Natura non facit saltus: Conserved and rewired molecular pathways underpinning root ground tissue evolution in vascular plants

The emergence of roots during the Devonian period represented a key evolutionary innovation, improving anchorage, nutrient uptake, and stress resilience, hence easing plant colonization of diverse terrestrial environments. The early establishment of the root ground tissue (GT), particularly the cortex, also contributed to this transition as through its diversification underlies the vast anatomical and ecological breadth observed across vascular plant roots. Molecular studies have revealed that GT patterning is governed by gene regulatory networks (GRNs) deeply conserved across land plants. Central among these are the SHORT-ROOT (SHR)-SCARECROW (SCR) module and regulatory cascades shaped by hormonal signals and noncoding RNAs. These pathways act through gradients of transcription factors, mobile proteins, miRNAs, and hormone responses to control GT identity, cortex layer number, and postembryonic/age-dependent reprogramming of GT patterning. Comparative analyses across lycophytes, ferns, eudicots, and monocots suggest that while SHR-SCR is ancestral, it has been repeatedly rewired-via changes in protein mobility, miRNA expression domains, developmental phase-change regulators, and hormone-cell cycle crosstalk-driving lineage-specific cortical patterns. Notably, cortical layer number remains a highly plastic trait responsive to drought, flooding, salinity, and soil compaction, pointing to strong selective pressures on these molecular pathways. Here, we review the recent advances in the molecular mechanisms and evolutionary trajectories that have sculpted cortex patterning across land plants.