A WOX5/7–SCRReciprocal Feedback Loop in Middle Cell Layer Drives Callus Proliferation

In plant tissue culture, the middle cell layer of the callus is crucial for establishing pluripotency and serves as the foundation for subsequent organ regeneration. Although several root apical stem cell regulators have been implicated in maintaining callus pluripotency, how they functionally coordinate to control the formation and proliferation of the middle callus layer remains unclear. Here, we identify a reciprocal transcriptional activation between the root stem cell regulators WOX5/7 and SCR in the middle callus layer of Arabidopsis. We further show that WOX5/7 and SCR form protein complexes that mutually enhance their transcriptional activities. Transcriptomic analysis reveals that WOX5/7 and SCR co-regulate a subset of cell cycle-related genes, explaining the reduced mitotic activity observed in the callus of wox5 wox7 double mutants and scr mutants. Together, these findings support a model in which WOX5/7 and SCR establish a reciprocal positive feedback loop in the middle cell layer that drives the robust callus proliferation by promoting cell cycle progression.