crown gall diseased plants have increased populations of A. tumefaciens , 
with galled plants being the most common source of A. tumefaciens . 
Plants may be colonized by A. tumefaciens, but not infected until 
wounded. Some strains of A. tumefaciens have a broad host range while 
others have a very narrow host range. 
Two characteristics of gall cells not shared by normal cells are the ability 
of gall cells to synthesize certain compounds known as opines and the 
ability of gall cells to proliferate in culture in the absence of plant growth 
hormones. It seems that an unusual ratio of plant growth hormones (auxin 
and cytokinin) in infected cells plays a major role in gall formation. 
The ability of A. tumefaciens to produce galls requires the presence in the 
bacterium of a very large plasmid (Ti plasmid). A small region of DNA 
(T-DNA) in the Ti plasmid is transferred into wounded plant cells, and 
becomes integrated into the plant's chromosomal DNA. Expression of 
genes within the T-DNA results in an alteration of the plant cells' hormonal 
balance, causing the crown gall growth. This "natural genetic engineering" 
is the basis for the technique used in this request to incorporate foreign 
genes into plants. 
53 . 1 Biochemical Genetics of T-DNA 
Ti plasmids have the unique natural ability to transform cells of 
susceptible host plants by the insertion of an 8 to 23 kilobase (kb) 
sector of plasmid DNA into host chromosomal DNA. This 
transferred DNA (T-DNA) causes the transformed cells to 
synthesize new metabolites called opines, with the synthase 
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