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. Expected final online publication date for the Annual Review of Plant Biology, Volume 73 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.