CsBRC1 inhibits axillary bud outgrowth by directly repressing the auxin efflux carrier <i>CsPIN3</i> in cucumber

Abstract

Shoot branching is an important agronomic trait that directly determines plant architecture and affects crop productivity. To promote crop yield and quality, axillary branches need to be manually removed during cucumber production for fresh market and thus are undesirable. Auxin is well known as the primary signal imposing for apical dominance and acts as a repressor for lateral bud outgrowth indirectly. The <i>TEOSINTE BRANCHED1</i>/<i>CYCLOIDEA/PCF</i> (<i>TCP</i>) family gene <i>BRANCHED1</i> (<i>BRC1</i>) has been shown to be the central integrator for multiple environmental and developmental factors that functions locally to inhibit shoot branching. However, the direct molecular link between auxin and BRC1 remains elusive. Here we find that cucumber <i>BRANCHED1</i> (<i>CsBRC1</i>) is expressed in axillary buds and displays a higher expression level in cultivated cucumber than in its wild ancestor. Knockdown of <i>CsBRC1</i> by RNAi leads to increased bud outgrowth and reduced auxin accumulation in buds. We further show that CsBRC1 directly binds to the auxin efflux carrier <i>PIN-FORMED</i> (<i>CsPIN3</i>) and negatively regulates its expression in vitro and in vivo. Elevated expression of <i>CsPIN3</i> driven by the <i>CsBRC1</i> promoter results in highly branched cucumber with decreased auxin levels in lateral buds. Therefore, our data suggest that CsBRC1 inhibits lateral bud outgrowth by direct suppression of <i>CsPIN3</i> functioning and thus auxin accumulation in axillary buds in cucumber, providing a strategy to breed for cultivars with varying degrees of shoot branching grown in different cucumber production systems.