Mutations in the dwarf3 gene confer height stability in sorghum

Dwarfism is a useful trait in many crop plants because it contributes to improved lodging resistance and harvest index. The mutant allele dw₃-ref (dwarf₃-reference) of sorghum [Sorghum bicolor (L.) Moench] is characterized by an 882 bp tandem duplication in the fifth exon of the gene that is unstable and reverts to wild-type at a frequency greater than 0.001 in many genetic backgrounds. The goal of this research was to identify stable alleles of dw₃ (dwarf3) that could be backcrossed into elite parent lines to improve height stability of the crop. To discover new alleles of dw₃, a panel consisting mostly of sorghum conversion lines (SC-lines) was screened by polymerase chain reaction for the 882 bp tandem duplication in the fifth exon of dw₃-ref. Sanger sequencing was used to characterize the DNA sequence of this fragment in genotypes that did not contain the 882 bp tandem duplication. Sequence analysis identified three indel mutations, including an 82 bp deletion, a 6 bp duplication, and a 15 bp deletion in this region of the gene. Field trials of the donor genotypes with these new alleles indicated no wild-type revertants of dw₃-sd3 (dwarf₃-stable dwarf), dw₃-sd4, and dw₃-sd5. These alleles were backcrossed into Tx430. Field trials of backcross progeny (BC₂F₄) with the dw₃-sd3, dw₃-sd4, and dw₃-sd5 alleles indicated no revertants. The plant height and flowering time characteristics of the backcross progeny were similar or slightly shorter and earlier than the recurrent parent. These findings demonstrate that dw₃-sd3, dw₃-sd4, and dw₃-sd5 alleles will be useful in breeding for the stable dwarf trait.