Chromosome 4AL Research Articles

Development of an STS Marker Linked to Powdery Mildew Resistance Genes PmLK906 and Pm4a by Gene Chip Hybridization

Wheat ( Triticum aestivum L.) line Lankao 90(6) carries a recessive powdery mildew resistance gene temporarily named PmLK906 on chromosome 2AL. Near PmLK906 there is another known powdery mildew resistance gene locus Pm4. To track the two powdery mildew resistance genes in wheat breeding program by marker assisted selection (MAS), a linked molecular marker was developed in this study. Wheat gene chip hybridization combined with bulked segregant analysis (BSA) was used to develop an sequence-tagged sites (STS) marker for PmLK906 and Pm4. A new 2 125 bp full-length cDNA clone (GenBank accession no. EU082094) similar to csAtPR5 of Aegilops tauschii was isolated from Lankao 90(6) 21-12, and temporarily named TaAetPR5. Specific products could be amplified from cultivars or lines possessing Pm4a, Pm4b and PmLK906 with primers p9-7p1 and p9-7p2 derived from TaAetPR5. TaAetPR5 was linked to PmLK906 at a genetic distance of 7.62 cM, and cosegregated with Pm4a. The p9-7p1 and p9-7p2 could be used as an STS marker for these resistance genes in wheat breeding. Because this marker was cosegregated with Pm4a, it can be used in map-based cloning of the alleles at Pm4 locus also.

Relationship between wheat rust resistance genes Yr1 and Sr48 and a microsatellite marker

A recombinant inbred line population derived from a cross between the winter wheat cultivars Arina and Forno was used to study the genetic relationship between genes Yr1 and SrAn1 in chromosome 2AL. Yr1 from Forno showed repulsion linkage (16·5 cM) with SrAn1 carried by Arina and was proximal to SrAn1. Based on the chromosomal location of SrAn1 with respect to Yr1 and its ineffectiveness against the Australian Puccinia graminis f. sp. tritici pathotype 34‐1,2,7 + Sr38, it was concluded that SrAn1 is distinct from Sr21 and was designated as Sr48. An attempt was made to map more markers in the distal region of chromosome 2AL in order to determine the precise location of Sr48. Unfortunately none of the markers tested mapped between Yr1 and Sr48, with the latter being the most distal locus. However, close linkage was identified between Yr1 and the PCR‐based molecular marker stm673acag. Genotyping with stm673acag amplified a 120‐bp fragment in eight of nine genotypes carrying Yr1. Avocet S was the only line tested without Yr1 that also amplified the 120‐bp product. A 124‐bp product was amplified in 40 Australian wheat cultivars known to lack Yr1. These results suggest that stm673acag could be used for marker assisted selection of Yr1. Genotypes carrying both Yr1 and Sr48 were identified.

Allelic variations of functional markers for polyphenol oxidase (PPO) genes in Indian bread wheat (Triticum aestivum L.) cultivars

Polyphenol oxidase (PPO) activity causes undesirable browning and discolouration of products manufactured from bread wheat (Triticum aestivumL.) during processing or storage. PPO is a copper-containing metalloprotein which catalyses hydroxylation of o-monophenols to o-diphenols and oxidation of o-diphenols to o-quinones. Auto-oxidation and polymerization of quinones with amino acid group of cellular proteins results in dark and brown discolouration of products made from bread wheat grains (Anderson and Morris 2001). The darkening phenomena of such products may reduce the quality of products and thus affect consumer acceptance. Flour protein content has a negative association with flour PPO activity (Park et al. 1997) presumably because of reactivity of phenolic side groups (Demeke et al. 2001). In wheat, PPO genes belong to a multi-gene family and are classified into two clusters: kernel and nonkernel, based on gene expression sites (Anderson et al. 2006; Jukanti et al. 2006). Recently, Massa et al. (2007) reported 21 distinct PPO sequences in kernel-type genes in wheat and wild relatives. Many studies have implied that PPO activity is mainly conditioned by the genes located on homoeologous group 2 chromosomes in wheat and wild relatives (Demeke et al. 2001; Watanabe et al. 2004, 2006; Sun et al. 2005; He et al. 2007, 2009; Raman et al. 2007). The high activity PPO alleles on chromosomes 2AL and 2DL are most thoroughly studied and were first reported by Wrigley and McIntosh (1975). A relatively low PPO activity was also found to be associated with chromosomes 2B, 3D and 6B (Demeke et al. 2001; Fuerst et al. 2008). Functional Sequence tagged site (STS) markers for PPO genes on chromosomes 2A and 2D have been developed, based on DNA sequences in GenBank (Sun et al. 2005; He et al. 2007;

