RIBOSOME-DEFICIENT PLASTIDS AFFECT TRANSCRIPTION OF LIGHT-INDUCED NUCLEAR GENES - GENETIC-EVIDENCE FOR A PLASTID-DERIVED SIGNAL

Transcription of ten nuclear genes was analysed in the albostrians mut ant of barley (Hordeum vulgare L.). The lack of plastid ribosomes in w hite seedlings of this mutant results in a complex alteration of nucle ar gene expression at the transcriptional level. We found a strong red uction in the accumulation of mRNAs transcribed from nuclear genes enc oding chloroplast enzymes involved in the Calvin cycle, the chlorophyl l a/b binding protein, and the cytosolic enzyme nitrate reductase. In contrast, the levels of transcripts of the genes encoding the cytosoli c glycolytic enzymes glyceraldehyde phosphate dehydrogenase and phosph oglycerate kinase were slightly enhanced. Accumulation of chalcone syn thase mRNA even reaches much higher levels in white than in green leav es. Ribosome-deficient plastids were combined by crossing with a nucle ar genotype heterozygous for the albostrians allele. Analysis of trans cript levels in F-1 plants having the same nuclear genotype and differ ing only with respect to their content of normally developed chloropla sts versus undifferentiated mutant plastids, provided strong genetic e vidence for the plastid being the origin of a signal (chain) involved in regulation of nuclear gene expression. Results of run-on transcript ion in isolated nuclei demonstrated that the plastid signal acts at th e level of transcription; it does not interfere with gene regulation i n general. Mechanisms triggering nuclear gene expression in response t o light operate in white mutant leaves: the very low levels of mRNAs d erived from nuclear genes encoding chloroplast proteins and the strong ly enhanced level of chalcone synthase mRNA were both light inducible. Also the negative regulation of leaf thionin gene expression by light is observed in white albostrians seedlings. Furthermore, in spite of its low absolute level, the circadian rhythm in the abundance of the c ab mRNA is still detectable in plastid ribosome-deficient seedlings. T hus, functional plastid protein biosynthesis and photosynthesis are no t preconditions for the circadian oscillations in the level of mRNA tr anscribed from this gene (family).