Differential transcription of plastome-encoded genes in the mesophyll and bundle-sheath chloroplasts of the monocotyledonous NADP-malic enzyme-type C4 plants maize and Sorghum

Abstract

The transcription of plastome-encoded genes in mesophyll and bundle-sheath chloroplasts of the monocotyledonous NADP-malic enzyme-type C4 species Zea mays L. (maize) and Sorghum bicolor (L.) Moench. was investigated. RNA accumulation and transcription were assayed starting from isolated mesophyll and bundle-sheath chloroplasts and using quantitative northern and run-on transcription analysis. Determination of the mesophyll to bundle-sheath ratios of transcript abundance in maize and Sorghum chloroplasts showed that the mRNAs of the plastome-encoded photosystem II genes analysed (psbA, psbB, psbD, psbH and psbE/F) varied from 2.5- to 4.0-fold (maize) and 3.1- to 5.2-fold (Sorghum), respectively. The rbcL transcript, in contrast, was more abundant in bundle-sheath chloroplasts of both species, about 3-fold in maize and more than 10-fold in Sorghum. On the other hand, transcripts of genes encoding the 16S ribosomal RNA (r16) and subunits of photosystem I (psaA) and the cytochrome b/f complex (petB, petA) accumulated to similar levels in both types of chloroplasts. Determination of absolute transcript levels for rbcL and psbA in chloroplasts from maize and Sorghum demonstrated that for both genes, the mesophyll to bundle-sheath differences in transcript abundance were more pronounced in Sorghum. Measurements of the transcriptional activities of rbcL and psbA showed that the transcription rate of rbcL is higher in bundle-sheath chloroplasts while psbA is more actively transcribed in mesophyll chloroplasts. The differences in the transcription rates between the two chloroplast types were again more pronounced in Sorghum, thus reflecting the differences between maize and Sorghum in the relative levels of the rbcL and psbA transcripts. However, although transcription rate and mRNA abundance are correlated, they did not exactly match one another. This indicates additional regulation of transcript abundance at the level of RNA stability.

Differential use of two cyclic electron flows around photosystem I for driving CO 2 -concentration mechanism in C 4 photosynthesis

“…2 a and d), consistent with previous results (23). Because the intersystem electron carrier cytochrome b 6 ͞f complex participates in both LEF and CEF, its contents should not be affected by PSI͞PSII and show good correlation with PSI (24).…”

Complementary Expression of Two Plastid-Localized ς-Like Factors in Maize

“…Leaves of C4 plants, such as maize, have agranal chloroplasts in the bundle sheath cells encircling the leaf vascular tissue, and granal chloroplasts in the mesophyll cells surrounding the bundle sheath (Langdale and Nelson, 1991;Furbank and Taylor, 1995). Consistent with their different roles in photosynthesis, bundle sheath and mesophyll chloroplasts differ in their expression of plastid-encoded genes (Link et al, 1978;Kubicki et al, 1994). Since these transcription differences may be mediated by different -factors in the two cell types, we asked whether ZmSig1 protein accumulated preferentially in one or the other cell type.…”

bundle sheath defective2, a Mutation That Disrupts the Coordinated Development of Bundle Sheath and Mesophyll Cells in the Maize Leaf

“…The molecular mechanisms that allow bundle sheath cell-specific accumulation of the chloroplast-encoded large subunit and nuclear-encoded small subunit of RuBPCase have been studied in maize, amaranth, and sorghum leaves (Boinski et al, 1993;Kubicki et al, 1994). For example, transcriptional run-on analyses of RuBPCase large subunit (rbcL) mRNAs in separated bundle sheath and mesophyll chloroplasts of lightgrown amaranth leaves demonstrated that the rbcL gene is transcribed in both cell types, even though rbcL transcripts and peptides accumulate in a bundle sheath cell-specific manner (Boinski et al, 1993).…”