Genome-Wide Identification and Characterization of the Dehydrin Gene Family in Sesame (Sesamum indicum): Structural Divergence and Differential Expression Under Drought Stress

Background/Objectives: Dehydrins (DHNs) are late embryogenesis abundant proteins that play protective roles under water-deficit conditions; however, their organization and function remain unexplored in sesame (Sesamum indicum), an important oilseed crop frequently cultivated in arid and semi-arid regions. This study aimed to identify and characterize the DHN gene family in sesame and evaluate the expression of its members under drought stress. Methods: Genome-wide identification was performed using the Dehydrin domain HMM profile, followed by phylogenetic analysis, conserved motif and gene structure characterization, synteny analysis, promoter cis-element profiling, and secondary/tertiary structure prediction. Transcriptional responses were profiled by qPCR in two sesame cultivars (drought-sensitive and drought-tolerant) under PEG-induced osmotic stress at germination and seedling stages. Results: Four DHN genes were identified, spanning three phylogenetic subfamilies (I–III) and three DHN subclasses: SKn (SiDHN1/2), YnKn (SiDHN3), and YnSKn (SiDHN4). SiDHN1 and SiDHN2 likely arose from a segmental duplication, and a single conserved syntenic pair was found between SiDHN4 and olive (Olea europaea). Secondary structure predictions uncovered contrasting structural propensities: SiDHN1/2 are predicted to be α-helix-rich, partially ordered proteins (41–44% predicted α-helix), whereas SiDHN3/4 are predicted to be predominantly intrinsically disordered (~75–80% random coil). SiDHN3 was consistently upregulated across all conditions (1.84–9.12-fold), while SiDHN4 exhibited strong genotype-specific induction of 9.39-fold exclusively in the drought-tolerant cultivar during germination. Conclusions: The sesame DHN family achieves functional breadth through structural diversification—an ordered–disordered continuum mirrored by divergent expression programming—rather than through numerical expansion. SiDHN3 and SiDHN4 are identified as primary candidates for drought tolerance improvement in sesame.