Genetic mapping and candidate gene identification for key physiological traits associated with heat tolerance in wheat (Triticum aestivum L.) using a MAGIC population

BackgroundHigh temperature (heat) stress during grain filling is a major problem in most of the wheat growing areas. Developing heat tolerant cultivars has become a principal breeding goal in the Southern and Central Great Plain areas of the USA. Traits associated with high temperature tolerance can be used to develop heat tolerant cultivars in wheat. The present study was conducted to identify chromosomal regions associated with thylakoid membrane damage (TMD), plasmamembrane damage (PMD), and SPAD chlorophyll content (SCC), which are indicative of high temperature tolerance.ResultsIn this study we have reported one of the first linkage maps in wheat using genotype by sequencing SNP (GBS-SNP) markers to extreme response to post anthesis heat stress conditions. The linkage map was comprised of 972 molecular markers (538 Bin, 258 AFLPs, 175 SSRs, and an EST). The genotypes of the RIL population showed strong variation for TMD, SCC and PMD in both generations (F10 and F9). Composite interval mapping identified five QTL regions significantly associated with response to heat stress. Associations were identified for PMD on chromosomes 7A, 2B and 1D, SCC on 6A, 7A, 1B and 1D and TMD on 6A, 7A and 1D. The variability (R2) explained by these QTL ranged from 11.9 to 30.6% for TMD, 11.4 to 30.8% for SCC, and 10.5 to 33.5% for PMD. Molecular markers Xbarc113 and AFLP AGCTCG-347 on chromosome 6A, Xbarc121 and Xbarc49 on 7A, gwm18 and Bin1130 on 1B, Bin178 and Bin81 on 2B and Bin747 and Bin1546 on 1D were associated with these QTL.ConclusionThe identified QTL can be used for marker assisted selection in breeding wheat for improved heat tolerance in Ventnor or Karl 92 genetic background.Electronic supplementary materialThe online version of this article (doi:10.1186/s12863-014-0097-4) contains supplementary material, which is available to authorized users.

Heat stress during grain filling substantially decreases wheat productivity; thus, to ensure food security, heat tolerance in wheat needs to be developed. In this study, we evaluated the effect of heat priming applied during the stem-elongation stage, booting and anthesis, followed by 5 days of severe heat stress (a 7.86°C rise in temperature) during the grain-filling stage on physiological activities and grain yield of winter wheat in pot experiments during the 2015-2017 growing seasons using the winter wheat cultivars Yangmai 18 (a vernal type) and Yannong 19 (a facultative type). Compared with the damage observed in non-primed plants, heat priming during the stem-elongation stage and booting significantly prevented the grain-yield damage caused by heat stress during grain filling. Heat-primed plants displayed higher sucrose contents and sucrose-phosphate activity in leaves and greater above-ground dry matter than non-primed plants. Priming during stem elongation and booting led to increased photosynthetic capacity, stomatal conductance and chlorophyll contents in comparison with non-priming. Improved tolerance to heat stress due to the enhanced activities of antioxidant enzymes superoxide dismutase and peroxidase and reductions in reactive oxygen species and malondialdehyde production was observed in primed plants compared with non-primed plants of both cultivars. The positive effect of heat priming on the response to heat stress during grain filling was more pronounced in plants primed at the booting stage than in those primed at the stem-elongation or anthesis stage. Moreover, the vernal-type Yangmai 18 benefited more from heat priming than did Yannong 19, as evidenced by its higher productivity. We conclude that heat priming during early reproductive-stage growth can improve post-anthesis heat tolerance in winter wheat.

