On March 3, 1998, a joint patent (US patent No: 5723765) has been granted to United States Department of Agriculture (USDA) and the Delta and Pine Land Company, Mississippi in the name of ‘Control of Plant Gene Expression’. Mr. Hope Shand, Research Director, Rural Advancement Foundation International (RAFI), christened it as ‘Terminator technology’ as the hybrid seeds containing it do not germinate after one generation.
The terminator technology is an extremely complex technology in which two gene systems are brought together to stop the normal process of embryo development, leading to the failure of seed germination. The gene systems are: Gene System I (Gene A) and Gene System II (Gene B and C).
The gene system I consist of a gene ‘A’ which produces the ribosome inactivating protein (RIP), which is lethal to the growing embryo. Gene ‘A’ is linked to a transistently active LEA Promoter, ‘PA’, through a blocking sequence. A recombinase specific excision sequence (LOX sequence) flanks the blocking sequence on either side.
The gene system II consists of a gene B linked to a promoter, PB the gene B encodes for a recombinase which is specific to the LOX sequence of the gene system I. A third gene C produces a repressor protein which blinds to the promoter PB and prevents the expression of gene B. The gene B can be depressed by exogenous application of tetracycline.
To develop a variety of seeds with functional terminator system, two cells of the same crop are transferred with the gene system I and II separately. As a result, one transgenic is obtained with unexpressed gene A due to the presence of blocking sequence between gene A and its promoter PA and another transgenic is obtained with gene system II having repressor of gene B. To recombine these two systems into one, the obtained transgenic are hybridized and normal hybrid seeds are obtained. Since the gene A does not express, the seeds obtained remain viable. Upon treatment of the seeds with tetracycline, the antibiotic is absorbed by the seeding tissue. Since tetracycline acts as an inducer of gene B, it depresses the gene and recombinase is produced. The recombinase removes the intervening blocking sequence between gene A and its promoter PA. Thus PA comes in proper orientation with gene A and the gene ready for expression. The promoter specifically expresses during early embryo development. As a result, the seeds germinate normally in that generation and give rise to normal crop and seeds. But the seeds obtained do not germinate as the embryo gets aborted due to expression of gene A. so long as gene B remains repressed in absence of tetracycline, gene A is not expressed leading to production of viable seed.
There are reports that this technology is presently being incorporated into two crops viz., tobacco and cotton. But it is a matter of time that it can be incorporated into other crops as well. This technology is not yet introduced into our country.
It is premature to predict its impact on our farmers. However, we can visualize its utility in curbing the spurious practice of selling F2 seeds of a hybrid variety as F1 seed. 