The Ti plasmid, or tumor-inducing plasmid, is a circular piece of DNA found in certain strains of the bacterium Agrobacterium tumefaciens. This plasmid is responsible for causing crown gall disease in plants by transferring a portion of its DNA into the plant's genome, leading to uncontrolled growth and tumor formation. Researchers utilize the Ti plasmid in genetic engineering to introduce new genes into plants. By modifying the Ti plasmid, scientists can insert desired traits, such as pest resistance or improved nutritional content, into crops, making it a valuable tool in biotechnology and agriculture.