CRISPR-knockout of β-kafirin in sorghum does not recapitulate the grain quality of natural mutants

When gene editing was applied to knockout beta-kafirin, there was a compensatory increase of gamma-kafirin which does not occur in domesticated null varieties, so enhanced grain quality was not achieved. Sorghum bicolor is an important animal feedstock cereal crop throughout Australia and the southern United States, where its use as a food product is limited by issues with low calorific and nutritive value. Qualities such as reduced digestibility and low essential amino acid content are directly attributed to the kafirin grain storage proteins, the major components of protein bodies within the endosperm. Specifically, the β- and γ-kafirins have few protease cleavage sites and high levels of cysteine residues which lead to a highly cross-linked shell of intra- and inter-molecular disulphide linkages that encapsulate the more digestible α- and δ-kafirins in the core of the protein bodies. Naturally occurring β-kafirin mutants exist and are known to have improved grain quality, with enhanced protein contents and digestibility, traits which are often attributed to the lack of this cysteine-rich kafirin in the mature grain. However, when CRISPR/Cas9 editing was used to create β-kafirin knockout lines, there was no improvement to grain quality in the Tx430 background, although they did have unique protein composition and changes to protein body morphology in the vitreous endosperm. One explanation of the divergence in quality traits found the lines lacking β-kafirin are due to a drastic increase of γ-kafirin which was only found in the gene edited lines. This study highlights that in some germplasm, there is a level of redundancy between the peripheral kafirins, and that improvement of grain protein digestibility cannot be achieved by simply removing the β-kafirin protein in all genetic backgrounds.

Effect of wet and dry milling on the functional properties of whole sorghum grain flour and kafirin

The effect of wet and dry milling on the functional properties of whole sorghum grain flour (SF) and extracted kafirin were assessed. White sorghum landrace was used to prepare two SFs by wet milling (SF1) or dry milling (SF2) and to extract their respective kafirins SK1 and SK2. Protein contents of SK1 and SK2 were 90.07 and 94.23%. Wet milling of SF allowed increasing the oil binding capacity, the least gelling concentration and the instant foam capacity and decreasing the water binding capacity and foam stability. The emulsifying activity index of SF1 and SF2 were in the same range, while emulsion stability was two time higher in SF2. Functional properties of SK1 and SK2 showed an appropriate water binding capacity of 2.20 ± 0.10 and 1.82 ± 0.22 (g water/g dry mater), respectively. Both SK1 and SK2 showed higher oil binding capacity than SF1 and SF2 with no gel and foam formation. The wet milling improved water and oil binding capacities of SK1 by 17 and 5%. The emulsifying activity indexes were approximately similar for SK1 and SK2 with emulsion stability exceeding 60%. Fourier transform infrared spectroscopy of SK1 and SK2 showed that wet milling induced a decrease of α-helical structure and an increase of intermolecular β-sheet and β-turns and no change in the anti parallel β-sheet. This study showed that wet milling could allow extracting kafirin with preserving the most important functional properties of SF and kafirin and could constitute an interesting method for protein recovery and starch isolation.

Physicochemical properties of kafirin protein and its applications as building blocks of functional delivery systems

The unique physicochemical properties of kafirin highlight its potential as an attractive resource for gluten-free products and building blocks for functional delivery systems.

In vitro starch digestion in sorghum flour from Algerian cultivars

This work aims to evaluate starch digestion in whole sorghum grains. Nine sorghum cultivars were sampled from the Sahara of Algeria. The structural characteristics of sorghum grains were measured. Total starch (TS) varied between 67.67% and 74.82%, digestible starch (DS) between 64.34% and 69.70%, and resistant starch (RS) ranged from 2.55% to 7.98%. The kinetic of starch digestion displayed first-order model. For all sorghum cultivars, starch were digested with different extents, DS at infinite time (D ∞) ranged from 52.58 to 102.13 g/100 g dry starch, while the hydrolysis index (HI) ranged from 41.55% to 76.93% and high average glycemic index (GIavg) ranged from 65.97 to 94.14. The results showed that there are differences in grain quality of Algerian sorghum cultivars. The starch fractions have acceptable nutritional value with good in vitro digestibility characteristics suitable for human health and nutrition.

Allelic variation of the β-, γ- and δ-kafirin genes in diverse Sorghum genotypes

The β-, γ- and δ-kafirin genes were sequenced from 35 Sorghum genotypes to investigate the allelic diversity of seed storage proteins. A range of grain sorghums, including inbred parents from internationally diverse breeding programs and landraces, and three wild Sorghum relatives were selected to encompass an extensive array of improved and unimproved germplasm in the Eusorghum. A single locus exists for each of the expressed kafirin-encoding genes, unlike the multigenic α-kafirins. Significant diversity was found for each locus, with the cysteine-rich β-kafirin having four alleles, including the first natural null mutant reported for this prolamin subfamily. This allele contains a frame shift insertion at +206 resulting in a premature stop codon. SDS-PAGE revealed that lines with this allele do not produce β-kafirin. An analysis of flour viscosity reveals that these β-kafirin null lines have a difference in grain quality, with significantly lower viscosity observed over the entire Rapid ViscoAnalyser time course. There was less diversity at the protein level within the cysteine-rich γ-kafirin, with only two alleles in the cultivated sorghums. There were only two alleles for the δ-kafirin locus among the S. bicolor germplasm, with one allele encoding ten extra amino acids, of which five were methionine residues, with an additional methionine resulting from a nucleotide substitution. This longer allele encodes a protein with 19.1% methionine. The Asian species, S. propinquum, had distinct alleles for all three kafirin genes. We found no evidence for selection on the three kafirin genes during sorghum domestication even though the δ-kafirin locus displayed comparatively low genetic variation. This study has identified genetic diversity in all single copy seed storage protein genes, including a null mutant for β-kafirin in Sorghum.