MlAG12: a Triticum timopheevii-derived powdery mildew resistance gene in common wheat on chromosome 7AL

Wheat powdery mildew is an economically important disease in cool and humid environments. Powdery mildew causes yield losses as high as 48% through a reduction in tiller survival, kernels per head, and kernel size. Race-specific host resistance is the most consistent, environmentally friendly and, economical method of control. The wheat (Triticum aestivum L.) germplasm line NC06BGTAG12 possesses genetic resistance to powdery mildew introgressed from the AAGG tetraploid genome Triticum timopheevii subsp. armeniacum. Phenotypic evaluation of F(3) families derived from the cross NC06BGTAG12/'Jagger' and phenotypic evaluation of an F(2) population from the cross NC06BGTAG12/'Saluda' indicated that resistance to the 'Yuma' isolate of powdery mildew was controlled by a single dominant gene in NC06BGTAG12. Bulk segregant analysis (BSA) revealed simple sequence repeat (SSR) markers specific for chromosome 7AL segregating with the resistance gene. The SSR markers Xwmc273 and Xwmc346 mapped 8.3 cM distal and 6.6 cM proximal, respectively, in NC06BGTAG12/Jagger. The multiallelic Pm1 locus maps to this region of chromosome 7AL. No susceptible phenotypes were observed in an evaluation of 967 F(2) individuals in the cross NC06BGTAG12/'Axminster' (Pm1a) which indicated that the NC06BGTAG12 resistance gene was allelic or in close linkage with the Pm1 locus. A detached leaf test with ten differential powdery mildew isolates indicated the resistance in NC06BGTAG12 was different from all designated alleles at the Pm1 locus. Further linkage and allelism tests with five other temporarily designated genes in this very complex region will be required before giving a permanent designation to this gene. At this time the gene is given the temporary gene designation MlAG12.

Identification of novel tan spot resistance QTLs using an SSR-based linkage map of tetraploid wheat

Durum wheat (Triticum turgidum L. subsp. durum ,2 n= 4x = 28, AABB) is an important cereal used for making pasta products. Compared with bread wheat, durum wheat receives less attention in genetic and genomic studies. In this research, a tetraploid wheat doubled haploid (DH) population derived from the cross between the durum wheat cultivar 'Lebsock' and the T. turgidum subsp. carthlicum (2n = 4x = 28, AABB) accession PI 94749 was developed. The population consisted of 146 lines and was used to construct linkage maps of all 14 chromosomes. The maps consisted of 280 SSR markers and spanned 2,034.1 cM with an average density of one marker per 7.2 cM. The DH population and the whole genome linkage maps were then used to identify QTLs associ- ated with tan spot resistance. The DH population was inoculated separately with two Ptr ToxA-producing isolates (Pti2 and 86-124) representing races 1 and 2, respectively, of Pyrenophora tritici-repentis, and five resistance QTLs were detected on chromosome arms 3AS, 3BL, 5AL and 7BL. Together, the QTLs explained a total of 46 and 41% of the phenotypic variation for reaction to Pti2 and 86-124, respectively. The Tsn1-Ptr ToxA interaction was not a significant factor in tan spot development in this population, and none of the QTLs corresponded to previously identified loci known to confer insensitivity to host-selective toxins (HSTs) produced by P. tritici-repentis. This result, together with those of other similar studies, indicates that the wheat-P. tritici-repentis pathosystem involves more factors than currently published host-toxin interactions. The DH population and genetic maps reported here will be useful for genetic dissection of important agronomic traits as well as the identification and development of markers for marker-assisted selection (MAS).