Identification of marker trait association is a prerequisite for marker-assisted breeding. To find markers linked with traits under heat and drought stress in bread wheat (Triticum aestivum L.), we performed a genome-wide association study (GWAS). GWAS mapping panel used in this study consists of advanced breeding lines from the IARI stress breeding programme produced by pairwise and complex crosses. Phenotyping was done at multi locations namely New Delhi, Karnal, Indore, Jharkhand and Pune with augmented-RCBD design under different moisture and heat stress regimes, namely timely sown irrigated (IR), timely sown restricted irrigated (RI) and late sown (LS) conditions. Yield and its component traits, viz., Days to Heading (DH), Days to Maturity (DM), Normalized Difference Vegetation Index (NDVI), Chlorophyll Content (SPAD), Canopy temperature (CT), Plant Height (PH), Thousand grain weight (TGW), Grain weight per spike (GWPS), Plot Yield (PLTY) and Biomass (BMS) were phenotyped. Analysis of variance and descriptive statistics revealed significant differences among the studied traits. Genotyping was done using the 35k SNP Wheat Breeder's Genotyping Array. Population structure and diversity analysis using filtered 10,546 markers revealed two subpopulations with sufficient diversity. A large whole genome LD block size of 7.15 MB was obtained at half LD decay value. Genome-wide association search identified 57 unique markers associated with various traits across the locations. Twenty-three markers were identified to be stable, among them nine pleiotropic markers were also identified. In silico search of the identified markers against the IWGSC ref genome revealed the presence of a majority of the SNPs at or near the gene coding region. These SNPs can be used for marker-assisted transfer of genes/QTLs after validation to develop climate-resilient cultivars.

Twelve accessions including two diploid, four tetraploid and six hexaploid genotypes were used in this study to assess heat tolerance in wheat. All possible crosses were made and the resulted hybrids with their parents were subsequently evaluated for heat tolerance in two sowing dates. Evaluation was based on 1000 grain weight, grain yield per plant, cell membrane thermostability and tetrazolium chloride reduction. Results revealed significant variability among wheat genotypes and hybrids in all evaluated traits under favorable and heat stress conditions. However, tetraploid genotypes showed the highest performance under heat stress followed by hexaploid and diploid genotypes, respectively. According to results of heat tolerance index, one tolerant and one susceptible parent from each ploidy level along with 6 related interspecific hybrids were chosen to be involved in molecular analysis. Target region amplification polymorphism (TRAP) markers were used based on fixed primers of heat shock protein genes. TRAP clearly confirmed the morpho-physiological findings and generated a dendrogram which was quietly like that of morpho-physiological data. TRAP was able to generate one and five specific bands for diploid and hexaploid genotypes respectively, while no specific bands were generated for tetraploid. In addition, the different genomes showed some shared bands between each other revealing their relationship. Interestingly, two specific bands for tolerant genotypes of different ploidy level were generated which were absent in all susceptible genotypes. Findings herein are of high importance and could help in successive breeding programs for wheat improvement.

The grain-filling rate (GFR) plays an important role in determining grain yield. An F2 population of wheat was developed from a cross between the 2 wheat cultivars, Ksu106 (heat-tolerant) and Yecora Rojo (heat-sensitive). The parents and 205 F 2 plants were planted on the 20 th of January during the winter season of 2009 to evaluate heat tolerance during the grain-filling period. The sowing date in the present investigation represents the heat stress conditions in Saudi Arabia. Bulked – segregant analyses (BSA) was used in conjunction with simple sequence repeats (SSR) analysis to find markers linked to genes of heat tolerance. Composite interval mapping was used for mapping quantitative trait loci (QTL). The results reveal that 12 SSR markers: Wmc24, Wmc168, Wmc326, Xgwm30, Xgwm456, Wmc25, Wmc44, Wmc94, Wmc161, Wmc273, Wmc327 and Xgwm566 were linked to GFR by QTLs analysis of the F 2 population. The results show that regression analysis for the relationship between the 12 markers and the phenotypes of F 2 individuals were highly significant. The results demonstrate that SSR markers combined with bulked segregant analysis could be used to identify molecular markers linked to the grain filling rate as an indicator for heat tolerance in wheat. Keywords: Grain filling rate, QTL analysis, SSR marker, wheat

