Seed composition

Soybean seed is a major source of oil (20%) and protein (40%). In addition to these two components, it is rich in isoflavone, phytate, sugar and other nutritional components. Soybean breeding for oil and protein has been a major objective of several breeding programmes throughout the world, and is still valid. There are rarely any soybean breeding programmes without this objective. However, at present there is a trend towards breeding soybean for quality traits, including oil quality (Pantalone et al., 2004) and protein quality (Panthee et al., 2006a, 2006c).

Oil quality consists of fatty acid composition. There are five predominant fatty acids in soybean oil: palmitic (C16:0), stearic (C18:0), oleic (C18:1), linoleic (C18:2) and linolenic (C18:3) acids (Wilson, 2004). Depending upon the use of the soybean oil, different concentrations of a particular fatty acid are desirable. The higher the number of carbon bonds, the greater the level of unsaturation, indicating that the oil is more reactive. It is known from past research that saturated and polyunsaturated fatty acids are not desirable for human consumption because they become rancid in a short time. Increases in oleic acid and decreases in linolenic acid make the oil better for human consumption. Increases in saturated fatty acids may improve applications of soybean oil such as in cosmetics. The fatty acid concentration can be manipulated in a breeding programme if the genetics of the trait are well known and desired sources of germplasm are available. It is known that fatty acid composition is a quantitative trait (Diers and Shoemaker, 1992; Wilson et al., 2002). Realizing this fact, researchers have identified the quantitative trait loci (QTL) associated with various fatty acids (Spencer et al., 2003; Panthee et al., 2006b). This has provided important information for the manipulation of fatty acid profiles in soybean through marker-assisted selection (MAS). Tremendous progress has been made in identifying and manipulating fatty acid composition by breeding, as reported in several studies (Johnson et al., 2001; Alt et al, 2002; Wilson et al, 2002; Hyten et al, 2004).

Breeding for protein quality consists of improving amino acid composition (Kwanyuen et al., 1998). A major function of proteins in nutrition is to supply adequate amounts of required amino acids (Friedman and Brandon, 2001). Amino acids are the principal building blocks of enzymes and other proteins. Twenty different amino acids are required for the growth and development of human and animal bodies. These amino acids are classified into two groups: essential and non-essential. Non-essential amino acids are readily available or can be synthesized by animals, hence they need not be supplied from external sources. Essential amino acids cannot be synthesized by animals, but play a crucial role in metabolic processes. The essential amino acids are lysine, histidine, leucine, isoleucine, valine, methionine, threonine, tryptophan and phenylalanine (D'Mello, 2003), although some nutritionists do not include histidine as an essential amino acid. Differences in classifying the amino acids as essential or non-essential are based on the type of animal and its nutritional requirements. For example, humans can produce ten of the 20 amino acids, whereas swine can produce only nine. Other amino acids must be supplied in the feed. Failure to obtain an adequate quantity of even a single essential amino acid leads to degradation of the body's proteins to obtain the deficient amino acid. Unlike fat and starch, the body does not store excess amino acids for later use. Therefore, the amino acids must be obtained from food every day. As mentioned before, soybean is rich in protein, but it does not contain a balanced composition of amino acids. Mainly, it is deficient in the sulphur-containing amino acids methionine and cysteine. Recently, soybean breeders have started to address this issue and breeding for protein quality has been a breeding objective (Panthee et al., 2006c). As a result of these efforts, three breeding lines have been released (Panthee and Pantalone, 2006). These lines are being used in other breeding programmes to improve the protein quality.

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