Preharvest sprouting of wheat (Triticum aestivum L.) involves several plant hormones, but a role for indoleacetic acid (IAA) and its precursor, tryptophan, has not been demonstrated. Our objectives were to determine the roles of IAA, tryptophan, and related compounds in germination of cultivars that differed in susceptibility to preharvest sprouting. L-Tryptophan strongly inhibited germination of embryos excised from caryopses that were highly dormant at harvest but not of embryos from caryopses that had little innate dormancy. The embryos responded similarly to indoleacetaldehyde, IAA, and synthetic auxins, suggesting that tryptophan functioned as a precursor of IAA. Indoleacetaldehyde oxidase inhibitors alleviated the adverse effects of tryptophan and indoleacetaldehyde, and an auxin antagonist decreased the inhibitory action of tryptophan and IAA on embryos from dormant caryopses, further suggesting that IAA was involved. Changes in sensitivity to IAA during afterripening also supported a role for auxin in dormancy. Embryos from caryopses that were highly dormant at harvest gradually lost sensitivity to IAA during afterripening, whereas intact caryopses were insensitive to IAA. The results implicated IAA in dormancy of wheat caryopses and indicated that the auxin might complement the role of abscisic acid in germination. The importance of using dormant caryopses and arresting afterripening in investigations of seed dormancy was noted.

Over 70% of wheat (Triticum aestivum L.) cultivars grown worldwide have a semidwarf phenotype controlled by the major genes Rht‐B1, Rht‐D1, and Rht8c The objective of this study was to determine their frequency in a set of historic and modern soft and hard winter wheat cultivars grown in the central and eastern USA. Three hundred sixty‐two cultivars that were developed from 1808 to 2008 were evaluated with molecular markers for Rht‐B1, Rht‐D1, and Rht8c All cultivars released before 1964 (41 soft winter wheat and 6 hard winter wheat) had wild‐type (tall) alleles at all three loci. After introduction of the dwarfing genes, the percentage of tested lines carrying either Rht‐B1b or Rht‐D1b increased rapidly to greater than 90% of modern varieties. Among soft winter wheat cultivars, the Rht‐D1b dwarfing gene was the most frequent being present in 45% of all lines tested and Rht‐B1b was present in 28%, while in the hard winter wheat cultivars the Rht‐B1b allele is the most prevalent in 77% of lines. Only 8% of the hard cultivars tested had the Rht‐D1b allele. The presence of the 192‐base pair (bp) allele of the microsatellite marker Xgwm261 indicated that Rht8c was less frequently used as a source of dwarfing in U.S. winter wheat germplasm, being present in 8 and 3% of the soft winter wheat and the hard winter wheats, respectively. A number of modern cultivars were identified that did not carry any of the dwarfing genes assayed and may possess alternative reduced height genes.

In wheat (Triticum aestivum L.), time from planting to spike emergence is influenced by genes controlling vernalization requirement and photoperiod response. Characterizing the available genetic diversity of known and novel alleles of VERNALIZATION1 (VRN1) and PHOTOPERIOD1 (PPD1) in winter wheat can inform approaches for breeding climate resilient cultivars. This study identified QTL for heading date (HD) associated with multiple VRN1 and PPD1 loci in a population developed from a cross between two early flowering winter wheat cultivars. When the population was grown in the greenhouse after partial vernalization treatment, major heading date QTLs co-located with the VRN-A1 and VRN-B1 loci. Copy number variation at the VRN-A1 locus influenced HD such that RIL having three copies required longer cold exposure to transition to flowering than RIL having two VRN-A1 copies. Sequencing vrn-B1 winter alleles of the parents revealed multiple polymorphisms in the first intron that were the basis of mapping a major HD QTL coinciding with VRN-B1. A 36 bp deletion in the first intron of VRN-B1 was associated with earlier HD after partial vernalization in lines having either two or three haploid copies of VRN-A1. The VRN1 loci interacted significantly and influenced time to heading in field experiments in Louisiana, Georgia and North Carolina. The PPD1 loci were significant determinants of heading date in the fully vernalized treatment in the greenhouse and in all field environments. Heading date QTL were associated with alleles having large deletions in the upstream regions of PPD-A1 and PPD-D1 and with copy number variants at the PPD-B1 locus. The PPD-D1 locus was determined to have the largest genetic effect, followed by PPD-A1 and PPD-B1. Our results demonstrate that VRN1 and PPD1 alleles of varying strength allow fine tuning of flowering time in diverse winter wheat growing environments.

Summary Awns are stiff, hair‐like structures which grow from the lemmas of wheat (Triticum aestivum) and other grasses that contribute to photosynthesis and play a role in seed dispersal. Variation in awn length in domesticated wheat is controlled primarily by three major genes, most commonly the dominant awn suppressor Tipped1 (B1). This study identifies a transcription repressor responsible for awn inhibition at the B1 locus. Association mapping was combined with analysis in biparental populations to delimit B1 to a distal region of 5AL colocalized with QTL for number of spikelets per spike, kernel weight, kernel length, and test weight. Fine‐mapping located B1 to a region containing only two predicted genes, including C2H2 zinc finger transcriptional repressor TraesCS5A02G542800 upregulated in developing spikes of awnless individuals. Deletions encompassing this candidate gene were present in awned mutants of an awnless wheat. Sequence polymorphisms in the B1 coding region were not observed in diverse wheat germplasm whereas a nearby polymorphism was highly predictive of awn suppression. Transcriptional repression by B1 is the major determinant of awn suppression in global wheat germplasm. It is associated with increased number of spikelets per spike and decreased kernel size.

