Heat Shock Transcription Factor B1 Coordinates Distinct bHLH Modules to Orchestrate Land Plant Adaptation

Adaptation to terrestrial environments required the evolution of morphological structures, such as rhizoids and air pores, and robust tolerance mechanisms to environmental stress. While these innovations are well-described, the regulatory logic coordinating development with stress signaling during this transition remains largely unknown. Here, we identify Heat Shock Transcription Factor B1 (HSFB1) as a central evolutionary node that bridges this gap in Marchantia polymorpha. Using single-nucleus RNA sequencing (snRNA-seq) and gene regulatory network analysis, we reveal that MpHSFB1 is required to activate cell-type-specific programs for terrestrial adaptation, functioning via the regulation of cytokinin and reactive oxygen species (ROS) homeostasis. We find that MpHSFB1 functions through a modular logic: it co-opts a specific set of bHLH transcription factors (including MpbHLH21/40) to drive developmental morphogenesis, while recruiting a distinct partner (MpbHLH51) to confer resistance to prolonged heat stress. This functional uncoupling allows a single ancestral regulator to manage disparate physiological needs. By reconstructing HSFB1 regulons across diverse plant lineages, we propose that MpHSFB1 acted as an ancestral pleiotropic hub that was effectively rewired and subfunctionalized in later angiosperm lineages for land plant evolution.