Genetic mapping of quantitative trait loci (QTL) for resistance to septoria tritici blotch in a winter wheat cultivar Liwilla

Septoria tritici blotch (STB), caused by Mycosphaerella graminicola (anamorph Septoria tritici, syn. Zymoseptoria tritici), is present in most wheat-growing areas worldwide. Resistance breeding appears to be the most sensible approach to disease control. An attempt was made to identify loci associated with resistance to STB in a resistant winter wheat cultivar Liwilla. In the study we used a set of 74 doubled-haploid lines generated from anthers of F₁ hybrids between the resistant cultivar Liwilla and susceptible cultivar Begra. Four monopycnidiospore isolates of M. graminicola with diverse pathogenicity were used in tests on seedlings under controlled growth conditions and on adult plants under polytunnel conditions over a six year period. In both environments, the percentage leaf area covered by necrosis and covered by pycnidia were measured; time to heading and plant height were also recorded for the polytunnel experiments. Seven isolate-specific quantitative trait loci (QTLs) were associated with STB resistance: QStb.ihar-3A.2, QStb.ihar-6A, QStb.ihar-7A.2, QStb.ihar-1B, QStb.ihar-2B.2, QStb.ihar-3B, and QStb.ihar-5D. QTL on chromosome 5D and 7A represent novel STB resistance loci. The phenotypic variance explained by individual QTLs ranged from 9.5 % to 50.3 %. Three QTLs detected on chromosomes 3A, 7A and 1B showed major effects and were detected consistently in different environments. The locations of QStb.ihar-3A.2 and QStb.ihar-1B coincide with the resistance genes Stb6 and Stb11, respectively. Locus QStb.ihar-3B and a QTL for time to heading mapped to the same location, but are most likely not associated. Most of the mapped QTLs explain the resistance associated with both low necrosis and low pycnidia coverage.     

Single-gene resistance to Septoria tritici blotch in the spring wheat cultivar 'Tadinia'

Resistance to Mycosphaerella graminicola causal agent of Septoria tritici blotch, was identified in progenies of crosses with European winter wheat cultivars, Tadorna and Cleo. This resistance was used to develop the resistant spring wheat cultivar Tadinia, selected from ‘Tadorna’/‘Inia 66’ released in 1985. Evaluation of the progeny of intercrosses between ‘Tadorna’, ‘Cleo’, ‘Tadinia’, and two short-statured resistant lines derived from hybrids with ‘Tadinia’ as one parent indicate the resistance was inherited as a single gene showing partial to strong dominance. The gene in ‘Tadinia’ was designated Stb4. Crosses between another resistant cultivar, ‘Bulgaria 88’, and ‘Tadinia’ suggest that ‘Bulgaria 88’ does not have Stb4. Successful introgression of Stb4 into Rht1+Rht2 short-statured lines revealed that plant stature and resistance to M. graminicola segregated independently. The Stb4 gene has been effective since 1975 against M. graminicola extant in California. A high positive correlation between seedling and adult plant disease scores, based on scoring of lesions producing pycnidia, indicated that the Stb4 gene is expressed throughout the life cycle under both field and greenhouse conditions, confirming that disease screening can be carried out on seedling plants. Separate assessment of necrotic lesions with and without pycnidia indicated that leaf necrosis without pycnidia, observed, especially under greenhouse conditions, can confound disease evaluations and lead to inaccurate genotype classification.     

Genomewide association study reveals novel quantitative trait loci associated with resistance towards Septoria tritici blotch in North European winter wheat

Fungal diseases are a major constraint for wheat production. Effective disease resistance is essential for ensuring a high production quality and yield. One of the most severe fungal diseases of wheat is Septoria tritici blotch (STB), which influences wheat production across the world. In this study, genomewide association mapping was used to identify new chromosomal regions on the wheat genome conferring effective resistance towards STB. A winter wheat population of 164 North European varieties and breeding lines was genotyped with 15K single nucleotide polymorphism (SNP) wheat array. The varieties were evaluated for STB in field trials at three locations in Denmark and across 3 years. The association analysis revealed four quantitative trait loci, on chromosomes 1B, 2A, 5D and 7A, highly associated with STB resistance. By comparing varieties containing several quantitative trait loci (QTL) with varieties containing none of the found QTL, a significant difference was found in the mean disease score. This indicates that an effective resistance can be obtained by pyramiding several QTL.     

