Heat Tolerant Lines Research Articles

CROP ADAPTATION TO CLIMATE CHANGE: High-Temperature Stress in Drought-Prone Areas

The papers presented in this special issue are focused on the development of mutant lines and new crop varieties of rice and common bean with improved adaptation to climate change. Also included are the development or adaptation of screening techniques that enable efficient selection of desired phenotypes in plant breeding programmes. The breeding methods used are based on mutation induction and mutation detection, where mutation induction is via physical mutagenesis using gamma and X-ray irradiation. Selection for improved mutant lines is achieved through screening for plant performance in the laboratory, green house and field under hight temperature and/or drought stress. The papers presented are the result of a 5-year coordinated research project (CRP) on Climate Proofing Crops: Genetic Improvement for Adaptation to High Temperatures in Drought Prone Areas and Beyond, funded by the IAEA. The CRP, initiated in 2011, focused on tolerance to increased temperatures in rice and common bean. All participating countries generated new mutant populations in rice and bean and identified heat-tolerant lines with better yields than local standard varieties

Evaluation of some Rice Genotypes (Oryza sativa L.) Under Normal and Heat Stress Conditions تقييم تراکيب وراثية مختلفه من الأرز تحت الظروف الطبيعية والإجهاد الحراري

The aim of the current study was to evaluate 25 rice genotypes under normal at Sakha, Kafr El-Sheikh, Egypt and under heat stress conditions at New Valley, Egypt during two rice growing seasons of 2017 and 2018. Data were collected on eight traits i.e., flag leaf area, flag leaf angle, plant height, panicle length, panicles plant-1, 1000-grain weight, fertility percentage and grain yield plant-1. High amount of genetic variation was found among the studied genotypes under normal and heat stress conditions. All studied genotypes were negatively affected by high temperature. The most affected traits by heat stress were fertility percentage, grain yield plant-1, flag leaf area and panicles plant-1 with average reductions of 73.59, 64.22, 42.46 and 41.31%, respectively. Correlation and path coefficient analysis showed that number of panicles plant-1 and fertility percentage were good indicators for grain yield under heat stress condition. Based on the HSI values of grain yield plant-1 and fertility percentage, both genotypes N22 and Giza178 could be identified as heat tolerant (HSI value was ≤0.5 for both traits). Based on the results of PCA analysis, the most desirable genotypes under heat stress were Giza178, N22, Bala, Dular, WAB56-104, Sakha104 and Egyptian Yasmin. According to the current investigation, it could be concluded that those eight genotypes could be used in breeding programs as donors for developing high yielding heat tolerant promising lines. In addition, Egyptian rice varieties; Giza178, Sakha104 and Egyptian Yasmin are suitable for cultivating under both normal and heat stress conditions in Egypt.

Resistance Response of Drought and Heat Tolerant Spring Wheat Lines against the Cereal Cyst Nematode Heterodera filipjevi

Wheat (Triticum aestivum L.) is extremely affected by several abiotic and biotic stresses. Drought and/or heat alongside the parasitism of cereal cyst nematodes of the Heterodera genera can have a combined destructive impact on wheat. Solely, the cereal cyst nematode species Heterodera filipjevi can cause wheat yield losses of up to 50%. Several control measures have been implemented, yet, the most economical and convenient control strategy is the use of resistant hosts. Therefore, the main aim of this study was to evaluate the resistant response of 257 spring wheat lines obtained from CIMMYT to Heterodera filipjevi that might contain novel sources of resistance and be added as genetic resources for future breeding programs. Also, provide a base for future research to understand the relationship between nematode resistances and drought and heat tolerance. The results indicated that 11 lines (4%) and 36 lines (14%) were resistant and moderately resistant, respectively. High frequency of susceptible and highly susceptible lines and low frequency of resistant lines within this set were also recorded. The linear regression analysis between the number of cysts formed and the resistance response grouping showed a strong, positive, linear correlation. Log-linear regression analysis showed that there is a weak positive correlation between the yield of heat tolerant wheat lines and their resistance to the cyst nematodes as these lines showed tolerance, while there was a weak negative correlation of formed cyst nematodes on the yield of drought tolerant lines. This study was able to add new genetic sources of resistance to Heterodera filipjevi for upcoming breeding programs.

