Heat Tolerance In Rice Research Articles

Identification of simple sequence repeat markers linked to heat tolerance in rice using bulked segregant analysis in F2 population of NERICA-L 44 × Uma.

The damage caused by high temperature is one of the most important abiotic stress affecting rice production. Reproductive stage of rice is highly susceptible to high temperature. The present investigation was undertaken to identify polymorphic microsatellite markers (SSR) associated with heat tolerance. The rice cultivars NERICA- L 44 (heat tolerant) and Uma (heat susceptible) were crossed to generate F1 and F2 populations. The F2 population was subjected to heat stress at >38°C and the 144 F2 plants were evaluated for their tolerance. The results note that the mean of the F2 population was influenced by the tolerant parent with regards to the traits of plant height, membrane stability index, photosynthetic rate, stomatal conductance, evapotranspiration rate, pollen viability, spikelet fertility and 1000 grain weight. Ten each of the extremely susceptible and tolerant plants were selected based on the spikelet fertility percentage. Their DNA was pooled into tolerant and susceptible bulks and Bulked Segregant Analysis (BSA) was carried out using 100 SSR markers to check for polymorphism. The survey revealed a polymorphism of 18% between the parents. RM337, RM10793, RM242, RM5749, RM6100, RM490, RM470, RM473, RM222 and RM556 are some of the prominent markers that were found to be polymorphic between the parents and the bulks. We performed gene annotation and enrichment analysis of identified polymorphic markers. Result revealed that the sequence specific site of that chromosome mostly enriched with biological processes like metabolic pathway, molecular mechanism, and subcellular function. Among that RM337 was newly reported marker for heat tolerance. Expression analysis of two genes corresponds to RM337 revealed that LOP1 (LOC_Os08g01330) was linked to high temperature tolerance in rice. The results demonstrate that BSA using SSR markers is useful in identifying genomic regions that contribute to thermotolerance.

Exogenous Kinetin Modulates ROS Homeostasis to Affect Heat Tolerance in Rice Seedlings.

Heat stress caused by rapidly changing climate warming has become a serious threat to crop growth worldwide. Exogenous cytokinin (CK) kinetin (KT) has been shown to have positive effects in improving salt and drought tolerance in plants. However, the mechanism of KT in heat tolerance in rice is poorly understood. Here, we found that exogenously adequate application of KT improved the heat stress tolerance of rice seedlings, with the best effect observed when the application concentration was 10-9 M. In addition, exogenous application of 10-9 M KT promoted the expression of CK-responsive OsRR genes, reduced membrane damage and reactive oxygen species (ROS) accumulation in rice, and increased the activity of antioxidant enzymes. Meanwhile, exogenous 10-9 M KT treatment significantly enhanced the expression of antioxidant enzymes, heat activation, and defense-related genes. In conclusion, exogenous KT treatment regulates heat tolerance in rice seedlings by modulating the dynamic balance of ROS in plants under heat stress.

Current Status and Future Prospects of Head Rice Yield

Global warming poses a serious threat to food security because of its impacts on thermosensitive food crop production. Rice is of paramount significance due to the world’s three-billion-population dependence on it as a staple food. It is well established that the high temperatures at day or night times during the grain-filling period can reduce rice grain yield, although the intriguing impact of high temperatures on head rice yields (HRY) is poorly discussed. This is because high and stable HRY is vital to meet the demand for rice grain, which is a staple food for many developing and developed nations. Hence, identifying the novel heat-tolerant rice germplasm with higher head rice yields may help mitigate a critical problem threatening global food security resulting from climate change. This review addresses the key factors, including pre-and-post-harvest scenarios related to overall reductions in the HRY and how grain molecular composition can play a significant role in determining head rice yields. Moreover, the underlying genetics of head rice is discussed as and possible mechanism to breach the complexity of HRY before identifying the key alleles and genomic regions related to the reduction in the HRY. Future research should focus on understanding the mechanisms of tolerating heat stress in rice by combining modern statistical, physiological, and molecular techniques to increase HRY. This may include high-throughput phenotyping techniques, mapping quantitative loci affecting HRY loss processes and genomic prediction using a broad wild and cultivated rice germplasm.