Locating stable across environment QTL involved in the determination of agronomic characters in wheat

The International Triticeae Mapping Initiative (ITMI) recombinant inbred line (RIL) population was used to detect quantitative trait loci (QTL) underlying some key agronomic characters in bread wheat ( Triticum aestivum L.). Trait measurements were taken from five independent field experiments performed in Serbia. Stable across environment QTL involved in the determination of heading/flowering time and ear morphology/grain yield were detected on, respectively, chromosome arms 2DS and 4AL. These map locations are consistent with those obtained where the same population has been grown in contrasting geographical sites. However, as a result of QTL × environment interactions, not all these QTL are expressed in all environments. Nevertheless the (pleiotropic) effect on ear morphology appears to be expressed in almost all environments, and so represents a high value target for wheat improvement.

Ent-kaurenoic acid oxidase genes in wheat

Ent-kaurenoic acid oxidase (KAO) catalysis three steps in the gibberellin (GA) biosynthesis pathway, which yields a large hormone family affecting plant growth and development. In the current study, we performed partial gene cloning and comparative structural and mapping analysis among three Kao genes in bread wheat (Triticum aestivum L.). Molecular-marker-based mapping demonstrated that the Kao loci map to the distal ends of the chromosome arms 7AS, 4AL and 7DS, corresponding to the 7BS/4AL translocation region. Co-linearity of the chromosomal regions carrying the Kao genes was shown. It was concluded that the Kao genes we mapped represent a homoeoloci set, therefore the genes were designated Kao-A1 (chromosome 7AS), Kao-B1 (4AL), and Kao-D1 (7DS). It was found that exonic sequences of the three homoeologues differ from each other mainly by silent mutations, and each homoeologue is expressed.

Genome-wide QTL analysis for pre-harvest sprouting tolerance in bread wheat

A framework linkage map comprising 214 molecular marker (SSR, AFLP, SAMPL) loci was prepared using an intervarietal recombinant inbred line (RIL) mapping population of bread wheat. The RIL population that was developed from the cross SPR8198 (red-grained and PHS tolerant genotype) × HD2329 (white-grained and PHS susceptible genotype) following single seed descent segregated for pre-harvest sprouting (PHS). The RIL population and parental genotypes were evaluated in six different environments and the data on PHS were collected. Using the linkage map and PHS data, genome-wide single-locus and two-locus QTL analyses were conducted for PHS tolerance (PHST). Single-locus analysis following composite interval mapping (CIM) detected a total of seven QTL, located on specific arms of five different chromosome (1AS, 2AL, 2DL, 3AL and 3BL). These seven QTL included two major QTL one each on 2AL and 3AL. Two of these seven QTL were also detected following two-locus analysis, which resolved a total of four main-effect QTL (M-QTL), and 12 epistatic QTL (E-QTL), the latter involved in 7 QTL × QTL interactions. Interestingly, none of these M-QTL and E-QTL detected by two-locus analysis was involved in Q × E and Q × Q × E interactions, supporting the results of ANOVA, where genotype × environment interaction were non-significant. The QTL for PHS detected in the present study may be efficiently utilized for marker-aided selection for enhancing PHST in bread wheat.

Allelic variation at Psy1-A1 and association with yellow pigment in durum wheat grain

The yellow pigment (YP) of durum wheat (Triticum turgidum L. var durum) semolina is due in part to the presence of carotenoid pigments found in the endosperm and is an important end-use quality trait. Phytoene synthase (Psy) is considered a rate-limiting enzyme in the carotenoid biosynthetic pathway and in this study, three alleles of Psy1-A1 were sequenced from four durum wheat cultivars and a co-dominant marker was developed for genetic mapping. Psy1-A1 mapped to chromosome 7AL near Xwmc809 in three durum mapping populations and was significantly associated with a pigment quantitative trait loci (QTL) identified on that chromosome. A second QTL localized 25 cM proximal to Psy1-A1 in two populations, and the interaction between the two QTL was not significant. Consistent with QTL mapping data, the Psy1-A1o allele was associated with elevated pigment in a validation population comprising 93 diverse cultivars and breeding lines. These results confirm an earlier hypothesis that Psy1, and at least one additional gene in the distal region of 7AL, are associated with grain YP differences in durum wheat. The functional co-dominant marker developed in this study differentiates the Psy1-A1 alleles reported here and could be used as a target to enhance YP selection in durum wheat breeding programs.

