Dynamic Transcriptome Analysis of Anther Response to Heat Stress during Anthesis in Thermotolerant Rice (Oryza sativa L.)

Integrating physiological and multi-omics methods to elucidate heat stress tolerance for sustainable rice production

Heat stress presents unique challenges compared to other environmental stressors, as predicting crop responses and understanding the mechanisms for heat tolerance are complex tasks. The escalating impact of devastating climate changes heightens the frequency and intensity of heat stresses, posing a noteworthy threat to global agricultural productivity, especially in rice-dependent regions of the developing world. Humidity has been demonstrated to negatively affect rice yields worldwide. Plants have evolved intricate biochemical adaptations, involving intricate interactions among genes, proteins, and metabolites, to counter diverse external signals and ensure their survival. Modern-omics technologies, encompassing transcriptomics, metabolomics, and proteomics, have revolutionized our comprehension of the intricate biochemical and cellular shifts that occur in stressed agricultural plants. Integrating these multi-omics approaches offers a comprehensive view of cellular responses to heat stress and other challenges, surpassing the insights gained from multi-omics analyses. This integration becomes vital in developing heat-tolerant crop varieties, which is crucial in the face of increasingly unpredictable weather patterns. To expedite the development of heat-resistant rice varieties, aiming at sustainability in terms of food production and food security globally, this review consolidates the latest peer-reviewed research highlighting the application of multi-omics strategies.

Ambient temperature regulates root circumnutation in rice through the ethylene pathway: transcriptome analysis reveals key genes involved

Plant roots are constantly prepared to adjust their growth trajectories to avoid unfavorable environments, and their ability to reorient is particularly crucial for survival. Under laboratory conditions, this continuous reorientation of the root tip is manifested as coiling or waving, which we refer to as root circumnutation. However, the effect of ambient temperature (AT) on root circumnutation remains unexplored. In this study, rice seedlings were employed to assess the impact of varying ATs on root circumnutation. The role of ethylene in mediating root circumnutation under elevated AT was examined using the ethylene biosynthesis inhibitor aminooxyacetic acid (AOA) and the ethylene perception antagonist silver thiosulfate (STS). Furthermore, transcriptome sequencing, weighted gene co-expression network analysis, and real-time quantitative PCR were utilized to analyze gene expressions in rice root tips under four distinct treatments: 25°C, 35°C, 35°C+STS, and 35°C+AOA. As a result, genes associated with ethylene synthesis and signaling (OsACOs and OsERFs), auxin synthesis and transport (OsYUCCA6, OsABCB15, and OsNPFs), cell elongation (OsEXPAs, OsXTHs, OsEGL1, and OsEXORDIUMs), as well as the inhibition of root curling (OsRMC) were identified. Notably, the expression levels of these genes increased with rising temperatures above 25°C. This study is the first to demonstrate that elevated AT can induce root circumnutation in rice via the ethylene pathway and proposes a potential molecular model through the identification of key genes. These findings offer valuable insights into the growth regulation mechanism of plant roots under elevated AT conditions.

Transcriptomic and physiological analyses reveal different grape varieties response to high temperature stress

High temperatures affect grape yield and quality. Grapes can develop thermotolerance under extreme temperature stress. However, little is known about the changes in transcription that occur because of high-temperature stress. The heat resistance indices and transcriptome data of five grape cultivars, 'Xinyu' (XY), 'Miguang' (MG), 'Summer Black' (XH), 'Beihong' (BH), and 'Flame seedless' (FL), were compared in this study to evaluate the similarities and differences between the regulatory genes and to understand the mechanisms of heat stress resistance differences. High temperatures caused varying degrees of damage in five grape cultivars, with substantial changes observed in gene expression patterns and enriched pathway responses between natural environmental conditions (35 °C ± 2 °C) and extreme high temperature stress (40 °C ± 2 °C). Genes belonging to the HSPs, HSFs, WRKYs, MYBs, and NACs transcription factor families, and those involved in auxin (IAA) signaling, abscisic acid (ABA) signaling, starch and sucrose pathways, and protein processing in the endoplasmic reticulum pathway, were found to be differentially regulated and may play important roles in the response of grape plants to high-temperature stress. In conclusion, the comparison of transcriptional changes among the five grape cultivars revealed a significant variability in the activation of key pathways that influence grape response to high temperatures. This enhances our understanding of the molecular mechanisms underlying grape response to high-temperature stress.