A new powdery mildew resistance gene Pm54 was identified on chromosome 6BL in soft red winter wheat. Powdery mildew is causing increasing damage to wheat production in the southeastern USA. To combat the disease, a continuing need exists to discover new genes for powdery mildew resistance and to incorporate those genes into breeding programs. Pioneer(®) variety 26R61 (shortened as 26R61) and AGS 2000 have been used as checks in the Uniform Southern Soft Red Winter Wheat Nursery for a decade, and both have provided good resistance across regions during that time. In the present study, a genetic analysis of mildew resistance was conducted on a RIL population developed from a cross of 26R61 and AGS 2000. Phenotypic evaluation was conducted in the field at Plains, GA, and Raleigh, NC, in 2012 and 2013, a total of four environments. Three quantitative trait loci (QTL) with major effect were consistently detected on wheat chromosomes 2BL, 4A and 6BL. The 2BL QTL contributed by 26R61 was different from Pm6, a widely used gene in the southeastern USA. The other two QTL were identified from AGS 2000. The 6BL QTL was subsequently characterized as a simple Mendelian factor when the population was inoculated with a single Blumeria graminis f. sp. tritici (Bgt) isolate in controlled environments. Since there is no known powdery mildew resistance gene (Pm) on this particular location of common wheat, the gene was designated Pm54. The closely linked marker Xbarc134 was highly polymorphic in a set of mildew differentials, indicating that the marker should be useful for pyramiding Pm54 with other Pm genes by marker-assisted selection.

Winter wheats require a long exposure to cold temperatures (vernalization) to accelerate flowering. However, varieties differ in the length of the period of cold required to saturate the vernalization response. Here we show that single nucleotide polymorphisms (SNP) at the binding site of the GRP2 protein in the VRN-A1 first intron (henceforth, RIP3) are associated with significant differences in heading time after a partial vernalization treatment. The ancestral winter VRN-A1 allele in ‘Triple Dirk C’ has one SNP in the RIP3 region (1_SNP) relative to the canonical RIP3 sequence, whereas the derived ‘Jagger’ allele has three SNPs (3_SNPs). Both varieties have a single VRN-A1 copy encoding identical proteins. In an F2 population generated from a cross between these two varieties, plants with the 3_SNPs haplotype headed significantly earlier (P < 0.001) than those with the 1_SNP haplotype, both in the absence of vernalization (17 days difference) and after 3-weeks of vernalization (11 days difference). Plants with the 3_SNPs haplotype showed higher VRN-A1 transcript levels than those with the 1_SNP haplotype. The 3_SNPs haplotype was also associated with early heading in a panel of 127 winter wheat varieties grown in three separate controlled-environment experiments under partial vernalization (36 to 54 days, P < 0.001) and one experiment under field conditions (21 d, P < 0.0001). The RIP3 polymorphisms can be used by wheat breeders to develop winter wheat varieties adapted to regions with different duration or intensity of the cold season.Electronic supplementary materialThe online version of this article (10.1007/s00438-018-1455-0) contains supplementary material, which is available to authorized users.

The aim of this study was to identify quantitative trait locus (QTL) associated with grain yield (GY) in a recombinant inbred line (RIL) population from a cross between two elite soft red winter wheat (SRWW) cultivars ('Pioneer 26R61' and 'AGS2000'). The RIL population was grown from 2011 to 2014 in 12 site-year combinations throughout the southeastern US. Overall, AGS2000 was the higher yielding parental line, out-performing 26R61 in seven of the 12 environments. Mean GY for the RILs ranged from 3.39 to 7.16 t ha -1 with significant genotype, environment and genotype by environmental interaction effects. Nine stable QTL were detected for yield, explaining up to 53 % of the phenotypic variation when fit into a multiple-QTL model. The QTL with the largest effect was detected at the Vrn-B1 locus with the short vernalization winter allele from AGS2000 favorable for yield. In addition, vrn-B1 acted additively with a region on chromosome 2B near the Ppd-B1 locus, indicating that a shorter vernalization requirement combined with the Ppd-B1b allele for photoperiod sensitivity may play a key role in adaptation of SRWW to the southern US. Single nucleotide polymorphism markers linked to additional QTL on chromosomes 3A and 3B were in agreement with a previous genome-wide association study in spring wheat, confirming the importance of these regions for yield across environments and germplasm pools. Overall the stable QTL were more predictive of GY compared to individual site-year QTL, indicating that a targeted QTL approach can be utilized by breeding programs to enrich for favorable loci.Electronic supplementary material The online version of this article (

The impact of high temperatures on accumulation of starch in the grain of wheat (Triticum aestivum L.) is usually attributed to direct effects of the stress on the enzymes involved. However, roots are extremely sensitive to temperatures that can be as high as those experienced by the shoots, and their role in whole-plant responses should be considered. Wheat (cv. Len) was grown at 15/15, 30/15, 15/30, and 30/30˚C shoot/root temperatures during maturation, and accumulation of dry matter and N, contents of sucrose and starch, and activities of enzymes in the pathway of starch assimilation in the endosperm, were measured weekly. Dry matter and N accumulation were affected more by root than by shoot temperatures. High whole-plant temperatures (30/30˚C) accelerated linear grain growth but diminished the duration of assimilation, the contents of sucrose and starch, and the activities of the enzymes involved. The effects of high root temperature (15/30˚C) resembled those of high whole-plant temperature, whereas low root temperature (30/15˚C) tended to ameliorate them. Sucrose synthase and soluble starch synthase were affected more than the other enzymes by high shoot and/or root temperature. However, treatments that caused the lowest activities resulted in the fastest, but briefest, linear rates of grain growth. We concluded that shoots and roots interact in the response of wheat to high temperature, and that stress on both organs affects accumulation of starch in grain.