Differential Expression in Bundle Sheath and Mesophyll Cells of Maize of Genes for Photosystem II Components Encoded by the Plastid Genome

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Abstract
We have explored the cell-specific expression of plastid genes encoding five photosystem II proteins (PSII-A, -B, -C, -D and cytochrome (Cyt) b-559 encoded by plastid genes psbA, B, C, D and E plus F, respectively) and one encoding the photosystem I protein PSI-A1 in bundle sheath and mesophyll cells (BSC and MC) at different stages of photoregulated development in leaves of dark-grown maize seedlings. Two types of cell-specific mRNA ccumulation patterns have been discerned. First is the PSII-A type: the mRNA is present in small but about equal amounts in both cell types in etiolated leaves; then, upon illumination, the level of mRNA increases progressively and remains high in MC but not BSC of green leaves. Second is the Cyt b-559 type: MC of etiolated leaves have about twice as much of this mRNA as do BSC; about a twofold to threefold transient increase is induced by light in MC but not in BSC. The genes for the PSII-C and -D proteins are complementary to a sizable family of transcripts over a large range of sizes; the patterns of changes in pools of some of these transcripts are of the PSII-A type and others are of the Cyt b-559 type. The DNA region encoding the PSII-B protein also hybridizes to multiple transcripts; among them is a prominent 2.2 kilobase transcript that follows the Cyt b-559 accumulation pattern. In contrast, the pattern of PSI-A1 mRNA accumulation is the same in both cell types. Accumulation of the PSII-B and -D proteins is light-dependent and coordinate. Both are much more abundant in MC than in BSC. PSII-A is detectable in MC of etiolated leaves and increases during photoregulated chloroplast maturation in both cell types, however, to a much greater extent in MC. We conclude that the expression of each of these plastidencoded photosystem II genes in BSC and MC is photoregulated differently; both transcript abundance and translation are regulated.

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