Histone acetylation of 45S rDNA correlates with disrupted nucleolar organization during heat stress response in Zea mays L.

The role of the nucleolus in plant response to heat stress remains largely obscure. Our current efforts focused on exploring the underlying mechanism by which nucleolar disorganization is regulated in heat stressed-maize lines. Here, two maize lines, a heat-sensitive line, ZD958, and a heat-tolerant line, ZDH, were submitted to heat stress for investigating their association with the nucleolar disruption. Immunofluorescence staining showed that nucleolar disruption increased with prolonged treatment time. After heat treatment, a significant change in nucleolus organization was observed in the ZD958 line, but the ZDH line showed mild alteration. Moreover, actinomycin D (ActD)-induced nucleolus fission led to inhibition of maize growth under the normal condition. The ZD958 line exhibited a significant increase in the level of H3K9ac and H4K5ac of the 45S rDNA accompanied by a higher transcription of the 5'-external transcribed spacer (ETS) region, while the line ZDH showed a slight increase in histone acetylation levels and the transcriptional initiation at this site after heat treatment. To our knowledge, this is the first report providing a comparative insight between heat stress, rDNA histone modifications, and nucleolus disintegration in a heat-tolerant ZDH compared with a heat-sensitive line ZD958. Our investigation might assist maize breeders in obtaining heat-tolerant lines by targeting nucleoli using epigenetics.

Development of Heat Tolerant Two-Line Hybrid Rice Restorer Line Carrying Dominant Locus of OsHTAS

In order to create novel germplasm resources for breeding heat tolerant variety, we transferred a dominant allele OsHTAS, previously characterized and cloned from a high-temperature tolerant local variety HT54, which was collected from the rice production area of southern China, into a high- temperature sensitive intermediate breeding line HT13 through six rounds of successive backcross by using marker-assisted selection. The molecular analysis showed that the recovery of genetic background of a resultant near isogenic line (NIL), MHT13, was around 99.8%. The OsHTAS gene introduced in the MHT13 expressed normally in the HT13 genetic background, mediating heat tolerance and phenotype similar to those of the donor parent HT54. The major agronomic traits of MHT13 resembled those of the recurrent parent HT13. Moreover, MHT13 had high general combining ability and its rice quality reached the grade 3 standard of edible high-quality rice issued by Ministry of Agriculture of the People's Republic of China, which greatly improved its application value in rice production.

Molecular characterization of heat tolerance in eggplant (Solanum melongena) lines using SSR markers

High temperature is one of the most important environmental stresses causing heavy reduction in yield of eggplant (Solanum melongena L.) in India. Therefore, development of heat tolerant eggplant variety is an important breeding objective. The present investigation was conducted to assess genetic variation among 62 eggplant lines including heat tolerant and susceptible lines in order to identify novel heat tolerant lines in changing climate in India. We employed 15 simple sequence repeat (SSR) markers to study polymorphisms among eggplant lines obtained from eggplant breeding program. The observations were analyzed using Power Marker version 3.5. The range of amplified number of bands was from 2 to 3 in each of 15 markers. The allele frequency was ranged from 0.5 to 0.7931 with an average of 2.067 alleles per locus. Among the 15 polymorphic SSR markers, 67% markers had PIC value higher than 0.3 with heterozygosity of individual loci was ranged from 0.00 to 0.1667. The marker emf01C03 had higher number of allele (3) and the marker emd03C01 had maximum PIC value and gene diversity. The NJ dendrogram obtained by 15 SSR markers divides 62 lines into 7 clusters where the cluster VII comprised of most heat tolerant lines (DBL-21, Guhala Chatua Local). The amplification pattern of heat tolerant lines also differ which implies different genetic architecture of heat tolerant lines. The research work will be very useful to combine these diverse sources of genetic variability to develop heat tolerant variety (s) in eggplant.