Identification and analysis of genes encoding a novel ER-localised Cyclophilin B in wheat potentially involved in storage protein folding

The storage protein deposition processes in cereal endosperm follow a strict script involving protein folding and refolding, assortment and protein body deposition by two distinct pathways. The endoplasmic reticulum-localised enzymes called peptidyl prolyl cis– trans isomerases (PPIases) are expected to play crucial ‘foldase’ and chaperone roles in the deposition of wheat prolamins due to their proline-rich nature. The storage protein quality of wheat is important for nutritional and food technological purposes; however, little information exists on genetic control of protein sorting and deposition processes. In this study, CypB genes encoding an ER-localised cyclophilin (a PPIase) have been characterised from common wheat ( Triticum aestivum L. AABBDD) and its progenitors for the first time. The full-length genes contain seven exons and six introns, the exons being significantly more conserved. The putative CypB protein contains all PPIase signature sites including the putative active site and cyclosporin-binding residues, but an atypical ER localisation signal. Using the sequence variations, the common wheat genes were localized to chromosomes 7AL, 7BL and 7DL. The promoters of wheat genes were identified by inverse-PCR and show a number of potential tissue specific regulatory elements. The wheat genes are similar in structure to rice CypB and the putative proteins are 83% identical. Further, certain QTLs related to protein quality occur at the rice and wheat CypB loci. The results support this enzyme being a strong candidate for regulating storage protein quality in wheat.

Mapping of resistance to spot blotch disease caused by Bipolaris sorokiniana in spring wheat

Spot blotch caused by Bipolaris sorokiniana is a destructive disease of wheat in warm and humid wheat growing regions of the world. The development of disease resistant cultivars is considered as the most effective control strategy for spot blotch. An intervarietal mapping population in the form of recombinant inbred lines (RILs) was developed from a cross 'Yangmai 6' (a Chinese source of resistance) x 'Sonalika' (a spot blotch susceptible cultivar). The 139 single seed descent (SSD) derived F(6), F(7), F(8) lines of 'Yangmai 6' x 'Sonalika' were evaluated for resistance to spot blotch in three blocks in each of the 3 years. Joint and/or single year analysis by composite interval mapping (CIM) and likelihood of odd ratio (LOD) >2.2, identified four quantitative trait loci (QTL) on the chromosomes 2AL, 2BS, 5BL and 6DL. These QTLs were designated as QSb.bhu-2A, QSb.bhu-2B, QSb.bhu-5B and QSb.bhu-6D, respectively. A total of 63.10% of phenotypic variation was explained by these QTLs based on the mean over years. Two QTLs on chromosomes 2B and 5B with major effects were consistent over 3 years. All QTL alleles for resistance were derived from the resistant parent 'Yangmai 6'.

Pm23: a new allele of Pm4 located on chromosome 2AL in wheat

Powdery mildew, caused by Blumeria graminis f. sp. tritici, is one of the major diseases of common wheat (Triticum aestivum) worldwide. The powdery mildew resistance gene Pm23, identified in the common wheat Line 81-7241 and originally assigned to wheat chromosome 5A, was relocated on chromosome 2AL with the aid of molecular markers. Mapping of microsatellite markers in two wheat crosses segregating for Pm23 and Pm4b, respectively, in combination with the reported mapping of Pm4a, indicated that the three genes were all linked to the marker Xgwm356 with a distance of 3-5 cM. Allelism between Pm4b and Pm23 was then confirmed, when the progenies of a cross between VPM1 (Pm4b) and Line 81-7241, were shown to be all resistant to a B. graminis isolate avirulent to the both parents. Pm23 is therefore a new allele of the Pm4 locus, and was redesignated as Pm4c.

