Benzyladenine modulation of the expression of two genes for nuclear-encoded chloroplast proteins in Lemna gibba: Apparent post-transcriptional regulation

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Abstract
Cytokinins and phytochrome have both been reported to promote chloroplast development, and possible interactions between the two have been suggested. We have examined the effects of red light (R) and a cytokinin, benzyladenine (N6-benzylaminopurine; BA), on the levels of four mRNAs coding for chloroplast proteins in Lemna gibba L. The amounts of hybridizable RNA coding for both the major chlorophyll a/b-binding protein and for the small subunit of ribulose-1,5-bisphosphate (RuBP) carboxylase decrease to a low level when white-light-grown L. gibba plants are placed in the dark. We have previously shown that a subsequent R treatment causes a several-fold increase in the levels of these two messages, and this increase is phytochrome-mediated. We have now found that addition of submicromolar concentrations of BA to plants kept in total darkness also results in an increase in levels of these two mRNAs. Furthermore, BA treatment magnifies the extent of the response to R treatment. However, the levels of mRNAs encoding the large subunit of RuBP carboxylase and the 32-kDa herbicide-binding protein, which are both chloroplastsynthesized messages, are not significantly altered by either R or BA treatment during the same time period. The relative amount of β-actin mRNA, a nuclear-encoded message for a cytoplasmic protein, is also not altered either by R or BA treatment. Thus, BA treatment does not simply alter the proportion of mRNA to total RNA. This conclusion is also supported by the observation that levels of mRNA hybridizing to a sequence abundant in dark-treated plants are not altered by BA treatment. The amplification by BA of the R-induced increase in the level of chlorophyll a/b-binding protein mRNA, consistently seen in total RNA, is not observed in RNA isolated from nuclei from plants receiving the same treatments. We therefore suggest that cytokinin is regulating expression of this message at a post-transcriptional level, possibly by affecting the stability of the RNA.