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

Abstract

Abstract C4 plants are believed to have evolved from C3 plants through various C3–C4 intermediate stages in which a photorespiration‐dependent CO2 concentration system known as C2 photosynthesis operates. Genes involved in the C4 cycle were thought to be recruited from orthologs present in C3 species and developed cell‐specific expression during C4 evolution. To understand the process of establishing C4 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 C3, C3–C4 intermediate, C4–like, and C4 Flaveria species. While genome sizes vary greatly, the number of predicted protein‐coding genes was similar among C3, C3–C4 intermediate, C4–like, and C4 Flaveria species. Cell‐specific localization of the PEPC, NADP‐ME, and PPDK transcripts was insignificant or weak in C3–C4 intermediate species, whereas these transcripts were expressed cell‐type specific in C4–like species. These results showed that elevation of gene expression and cell‐specific control of pre‐existing C4 cycle genes in C3 species was involved in C4 evolution. Gene expression was gradually enhanced during C4 evolution, whereas cell‐specific control was gained independently of quantitative transcriptional activation during evolution from C3–C4 intermediate to C4 photosynthesis in genus Flaveria.