A retinoblastoma‐related protein promotes adventitious root development and secondary wall formation in Populus through the SHR/SCR network.

SUMMARY: Retinoblastoma‐Related (RBR) proteins, evolutionarily conserved homologs of animal RB tumor suppressor, are involved in cell cycle regulation, differentiation, and stress responses. This study systematically investigates the functional characterization of PeRBR in hybrid poplar (Populus deltoides × P. euramericana, clone "Nanlin 895") and its regulatory interactions with the SHR/SCR network governing adventitious root (AR) morphogenesis and secondary wall biogenesis. Transgenic poplar overexpressing PeRBR exhibited significant enhancement in AR system architecture and secondary xylem development, manifesting increased cambial cell layers (1.5–2.2 fold) and elevated lignin deposition (35% increase). Molecular analyses employing bimolecular fluorescence complementation (BiFC) and quantitative real‐time PCR (qRT‐PCR) revealed that PeRBR directly interacts with PeSCR in the nucleus while transcriptionally upregulating PeSHR, PeCYCD6;1, and PeWOX5 expression. Transcriptomic profiling identified 817 differentially expressed genes (DEGs) between WT plants and overexpression transgenic lines (OE_PeRBR), with notable enrichment in phenylpropanoid biosynthesis pathways. Key lignin biosynthesis genes (PAL, 4CL, CAD) and cellulose synthase (CesA) family members showed significant upregulation in OE_PeRBR lines compared to WT. These findings establish PeRBR as a central regulatory node within the SHR/SCR network, coordinating both AR development and secondary wall formation through transcriptional reprogramming of cell cycle regulators and cell wall biosynthesis machinery in woody species. Significance Statement: PeRBR as a central regulator within the SHR/SCR network governs AR development and secondary wall formation in Populus. [ABSTRACT FROM AUTHOR]