Sorghum grain biofortification by SbSTP20-mediated hexose influx and sub-aleurone thickening

Biofortification offers a sustainable solution to "hidden hunger, " yet the trade-off between grain yield and nutritional quality remains a persistent breeding bottleneck. Modern crop breeding emphasises the balance between high yield and nutritional values. Use of novel genes/alleles for grain quality lost during domestication and genetic improvement has been proven an efficient way to help achieve the dual goals. In sorghum, we report here the discovery of SbSTP20, encoding a sorghum monosaccharide transporter, as an essential component for biofortification by mediating the endosperm hexose influx and sub-aleurone (SA) thickness. SbSTP20 was ectopically expressed in sorghum developing grains and its expression levels were negatively associated with SA thickness. CRISPR edited Sbstp20 lines decreased the influx of hexoses, and exhibited thicker sub-aleurone layers and 6.6-7.5% higher grain protein and enriched micronutrients (Fe, Zn). SbSTP20thick natural variant increased sub-aleurone layer thickness and improved grain nutritional quality without altering grain size or thousand-grain weight. The SbSTP20thick allele was absent in 93% of modern improved lines, implying that the genomic region containing SbSTP20 showed a moderate selection signature. The metabolizable energy of SbSTP20thick containing sorghum lines was up to 6% higher than that of commercial corn standards in swine digestion and metabolism experiments. Reintroducing this haplotype via marker-assisted selection could provide a molecular strategy for nutritional enhancement.