Integrating GWAS and transcriptomics to identify candidate genes conferring heat tolerance in rice.

Rice (Oryza sativa L.) production is being challenged by global warming. Identifying new loci and favorable alleles associated with heat tolerance is crucial to developing rice heat-tolerant varieties. We evaluated the heat tolerance at the seedling stage using 620 diverse rice accessions. A total of six loci associated with heat tolerance were identified by a genome-wide association study (GWAS) with ~2.8 million single nucleotide polymorphisms (SNPs). Among the six detected loci, qHT7 harbored the strongest association signal and the most associated SNPs. By comparing the transcriptomes of two representative accessions with contrasting heat tolerance, LOC_Os07g48710 (OsVQ30) was selected as a promising candidate gene in qHT7 due to the significant difference in its expression level between the two accessions. Haplotype 4 (Hap4) of LOC_Os07g48710 was determined as the favorable haplotype for heat tolerance via the gene-based haplotype analysis. The heat-tolerant haplotype LOC_Os07g48710Hap4 is highly enriched in the tropical Geng/Japonica accessions, and its frequency has decreased significantly during the improvement process of rice varieties. Based on the GWAS and transcriptomics integrated results, a hypothetical model modulated by qHT7 in response to heat stress was proposed. Our results provide valuable candidate genes for improving rice heat tolerance through molecular breeding.

Individual and Combined Effects of High-Temperature Stress at Booting and Flowering Stages on Rice Grain Yield

Extreme temperature events as a consequence of global climate change result in a significant decline in rice yield. A two year phytotron experiment was conducted using three temperature levels and two heating durations to compare the effects of heat stress at booting (BT), flowering (FL), and combined (BT + FL) stages on rice yield and yield components. Compared with T1 (Tmean of 27 °C), heat stress at BT + FL and BT stages produced more regenerated tillers and compensated more for yield loss than heat stress at FL. Heat stress at BT + FL stages alleviated spikelet sterility and yield loss of original tillers compared to heat stress at FL. The greater variation of yield per plant (YPP) under heat stress at flowering as compared to BT and BT + FL stages was accompanied by a higher decrease in spikelet fertility, while, at BT and BT + FL stages, spikelet number per plant and 1000-grain weight also contributed well to variation in yield. Furthermore, heat stress during BT and BT + FL stages caused a significant decline in spikelet fertility of the upper part of panicles, followed by middle and lower parts, while heat stress at the FL stage responded inversely. For every 1 °C day increase in heat degree days at BT, FL, and BT + FL stages, YPPO (only original tillers) declined by 2.9%, 2.5%, and 6.0%, and YPPT (including original + regenerated tillers) decreased by 5.8%, 2.7%, and 2.2%, respectively. The projected alleviation effects under BT + FL stages of heat stress in contrast to single-stage heat stress would help to accurately estimate rice yield under extreme temperature events, as well as to develop a heat-tolerant rice cultivar.

Identification of QTLs for Heat Tolerance at the Flowering Stage Using Chromosome Segment Substitution Lines in Rice

High temperature is a major stress in rice production. Although considerable progress has been made in investigating heat tolerance (HT) in rice, the genetic basis of HT at the heading stage remains largely unknown. In this study, a novel set of chromosome segment substitution lines (CSSLs) consisting of 113 lines derived from a heat-resistant indica variety N22 and a heat-sensitive indica variety 9311 was developed and used for the analysis of the genetic basis of HT. The heat sensitivity index (HSI) calculated based on seed-setting rates under natural and high-temperature environments was used to evaluate the influence of HT at the rice heading stage. In total, five quantitative trait loci (QTLs) associated with HT were detected based on seed-setting rate (SSR) evaluation; these were named qSSR6-1, qSSR7-1, qSSR8-1, qSSR9-1 and qSSR11-1 located on chromosomes 6, 7, 8, 9 and 11, respectively. Heat-tolerant alleles of the QTLs were all derived from N22. Among them, qSSR9-1 overlapped with QTLs identified previously, while the remaining QTLs were found novel. In particular, qSSR7-1 explained a high phenotypic variation of 26.35% with a LOD score of 10.75, thus deserved to be further validated. These findings will increase our understanding of the genetic mechanism underlying HT and facilitate the breeding of heat-tolerant rice varieties.

