Cytokinin modulation of LHCP mRNA levels: the involvement of post-transcriptional regulation.

When white-light-grown Lemna gibba plants are placed in the dark, the levels of mRNAs for two nuclear-encoded chloroplast proteins, the small subunit of ribulose-1,5-bisphosphate carboxylase (SSU) and the major chlorophyll a/b-binding protein of light-harvesting complex II (LHCP), decline to a small fraction of their previous level. We have reported [4] that red light (R), acting through phytochrome, and benzyladenine (BA), a synthetic cytokinin, independently stimulate accumulation of both mRNAs in the dark. Here, we have analyzed the products of transcription in isolated nuclei to determine if cytokinins act primarily through stimulation of transcription or if post-transcriptional processes are involved. We find that BA pretreatment may slightly stimulate transcription of LHCP RNA either with or without a red-light treatment. However, the effects of BA on the LHCP RNA accumulation were much greater than on transcription. Two naturally occurring cytokinins are also effective in increasing the mRNA abundance. We therefore conclude that, in Lemna, post-transcriptional processes are important in regulation of the LHCP RNA by cytokinins.