Abstract
Six subspecies of hexaploid wheat (Triticum aestivum) have been identified, but the origin of Indian dwarf wheat (Triticum sphaerococcum), the only subspecies with round grains, is currently unknown. Here, we isolated the grain-shape gene Tasg-D1 in T sphaerococcum via positional cloning. Tasg-D1 encodes a Ser/Thr protein kinase glycogen synthase kinase3 (STKc_GSK3) that negatively regulates brassinosteroid signaling. Expression of TaSG-D1 and the mutant form Tasg-D1 in Arabidopsis (Arabidopsis thaliana) suggested that a single amino acid substitution in the Thr-283-Arg-284-Glu-285-Glu-286 domain of TaSG-D1 enhances protein stability in response to brassinosteroids, likely leading to formation of round grains in wheat. This gain-of-function mutation has pleiotropic effects on plant architecture and exhibits incomplete dominance. Haplotype analysis of 898 wheat accessions indicated that the origin of T sphaerococcum in ancient India involved at least two independent mutations of TaSG-D1 Our results demonstrate that modest genetic changes in a single gene can induce dramatic phenotypic changes.
Highlights
Hexaploid wheat (Triticum aestivum) is a typical allopolyploid species containing three distinct genomes (A, B and D; AABBDD) resulting from two sequential allopolyploidization events
The corresponding segregation ratio fits a Mendelian model of 3:1 (208 long grains/62 semispherical grains; x2 < x20.05, 1 5 3. 84), indicating that the semispherical grain trait is controlled by a single nuclear gene, which is consistent with previous findings (Sears, 1947; Salina et al, 2000)
The gene that determines semispherical grain was mapped between markers Xgwm341 and Xgdm72 on the short arm of chromosome 3D near the centromeric region (Figure 1C)
Summary
Hexaploid wheat (Triticum aestivum) is a typical allopolyploid species containing three distinct genomes (A, B and D; AABBDD) resulting from two sequential allopolyploidization events. To perform fine mapping of Tasg-D1, we generated an F7 recombinant inbred line (RIL) population of 247 lines derived from a cross between HS2 and ND4332 and confirmed that the locus is located between markers 3DS-68 and 3DS-44 (Figure 1D; Supplemental Data Set 2).
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