Quantitative trait locus analysis for flowering-related traits using two F2 populations derived from crosses between Japanese common wheat cultivars and synthetic hexaploids.

Flowering time is an important trait for Japanese wheat breeding. Aegilops tauschii, the D-genome donor of hexaploid wheat, is a useful resource to enlarge the D-genome diversity of common wheat. Previously, we identified flowering-related QTLs in F2 populations of synthetic hexaploid wheat lines between the tetraploid wheat cultivar Langdon and Ae. tauschii accessions. Here, to evaluate the usefulness of the early-flowering alleles from Ae. tauschii for Japanese wheat breeding, QTL analyses were conducted in two F2 populations derived from crosses between Japanese wheat cultivars and early-flowering lines of synthetic hexaploid wheat. Only two chromosomal regions controlling flowering-related traits were identified, on chromosomes 2DS and 5AL in the mapping populations, and no previously identified QTLs were found in the synthetic hexaploid lines. The strong effect of the 2DS QTL, putatively corresponding to Ppd-D1, was considered to hide any significant expression of other QTLs with small effects on flowering-related traits. When F2 individuals carrying Ae. tauschii-homozygous alleles around the 2DS QTL region were selected, the Ae. tauschii-derived alleles of the previously identified flowering QTLs partly showed an early-flowering phenotype compared with the Japanese wheat-derived alleles. Thus, some early-flowering alleles from Ae. tauschii may be useful for production of early-flowering Japanese wheat cultivars.

Evidence from principal component analysis for improvement of grain shape- and spikelet morphology-related traits after hexaploid wheat speciation.

Grain shape and size are involved in the main components of the domestication syndrome in cereals. Wheat grain shape has been dramatically altered at each stage of the domestication of tetraploid wheat and through common wheat speciation. To elucidate the evolutionary change of wheat grain shape, principal component (PC) analysis of grain shape-related traits was first conducted using wild and cultivated tetraploid, synthetic hexaploid, and common wheat accessions. The synthetic hexaploid wheat lines were previously produced through interspecific crosses between two common wheat progenitors, tetraploid wheat and Aegilops tauschii, and produced grains similar to those of cultivated tetraploid wheat. To identify genetic loci related to the difference in grain shape between common wheat and the synthetic wheat, the 15 traits related to grain and spikelet shape were measured in 108 F(2) individuals between Norin 61 and a synthetic wheat line, and the first three PC values for the 15 traits, PC1, PC2 and PC3, were mapped as quantitative traits in the F(2) population. In total, six QTLs, found on chromosomes 1A, 5A, 1D, 2D and 7D, showed significant LOD scores. Among them, a QTL for PC2, located on the 2DS chromosomal region near the Ppd-D1 locus, mainly contributed to the phenotypic difference in grain shape. Tg-D1, controlling tenacious glume phenotype, was located at a similar region to the 2DS QTL, which suggested that the Tg-D1 locus pleiotropically affects not only glume toughness but also spikelet and grain shape in hexaploid wheat. Therefore, it was predicted that wheat grains were rapidly improved toward a shorter and rounder phenotype accompanied with free-threshing wheat formation.

Utility of leaf senescence-associated gene homologs as developmental markers in common wheat.

Senescence is the final stage of development in plant tissues. In the senescence process, many senescence-associated genes (SAGs) are reportedly transcriptionally up-regulated. Here, we reported the isolation of nine wheat SAG cDNA clones named TaSAG1 to TaSAG9, and evaluated the usefulness of the SAG homologs for wheat developmental molecular markers based on their expression patterns. The nine wheat SAGs were identified in wheat EST libraries based on their homology to rice SAGs. All wheat SAG transcripts were up-regulated during natural senescence as well as during dark-induced senescence in seedling leaves. However, the expression patterns of wheat SAGs in the flag leaf did not necessarily correspond to those in seedling leaves during senescence. The nine wheat SAGs also showed variable expression patterns in developing and ripening seeds. The transcript accumulation patterns of TaSAG5 and TaSAG6 increased linearly during the period examined in the flag leaf and seed, and are therefore available as molecular markers to respectively evaluate the degree of wheat flag leaf senescence and seed maturation. Transcript accumulation levels of the six SAGs were increased before apparent necrotic cell death of seeding leaves exhibiting wheat hybrid necrosis. These results suggested that necrotic cell death in wheat hybrid necrosis could be closely related not only to senescence but also to defense responses.