Evolution of c4 phosphoenolpyruvate carboxylase. Genes and proteins: a case study with the genus Flaveria.

Abstract

C4 photosynthesis is characterized by a division of labour between two different photosynthetic cell types, mesophyll and bundle-sheath cells. Relying on phosphoenolpyruvate carboxylase (PEPC) as the primary carboxylase in the mesophyll cells a CO2 pump is established in C4 plants that concentrates CO2 at the site of ribulose 1,5-bisphosphate carboxylase/oxygenase in the bundle-sheath cells. The C4 photosynthetic pathway evolved polyphyletically implying that the genes encoding the C4 PEPC originated from non-photosynthetic PEPC progenitor genes that were already present in the C3 ancestral species. The dicot genus Flaveria (Asteraceae) is a unique system in which to investigate the molcular changes that had to occur in order to adapt a C3 ancestral PEPC gene to the special conditions of C4 photosynthesis. Flaveria contains not only C3 and C4 species but also a large number of C3-C4 intermediates which vary to the degree in which C4 photosynthetic traits are expressed. The C4 PEPC gene of Flaveria trinervia, which is encoded by the ppcA gene class, is highly expressed but only in mesophyll cells. The encoded PEPC protein possesses the typical kinetic and regulatory features of a C4-type PEPC. The orthologous ppcA gene of the C3 species Flaveria pringlei encodes a typical non-photosynthetic, C3-type PEPC and is weakly expressed with no apparent cell or organ specificity. PEPCs of the ppcA type have been detected also in C3-C4 intermediate Flaveria species. These orthologous PEPCs have been used to determine the molecular basis for C4 enzyme characteristics and to understand their evolution. Comparative and functional analyses of the ppcA promoters from F. trinervia and F. pringlei make it possible to identity the cis-regulatory sequences for mesophyll-specific gene expression and to search for the corresponding trans-regulatory factors.