Avenues to realize potential of phenomics to accelerate crop breeding for heat tolerance

High-temperature stress is one of the significant abiotic stresses that reduce crop yields across the world. Heat stress is more prevalent in arid and semiarid regions of the tropics, and its occurrence has become more frequent in the subtropical areas. However, concerns related to this stress are significant due to the predicted rise in ambient temperatures due to global warming. It necessitates renewed phenotyping methods and crop breeding strategies to develop high-temperature tolerant crop cultivars. These strategies have a higher chance of success if the trait-based selection approach is implemented to achieve higher productivity under hotter environments. Hence, trait identification and phenotyping for key traits will play a crucial role in breeding programmes aiming at developing heat-tolerant crops. Although the concept has been around for decades, trait-based breeding has always been a challenge as screening large number genotypes for traits of interest is laborious and time-intensive. However, recent advances in phenomics have opened up new avenues to address this bottleneck efficiently and rapidly. It is attributed to the potential of phenomics tools to capture temporal and spatial changes in morphology and physiology and then related to the biochemistry of plants. These changes can provide clues about useful traits that can be used for selection of heat-tolerant lines in breeding programs. For this purpose, however, intensified efforts are needed to translate existing knowledge of mechanisms underlying heat tolerance into heritable traits and also into protocols for high throughput screening. In this regard, this review attempts to summarize the current status of breeding efforts to improve heat tolerance in crop plants and avenues for employing phenomic tools.

QTL-Seq identifies quantitative trait loci of relative electrical conductivity associated with heat tolerance in bottle gourd (Lagenaria siceraria)

Heat is a major abiotic stress that seriously affects watermelon (Citrullus lanatus) production. However, its effects may be mitigated through grafting watermelon to heat tolerant bottle gourd (Lagenaria siceraria) rootstocks. Understanding the genetic basis of heat tolerance and development of reliable DNA markers to indirectly select for the trait are necessary in breeding for new varieties with heat tolerance. The objectives of this study were to investigate the inheritance of heat tolerance and identify molecular markers associated with heat tolerance in bottle gourd. A segregating F2 population was developed from a cross between two heat tolerant and sensitive inbred lines. The population was phenotyped for relative electrical conductivity (REC) upon high temperature treatment which was used as an indicator for heat tolerance. QTL-seq was performed to identify regions associated with heat tolerance. We found that REC-based heat tolerance in this population exhibited recessive inheritance. Seven heat-tolerant quantitative trait loci (qHT1.1, qHT2.1, qHT2.2, qHT5.1, qHT6.1, qHT7.1, and qHT8.1) were identified with qHT2.1 being a promising major-effect QTL. In the qHT2.1 region, we identified three non-synonymous SNPs that were potentially associated with heat tolerance. These SNPs were located in the genes that may play roles in pollen sterility, intracellular transport, and signal recognition. Association of the three SNPs with heat tolerance was verified in segregating F2 populations, which could be candidate markers for marker assisted selection for heat tolerance in bottle gourd. The qHT2.1 region is an important finding that may be used for fine mapping and discovery of novel genes associated with heat tolerance in bottle gourd.

Genome-Wide Association Study Reveals Genomic Regions Associated With Ten Agronomical Traits in Wheat Under Late-Sown Conditions.

