Quality traits. Grain Texture
Contributed by Gabriela Tranquilli (email@example.com)
Wheat marketing systems established a primary classification of hexaploid wheat (Triticum aestivum L.) based on endosperm texture, i.e. the hardness or softness of the grain, because this trait determines many of its potential end-uses. Hard textured grains require more grinding energy than soft textured grains to reduce endosperm into flour, and during this milling process a larger number of starch granules become physically damaged. Since damaged starch granules absorb more water than undamaged granules, flours from hard wheats are preferred for yeast-leavened bread baking, while flours from soft wheats are preferred for manufacturing cookies and cakes.
Endosperm texture is primary controlled by the Hardness (Ha) locus on the short arm of chromosome 5D. It is a simply inherited character and although the main locus is referred as hardness, softness is in fact the dominant trait.
The lipid binding proteins puroindolines a (PINA) and b (PINB) have been identified as responsible in determining differences between hard and soft textured wheats. These proteins are coded by the Pina-D1 and Pinb-D1 genes, tightly linked to the Ha locus on chromosome 5DS (reviewed in 1, 2).
These two genes act together in the determination of grain texture. The presence of wild-type puroindoline alleles at both loci (Pina-D1a and Pinb-D1a) determines softness, while either a deletion resulting in the complete lack of PINA protein, or single -nucleotide mutations resulting in a modified amino acid sequence or null expression of the PINB protein were shown to be inseparably linked to hard-textured grains in surveys of American and European wheats (3-6). It has been observed that genotypes having the deletion at Pina-D1 were harder than those having the single nucleotide mutation in Pinb-D1, and that genotypes having deleted both Pina-D1 and Pinb-D1 were even harder (7, 8). On the other hand, the incorporation of additional copies of Pina-Am1 and Pinb-Am1 from T. monococcum in hexaploid wheat resulted in significantly softer grains than those from the soft control (8). Complementation tests using over-expression of the pinB gene in transgenic wheat plants carrying the soft pinB-D1b allele demonstrated that the puroindoline genes are responsible for the soft texture (9).
Small modulations of this trait might also be achieved by manipulating minor QTLs like the one reported by Campbell et al (10) on chromosome 3AS.
A valuable resource for breeders and researchers working with wheat puroindolines genes is the puroindoline genotype database (Western Wheat Quality Laboratory), which provides the Pina-D1 and Pinb-D1 alleles and hardness phenotype data for many varieties.
There are PCR markers available for both Pina-D1 and Pinb-D1 loci. For details go to the methods section. For the QTL on chromosome 3AS Campbell et al. (10) reported an RFPL marker.
1. Puroindolines: the molecular basis of wheat grain hardness. Morris CF In: Plant Molecular Biology, 2002, 48: 633-647. [abstract]
2. Endosperm texture in wheat. Turnbull KM, Rahman S. In: Journal of Cereal Science, 2002, 36: 327-337. [abstract]
3. A glycine to serine change in puroindoline b is associated with wheat grain hardness and low levels of starch-surface friabilin. Giroux MJ, Morris CF. In: Theoretical and Applied Genetics, 1997, 95: 857:864. [abstract]
4. Wheat grain hardness results from highly conserved mutations in the friabilin components puroindoline a and b. Giroux MJ, Morris CF. In: Proceedings of the National Academy of Sciences, 1998, 95: 6262-6266. [abstract]
5. A leucine to proline mutation in puroindoline b is frequently present in hard wheats from Northern Europe. Lillemo M, Morris CF. In: Theoretical and Applied Genetics, 2000, 100:1100-1107. [abstract]
6. Prevalence of puroindolines grain hardness genotypes among historically significant North American spring and winter wheats. Morris CF, Lillemo M, Simeone MC, Giroux MJ, Babb SL, Kidwell KK. In: Crop Science, 2001, 41: 218-228. [abstract]
7. Association of puroindoline sequence type and grain hardness in hard red spring wheat Giroux MJ, Talbert L, Habernicht DK, Lanning S, Hemphill A, Martin JM. In: Crop Science, 2000, 40: 370-374. [abstract]
8. Substitutions and deletions of genes related to grain hardness in wheat and their effect on grain texture Tranquilli G, Heaton J, Chicaiza O, Dubcovsky J. In: Crop Science, 2002, 42: 1812-1817. [abstract]
9. Expression of wild-type pinB sequence in transgenic wheat complements a hard phenotype. Beecher B, Bettge A, Smidansky E, Giroux, MJ. In: Theoretical and Applied Genetics, 2002, 105: 870-877. [abstract]
10. Quantitative Trait Loci Associated with Kernel Traits in a Soft x Hard Wheat Cross. Campbell KG, Bergman CJ, Gualberto DG, Anderson JA, Giroux MJ, Hareland G, Fulcher RG, Sorrells ME, Finney PL..In: Crop Science, 1999, 39:1184–1195. [abstract]
11. Triticum aestivum puroindolines, two cystine-rich seed proteins: cDNA sequence analysis and developmentar gene expression Gautier MF, Aleman ME, Guirao A, Marion D, Joudrier P. In: Plant Molecular Biology, 1994, 25: 43-57. [abstract]
12. Genetic and physical characterization of grain texture-related loci in diploid wheat. Tranquilli G, Lijavetzky D, Muzzi G, Dubcovsky J. In: Molecular and General Genetics, 1999, 262: 846 - 850. [abstract]