Elevated temperatures during grain filling stage, exceeding the optimal range by 3-4°C, not only results in a substantial yield reduction in wheat by 10-50% but activates disease and insect infestation. In this research, we introduced heat-tolerant MYB36 and APX-1 gene cassettes into wheat, employing an efficient Agrobacterium mediated transformation protocol, demonstrating higher transformation efficiency. The study encompassed the assembly of MYB36 and APX-1 gene cassettes, and confirmation of gene products in Agrobacterium, followed by the transformation of the MYB36 and APX-1 genes into wheat explants. We were able to select transgenic plant with various combinations. The transgenic plants with APX-1 gene alone produced medium sized grain and spike whereas with both APX-1 and MYB36 genes expressed individually under SPS and rd29a promoter respectively showed good tolerance to heat at 32oC at grain filling/milking stage and produced relatively bold grains. While non-transgenic plants grains were wrinkled with thin spike showing susceptibility to heat. This research contributes to the broader scientific understanding of plant stress responses and the combined effectiveness of MYB36 and APX-1 genes in crop improvement without disturbing normal nutritional values. The gene integration can serve as a valuable tool in breeding programs aimed at developing heat-tolerant wheat varieties. These findings also advance our comprehension of the functions of heat-induced genes and lay the foundation for selecting optimal candidates for in-depth functional studies of heat-responsive MYB36 and APX-1 genes in wheat.

Drought and heat tolerance tests that were developed for sorghum (Sorghum bicolor L. Moench) were adapted to and evaluated in field grown wheat (Triticum aestivum L. and T. durum Desf.) during 1977/1978 and 1978/1979.The drought tolerance test is based on the measurement of the electroconductivity of aqueous media containing leaf discs that were previously water stressed in vitro by exposure to a solution of polyethylene glycol‐6,000 (PEG). The heat tolerance test is similarly based on exposure of leaf discs to heating, in vitro, to 44 C.Drought tolerance of wheat leaves decreased with plant age. For a given plant growth stage, some variation was revealed in drought tolerance, according to leaf position. Maximal separation of wheat cultivars in drought tolerance was obtained with 40% (w/v) solution of PEG, when plants were grown under conditions of favorable moisture regime and sampled during the late jointing growth stage.Wheat was more drought tolerant than maize, sorghum or millet, based on published data.When plants were sampled during a period of water stress, they were more drought tolerant than well‐watered plants, indicating adjustment of cell membrane stability to drought stress. Wheat cultivars varied in their ability to adjust, in this respect.Unlike in sorghum, drought and heat tolerance were not correlated in wheat.The inclusion of a limited number of barley (Hordeum vulgare) and triticale (X Triticosecale Wittmark) genotypes in this study indicated that the methods discussed work equally well with these crop plants.

Key message This study integrated RNA-seq and WGCNA to identify TaBI-1 as a key thermotolerance gene in wheat, with preliminary functional validation through yeast assays and nano-mediated transient expression. Heat stress severely limits global wheat production. To identify key thermotolerance genes, RNA-seq was performed on wheat flag leaves and grains at the grain-filling stage under 37 ℃ treatment with five time points (0, 0.5h, 1 h, 2 h and 4 h). Differential expression analysis revealed 16,228 and 5,480 differentially expressed genes (DEGs) in flag leaves and grains, respectively. Weighted gene co-expression network analysis (WGCNA) identified tissue-specific modules correlated with early heat stress (0.5h and 1 h), yielding 922 and 454 core genes, including the previously reported genes TaMBF1c and TaHSFA6e . Gene Ontology enrichment indicated these genes are involved in "response to heat", "protein folding", and "response to reactive oxygen species". Integrating DEGs, core genes, and prior data, we selected three candidate genes: TaCPD-1 , TaHSF-1 , and TaBI-1 , with TaMBF1c employed as the positive control in the experiment. Yeast assays showed that TaMBF1c , TaBI-1 , and TaCPD-1 overexpression enhanced thermotolerance under 50 ℃. Transient expression in wheat via nano-carriers confirmed that TaMBF1 c and TaBI-1 overexpression improve heat tolerance under 42 ℃, outperforming wild-type and control plants. Our results demonstrate that TaMBF1c and TaBI-1 enhance thermotolerance in both heterologous and native systems. This study provides a theoretical basis for elucidating the molecular mechanisms of heat stress responses in wheat and for cultivating heat-tolerant wheat varieties.

