Soil-grain selenium dynamics and marker-assisted breeding for high-selenium rice in selenium-rich soils of Guangxi, China

Introduction Developing selenium-biofortified rice represents a sustainable public health strategy to address the widespread deficiency in dietary selenium (Se) intake. Methods This study investigated total Se and bioavailable Se concentrations across five regions in Guangxi, China, and evaluated grain Se content and speciation in 80 rice cultivars. Furthermore, we assessed the effects of foliar selenium application on grain selenium accumulation and speciation. Subsequently, association mapping using 48 SSR markers was performed to identify loci associated with grain selenium content, and marker-assisted selection (MAS) was used to develop new high-Se rice lines. Results Across the five sites, soil total Se (0.249–0.563 mg/kg) correlated highly positively (r = 0.9445, P <0.01) with soil bioavailable Se (0.029–0.062 mg/kg), and grain Se concentrations (0.027–0.148 mg/kg) correlated positively with soil total Se (r = 0.503) and soil bioavailable Se (r = 0.505). Within each site, the grain Se varied dramatically among cultivars. Foliar application of exogenous Se raised grain Se 7.55-fold on average but caused 76.25% of cultivars to exceed the national safety ceiling (0.30 mg/kg); it also shifted the SeMet/SeCys 2 balance and elevated the concentrations of Se(IV). Preliminary association mapping using 48 simple sequence repeats (SSR) markers across the 80 cultivars identified five exploratory candidate loci linked to grain Se levels (P < 0.01). Markers RM71 (Chr. 2) and RM542 (Chr. 7) were detected across multiple environments with high phenotypic variance explained (PVE 8.56%–12.50%). Using marker-assisted selection (MAS) and genotypic screening based on molecular markers linked to these two exploratory candidate loci, 34 new high-Se varieties/lines were developed and identified, including the officially released cultivar “Guifengjinxiang”. Among them, 85.29%–97.06% of the varieties/lines met the national Se-enriched standard across five experimental sites with different soil Se contents. Discussion These findings establish a theoretical and technical framework for the sustainable exploitation of Guangxi’s natural Se resources and the molecular breeding of Se-biofortified rice.