Breeding targets for heat-tolerant rice varieties in Japan in a warming climate

The breeding of heat-tolerant (HT) varieties is considered an effective adaptation measure to reduce the impacts of global warming on crops. However, little is known about how much heat tolerance needs to be attained in the breeding of HT varieties and by when the HT varieties should be introduced in a warming climate. Here, we assessed the effect of the introduction of HT rice varieties on the incidence of low-quality rice grains (chalky rice grain, CRG, as a result of high temperatures), and formulated a breeding target for rice in Japan, where high incidence of CRG is already a concern. Varieties with a 3 °C higher temperature tolerance should be bred and introduced until the 2040s under representative concentration pathways (RCPs) 2.6 and 8.5, in the pessimistic scenario, considering the uncertainties in climate models and an impact assessment model, which were large. In the median case, varieties with a 1 and 2 °C higher temperature tolerance are needed until the 2040s under RCPs 2.6 and 8.5, respectively. The findings revealed necessary efforts in breeding research and programs under conditions of global warming.

Potential roles of stigma exsertion on spikelet fertility in rice (Oryza sativa L.) under heat stress.

Heat stress during the flowering stage induces declining spikelet fertility in rice plants, which is primarily attributed to poor pollination manifesting as insufficient pollen deposited on the stigma. Plant pollination is associated with anther dehiscence, pollen dispersal characteristics, and stigma morphology. The mechanisms underlying the responses of spikelet fertility to heat stress have been clarified in depth in terms of the morphological and behavioral characteristics of the male reproductive organs in rice. However, the roles of female reproductive organs, especially the stigma, on spikelet fertility under heat conditions are unclear. The present study reviews the superiority of stigma exsertion on pollen receptivity under heat during the flowering stage and discusses the variations in the effects of exserted stigma on alleviating injury under asymmetric heat (high daytime and high nighttime temperatures). The pollination advantages of exserted stigmas seem to be realized more under high nighttime temperatures than under high daytime temperatures. It is speculated that high stigma exsertion is beneficial to spikelet fertility under high nighttime temperatures but detrimental under high daytime temperatures. To cope with global warming, more attention should be given to rice stigma exsertion, which can be manipulated through QTL pyramiding and exogenous hormone application and has application potential to develop heat-tolerant rice varieties or innovate rice heat-resistant cultivation techniques, especially under high nighttime temperatures.

QTL mapping and identification of candidate genes using a genome-wide association study for heat tolerance at anthesis in rice (Oryza sativa L.).

Heat tolerance (HT) of rice at anthesis is a key trait that ensures high and stable yields under heat stress. Finding the quantitative trait loci (QTLs) and gene loci controlling HT is crucial. We used relative spikelet fertility (RSF) as a measure of HT. The phenotypic values of RSF in 173 rice accessions were investigated in two environments and showed abundant variations. We performed a genome-wide association study on RSF using 1.2 million single nucleotide polymorphisms (SNPs). Five QTLs were significantly associated with RSF were identified, four were found in previously reported QTLs/genes, and one was novel. The novel QTL qRSF9.2 was mapped into the 22,059,984-22,259,984 bp region, which had 38 positional candidate genes. By combining the linkage disequilibrium analysis, the QTL region was narrowed to 22,110,508–22,187,677 bp, which contained 16 candidate genes. Among them, only gene LOC_Os09g38500 contained nonsynonymous SNPs that were significantly associated with RSF. In addition, accessions with large and small RSF values had corresponding respective high and low gene expression levels. Furthermore, the RSF of the CC allele was significantly higher than that of the TT allele. Hap 2 and Hap 3 can increase heat tolerance by 7.9 and 11.3%, respectively. Our results provide useful information that recommends further cloning of qRSF9.2 and breeding heat-tolerant rice varieties by marker-assisted selection.

