Submitted to: Bioscience Biotechnology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/1998
Publication Date: N/A
Interpretive Summary: A number of herbicide resistant crops (HRCs) have been developed that are resistant to phosphinothricin (PPT), the metabolite of the natural herbicide bialaphos. PPT is a potent inhibitor of glutamine synthetase (GS), a critical enzyme in plant metabolism. The resistance results from the expression of a gene, named bar, that encodes an enzyme the detoxifies PPT. A gene, named ttr, that encodes a similar enzyme capable of detoxifying tabtoxin, another GS inhibitor, has also been used to transform plants. While it has been demonstrated that the ttr gene does not protect against PPT, it has not been demonstrated if the bar gene provides cross-resistance to tabtoxin. This report describes experiments to test if plants that express the bar gene are cross-resistant to tabtoxin. The results indicate that plants that express the bar gene and are resistant to PPT are not resistant to tabtoxin. These results will be of benefit to researchers in government and industry interested in herbicide resistance mechanisms in plants.
Technical Abstract: Glutamine synthetase (GS) in plants have proven to be an important target enzyme in herbicide development. The most potent GS inhibitor, phosphinothricin (PPT), is a metabolite of the natural tripeptide bialaphos. Other natural GS inhibitors, such as tabtoxin (TB)and oxetin, also exist. Transgenic plants resistant to PPT and TB have been developed through modification with the acetyltransferase genes bar and ttr, respectively. Although plants expressing the ttr gene are not protected against PPT, it has not been determined if plants expressing the bar gene are protected against TB. A 20 æl drop of cell free culture filtrate (cfcf) from either a TB+ or TB- strain of Pseudomonas syringae was applied to leaves of growth chamber-grown transgenic soybean plants expressing bar gene and the leaf pricked through the drop of solution with a 22 gauge syringe needle. In a similar way, leaves were also treated with 10 æM glufosinate. Plants were grown for 1 to 2 days after treatment and the presence and size of chlorotic zones recorded. The none transformed soybean plants developed chlorotic zones on the leaves treated with 10 æM glufosinate, however, no chlorotic zones were present on the leaves of bar-transformed soybean plants. Treatments with cfcfs from the TB- Pseudomonade did not produce chlorotic zones on leaves of nontransformed or bar-transformed soybean plants. However, chlorotic zones were produced on leaves of both the nontransformed and bar-transformed soybean plants treated with the cfcf to the TB+ Pseudomonade. The results demonstrate that plants transformed with the bar gene are not cross-resistant to TB. These results will be of benefit to researchers in government and industry interested in herbicide resistance mechanisms in plants.