Novel Hina alleles created by genome editing increase grain hardness and reduce grain width in barley

The hordoindolina genes (Hina and Hinb) are believed to play critical roles in barley (Hordeum vulgare L.) grain texture. In this study, we created novel alleles of the Hina gene using CRISPR/Cas9 (Clustered regularly inter spaced short palindromic repeat-associated protein, CRISPR-Cas) genome editing. Mutagenesis of single bases in these novel alleles led to loss of Hina protein function in edited lines. The grain hardness index of hina mutants was 95.5 on average, while that of the wild type was only 53.7, indicating successful conversion of soft barley into hard barley. Observation of cross-sectional grain structure using scanning electron microscopy revealed different adhesion levels between starch granules and protein matrix. Starch granules were loose and separated from the protein matrix in the wild type, but deeply trapped and tightly integrated with the protein matrix in hina02 mutants. In addition, the grain width and thousand-grain weight of the hina02 mutant were significantly lower than those of the wild type.

Biochemical and molecular characterization of Avena indolines and their role in kernel texture

Among cereals, Avena sativa is characterized by an extremely soft endosperm texture, which leads to some negative agronomic and technological traits. On the basis of the well-known softening effect of puroindolines in wheat kernel texture, in this study, indolines and their encoding genes are investigated in Avena species at different ploidy levels. Three novel 14 kDa proteins, showing a central hydrophobic domain with four tryptophan residues and here named vromindoline (VIN)-1,2 and 3, were identified. Each VIN protein in diploid oat species was found to be synthesized by a single Vin gene whereas, in hexaploid A. sativa, three Vin-1, three Vin-2 and two Vin-3 genes coding for VIN-1, VIN-2 and VIN-3, respectively, were described and assigned to the A, C or D genomes based on similarity to their counterparts in diploid species. Expression of oat vromindoline transgenes in the extra-hard durum wheat led to accumulation of vromindolines in the endosperm and caused an approximate 50 % reduction of grain hardness, suggesting a central role for vromindolines in causing the extra-soft texture of oat grain. Further, hexaploid oats showed three orthologous genes coding for avenoindolines A and B, with five or three tryptophan residues, respectively, but very low amounts of avenoindolines were found in mature kernels. The present results identify a novel protein family affecting cereal kernel texture and would further elucidate the phylogenetic evolution of Avena genus.

A barley Hordoindoline mutation resulted in an increase in grain hardness

Barley seed proteins, Hordoindolines, are homologues of wheat Puroindolines, which are associated with grain hardness. Barley Hordoindoline genes are known to comprise Hina and Hinb, and Hinb consists of two Hinb genes, Hinb-1 and Hinb-2. Two types of allele were found for Hina, Hinb-1 and Hinb-2 genes, respectively, among Japanese two- and six-rowed barley lines. One of the alleles of Hinb-2 (Hinb-2b) had a frame-shift mutation resulting in an in-frame stop codon. For two-rowed barley lines, grain hardness was significantly higher among lines with the Hinb-2b than those with the wild type Hinb-2 gene (Hinb-2a). Protein spots corresponding to HINa, HINb-1, and HINb-2 were identified by 2D-gel electrophoresis among barley lines with Hinb-2a. Among the lines with Hinb-2b, HINa and HINb-1 were expressed at similar levels as those in the wild type, but HINb-2 was not detected. A DNA (cleaved amplified polymorphic sequence) marker was developed to distinguish between the Hinb-2a and Hinb-2b gene sequences. Analysis of grain hardness among F(2) lines derived from a cross between a line with Hinb-2a (Shikoku hadaka 115) and a line with the Hinb-2b (Shikoku hadaka 84) showed significantly higher grain hardness in the mutant lines. From these results, the Hinb-2b frame-shift (null) mutation might play a critical role in barley grain hardness. The DNA marker will be useful in barley breeding to select lines having harder grain texture.