Mapping gibberellin-sensitive dwarfing locus <Emphasis Type="Italic">Rht18</Emphasis> in durum wheat and development of SSR and SNP markers for selection in breeding

Gibberellin-sensitive dwarfing gene Rht18 was mapped in two durum wheat recombinant inbred lines (RIL) populations developed from crosses, Bijaga Yellow/Icaro and HI 8498/Icaro. Rht18 was mapped within genetic interval of 1.8 cM on chromosome 6A. Simple sequence repeat (SSR) markers S470865SSR4, barc37 and TdGA2ox-A9 specific marker showed co-segregation with Rht18 in Bijaga Yellow/Icaro population consisting 256 RILs. Effect of Rht18 on plant height was validated in HI 8498/Icaro RIL population which segregated for Rht18 and Rht-B1b. Rht-B1b from HI 8498 showed pleiotropic effect on plant height and coleoptile length, on the other hand, Rht18 did not show effect on coleoptile length. The SSR and SNP markers linked to Rht18 were also validated by assessing their allelic frequency in 89 diverse durum and bread wheat accessions. It was observed that 204 bp allele of S470865SSR4 could differentiate Icaro from rest of the wheat accessions except HI 8498, suggesting its utility for selection of Rht18 in wheat improvement programs. Rht18 associated alleles of TdGA2ox-A9, IAW4371 and IAW7940 were absent in most of the tall Indian local durum wheat and bread wheat, hence could be used to transfer Rht18 to bread wheat and local durum wheat. SSR marker barc3 showed high recombination frequency with Rht18, though it showed allele unique to Icaro. Since semidwarf wheat with GA-sensitive dwarfing genes are useful in dry environments owing to their longer coleoptile, better emergence and seedling vigor, Rht18 may provide a useful alternative to widely used GA-insensitive dwarfing genes under dry environments.     

Identification and characterization of <Emphasis Type="Italic">Rht25</Emphasis>, a locus on chromosome arm 6AS affecting wheat plant height,heading time,and spike development

Key message

This study identified Rht25, a new plant height locus on wheat chromosome arm 6AS, and characterized its pleiotropic effects on important agronomic traits.

Abstract

Understanding genes regulating wheat plant height is important to optimize harvest index and maximize grain yield. In modern wheat varieties grown under high-input conditions, the gibberellin-insensitive semi-dwarfing alleles Rht-B1b and Rht-D1b have been used extensively to confer lodging tolerance and improve harvest index. However, negative pleiotropic effects of these alleles (e.g., poor seedling emergence and reduced biomass) can cause yield losses in hot and dry environments. As part of current efforts to diversify the dwarfing alleles used in wheat breeding, we identified a quantitative trait locus (QHt.ucw-6AS) affecting plant height in the proximal region of chromosome arm 6AS (<?0.4 cM from the centromere). Using a large segregating population (~?2800 gametes) and extensive progeny tests (70–93 plants per recombinant family), we mapped QHt.ucw-6AS as a Mendelian locus to a 0.2 cM interval (144.0–148.3 Mb, IWGSC Ref Seq v1.0) and show that it is different from Rht18. QHt.ucw-6AS is officially designated as Rht25, with Rht25a representing the height-increasing allele and Rht25b the dwarfing allele. The average dwarfing effect of Rht25b was found to be approximately half of the effect observed for Rht-B1b and Rht-D1b, and the effect is greater in the presence of the height-increasing Rht-B1a and Rht-D1a alleles than in the presence of the dwarfing alleles. Rht25b is gibberellin-sensitive and shows significant pleiotropic effects on coleoptile length, heading date, spike length, spikelet number, spikelet density, and grain weight. Rht25 represents a new alternative dwarfing locus that should be evaluated for its potential to improve wheat yield in different environments.

