Influence of single-nucleotide polymorphisms in the gene encoding granule-bound starch synthase I on amylose content in Vietnamese rice cultivars

Granule-bound starch synthase in plants: Towards an understanding of their evolution, regulatory mechanisms, applications, and perspectives

Amylose content (AC) is a significant quality trait in starchy crops, affecting their processing and application by the food and non-food industries. Therefore, fine-tuning AC in these crops has become a focus for breeders. Granule-bound starch synthase (GBSS) is the core enzyme that directly determines the AC levels. Several excellent reviews have summarized key progress in various aspects of GBSS research in recent years, but they mostly focus on cereals. Herein, we provide an in-depth review of GBSS research in monocots and dicots, focusing on the molecular characteristics, evolutionary relationships, expression patterns, molecular regulation mechanisms, and applications. We also discuss future challenges and directions for controlling AC in starchy crops, and found simultaneously increasing both the PTST and GBSS gene expression levels may be an effective strategy to increase amylose content.

The kinase OsSK41/OsGSK5 negatively regulates amylose content in rice endosperm by affecting the interaction between OsEBP89 and OsBP5

Amylose content (AC) is the main factor determining the palatability, viscosity, transparency, and digestibility of rice (Oryza sativa) grains. AC in rice grains is mainly controlled by different alleles of the Waxy (Wx) gene. The AP2/EREBP transcription factor OsEBP89 interacts with the MYC-like protein OsBP5 to synergistically regulate the expression of Wx. Here, we determined that the GLYCOGEN SYNTHASE KINASE 5 (OsGSK5, also named SHAGGY-like kinase 41 [OsSK41]) inhibits the transcriptional activation activity of OsEBP89 in rice grains during amylose biosynthesis. The loss of OsSK41 function enhanced Wx expression and increased AC in rice grains. By contrast, the loss of function of OsEBP89 reduced Wx expression and decreased AC in rice grains. OsSK41 interacts with OsEBP89 and phosphorylates four of its sites (Thr-28, Thr-30, Ser-238, and Thr-257), which makes OsEBP89 unstable and attenuates its interaction with OsBP5. Wx promoter activity was relatively weak when regulated by the phosphomimic variant OsEBP89^E -OsBP5 but relatively strong when regulated by the nonphosphorylatable variant OsEBP89^A -OsBP5. Therefore, OsSK41-mediated phosphorylation of OsEBP89 represents an additional layer of complexity in the regulation of amylose biosynthesis during rice grain development. In addition, our findings provide four possible sites for regulating rice grain AC via precise gene editing.

Mechanistic insights into granule-bound starch synthase I (GBSSI.L539P) allele in high amylose starch biosynthesis in wheat (Triticum aestivum L.)

Amylose fraction of grain starch is correlated with a type of resistant starch with better nutritional quality. Granule-bound starch synthase I (GBSSI) is the known starch synthase, responsible for elongation of linear amylose chains. GBSSI expression, activity, and binding to starch and other proteins are the key factors that can affect amylose content. Previously, a QTL, qhams7A.1 carrying GBSSI mutant allele, was identified through QTL mapping using F2 population of the high amylose mutant line, 'TAC 75'. This high amylose mutant line has >2-fold higher amylose content than wild variety 'C 306'. In this study, we characterized this novel mutant allele, GBSSI.L539P. In vitro starch synthase activity of GBSSI.L539P showed improved activity than the wild type (GBSSI-wt). When expressed in yeast glycogen synthase mutants (Δgsy1gsy2), GBSSI-wt and GBSSI.L539P partially complemented the glycogen synthase (gsy1gsy2) activity in yeast. Structural analysis by circular dichroism (CD) and homology modelling showed no significant structural distortion in the mutant enzyme. Molecular docking studies suggested that the residue Leu539 is distant from the catalytic active site (ADP binding pocket) and had no detectable conformational changes in active site. Both wild and mutant enzymes were assayed for starch binding in vitro, and demonstrating higher affinity of the GBSSI.L539P mutant for starch than the wild type. The present study indicated that distant residue (L539P) influenced GBSSI activity by affecting its starch-binding ability. Therefore, it may be a potential molecular target for enhanced amylose content in grain.

Haplotype Variations and Evolutionary Analysis of the Granule-Bound Starch Synthase I Gene in the Korean World Rice Collection

Granule-bound starch synthase I (GBSSI) is responsible for Waxy gene encoding the, which is involved in the amylose synthesis step of starch biosynthesis. We investigated the genotypic and haplotypic variations of GBSSI (Os06g0133000) gene, including its evolutionary relatedness in the nucleotide sequence level using single-nucleotide polymorphisms (SNPs), indels, and structural variations (SVs) from 475 Korean World Rice Collection (KRICE_CORE), which comprised 54 wild rice and 421 cultivated represented by 6 ecotypes (temperate japonica, indica, tropical japonica, aus, aromatic, and admixture) or in another way by 3 varietal types (landrace, weedy, and bred). The results revealed that 27 of 59 haplotypes indicated a total of 12 functional SNPs (fSNPs), identifying 9 novel fSNPs. According to the identified novel fSNPs, we classified the entire rice collection into three groups: cultivated, wild, and mixed (cultivated and wild) rice. Five novel fSNPs were localized in wild rice: four G/A fSNPs in exons 2, 9, and 12 and one T/C fSNP in exon 13. We also identified the three previously reported fSNPs, namely, a G/A fSNP (exon 4), an A/C fSNP (exon 6), and a C/T fSNP (exon 10), which were observed only in cultivated rice, whereas an A/G fSNP (exon 4) was observed exclusively in wild rice. All-against-all comparison of four varietal types or six ecotypes of cultivated rice with wild rice showed that the GBSSI diversity was higher only in wild rice (π = 0.0056). The diversity reduction in cultivated rice can be useful to encompass the origin of this gene GBSSI during its evolution. Significant deviations of positive (wild and indica under balancing selection) and negative (temperate and tropical japonica under purifying selection) Tajima's D values from a neutral model can be informative about the selective sweeps of GBSSI genome insights. Despite the estimation of the differences in population structure and principal component analysis (PCA) between wild and subdivided cultivated subgroups, an inbreeding effect was quantified by F _ST statistic, signifying the genetic relatedness of GBSSI. Our findings of a novel wild fSNPS can be applicable for future breeding of waxy rice varieties. Furthermore, the signatures of selective sweep can also be of informative into further deeper insights during domestication.

