For many plant species, the easiest method is Agrobacterium-mediated gene transfer. The normal T-DNA contains genes that encode plant growth hormones and cause the production of a plant tumor called a crown gall. Using recombinant DNA methods, the tumor causing genes were deleted from the T-DNA and any DNA of interest can be inserted. In addition to the gene of interest, the gene for a selectable marker is put between the T-DNA borders. This allows the selection of plant cells that have been genetically transformed.
Selectable markers used include antibiotic resistance genes and herbicide resistance genes.
The modified T-DNA is put back into Agrobacterium and is transferred to the plant by the normal infection process. Plants are regenerated, self-pollinated, and seed from these plants is germinated on the selective media. Many plants respond to low, nonfreezing temperatures by changing the intracellular concentrations of carbohydrates and free amino acids, their isozyme patterns, and their membrane composition. In addition, there is an alteration in the activity of many genes in response to low temperatures. Dr. Michael Thomashow of Michigan State University studies how plants respond to cold temperatures.