New DNA markers for high molecular weight glutenin subunits in wheat

End-use quality is one of the priorities of modern wheat (Triticum aestivum L.) breeding. Even though quality is a complex trait, high molecular weight (HMW) glutenins play a major role in determining the bread making quality of wheat. DNA markers developed from the sequences of HMW glutenin genes were reported in several previous studies to facilitate marker-assisted selection (MAS). However, most of the previously available markers are dominant and amplify large DNA fragments, and thus are not ideal for high throughput genotyping using modern equipment. The objective of this study was to develop and validate co-dominant markers suitable for high throughput MAS for HMW glutenin subunits encoded at the Glu-A1 and Glu-D1 loci. Indels were identified by sequence alignment of allelic HMW glutenin genes, and were targeted to develop locus-specific co-dominant markers. Marker UMN19 was developed by targeting an 18-bp deletion in the coding sequence of subunit Ax2* of Glu-A1. A single DNA fragment was amplified by marker UMN19, and was placed onto chromosome 1AL. Sixteen wheat cultivars with known HMW glutenin subunits were used to validate marker UMN19. The cultivars with subunit Ax2* amplified the 362-bp fragment as expected, and a 344-bp fragment was observed for cultivars with subunit Ax1 or the Ax-null allele. Two co-dominant markers, UMN25 and UMN26, were developed for Glu-D1 by targeting the fragment size polymorphic sites between subunits Dx2 and Dx5, and between Dy10 and Dy12, respectively. The 16 wheat cultivars with known HMW glutenin subunit composition were genotyped with markers UMN25 and UMN26, and the genotypes perfectly matched their subunit types. Using an Applied Biosystems 3130xl Genetic Analyzer, four F(2) populations segregating for the Glu-A1 or Glu-D1 locus were successfully genotyped with primers UMN19, UMN25 and UMN26 labeled with fluorescent dyes.

The chromosome region including the earliness per se locus Eps-Am1 affects the duration of early developmental phases and spikelet number in diploid wheat

Earliness per se genes are those that regulate flowering time independently of vernalization and photoperiod, and are important for the fine tuning of flowering time and for the wide adaptation of wheat to different environments. The earliness per se locus Eps-Am1 was recently mapped within a 0.8 cM interval on chromosome 1AmL of diploid wheat Triticum monococcum L., and it was shown that its effect was modulated by temperature. In this study, this precise mapping information was used to characterize the effect of the Eps-Am1 region on both duration of different developmental phases and spikelet number. Near isogenic lines (NILs) carrying the Eps-Am1-l allele from the cultivated accession DV92 had significantly longer vegetative and spike development phases (P <0.0001) than NILs carrying the Eps-Am1-e allele from the wild accession G3116. These differences were paralleled by a significant increase in the number of spikelets per spike, in both greenhouse and field experiments (P <0.0001). Significant interactions between temperature and Eps-Am1 alleles were detected for heading time (P <0.0001) but not for spikelet number (P=0.67). Experiments using NILs homozygous for chromosomes with recombination events within the 0.8 cM Eps-Am1 region showed that the differences in number of spikelets per spike were linked to the differences in heading time controlled by the Eps-Am1 locus. These results indicate that the differences in these two traits are either pleiotropic effects of a single gene or the effect of closely linked genes. A similar effect on spikelet number was detected in the distal region of chromosome 1AL in common wheat (T. aestivum L.).

Molecular mapping of genomic regions associated with wheat seedling growth under osmotic stress

A quantitative trait loci (QTL) approach was applied to dissect the genetic control of the common wheat seedling response to osmotic stress. A set of 114 recombinant inbred lines was subjected to osmotic stress from the onset of germination to the 8th day of seedling development, induced by the presence of 12 % polyethylene glycol. Root, coleoptile and shoot length, and root/shoot length ratio were compared under stress and control conditions. In all, 35 QTL mapping to ten chromosomes, were identified. Sixteen QTL were detected in controls, 17 under stressed conditions, and two tolerance index QTL were determined. The majority of the QTL were not stress-specific. In regions on five chromosome arms (1AS, 1BL, 2DS, 5BL and 6BL) the QTL identified under stress co-mapped with QTL affecting the same trait in controls, and these were classified as seedling vigour QTL, in addition to those expressed in controls. Tolerance-related QTL were detected on four chromosome arms. A broad region on chromosome 1AL, including five QTL, with a major impact of the gene Glu-A1 (LOD 3.93) and marker locus Xksuh9d (LOD 2.91), positively affected root length under stress and tolerance index for root length, respectively. A major QTL (LOD 3.60), associated with marker locus Xcdo456a (distal part of chromosome arm 2BS) determined a tolerance index for shoot length. Three minor QTL (LOD < 3.0) for root length and root/shoot length ratio under osmotic stress were identified in the distal parts of chromosome arms 6DL (marker locus Xksud27a) and 7DL (marker locus Xksue3b). Selecting for the favourable alleles at marker loci associated with the detected QTL for growth traits may represent an efficient approach to enhance the plants’ ability to maintain the growth of roots, coleoptile and shoots in drought-prone soils at the critical early developmental stages.

