The study introduces Transposase-Accessible Chromosome Conformation Capture (TAC-C), which combines ATAC‑seq and Hi‑C to map fine‑scale chromatin interactions in rice, sorghum, maize, and wheat, revealing genome‑size‑correlated loop structures and distinct C3 vs. C4 patterns. Integration with population genetics shows that loops link distal regulatory elements to phenotypic variation, and SPL transcription factors (TaSPL7/15) modulate photosynthesis‑related genes via these interactions, enhancing photosynthetic efficiency and starch content in wheat mutants.