Protein modification

Research in this area has a number of objectives including:

• improving the functionality of a target crop protein
• increasing the essential amino acid content of the crop
• expressing the storage protein gene in parts of the plant other than seeds
• reducing the content of those proteins with specific allergenic properties.

Work has been done, for example, on the transfer of genes encoding proteins rich in desirable amino acids (usually methionine and lisine) from other species.

The maize gene encoding the protein zein increased methionine content by over 80% in transgenic soybean seeds. Allergenic protein has been reduced by the use of an antisense gene in rice (Tada et al. 1996) and the same strategies could be applied in fruit crops. It has also been possible to produce the β-casein protein in plants, avoiding the gastric and intestinal disorders in some children when fed with bovine milk (Arakawa et al. 1998).

Sweeter fruits, but with low calories, can now be produced by expressing the super-sweet protein thaumatin, isolated first in Thaumatococcus daniellii Benth. The gene has been isolated and sequenced (Edens et al. 1982) and introduced in potato and cucumber (Witty and Harvey, 1990; Szwacka et al. 1996) inducing sweet tasting phenotype. Thaumatin-like protein was isolated also from maize (Malehorn et al. 1994) which is similar to zeamatin and to α-amylase trypsin inhibitor. Overexpression in insect cells and in plants showed antifungal activity. Similar proteins were noted in cherry (Fils-Lycaon et al. 1996) and grapefruit (Tattersall et al. 1997) during ripening, demonstrating a probable antifungal role in ripened fruits and, at the same time, conferring a sweet taste.