Cytokinin-controlled ribulose 1,5-bisphosphate carboxylase gene expression in pumpkin cotyledons

Treatment of etiolated and excisedCucurbita cotyledons with exogenous cytokinin (benzyladenine) in darkness or light results in a marked stimulation of Rubisco activity, content of enzyme protein, and incorporation of labelled precursors into it, indicating cytokinin-stimulatedde novo synthesis of the enzyme. Cell-free translations of RNA in the wheat germ andE. coli systems show an increase in both large and small subunit mRNA amounts relative to the increase of total RNA under the influence of the phytohormone and light. This increase in the level of translatable RNA is confirmed by RNA hybridization with the Rubisco large subunit gene of spinach. In addition, our results demonstrating additive effects of benzyladenine and light in cotyledon and chloroplast development suggest that the two factors co-act independently in the causal sequence of Rubisco gene expression. The data are discussed in a general view of cytokinin action in gene expression steps. Parts of the results have been obtained by cooperation with Drs. N. L. Klyachko, E. Romanko, and O. N. Kulaeva, Institute Plant Physiology, Acad. Sci. USSR, Moscow (cf. Lerbset al. 1984).