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Modification of Seed Protein Quality -
Cereal seed proteins are deficient in lysine, while those of pulses are deficient in sulphur containing amino acids, e.g., methionine, and tryptophan. This limits their nutritional value for man since these amino acids are essential for man.

Therefore, improvement of seed storage protein quality is an important and seemingly feasible objective. The approaches to achieve this objective may be grouped into the following two broad categories:
(1) introduction of an appropriate transgene, and
(2) modification of the endogenous protein encoding gene.

Introduction of An Appropriate Transgene. In this approach a new gene encoding a storage protein, which is rich in the deficient amino acids, is introduced into the crop to correct its amino acid deficiency. The transgene is linked to a seed-specific promoter to ensure its expression only in seeds.
Vicilin is the major seed storage protein of pea; it contains 7% lysine but no sulphur containing amino acid (methionine and cysteine). Therefore, pea seed protein is generally low in sulphur containing amino acids, which needs to be ameliorated. In contrast, a sunflower seed storage protein, sunflower albumin 8 (SFA8), contains 23% methionine plus cysteine.

The gene coding for SF A8 has been isolated. The SF A8 gene has been fused with the vicilin gene promoter and expressed in tobacco, which showed the accumulation of the protein in seeds of transgenic tobacco.

Modification of Endogenous Genes. This approach to improve seed storage protein quality is based on the isolation and modification of the concerned protein encoding gene sequence either by
(i) replacing one or few codons with the selected codons or by
(ii) inserting one or few selected additional codons at appropriate sites.

For example, prolamine storage proteins e.g., zein, of cereals are deficient in the essential amino acids lysine and tryptophan. Single lysine replacements in the N-terminal coding sequence as well as within and between the peptide repeats, and double lysine replacement constructs of prolamine genes have been prepared.

In addition, short oligo­nucleotides encoding lysine-and tryptophan-rich peptides were inserted separately at several different points in the coding sequence. These constructs were shown to express well in Xenopus oocytes and their polypeptides were able to form normal aggregates of protein bodies.

Some Successful Examples. In a recent study, the 7 S legume seed storage protein, β-phaseolin, gene (driven by rice storage protein gene gt1 (glutein 1) promoter was transferred in rice. Transgenic rice plants expressed the gene in their endosperm, and some plants showed up to 4% of their total proteins to be β-phaseolin.

The 11S legumin protein gene driven by gt1 promoter has also been transferred, and expressed in rice endosperm. In another study, Du Pont (USA) scientists have synthesized and patented a gene encoding a protein, called CP 3-5, containing 35% lysine and 22% methiomine.
The CP 3-5 gene was coupled with seed specific promoters and transferred into maize. Rice gt1 gene encodes the major rice seed storage protein. It has been modified to encode higher levels of lysine, tryptophan and methionine.

The modified gtl gene, driven by its own promoter, was transferred into rice protoplasts; the resulting transgenic rice plants express the modified gene in their developing endosperm. Similarly, a modified zein protein gene encodes a protein having improved methionine. This gene was introduced in maize rice and wheat the transgenic plants showed upto 3.8% methionine in their seed proteins.

 
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