| Summary: | <p>The transition of C<sub>3</sub>, via C<sub>2</sub> towards C<sub>4</sub> 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<sub>2</sub> concentration mechanism around Rubisco. In C<sub>2</sub> 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<sub>3</sub> leaves. This limits photorespiratory CO<sub>2</sub> release to BS cells. Because BS cells are surrounded by photosynthetically active mesophyll cells, this arrangement enhances the probability of re‐fixation of CO<sub>2</sub>. 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<sub>2</sub>‐type gene expression by studying the upstream sequences of C<sub>3</sub> <em>Gldp</em> genes in <em>Arabidopsis thaliana</em>. Deletion of 59 bp in the upstream region of <em>AtGldp1</em> 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 <em>AtGldp2</em> 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. <em>In silico</em> analysis defined a possible consensus for the element that was conserved across the Brassicaceae, but not in <em>Moricandia nitens</em>, a C<sub>2</sub> plant. Collective results provide evidence that a simple mutation is sufficient for establishment of C<sub>2</sub>‐specific gene expression in a C<sub>3</sub> plant.</p>
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