ARC Centre of Excellence in Plant Energy Biology, The University of Western Australia.
The C4 cycle enhances photosynthesis by concentrating CO2 from mesophyll (M) cells into RuBisCO-containing bundle sheath (BS) cells. Cell-specific accumulation of proteins, especially in the chloroplast, is critical for an efficient C4 cycle. Although the mechanisms governing cell-specific expression of nuclear genes have been extensively investigated, little is known about the mechanisms that give rise to M- and BS-specific expression of chloroplast genes, despite the primordial role of chloroplast-encoded RuBisCO in C4 photosynthesis. To complete our understanding of the processes that generate cell-specific accumulation of proteins used in the C4 pathway, we have analysed transcript abundances across the whole chloroplast genome in both M and BS cells in Cleome, the genus containing the closest known C4 relatives of Arabidopsis. Chloroplast transcripts show up to 35-fold differences in leaves of the closely related species C. gynandra (C4) and C. hassleriana (C3). However, these differences are mostly not due to M- or BS-specific accumulation of chloroplast transcripts. The similarity of the M and BS chloroplast transcriptomes in C. gynandra contrasts with the striking differences at the protein level and allows us to rule out transcriptional regulation and RNA degradation as major determinants of cell specific expression, as they would lead to observable differences in steady-state RNA levels. Most likely, cell-specific chloroplast gene expression in C. gynandra is regulated at the post-transcriptional or translational level.