Blue-light control of mRNA level and transcription during chloroplast differentiation in photomixotrophic and photoautotrophic cell cultures (Chenopodium rubrum L.).

In cell suspension cultures of Chenopodium rubrum maintained under photomixotrophic or photoautotrophic growth conditions the differentiation of chloroplasts is strictly blue-light-dependent. During this process of greening the steady-state concentration of mRNAs coding for plastid proteins increases rapidly in response to blue-light exposure as was determined by a dothybridization technique employing cloned DNA sequences complementary to these nuclear and plastid transcripts (light-harvesting chlorophyll a/b protein, rbcs, rbcl, psbA, atpB, atpE). Red light suppresses this response when applied at an advanced stage of chloroplast development. Indications are that blue-light dependency of chloroplast differentiation is a common feature of cultured plant cells irrespective of their metabolism. A DNA-protein complex with an active RNA polymerase ("transcriptionally active chromosome"; TAC) which specifically transcribes the plastid genes of its endogenous DNA has been isolated and purified from chloroplasts of light-grown cells. Quantitative analyses of these in-vitro transcripts show that the activity of TAC from cells grown in blue light prior to isolation is significantly higher than that of TAC from cells raised in red light under the same conditions. The results indicate that blue light enhances the transcription of plastid genes encoding prominent proteins. This response could account for the observed rise in transcript level in vivo.