RSL4-regulated transcription and ROP signaling coordinate root hair growth via BOUNDARY OF ROP DOMAIN proteins

In vascular plants, root hairs (RHs) increase plant adaptability by facilitating the uptake of water and nutrients. RH initiation and elongation require the establishment and maintenance of cellular polarity. The ROOT HAIR DEFECTIVE 6-LIKE (RSL) transcription factor RSL4 controls RH-specific gene expression and ROP GTPases (Rho-related GTPases from Plants) direct polar growth, but how these pathways integrate to spatiotemporally orchestrate RH initiation and elongation remains unknown. We identify Arabidopsis BOUNDARY OF ROP DOMAIN (BDR) protein BDR6/7 as positive regulators of RH initiation and growth. BDR6/7 are targeted both to the plasma membrane (PM) and in the nucleus of RHs. By genetic approaches, we demonstrate that ROP signaling is essential for BDR6/7-promoted RH growth whereas ROP2-driven RH growth also requires BDR6/7. The BDR6/7 proteins associate and co-localize with RSL2/4 to nuclei in heterologous plant cells. Genetic dissections uncover an interdependency between BDR6/7 and RSL2/4. Indeed, BDR6/7 enhances RSL4-associated RHE reporter activity in protoplasts and supports RSL4 binding at target promoters whereas RSL4 binds BDR6/7 promoters and is required for their expression, creating a positive feedback loop that may integrate transcriptional and signaling dynamics. Genetic evidence also supports the dependence of ROP signaling on RSL2/4. The dual-targeted BDR6/7, RSL2/4-mediated transcription, and apical PM-associated ROP signaling may form a tripartite module, scaling developmental outputs for spatiotemporal coordination of plant cell morphogenesis.