Abstract

Leaves of almost all C4 lineages separate the reactions of photosynthesis into the mesophyll (M) and bundle sheath (BS). The extent to which mRNA profiles of M and BS cells from independent C4 lineages resemble each other is not known. To address this we conducted deep sequencing of RNA isolated from the M and BS of Setaria viridis and compared these data with publically available information from maize. This revealed a high correlation (r = 0.89) between the relative abundance of transcripts encoding proteins of the core C4 pathway in M and BS cells in these species, indicating significant convergence in transcript accumulation in these evolutionarily independent C4 lineages. We also found that the vast majority of genes encoding proteins of the C4 cycle in S. viridis are syntenic to homologues used by maize. In both lineages, 122 and 212 homologous transcription factors were preferentially expressed in the M and BS respectively. Sixteen shared regulators of chloroplast biogenesis were identified, fourteen of which were syntenic homologues in maize and S. viridis. In Sorghum bicolor, a third C4grass, we found that 82% of these trans-factors were also differentially expressed in either M or BS cells. Taken together these data provide the first quantification of convergence in transcript abundance in the M and BS cells from independent lineages of C4 grasses. Furthermore, the repeated recruitment of syntenic homologues from large gene families strongly implies that parallel evolution of both structural genes and trans-factors underpins the polyphyletic evolution of this highly complex trait in the monocotyledons.