Changes in regulatory regions shape C3 to C4 evolution

In most land plants, carbon fixation into a three-carbon compound by the enzyme Rubisco takes place in the leaf mesophyll cells; these are called C3 plants. However, a different and more efficient pathway has evolved independently many times, in which a four-carbon metabolite is first produced in the mesophyll cells, followed by Rubisco activity in the bundle sheath cells surrounding the leaf veins; such plants are called C4 plants. While mesophyll and bundle sheath cells exist in both C3 and C4 plants, the specialization of bundle sheath cells for photosynthesis is a particular feature of C4 species. Swift, Luginbuehl et al. focused on rice (C3) and sorghum (C4) to elucidate the molecular changes underlying the specialization of bundle sheath cells for photosynthetic functions. The authors found that genes showing specialized expression in sorghum bundle sheath cells show an enrichment in DOF (DNA binding with one finger) binding motifs. Such motifs are absent in the regulatory regions of their mesophyll-specific rice counterparts, despite a similar expression of DOF transcription factors in the two species. Based on their findings, the authors propose a general model whereby changes in the promoter of photosynthesis genes of C4 plants, and in particular an enrichment in DOF motifs, control expression of such genes in bundle sheath cells instead of mesophyll cells. In C3 plants, different motifs in the promoter of photosynthesis genes drive their expression in mesophyll cells but not in bundle sheath cells. This work provides new insights into the transcriptional changes associated with C4 photosynthesis at the single-cell level, and will contribute to international efforts to engineer C4 photosynthesis in C3 plants to increase their efficiency. Moreover, it provides an example of the importance of changes in cis-regulatory sequences to shape important evolutionary traits. Summary by Laura Turchi, @turchil.bsky.social @turchi_l) Nature 10.1038/s41586-024-08204-3