Transgenic sorghum with improved digestibility of storage proteins obtained by Agrobacterium-mediated transformation

Development of transgenic plants with modified seed storage protein composition and increased nutritive value is one of the most promising areas of genetic engineering. This task is especially important for sorghum—a unique drought tolerant cereal crop that is characterized, however, by a relatively poor nutritive value in comparison with other cereals. It is considered that one of the reasons of the low nutritive value of the sorghum grain is the resistance of one of its seed storage proteins, γ-kafirin, located in the outer layer of endosperm protein bodies, to protease digestion. Using Agrobacterium-mediated genetic transformation, we obtained transgenic sorghum plants (Sorghum bicolor (L.) Moench) harboring a genetic construct for RNAi silencing of the γ-kafirin gene. In the T₁ generation, the plants with almost floury or modified endosperm texture of kernels were found. In these kernels, the vitreous endosperm layer has been reduced and/or covered by a thin layer of floury endosperm. In vitro protein digestibility (IVPD) analysis showed that the amount of undigested protein in transgenic plants from the T₃ generation was reduced by 2.9–3.2 times, in comparison with the original non-transgenic line, and the digestibility index reached 85–88% (in comparison with 59% in the original line). In T₂ families, the plants combining high IVPD with vitreous endosperm type were found. In the electrophoretic spectra of endosperm proteins of transgenic plants with increased digestibility, the proportion of 20 kD protein that is encoded by the γ-kafirin gene, was significantly reduced, in comparison with the original non-transgenic line. HPLC analysis showed total amino acid content in two out of the three studied transgenic plants from the T₂ generation was reduced in comparison with the original non-transgenic line, while the lysine proportion increased by 1.6–1.7 times. The mechanisms conditioning improved digestibility of storage proteins in transgenic plants are discussed. The results of experiments demonstrate that it is feasible to develop sorghum lines combining high protein digestibility and vitreous endosperm that has a high breeding value.