Many seeds contain components that are antinutritional and therefore restrict grain utilization for human or livestock consumption. Transgenic approaches have the potential to selectively reduce or remove these components, thereby increasing the availability of seed storage proteins for nutrition.
In order to use soybean meal in animal feed, it must be heat treated first to inactivate the endogenous trypsin inhibitor (TI) and chymotrypsin inhibitor (CI) proteins, which otherwise reduce protein digestibility. Identification of soybean lines without TI and CI activities could reduce soybean processing costs and increase amino acid availability, which can be reduced by excessive heat treatment (Herkelman et al., 1993; Lee and Garlich, 1992). Screening of the USDA soybean germplasm collection led to the discovery of one line (ti) that lacked the A2 TI and manifested a 30–50% reduction in TI activity (Orf and Hymowitz, 1979). Expression of the gene encoding BNA in soybean, originally intended as a means of increasing the methionine level as described above, also resulted in a reduction in TI and CI activities (Streit et al., 2001). To take advantage of both of these traits, transgenic soybean lines were created that express both BNA and the mutant ti allele of the Kunitz TI (Streit et al., 2001). Compared with control plants, average reductions of 85% in TI and 61% in CI activities were observed in the absence of any significant changes in plant yield and size, maturation time, and protein and oil deposition (Streit et al., 2001). While attempting to reduce seed TI and CI levels through either breeding or transgenic means, it should be considered that both proteins make substantial contributions to seed sulfur amino acid levels. Furthermore, these proteins are thought to be part of a plant defense mechanismand may need to be compensated with alternative mechanisms (Clarke and Wiseman, 2000).
In recent years, rape seed (Canola/B. napus) has become one of the most important oilseed crops in the world, as the healthful characteristics of the largely monounsaturated fatty acid content of its oil are widely recognized. Rape seed meal is also an important source of protein for animal feed, since its 2S albumins (napins) are rich in sulfur-containing amino acids. However, the meal is not suitable for human nutrition due to the high levels of antinutritional compounds, like sinapine esters. Sinapine is therefore a target for reduction or removal (Leckband et al., 2002) and this may be achieved by careful screening and breeding of low sinapine cultivars (Velasco and Mollers, 1998) and by genetic engineering. Nair et al. (2000) demonstrated a 40% reduction in sinapine by expressing Cauliflower mosaic virus 35Santisense B. napus ferulate-5-hydroxylase (BNF5H) transgene in B. napus. More modest reductions (17%) were achieved when the seed-specific napin promoter was used. BNF5H has an as yet undefined role in sinapine synthesis (Nair et al., 2000).
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