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The adaptation of germinating spinach seedlings to yellow and red light was studied and compared with plants grown in white light. Spinach chloroplasts isolated from cotyledons and leaves of yellow and white light-grown plants showed similar membrane structures and compositions, while chloroplasts from plants grown in red light have significant adaptive changes. Based on an equal amount of chlorophyll, these changes include a reduction in the number of photosystem I complexes, an increase of photosystem II antenna size, and an increased ratio of stacked to unstacked membranes in red light-adapted chloroplasts. The decrease in the number of photosystem I complexes per unit of chlorophyll in these chloroplasts was qualitatively correlated with an approximately 10-fold decrease in the level of the psaA mRNA encoding the photosystem I 65-kilodalton to 70-kilodalton chlorophyll apoprotein, as well as with a differential decrease in mRNA levels of other photosynthetic proteins. Light quality adaptations do not significantly affect the plastid to nuclear DNA ratio or the overall chloroplast transcription activity. The relative transcriptional activities of 10 plastid genes, as determined by run-on transcription assays, are similar in chloroplasts from cotyledons and leaves of plants grown under the three light qualities. Only the psaA gene shows a 30% to 40% decrease in transcription activity in chloroplasts of plants adapted to red light. This decrease in psaA transcription activity, however, cannot fully account for the decrease of its mRNA level. We conclude, therefore, that post-transcriptional mechanisms are primarily responsible for the control of differential chloroplast mRNA accumulation in light quality adaptations.

We have used the carotenoid biosynthesis inhibitor norflurazon to study the relationship between chloroplast and nuclear gene expression and the mechanisms by which plastid mRNA accumulation is regulated in response to photooxidative stress. By treating 4-week-old hydroponic spinach plants (Spinacea oleracea), we were able to determine the response at two distinct stages of chloroplast development. For all parameters studied, differences were found between the norflurazon-treated young and mature leaves. Young leaves lost essentially all pigment content in the presence of norflurazon, whereas mature leaves retained more than 60% of their chlorophyll and carotenoids. The accumulation of plastid mRNA was determined for several genes, and we found a decrease in mRNA levels for all genes except psbA in herbicidetreated young leaves. For genes such as atpB, psbB, and psaA, there was a corresponding change in the relative level of transcription, but for psbA and rbcL, transcription and mRNA accumulation were uncoupled. In norflurazon-treated mature leaves, all plastid mRNAs except psaA accumulated to normal levels, and transcription levels were either normal or higher than corresponding controls. This led to the conclusion that plastid mRNA accumulation is regulated both transcriptionally and posttranscriptionally in response to photooxidative stress. Although direct photooxidative damage is confined to the plastid and peroxisome, there is a feedback of information controlling the transcription of nuclearencoded plastid proteins. Considerable evidence has accumulated implicating a "plastid factor" in this control. Therefore, the expression of several nuclear-encoded plastid proteins and the corresponding mRNAs were determined. Although the levels of both the small subunit of ribulose-1,5-bisphosphate carboxylase and the light harvesting chlorophyll a/b-binding protein and corresponding mRNAs were reduced, a 28-kilodalton chloroplast RNA-binding protein and corresponding mRNA were at normal levels in norflurazon-treated plants. Changes in mRNA and protein levels were not the result of a general loss due to photooxidation but rather the result of selective stabilization of certain components. The response of both genomes to photooxidative stress is discussed in terms of the postulated plastid factor.Carotenoids in higher plants protect Chl from photooxidation under normal or high light conditions (1). There is 'This work was supported by the