Agronomic and Physiological Indices for Reproductive Stage Heat Stress Tolerance in Green Super Rice

Optimum growing temperature is necessary for maximum yield-potential in any crop. The global atmospheric temperature is changing more rapidly and irregularly every year. High temperature at the flowering/reproductive stage in rice causes partial to complete pollen sterility, resulting in significant reduction in grain yield. Green Super Rice (GSR) is an effort to develop an elite rice type that can withstand multiple environmental stresses and maintain yield in different agro-ecological zones. The current study was performed to assess the effect of heat stress on agronomic and physiological attributes of GSR at flowering stage. Twenty-two GSR lines and four local checks were evaluated under normal and heat-stress conditions for different agro-physiological parameters, including plant height (PH), tillers per plant (TPP), grain yield per plant (GY), straw yield per plant (SY), harvest index (HI), 1000-grain weight (GW), grain length (GL), cell membrane stability (CMS), normalized difference vegetative index (NDVI), and pollen fertility percentage (PFP). Genotypes showed high significant variations for all the studied parameters except NDVI. Association and principal component analysis (PCA) explained the genetic diversity of the genotypes, and relationship between the particular parameters and grain yield. We found that GY, along with other agronomic traits, such as TPP, SY, HI, and CMS, were greatly affected by heat stress in most of the genotypes, while PH, GW, GL, PFP, and NDVI were affected only in a few genotypes. Outperforming NGSR-16 and NGSR-18 in heat stress could be utilized as a parent for the development of heat-tolerant rice. Moreover, these findings will be helpful in the prevention and management of heat stress in rice.

Identification of heat-tolerant rice genotypes and their molecular characterisation using SSR markers

The effect of high-temperature stress has a critical impact in causing reduced crop yield. The focus of the current investigation is the identification of heat-tolerant rice varieties that can alleviate the effects of stress. Among the ten genotypes evaluated across various parameters such as leaf area, dry weight, photosynthetic rate, stomatal conductance and spikelet fertility, N-22 showed superior characteristics for the grain filling parameters along with CR-Dhan 307. The variety CR-Dhan 307 with significantly higher mean pollen viability (80.23%), spikelet fertility (81.18%) and 1000 grain weight (25.45 gm) can be utilized as a heat-tolerant variety. Other genotypes Ptb-7 and CR-Dhan 202 seemed to have tolerance traits beneficial at the vegetative stage. The genotype Rajalakshmi can be characterised as heat susceptible as it had significantly lower values for all parameters. Polymorphic analysis was carried out to validate SSR markers associated with heat tolerance. The polymorphic information content (PIC) was found to be the highest for RM236 and RM6100. The SSR marker RM6100 has been validated in the current investigation to be associated with heat tolerance. As the PIC value is an indication of the ability of the marker in indicating genetic diversity, the PIC values of the 11 polymorphic markers is useful for identify heat-tolerant genotypes. The genetic diversity analysis was carried out using DendroUPGMA to establish the relationship between the genotypes. The genotypes Ptb-7 and CR-Dhan 204 were thus found to be closely related to the heat-tolerant check variety, N-22 indicating genetically related traits for tolerance to heat.

Antioxidative Defense System, Hormones, and Metabolite Accumulation in Different Plant Parts of Two Contrasting Rice Cultivars as Influenced by Plant Growth Regulators Under Heat Stress.

