SUMMARYThe transition of C 3 , via C 2 towards C 4 photosynthesis is an important example of stepwise evolution of a complex genetic trait. A common feature that was gradually emphasized during this trajectory is the evolution of a CO 2 concentration mechanism around Rubisco. In C 2 plants, this mechanism is based on tissue-specific accumulation of glycine decarboxylase (GDC) in bundle sheath (BS) cells, relative to global expression in the cells of C 3 leaves. This limits photorespiratory CO 2 release to BS cells. Because BS cells are surrounded by photosynthetically active mesophyll cells, this arrangement enhances the probability of re-fixation of CO 2 . The restriction of GDC to BS cells was mainly achieved by confinement of its P-subunit (GLDP). Here, we provide a mechanism for the establishment of C 2 -type gene expression by studying the upstream sequences of C 3 Gldp genes in Arabidopsis thaliana. Deletion of 59 bp in the upstream region of AtGldp1 restricted expression of a reporter gene to BS cells and the vasculature without affecting diurnal variation. This region was named the 'M box'. Similar results were obtained for the AtGldp2 gene. Fusion of the M box to endogenous or exogenous promoters supported mesophyll expression. Nucleosome densities at the M box were low, suggesting an open chromatin structure facilitating transcription factor binding. In silico analysis defined a possible consensus for the element that was conserved across the Brassicaceae, but not in Moricandia nitens, a C 2 plant. Collective results provide evidence that a simple mutation is sufficient for establishment of C 2 -specific gene expression in a C 3 plant.

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