Stay green trait: variation, inheritance and its association with spot blotch resistance in spring wheat (Triticum aestivum L.)

One thousand four hundred and seven spring wheat germplasm lines belonging to Indian and CIMMYT wheat programs were evaluated for stay green (SG) trait and resistance to spot blotch caused by Bipolaris sorokiniana during three consecutive crop seasons, 1999–2000, 2000–2001 and 2001–2002. Disease severity was recorded at six different growth stages beginning from tillering to late milk stage. SG trait was measured by following two approaches: difference for 0–9 scoring of green coloration (chlorophyll) of flag leaf and spike at the late dough stage (GS 87) and a new approach of leaf area under greenness (LAUG). Germplasm lines showed a wide range (7–89) for LAUG and were grouped into four viz., SG, moderately stay green, moderately non-stay green and non-stay green (NSG). However, very few (2.2%) lines showed high expression of SG trait, i.e., LAUG >60. LAUG appeared to be a better measure of SG trait than a 0–9 scale. Mean spot blotch ratings of SG genotypes were significantly lower than those of NSG genotypes at all growth stages. Two spot blotch resistant genotypes (Chirya 3 and Chirya 7) having strong expressions of SG trait were crossed with NSG, spot blotch susceptible cv. Sonalika. Individually threshed F₂ plants were used to advance the generations. SG trait and spot blotch severity were recorded in the parents and F₁, F₃, F₄, F₅, F₆ and F_6–7 generations under disease-protected and inoculated conditions. SG trait in the F₁ generation was intermediate and showed absence of dominance. Evaluation of progenies (202–207) in the segregating generations revealed that SG trait was under the control of around four additive genes. Lines homozygous for SG trait in F₄, F₅, F₆ and F_6–7 generations showed significantly lower mean area under disease progress curve (AUDPC) for spot blotch than those with NSG expression. A positive correlation (0.73) between SG trait and AUDPC further indicated a positive influence of SG on severity of spot blotch. The study established that variation for SG trait exists in spring wheat; around four additive genes control its inheritance in the crosses studied and there is positive association between SG trait and resistance to spot blotch.     

Diallel analysis of four resistance components to Septoria tritici in six crosses of wheat (Triticum aestivum)

Four resistance components, i.e. incubation period, latent period, maturation period and pycnidial coverage, were studied in a diallel cross of four wheat cultivars differing in susceptibility to Septoria tritici. Inoculations and evaluations were carried out at the fourth leaf stage for two years. A combined ANOVA for both years exhibited a significant interaction between crosses and years for maturation period. A separate ANOVA for each year was done for this trait. The combined ANOVA showed that crosses were significantly different for incubation, latent period and pycnidial coverage. Cross effects were also significant for maturation period in each year for the separate ANOVA. Year effects were significant for latent period and maturation period. General combining ability (GCA) was significant and preponderant for incubation period, latent period and pycnidial coverage. For maturation period, the separated ANOVA showed the significance of the GCA for both years. Specific combining ability (SCA) was significant for incubation and pycnidial coverage and for maturation in one year. The preponderance of the additive genetic variance for all traits indicates the possibility of selecting for them in order to obtain improved cultivars. Incubation period was inherited independently of maturation period and pycnidial coverage, indicating that combinations of some of those characters may lead to more effective and durable resistance.     