Transcriptome Analysis Reveals the Dynamic and Rapid Transcriptional Reprogramming Involved in Heat Stress and Identification of Heat Response Genes in Rice

High temperature is one of the most important environmental factors influencing rice growth, development, and yield. Therefore, it is important to understand how rice plants cope with high temperatures. Herein, the heat tolerances of T2 (Jinxibai) and T21 (Taizhongxianxuan2hao) were evaluated at 45 °C, and T21 was found to be sensitive to heat stress at the seedling stage. Analysis of the H2O2 and proline content revealed that the accumulation rate of H2O2 was higher in T21, whereas the accumulation rate of proline was higher in T2 after heat treatment. Meanwhile, transcriptome analysis revealed that several pathways participated in the heat response, including "protein processing in endoplasmic reticulum", "plant hormone signal transduction", and "carbon metabolism". Additionally, our study also revealed that different pathways participate in heat stress responses upon prolonged stress. The pathway of "protein processing in endoplasmic reticulum" plays an important role in stress responses. We found that most genes involved in this pathway were upregulated and peaked at 0.5 or 1 h after heat treatment. Moreover, sixty transcription factors, including the members of the AP2/ERF, NAC, HSF, WRKY, and C2H2 families, were found to participate in the heat stress response. Many of them have also been reported to be involved in biotic or abiotic stresses. In addition, through PPI (protein-protein interactions) analysis, 22 genes were identified as key genes in the response to heat stress. This study improves our understanding of thermotolerance mechanisms in rice, and also lays a foundation for breeding thermotolerant cultivars via molecular breeding.

Chlorophyll fluorescence-based high-throughput phenotyping facilitates the genetic dissection of photosynthetic heat tolerance in African (Oryza glaberrima) and Asian (Oryza sativa) rice

Rising temperatures and extreme heat events threaten rice production. Half of the global population relies on rice for basic nutrition, and therefore developing heat-tolerant rice is essential. During vegetative development, reduced photosynthetic rates can limit growth and the capacity to store soluble carbohydrates. The photosystem II (PSII) complex is a particularly heat-labile component of photosynthesis. We have developed a high-throughput chlorophyll fluorescence-based screen for photosynthetic heat tolerance capable of screening hundreds of plants daily. Through measuring the response of maximum PSII efficiency to increasing temperature, this platform generates data for modelling the PSII-temperature relationship in large populations in a small amount of time. Coefficients from these models (photosynthetic heat tolerance traits) demonstrated high heritabilities across African (Oryza glaberrima) and Asian (Oryza sativa, Bengal Assam Aus Panel) rice diversity sets, highlighting valuable genetic variation accessible for breeding. Genome-wide association studies were performed across both species for these traits, representing the first documented attempt to characterize the genetic basis of photosynthetic heat tolerance in any species to date. A total of 133 candidate genes were highlighted. These were significantly enriched with genes whose predicted roles suggested influence on PSII activity and the response to stress. We discuss the most promising candidates for improving photosynthetic heat tolerance in rice.

High-temperature stress in crops: male sterility, yield loss and potential remedy approaches

Global food security is one of the utmost essential challenges in the 21st century in providing enough food for the growing population while coping with the already stressed environment. High temperature (HT) is one of the main factors affecting plant growth, development and reproduction and causes male sterility in plants. In male reproductive tissues, metabolic changes induced by HT involve carbohydrates, lipids, hormones, epigenetics and reactive oxygen species, leading to male sterility and ultimately reducing yield. Understanding the mechanism and genes involved in these pathways during the HT stress response will provide a new path to improve crops by using molecular breeding and biotechnological approaches. Moreover, this review provides insight into male sterility and integrates this with suggested strategies to enhance crop tolerance under HT stress conditions at the reproductive stage.

