SPL13 controls tomato lateral branch outgrowth by regulating brassinosteroid biosynthesis and signal transduction

Plant architecture can directly impact on tomato (Solanum lycopersicum) fruit production. Our previous work has demonstrated an important role of microRNA156a (miR156a) in determining the lateral branches mainly by regulating SQUAMOSA PROMOTER BINDING PROTEIN LIKE 13 (SPL13) expression in tomato. However, the regulatory pathway by which the miR156a-SPL13 module regulates branching remains obscure. In this study, we examined the relationship between SPL13 and two other genes, BRANCHED1b (BRC1b) and DWARF (DWF), which have previously been known to regulate lateral branch development in tomato. Our findings indicate that SPL13 directly interacts with the promoters of BRC1b and DWF, enhancing BRC1b expression while inhibiting DWF expression, thereby finely regulating lateral branch development in tomatoes. Additionally, overexpression of BRC1b or knockout of DWF were found to rescue the increased lateral branch outgrowth phenotype of the spl13 mutant plants. Furthermore, brassinosteroid (BR) content was high in spl13 mutant plants, supporting the notion that SPL13 negatively regulates BR biosynthesis. These findings suggest that SPL13 serves as an activator of the BR signaling downstream gene BRC1b and a repressor of the BR biosynthesis gene DWF to regulate lateral branch outgrowth in tomato. One-sentence summary: The miR156a-targeted transcription factor SlSPL13 regulates plant architecture in tomato.