Towards the Systematic Mutagenesis of Sweet Sorghum Genome by Gamma Ray: Screening and Analysis of Dwarf and Slender Mutants

Sweet sorghum accumulates high sugar content in the stalk, making it one of the most viable energy crop candidates. However, many varieties of sweet sorghum are usually tall, thereby easily lodged. Because the control of internode elongation is well studied to improve lodging resistance in rice and wheat, we carried out gamma ray mutagenesis of sweet sorghum (cv. SIL-05) and screened ~5,000 mutant lines at the M₂ and M₃ generation for (i) severe mutants (e.g. dwarf and slender; to study the mechanism of internode elongation) and (ii) mild mutants (e.g. semi-dwarf; for use in sweet sorghum breeding). As a result, we isolated 211 mutants, of which, 81 were severe and 130 were mild. Among them, we identified mutants that show striking phenotypic similarities with known rice mutants. Genetic analyses on the corresponding genes of these mutants confirmed that sorghum mutants with defective gibberellin (GA) biosynthetic genes generally exhibit severe dwarfism and dark green leaves, very similar to rice GA mutants. Genetic and phenotypic similarity between sorghum and rice is also observed in the slender mutants. However, phenotypic differences still exist. The most interesting point is that the GA-deficient mutants of sorghum, in contrast with those of rice, show an obvious abnormal stem bending at the onset of the vegetative stage. We also selected 3 mutants with semi-dwarfism, which are used as prototypes for the lodging resistance breeding of sweet sorghum. In summary, systematic mutagenesis is an effective approach for both physiological study and breeding of sweet sorghum.