Nicole L. Arnold

Robust crop resistance to broadleaf and grass herbicides provided by aryloxyalkanoate dioxygenase transgenes

Engineered glyphosate resistance is the most widely adopted genetically modified trait in agriculture, gaining widespread acceptance by providing a simple robust weed control system. However, extensive and sustained use of glyphosate as a sole weed control mechanism has led to field selection for glyphosate-resistant weeds and has induced significant population shifts to weeds with inherent tolerance to glyphosate. Additional weed control mechanisms that can complement glyphosate-resistant crops are, therefore, urgently needed. 2,4-dichlorophenoxyacetic acid (2,4-D) is an effective low-cost, broad-spectrum herbicide that controls many of the weeds developing resistance to glyphosate. We investigated the substrate preferences of bacterial aryloxyalkanoate dioxygenase enzymes (AADs) that can effectively degrade 2,4-D and have found that some members of this class can act on other widely used herbicides in addition to their activity on 2,4-D. AAD-1 cleaves the aryloxyphenoxypropionate family of grass-active herbicides, and AAD-12 acts on pyridyloxyacetate auxin herbicides such as triclopyr and fluroxypyr. Maize plants transformed with an AAD-1 gene showed robust crop resistance to aryloxyphenoxypropionate herbicides over four generations and were also not injured by 2,4-D applications at any growth stage. Arabidopsis plants expressing AAD-12 were resistant to 2,4-D as well as triclopyr and fluroxypyr, and transgenic soybean plants expressing AAD-12 maintained field resistance to 2,4-D over five generations. These results show that single AAD transgenes can provide simultaneous resistance to a broad repertoire of agronomically important classes of herbicides, including 2,4-D, with utility in both monocot and dicot crops. These transgenes can help preserve the productivity and environmental benefits of herbicide-resistant crops.

Whiskers-Mediated Maize Transformation

There has been rapid progress in recent years in extending gene transfer capabilities to include plant species that fall outside the normal host range of Agrobacterium. Methods that allow direct DNA delivery into plant cells have contributed significantly to this expanded capability. Whiskers treatment is one means of delivering macromolecules, including DNA, to plant cells. Using relatively simple equipment and inexpensive materials, whiskers-mediated transformation of maize is possible. A critical prerequisite, however, is the establishment and maintenance of embryogenic tissue cultures as a source of totipotent, transformation-competent cells. Within hours of agitation in the presence of silicon carbide whiskers and DNA, embryogenic maize tissue cultures display transient gene expression, providing evidence for DNA uptake. Using appropriate selectable marker genes, following in vitro selection on inhibitory levels of a corresponding selection agent, stably transgenic tissue cultures can be generated from which fertile plants can be recovered. The timeline from whiskers treatment of embryogenic maize tissue cultures to fertile seed recovery is approximately 9 months, which is competitive with other methods of maize transformation.