Gene expression in cytokinin- and light-mediated plastogenesis of Cucurbita cotyledons: ribulose-1,5-bisphosphate carboxylase/oxygenase

The effects of a cytokinin (N6-benzyladenine, BA) and light on plastogenesis have been studied in detached Cucurbita cotyledons using the key enzyme of photosynthetic CO2 fixation, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCase), as an example of a coordinated program of plastid and nucleo-cytoplasmic gene expression. Treatment of etiolated cotyledons with either BA in darkness or in light or light alone results in a marked and correlated stimulation of enzyme activity, quantity and biosynthesis (in-vivo [14C]leucine incorporation into immunoprecipitated enzyme protein), indicating an increase of de-novo synthesis under the influence of the two effectors. Cell-free translation of non-polyadenylated (poly(A)-)RNA in an Escherichia coli system and total RNA in a wheat-germ system likewise demonstrate a light and hormone-dependent increase in the amounts of translatable mRNAs for the large (LS) and small subunits (SS) of RuBPCase (among other polypeptides). Hybridisation of poly(A)-RNA with a nick-translated LS gene of spinach RuBPCase reveals also two- to three-fold BA- or light-induced enhancement of LS mRNA content. Indications for stimulation of SS mRNA transcription are derived from inhibitor experiments with cordycepin. We conclude that the observed stimulation of de-novo RuBPCase synthesis by cytokinin and by light can be referred to the level of mRNA transcription, but it remains open whether this is a primary action. Furthermore, our results indicate that the coaction of the two exogenous factors is additive at different steps of RuBPCase formation, indicating independent actions in the causal chain between effector-signal transduction and RuBPCase gene expression.