Experimental
Plant material
The Elite-II subset studied is comprised of 33 primary synthetic hexaploid wheats. These synthetic hexaploids (SHs) were derived from the combinations of 14 durum wheats and 32 Aegilops tauschii accessions. Their production and the protocol have been reported earlier (Mujeeb-Kazi et al., Reference Mujeeb-Kazi, Rosas and Roldan1996). Six wheat varieties (Chinese Spring, Pavon-76, C-591, SH-231, SH-61 and SH-139) known to carry identified high-molecular-weight (HMW) glutenin subunits were used as controls. The 14 durum parents within these SHs were also characterized separately for the Glu-A1 and Glu-B1 subunits to confirm the respective subunits in the synthetic hexaploid wheats.
Electrophoresis and nomenclature
Protein extraction and electrophoresis procedures were carried out according to the method of Xu et al. (Reference Xu, Khan, Klindworth and Nygard2010). The nomenclature assigned to Glu-A1 and Glu-B1 was adopted from Payne and Lawrence (Reference Payne and Lawrence1983). The alleles at the Glu-D t1 locus were identified according to William et al. (Reference William, Peña and Mujeeb-Kazi1993). All the allele names were obtained from MacGenes. The Glu-D t1-encoded y-type subunit, initially named T2, was replaced by 12.2 according to Gianibelli et al. (Reference Gianibelli, Gupta, Lafiandra, Margiotta and MacRitchie2001).
Discussion
Although an Elite-I set has a wide global distribution and utilization, the Elite-II set has had lesser emphasis but has been structured based on multiple stress resistance that is crucial for crop improvement. Recent investigations have reported the resistance to barley yellow dwarf virus (Saffdar et al., Reference Saffdar, Ashfaq, Hameed, Ul-Haq and Mujeeb-Kazi2009) and stripe rust (Tariq-Khan and Ul-Haq, Reference Tariq-Khan and Ul-Haq2011) in the Elite-II set and suggested its wider use in breeding. This study focuses on the delineation of the 33 entries based on quality components for targeted use in breeding, emphasizing the significance of quality diversity along with stress resistance.
The results regarding the HMW glutenin composition in SHs are presented in Table 1. The allelic variants observed in these SHs are presented in Fig. 1. Fourteen different allelic variants at Glu-1 loci were observed in these SHs. At the Glu-A1 locus, only two alleles, 1AxNull and 1Ax1, were found. The other subunit generally found in bread wheat germplasm, designated as 1Ax2*, was absent in these SHs. For the Glu-B1 allelic variants, both the x- and y-type subunits were observed except for 1Bx20, which was found only in one SH. Although it was initially considered to occur with a 1By null gene, analysis using reverse phase high-performance liquid chromatography showed the presence of a y-type subunit, which accounts for about one-third of the total protein and co-migrates with the subunit 1Bx20 on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (Margiotta et al., Reference Margiotta, Colaprico, D'Ovidio and Lafiandra1993). In the observed allelic variants, the subunit 1Bx6+1By8 encoded by the allele Glu-B1d was predominant, appearing in 17 (50%) SHs. This subunit has been reported to have a positive influence on most of the bread-making quality parameters (Tang et al., Reference Tang, Yang, Wu, Li, Li, Zou, Chen and Mares2010). The other subunits observed were 1Bx7+1By8 in 11 (32.2%) and 1Bx13+1By16 in 4 (12.1%) SHs.
Table 1 High-molecular-weight glutenin subunit combinations in Elite-II D-genome synthetic hexaploids (Triticum turgidum×Aegilops tauschii; 2n=6x=42; AABBDD)
a E-II-1: to be read as Elite two entry number 1 up to Elite two entry number 33.
b The accession numbers of Ae. squarrosa in the Wheat Wide Crosses working collection at CIMMYT, Mexico and NARC Islamabad, Pakistan are indicated in parentheses.
