Developments in plant breeding for improved nutritional quality of soya beans II. Anti-nutritional factors

Nutritional value of most plant materials is limited by the presence of num erous naturally occurring compounds which interfere with nutrient digestion and absorption. Although processing is employed widely in removal of these factors, selection of cultivars of soya beans with inherently low levels w ould have a considerable impact on efficiency of non-ruminant livestock pro duction. The review considers the role of plant breeding in achieving this objective. The most abundant trypsin inhibitors are the Kunitz and the Bowm an-Birk inhibitors, containing 181 and 71 amino acids respectively. The Kun itz inhibitor is present at a concentration of 1.4 g/kg of total seed conte nts and the Bowman-Birk inhibitor 1.6 g/kg. A large number of isoforms of t he Bowman-Birk inhibitor have been described in soya bean cultivars and it has been shown that the general properties of the inhibitor an, in fact, at tributable to different isoforms. Nulls for both Bowman-Birk and Kunitz try psin inhibitors have been identified, allowing new low trypsin inhibitor cu ltivars to be produced. However, research into breeding for low trypsin inh ibitor cultivars currently has limited application as trypsin inhibitors co ntribute a major proportion of the methionine content of soya beans. Trypsi n inhibitors are thought to be involved in the regulation of and protection against unwanted proteolysis in plant tissues and also act as a defence me chanism against attack from diseases, insects and animals. Hence, in breedi ng programmes for low trypsin inhibitor cultivars, alternative protection f or growing plants must be considered. Use of soya beans in non-ruminant ani mal feeds is limited by the flatulence associated with their consumption. T he principal causes appear to be the low molecular weight oligosaccharides containing alpha-galactosidic and beta-fructosidic linkages; raffinose and stachyose. Non-ruminants do not have the alpha-galactosidase enzyme necessa ry for hydrolysing the alpha-galactosidic linkages of raffinose and stachyo se to yield readily absorbable sugars. Soya beans contain between 6.8 and 1 7.5 g of phytic acid/kg; a ring form of phosphorus (P) which chelates with proteins and minerals to form phytates not readily digested within the gut of non-ruminants. One approach for over-coining the effects of phytic acid is through synthesis of phytase in the seeds of transgenic plants. Currentl y, recombinant phytase produced in soya beans is not able to withstand the processing temperatures necessary to inactivate proteinaceous anti-nutritio nal factors present. Soya bean lectins have the ability to bind with certai n carbohydrate molecules (N-acetyl-D-galactosamine and galactose) without a ltering the covalent structure. Lectins are present in raw soya bean at a c oncentration of between 10 and 20 g/kg. Purified soya bean agglutinin is ea sily inactivated by hydrothermal treatment but in complex diets binding wit h haptenic carbohydrates may confer protection against denaturation. The ma jority of research into soya bean lectins is carried out using laboratory a nimals so very limited information is available on their in vivo effects in farm animals. This review is concerned specifically with breeding but ther e are other means of improving nutritive value, for example processing whic h may alter protein structure and therefore functionality of proteinaceous anti-nutritional factors present.