The hot science in rice research: How rice plants cope with heat stress

Global climate change has great impacts on plant growth and development, reducing crop productivity worldwide. Rice (Oryza sativa L.), one of the world's most important food crops, is susceptible to high-temperature stress from seedling stage to reproductive stage. In this review, we summarize recent advances in understanding the molecular mechanisms underlying heat stress responses in rice, including heat sensing and signalling, transcriptional regulation, transcript processing, protein translation, and post-translational regulation. We also highlight the irreversible effects of high temperature on reproduction and grain quality in rice. Finally, we discuss challenges and opportunities for future research on heat stress responses in rice.

OsGRF4AA compromises heat tolerance of developing pollen grains in rice

Extreme high temperature at the meiosis stage causes a severe decrease in spikelet fertility and grain yield in rice. The rice variety grain size on chromosome 2 (GS2) contains sequence variations of OsGRF4 (Oryza sativa growth-regulating factor 4; OsGRF4^AA ), escaping the microRNA miR396-mediated degradation of this gene at the mRNA level. Accumulation of OsGRF4 enhances nitrogen usage and metabolism, and increases grain size and grain yield. In this study, we found that pollen viability and seed-setting rate under heat stress (HS) decreased more seriously in GS2 than in its comparator, Zhonghua 11 (ZH11). Transcriptomic analysis revealed that, following HS, genes related to carbohydrate metabolic processes were expressed and regulated differentially in the anthers of GS2 and ZH11. Moreover, the expression of genes involved in chloroplast development and photosynthesis, lipid metabolism, and key transcription factors, including eight male sterile genes, were inhibited by HS to a greater extent in GS2 than in ZH11. Interestingly, pre-mRNAs of OsGRF4, and a group of essential genes involved in development and fertilization, were differentially spliced in the anthers of GS2 and ZH11. Taken together, our results suggest that variation in OsGRF4 affects proper transcriptional and splicing regulation of genes under HS, and that this can be mediated by, and also feed back to, carbohydrate and nitrogen metabolism, resulting in a reduction in the heat tolerance of rice anthers.

M, Paudel K. Influence of high-temperature stress on rice growth and development. A review

High-temperature stress (HS) has become an alarming threat to the global food system. Rice, an important crop that supports almost half of the global population, is vulnerable to heat stress. Under the influence of HS, it shows various physiological and morphological symptoms that increase spikelet sterility, reduce grain yield, and even cause total crop failure. HS affects growth and yield in two ways: hindrance in the process of pollination and fertilization and reduction of the grain weight. The former is caused by (i) distortion of floral organs, (ii) tapetum degeneration, (iii) low pollen protein concentration, (iv) decline in pollen viability, (v) reduction in dehiscence of anther, (vi) low pollen dispersal, (vii) decrease in number of pollens on stigma, (viii) reduction in pollen grain germination, (ix) hindrance in extension of pollen tubes, and (x) shrinkage of stigma which ultimately cause spikelet infertility. The latter is caused by (i)reduced photosynthetic rate, (ii) a boost in senescence of functional leaves, (iii) reduction of biological synthesis of starch, (iv)reduced starch augmentation, (v) shrunk duration of grain filling, and (vi) declined grain weight which ultimately reduce the grain yield. However, some agronomic and breeding approaches have been adopted for developing thermo-resistant cultivars but the success is limited. In this paper, we have summarized the the morpho-physiological and molecular response of plant to HS, and a few possible management strategies.

Comparative transcriptomic and epigenetic analyses reveal conserved and divergent regulatory pathways in barley response to temperature stresses

