C4 Photosynthesis Mechanisms and Evolution

C4 photosynthesis is a sophisticated adaptation that enhances carbon fixation efficiency by partitioning the photosynthetic process between two specialised cell types. In mesophyll cells, atmospheric CO₂ is initially fixed by phosphoenolpyruvate carboxylase into four-carbon acids, which are then shuttled to bundle sheath cells. There, decarboxylation elevates CO₂ concentration around Rubisco, suppressing photorespiration and enabling higher productivity under high light, temperature and water-limited conditions. This two-cell mechanism, known as Kranz anatomy, has arisen independently over 60 times in angiosperms through the recruitment and rewiring of ancestral C3 regulatory networks. Recent single-cell and chromatin studies reveal that C4 evolution frequently relies on exaptation of pre-existing cis-regulatory codes, guided by DNA One Finger (DOF) motifs, to activate photosynthetic genes in bundle sheath cells. Intermediates such as the C2 photorespiratory pump are seen as evolutionary stepping-stones that rebalance nitrogen and carbon metabolism, predisposing lineages to full C4 function. Understanding these molecular and anatomical innovations offers routes to engineer C4 traits into C3 crops like rice or wheat, promising gains in yield and resilience in the face of rising atmospheric CO₂ and global warming.

Research from Nature Portfolio

Recent studies have uncovered the role of exaptation of ancestral cell identity networks in activating photosynthesis within bundle sheath cells. Single-nucleus transcriptomic and chromatin accessibility maps demonstrated that genes encoding core C4 enzymes acquire pre-existing cis-codes originally associated with C3 bundle sheath identity, guided by DNA One Finger (DOF) motifs. This molecular recruitment model explains how a stable pattern of transcription factor deployment in C3 leaves was co-opted to establish the C4 pathway, offering targets for rational engineering of photosynthetic efficiency in crops. A foundational analysis of nitrogen distribution along the sugarcane leaf has also illuminated how nutrient supply modulates the establishment of the C4 apparatus, showing that genotypic responsiveness to nitrogen alters the activity of phosphoenolpyruvate carboxylase and the accumulation of photosynthetic metabolites in specific leaf zones, thereby linking nutrient status to C4 functionality and crop productivity under variable soil conditions.

C4 Photosynthesis Mechanisms and Evolution publication trend

The graph below shows the total number of articles in c4 photosynthesis mechanisms and evolution across all publications each year (not limited to Nature Index journals).

Technical terms

Kranz anatomy: Specialised leaf organisation in which mesophyll and bundle sheath cells form a concentric arrangement to facilitate spatial separation of steps in C4 photosynthesis.

Mesophyll cells: Photosynthetic cells that capture atmospheric CO₂ via phosphoenolpyruvate carboxylase, generating four-carbon acids.

Bundle sheath cells: Internal leaf cells where decarboxylation of four-carbon acids concentrates CO₂ around Rubisco, minimising photorespiration.

Phosphoenolpyruvate carboxylase (PEPC): Enzyme that catalyses the initial fixation of CO₂ into oxaloacetate in the C4 pathway.

Cis-regulatory element: DNA sequence segment that controls the timing, location and level of gene transcription, crucial for cell-specific expression of C4 genes.

DOF transcription factors: Plant-specific proteins that bind to DNA One Finger motifs to regulate gene expression in bundle sheath cells.

De-etiolation: Transition of plant tissues from dark-grown to light-exposed state, revealing mechanisms of photosynthetic gene induction.

References

1. Exaptation of ancestral cell-identity networks enables C4 photosynthesis. Nature (2024).
2. C4 gene induction during de-etiolation evolved through changes in cis to allow integration with ancestral C3 gene regulatory networks. Science Advances (2023).
3. C4 maize and sorghum are more sensitive to rapid dehydration than C3 wheat and sunflower. New Phytologist (2023).
4. Nitrogen supply influences photosynthesis establishment along the sugarcane leaf. Scientific Reports (2018).
5. On the road to C4 rice: advances and perspectives. The Plant Journal (2019).
6. The role of photorespiration during the evolution of C4 photosynthesis in the genus Flaveria. eLife (2014).