Shared characteristics underpinning C4 leaf maturation derived from analysis of multiple C3 and C4 species of Flaveria

Britta M.C. Kümpers, Steven J. Burgess, Ivan Reyna-Llorens, Richard Smith-Unna, Chris Boursnell, Julian M. Hibberd

Most terrestrial plants use C3 photosynthesis to fix carbon. In multiple plant lineages a modified system known as C4 photosynthesis has evolved. To better understand the molecular patterns associated with induction of C4 photosynthesis, the genus Flaveria that contains C3 and C4 species was used. A base to tip maturation gradient of leaf anatomy was defined, and RNA sequencing was undertaken along this gradient for two C3 and two C4 Flaveria species. Key C4 traits including vein density, mesophyll and bundle sheath cross-sectional area, chloroplast ultrastructure, and abundance of transcripts encoding proteins of C4 photosynthesis were quantified. Candidate genes underlying each of these C4 characteristics were identified. Principal components analysis indicated that leaf maturation and the photosynthetic pathway were responsible for the greatest amount of variation in transcript abundance. Photosynthesis genes were over-represented for a prolonged period in the C4 species. Through comparison with publicly available data sets, we identify a small number of transcriptional regulators that have been up-regulated in diverse C4 species. The analysis identifies similar patterns of expression in independent C4 lineages and so indicates that the complex C4 pathway is associated with parallel as well as convergent evolution.