| Summary: | <ul>
<li>Photosynthetic efficiency is reduced by the dual role of Rubisco, which acts either as a carboxylase or as an oxygenase, the latter leading to photorespiration. C<sub>4</sub> photosynthesis evolved as a carbon-concentrating mechanism to reduce photorespiration. To engineer C<sub>4</sub> into a C<sub>3</sub> plant, it is essential to understand how C<sub>4</sub> genes, such as <i>phosphoenolpyruvate carboxylase (PEPC1)</i>, are regulated to be expressed at high levels and in a cell-specific manner.</li>
<li>Yeast one-hybrid screening was used to show that OsPRI1, a rice bHLH transcription factor involved in iron homeostasis, binds to the <i>Setaria viridis PEPC1</i> promoter. This promoter drives mesophyll-specific gene expression in rice. The role of OsPRI1 <i>in planta</i> was characterized using a rice line harbouring <i>SvPEPC1<sub>pro</sub>::GUS</i>.</li>
<li>We show that OsPRI1 activates the <i>S. viridis PEPC1</i> promoter by binding to an N-box in the proximal promoter, and that GUS activity is highly reduced in <i>SvPEPC1<sub>pro</sub>::GUS</i> lines when <i>OsPRI1</i> is mutated. Cross-species comparisons showed that the SvPRI1 homolog binds to the <i>SvPEPC1</i> promoter but the maize ZmPRI1 does not bind to the <i>ZmPEPC1</i> promoter.</li>
<li>Our results suggest that elements of the iron homeostasis pathway were co-opted to regulate <i>PEPC1</i> gene expression during the evolution of some but not all C<sub>4</sub> species.</li>
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