Molecular mapping of quantitative trait loci determining resistance to septoria tritici blotch caused by Mycosphaerella graminicola in wheat

A set of 65 recombinant inbred lines of the ‘International Triticeae Mapping Initiative’ mapping population (‘W7984’בOpata 85’) was analysed for resistance to septoria tritici blotch at the seedling and adult plant stages. The mapping population was inoculated with two Argentinean isolates (IPO 92067 and IPO 93014). At the seedling stage, three loci were discovered on the short arms of chromosomes 1D, 2D and 6B. All three loci were detected with both isolates. At the adult plant stage, two isolate-specific QTL were found. The loci specific for isolates IPO 92067 and IPO 93014 were mapped on the long arms of chromosomes 3D and 7B, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mapping quantitative trait loci associated with grain filling duration and grain number under terminal heat stress in bread wheat (Triticum aestivum L.)

Terminal heat stress has the potential negative impact on wheat production across the world, especially in South Asia. Under the threat of terminal heat stress, wheat genotypes with stay green trait would suffer from high temperature stress during their long grain filling duration (GFD). The genotypes with short GFD would be advantageous. To identify quantitative trait loci (QTL) for heat tolerance, a RIL population of K 7903 (heat tolerant) and RAJ 4014 (heat sensitive) wheat genotypes was investigated under timely and late‐sown conditions. Heat susceptibility index of GFD, yield components and traits under late‐sown condition were used as phenotypic data for QTL identification. Stable QTLs associated with these traits were identified on chromosomes 1B, 2B, 3B, 5A and 6B. The LOD value ranged from 2.9 to 5.0 and the corresponding phenotyping variation explained ranged from 12.0–22%. QTL for heat susceptibility index for the grain filling duration were colocalized with QTL for productive tillers under late sown and GFD under late‐sown condition on chromosomes 1B and 5A, respectively. These genomic regions could be exploited for molecular wheat breeding programmes targeting heat tolerance.     

Practical breeding strategies to improve resistance to Septoria tritici blotch of wheat

Septoria tritici blotch (STB), caused by fungal agent Zymoseptoria tritici (previously known as Mycosphaerella graminicola) is a devastative foliar wheat diseases globally. Importance and potential threat of STB have been discussed historically and geographically. This paper reviews information on the Z. tritici—wheat pathosystem and proposes approaches to identify resistance genes and to advance in breeding for STB resistance. Screening of resistant lines/cultivars, QTL mapping analysis within genetic populations derived from crosses, detection of new resistance gene(s) and finally application of Stb gene carrier line/cultivar in crosses are the major stages of a practical wheat-breeding program against STB of wheat. Phenotyping and genotyping outputs on the top of each other should confirm each other, so it needs to expose a resistance gene carrier line/cultivar in the epidemic condition at seedling/adult plant stage to confirm resistance performance of detected gene(s) in the real condition. On the other word, detecting an associated QTL to resistance should not be considered as the end of investigation. Climate change resulted geographical disease pattern conversion where some diseases became more important in some area where they had not been serious in the past and vice versa. Hence, a reconsideration of wheat disease importance zone is necessary to predict regions where STB is and will be a limitation for wheat yield improvement.     

Sources of resistance in wheat to speckled leaf blotch caused by Septoria tritici

Summary Seven and a half thousand wheat varieties, including over 2,000 durums, were screened for resistance to Septoria tritici under conditions of artificial inoculation in the field. From this screening, 460 varieties were selected on the basis of resistance and earliness and were subject to more detailed observation.Selected varieties were tested in the field with two replications under both natural and artificial epidemic conditions. The agreement between varietal scores in the two replications and under the two conditions was good; 91–94% of varieties showing consistent disease scores. The correlation coefficient between varietal scores for the natural epidemic and the artificial epidemic conditions was 0.63^***.A number of the very early maturing varieties of T. aestivum showed good resistance as measured by pycnidial production but exhibited extensive leaf necrosis. This reaction was not observed in later varieties and in accessions of T. durum.Varieties of Triticum durum were typically more resistant than varieties of T. aestivum. Thirty-four varieties of T. aestivum and 266 varieties of t. durum showed a consistent expression of resistance. A list of these 34 accessions of T. aestivum and selected accessions of T. durum is provided, along with agronomic data.     

