Light and benzylaminopurine induce changes in ultrastructure and gene expression in plastids of Petunia hybrida cell cultures

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Abstract
The regulatory effect of light and the cytokinin 6-benzylaminopurine (BA) on the plastid ultrastructure and plastid DNA gene expression is studied in white and mutant green cell suspension cultures of Petunia hybrida. By electron microscopy we show that both light and 6-benzylaminopurine induce the formation of thylakoid membranes and grana structures in plastids of the green cultures. For membrane formation in plastids of white cultures, light in combination with BA is required. Light and benzylaminopurine also influence the plastid DNA gene expression. By in-organello protein synthesis with isolated plastids we show that light as well as benzylaminopurine affects the synthesis of plastid DNA encoded proteins. A characteristic effect of benzylaminopurine on plastids from white and green cultures is the reduction in the synthesis of the CFI subunits of 55,000 and 57,000 D, and the reduction in the synthesis of large polypeptides with a molecular weight higher than 67,000 D. In contrast to benzylaminopurine, light only affects the DNA gene expression of plastids from white cell cultures, that are in a very early stage of plastid development. Light stimulates the synthesis of polypeptides with a molecular weight of 84,000, 70,000 and 46,000 D which are encoded by cpDNA in these white culture plastids. In green cell cultures both plastids with a etioplast-like phenotype and with a chloroplast like morphology synthesize similar polypeptides, resulting in the same polypeptide pattern. Our results indicate that qualitative differences in plastid DNA gene expression as an effect of light do occur but only in plastids at very early stages of chloroplast development. We observe a gradual reduction in the number of high molecular weight polypeptides at later stages of chloroplast development. This suggests that these large polypeptides are characteristic for plastids at an early developmental stage.