We examined the metabolic, hormonal, enzymatic, and non-enzymatic responses of various plant components (leaf, root, and xylem sap) to plant growth regulators [methyl jasmonate (MeJA), ascorbic acid (Vc), brassinosteroids (Br), triazoles (Tr), alpha-tocopherol (Ve), and control] under heat stress [ambient temperature (AT), heat stress at night time (HNT), and heat stress at day (HDT)] in heat-sensitive (IR-64) and heat-tolerant (Huanghuazhan) rice cultivars under greenhouse conditions. Our results showed that heat stress altered the antioxidant activities and hormonal balance and rigorously reduced total soluble sugars, proteins, and proline, whereas increases were observed in H2O2 and Malondialdehyde (MDA) content accumulation in the plant xylem sap and leaves of both tested cultivars; however, the impact was more pronounced in IR-64. The superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), Glutathione (GSH), dehydroascorbate reductase (DHAR), and monodehydroascorbate reductase (MDHAR) activities were higher in Huanghuazhan than in IR-64 in response to temperature stress, when compared to AT. Additionally, heat stress increased abscisic acid (ABA) levels in both rice cultivars, especially in IR-64. The highest concentrations of hormones were recorded in the roots, followed by the leaves and xylem sap, in both cultivars. HDT and HNT stresses severely reduced the concentrations of all of the cytokinin types (except for iP9G and tZ9G) and IAA in the different plant parts of rice cultivars. Moreover, HNT was more detrimental for hormone and metabolite synthesis in both cultivars. The growth regulators (especially Vc + Br + Ve + MeJA) were comparatively more effective in minimizing the hostile impact of heat stress on most of the studied traits and should be applied to obtain the optimum yield of rice in subtropical and tropical areas under changing climatic conditions.

Physiological and agronomical evaluation of elite rice varieties for adaptation to heat stress

BackgroundThe increasing temperatures due to climate change around the world poses a serious threat to sustainable crop production. The growing adverse effects of heat stress are putting global food security at great risk. Crop improvement for adaptation to increased temperatures is therefore of paramount importance. This study aims at assessing the effects of heat stress in relation to agro-morphological and physiological traits of six rice varieties. The study was carried out in the Township of Glazoué, a rice-growing area in Benin. The experiments were laid in randomized complete block design with three replications. Two types of stress were imposed: high-temperature stress in the dry season and optimal temperatures in the rainy season. The calculated mean values of morphological, physiological, and agronomic traits were used to estimate heritability, genetic advance, PCA, and correlation.ResultsThe results showed that heat stress had a significant (p ≤ 0.01) influence on plant height, leaf length, number of tillers, number of internodes, days to flowering, and days to maturity, 1000-seed weight, and yield per plant. The heat stress had significantly delayed the flowering of all the varieties when compared to the controls. The highest values of 1000-seed weight (34. 67 g) were recorded for BRIZ-8B while the lowest (25.33 g) were recorded for NERICA-L20. The highest values for the genotypic coefficient of variation (43.05%) and phenotypic coefficient of variation (99.13%) were recorded for yield per plant under heat stress. The topmost broad-sense heritability was recorded for grain width (92.72%), followed by days to maturity (69.33%), days to flowering (68.50%), number of grains per panicle (57.35%), and yield (54.55%).ConclusionsThese results showed that BRIZ-8B and BRIZ-10B were the most tolerant to high temperature amongst the six varieties assessed and potentially could be recommended to farmers for production under high temperature and be used in breeding programs to improve heat tolerance in rice.

Identifying candidate genes and patterns of heat-stress response in rice using a genome-wide association study and transcriptome analyses

Because high temperatures impair rice production, it is desirable to elucidate the regulatory mechanisms involved in rice response to heat stress. The objectives of this study were to identify candidate genes and characterize their response patterns during rice adaptation to high temperatures at the seedling stage. Ten heat-associated quantitative-trait loci were identified in a genome-wide association study. Comparison of transcript abundances in heat-sensitive and heat-tolerant rice pools under heat stress revealed approximately 400 differentially expressed genes. The expression of genes from heat-sensitive accessions changed more than those from heat-tolerant accessions under heat stress. Alternative splicing (AS) events responded to heat stress in rice. The types of AS variants significant different between the heat-sensitive and heat-tolerant accessions. Expression patterns differing between the heat-sensitive and heat-tolerant accessions were identified for genes known to be involved in heat stress. We identified eleven genes associated with rice heat stress response. These genes could be pyramided to breed heat-tolerant rice accessions.

Selection of Candidate Genes Conferring Blast Resistance and Heat Tolerance in Rice through Integration of Meta-QTLs and RNA-Seq.

