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

The discovery of C₃-C₄ 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 C₃ to C₄. A key feature of C₃-C₄ intermediates that utilize C₂ photosynthesis is the improvement in photosynthetic efficiency compared with C₃ 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 C₃ to C₄ photosynthesis. Despite the many evolutionary trajectories, the recruitment of glycine decarboxylase for C₂ photosynthesis is likely required. As technologies enable high-throughput genotyping and phenotyping, the discovery of new C₃-C₄ intermediates species will enrich comparisons between evolutionary lineages. The investigation of C₃-C₄ intermediate species will enhance our understanding of photosynthetic mechanisms and evolutionary processes and will potentially aid in crop improvement.