The phytochrome B mediated light signaling integrates with various phytohormone signalings to control plant immune response. However, it is still unclear whether phyB-mediated light signaling has an effect on the biosynthesis of jasmonate during plant defense response against Botrytis cinerea. In this study, we demonstrated that phyB-mediated light signaling has a role in this process. Initially, we confirmed that phyb plants were obviously less resistant to B. cinerea while phyB overexpressing plants showed significantly enhanced resistance. We also found that the expression of numerous JA biosynthesis genes was promoted upon treatment with red or white light when compared to that of darkness, and that this promotion is dependent on phyB. Consistent with the gene expression results, phyb plants accumulated reduced pool of JA-Ile, indicating that phyB-mediated light signaling indeed increased JA biosynthesis. Further genetic analysis showed that light-mediated JAZ9 degradation and phyB-enhanced resistance were dependent on the receptor COI1, and that pif1/3/4/5 (pifq) can largely rescue the severe symptom of phyb. Taken together, our study demonstrates that phyB may participate in plant defense against B. cinerea through the modulation of the biosynthesis of JA. Copyright (C) 2021 Kunming Institute of Botany, Chinese Academy of Sciences. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.