Hybrid rice breeding relies on cytoplasmic male sterility (CMS), but the metabolic basis of sterility is still poorly understood. From 2014 to 2024, a new CMS line (HURS1-A) was developed and evaluated for sterility, floral traits, and agronomic performance. Despite its importance in breeding, the metabolic basis underlying CMS expression remains unclear. Therefore, in 2024, metabolomic profiling of HURS1-A and its maintainer (HURS1-B) was conducted at defined panicle stages under controlled conditions. The newly developed CMS line HURS1-A showed complete pollen and spikelet sterility across multi-season evaluations (2021–2024), whereas its isogenic maintainer HURS1-B remained fully fertile. HURS1-A exhibited superior floral traits essential for hybrid seed production, including 43.5% stigma exsertion, longer stigma and style length and 95.5% panicle exertion, confirming its strong natural outcrossing efficiency. Ultra-High-Performance Liquid Chromatography-High-Resolution Mass Spectrometry (UHPLC-HRMS) metabolomic profiling at pre-meiosis (PRM) and post-meiosis (POM) stages revealed substantial metabolic difference between the sterile and fertile lines, with 252 and 287 differentially accumulated metabolites (DAMs), respectively. Multivariate analyses, principal component analysis, Partial Least Squares-Discriminant Analysis (PLS-DA), and Orthogonal Partial Least Squares–Discriminant Analysis (OPLS-DA) clearly separated the two lines, and specific metabolites associated with the PRM stage (Ubiquinol-10, Ethyl aconitate, Arginyl lysine) and the POM stage (5-O-caffeoylshikimic acid, hypoxanthine, and N-oleoyl glutamine) have been emerged as accurate stage-specific biomarkers (Variable Importance in Projection (VIP) > 1.0; Receiver Operating Characteristic (ROC) and Area Under the Curve (AUC) = 1.0). Pathway enrichment showed major disruptions in Tricarboxylic Acid cycle (TCA) cycle activity, redox regulation, amino-acid and nitrogen metabolism, and phenylpropanoid and flavonoid pathways, all central to tapetal function, pollen wall formation, and microspore development. This study highlighted HURS1-A as a breakthrough novel CMS line, not only for its strong outcrossing traits (High stigma exsertion) but also for offering stage-specific novel metabolite-based markers such as Ubiquinol-10 and key pathways like phenylpropanoid biosynthesis, which simplify sterile line identification. This makes HURS1A a milestone as a novel CMS for hybrid rice breeding.
Singh et al. (Thu,) studied this question.