The origin of Wxla provides new insights into the improvement of grain quality in rice

Appearance and taste are important factors in rice (Oryza sativa) grain quality. Here, we investigated the taste scores and related eating-quality traits of 533 diverse cultivars to assess the relationships between-and genetic basis of-rice taste and eating-quality. A genome-wide association study highlighted the Wx gene as the major factor underlying variation in taste and eating quality. Notably, a novel waxy (Wx) allele, Wx^la , which combined two mutations from Wx^b and Wxⁱⁿ , exhibited a unique phenotype. Reduced GBSSI activity conferred Wx^la rice with both a transparent appearance and good eating quality. Haplotype analysis revealed that Wx^la was derived from intragenic recombination. In fact, the recombination rate at the Wx locus was estimated to be 3.34 kb/cM, which was about 75-fold higher than the genome-wide mean, indicating that intragenic recombination is a major force driving diversity at the Wx locus. Based on our results, we propose a new network for Wx evolution, noting that new Wx alleles could easily be generated by crossing genotypes with different Wx alleles. This study thus provides insights into the evolution of the Wx locus and facilitates molecular breeding for quality in rice.

A rare Waxy allele coordinately improves rice eating and cooking quality and grain transparency

In rice (Oryza sativa), amylose content (AC) is the major factor that determines eating and cooking quality (ECQ). The diversity in AC is largely attributed to natural allelic variation at the Waxy (Wx) locus. Here we identified a rare Wx allele, Wx^mw , which combines a favorable AC, improved ECQ and grain transparency. Based on a phylogenetic analysis of Wx genomic sequences from 370 rice accessions, we speculated that Wx^mw may have derived from recombination between two important natural Wx alleles, Wxⁱⁿ and Wx^b . We validated the effects of Wx^mw on rice grain quality using both transgenic lines and near-isogenic lines (NILs). When introgressed into the japonica Nipponbare (NIP) background, Wx^mw resulted in a moderate AC that was intermediate between that of NILs carrying the Wx^b allele and NILs with the Wx^mp allele. Notably, mature grains of NILs fixed for Wx^mw had an improved transparent endosperm relative to soft rice. Further, we introduced Wx^mw into a high-yielding japonica cultivar via molecular marker-assisted selection: the introgressed lines exhibited clear improvements in ECQ and endosperm transparency. Our results suggest that Wx^mw is a promising allele to improve grain quality, especially ECQ and grain transparency of high-yielding japonica cultivars, in rice breeding programs.

Wxlv, the Ancestral Allele of Rice Waxy Gene

In rice grains, the Waxy (Wx) gene is responsible for the synthesis of amylose, the most important determinant for eating and cooking quality. The effects of several Wx alleles on amylose content and the taste of cooked rice have been elucidated. However, the relationship between artificial selection and the evolution of various Wx alleles as well as their distribution remain unclear. Here we report the identification of an ancestral allele, Wx^lv, which dramatically affects the mouthfeel of rice grains by modulating the size of amylose molecules. We demonstrated that Wx^lv originated directly from wild rice, and the three major Wx alleles in cultivated rice (Wx^b, Wx^a, and Wxⁱⁿ) differentiated after the substitution of one base pair at the functional sites. These data indicate that the Wx^lv allele played an important role in artificial selection and domestication. The findings also shed light on the evolution of various Wx alleles, which have greatly contributed to improving the eating and cooking quality of rice.

Three Major Nucleotide Polymorphisms in the Waxy Gene Correlated with the Amounts of Extra-long Chains of Amylopectin in Rice Cultivars with S or L-type Amylopectin

Extra-long chains (ELC) of amylopectin in rice endosperm are synthesized by granule-bound starch synthase I encoded by the Waxy (Wx) gene, which primarily synthesizes amylose. Previous studies showed that single nucleotide polymorphisms (SNP) in intron 1 and exon 6 of the Wx gene influences ELC amount. However, whether these SNPs are conserved among rice cultivars and if any other SNPs are present in the Wx gene remained unknown. Here, we sequenced the Wx gene from 17 rice cultivars with S or L-type amylopectin, including those with known ELC content and those originating in China with unique starch properties, as well as typical japonica and indica cultivars. In addition to the two SNPs described above, an additional SNP correlating with ELC content was found in exon 10. Low ELC cultivars (<3.0 %) had thymine at the splicing donor site of intron 1, Tyr224 in exon 6, and Pro415 in exon 10. Cultivars with moderate ELC content (4.1–6.9 %) had guanine at the splicing donor site of intron 1, Ser224 in exon 6, and Pro415 in exon 10. Cultivars with high ELC content (7.7–13.9 %) had guanine at the splicing donor site of intron 1, Tyr224 in exon 6, and Ser415 in exon 10. The chain length distribution pattern of amylopectin was correlated with the amounts of SSIIa found in starch granules and gelatinization temperature, but not with ELC content. The combinations of SNPs in the Wx gene found in this study may provide useful information for screening specific cultivars with different ELC content.