Transfer of a starch phenotype from wild wheat to bread wheat by deletion of a locus controlling B-type starch granule content

Abstract

Our previous genetic analysis of a tetraploid wild wheat species, Aegilops peregrina, predicted that a single gene per haploid genome, Bgc-1, controls B-type starch granule content in the grain. To test whether bread wheat (Triticum aestivum L.) has orthologous Bgc-1 loci, we screened a population of γ-irradiated bread wheat cv. Paragon for deletions of the group 4 chromosomes spanning Bgc-1. Suitable deletions, each encompassing ~600-700 genes, were discovered for chromosomes 4A and 4D. These two deletions are predicted to have 240 homoeologous genes in common. In contrast to single deletion mutant plants, double deletion mutants were found to lack B-type starch granules. The B-less grains had normal A-type starch granule morphology, normal overall starch content, and normal grain weight. In addition to variation in starch granule size distribution, the B-less wheat grains differed from controls in grain hardness, starch swelling power, and amylose content. We believe that these B-less wheat plants are the only Triticeae cereals available that combine substantial alterations in starch granule size distribution with minimal impact on starch content.

Keywords: B-type starch granules; Breeding; Triticeae; deletion mutant; grain hardness; granule size distribution; starch granule initiation; starch swelling power; wheat grain.

Fig. 1.

The size and position of deletions in two Paragon mutant lines. The two Paragon deletion mutant lines, A1 (A) and D4 (B), which were crossed to generate the B-granule-less mutant were exome sequenced. The graphs show the number of counts (normalized against Paragon wild type) on the y-axis against the chromosomal position (IWGSC RefSeq v1.0) of the read (in million base pairs) on the x-axis. The dotted line represents the 0.1 normalized coverage cut-off. For each panel, the rows show chromosomes 1–7 (top to bottom, respectively) and the A-genome data are shown on the left, the B-genome in the middle, and the D-genome on the right. Deletions are indicated by regions of reduced counts. Deletions in line A1 are on Chr 1B, 4A, 5D, and 6A and in line D4 on Chr 1A, 1B, 4B, 4D, 6D, and 7A. (This figure is available in colour at JXB online.)

Fig. 2.

Identification of genes flanking the Chr 4AL and 4DS deletions. The read coverage at the borders of the 4AL and 4DS deletions in lines A1 (A) and D4 (B), respectively, was examined to determine the precise position at which the coverage in the mutant line drops off. The genes closest to the borders of each deletion were identified manually using the TGAC_v1 gene models. In each panel, the graph at the top shows the exome sequencing data for the whole chromosome. The bottom panel in (A) shows the normalized counts for the two genes surrounding the left border and for the two genes surrounding the right border (on the left and right hand sides, respectively). In (B), three gene models distributed along the deletion are shown as well as the closest gene model outside of the deletion. (This figure is available in colour at JXB online.)

Fig. 3.

Microscopy of mature endosperm and purified starch granules. (A–D) Mature grains observed by SEM. (A) Wild type: Paragon. (B) A1 single deletion mutant. (C) D4 single deletion mutant. (D) AD double deletion mutant. (E–G) Purified starch observed by light microscopy. (E) Paragon. (F) Control (wild-type segregant). (G) AD double deletion mutant.