DNA methylation and histone modification enable plants to rapidly adapt to adverse temperature stresses, including low temperature (LT) and high temperature (HT) stress. In this study, we conducted physiological, epigenetic, and transcriptomic analyses of barley seedlings grown under control (22°C), mild low temperature (MLT, 14°C) and HT (38°C) conditions to elucidate the underlying molecular mechanisms. Compared to MLT, HT implies greater deleterious effects on barley seedlings' growth. The methylation-sensitive amplification polymorphism analysis showed that MLT induced more DNA methylation and HT more DNA demethylation compared to control. Besides, the higher levels of H3K9ac and H3K4me3 under HT compared to MLT stresses might lead to the loosening of chromatin and, subsequently, the activation of gene expression. Consistently, the transcriptome analysis revealed that there were more differentially expressed genes (DEGs) in plants subjected to HT stress than MLT stress compared to control. The common and unique pathways of these DEGs between MLT and HT were also analyzed. Transcription factors, such as ERF, bHLH, NAC, HSF, and MYB, were most involved in MLT and HT stress. The underlying gene regulation networks of epigenetic modulation-related genes were further explored by weight gene co-expression network analysis. Our study provides new insights into the understanding of epigenetic regulation responses to temperature stress in barley, which will lead to improved strategies for the development of cold- and heat-tolerant barley varieties for sustainable barley production in a climate-changing world.

Genome-Wide Association Mapping Reveals Novel Putative Gene Candidates Governing Reproductive Stage Heat Stress Tolerance in Rice

Temperature rise predicted for the future will severely affect rice productivity because the crop is highly sensitive to heat stress at the reproductive stage. Breeding tolerant varieties is an economically viable option to combat heat stress, for which the knowledge of target genomic regions associated with the reproductive stage heat stress tolerance (RSHT) is essential. A set of 192 rice genotypes of diverse origins were evaluated under natural field conditions through staggered sowings for RSHT using two surrogate traits, spikelet fertility and grain yield, which showed significant reduction under heat stress. These genotypes were genotyped using a 50 k SNP array, and the association analysis identified 10 quantitative trait nucleotides (QTNs) for grain yield, of which one QTN (qHTGY8.1) was consistent across the different models used. Only two out of 10 MTAs coincided with the previously reported QTLs, making the remaing eight novel. A total of 22 QTNs were observed for spikelet fertility, among which qHTSF5.1 was consistently found across three models. Of the QTNs identified, seven coincided with previous reports, while the remaining QTNs were new. The genes near the QTNs were found associated with the protein–protein interaction, protein ubiquitination, stress signal transduction, and so forth, qualifying them to be putative for RSHT. An in silico expression analysis revealed the predominant expression of genes identified for spikelet fertility in reproductive organs. Further validation of the biological relevance of QTNs in conferring heat stress tolerance will enable their utilization in improving the reproductive stage heat stress tolerance in rice.

Genome-Wide Analysis of AP2/ERF Superfamily Genes in Contrasting Wheat Genotypes Reveals Heat Stress-Related Candidate Genes

The AP2/ERF superfamily is one of the largest groups of transcription factors (TFs) in plants, which plays important roles in regulating plant growth and development under heat stress. A complete genome-wide identification, characterization, and expression analysis of AP2/ERF superfamily genes focusing on heat stress response were conducted in bread wheat. This study identified 630 putative AP2/ERF superfamily TF genes in wheat, with 517 genes containing well-defined AP2-protein domains. They were classified into five sub-families, according to domain content, conserved motif, and gene structure. The unique genes identified in this study were 112 TaERF genes, 77 TaDREB genes, four TaAP2 genes, and one TaRAV gene. The chromosomal distribution analysis showed the unequal distribution of TaAP2/ERF genes in 21 wheat chromosomes, with 127 pairs of segmental duplications and one pair of tandem duplication, highly concentrated in TaERF and TaDREB sub-families. The qRT-PCR validation of differentially expressed genes (DEGs) in contrasting wheat genotypes under heat stress conditions revealed that significant DEGs in tolerant and susceptible genotypes could unequivocally differentiate tolerant and susceptible wheat genotypes. This study provides useful information on TaAP2/ERF superfamily genes and reveals candidate genes in response to heat stress, which forms a foundation for heat tolerance breeding in wheat.

Comparative transcriptome analysis of heat stress responses of Clematis lanuginosa and Clematis crassifolia

BACKGROUND

Clematis species are attractive ornamental plants with a variety of flower colors and patterns. Heat stress is one of the main factors restricting the growth, development, and ornamental value of Clematis. Clematis lanuginosa and Clematis crassifolia are large-flowered and evergreen Clematis species, respectively, that show different tolerance to heat stress. We compared and analyzed the transcriptome of C. lanuginose and C. crassifolia under heat stress to determine the regulatory mechanism(s) of resistance.

