Genetic variability in cereal carbohydrate compositions and potentials for improving nutritional value

Mono-gastric animals and ruminants require cereal starch with different degradation characteristics because they degrade and utilize starch differently. Characteristics affecting starch digestion are the amylose/amylopectin ratio, proportion A-/B-starch granules, starch granule shape, crystallinity, lipid content, nature of the protein matrix surrounding starch granules and the overall architecture of the starch granules. These are all characteristics of cereal grain that can vary within species according to genotype, and can be manipulated through plant breeding.A large collection of barley mutants affecting the endosperm phenotype is available and constitutes an enormous potential resource to improve nutritive values of cereals tailored for different nutritional purposes. In the starchy endosperm, starch biosynthesis is characterised by a committed pathway of different enzyme classes, and all together 14 different isoform classes are found in higher plants with specific functions. A wide genetic variation in starch polymer composition could therefore be obtained by searching for mutations in these genes. However, while the starch biosynthetic pathways are well known, the biosynthetic pathway of β-glucan and the accumulation in endosperm cell walls are poorly understood. Some interesting barley mutants exist that combine low starch synthesis with excessive β-glucans synthesis completely or partly compensating for the decrease in starch formation. Recent knowledge on the biosynthesis of starch together with methods in biotechnology and bioinformatics, molecular genetics and comparative genomics have opened new possibilities to provide a more complete understanding of starch synthesis as well as synthesis of β-glucan.In this review, knowledge on carbohydrate biosynthesis and the use of new biotechnological methods, are coupled with animal feed science to reveal the potential for optimising feed nutritional value through plant breeding. This can as well be exploited to obtain cereal-based food with less starch and more β-glucan that may be of major importance in low calorie foods for human nutrition.