Because the F2 and subsequent generations of any hybrid crop show reduced vigour and genotypic variance to the F1 parent (Richards 1997; Agrawal 1998; Gupta 1998), there is little value for farmers in keeping seeds of these crops for sowing the following year.
Instead, farmers are compelled to purchase new seed annually if they wish to continue using that particular hybrid cultivar. There is, however, no biological mechanism preventing farmers from using that hybrid seed in breeding programmes of their own.
This article explores the application of Genetic Use Restriction Technologies as it applies to hybrids.
The only researcher who has reviewed the Technology Protection System as it applies to hybrid crops is Gupta (1998). The molecular basis is very similar to that in non-hybrid plants.
Only two genes are required for TPS in hybrids.
One of the hybrid parents contains the Ribosomal Inactivating Protein “terminator” gene which is separated from its promoter (Late Embryogenesis Abundant) by the LOX blocking sequence (Gupta 1998). This is exactly as described in the previous article. The other hybrid parent is transformed with the Cre recombinase gene.
When the two parent plants are crossed, the Cre recombinase gene, Late Embryogenesis Abundant promoter, LOX sequence and Ribosome Inactivating Protein gene are integrated into the same genome.
Unlike in non-hybrid plants, there is no repressor for the Cre Recombinase gene so it is transcribed immediately and Cre is produced (Gupta 1998). As a result, the LOX sequence is cut from the genome and the LEA promoter regulates the production of Ribosome Inactivating Protein from the Ribosome Inactivating Protein gene, causing the embryos of the hybrid plant’s seed to abort.
Figure 3: A schematic diagram of the Technology Protection System as applied to hybrids. A Late Embryogenesis Abundant (LEA) promoter regulates the expression of a LOX sequence (itself containing a Cre recognition sequence), followed by a Ribosomal Inactivating Protein ("terminator") gene. A Cre Recombinase gene, under the control of a Late Embryogenesis Abundant promoter synthesies Cre Recombinase. The Cre enzyme recognises the Cre blocking sequence in the LOX sequence and splices LOX from the genome, thus placing the Ribosomal Inactivating Protein under the direct control of the Late Embryogenesis Abundant promoter. During late embryogenesis, the Ribosomal Inactivating Protein "Terminator gene" is expressed, leading to the abortion of all embryos.