A tripartite transcriptional module regulates protoderm specification during embryogenesis in <i>Arabidopsis</i>

Protoderm formation is a crucial step in early embryo patterning in plants, separating the precursors of the epidermis and the inner tissues. Although key regulators such as ARABIDOPSIS THALIANA MERISTEM LAYER1 (ATML1) and PROTODERMAL FACTOR2 (PDF2) have been identified in the model plant Arabidopsis thaliana, the genetic pathways controlling protoderm specification remain largely unexplored. Here, we combined genetic, cytological, and molecular approaches to investigate the regulatory mechanisms of protoderm specification in Arabidopsis thaliana. We report a novel role of the β-importin KETCH1 in protoderm specification. KETCH1 loss-of-function leads to aberrant protoderm cell morphology and absent ATML1 transcription in embryos. We further demonstrate that KETCH1 directly interacts with an RNA Polymerase II (Pol-II) cofactor JANUS, mediating its nuclear accumulation. Furthermore, JANUS directly interacts with the WUS HOMEOBOX2 (WOX2) protein, which is critical for WOX2-activated ATML1 expression. Consequently, JANUS, KETCH1, and WOX2 loss-of-function results in similar protoderm defects. Our results identify the tripartite KETCH1/JANUS/WOX2 transcriptional module as a novel regulatory axis in Arabidopsis protoderm specification.