Cracking the Hard Seed: Molecular Mechanisms and Multi-Omics Insights into Seed Dormancy and Germination in the Genus Astragalus

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The genus Astragalus comprises one of the largest and most ecologically diverse groups of flowering plants and includes species of major medicinal, forage, and restoration value, particularly Astragalus membranaceus and allied taxa. However, effective germplasm utilization and large-scale cultivation remain constrained by strong seed dormancy, most commonly expressed as physical dormancy imposed by a water-impermeable seed coat. In some species, this coat-imposed barrier is further complicated by an additional physiological component, resulting in combinational dormancy. This review synthesizes current knowledge on the structural, molecular, and multi-omics basis of seed dormancy and germination in Astragalus, with emphasis on the dormancy-to-germination transition. We first examine the anatomical basis of hardseededness, including the palisade layer, light line, and associated seed-coat barrier domains, and discuss the likely contributions of phenylpropanoid, lignin, suberin, and cutin biosynthesis to the establishment of physical dormancy. We then summarize current understanding of hormone crosstalk, highlighting the central roles of abscisic acid and gibberellins together with ethylene, brassinosteroids, reactive oxygen species, and nitric oxide in regulating post-dormancy germination. Recent transcriptomic, metabolomic, and emerging integrative omics studies reveal that Astragalus germination involves not only reserve mobilization and energy activation but also early induction of characteristic secondary-metabolite pathways, particularly flavonoids and isoflavonoids, which distinguishes medicinal Astragalus seeds from many conventional crop seeds. Finally, we discuss current bottlenecks, including the limited availability of robust functional-genomics tools, and outline future directions involving CRISPR-based validation, epigenetic regulation, and marker-assisted breeding for improved germination uniformity and reduced hardseededness. This review provides a mechanistic framework for unlocking Astragalus seed dormancy and accelerating the conservation, breeding, and medicinal utilization of this important genus.