Two varieties of wheat differing in heat tolerance were exposed to very high temperature (40/19°C day/night) for periods of 1–10 days duration. Responses of grain dry matter, water and fractional protein accumulation to high temperature were monitored throughout grain filling in the heat- sensitive variety, and at maturity only in the heat-tolerant variety. Results are compared with controls maintained at 21/16°C day/night. As little as 1 day of heat treatment reduced kernel mass by 14% in the heat-sensitive variety (Oxley), but by only 5% in the heat-tolerant variety (Egret). In both varieties, the reduction of individual kernel mass due to high temperature increased linearly with increased duration of heat treatment, such that after the first day of heat stress, each additional day of treatment reduced mature individual kernel mass by a further 1.6%. For a given duration of heat treatment, the difference in response of the two varieties was constant (9%), indicating that the varietal difference in heat tolerance was maintained for both brief and extended periods of very high temperature. Responses of grain water content and fractional protein accumulation to duration of heat stress are discussed.

Farmers in northwestern and central India have been exploring to sow their wheat much earlier (October) than normal (November) to sustain productivity by escaping terminal heat stress and to utilize the available soil moisture after the harvesting of rice crop. However, current popular varieties are poorly adapted to early sowing due to the exposure of juvenile plants to the warmer temperatures in the month of October and early November. Therefore, a study was undertaken to identify wheat genotypes suited to October sowing under warmer temperatures in India. A diverse collection of 3322 bread wheat varieties and elite lines was prepared in CIMMYT, Mexico, and planted in the 3rd week of October during the crop season 2012–2013 in six locations (Ludhiana, Karnal, New Delhi, Indore, Pune and Dharwad) spread over northwestern plains zone (NWPZ) and central and Peninsular zone (CZ and PZ; designated as CPZ) of India. Agronomic traits data from the seedling stage to maturity were recorded. Results indicated substantial diversity for yield and yield-associated traits, with some lines showing indications of higher yields under October sowing. Based on agronomic performance and disease resistance, the top 48 lines (and two local checks) were identified and planted in the next crop season (2013–2014) in a replicated trial in all six locations under October sowing (third week). High yielding lines that could tolerate higher temperature in October sowing were identified for both zones; however, performance for grain yield was more promising in the NWPZ. Hence, a new trial of 30 lines was planted only in NWPZ under October sowing. Lines showing significantly superior yield over the best check and the most popular cultivars in the zone were identified. The study suggested that agronomically superior wheat varieties with early heat tolerance can be obtained that can provide yield up to 8 t/ha by planting in the third to fourth week of October.

Globally, insects have been impacted by climate change, with bumble bees in particular showing range shifts and declining species diversity with global warming. This suggests heat tolerance is a likely factor limiting the distribution and success of these bees. Studies have shown high intraspecific variance in bumble bee thermal tolerance, suggesting biological and environmental factors may be impacting heat resilience. Understanding these factors is important for assessing vulnerability and finding environmental solutions to mitigate effects of climate change. In this study, we assess whether geographic range variation in bumble bees in the eastern United States is associated with heat tolerance and further dissect which other biological and environmental factors explain variation in heat sensitivity in these bees. We examine heat tolerance by caste, sex, and rearing condition (wild/lab) across six eastern US bumble bee species, and assess the role of age, reproductive status, body size, and interactive effects of humidity and temperature on thermal tolerance in Bombus impatiens. We found marked differences in heat tolerance by species that correlate with each species' latitudinal range, habitat, and climatic niche, and we found significant variation in thermal sensitivity by caste and sex. Queens had considerably lower heat tolerance than workers and males, with greater tolerance when queens would first be leaving their natal nest, and lower tolerance after ovary activation. Wild bees tended to have higher heat tolerance than lab reared bees, and body size was associated with heat tolerance only in wild‐caught foragers. Humidity showed a strong interaction with heat effects, pointing to the need to regulate relative humidity in thermal assays and consider its role in nature. Altogether, we found most tested biological conditions impact thermal tolerance and highlight the stages of these bees that will be most sensitive to future climate change.

