Fungal resistance in potatoes:
The two-component barnase-barstar system

Potatoes have a natural defence mechanism against pathogens like Phytophthora. It forms a protective wall of dead plant cells around the seat of infection that is designed to prevent the fungus penetrating any further. It is this natural plant ability that is being enhanced with the help of genetic engineering. Genetic engineers use a gene from the soil bacterium Bacillus amyloliquefaciens, which leads to the death of cells. This toxic effect is triggered by a special enzyme (barnase), which cleaves the RNA molecules in the cells. However, to ensure that this affects only those parts of the plant that are infected with the fungus, the toxin gene is linked to the genetic switch (promoter) of the plant’s own defence gene. This switch activates the gene only in the affected cells, thereby preventing the infection from spreading.

Now scientists have established that barnase is nevertheless active to a limited extent outside the infection sites as well. They have therefore transferred a second gene from Bacillus amyloliquefaciens that can inhibit the effect of the barnase gene. Wherever only a small amount of barnase is active, i.e. in healthy plant regions, barstar can offset the toxic effect of barnase.

The barnase inhibitor, barstar, is of course also produced in the infected cells. However, here the amount of barnase produced as a result of the infection-specific promoter (gst1) is so high that the presence of a small volume of barstar does not prevent cell death.