     

Pleiotropic effects of gibberellin-sensitive and giberrellin-insensitive dwarfing genes in bread wheat of the southern step region of the Black Sea

Investigation of the pleiotropic effects of GA-sensitive (Rht8) and GA-insensitive (Rht-B1 and Rht-D1) winter bread wheat dwarfing genes and the gene that determines the response of plants to photoperiod—Ppd-D1—were carried out for 3 years in the southern step region of the Black Sea bank on five different genetic backgrounds. It is shown that, in addition to direct effects on plant height, GA-sensitive and GA-insensitive dwarfing genes have pleiotropic effects on all studied traits except the number of fertile spikelets. Presence of the dwarfing genes in the genotype of tall forms led to the decrease of stem and ear length, and, at the same time, to the increase of ear density. The number of spikelets per spike decreased due to sterile spikelets, whereas the number of fertile spikelets did not change. There was a significant increase in the number of grains per ear as a result of increasing of spikelets in ears. The number and weight of grains did not decrease, even though the plants were characterized by a smaller number of productive tillers. The presence of Rht8x allele on genetic background of variety Stepnyak resulted in a significant decrease of plants productivity. However, in combination with Ppd-D1a allele, plants with Rht8x increased the potential productivity and surpassed the parental form (Rht8x Ppd-D1a). The presence of Rht-Ble allele resulted in reduction of weight of kernels from the main ear and 1000-kernels weight, increase of l/h, and left the number of seeds per spikelet stable in comparison with Rht8x.     

Evolutionary divergence of the rye <Emphasis Type="Italic">Pm17</Emphasis> and <Emphasis Type="Italic">Pm8</Emphasis> resistance genes reveals ancient diversity

Key message

We have isolated a novel powdery mildew resistance gene in wheat that was originally introgressed from rye. Further analysis revealed evolutionary divergent history of wheat and rye orthologous resistance genes.

Abstract

Wheat production is under constant threat from a number of fungal pathogens, among them is wheat powdery mildew (Blumeria graminis f. sp. tritici). Deployment of resistance genes is the most economical and sustainable method for mildew control. However, domestication and selective breeding have narrowed genetic diversity of modern wheat germplasm, and breeders have relied on wheat relatives for enriching its gene pool through introgression. Translocations where the 1RS chromosome arm was introgressed from rye to wheat have improved yield and resistance against various pathogens. Here, we isolated the Pm17 mildew resistance gene located on the 1RS introgression in wheat cultivar ‘Amigo’ and found that it is an allele or a close paralog of the Pm8 gene isolated earlier from ‘Petkus’ rye. Functional validation using transient and stable transformation confirmed the identity of Pm17. Analysis of Pm17 and Pm8 coding regions revealed an overall identity of 82.9% at the protein level, with the LRR domains being most divergent. Our analysis also showed that the two rye genes are much more diverse compared to the variants encoded by the Pm3 gene in wheat, which is orthologous to Pm17/Pm8 as concluded from highly conserved upstream sequences in all these genes. Thus, the evolutionary history of these orthologous loci differs in the cereal species rye and wheat and demonstrates that orthologous resistance genes can take different routes towards functionally active genes. These findings suggest that the isolation of Pm3/Pm8/Pm17 orthologs from other grass species, additional alleles from the rye germplasm as well as possibly synthetic variants will result in novel resistance genes useful in wheat breeding.

     

Molecular characterization of a novel vernalization allele <Emphasis Type="Italic">Vrn-B1d</Emphasis> and its effect on heading time in Chinese wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) landrace Hongchunmai

Flowering time of wheat cultivars contributes greatly to the adaptability to environmental conditions and it is largely controlled by vernalization genes. In this study, 262 Chinese mini-core wheat cultivars were used to identify the allelic variation at VRN-B1 locus. A novel dominant allele Vrn-B1d was found in Chinese spring wheat landrace cultivar Hongchunmai. This allele contained several genetic divergence within the first intron comparing to the recessive allele vrn-B1, including one large 6850-bp deletion (670–7519 bp), one small 187-bp deletion (7851–8037 bp), one unique SNP (T to C, 7845 bp), and one 4-bp mutation (TTTT to ACAA, 7847–7850 bp). Meanwhile, it was also different from the three known dominant alleles at VRN-B1 locus. Two pairs of primers were designed to identify the novel allele Vrn-B1d and other four known alleles of VRN-B1. A multiplex PCR was established to discriminate all five alleles simultaneously. The greenhouse experiment with high temperature (non-vernalizing condition) and long light showed that F₂ plants containing Vrn-B1d allele headed significantly earlier than those with recessive vrn-B1 allele, suggesting that Vrn-B1d is a dominant allele conferring the spring growth habit. This study provides a useful germplasm and molecular markers for wheat breeding.     

