PLANT PHYSIOLOGY , Vol 105, Issue 3 989-998, Copyright © 1994 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Cell-Autonomous Cytokinin-Independent Growth of Tobacco Cells Transformed by Agrobacterium tumefaciens Strains Lacking the Cytokinin Biosynthesis Gene
R. C. Black, A. N. Binns, C. F. Chang and D. G. Lynn
Biology Department, Pennsylvania State University, Media, Pennsylvania 19063 (R.C.B.)
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.