Poor understanding of the genetic and molecular basis of heat tolerance component traits is a major bottleneck in designing heat tolerant wheat cultivars. The impact of terminal heat stress is generally reported in the case of late sown wheat. In this study, our aim was to identify genomic regions for various agronomic traits under late sown conditions by using genome-wide association approach. An association mapping panel of 205 wheat accessions was evaluated under late sown conditions at three different locations in India. Genotyping of the association panel revealed 15,886 SNPs, out of which 11,911 SNPs with exact physical locations on the wheat reference genome were used in association analysis. A total of 69 QTLs (10 significantly associated and 59 suggestive) were identified for ten different traits including productive tiller number (17), grain yield (14), plant height (12), grain filling rate (6), grain filling duration (5), days to physiological maturity (4), grain number (3), thousand grain weight (3), harvest index (3), and biomass (2). Out of these associated QTLs, 17 were novel for traits, namely PTL (3), GY (2), GFR (6), HI (3) and GNM (3). Moreover, five consistent QTLs across environments were identified for GY (4) and TGW (1). Also, 11 multi-trait SNPs and three hot spot regions on Chr1Ds, Chr2BS, Chr2DS harboring many QTLs for many traits were identified. In addition, identification of heat tolerant germplasm lines based on favorable alleles HD2888, IC611071, IC611273, IC75240, IC321906, IC416188, and J31-170 would facilitate their targeted introgression into popular wheat cultivars. The significantly associated QTLs identified in the present study can be further validated to identify robust markers for utilization in marker-assisted selection (MAS) for development of heat tolerant wheat cultivars.

Genetic Diversity, Population Structure and Inter-Trait Relationships of Combined Heat and Drought Tolerant Early-Maturing Maize Inbred Lines from West and Central Africa

Adequate knowledge and understanding of the genetic diversity and inter-trait relationships among elite maize inbred lines are crucial for determining breeding strategies and predicting hybrid performance. The objectives of this study were to investigate the genetic diversity of 162 early maturing white and yellow tropical maize inbred lines, and to determine the population structure, heterotic groups and inter-trait relationships among the lines. Using 9684 DArT single nucleotide polymorphism (SNP) markers, a gene diversity (GD) of 0.30 was recorded for the inbred lines with polymorphic information content (PIC) ranging from 0.08 to 0.38. The genetic relatedness among the inbred lines evaluated revealed six different groups based on the history of selection, colour of endosperm and pedigree. The genotype-by-trait (GT) biplot analysis identified inbred 1 (TZEI 935) as outstanding in terms of combined heat and drought (HD) tolerance with the base index analysis identifying 15 superior inbreds in the HD environment. A wide range of genetic variability was observed among the inbred lines, indicating that they are an invaluable resource for breeding for HD tolerance in maize breeding programmes, especially in West and Central Africa.

Backcross breeding for improvement of heat tolerance at reproductive phase in Thai rice (Oryza sativaL.) varieties

AbstractHeat stress during the reproductive and grain-filling stages leads to severe yield losses in rice, especially in irrigated areas during the dry season in Thailand. Thus, breeding for heat-tolerant rice is one of the strategies for developing rice crops adapted to the effects of high temperature in major growing regions. M9962, a fast neutron-induced mutant, was identified as a heat-tolerant line that was then used as a donor parent and crossed with the Phisanulok 2 (PSL2) cultivar (the recurrent parent) in a backcross breeding programme. Heat tolerance scoring under controlled greenhouse and field conditions was based on the percentage of spikelet fertility. Yield trials for the BC2F7and BC3F6lines were subsequently conducted under field conditions at three locations and under controlled greenhouse conditions during the dry season of 2019 (DS2019). In the early generation, 210 plants of the BC1F1population and 440 plants of BC2F1population were evaluated for their heat tolerance, and the spikelet fertility distribution within the BC1F1, BC2F1and BC3F1populations showed a skewed-right pattern. In addition, most rice plants in every generation were heat sensitive. Finally, compared with PSL2, the four-candidate heat-tolerant lines exhibited a high percentage of spikelet fertility and high yield. In addition, the one candidate line, BC2F7-6-5-4-1-1-21 was most similar to PSL2 in terms of genetic background, plant type and grain quality. Thus, backcross breeding programmes via the spikelet fertility can be used as an indirect trait to select for heat tolerance.

