The OsbZIP35-COR1-OsTCP19 module modulates cell proliferation to regulate grain length and weight in rice.

Grain size substantially influences rice quality and yield. In this study, we identified CONTROL OF SLENDER RICE 1 (COR1), a quantitative trait locus encoding an F-box protein that enhances grain length by promoting cell proliferation. The transcription factor OsbZIP35 represses COR1 expression, while COR1 interacts with OsTCP19, leading to its degradation. Knockout of either OsbZIP35 or OsTCP19 results in increased grain length, confirming their regulatory roles in grain development. The OsbZIP35-COR1-OsTCP19 module controls the expression of the cell cycle gene OsCycB1;4, thereby modulating cell proliferation and ultimately determining grain size. Five haplotypes of COR1 were identified, with COR1-Hap1 and COR1-Hap3 being elite alleles associated with longer grains. Field plot trials demonstrated that the near-isogenic line NIL-COR1(SLG) increased yield by ~6.6% compared to NIL-COR1(Nip). These findings elucidate the genetic mechanisms underlying grain size regulation and offer promising strategies for improving rice yield.