Heatwaves and soil droughts are increasing in frequency and intensity, leading many tree species to exceed their thermal thresholds, and driving wide-scale forest mortality. Therefore, investigating heat tolerance and canopy temperature regulation mechanisms is essential to understanding and predicting tree vulnerability to hot droughts. We measured the diurnal and seasonal variation in leaf water potential (Ψ), gas exchange (photosynthesis Anet and stomatal conductance gs), canopy temperature (Tcan), and heat tolerance (leaf critical temperature Tcrit and thermal safety margins TSM, i.e., the difference between maximum Tcan and Tcrit) in three oak species in forests along a latitudinal gradient (Quercus petraea in Switzerland, Quercus ilex in France, and Quercus coccifera in Spain) throughout the growing season. Gas exchange and Ψ of all species were strongly reduced by increased air temperature (Tair) and soil drying, resulting in stomatal closure and inhibition of photosynthesis in Q. ilex and Q. coccifera when Tair surpassed 30°C and soil moisture dropped below 14%. Across all seasons, Tcan was mainly above Tair but increased strongly (up to 10°C > Tair) when Anet was null or negative. Although trees endured extreme Tair (up to 42°C), positive TSM were maintained during the growing season due to high Tcrit in all species (average Tcrit of 54.7°C) and possibly stomatal decoupling (i.e., Anet ≤0 while gs >0). Indeed, Q. ilex and Q. coccifera trees maintained low but positive gs (despite null Anet), decreasing Ψ passed embolism thresholds. This may have prevented Tcan from rising above Tcrit during extreme heat. Overall, our work highlighted that the mechanisms behind heat tolerance and leaf temperature regulation in oak trees include a combination of high evaporative cooling, large heat tolerance limits, and stomatal decoupling. These processes must be considered to accurately predict plant damages, survival, and mortality during extreme heatwaves.

Global warming is causing changes in tempera- ture rapidly for over two decades. The increased temperature during reproductive phase of plant growth has emerged as a serious problem all over the world. Constant or transitory high temperatures may affect the plant growth and develop- ment which may lead to diverse morphological, physiolog- ical and biochemical changes in plants ultimately decrease in yield. Genetic approaches leading to improved thermo- tolerance can mitigate the reduction in yield. In this back- drop, several indirect traits or parameters have been devel- oped for identification of heat tolerant plants/lines. The traits like stay green/delayed senescence are reported to contribute toward capability of plants to tolerate heat stress. In addition, understanding of biochemical and molecular basis of thermo-tolerance in combination with genetic approaches like identification and mapping of heat tolerant QTLs will not only assist conventional breeders to develop heat tolerant cultivars but also help molecular biologists to clone and characterize genes associated with heat tolerance, which could be used in genetically modified heat tolerant plants. Therefore, overviews of different strategies for developing heat tolerant wheat are discussed in this review.

Molecular mapping of high temperature tolerance in bread wheat adapted to the Eastern Gangetic Plain region of India

IPCC has predicted increase in drought and heat affected areas under wheat. To combat this challenge, development of heat and drought tolerant wheat genotypes is on priority. Landraces are unexploited genetic resources for various agronomic traits contributing tolerance to abiotic stress. Twenty-one wheat genotypes were evaluated in irrigated timely, rainfed timely and irrigated late field conditions for two years for their response to drought and heat stress. Genotypes differed significantly in their response to 1000-grain weight under stress conditions. Stress susceptibility index (SSI) was used to identify drought and heat tolerant genotypes. Nine genotypes were tolerant to drought stress and 11 to heat stress. Six of these genotypes were tolerant to both the stresses. SSR and allele specific markers were used to validate the results. Allelic variation for heat tolerance was observed in two genotypes IC 36761A and IC 41504 with gwm 63. Similarly, SNP marker for distinguishing drought tolerance in bread wheat was used and bi-allelic variation with this primer was observed in genotypes IC 57586, IC 30276A, IC 36761A and IC 31405B. IC 36761A was tolerant to both the stresses and was differentiated using molecular markers also. This genotype can be used for breeding drought and heat tolerant cultivars.