Improvement of grain sorghum nutritive properties using modern genetic and biotechnological methods

The paper presents a review of the studies on the use of genetic engineering and genome editing tools for improving nutritional properties of sorghum grain. As a result of experiments performed over the past 5-7 years by several research groups, the created transgenic lines carry genetic constructs for RNA silencing of different kafirin sub-classes (prolamins of sorghum grain). The CRISPR/Cas9 genome editing experiments have yielded mutants with deletions and insertions in the signal sequence of the gene encoding the 22 kDa α-kafirin in sorghum. These lines and mutants were characterized by improved in vitro digestibility of grain proteins, altered ultrastructure of protein bodies and an increased content of lysine. RNA silencing of α-kafirin increased the digestibility of proteins of both raw and cooked flour, while silencing of γ-kafirin led to improved digestibility of proteins of only raw flour. The lines with α-kafirin silencing have kernels with the floury endosperm type that discourages their direct commercial use because of fragility and reduced tolerance to fungal contamination; however, these lines can be used as donors of high digestibility trait when crossed with sorghum lines adapted to local conditions to improve their nutritional value. Kernels of the lines with γ-kafirin silencing may have different endosperm types: floury, vitreous, or a modified type with vitreous endosperm interspersed in the floury endosperm. This fact indicates the possibility of producing agronomically important sorghum lines with high kafirin digestibility and hard endosperm. The increased lysine level in kernels of sorghum lines with the suppressed synthesis of kafirins may be caused by rebalancing of protein synthesis in endosperm of developing kernels due to the synthesis of other proteins, including those with a higher content of essential amino acids. Alongside with improving the digestibility of kafirins, the genetic engineering approach allowed the creation of sorghum lines with a high content of provitamin A in grain and its increased stability during long-term storage. The results of these works show that it is promising to use RNA-interference and genome editing for creating sorghum lines with improved nutritional value of grain.