ABSTRACT
Cells cultured from explants of tobacco leaf require exogenous cell-division factors such as the cytokinin kinetin for sustained proliferation. When cytokinin-requiring (C− ) cells are cultured on medium containing 1/100 the optimum cytokinin concentration they rapidly give rise to cytokinin-autotrophic (C+ ) variants. Some of these variants result from a meiotically transmitted change at the Habituated leaf-2 locus. We measured the rate of phenotypic variation by a simple, quantitative method and found that cultured tobacco cells alternate between the C− and C+ states at extremely high rates of approx. 10−2 per cell generation, which is 102 to 103 -fold more rapid than most somatic mutations in tobacco. These changes are so rapid that the classical distinction between random and induced events is blurred. Selection of alternate phenotypes arising by rapid, reversible cellular variation results in changes that appear to be directed at the tissue level. This phenomenon, called pseudodirected variation, is of particular interest because it suggests novel stochastic mechanisms of cytokinin action and a plausible explanation for the directed, but plastic nature of development in plants.
