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. The T-DNA carries several genes, none of which is necessary for its own transfer, but which confer special properties to the tumor cells. These genes are all of the eukaryotic type with a typical plant promoter and terminator. As such, they do not function in bacteria. These genes were given names based on the tumor phenotype they show when the named gene is knocked out.