Plant ALS genes

Mutations can also be incorporated into the ALS gene in order to alter the specific interactions between the enzyme and its inhibitors. Site directed mutagenesis has been used to incorporate mutations into cloned plant ALS genes based both on previously characterized mutants and on computer modeling information. These altered genes have been transformed into plants, and the transgenic plants have been tested for their sensitivity to a variety of... 

Figure 5. Disk assays of plant ALS genes expressed in bacteria. E. coli expressing a wild type or a herbicide resistant ALS gene were tested for their ability to grow in the presence of the active ingredients from two commercial herbicides.

Gallagher, C.E. et al.. Gene duplication in the carotenoid biosynthetic pathway preceded evolution of the grasses (Poaceae). Plant Physiol. 135, 1776, 2004. 

The sulfonylureas, an extremely potent class of herbicides, act by inhibiting acetolactate synthase (ALS), which is the first common enzyme in the biosynthetic pathways leading to the branched chain amino acids. Two other unrelated classes of herbicides also act by interfering with this enzyme. We have cloned and characterized the genes encoding ALS from several higher plants. The ALS genes isolated from herbicide sensitive and herbicide resistant plants have been compared, and several mutations which confer the herbicide resistant phenotype have been identified. 

Cloned herbicide resistant ALS genes have been used to transform both homologous and heterologous plant species. ALS genes can be modified in vitro in order to achieve selective resistance toward broad or narrow classes of inhibitors. The modified genes can be introduced into a variety of commercial crops. 

These cloned Arabidopsis and tobacco ALS genes have been used as hybridization probes to isolate ALS genes from other crop species and to isolate ALS genes from plants selected for resistance to sulfonylurea herbicides. 

Figure 2. Assays of tobacco transformed with Arabidopsis ALS genes. Tobacco containing either the herbicide sensitive (S) or resistant (R) ALS gene was tested for the ability of transformed callus to grow in the presence of herbicide (slashed boxes) and the herbicide resistance of enzyme activity in plant extracts (solid boxes). Each measurement is expressed as a percentage of the value that was obtained in the absence of herbicide.

Figure 3. Assays of tobacco transformed with a mutant tobacco ALS gene. Enzyme activity in the presence of herbicide was measured in leaves of commercial tobacco cultivars transformed with the HRA gene (plants 7-54), and in untransformed plants (WT2, WT4). Activity is expressed as a percentage of the activity measured in the absence of herbicide.

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