Sulphur Nutrition and Legume Seed Quality

Abstract

Plants are the major food source for humans and other animals, providing carbohydrates, protein, lipids and vitamins. This chapter describes the very important role plants play in providing the protein required in animal diets. Protein accounts for approximately 10 to 15% of the dry weight of cereal grains, 20 to 25% of the dry weight of many legume seeds, and 40% of the dry weight of a typical soybean seed. Even though cereal grains and legume seeds are excellent sources of protein, many are deficient in at least some of the ten essential amino acids. Maize, for example, is deficient in both lysine and tryptophan, whereas legume seeds are frequently deficient in the sulphur amino acids, methionine and cysteine. Clearly, a plant must coordinate its effort to obtain reduced nitrogen and reduced sulphur if it is to synthesize large quantities of seed protein. On the other hand, the seed must contain sufficient nitrogen to nourish the developing seedling. Nitrogen-fixing legumes have evolved so that they can produce a large quantity of two different classes of seed storage proteins, those that are rich in both nitrogen and sulphur and others that are sulphur-deficient. Glycinins, which account for approximately 40% of soybean seed protein and are rich in both nitrogen and sulphur, represent the former class. On the other hand, the β-chain of soybean (β-conglycinin lacks methionine and cysteine and normally accounts for approximately 10% of soybean seed protein. When a plant is grown on a sulphur-deficient regime, however, the β-chain of β-conglycinin can account for 30% of total seed protein. Mechanisms by which the plant regulates the relative abundance of these two protein classes are proposed. Also, efforts to enhance the relative abundance of the sulphur amino acids in seed storage proteins are described.