CYTOKININS MODULATE THE STEADY-STATE LEVELS OF LIGHT-DEPENDENT AND LIGHT-INDEPENDENT PROTEINS AND MESSENGER-RNAS IN TOBACCO CELL-SUSPENSIONS

Cytokinin modulation of chloroplast differentiation in light-grown tobacco (Nicotiana tabacum cv Wisconsin 38) cell suspensions was previously shown to involve the control of specific nuclear-encoded plastid proteins, including light-harvesting complex chlorophyll a/b binding protein (Cab) and the small subunit of ribulose 1,5-bisphosphate carboxylase/oxygenase (SSU). The steady-state levels of the corresponding mRNAs was enhanced in response to the hormone. Patterns of gene expression in response to cytokinin and light were further compared. Cytokinin-deprived, light-grown tobacco cells were subcultured, either in the dark or under white light, in a kinetin-supplemented medium or in a cytokinin-free medium. Total cell proteins were separated by two-dimensional electrophoresis. Amongst more than 500 distinguishable silver stained polypeptide spots, only about twenty were found to vary as a function of culture time and of light or cytokinin supply. These variations were quantified by image analysis. Various interactions between light and cytokinin in modulating the steady-state levels of some polypeptides were observed. Independently of light, cytokinin increased or decreased the levels of some polypeptides. These polypeptides were different from pathogenesis-related polypeptides detected in virus infected tobacco leaves. In vitro translation products of poly(A)+RNAs purified from cells harvested during the growth phase were fractionated by two-dimensional electrophoresis: some translatable mRNAs were modulated by kinetin but not by light. Northern blot and slot-blot hybridization experiments showed that kinetin enhanced the steady-state levels of Cab and SSU encoding mRNAs in light-grown cells only and had no detectable effect on these mRNAs in the dark.