Crown gall refers to the formation of tumors on roots and crown of many dicots.
The tumors contain enzymes, not seen in normal plants, which synthesize aminoacid and sugar conjugates called Opines. Opines are catabolized, i.e. used as food, by Agrobacterium , which, therefore, can take unique advantage of the tumor tissue. Agrobacterium-induced tumors have distinct properties. Intermediate ratios give undifferentiated tissue called callus. Virulent Agrobacterium strains harbor a large plasmid, called TI plasmid, which is necessary for tumor formation.