Evolution of <Emphasis Type="Italic">VRN-1</Emphasis> homoeologous loci in allopolyploids of <Emphasis Type="Italic">Triticum</Emphasis> and their diploid precursors

Background

The key gene in genetic system controlling the duration of the vegetative period in cereals is the VRN1 gene, whose product under the influence of low temperature (vernalization) promotes the transition of the apical meristem cells into a competent state for the development of generative tissues of spike. As early genetic studies shown, the dominant alleles of this gene underlie the spring forms of plants that do not require vernalization for this transition. In wheat allopolyploids various combinations of alleles of the VRN1 homoeologous loci (VRN1 homoeoalleles) provide diversity in such important traits as the time to heading, height of plants and yield. Due to genetical mapping of VRN1 loci it became possible to isolate the dominant VRN1 alleles and to study their molecular structure compared with the recessive alleles defining the winter type of plants. Of special interest is the process of divergence of VRN1 loci in the course of evolution from diploid ancestors to wheat allopolyploids of different levels of ploidy.

Results

Molecular analysis of VRN1 loci allowed to establish that various dominant alleles of these loci appeared as a result of mutations in two main regulatory regions: the promoter and the first intron. In the diploid ancestors of wheat, especially, in those of A- genome (T. boeoticum, T. urartu), the dominant VRN1 alleles are rare in accordance with a limited distribution of spring forms in these species. In the first allotetraploid wheat species including T. dicoccoides, T. araraticum (T. timopheevii), the spring forms were associated with a new dominant alleles, mainly, within the VRN-A1 locus. The process of accumulation of new dominant alleles at all VRN1 loci was significantly accelerated in cultivated wheat species, especially in common, hexaploid wheat T. aestivum, as a result of artificial selection of spring forms adapted to different climatic conditions and containing various combinations of VRN1 homoeoalleles.

Conclusions

This mini-review summarizes data on the molecular structure and distribution of various VRN1 homoeoalleles in wheat allopolyploids and their diploid predecessors.

     

Exploiting the <Emphasis Type="Italic">Rht</Emphasis> portfolio for hybrid wheat breeding

Key message

The portfolio of available Reduced height loci (Rht-B1, Rht-D1, and Rht24) can be exploited for hybrid wheat breeding to achieve the desired heights in the female and male parents, as well as in the hybrids, without adverse effects on other traits relevant for hybrid seed production.

Abstract

Plant height is an important trait in wheat line breeding, but is of even greater importance in hybrid wheat breeding. Here, the height of the female and male parental lines must be controlled and adjusted relative to each other to maximize hybrid seed production. In addition, the height of the resulting hybrids must be fine-tuned to meet the specific requirements of the farmers in the target regions. Moreover, this must be achieved without adversely impacting traits relevant for hybrid seed production. In this study, we explored Reduced height (Rht) loci effective in elite wheat and exploited their utilization for hybrid wheat breeding. We performed association mapping in a panel of 1705 wheat hybrids and their 225 parental lines, which besides the Rht-B1 and Rht-D1 loci revealed Rht24 as a major QTL for plant height. Furthermore, we found that the Rht-1 loci also reduce anther extrusion and thus cross-pollination ability, whereas Rht24 appeared to have no adverse effect on this trait. Our results suggest different haplotypes of the three Rht loci to be used in the female or male pool of a hybrid breeding program, but also show that in general, plant height is a quantitative trait controlled by numerous small-effect QTL. Consequently, marker-assisted selection for the major Rht loci must be complemented by phenotypic selection to achieve the desired height in the female and male parents as well as in the wheat hybrids.

     

Pyramiding of transgenic <Emphasis Type="Italic">Pm3</Emphasis> alleles in wheat results in improved powdery mildew resistance in the field

Key message

The combined effects of enhanced total transgene expression level and allele-specificity combination in transgenic allele-pyramided Pm3 wheat lines result in improved powdery mildew field resistance without negative pleiotropic effects.

