A chromosome-scale reference genome and single-nucleus root atlas reveal the cellular and molecular basis of coastal adaptation in sand bean (Strophostyles helvola)

Plants adapted to saline coastal habitats provide valuable systems for understanding how natural selection reshapes stress-response programs, yet the cellular and molecular basis of this adaptation remains poorly resolved. Here, we generated a chromosome-scale reference genome and a single-nucleus transcriptomic atlas of roots from sand bean (Strophostyles helvola), a wild legume represented by salt-tolerant Beach and salt-sensitive Inland ecotypes. The atlas comprised 87,901 nuclei assigned to 25 transcriptional clusters representing 16 major root cell types. Salt exposure elicited cell-type-specific transcriptional responses that differed between Beach and Inland roots. Beach roots preferentially maintained respiration-, energy metabolism-, and protein-homeostasis-associated functions, whereas Inland roots showed stronger induction of canonical abiotic-stress, ABA, water-deficit, hypoxia, and oxidative-stress programs. Integrating baseline ecotype differences with salt-responsive expression revealed that many genes induced by salt in Inland roots were already expressed at higher levels in untreated Beach roots. This baseline-enriched configuration comprised a broadly distributed regulatory backbone, including ERF-family transcription factors and RING-type E3 ubiquitin ligases, together with cell-type-associated modules involving ion and water transport at the root-soil interface, redox and dehydration protection in outer-root tissues, endodermal barrier-associated genes, and vascular regulatory candidates. These findings support a model in which Beach salt tolerance is associated not with entirely distinct stress-response pathways, but with the pre-existing and spatially organized deployment of conserved protective programs that remain responsive to salt exposure. Our study establishes genomic and cellular resources for sand beans and provides a framework for investigating how natural variation in the regulation and cellular organization of conserved pathways contributes to environmental stress tolerance.