Abstract
BackgroundThe granule-bound starch synthase I (GBSSI) enzyme is responsible for the synthesis of amylose, and therefore, its absence results in individuals with a waxy starch phenotype in various amylaceous crops. The validation of mutation points previously associated with the waxy starch phenotype in cassava, as well as the identification of alternative mutant alleles in the GBSSI gene, can allow the development of molecular-assisted selection to introgress the waxy starch mutation into cassava breeding populations.ResultsA waxy cassava allele has been identified previously, associated with several SNPs. A particular SNP (intron 11) was used to develop SNAP markers for screening heterozygote types in cassava germplasm. Although the molecular segregation corresponds to the expected segregation at 3:1 ratio (dominant gene for the presence of amylose), the homozygotes containing the SNP associated with the waxy mutation did not show waxy phenotypes. To identify more markers, we sequenced the GBSS gene from 89 genotypes, including some that were segregated from a cross with a line carrying the known waxy allele. As a result, 17 mutations in the GBSSI gene were identified, in which only the deletion in exon 6 (MeWxEx6-del-C) was correlated with the waxy phenotype. The evaluation of mutation points by discriminant analysis of principal component analysis (DAPC) also did not completely discriminate the waxy individuals. Therefore, we developed Kompetitive Allele Specific PCR (KASP) markers that allowed discrimination between WX and wx alleles. The results demonstrated the non-existence of heterozygous individuals of the MeWxEx6-del-C deletion in the analyzed germplasm. Therefore, the deletion MeWxEx6-del-C should not be used for assisted selection in genetic backgrounds different from the original source of waxy starch. Also, the alternative SNPs identified in this study were not associated with the waxy phenotype when compared to a panel of accessions with high genetic diversity.ConclusionAlthough the GBSSI gene can exhibit several mutations in cassava, only the deletion in exon 6 (MeWxEx6-del-C) was correlated with the waxy phenotype in the original AM206–5 source.
Highlights
The granule-bound starch synthase I (GBSSI) enzyme is responsible for the synthesis of amylose, and its absence results in individuals with a waxy starch phenotype in various amylaceous crops
This result showed that the mutation of the C by G allele at the 3197 bp position of the GBSSI gene is not a suitable molecular marker for use in molecular-assisted selection of waxy individuals when the genetic background is completely different from the original source of the mutation (AM206–5)
Genotypic evaluation of the germplasm bank via Kompetitive Allele Specific polymerase chain reaction (PCR) (KASP) After the complete sequencing of the GBSSI gene, we identified a deletion in exon 6 at position 1701 bp, previously described by Aiemnaka et al [27]
Summary
The granule-bound starch synthase I (GBSSI) enzyme is responsible for the synthesis of amylose, and its absence results in individuals with a waxy starch phenotype in various amylaceous crops. Starch is widely used for multiple commercial purposes Does it provide more than 80% of the calories in the human diet [1], it has many industrial applications, such as for glues and adhesives. It basically consists of two types of polymers: amylose (essentially α1,4polyglucans, linear) and amylopectin (α1,4-polyglucans and α-1,6-polyglucans, branched) [2], whose proportions and arrangements confer characteristics that define the starch’s commercial advantages and are the focus of breeding programs. The main commercial sources of starch are maize, cassava, rice, wheat and potato. Cassava is the second most important source of starch worldwide after maize. The functional and specific properties of each of these species define their industrial applications [3,4,5,6]
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