Here’s how Agrobacterium causes the formation of galls.  This clever bacterium carries around plant genes that it doesn’t use, but when it detects an opening -- meaning a wounded plant cell (shown by the breaks in the wall around the cell),  it attaches and slips those genes, themselves on a separate small plasmid called the Ti plasmid, into the plant.  These genes find their way into the cells nucleus and are inserted into its DNA within its chromosomes.  The bacterial genes reprogram the plant a bit, first to grow a tumor  and second to make special compounds that the bacteria in the vicinity can use for nourishment. The tumor doesn’t kill the plant, it just coaxes it to make food for the bacterium. What Schell and others did was remove the gall-forming genes, incorporate a new gene, putting Agrobacterium to work as a gene-delivery system in order  to improve crop plants.  The tumor-forming genes were replaced with genes that are beneficial for the plants, such as a gene that codes for a protein that is toxic to insects that feed on a plant.