Due to global warming, high temperature is a significant environmental stress for rice production. Rice (Oryza sativa L.), one of the most crucial cereal crops, is also seriously devastated by Magnaporthe oryzae. Therefore, it is essential to breed new rice cultivars with blast and heat tolerance. Although progress had been made in QTL mapping and RNA-seq analysis in rice in response to blast and heat stresses, there are few reports on simultaneously mining blast-resistant and heat-tolerant genes. In this study, we separately conducted meta-analysis of 839 blast-resistant and 308 heat-tolerant QTLs in rice. Consequently, 7054 genes were identified in 67 blast-resistant meta-QTLs with an average interval of 1.00 Mb. Likewise, 6425 genes were obtained in 40 heat-tolerant meta-QTLs with an average interval of 1.49 Mb. Additionally, using differentially expressed genes (DEGs) in the previous research and GO enrichment analysis, 55 DEGs were co-located on the common regions of 16 blast-resistant and 14 heat-tolerant meta-QTLs. Among, OsChib3H-c, OsJAMyb, Pi-k, OsWAK1, OsMT2b, OsTPS3, OsHI-LOX, OsACLA-2 and OsGS2 were the significant candidate genes to be further investigated. These results could provide the gene resources for rice breeding with excellent resistance to these 2 stresses, and help to understand how plants response to the combination stresses of blast fungus and high temperature.

Exogenously-Sourced Ethylene Positively Modulates Photosynthesis, Carbohydrate Metabolism, and Antioxidant Defense to Enhance Heat Tolerance in Rice.

The effect of exogenously-applied ethylene sourced from ethephon (2-chloroethyl phosphonic acid)was studied on photosynthesis, carbohydrate metabolism, and high-temperature stress tolerance in Taipei-309 and Rasi cultivars of rice (Oryza sativa L.). Heat stress increased the content of H2O2 and thiobarbituric acid reactive substances (TBARS)more in Rasi than Taipei-309. Further, a significant decline in sucrose, starch, and carbohydrate metabolism enzyme activity and photosynthesis was also observed in response to heat stress. The application of ethephon reduced H2O2 and TBARS content by enhancing the enzymatic antioxidant defense system and improved carbohydrate metabolism, photosynthesis, and growth more conspicuously in Taipei-309 under heat stress. The ethephon application enhanced photosynthesis by up-regulating the psbA and psbB genes of photosystem II in heat-stressed plants. Interestingly, foliar application of ethephoneffectively down-regulated high-temperature-stress-induced elevated ethylene biosynthesis gene expression. Overall, ethephon application optimized ethylene levels under high-temperature stress to regulate the antioxidant enzymatic system and carbohydrate metabolism, reducing the adverse effects on photosynthesis. These findings suggest that ethylene regulates photosynthesis via carbohydrate metabolism and the antioxidant system, thereby influencing high-temperature stress tolerance in rice.

Analyzing the impacts of climate change on rice supply in West Nusa Tenggara, Indonesia

Differing from other main grains, rice is mainly fed by humans. This means that rice is important for human food security. Rice also has a significant contribution to employment and the economy, especially in developing countries. Owing to this, understanding the negative impacts of climate change on rice supply is a critical issue. This study uses the system dynamics modeling to investigate the impacts of climate change on three factors: rice yield, harvested areas, and rice production in West Nusa Tenggara (WNT), Indonesia throughout this century. In doing so, this study incorporates production and climate factors in assessing the impacts of climate change on rice supply. Although climate change may allow a small increase in harvested areas, decreases in rice yield will likely outweigh this. Owing to this, policymakers should not rely on the farming land expansion. Rather, policymakers should enhance the research and development of heat-tolerant rice varieties to anticipate the negative impacts of climate change on rice supply. This study also claims that despite the negative impacts of climate change on rice production, WNT can fulfill its local rice demand under a changing climate. Nevertheless, WNT cannot provide a sufficient rice for its neighboring provinces: Bali and East Nusa Tenggara.