Abstract

Allelic Pm3 resistance genes of wheat confer race-specific resistance to powdery mildew (Blumeria graminis f. sp. tritici, Bgt) and encode nucleotide-binding domain, leucine-rich repeat (NLR) receptors. Transgenic wheat lines overexpressing alleles Pm3a, b, c, d, f, and g have previously been generated by transformation of cultivar Bobwhite and tested in field trials, revealing varying degrees of powdery mildew resistance conferred by the transgenes. Here, we tested four transgenic lines each carrying two pyramided Pm3 alleles, which were generated by crossbreeding of lines transformed with single Pm3 alleles. All four allele-pyramided lines showed strongly improved powdery mildew resistance in the field compared to their parental lines. The improved resistance results from the two effects of enhanced total transgene expression levels and allele-specificity combinations. In contrast to leaf segment tests on greenhouse-grown seedlings, no allelic suppression was observed in the field. Plant development and yield scores of the pyramided lines were similar to the mean scores of the corresponding parental lines, and thus, the allele pyramiding did not cause any negative effects. On the contrary, in pyramided line, Pm3b × Pm3f normal plant development was restored compared to the delayed development and reduced seed set of parental line Pm3f. Allele-specific RT qPCR revealed additive transgene expression levels of the two Pm3 alleles in the pyramided lines. A positive correlation between total transgene expression level and powdery mildew field resistance was observed. In summary, allele pyramiding of Pm3 transgenes proved to be successful in enhancing powdery mildew field resistance.

     

New data on the distribution of hybrid necrosis genes in winter bread wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) cultivars

Hybrid necrosis genotypes have been identified in 125 Russian cultivars of winter bread wheat. More than half of them (56%) carry the Ne2 gene (genotype ne1ne1Ne2Ne2); others are free of necrosis genes (genotype ne1ne1ne2ne2). The possible causes of the increase in the Ne2 allele frequency and the loss of the Ne1Ne1ne2ne2 genotype in modern Russian cultivars of winter wheat are discussed. The principal component method has been used to compare the structures of the genetic diversity of cultivars differing in the hybrid necrosis genotype. It has been found that the Ne2 allele in winter wheat cultivars from northern Russia has originated from the cultivar Mironovskaya 808, whereas the cultivar Bezostaya 1 is not a source of this gene. In cultivars from southern Russia, the presence of the Ne2 allele is also mainly accounted for by the use of Mironovskaya 808 wheat in their breeding. The recessive genotype is explained by the presence of descendants of the cultivar Odesskaya 16 in the pedigrees of southern Russian winter wheats. The genetic relationship of cultivars with identical and different necrosis genotypes has been analyzed in nine regions of the Russian Federation.     

Development of a robust marker for <Emphasis Type="Italic">Psy-1</Emphasis> homoeologs and its application in improvement of yellow pigment content in durum wheat

Phytoene synthase-1 (Psy-1) homoeologs are associated with yellow pigment content (YPC) in endosperm of durum and bread wheat. In the present study, microsatellite variation in promoter region of Psy-A1 was identified in durum wheat and marker Psy-1SSR, targeting the microsatellite variation was developed which amplifies variation in Psy-A1 and Psy-B1 loci simultaneously. Psy-A1SSR was mapped within QYp.macs-7A, a major QTL for YPC identified earlier in PDW 233/Bhalegaon 4 population. Marker Psy-A1SSR was further validated in two different RIL populations and a set of 222 tetraploid wheat accessions including less cultivated tetraploid wheat species. Eight alleles of Psy-A1SSR were identified in 222 wheat accessions, while seven alleles were observed for Psy-B1SSR. Variation at Psy-A1SSR showed significant association with YPC, whereas no association was observed with Psy-B1SSR. Marker-assisted introgression of Psy-A1SSRe allele from PDW 233, to durum wheat cultivars MACS 3125 and HI 8498 resulted in improvement of YPC. Backcrossed BC₃F_2:4 and BC₂F_2:3 lines selected using Psy-A1SSR showed 89 to 98% gain in YPC over recurrent parents indicating robustness of marker. The marker can thus be utilized in marker-assisted improvement of YPC in durum wheat cultivars.     

Characterization of grain protein content gene (<Emphasis Type="Italic">GPC</Emphasis>-<Emphasis Type="Italic">B1</Emphasis>) introgression lines and its potential use in breeding for enhanced grain zinc and iron concentration in spring wheat

