Dynamic changes of genome sizes and gradual gain of cell-specific distribution of C4 enzymes during C4 evolution in genus Flaveria

C-4 plants are believed to have evolved from C-3 plants through various C-3-C-4 intermediate stages in which a photorespiration-dependent CO2 concentration system known as C-2 photosynthesis operates. Genes involved in the C-4 cycle were thought to be recruited from orthologs present in C-3 species and developed cell-specific expression during C4 evolution. To understand the process of establishing C-4 photosynthesis, we performed whole-genome sequencing and investigated expression and mesophyll- or bundle-sheath-cell-specific localization of phosphoenolpyruvate carboxylase (PEPC), NADP-malic enzyme (NADP-ME), pyruvate, orthophosphate dikinase (PPDK) in C-3, C-3-C-4 intermediate, C-4-like, and C-4 Flaveria species. While genome sizes vary greatly, the number of predicted protein-coding genes was similar among C-3, C-3-C-4 intermediate, C4-like, and C4 Flaveria species. Cell-specific localization of the PEPC, NADP-ME, and PPDK transcripts was insignificant or weak in C-3-C-4 intermediate species, whereas these transcripts were expressed celltype specific in C-4-like species. These results showed that elevation of gene expression and cell-specific control of pre-existing C-4 cycle genes in C-3 species was involved in C-4 evolution. Gene expression was gradually enhanced during C-4 evolution, whereas cell-specific control was gained independently of quantitative transcriptional activation during evolution from C-3-C-4 intermediate to C-4 photosynthesis in genus Flaveria.

Automated summary

C-4 plants are believed to have evolved from C-3 plants through various intermediate stages, involving changes in gene expression and cell-specific control. Research results indicate that during the establishment of C-4 photosynthesis, gene expression increases gradually and cell-specific control strengthens.