enzymes for opine production encoded by T-DNA genes. 
Additional genes in T, DNA affect the phytohormone balance of 
transformed cells as evidenced by altered morphology of crown 
gall cells transformed by various mutant T-DNAs. Transformed 
plant cells containing wild type T-DNA grow in vitro without an 
exogenous supply of either auxin or cytokinin, whereas normal 
plant cells usually require both substances for growth in tissue 
culture. Mutations in one T-DNA locus result in callus cultures 
from which abundant roots proliferate ("rooty" mutants), while 
mutations in a second T-DNA locus result in callus cultures from 
which shoots proliferate ("shooty" mutants). Transformation of 
tobacco cells by shooty mutant T-DNA results in callus cultures 
which exhibit a complex auxin requirement. Such tissues will 
grow on hormone free medium under conditions which allow shoot 
proliferation, but otherwise require auxin for continuous growth. 
The rooty and shooty functions are coded in a T-DNA region 
common to octopine, nopaline, "unusual nopaline" and agropine Ti 
plasmids of wide host range, evidence that all such Ti plasmids 
promote crown gall growth of plant cells through a common 
mechanism. 
T-DNA acts as a natural gene vector for A. tumefaciens, 
producing transformed plant cells that display an abnormal 
hormonal balance and synthesize new metabolites. Recent work 
has shown that the Ti plasmid can be exploited as an artificial 
gene vector to introduce novel genes into plant tumor cells. 
Heretofore, a major obstacle to the exploitation of Ti plasmids as 
Environmental Assessment 
Pace 29 
[ 499 ] 
