The CsphyB–CsPIF4–CsBRC1 module regulates ABA biosynthesis and axillary bud outgrowth in cucumber

Shoot branching is an important crop agronomic trait that directly affects plant architecture and crop productivity. Although phytochrome B (phyB), BRANCHED1 (BRC1), and abscisic acid (ABA) mediate axillary bud outgrowth, it is unknown if there is any integrating factor among them in the Plantae. We report that mutation of CsphyB or inactivation of CsphyB by shade inhibits lateral bud outgrowth in cucumber. Cucumber PHYTOCHROME INTERACTING FACTOR 4 (CsPIF4) interacts with CsphyB and directly binds to the promoter of CsBRC1 to activate CsBRC1 expression. CsBRC1 also directly promotes the expression of ABA biosynthesis gene 9-CIS-EPOXICAROTENOID DIOXIGENASE 3 (CsNCED3). Functional disruption of CsPIF4 decreased expression of CsBRC1 and CsNCED3, reduced ABA accumulation, and increased bud outgrowth in cucumber. Csnced3 mutants had reduced ABA levels and increased lateral bud outgrowth. These results suggest that a regulatory network involving CsphyB-CsPIF4-CsBRC1 exists that integrates light signaling and ABA biosynthesis to modulate bud outgrowth. This provides a strategy to manipulate branch numbers in crop breeding to realize ideal branching characteristics to maximize yield.