Morphological and Ultra-Structural Characterization of Blackgram [Vigna mungo (L.) Hepper] Genotypes Grown under High Temperature Stress

Background: In this decade, increasing temperatures and adverse environmental factors are expected to influence the crop yield, seed quality and germination ability. Prolonged heat stress affects the pod setting, seed morphology and alters the starch granules.Methods: In this study, Scanning Electron Microscopy (SEM) and Light microscope was used to investigate the variations of morphological and ultra-structural characteristics in blackgram genotypes grown under high temperature (38+2°C) stress (HTS). Nineteen blackgram genotypes were evaluated under open top temperature controlled chamber (OTC) and the genotypes were classified into three categories such as heat tolerant, moderately heat tolerant and heat susceptible genotypes based on the Temperature Induction Response (TIR) and yield reduction percentage under HTS. Two genotypes were selected and examined the seed size, surface pattern, starch granule size and hilum from the each category.Result: Seed coat morphology was captured using SEM and light microscope showed that the tolerant genotype VBG-07-001 had a clear shiny surface, thin and reduced cotyledon fissures, compact surface devoid of surface deposits and pits whereas rest of the genotypes (VBG-06-001, VBN-6, CoBg-11-02, CoBg-11-03 and VBG-08-003) had a rough surface. A bold and well-structured starch granule was observed in heat tolerant lines whereas, unstructured and pleated granules were observed in moderately tolerant and heat stress lines. These morphological and ultra-structural studies revealed remarked diversity among different genotypes that can be regarded as useful to screen the genotypic tolerant characters for HTS.

Characterization of the multiple crossing tropical wheat “magic population” using SSR marker associated with physiological characters

Heat stress is a challenge in the development of wheat in tropical environments. Characterization of heat tolerant wheat lines based on physiological characters is still very rarely carried out in tropics, therefore early detection using heat stress linked markers related to physiological characters is very necessary in order to produce adaptive wheat varieties in tropics. The study aims to characterize wheat lines from the “magic population” crossing based on SSR markers related to physiological characters and identified by field observations. The research used 23 genotypes for SSR marker characterization and continued to field experiment using 10 selected genotypes with Guri-5 Agritan and Guri-6 Agritan varieties as check. The experiment was arranged in randomized block design with 3 replications. The results showed polymorphism on Xbarc84 and Xgwm285 markers at the genotypes number 203, 145 and 184. Xbarc84 markers could show a relation to flowering time on genotype 203 with longer flowering time and harvest time. Xgwm285 marker show relation to HTI at genotypes no.145 with no band was formed, and the genotypes show weakness on growing in the field and did not show any relation on genotypes no.184. Correlation analysis between characters showed that SRF, SL and NSp characters could be used as secondary characters for heat tolerance wheat selection.

Termotolerancia en líneas de sorgo [Sorghum bicolor (L.) Moench] para grano

El clima extremo como el aumento de la sequía y altas temperaturas han tenido un impacto significativo en el rendimiento de los cultivos. Por lo tanto, es importante identificar caracteres en los genotipos de sorgo que les ayuden a disminuir el impacto de estos factores y les permita ser productivos. El objetivo de este trabajo fue evaluar la tolerancia a calor de un grupo de líneas B y R de sorgo granífero bajo dos condiciones de humedad para seleccionar las mejores líneas tolerantes a calor. Se sembraron 28 líneas experimentales B y R el 30 de agosto de 2017 en Marín, NL, México. El experimento se sembró en un diseño completamente al azar con un arreglo en parcelas divididas y con dos repeticiones El tratamiento de humedad bajo estrés hídrico se aplicó 43 días después de la siembra (DDS) por un periodo de cuatro semanas sin riego. Se tomaron datos de días a floración, daño a la membrana celular a 40 ºC y contenido relativo de agua en hoja (CRA). Los resultados mostraron que el CRA no permitió establecer diferencias entre genotipos, mientras que el tratamiento de calor aplicado a 40 ºC, permitió establecer diferencias y clasificarlos de acuerdo con el porciento de daño causado a la membrana celular, ya que este es uno de los rasgos ampliamente utilizado para seleccionar genotipos tolerantes. Se identificaron como tolerantes al calor los genotipos 20 y 22 y como susceptibles los genotipos 5 y 8.

