes. In 2005 the US Food and Drugs Administration (FDA) granted that marketers of whole grain barley and barley-containing products could claim that these products reduce the risk of coronary heart disease, providing they comprised at least 0.75 grams of soluble (1,3;1,4)-β-glucan fibre per 228 g serving. While the use of barley as a food crop has decreased somewhat in recent years there is an opportunity now to reverse this decline and to simultaneously address the health agenda, through the direct use or incorporation of novel barleys or barley products into a wide range of staples. This is especially relevant in Northern Europe where the climate and soils are better suited to barley production than to wheat, largely due to barley’s better performance under marginal conditions.
Dietary fibre and plant cell walls: Plant cell walls comprise a strong, flexible and adaptable layer of material that surrounds individual cells in all land plants. These protective layers are composed predominantly of cellulose and a range of other polysaccharides. Many also contain lignin. To put this into context, cellulose is believed to be the single most abundant biological molecule on the planet (180 billion tonnes generated per annum through capture of light energy by plants) and an enormous sink for atmospheric CO2 (i.e. efficient carbon capture). Plant cell walls represent exceptionally strong bio-composites that allow the development of materials that range from water-proofing outdoor furniture to fibre glass manufacture and coatings for biomedical devices. Their tremendous benefits as components of human dietary fibre intake have been recognised, as described above
Mixed linkage (1,3;1,4)-β-glucans: Although the structure of wall cellulose is relatively constant across higher plant species, the non-cellulosic wall polysaccharides are complex and vary widely between species. In contrast to the dicots and many monocots, in which pectic polysaccharides, xyloglucans and heteromannans represent the major non-cellulosic wall components, in the grasses the levels of these polysaccharides are relatively low. In their place, much higher levels of heteroxylans and the appearance of a new class of wall polysaccharide, a hemicellulose called (1,3;1,4)-β-glucan, are observed. The effectiveness of non-cellulosic cell wall polysaccharides, including (1,3;1,4)-β-glucans, in improving health outcomes is related to their levels in grain, to their fine structure and to their associated physicochemical properties. It has been previously shown that the synthesis of barley (1,3;1,4)-β-glucans are mediated by members of the cellulose synthase gene superfamily, in particular the CslF and CslH sub-families. Allelic variation in the individual genes / gene families that regulate them, either directly or indirectly, control the relative abundance, composition and physicochemical properties of the (1,3;1,4)-β-glucans in the grain and the rest of the plant. Thus, different barley varieties contain different amounts of (1,3;1,4)-β-glucans and the levels observed are under both genetic and environmental control. Some barley varieties contain more than 30% total fibre (cf. 3.5% in brown rice, 10% in oats and 12% in wheat) and have been marketed as health promoting super-foods (e.g. Sustagrain and BarleyMax).
Potential for exploitation: A more thorough understanding of the gene families that are responsible for synthesising (1,3;1,4)-β-glucan, and how they are regulated in barley and other cereals, is highly likely to suggest innovative approaches to tailor (1,3;1,4)-β-glucan content and physicochemical properties in grain.
In the work proposed, we will define the genetic, cellular and biochemical regulatory factors that are responsible for the amounts and properties of (1,3;1,4)-β-glucan in grain, and for the partitioning of carbon between starch and cell wall (1,3;1,4)-β-glucan in barley. Understanding the diversity and regulation of the cellulose synthase superfamily, and the CslF and CslH gene families in particular, and showing a strong correlation between that variation and (1,3;1,4)-β-glucan structure and content, will provide an opportunity to directionally breed new varieties of barley.
In examining the regulatory factors that control cell wall biology in the grasses, we will also determine if any of these factors might contribute to the evolutionary success of the grasses and their clear competitive advantage over many other species of higher plants. Against this background, our broad objective is to define:
‘How regulation of synthesis of (1,3;1,4)-β-glucan cell wall polysaccharides is achieved in barley during growth and development, and how quantitative and qualitative variation in (1,3;1,4)-β-glucan production contribute to overall polysaccharide deposition and structure’
This BBSRC funded project was a collaboration between The James Hutton Institute, The University of Dundee, and The Australian Research Council Centre of Excellence in Plant Cell Walls.
For more information also see Kelly Houstons staff page.
Schreiber, M., Wright, F., MacKenzie, K., Hedley, P. E., Schwerdt, J. G., Little, A., … & Halpin, C. (2014). The barley genome sequence assembly reveals three additional members of the CslF (1, 3; 1, 4)-β-glucan synthase gene family. PloS one, 9(3), e90888.
Houston, K., Russell, J., Schreiber, M., Halpin, C., Oakey, H., Washington, J. M., … & Waugh, R. (2014). A genome wide association scan for (1, 3; 1, 4)-beta-glucan content in the grain of contemporary 2-row Spring and Winter barleys. BMC genomics, 15(1), 907.
Schwerdt, J., MacKenzie, K., Wright, F., Oehme, D., John, M. W., Andrew, J. H., … & Fincher, G. (2015). Evolutionary Dynamics of the Cellulose Synthase Gene Superfamily in Grasses. Plant physiology, pp-00140.
For further information on this project please contact Kelly Houston (firstname.lastname@example.org) from the James Hutton Institute.