Agronomic and genetic approaches for enhancing tolerance to heat stress in rice: a review

Rice is an important cereal crop worldwide that serves as a dietary component for half of the world’s population. Climate change, especially global warming is a rising threat to crop production and food security. Therefore, enhancing rice growth and yield is a crucial challenge in stress-prone environments. Frequent episodes of heat stress threaten rice production all over the world. Breeders and agronomists undertake several techniques to ameliorate the adverse effects of heat stress to safeguard global rice production. The selection of suitable sowing time application of plant hormones, osmoprotectants and utilization of appropriate fertilizers and signaling molecules are essential agronomic practices to mitigate the adverse effects of heat stress on rice. Likewise, developing genotypes with improved morphological, biochemical, and genetic attributes is feasible and practical way to respond to this challenge. The creation of more genetic recombinants and the identification of traits responsible for heat tolerance could allow the selection of early-flowering cultivars with resistance to heat stress. This review details the integration of several agronomic, conventional breeding, and molecular approaches like hybridization, pure line selection, master-assisted-selection (MAS), transgenic breeding and CRRISPR/Cas9 that promise rapid and efficient development and selection of heat-tolerant rice genotypes. Such information’s could be used to determine the future research directions for rice breeders and other researchers working to improve the heat tolerance in rice.

Field performance of heat tolerant mutant rice lines generated from Oryza sativa and Oryza glaberrima

This study evaluated mutant lines developed from two cultivated species of upland rice, Oryza sativa, and Oryza glaberrima, in field experiments conducted during the hot and dry seasons of 2014/2015 in Morogoro, Tanzania. The growth yield and yield components of 34 and 14 Gamma induced mutant upland rice lines developed from O. sativa (Kihogo red) and O. glaberrima were evaluated, respectively. The mutant lines were selected based on variable expression of heat shock protein genes (HSPs) in previously conducted heat tolerance studies. The minimum and maximum temperatures and rainfall during the field performance experiment were measured between 20 oC and 35 oC, and 32.7 mm and 155.5 mm, respectively. The data for 12 yield and yield component parameters such as days to early and 50% flowering, days to physical maturity, plant height, number of tillers, number of panicles, spikelets, filled grains, unfilled grains and 1,000 grain weights were collected and analysed using ANOVA and Principal Component Analysis. Significant differences (P≤0.05) were obtained among the mutant lines in terms of grain yield, spikelet sterility and other variables, which were further used as criteria for selection of heat and drought tolerant rice lines. Eight heat and drought tolerant mutant rice lines with high yields (over 3.5 ton/ha) and low spikelet sterility were selected for further advancement in breeding programmes

Expression of heat shock protein (HSP) genes and antioxidant enzyme genes in hybrid rice II YOU 838 during heat stress

II YOU 838 (Oryza sativa subsp. indica), crossed by the maternal II-32A and paternal Fu Hui 838, was one of the most widely cultivated hybrid rice in China. Fu Hui 838, which has resistance to high temperature, was generated by mutation technology in 1990. Previous field-testing showed that II YOU 838 had tolerance to high temperature stress and this was confirmed in the present study. The mechanism of heat tolerance of II YOU 838 is not understood. The present study reports gene expression of a representative sample of heat-responsive proteins in II YOU 838 flag leaves subjected to heat stress during flowering. Differential expression of the heat shock protein 70 (HSP70), heat shock protein 90 (HSP90), small heat shock protein (smHSP), superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) were studied under heat stress and optimum temperatures in flag leaves of II YOU 838. All six genes studied were responsive to high temperatures. Quantitative real-time PCR showed increased expression of the heat shock protein genes and antioxidant enzyme genes in flag leaves under heat stress. With increasing number of days gene expression decreased under high temperature. Peak expression of SOD, POD, hsp70 and hsp90 was on Day 2 under 39 ℃. On Day 3, the expression of CAT under 39 ℃ was the highest. The expression of smhsp was highest on Day 3 under 27 ℃, followed by that on Day 2 under 27 ℃. The maximum expression values were observed on Day 2 or Day 3 after beginning of heat stress. This suggests that hsp90, hsp70, SOD and POD are principally involved in early responses to heat in rice flag leaves, and that smhsp may play a role in the recovery mechanism in rice after heat stress. This may provide insights into the mechanism of heat-tolerance in rice