Redundant functions of miR156-targeted SQUAMOSA PROMOTER BINDING PROTEIN-LIKE transcription factors in promoting cauline leaf identity.

Cauline leaf development represents an intermediate phase between vegetative and reproductive stages. While extensive research has been conducted on the genetic and environmental factors that determine cauline leaf number, less attention has been given to the regulation of their morphology and the establishment of cauline leaf identity. In this study, we report that miR156-targeted SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factors, including SPL2, SPL9, SPL10, SPL11, SPL13, and SPL15, redundantly regulate cauline leaf identity, affecting both cauline leaf shape and the number of leaves on secondary inflorescences. This function is distinct from that of floral meristem identity genes, which affect the number of cauline leaves by promoting floral fate. We further show that the inducers of reproductive development SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) and FRUITFUL (FUL) directly bind to and activate SPL9 and SPL15, linking floral induction pathways to the regulation of cauline leaf identity. Additionally, we demonstrate that the brassinosteroid receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1) is co-expressed with miR156-targeted SPLs in cauline leaves and is a direct target of SPL9. Together, this study uncovers a SOC1/FUL-SPL-BRI1 module that governs cauline leaf identity, providing new insights into the regulatory networks that control plant inflorescence architecture.