Fig. 1 Allelic composition of Elite-II D-genome synthetic hexaploids (Triticum turgidum × Aegilops tauschii; 2n = 6x = 42; AABBDD). Lane 1 (from left): Pavon (Check), 2: E-II-1, 3: E-II-2, 4: E-II-3, 5: E-II-4, 6: E-II-5, 7: E-II-6, 8: E-II-7, 9: C-591 (Check), 10: Chinese spring (Check), 11: E-II-11, 12: blank, 13: E-II-12, 14: E-II-13, 15: SH-61 (Check), 16: E-II-14, 17: Pavon (Check).
At the Glu-D t1 locus, seven different allelic variants were observed. All these alleles were the combinations of the x- and y-type subunits except in the case of the entry SKARV_2/Aegilops squarrosa (304) where only the y-type subunit was observed. Due to the superior bread-making quality attributes of 1Dx5+1Dy10 encoded by the allele Glu-D1d, it was important to observe its high frequency in ten (29.4%) SHs. The inferior subunit at the Glu-D1 locus, designated as 1Dx2+1Dy12, which resulted in poor bread-making quality, was observed in five (14.7%) SHs. This subunit is generally predominant in bread wheat germplasm, and its replacement is only possible when other allelic variants of this locus are deployed frequently. The x-type subunit 1Dx1.5 was found in seven SHs and its association with the y-type subunit 1Dy12 perceived in two SHs. The other two y-type subunits associated with 1Dx1.5 include 1Dy10 and 1Dy12.2, which were also perceived in two SHs. The Glu-D t1ah (1Dx1.5+1Dy10) allele encodes positive quality attributes and has the potential to improve end-use quality as validated by Peña et al. (Reference Peña, Zarco-Hernandez and Mujeeb-Kazi1995) and Tang et al. (Reference Tang, Yang, Tian, Li and Chen2008). Similarly, the x-type subunit 1Dx2.1 was found in five SHs associated in all cases with the y-type subunit 1Dy10. This subunit has also been previously reported in a landrace from Afghanistan (Lagudah et al., Reference Lagudah, Flood and Halloran1987). The Glu-D1z-encoded subunit 1Dx3+1Dy10 was found in six (17.7%) SHs. This allele has been reported to contribute significantly to extensible gluten and bread loaf volume (Peña et al., Reference Peña, Zarco-Hernandez and Mujeeb-Kazi1995).
The Glu-D t1-encoded allelic variants are very important for their contribution to bread-making quality. The narrow genetic base of the Glu-D1 locus in bread wheat is usually accredited to the presence of 1Dx2+1Dy12 or 1Dx5+1Dy10. The occurrence of the array of the described D-genome-encoded subunits with many B- and A-genome subunits allows this germplasm set to be investigated thoroughly for its important functional properties, which in turn allows their possible utilization for improvement in grain quality. On the other hand, the subunit 1Dx1.5+1Dy12.2, encoding dough stickiness and low bread volume (Hsam et al., Reference Hsam, Kieffer and Zeller2001), was observed in only two SHs.
There is always a search for new alleles having favourable functional properties for bread-making qualities (Xu et al., Reference Xu, Khan, Klindworth and Nygard2010). In this context, SHs are very important, as they possess a large number of allelic variants of HMW glutenin subunits (Rehman et al., Reference Rehman, Evans, Gianibelli and Rose2008). Most wheat breeding programmes focus on allelic diversity for crop improvement, and when it comes to Ae. tauschii as a novel source, the preference has been towards exploiting its synthetic hexaploid wheats. SH wheats have unique AB-genome compositions complemented by diverse D-genome accessions. Thus, using SHs for wheat improvement allows their incorporation across all three wheat genomes. Hence, SH wheat utilization via ‘bridge crosses’ has been preferred, as it augments diversity across all three wheat genomes. However, where precision is required to target specific D-genome transfers, the ‘direct crossing’ holds significance. In such cases, a limited backcross strategy where the A- and B-genomes remain constant while the D-genome recombines, gives precise information about any D-genome transfers. This strategy is less exploited but has excellent scientific precision that could be explored by using the accessional Ae. tauschii information generated through the present findings on glutenin subunits associated with a D-genome chromosome.
