A number of genes have been employed for the generation of genetically-modified crops possessing tolerance to herbicides in an effort promote crop growth and discourage the growth of competing plants such as weeds. Herbicide-resistant genes are also invaluable for use as selectable markers in the genetic transformation of plants. The majority of herbicide-resistant genes are derived from soil bacteria such as Agrobacterium and Streptomyces, organisms which have never been utilized as ingredients in products for human consumption. With respect to the use of plant-derived genes for herbicide tolerance, attention may be paid in order to facilitate public awareness and acceptance of the technologies involved. These genes are also useful in strategies involving intragenic transformation through homologous recombination to generate plants free from any exogenous DNA fragments. Our research efforts have focused on ALS. Use of this gene has several advantages including: (i) a single locus is present in Arabidopsis and rice, thus allowing for the straightforward implementation of gene targeting strategies, (ii) multiple classes of herbicides which interfere with different domains of ALS molecules are available, thereby providing the opportunity to generate plants with selected tolerance so as to reduce the occurrence of herbicide-resistant weeds in programs employing the rotation supply of different herbicides, and (iii) availability as a sustainable marker in chloroplast transformation in addition to a selectable marker for nuclear transformation. We have introduced the mutations G121A, A122V, P197S, P197H, R198S, W574L, S653I and others into Arabidopsis ALS and delivered these genes into nuclear and chloroplast genomes of plants. Use of these nuclear and transplastomic markers in crop plants would facilitate the implementation of a new strategy based on the rotation of multiple combinations of herbicides and mALSs to prevent the generation of herbicide-resistant weeds. Furthermore, the use of mALSs in gene-targeting for nuclear transformation and homologous recombination in plastid engineering would bring us closer to our goal of an ultimate clean technology, and allow for the production of GM plants in which only the ALS gene is mutated without integration of any other external DNA sequences.
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