Mapping of quantitative trait loci conferring resistance to bacterial wilt in tobacco (Nicotiana tabacum L.)

Tobacco bacterial wilt (TBW) is one of the most serious tobacco diseases in the world. Studies have shown that tobacco resistance to TBW is quantitatively inherited. This study aimed to map quantitative trait loci (QTL) conferring TBW resistance. An F_2 : 3 population containing 237 lines was developed from a cross between two flue‐cured tobacco cultivars, ‘Yanyan 97’ (YY97; moderately resistant to TBW) and ‘Honghua Dajinyuan’ (HD; highly susceptible to TBW), and a linkage map consisting of 201 simple sequence repeats (SSR) markers and spanning a total length of 2326.7 cM was constructed based on the population. Field experiments were conducted 2011 and 2012, and disease symptoms were investigated three times in each year. The phenotypic data were analysed either separately or jointly for QTL mapping using the software QTLNetwork 2.1. Eight QTL with significant main effects were mapped on chromosomes 2, 6, 12, 17 and 24. A major QTL (qBWR17a) was detected on chromosome 17, which explained up to 30% of the phenotypic variation. The results can facilitate marker‐assisted selection (MAS) in TBW resistance breeding programme.     

Mapping quantitative trait loci in wheat for resistance against greenbug and Russian wheat aphid

Breeding for genetic resistance against greenbug and Russian wheat aphid (RWA) is the most effective way of controlling these widespread pests in wheat. Earlier work had shown that chromosome 7D of a synthetic hexaploid wheat, ‘Synthetic’ (T. dicoccoides × Ae. squarrosa) (AABB × DD) gave resistance when transferred into the genetic background of an aphid‐susceptible cultivar, ‘Chinese Spring’, as the recipient. To map the genes involved, a set of 103 doubled haploid recombinant substitution lines was obtained from crossing the 7D substitution line with the recipient, and used to determine the number and chromosomal location of quantitative trait loci (QTL) controlling antixenosis and antibiosis types of resistance. Antixenosis to RWA was significantly associated with marker loci Xpsr687 on 7DS, and Xgwm437 on 7DL. Antibiosis to greenbug was associated with marker loci Xpsr490, Rc3 (on 7DS), Xgwm44, Xgwm111, Xgwm437, Xgwm121 and D67 (on 7DL). Similarly, antibiosis to RWA was linked to loci Xpsr490, Rc3, Xgwm44, Xgwm437 and Xgwm121. At least two QTL in repulsion phase, one close to the centromere either on the 7DS or 7DL arms, and a second distal on 7DL could explain antibiosis to RWA and, partially, this mechanism against greenbug.     

Evaluation of the effects of five QTL regions on Fusarium head blight resistance and agronomic traits in spring wheat (Triticum aestivum L.)

Fusarium head blight (FHB) is an important disease of wheat (Triticum aestivum L.). The aim of this study was to determine the effects of quantitative trait locus (QTL) regions for resistance to FHB and estimate their effects on reducing FHB damage to wheat in Hokkaido, northern Japan. We examined 233 F₁-derived doubled-haploid (DH) lines from a cross between ‘Kukeiharu 14’ and ‘Sumai 3’ to determine their reaction to FHB during two seasons under field conditions. The DH lines were genotyped at five known FHB-resistance QTL regions (on chromosomes 3BS, 5AS, 6BS, 2DL and 4BS) by using SSR markers. ‘Sumai 3’ alleles at the QTLs at 3BS and 5AS effectively reduced FHB damage in the environment of Hokkaido, indicating that these QTLs will be useful for breeding spring wheat cultivars suitable for Hokkaido. Some of the QTL regions influenced agronomic traits: ‘Sumai 3’ alleles at the 4BS and 5AS QTLs significantly increased stem length and spike length, that at the 2DL QTL significantly decreased grain weight, and that at the 6BS QTL significantly delayed heading, indicating pleiotropic or linkage effects between these agronomic traits and FHB resistance.     

