PSEUDODIRECTED VARIATION IN THE REQUIREMENT OF CULTURED PLANT-CELLS FOR CELL-DIVISION FACTORS

Cells cultured from explants of tobacco leaf require exogenous cell-di vision factors such as the cytokinin kinetin for sustained proliferati on. When cytokinin-requiring (C-) cells are cultured on medium contain ing 1/100 the optimum cytokinin concentration they rapidly give rise t o cytokinin-autotrophic (C+) variants. Some of these variants result f rom a meioticaIly transmitted change at the Habituated leaf-2 locus. W e 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 gener ation, which is 10(2)- to 10(3)-fold more rapid than most somatic muta tions in tobacco. These changes are so rapid that the classical distin ction between random and induced events is blurred. Selection of alter nate phenotypes arising by rapid, reversible cellular variation result s in changes that appear to be directed at the tissue level. This phen omenon, called pseudodirected variation, is of particular interest bec ause it suggests novel stochastic mechanisms of cytokinin action and a plausible explanation for the directed, but plastic nature of develop ment in plants.