At least two billion people around the world suffer from micronutrient deficiency, or hidden hunger, which is characterized by iron-deficiency anemia, vitamin A and zinc deficiency. As a key staple food crop, wheat provides 20% of the world’s dietary energy and protein, therefore wheat is an ideal vehicle for biofortification. Developing biofortified wheat varieties with genetically enhanced levels of grain zinc (Zn) and iron (Fe) concentrations, and protein content provides a cost-effective and sustainable solution to the resource-poor wheat consumers. Large genetic variation for Fe and Zn were found in the primitive and wild relatives of wheat, the potential high Zn and Fe containing genetic resources were used as progenitors to breed high-yielding biofortified wheat varieties with 30–40% higher Zn content. Grain protein content (GPC) determines processing and end-use quality of wheat for making diverse food products. The GPC-B1 allele from Triticum turgidum L. var. dicoccoides have been well characterized for the increase in GPC and the associated pleiotropic effect on grain Zn and Fe concentrations in wheat. In this study effect of GPC-B1 allele on grain Zn and Fe concentrations in wheat were measured in different genetic backgrounds and two different agronomic management practices (with- and without foliar Zn fertilization). Six pairs of near-isogenic lines differing for GPC-B1 gene evaluated at CIMMYT, Mexico showed that GPC-B1 influenced marginal increase for grain Zn, Fe concentrations, grain protein content and slight reduction in kernel weight and grain yield. However, the magnitude of GPC and grain Zn and Fe reductions varied depending on the genetic background. Introgression of GPC-B1 functional allele in combination with normal or delayed maturity alleles in the CIMMYT elite wheat germplasm has the potential to improve GPC and grain Zn and Fe concentrations without the negative effect on grain yield due to early senescence and accelerated maturity.     

Prevalence of <Emphasis Type="Italic">VRN1</Emphasis> locus alleles among spring common wheat cultivars cultivated in Western Siberia

With the use of allele-specific primers developed for the VRN1 loci, the allelic diversity of the VRN-A1, VRN-B1, and VRN-D1 genes was studied in 148 spring common wheat cultivars cultivated under the conditions of western Siberia. It was demonstrated that modern Western Siberian cultivars have the VRN-A1a allele, which is widely distributed in the world (alone or in combination with the VRN-B1a and VRN-B1c alleles). It was established that the main contribution in acceleration of the seedling–heading time is determined by a dominant VRN-A1a allele, while the VRNA1b allele, on the contrary, determines later plant heading. Cultivars that have the VRN-A1b allele in the genotype are found with a frequency of 8%. It was shown that cultivars with different allele combinations of two dominant genes (VRN-A1a + VRN-B1c and VRN-A1a + VRN-B1a) are characterized by earlier heading and maturing.     

Analysis of compound heterozygotes reveals that the mouse floxed <Emphasis Type="Italic">Pax6</Emphasis><Superscript><Emphasis Type="Italic">tm1Ued</Emphasis></Superscript> allele produces abnormal eye phenotypes

Analysis of abnormal phenotypes produced by different types of mutations has been crucial for our understanding of gene function. Some floxed alleles that retain a neomycin-resistance selection cassette (neo cassette) are not equivalent to wild-type alleles and provide useful experimental resources. Pax6 is an important developmental gene and the aim of this study was to determine whether the floxed Pax6 ^tm1Ued (Pax6 ^fl ) allele, which has a retained neo cassette, produced any abnormal eye phenotypes that would imply that it differs from the wild-type allele. Homozygous Pax6 ^fl/fl and heterozygous Pax6 ^fl/+ mice had no overt qualitative eye abnormalities but morphometric analysis showed that Pax6 ^fl/fl corneas tended be thicker and smaller in diameter. To aid identification of weak effects, we produced compound heterozygotes with the Pax6 ^Sey-Neu (Pax6 ^?) null allele. Pax6 ^fl/? compound heterozygotes had more severe eye abnormalities than Pax6 ^+/? heterozygotes, implying that Pax6 ^fl differs from the wild-type Pax6 ⁺ allele. Immunohistochemistry showed that the Pax6 ^fl/? corneal epithelium was positive for keratin 19 and negative for keratin 12, indicating that it was abnormally differentiated. This Pax6 ^fl allele provides a useful addition to the existing Pax6 allelic series and this study demonstrates the utility of using compound heterozygotes with null alleles to unmask cryptic effects of floxed alleles.     

<Emphasis Type="Italic">VRN1</Emphasis> genes variability in tetraploid wheat species with a spring growth habit

Background

Vernalization genes VRN1 play a major role in the transition from vegetative to reproductive growth in wheat. In di-, tetra- and hexaploid wheats the presence of a dominant allele of at least one VRN1 gene homologue (Vrn-A1,?Vrn-B1, Vrn-G1 or Vrn-D1) determines the spring growth habit. Allelic variation between the Vrn-1 and vrn-1 alleles relies on mutations in the promoter region or the first intron. The origin and variability of the dominant VRN1 alleles, determining the spring growth habit in tetraploid wheat species have been poorly studied.