The development of PCR-based markers for the selection of eyespot resistance genes Pch1 and Pch2

Two eyespot resistance genes (Pch1 and Pch2) have been characterised in wheat. The potent resistance gene Pch1, transferred from Aegilops ventricosa, is located on the distal end of the long arm of chromosome 7D (7DL). Pch2 derives from the variety Cappelle Desprez and is located at the distal end of chromosome 7AL. The RFLP marker Xpsr121 and the endopeptidase isozyme allele Ep-D1b, are very closely linked to Pch1, probably due to reduced recombination in the region of the introgressed A. ventricosa segment. Pch2 is less closely linked to these markers but is thought to be closer to Xpsr121 than to Ep-A1b. In the present study simple sequence repeat (SSR) markers were integrated into the genetic map of a single chromosome (7D) recombinant (RVPM) population segregating for Pch1. Sequence-tagged-site (STS)-based assays were developed for Xpsp121 and a 7DL wheat EST containing a SSR. SSR markers Xwmc14 and Xbarc97 and the Xpsr121-derived marker co-segregated with Pch1 in the RVPM population. A single chromosome (7A) recombinant population segregating for Pch2 was screened for eyespot resistance and mapped using SSRs. QTL interval mapping closely associated Pch2 with the SSR marker Xwmc525.

Identification and Molecular Mapping of the Powdery Mildew Resistance Gene in Wheat Cultivar Yumai 66

豫麦66是对小麦白粉病（Blumeria graminis f.sp.tritici）具有良好抗性的小黑麦后代品种。本试验通过抗病鉴定与遗传分析,明确了豫麦66携带1个抗白粉病显性单基因,暂命名为PmYm66。采用2个以豫麦66为抗病亲本的杂交组合（豫麦66/铭贤169和豫麦66/ND3509）F2代抗、感病分离群体和F3代家系,利用集群分离分析法（BSA）找到了与PmYm66连锁的分子标记XKsum193、EST48、EST83和EST84,抗病基因和分子标记的顺序为EST48—EST83（EST84）—Xksum193—PmYm66,并通过中国春缺体-四体、双端体和缺失系将PmYm66基因及其连锁的分子标记定位在2AL染色体臂末端。多小种鉴定结果表明PmYm66（豫麦66）与2AL染色体臂上已有的Pm4a（Khapli/8Cc）和Pm4b（Armada）基因存在致病反应型差异。

Genetic and QTL analyses of seed dormancy and preharvest sprouting resistance in the wheat germplasm CN10955

The inheritance and genetic linkage analysis for seed dormancy and preharvest sprouting (PHS) resistance were carried out in an F8 recombinant inbred lines (RILs) derived from the cross between "CN19055" (white-grained, PHS-resistant) with locally adapted Australian cultivar "Annuello" (white-grained, PHS-susceptible). Seed dormancy was assessed as germination index (GI7) while assessment for preharvest sprouting resistance was based on whole head assay (sprouting index, SI) and visibly sprouted seeds (VI). Segregation analysis of the F2, F3 data from the glasshouse and the RIL population in 2004 and 2005 field data sets indicated that seed dormancy and PHS resistance in CN19055 is controlled by at least two genes. Heritabilities for GI7 and VI were high and moderate for SI. The most accurate method for assessing PHS resistance was achieved using VI and GI7 while SI exhibited large genotype by environment interaction. Two quantitative trait loci (QTLs) QPhs.dpivic.4A.1 and QPhs.dpivic.4A.2 were identified. On pooled data across four environments, the major QTL, QPhs.dpivic.4A.2, explained 45% of phenotypic variation for GI7, 43% for VI and 20% for SI, respectively. On the other hand, QPhs.dpivic.4A.1 which accounted for 31% of the phenotypic variation in GI7 in 2004 Horsham field trial, was not stable across environments. Physical mapping of two SSR markers, Xgwm937 and Xgwm894 linked to the major QTL for PHS resistance, using Chinese Spring deletions lines for chromosome 4AS and 4AL revealed that the markers were located in the deletion bins 4AL-12 and 4AL-13. The newly identified SSR markers (Xgwm937/Xgwm894) showed strong association with seed dormancy and PHS resistance in a range of wheat lines reputed to possess PHS resistance. The results suggest that Xgwm937/Xgwm894 could be used in marker-assisted selection (MAS) for incorporating preharvest sprouting resistance into elite wheat cultivars susceptible to PHS.

