Meeting in the Middle: Lessons and Opportunities from Studying C3-C4 Intermediates

The discovery of C 3 -C 4 intermediate species nearly 50 years ago opened up a new avenue for studying the evolution of photosynthetic pathways. Intermediate species exhibit anatomical, biochemical, and physiological traits that range from C 3 to C 4 . A key feature of C 3 -C 4 intermediates that utilize C 2 photosynthesis is the improvement in photosynthetic efficiency compared with C 3 species. Although the recruitment of some core enzymes is shared across lineages, there is significant variability in gene expression patterns, consistent with models that suggest numerous evolutionary paths from C 3 to C 4 photosynthesis. Despite the many evolutionary trajectories, the recruitment of glycine decarboxylase for C 2 photosynthesis is likely required. As technologies enable high-throughput genotyping and phenotyping, the discovery of new C 3 -C 4 intermediates species will enrich comparisons between evolutionary lineages. The investigation of C 3 -C 4 intermediate species will enhance our understanding of photosynthetic mechanisms and evolutionary processes and will potentially aid in crop improvement.