Mutations at the
cytokinin biosynthesis locus (tmr) of Agrobacterium tumefaciens usually result in strains that induce
tumors exhibiting the rooty phenotype associated with high
auxin-to-
cytokinin ratios. However, tobacco (Nicotiana tabacum cv Havana 425) leaf disc explants responded to tmr- mutant strain A356 by producing rapidly growing, unorganized
tumors, indicating that these lines can grow in a
cytokinin-independent fashion despite the absence of a functional tmr gene. Several methods have been used to characterize the physiological and cellular basis of this phenotype. The results indicate that tmr-
tumors have a physiologically distinct mechanism for
cytokinin-independent growth in comparison to
tumors induced by wild-type bacteria. The
cytokinin-independent phenotype of the tmr- transformants appears to be cell autonomous in nature: only the transformed cells and their progeny were capable of
cytokinin-independent growth. Specifically, the tmr-
tumors did not accumulate
cytokinin, and clonal analysis indicated the tmr- transformed cells were not capable of stimulating the growth of neighboring nontransformed cells. Finally, the
cytokinin-independent phenotype of the tmr- transformants was shown to be cold sensitive, whereas the wild-type
tumors exhibited a cold-resistant
cytokinin-independent phenotype. Potential mechanisms for this novel form of
cytokinin-independent growth, including the role of the
dehydrodiconiferyl alcohol glucosides found in both
tumor types, are discussed.