Chromosomal location of resistance to Septoria tritici in seedlings of a synthetic hexaploid wheat, Triticum spelta and two cultivars of Triticum aestivum

Chromosomal location of resistance to two virulent Argentinean isolatesof Septoria tritici was studied in two wheat (Triticum aestivumL.) cultivars (Cappelle-Desprez & Cheyenne), a synthetic hexaploid(Synthetic 6x) and Triticum spelta in seedlings. Substitution lines of these(resistant or moderately resistant) genotypes into (susceptible) ChineseSpring were selected from a previous screening. For Synthetic 6x,resistance was clearly located in chromosome 7D. Chinese Spring with the7D chromosome substituted by Synthetic 6x showed almost completeresistance, similar to the level of Synthetic 6x. For the substitutions withCappelle-Desprez, Cheyenne, and T.spelta there were no lines with abehaviour similar to the resistant parent. However, some substitutions weremore resistant than the susceptible parent suggesting that severalchromosomes could be involved in the resistance of these genotypes toSeptoria leaf blotch.     

Reciprocal substitutions analysis of freezing resistance in wheat (Triticum aestivum L.)

To investigate the effects of individual chromosomes on freezing resistance, as well as their interactions with the genetic background, reciprocal sets of chromosome substitution lines between two hard red winter wheat cultivars, ‘Cheyenne’ and ‘Wichita’, were used in this study. Duplicate lines for each chromosome were included to check background homogeneity. Two experiments were carried out in complete block designs with two replications for each duplicate. Crown and leaf water content and leaf wet weight were measured in the field experiments. Crown survival, electrolyte leakage and 50% lethality temperature (LT50) were measured in the laboratory. The results showed that ‘Cheyenne’ was more resistant than ‘Wichita’. Crown survival was significantly correlated with crown water content, crown wet weight and electrolyte leakage. Chromosomes 6A, 3B and 5D substituted from ‘Wichita’ into ‘Cheyenne’ (‘CNN‐WI’), decreased the crown survival, and increased membrane stability, crown water content and crown wet weight of ‘Cheyenne’. Thus, these chromosomes from ‘Wichita’ decreased freezing resistance in ‘Cheyenne’. Reciprocally, chromosomes 5A, 5D, 3B and 4D from ‘Cheyenne’ into ‘Wichita’ increased crown survival and decreased crown water content and crown wet weight of ‘Wichita’. It was concluded that these chromosomes from ‘Cheyenne’ cause freezing resistance in ‘Wichita’ and carry freezing‐resistance genes.     

Mapping quantitative trait loci for yield-related traits in soybean (Glycine max L.)

Development of soybean cultivars with high seed yield is a major focus in soybean breeding programs. This study was conducted to identify genetic loci associated with seed yield-related traits in soybean and also to clarify consistency of the detected QTLs with QTLs found by previous researchers. A population of 135 F_2:3 lines was developed from a cross between a vegetable soybean line (MJ0004-6) and a landrace cultivar from Myanmar ({"type":"entrez-nucleotide","attrs":{"text":"R18500","term_id":"772110","term_text":"R18500"}}R18500). They were evaluated in the experimental field of Kasetsart University, Kamphaeng Saen, Nakhon Pathom, Thailand in a randomized complete block design with two replications each in 2011 and 2012 growing seasons. The two parents exhibited contrasting characteristics for most of the traits that were mapped. Analysis of variance showed that the main effects of genotype and environment (year) were significant for all studied traits. Genotype by environment interaction was also highly significant for all the traits. The population was genotyped by 149 polymorphic SSR markers and the genetic map consisted of 129 SSR loci which converged into 38 linkage groups covering 1156 cM of soybean genome. There were 10 QTLs significantly associated with seed yield-related traits across two seasons with single QTLs explaining between 5.0% to 21.9% of the phenotypic variation. Three of these QTLs were detected in both years for days to flowering, days to maturity and 100 seed weight. Most of the detected QTLs in our research were consistent with earlier QTLs reported by previous researchers. However, four novel QTLs including SF1, SF2 and SF3 on linkage groups L and N for seed filling period and PN1 on linkage group D1b for pod number were identified in the present study.     