A new PCR-based marker on chromosome 4AL for resistance to pre-harvest sprouting in wheat (Triticum aestivum L.)

Pre-harvest sprouting (PHS) is a complex trait controlled by multiple genes with strong interaction between environment and genotype that makes it difficult to select breeding materials by phenotypic assessment. One of the most important genes for pre-harvest sprouting resistance is consistently identified on the long arm of chromosome 4A. The 4AL PHS tolerance gene has therefore been targeted by Australian white-grained wheat breeders. A new robust PCR marker for the PHS QTL on wheat chromosome 4AL based on candidate genes search was developed in this study. The new marker was mapped on 4AL deletion bin 13-0.59-0.66 using 4AL deletion lines derived from Chinese Spring. This marker is located on 4AL between molecular markers Xbarc170 and Xwg622 in the doubled-haploid wheat population Cranbrook × Halberd. It was mapped between molecular markers Xbarc170 and Xgwm269 that have been previously shown to be closely linked to grain dormancy in the doubled haploid wheat population SW95-50213 × Cunningham and was co-located with Xgwm269 in population Janz × AUS1408. This marker offers an additional efficient tool for marker-assisted selection of dormancy for white-grained wheat breeding. Comparative analysis indicated that the wheat chromosome 4AL QTL for seed dormancy and PHS resistance is homologous with the barley QTL on chromosome 5HL controlling seed dormancy and PHS resistance. This marker will facilitate identification of the gene associated with the 4A QTL that controls a major component of grain dormancy and PHS resistance.

Characterization of quantitative trait loci controlling genetic variation for preharvest sprouting in synthetic backcross-derived wheat lines.

Aegilops tauschii, the wild relative of wheat, has stronger seed dormancy, a major component of preharvest sprouting resistance (PHSR), than bread wheat. A diploid Ae. tauschii accession (AUS18836) and a tetraploid (Triticum turgidum L. ssp. durum var. Altar84) wheat were used to construct a synthetic wheat (Syn37). The genetic architecture of PHS was investigated in 271 BC(1)F(7) synthetic backcross lines (SBLs) derived from Syn37/2*Janz (resistant/susceptible). The SBLs were evaluated in three environments over 2 years and PHS was assessed by way of three measures: the germination index (GI), which measures grain dormancy, the whole spike assay (SI), which takes into account all spike morphology, and counted visually sprouted seeds out of 200 (VI). Grain color was measured using both Chroma Meter- and NaOH-based approaches. QTL for PHSR and grain color were mapped and their additive and epistatic effects as well as their interactions with environment were estimated by a mixed linear-model approach. Single-locus analysis following composite interval mapping revealed four QTL for GI, two QTL for SI, and four QTL for VI on chromosomes 3DL and 4AL. The locus QPhs.dpiv-3D.1 on chromosome 3DL was tightly linked to the red grain color (RGC) at a distance of 5 cM. The other locus on chromosome 3D, "QPhs.dpiv-3D.2" was independent of RGC locus. Two-locus analysis detected nine QTL with main effects and 18 additive x additive interactions for GI, SI, and VI. Two of the nine main effects QTL and two epistatic QTL showed significant interactions with environments. Both additive and epistatic effects contributed to phenotypic variance in PHSR and the identified markers are potential candidates for marker-assisted selection of favorable alleles at multiple loci. SBLs derived from Ae. tauschii proved to be a promising tool to dissect, introgress, and pyramid different PHSR genes into adapted wheat genetic backgrounds. The enhanced expression of PHS resistance in SBLs enabled us to develop white PHS-resistant wheat germplasm from the red-grained Ae. tauschii accession.