We have found that the unorganized tobacco crown-gall tumor, isolated in our laboratory, and Braun's teratomatic tumor both exhibit the capacity to grow in the presence of high toxic concentrations of BUdR (up to 10-3mol·l-1). Double cytokinin-auxin habituated tissues also exhibit a constitutive BUdR-tolerance. In cytokinin-habituated tissues the BUdR-tolerance seems to depend on the degree of cytokinin autonomy. The presence of exogenous cytokinin significantly increases BUdR-tolerance, and the presence of BUdR suppresses the inhibitory effect of exogenous cytokinin upon habituated tissues. The results clearly show that the disposition of a cell for transient BUdR tolerance requires at least active expression of genes for cytokinin synthesis and that the tolerance becomes permanent when the auxin system is turned on. We conclude that BUdR-tolerance is connected with the establishment of a certain hormonal regulator pattern, depending on the state of cellular differentiation
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