A carbohydrate-binding protein, B-GRANULE CONTENT 1, influences starch granule size distribution in a dose-dependent manner in polyploid wheat

Abstract

In Triticeae endosperm (e.g. wheat and barley), starch granules have a bimodal size distribution (with A- and B-type granules) whereas in other grasses the endosperm contains starch granules with a unimodal size distribution. Here, we identify the gene, BGC1 (B-GRANULE CONTENT 1), responsible for B-type starch granule content in Aegilops and wheat. Orthologues of this gene are known to influence starch synthesis in diploids such as rice, Arabidopsis, and barley. However, using polyploid Triticeae species, we uncovered a more complex biological role for BGC1 in starch granule initiation: BGC1 represses the initiation of A-granules in early grain development but promotes the initiation of B-granules in mid grain development. We provide evidence that the influence of BGC1 on starch synthesis is dose dependent and show that three very different starch phenotypes are conditioned by the gene dose of BGC1 in polyploid wheat: normal bimodal starch granule morphology; A-granules with few or no B-granules; or polymorphous starch with few normal A- or B-granules. We conclude from this work that BGC1 participates in controlling B-type starch granule initiation in Triticeae endosperm and that its precise effect on granule size and number varies with gene dose and stage of development.

STARCH SYNTHASE 4 is required for normal starch granule initiation in amyloplasts of wheat endosperm

“…The mutants had highly abnormal endosperm starch morphology, resulting from the formation of compound starch granules. Interestingly, this phenotype resembled mutants defective in Ta BGC1 (Chia et al ., 2020). Our work demonstrates that both Ta SS4 and Ta BGC1 are required for the control of granule initiation in endosperm amyloplasts.…”

“…The elimination of another component of the putative granule initiation complex defined in Arabidopsis, PTST2/FLO6/BGC1, results in strong granule morphology defects in endosperms of barley and hexaploid wheat, including the occurrence of “semi-compound” granules (Suh et al ., 2004; Chia et al ., 2020). To discover the relationship between the roles of Ta SS4 and Ta BGC1 in wheat endosperm, we compared phenotypes of Tass4 and Tabgc1 mutants in the same Kronos background, and tested whether Ta SS4 and Ta BGC1 proteins can interact with each other.…”

“…Granule initiation patterns in the endosperm of the Triticeae are radically different from those in Arabidopsis leaves: large, flattened A-type granules initiate early during grain development, and small round B-type granules initiate 10-15 days after the A-type granules (Bechtel et al ., 1990; Howard et al ., 2011; Chia et al ., 2020). Nonetheless, the loss of PTST2 orthologs – FLOURY ENDOSPERM 6 (FLO6) in barley and B-GRANULE CONTENT 1 (BGC1) in wheat - has major effects on granule initiation in the endosperm (Suh et al ., 2004; Saito et al ., 2017; Chia et al ., 2020). This discovery raises the possibility that granule initiation in wheat endosperm is via an SS4-containing complex, similar to that in Arabidopsis leaves.…”

Figure 1: Manhattan plots for “Preparative tuber starch yield” trait; (A) GLM; (B) GLM+PCA; (C) GLM+Q.

“…Recently, several genome association studies with starch granules size distribution were published for maize (Liu et al, 2018), wheat (Li et al, 2017;Chia et al, 2020) and Japonica rice (Biselli et al, 2019). In the present study, no noticeable SNPs with p-values higher than Bonferroni or FDR levels were found for average values for the "Area", "Feret's diameter", "Minimal Feret's diameter" and "Solidity" traits.…”

Peer Review #2 of "Genetic loci determining potato starch yield and granule morphology revealed by genome-wide association study (GWAS) (v0.1)"

“…In total, 53 significant SNPs were associated with morphological and starch yield traits ( Manuscript to be reviewed Recently, several genome association studies with starch granules size distribution were published for maize [21], wheat [22,23] and Japonica rice [24]. In the present study, no noticeable SNPs with p-values higher than Bonferroni or FDR levels were found for average values for the "Area", "Feret's diameter", "Minimal Feret's diameter" and "Solidity" traits.…”