Results

Here we analyzed the growth habit of 228 tetraploid wheat species accessions and 25 % of them were spring type. We analyzed the promoter and first intron regions of VRN1 genes in 57 spring accessions of tetraploid wheats. The spring growth habit of most studied spring accessions was determined by previously identified dominant alleles of VRN1 genes. Genetic experiments proof the dominant inheritance of Vrn-A1d allele which was widely distributed across the accessions of Triticum dicoccoides. Two novel alleles were discovered and designated as Vrn-A1b.7 and Vrn-B1dic. Vrn-A1b.7 had deletions of 20 bp located 137 bp upstream of the start codon and mutations within the VRN-box when compared to the recessive allele of vrn-A1. So far the Vrn-A1d allele was identified only in spring accessions of the T. dicoccoides and T. turgidum species. Vrn-B1dic was identified in T. dicoccoides IG46225 and had 11 % sequence dissimilarity in comparison to the promoter of vrn-B1. The presence of Vrn-A1b.7 and Vrn-B1dic alleles is a predicted cause of the spring growth habit of studied accessions of tetraploid species. Three spring accessions T. aethiopicum K-19059, T. turanicum K-31693 and T. turgidum cv. Blancal possess recessive alleles of both VRN-A1 and VRN-B1 genes. Further investigations are required to determine the source of spring growth habit of these accessions.

Conclusions

New allelic variants of the VRN-A1 and VRN-B1 genes were identified in spring accessions of tetraploid wheats. The origin and evolution of VRN-A1 alleles in di- and tetraploid wheat species was discussed.

     

Molecular mapping of a new temperature-sensitive gene <Emphasis Type="Italic">LrZH22</Emphasis> for leaf rust resistance in Chinese wheat cultivar Zhoumai 22

Leaf rust, caused by Puccinia triticina, is one of the most widespread diseases in common wheat globally. The Chinese wheat cultivar Zhoumai 22 is highly resistant to leaf rust at the seedling and adult stages. Seedlings of Zhoumai 22 and 36 lines with known leaf rust resistance genes were inoculated with 13 P. triticina races for gene postulation. The leaf rust response of Zhoumai 22 was different from those of the single gene lines. With the objective of identifying and mapping, the new gene(s) for resistance to leaf rust, F₁, F₂ plants and F_2:3 lines from the cross Zhoumai 22/Chinese Spring were inoculated with Chinese P. triticina race FHDQ at the seedling stage. A single dominant gene, tentatively designated LrZH22, conferred resistance. To identify other possible genes in Zhoumai 22, ten P. triticina races avirulent on Zhoumai 22 were used to inoculate 24 F_2:3 lines. The same gene conferred resistance to all ten avirulent races. A total of 1300 simple sequence repeat (SSR) markers and 36 EST markers on 2BS were used to test the parents, and resistant and susceptible bulks. Resistance gene LrZH22 was mapped in the chromosome bin 2BS1-0.53-0.75 and closely linked to six SSR markers (barc183, barc55, gwm148, gwm410, gwm374 and wmc474) and two EST markers (BF202681 and BE499478) on chromosome arm 2BS. The two closest flanking SSR loci were Xbarc55 and Xgwm374 with genetic distances of 2.4 and 4.8 cM from LrZH22, respectively. Six designated genes (Lr13, Lr16, Lr23, Lr35, Lr48 and Lr73) are located on chromosome arm 2BS. In seedling tests, LrZH22 was temperature sensitive, conferring resistance at high temperatures. The reaction pattern of Zhoumai 22 was different from that of RL 4031 (Lr13), RL 6005 (Lr16) and RL 6012 (Lr23), Lr35 and Lr48 are adult-plant resistance genes, and Lr73 is not sensitive to the temperature. Therefore, LrZH22 is likely to be a new leaf rust resistance gene or allele.     

