Herbicide-resistance marker

Another major application of herbicide resistance is its utility as a selectable marker. Like antibiotic resistance in bacterial transformation, herbicide resistance should prove extremely useful for selecting transformants that are insect resistant, disease resistant, or engineered for other non-selectable traits. Many of these herbicide resistant markers have already been integrated into plant cloning vectors (32.43). 

DNA construct will often contain an effect gene and a selectable marker gene (such as antibiotic or herbicide resistance), both of which are bracketed by promoter and terminator sequences. A plasmid vector carries this cassette of genetic information into the plant genome by one of the above methods. 

In addition to being useful in the field, the herbicide resistance phenotype conferred by mutated ALS genes is a useful selectable marker in the laboratory. 

Li, Z.J., Hayashimoto, A., and Murai, N. 1992. A sulfonylurea herbicide resistance gene from arabidopsis-thaliana as a new selectable marker for production of fertile transgenic rice plants. Plant Physiol., 100, 662-668. 

Herbicide-resistant plant varieties have proven to be valuable experimental tools In determining the molecular mode of action of herbicides (2,8). In addition, such varieties are likely to be an Important source of selectable markers for use In plant molecular genetics and In the engineering of resistant crop species ( ) The earliest herbicide-resistant blotypes described arose spontaneously from weed populations which had been repeatedly exposed to a herbicide (10,11). More recently, mutagenesis and selection on defined media have been used to Isolate herbicide-resistant or herbicide-tolerant mutants of higher plants from populations of cells In tissue culture (4,5.7,12). While the use of plant tissue culture has proven useful for Isolation of some types of mutants, the... 

Nonetheless unanswered questions remain. What are the relative contributions of branched chain amino acid deficiency and AKB overabundance to the cytotoxic effects of the sulfonylurea and Imldazollnone herbicides Will the delineation of the cytotoxicity of AKB towards typhimurium, the only system in which it has been approached, provide relevant information towards the roles of this molecule in the Inhibition of plant growth Can herbicide-resistant alleles of ALS structural genes be used as dominant selectable markers In the transformation of a wide variety of sensitive cell lines What are the structural details of the interaction of ALS with the sulfonylurea and imldazollnone herbicides Are eukaryotic ALS Isozymes composed of nonldentlcal subunits How does SM cause cessation of DNA synthesis in plants In the next few years answers to some of these questions may emerge. 

Assuming that the chlorsulfuron resistance In the A. thallana mutants described here Is due to a mutation within the ALS structural locus. It may be possible to clone the resistant allele. Such a resistance gene should provide a very useful selectable marker for plant transformation studies. Indeed, transfer of the gene to other species may be useful In extending the agronomic utility of the herbicide. 

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