Jasmonic acid(JA) functions in plant development,including senescence and immunity. Arabidopsis thaliana CORONATINE INSENSITIVE 1 encodes a JA receptor and functions in the JA-responsive signaling pathway. The Arabidopsis genome harbors a single COI gene, but the rice(Oryza sativa) genome harbors three COI homologs, Os COI1 a,Os COI1 b, and Os COI2. Thus, it remains unclear whether each Os COI has distinct, additive, synergistic, or redundant functions in development. Here, we use the oscoi1b-1 knockout mutants to show that Os COI1 b mainly affects leaf senescence under senescence-promoting conditions. oscoi1b-1 mutants stayed green during dark-induced and natural senescence,with substantial retention of chlorophylls and photosynthetic capacity. Furthermore, several senescence-associated genes were downregulated in oscoi1b-1 mutants, including homologs of Arabidopsis thaliana ETHYLENE INSENSITIVE 3 and ORESARA 1, important regulators of leaf senescence. These results suggest that crosstalk between JA signaling and Resea ethylene signaling affects leaf senescence. The Arabidopsis coi1-1 plants containing 35S:Os COI1 a or 35S:Os COI1 b rescued the delayed leaf senescence during dark incubation, suggesting that both Os COI1 a and Os COI1 b are required for promoting leaf senescence in rice. oscoi1b-1 mutants showed significant decreases in spikelet fertility and grain weight,leading to severe reduction of grain yield, indicating that Os COI1-mediated JA signaling affects spikelet fertility and grain filling. Arabidopsis thaliana CORONATINE INSENSITIVE 1 encodes a JA receptor and functions in the JA-responsive signaling pathway. The Arabidopsis genome harbors a single COI gene, but the rice (Oryza sativa) genome harbors three COI homologs, Os COI1 a, Os COI1 b, and Os COI2. Thus, it remains unclear whether each Os COI has distinct, additive, synergistic, or redundant functions in development. Here, we use the oscoi1b-1 knockout mutants to show that Os COI1 b mainly affects leaf senescence under senescence-promoting conditions. oscoi1b-1 mutants stayed green during dark-induced and natural senescence, with substantial retention of chlorophylls and photosynthetic capacity. Furthermore, several senescence-associated genes were downregulated in oscoi1b -1 mutants, including homologs of Arabidopsis thaliana ETHYLENE INSENSITIVE 3 and ORESARA 1, important regulators of leaf senescence. These results suggest that cross talk between JA signaling and Resea ethylene signaling affects leaf senescence. The Arabidopsis coi1-1 plants containing 35S: Os COI1 a or 35S: Os COI1 b rescued the delayed leaf senescence during dark incubation, suggesting that both both COI1 a and Os COI1 b are required for promoting leaf senescence in rice. oscoi1b-1 mutants significant significant in spikelet fertility and grain weight, leading to severe reduction of grain yield, indicating that Os COI1-mediated JA signaling affects spikelet fertility and grain filling.