Genes <Emphasis Type="Italic">WHEAT FRIZZY PANICLE</Emphasis> and <Emphasis Type="Italic">SHAM RAMIFICATION 2</Emphasis> independently regulate differentiation of floral meristems in wheat

Background

Inflorescences of wheat species, spikes, are characteristically unbranched and bear one sessile spikelet at a spike rachis node. Development of supernumerary spikelets (SSs) at rachis nodes or on the extended rachillas is abnormal. Various wheat morphotypes with altered spike morphology, associated with the development of SSs, present an important genetic resource for studies on genetic regulation of wheat inflorescence development.

Results

Here we characterized diploid and tetraploid wheat lines of various non-standard spike morphotypes, which allowed for identification of a new mutant allele of the WHEAT FRIZZY PANICLE (WFZP) gene that determines spike branching in diploid wheat Ttiticum monococcum L. Moreover, we found that the development of SSs and spike branching in wheat T. durum Desf. was a result of a wfzp-A/TtBH-A1 mutation that originated from spontaneous hybridization with T. turgidum convar. сompositum (L.f.) Filat. Detailed characterization of the false-true ramification phenotype controlled by the recessive sham ramification 2 (shr2) gene in tetraploid wheat T. turgidum L. allowed us to suggest putative functions of the SHR2 gene that may be involved in the regulation of spikelet meristem fate and in specification of floret meristems. The results of a gene interaction test suggested that genes WFZP and SHR2 function independently in different processes during spikelet development, whereas another spike ramification gene(s) interact(s) with SHR2 and share(s) common functions.

Conclusions

SS mutants represent an important genetic tool for research on the development of the wheat spikelet and for identification of genes that control meristem activities. Further studies on different non-standard SS morphotypes and wheat lines with altered spike morphology will allow researchers to identify new genes that control meristem identity and determinacy, to elucidate the interaction between the genes, and to understand how these genes, acting in concert, regulate the development of the wheat spike.

     

Molecular and genetic characterization of the <Emphasis Type="Italic">Ry</Emphasis><Subscript><Emphasis Type="Italic">adg</Emphasis></Subscript> locus on chromosome XI from Andigena potatoes conferring extreme resistance to potato virus Y

Key message

We have elucidated the Andigena origin of the potato Ry_adg gene on chromosome XI of CIP breeding lines and developed two marker assays to facilitate its introgression in potato by marker-assisted selection.

Abstract

Potato virus Y (PVY) is causing yield and quality losses forcing farmers to renew periodically their seeds from clean stocks. Two loci for extreme resistance to PVY, one on chromosome XI and the other on XII, have been identified and used in breeding. The latter corresponds to a well-known source of resistance (Solanum stoloniferum), whereas the one on chromosome XI was reported from S. stoloniferum and S. tuberosum group Andigena as well. To elucidate its taxonomic origin in our breeding lines, we analyzed the nucleotide sequences of tightly linked markers (M45, M6) and screened 251 landraces of S. tuberosum group Andigena for the presence of this gene. Our results indicate that the PVY resistance allele on chromosome XI in our breeding lines originated from S. tuberosum group Andigena. We have developed two marker assays to accelerate the introgression of Ry_adg gene into breeding lines by marker-assisted selection (MAS). First, we have multiplexed RYSC3, M6 and M45 DNA markers flanking the Ry_adg gene and validated it on potato varieties with known presence/absence of the Ry_adg gene and a progeny of 6,521 individuals. Secondly, we developed an allele-dosage assay particularly useful to identify multiplex Ry_adg progenitors. The assay based on high-resolution melting analysis at the M6 marker confirmed Ry_adg plex level as nulliplex, simplex and duplex progenitors and few triplex progenies. These marker assays have been validated and can be used to facilitate MAS in potato breeding.

     

Mapping and characterization of the new adult plant leaf rust resistance gene <Emphasis Type="Italic">Lr77</Emphasis> derived from Santa Fe winter wheat

Key message

A new gene for adult plant leaf rust resistance in wheat was mapped to chromosome 3BL. This gene was designated as Lr77.

