For GM plants, the bacterium most frequently used is called Agrobacterium tumefaciens. The gene of interest is transferred into the bacterium and the bacterial cells then transfer the new DNA to the genome of the plant cells. The plant cells that have successfully taken up the DNA are then grown to create a new plant.
Related Theory
coli $\lambda$ bacteriophage is one of the most widely used vectors used to carry recombinant DNA into bacterial cells.