Branching is a major determinant of crop yield, and enables vigorous shoot growth and the production of a dense canopy. Phytochrome A signal transduction 1 (PAT1) positively regulates phytochrome A signal transduction in response to light, but its effects on branching remain unknown. In this study, we mapped PAT1, and revealed a previously unknown role related to branching and flowering in leafy Brassica juncea. Earlier and increased branching was observed when PAT1 expression was down-regulated, implying that PAT1 negatively regulates shoot branching. Additionally, down-regulated PAT1 expression reversed the inhibited branching induced by far-red light, suggesting PAT1 is involved in the shade avoidance response. PAT1 negatively regulated branching only after bud initiation. The observed interaction between PAT1 and BRC1 implied that PAT1 influences bud outgrowth in a BRC1-dependent manner. Biochemical and genetic evidence indicate that PAT1 directly interacts with CONSTANS-LIKE 13 (COL13), which negatively regulates flowering, with the resulting PAT1-COL13 complex mediating shoot branching and flowering. Our findings reveal a new crosstalk modality between phytochrome signaling and flowering pathways during the regulation of shoot branching and flowering. The data presented herein may be useful for future studies involving the editing of the GRAS family transcription factor PAT1 gene to enhance crop productivity and enable earlier harvesting. This article is protected by copyright. All rights reserved.