Selection of Korean heat tolerant rice lines in West Java irrigated lowland agro-ecosystem

One condition in global climate change era is increasing of world temperature which is approaching critical point of rice plant adaptation. Thus, heat tolerant rice variety is needed. Indonesian Centre for Rice Research (ICRR) and RDA (Rural Development Administration) of Korea through ASEAN-Korean Economic Cooperatives collaborates to breed heat tolerant rice variety. In this regard, 150 BC1F5 recombinant inbred lines (RIL) along with 5 checks was tested following Augmented Design of five blocks. Transplanting was conducted at 21 days after sowing on 25 cm x 25 cm planting space in 2 m x 5 m square plots. The results showed that SR34132(41)-1-1-1 (13.00 t/ha) and SR34132(11)-1-1 -1 (11.65 t/ha) had higher yield than the best check Inpari 42 Agritan GSR (8.23 t/ha). The first line had big grain size (27.72 g) and medium productive tiller (15 tillers), while the second had more productive tiller (23 tillers) even though had low filled grain/panilce, seed set, and grain size (21.60 g). There were 69 lines having comparable yield with Inpari 42 Agritan GSR. Combining high yield with heat tolerance traits is feasible for Indonesian agro ecosystem condition. Further testing to confirm the tolerance of the lines to heat stress is needed.

Investigating stress indices to discriminate the physiologically efficient heat tolerant genotypes of mungbean [Vigna radiata (L.) Wilczek

In present global climatic scenario, heat stress is one of the prime problems affecting production of mungbean crop during summer. A major objective of mungbean breeding programs to increase genetic potential of genotypes under heat stress environments. In order to identify heat tolerant promising lines, a total of 35 genotypes were evaluated in normal and heat stress environments during 2012–2013. Eight heat indices were used on the basis of grain yield in normal and heat stress environment. Highest significant positive correlations were observed among mean productivity, Geometric mean productivity, stress tolerant index and yield indices under both normal and heat stress environments. Based on 3-D plot, the genotypes i.e. SML 1186, NDM 12–308, IPM 02-4 and Smrat were found with high stress indices score and could be utilized in mungbean breeding programs to introduce and develop the heat tolerant promising varieties.

Identification of heat tolerance loci in broccoli through bulked segregant analysis using whole genome resequencing

Most broccoli cultivars are sensitive to high temperatures during the early stages of floral development causing a severe decline of head quality or even complete lack of head formation under superoptimal crop production temperatures. Several heat tolerant lines have been developed in recent years but there have been few studies of the genetic basis of this complex, polygenic trait. A doubled haploid population of broccoli was evaluated for head quality across two summer field trials with the phenotypic extremes validated in two additional summer fields. Whole-genome resequencing of the bulked segregants was used for a quantitative trait loci (QTL)-seq analysis of heat tolerance. Two novel QTL, which differ from previously reported QTL, were identified. Nonsynonymous SNPs were found in a block of flowering time genes within QHT_C09.2 and may explain the significant negative correlation between time to head maturity and heat tolerance. Breeding further genetic gains in this complex, polygenic trait could be expedited through marker assisted selection and gene pyramiding using markers developed from the QTL identified herein.

Different Transcriptomic Responses to Thermal Stress in Heat-Tolerant and Heat-Sensitive Pacific Abalones Indicated by Cardiac Performance.

