A dicotyledonous plant forms crown gall when

Crown gall is a disease caused by the bacterial plant pathogen, Agrobacterium tumefaciens. Crown gall bacteria enter plant roots through wounds. Wounds may have been created by planting, grafting, soil insect feeding, root damage from excavation or other forms of physical damage. Pathogenic agrobacteria are attracted to chemicals leaking from plant wound sites, then colonise and firmly attach to injured plant cells. The presence of these plant compounds can activate the transfer of part of their tumour-inducing plasmid (T-DNA) directly into the chromosomal DNA of the host plant cells. The T-DNA, once functional inside the plant cells, code for the production of plant hormones (e.g. indoleacetic acid and cytokinins) and carbon-nitrogen compounds called opines. Opines become food for the resident agrobacteria that colonise around the gall tissue.