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Quality traits. Waxy mutants

Background information

One of the main components of wheat flour is starch, its relative content and chemical composition affect the quality of the products obtained from wheat.

Starch is a glucose polymer composed of two different types of structures: amylose and amylopectin. Amylose is a linear polymer with D-glucosyl units linked by α1-4 bonds. Amylopectin has a branched structure: it also has α1-4 bonds but every 20-25 glucosyl there are ramifications due to α1-6 bonds.

The enzyme responsible for amylose biosynthesis in wheat is the granule-bound starch synthase (GBSS) or waxy protein. The synthesis of amylopectin is more complex and different enzymes are necessary. In wheat there are three genes that code for GBSS : Wx-A1, Wx-D1 and Wx-B1, with chromosomal localizations: 7AS, 7DS and 4AL, respectively.

Some mutants of wheat lack one or more GBSS proteins, they are called partial waxy mutants. In wheat lines with mutations in the three genes coding for GBSS, starch is only composed by amylopectin. When one or two genes are mutated the relative amount of amylopectin in starch increases compared to amylose. The largest effects on amylose content and quality are observed in Wx-B1 mutants, followed by Wx-D1 and Wx-A1 mutants (9).

Uses of partial waxy and waxy flours.

Consumers of Udon, one of the noodles produced from wheat, prefer a noodle with a firm surface and soft inside. These characteristics are associated with high swelling volumes and high peak pasting viscosities, which are observed in wheats with reduced amylose content and the presence of null alleles for GBSS (3). The null mutants for theWx-B1 locus are usually preferred for Japanese Udon noodles. A complete waxy mutant could not be used for noodle production, but could be used for other industrial applications.

Methods

There is a number of PCR markers available to analyze  the alleles of the different waxy genes. Some of them are described in the methods section. PCR markers usually require optimization to individual laboratory conditions. Conditions presented here should be consider only as a starting point of that optimization.

Available germplasm

Within the IFAFS project three lines are used as donor parents of waxy alleles: Ben-waxy,  IDO377S and Bai-Huo. Ben is a normal durum cultivar in which Dr. Shahryar Kianian (ND) has  introgressed the Wx-A1 and Wx-B1 mutations present in IkeIDO377S is a HWS with a null Wx-B1 allele and Bai-Huo (PI606717) is a HRS with a 588-bp deletion in the last exon of the Wx-D1 locus. The Wx-A1 null allele is frequent in Korean, Japanese and Turkish wheats, whereas the Wx-B1 null allele is common in Australian and Indian wheats. The natural 588-bp deletion in the Wx-D1 allele was found only in the Chinese landrace Bai-Huo (11). The triple mutant DHWx12 = TH7  can be used to breed full waxy hexaploid wheats (11).

References

1. A PCR-based marker for selection of starch and potential noodle quality in wheat. Briney, A; Wilson, R.; Potter, R. H.; Barclay, I.; Crosbie, G.; Appels, R.; Jones, M.G. J. In: Molecular Breeding, 1998, 4(5):427-433. [abstract]

2. Application of a high-throughput antibody-based assay for identification of the granule-bound starch synthase Wx-B1b allele in Australian wheat lines. Gale, K. R.; Panozzo, J. F.; Eagles, H. A.; Blundell, M.; Olsen, H.; Appels, R.  In: Australian Journal of Agricultural Research, 2001, 52(11-12):1417-1423. [abstract]

3. Waxy wheats: Origin, properties, and prospects Graybosch, R.A. In: Trends in Food Science & Technology, 1998, 9(4):135-142 [abstract]

4. Development of robust PCR-based DNA markers for each homoeo-allele of granule-bound starch synthase and their application in wheat breeding programs. McLauchlan, A.; Ogbonnaya, F. C.; Hollingsworth, B.; Carter, M.; Gale, K.R.; Henry, R.J.; Holton, T.A.; Morell, M. K.; Rampling, L.R.; Sharp, P. J.; Shariflou, M. R.; Jones, M.G.K.; Appels, R. In: Australian Journal of Agricultural Research, 2001, 52(11-12):1409-1416. [abstract]

5. Registration of D-null "Bai Huo" waxy wheat germplasm. . Morris, C.F.; Konzak, C.F. In: Crop Science, 2000, 40(1):304-305.

6. A polymorphic microsatellite in the 3' end of 'waxy' genes of wheat, Triticum aestivum. Shariflou, M. R.; Sharp, P. J. In: Plant Breeding, 1999, 118(3):275-277. [abstract]

7. Molecular characterization of waxy mutations in wheat. Vrinten, P.; Nakamura, T.; Yamamori M. In: Molecular and General Genetics, 1999, 261(3):463-471. [abstract]

8. The genes encoding granule-bound starch synthases at the waxy loci of the A, B, and D progenitors of common wheat.. Yan L.; Bhave M.; Fairclough R.; Konik C.; Rahman S.; Appels R. In: Genome, 2000, 43(2):264-272 [abstract]

9. Differential effects of Wx-A1,-B1 and-D1 protein deficiencies on apparent amylose content and starch pasting properties in common wheat. Yamamori, M.; Quynh, N.T. In: Theoretical and Applied Genetics, 2000, 100:32-38. [abstract]

10. A PCR-based DNA marker for detection of mutant and normal alleles of the Wx-D1 gene of wheat. Shariflou M.R.; Hassani M.E.; Sharp P.J. In: Plant Breeding, 2001, 120(2):121-124. [abstract]

11. Production of all eight genotypes of null alleles at 'waxy' loci in bread wheat, Triticum aestivum L. Zhao, X.C.; Sharp, P.J., In: Plant Breeding, 1998, 117:488-490. 

12. Isolation and characterization of the three Waxy genes encoding the granule-bound starch synthase in hexaploid wheat.. Murai J, Taira T, Ohta D. In: Gene, 1999, 234(1):71-79. [abstract]

13. Nucleotide sequence of a wheat (Triticum aestivum L.) cDNA clone encoding the waxy protein.. Clark JR, Robertson M, Ainsworth CC. In: Plant Molecular Biology, 1991, 16(6):1099-1101. [abstract]

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