1. β-Glucan 

Barley is one of the world's most significant crops and is widely used as a staple food, fodder, and in beer production. Barley grain contains a unique hemicellulose, scientifically known as mixed-linkage (1,3; 1,4) β-D-glucan (MLG) and commonly known as β-glucan. The backbone of β-glucan molecules is typically unbranched and contains a few consecutive β-(1,4) linkages interrupted by the presence of a single β-(1,3) linkage. Thus, in terms of chemical structure, β-glucan could be viewed as a mixture of cellulose (a β-1,4 glucan) and callose (a β-1,3 glucan) [Fig 1]. Barley cultivars with a reduced β-glucan content are desirable when the grain is to be used for beer brewery and animal feed, because the presence of β-glucan in malt could clog the filtration system during beer production and leads to the formation of undesirable haze (a turbidity) in the refrigerated beer. However, when barley grain is to be used as an ingredient in foods for human consumption, a high β-glucan content in grain is preferred, because the presence of β-glucan in human diets could reduce the levels of blood sugar and blood cholesterol, thus helping diet management of patients with diabetes and cardiovascular diseases (Jamalizadeh et al., 2025b). 

β-Glucan is synthesized by the MLG synthase which belongs to a subfamily of the cellulose synthase-like (CSL) proteins in plants [Fig 2]. CSL-F6 is the predominant form of the MLG synthase responsible for the synthesis of β-glucan in barley. Two other CSL family members (CSL-H1 and CSL-J1) also exhibit low MLG synthase activity when expressed in transgenic dicotyledon plants and may contribute, to a small extent if any, to the accumulation of MLG in cereals. CslH1 is present in the genomes of some cereals such as barley, while CslJ1 can be found in the genomes of some monocotyledon and some dicotyledon plants. CSL-H1 may also participate in the synthesis of β-glucan in barley (see figure below).  We have previously studied two different barley mutant lines (m38 and m351) (Hu et al., 2014a; 2014b). We have recently characterized a new high β-glucan mutant (CM1) (Jamalizadeh et al., 2025a). Genetic mapping of the CM1 locus led to the cloning of the AGPS1 gene (ADP-Glucose Pyrophosphorylase Small subunit 1), which encodes the small subunit of the ADP-glucose pyrophosphorylase (AGP) enzyme (Jamalizadeh et al., 2025c). We discovered that a nucleotide substitution in the AGPS1 gene in CM1 mutant leads to a reduction in the AGP enzyme activity and disrupts starch biosynthesis in the CM1 mutant grain. Sugars from photosynthesis is redirected to the synthesis of β-glucan in CM1 mutant, resulting in the accumulation of 20% β-glucan as compared with 8% β-glucan in its parental cultivar. Molecular markers of this gene have been developed and applied for barley breeding aiming at development of new food barley cultivars with elevated levels of β-glucan.

References

Jamalizadeh L, Hu G, Hong Z (2025a) A high mixed linkage glucan mutant of barley reveals the intertwinement between starch and mixed linkage glucan biosynthesis pathways during seed development. (manuscript in review)

Jamalizadeh L, Hu G, Hong Z (2025b) Biosynthesis and function of mixed-linkage glucan in cereals. (manuscript in review)

Jamalizadeh L, Hu G, Hong Z (2025c) Reassessing ADP-glucose pyrophosphorylase in starch synthesis. (manuscript in review)

Hu G, Burton C, Hong Z (2014) Molecular and chemical characterization of a new waxy allele in barley (Hordeum vulgare L.). Cereal Chem 91:438-444

Hu G, Burton C, Hong Z, Jackson E (2014) A mutation of the cellulose-synthase-like (CslF6) gene in barley (Hordeum vulgare L.) partially affects the β-glucan content in grains. J Cereal Sci 59: 189-195