The Pacific abalone Haliotis discus hannai is one of the most economically important mollusks in China. Even though it has been farmed in southern China for almost 20 years, summer mortality remains the most challengeable problem for Pacific abalone aquaculture recently. Here, we determined the different heat tolerance ability for five selective lines of H. discus hannai by measuring the cardiac performance and Arrhenius breakpoint temperature (ABT). The Red line (RL) and Yangxia line (YL) were determined as the most heat-sensitive and most heat-tolerant line, respectively. Heart rates for RL were significantly lower than those of the YL at the same temperature (p < 0.05). The differentially expressed genes (DEGs), which were enriched in several pathways including cardiac muscle contraction, glutathione metabolism and oxidative phosphorylation, were identified between RL and YL at control temperature (20°C) and heat stress temperature (28.5°C, the ABT of the RL) by RNA-seq method. In the RL, 3370 DEGs were identified between the control and the heat-stress temperature, while only 1351 DEGs were identified in YL between these two temperature tests. Most of these DEGs were enriched in the pathways such as protein processing in endoplasmic reticulum, nucleotide binding and oligomerization domain (NOD) like receptor signaling, and ubiquitin mediated proteolysis. Notably, the most heat-tolerant line YL used an effective heat-protection strategy based on moderate transcriptional changes and regulation on the expression of key genes.

Identification and characterization of novel homeodomain leucine zipper (HD-Zip) transcription factors associated with heat tolerance in perennial ryegrass

The homeodomain leucine zipper (HD-Zip) family transcription factors play roles in regulating plant development and responses to abiotic stress; however, how HD-Zip genes are associated with plant heat tolerance is unclear. The objectives of this study were to characterize HD-Zip I genes in perennial ryegrass (Lolium perenne), analyze the expression pattern under different abiotic stresses and identify novel HD-Zip I members associated with genetic variations in heat tolerance. A total of 13 HD-Zip I genes were identified in the transcriptome of perennial ryegrass. A comprehensive phylogenetic tree of HD-Zip I proteins in perennial ryegrass, Arabidopsis and rice was constructed. Motif analysis between LpHD-Zip I and OsHD-Zip I by MEME was characterized. LpHD-Zip I had close evolutionary relationship to OsHD-Zip I, and HD-Zip I may be subject to functional differentiation between monocotyledons and dicotyledons. qPCR analysis of 13 HD-Zip I genes showed these genes expressed at different levels in roots and leaves. These genes also showed various expression levels under different abiotic stresses (cold, heat, salt and osmotic). Physiological responses of heat-sensitive lines (321681 and 418714) and heat-tolerant lines (231565 and 275660) of perennial ryegrass to heat stress were evaluated. Differential expression levels of four genes (LpHOX6, -8, -21 and -24) were detected between heat-sensitive and heat-tolerant lines. LpHOX6, -8 and -24 expression levels were negatively associated with heat tolerance while LpHOX21 was positively associated with heat tolerance of perennial ryegrass. This is the first study demonstrating the association of four members of the HD-Zip I family (LpHOX6, -8, -21, and -24) with genetic variations in heat tolerance.

Molecular breeding approaches involving physiological and reproductive traits for heat tolerance in food crops

Heat stress is a significant threat that limits crop yield and fecundity all over the world. Prevalent strategies for heat adaptation that alter technical and management systems are inadequate to sustain yield. As such, the identification of heat-tolerant genotypes with improved yield potential is crucial. Raising tolerant and stable cultivars can be tedious as heat-stress responses are highly variable across different developmental stages. While molecular breeding has progressed in engineering heat-tolerant lines, the complexity of genetic networks and divergence of heat tolerance mechanisms is the main hindrance for plant breeders. Hence, insight into the physiological and reproductive traits associated with heat tolerance could assist in the development of strategies to screen germplasm for heat tolerance. Exploitation and use of landraces and wild relatives in breeding may enhance favorable genetic diversity in crop plants. A holistic approach to delineate molecular markers, where quantitative trait loci (QTLs) for different traits linked to heat tolerance involving physiological and reproductive traits are characterized in well-controlled field environments, may be an option for optimizing germplasm under heat stress. Here, we present an outline of the effects of heat stress and its associated tolerance mechanisms in food crops, along with some physiological, molecular and reproductive characteristics such as ‘stay-green,’ membrane thermostability, canopy temperature depression, metabolites, genes, QTLs, and pollen fertility. Further, we provide information on conventional and molecular breeding approaches as well as different selection strategies for heat stress tolerance.