Identification of quantitative trait loci (QTLs) associated with maintenance of wheat (Triticum aestivum Desf.) quality characteristics under heat stress conditions

The goal of this study was to identify quantitative trait loci (QTLs) associated with the maintenance of wheat grain quality following post-anthesis heat stress in a recombinant inbred line (RIL) population. The response to heat stress was measured using the sodium dodecyl sulfate sedimentation test (SDSS), a significant predictor of bread baking quality. SDSS scores were used to identify QTLs associated with grain quality and QTLs associated with quality stability were identified based on percent change in SDSS score between the heat stress and control treatments. Four QTLs were identified, located one each on linkage groups 1B, 1D, 4A, and 7A. The 1B, 1D, and 4A QTLs were associated with grain quality; the QTL on linkage group 7A was associated with quality stability. To confirm the detected QTLs, eighty advanced lines grown at three Texas nurseries were tested for relationships between allelic polymorphism at QTL linked markers and quality traits. Quality trait stability in the advanced lines was estimated using the coefficient of variability (CV%) of quality traits between nurseries. The analysis supported the relationship of the predicted QTLs on linkage groups 1B, 1D, and 4A with quality traits. The confirmed QTLs may be used in marker assisted selection (MAS) to develop wheat lines possessing superior quality traits. In addition, identification of genetic regions associated with this trait will aid the identification of the underlying genes.     

Mapping QTLs for potassium-deficiency tolerance at the seedling stage in wheat (Triticum aestivum L.)

Potassium (K) deficiency is a major factor limiting crop development and yield. In this study, 20 traits were investigated at the seedling stage using a doubled-haploid population subjected to normal K and K deprivation treatments. Evaluation of phenotypes expressed under the two K supply conditions revealed that K deprivation was able to decrease values of ten measured traits, whereas values of the other ten traits increased. A total of 65 QTLs were detected across all 21 chromosomes, except for chromosomes 2B, 5A, and 7B. Individual QTLs in the two K supply treatments explained between 5.35 and 39.64 % of the phenotypic variation. Nine QTL clusters (C1–C9) involving 34 QTLs under the different K supply treatments were mapped to chromosomes 1A, 1B, 1D, 3B, 4A, 5B, 6A, and 7B. Our results are helpful for understanding the genetic basis of K deficiency in wheat, and provide useful information for genetic improvement of K deficiency in wheat by marker-assisted selection.     

Inheritance of resistance to Karnal bunt (Tilletia indica Mitra) in bread wheat (Triticum aestivum L.)

The mode of inheritance and allelic relationships among genes conferring resistance to Karnal bunt were studied in seven bread-wheat (six resistant and one susceptible) genotypes. The resistant genotypes originated in China (‘Shanghai#8’), Brazil (PF71131), the USA (‘Chris’), and Mexico (‘Amsel’, CMH77.308 and ‘Pigeon’). The susceptible line WL711 was from India. Evaluation of these wheat lines and all possible crosses among their F₁ and F₃ generations (about 100 progenies in each cross) revealed that two partially recessive genes conferred the resistance to Karnal bunt in ‘Pigeon’, whereas four partially dominant genes were present in the other genotypes. ‘Chris’, ‘Amsel’ and PF71131 carry one gene, whereas ‘Shanghai#8’ and CMH77.308 have two genes. ‘Chris’, ‘Amsel’, and PF71131 have different genes, whereas one gene was common to PF71131, CMH77.308 and ‘Shanghai#8’, and another to ‘Chris’ and CMH77.308. Gene symbols were formally designated to the resistant stocks. Resistance was incomplete and stable.