As a first step in understanding the differential expression of the plastid-encoded photosystem II (PSII) genes in mesophyll and bundle-sheath cells, we have used RNA blotting techniques to investigate the transcript patterns of these genes in three NADP-malic enzymetype C4 species: Zea mays L., Sorghum bicolor (L.) Moench and Pennisetum americanum (L.) Leeke. Our comparison showed that in all three species the relative levels of transcripts encoding PSII proteins were diminished in bundle-sheath cells. No major differences, either in abundance or in the processing pathways, could be detected for transcripts encoding subunits of the PSI and ATP-synthase complexes. The transcript profiles of the psbB and psbD/C transcription units were particularly striking. These operons were of heterogeneous composition, i.e. they encode PSII subunits as well as proteins or RNAs which are involved in different functional entities. The transcript patterns of the psbB and psbD/C transcription units were complex and characterized by multiple, partially overlapping RNAs. Our analysis showed that the relative levels of the oligocistronic PSII transcripts derived from these transcription units with the exception of psbH were selectively reduced in bundlesheath cells. In contrast, RNAs carrying the non-PSII components were present in similar quantities in the two cell types. The data demonstrate that segmental RNAs within one single transcription unit can accumulate to different degrees. Regulatory mechanisms which may explain this expression behaviour are discussed.

We have investigated the photosystem-I1 organization in differentiating-bundle-sheath cells of the three malate dehydrogenase (oxaloacetate decarboxylating) (NADPf)-type C4 species maize, Sorghum and Pennisetum. Using a set of nine different antisera raised against individual subunits of photosystem-11, we demonstrate that photosystem-I1 components constitute a substantial part of the thylakoid membranes of young bundle-sheath chloroplasts. The abundance of subunits of the photosystem-I1 core, i.e. the 47-and 43-kDa chlorophyll-a-binding proteins, polypeptides D1 and D2, cytochrome b S s 9 , and the 34-kDa polypeptide, varies with the developmental state of the plant. However, the levels of the 23-kDa, 16-kDa and 10-kDa extrinsic polypeptides of the wateroxidation complex are drastically reduced in bundle-sheath chloroplasts of all three species analyzed, regardless of their state of differentiation. The reduction in protein abundance is also reflected at the transcript level: only traces of the nuclear-encoded mRNAs are found in differentiating bundle-sheath cells of Sorghum, suggesting that the transcription of these genes has been switched off. Our data are compatible with the idea that the wateroxidation complex is a prime site for initiating or maintaining the process leading to photosystem-I1 depletion during differentiation of bundle-sheath cells.

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