Abstract

‘Santa Fe’ is a hard red winter cultivar that has had long-lasting resistance to the leaf rust fungus, Puccinia triticina. The objective of this study was to determine the chromosome location of the adult plant leaf rust resistance in Santa Fe wheat. A partial backcross line of ‘Thatcher’ (Tc) wheat with adult plant leaf rust resistance derived from Santa Fe was crossed with Thatcher to develop a Thatcher//Tc*2/Santa Fe F₆ recombinant inbred line (RIL) population. The RIL population and parental lines were evaluated for segregation of leaf rust resistance in three field plot tests and in an adult plant greenhouse test. A genetic map of the RIL population was constructed using 90,000 single-nucleotide polymorphism (SNP) markers with the Illumina Infinium iSelect 90K wheat bead array. A significant quantitative trait locus for reduction of leaf rust severity in all four tests was found on chromosome 3BL that segregated as a single adult plant resistance gene. The RILs with the allele from the resistant parent for SNP marker IWB10344 had lower leaf rust severity and a moderately resistant to moderately susceptible response compared to the susceptible RILs and Thatcher. The gene derived from Santa Fe on chromosome 3BL was designated as Lr77. Kompetitive allele-specific polymerase chain reaction assay markers linked to Lr77 on 3BL should be useful for selection of wheat germplasm with this gene.

     

Loci on chromosomes 1A and 2A affect resistance to tan (yellow) spot in wheat populations not segregating for <Emphasis Type="Italic">tsn1</Emphasis>

Key message

QTL for tan spot resistance were mapped on wheat chromosomes 1A and 2A. Lines were developed with resistance alleles at these loci and at the tsn1 locus on chromosome 5B. These lines expressed significantly higher resistance than the parent with tsn1 only.

Abstract

Tan spot (syn. yellow spot and yellow leaf spot) caused by Pyrenophora tritici-repentis is an important foliar disease of wheat in Australia. Few resistance genes have been mapped in Australian germplasm and only one, known as tsn1 located on chromosome 5B, is known in Australian breeding programs. This gene confers insensitivity to the fungal effector ToxA. The main aim of this study was to map novel resistance loci in two populations: Calingiri/Wyalkatchem, which is fixed for the ToxA-insensitivity allele tsn1, and IGW2574/Annuello, which is fixed for the ToxA-sensitivity allele Tsn1. A second aim was to combine new loci with tsn1 to develop lines with improved resistance. Tan spot severity was evaluated at various growth stages and in multiple environments. Symptom severity traits exhibited quantitative variation. The most significant quantitative trait loci (QTL) were detected on chromosomes 2A and 1A. The QTL on 2A explained up to 29.2% of the genotypic variation in the Calingiri/Wyalkatchem population with the resistance allele contributed by Wyalkatchem. The QTL on 1A explained up to 28.1% of the genotypic variation in the IGW2574/Annuello population with the resistance allele contributed by Annuello. The resistance alleles at both QTL were successfully combined with tsn1 to develop lines that express significantly better resistance at both seedling and adult plant stages than Calingiri which has tsn1 only.

     

Interactions between <Emphasis Type="Italic">Glu-1</Emphasis> and <Emphasis Type="Italic">Glu-3</Emphasis> loci and associations of selected molecular markers with quality traits in winter wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) DH lines

The quality of wheat depends on a large complex of genes and environmental factors. The objective of this study was to identify quantitative trait loci controlling technological quality traits and their stability across environments, and to assess the impact of interaction between alleles at loci Glu-1 and Glu-3 on grain quality. DH lines were evaluated in field experiments over a period of 4 years, and genotyped using simple sequence repeat markers. Lines were analysed for grain yield (GY), thousand grain weight (TGW), protein content (PC), starch content (SC), wet gluten content (WG), Zeleny sedimentation value (ZS), alveograph parameter W (APW), hectolitre weight (HW), and grain hardness (GH). A number of QTLs for these traits were identified in all chromosome groups. The Glu-D1 locus influenced TGW, PC, SC, WG, ZS, APW, GH, while locus Glu-B1 affected only PC, ZS, and WG. Most important marker-trait associations were found on chromosomes 1D and 5D. Significant effects of interaction between Glu-1 and Glu-3 loci on technological properties were recorded, and in all types of this interaction positive effects of Glu-D1 locus on grain quality were observed, whereas effects of Glu-B1 locus depended on alleles at Glu-3 loci. Effects of Glu-A3 and Glu-D3 loci per se were not significant, while their interaction with alleles present at other loci encoding HMW and LMW were important. These results indicate that selection of wheat genotypes with predicted good bread-making properties should be based on the allelic composition both in Glu-1 and Glu-3 loci, and confirm the predominant effect of Glu-D1d allele on technological properties of wheat grains.