RESULTS

A total of 1720 and 6178 differentially expressed genes were identified from C. lanuginose and C. crassifolia, respectively. The photosynthesis and oxidation-reduction processes of C. crassifolia were more sensitive than C. lanuginose under heat stress. Glycine/serine/threonine metabolism, glyoxylic metabolism, and thiamine metabolism were important pathways in response to heat stress in C. lanuginose, and flavonoid biosynthesis, phenylalanine metabolism, and arginine/proline metabolism were the key pathways in C. crassifolia. Six sHSPs (c176964_g1, c200771_g1, c204924_g1, c199407_g2, c201522_g2, c192936_g1), POD1 (c200317_g1), POD3 (c210145_g2), DREB2 (c182557_g1), and HSFA2 (c206233_g2) may be key genes in the response to heat stress in C. lanuginose and C. crassifolia.

CONCLUSIONS

We compared important metabolic pathways and differentially expressed genes in response to heat stress between C. lanuginose and C. crassifolia. The results increase our understanding of the response mechanism and candidate genes of Clematis under heat stress. These data may contribute to the development of new Clematis varieties with greater heat tolerance.

Identification and characterization of heat-responsive lncRNAs in maize inbred line CM1

BACKGROUND

Frequent occurrence of extreme high temperature is a major threat to crop production. Increasing evidence demonstrates that long non-coding RNAs (lncRNAs) have important biological functions in the regulation of the response to heat stress. However, the regulatory mechanism of lncRNAs involved in heat response requires further exploration and the regulatory network remains poorly understood in maize.

RESULTS

In this research, high-throughput sequencing was adopted to systematically identify lncRNAs in maize inbred line CM1. In total, 53,249 lncRNAs (259 known lncRNAs and 52,990 novel lncRNAs) were detected, of which 993 lncRNAs showed significantly differential expression (DElncRNAs) under heat stress. By predicting the target genes, 953 common targets shared by cis- and trans-regulation of the DElncRNAs were identified, which exhibited differential expression between the control and the heat stress treatments. Functional annotation indicated that a number of important biological processes and pathways, including photosynthesis, metabolism, translation, stress response, hormone signal transduction, and spliceosome, were enriched for the common targets, suggesting that they play important roles in heat response. A lncRNA-mediated regulatory network was constructed to visualize the molecular response mechanism in response to heat stress, which represented the direct regulatory relationships of DElncRNAs, differentially expressed miRNAs, target genes, and functional annotations.

CONCLUSIONS

This study lays a foundation for further elucidation of the regulatory mechanism for the response to heat stress in the maize inbred line CM1. The findings provide important information for identification of heat-responsive genes, which will be beneficial for the molecular breeding in the cultivation of heat-tolerant maize germplasm.

Epigenetic Regulation of Heat Stress in Plant Male Reproduction

In flowering plants, male reproductive development is highly susceptible to heat stress. In this mini-review, we summarized different anomalies in tapetum, microspores, and pollen grains during anther development under heat stress. We then discussed how epigenetic control, particularly DNA methylation, is employed to cope with heat stress in male reproduction. Further understanding of epigenetic mechanisms by which plants manage heat stress during male reproduction will provide new genetic engineering and molecular breeding tools for generating heat-resistant crops.

Stress-Induced Changes in Alternative Splicing Landscape in Rice: Functional Significance of Splice Isoforms in Stress Tolerance

Improvements in yield and quality of rice are crucial for global food security. However, global rice production is substantially hindered by various biotic and abiotic stresses. Making further improvements in rice yield is a major challenge to the rice research community, which can be accomplished through developing abiotic stress-resilient rice varieties and engineering durable agrochemical-independent pathogen resistance in high-yielding elite rice varieties. This, in turn, needs increased understanding of the mechanisms by which stresses affect rice growth and development. Alternative splicing (AS), a post-transcriptional gene regulatory mechanism, allows rapid changes in the transcriptome and can generate novel regulatory mechanisms to confer plasticity to plant growth and development. Mounting evidence indicates that AS has a prominent role in regulating rice growth and development under stress conditions. Several regulatory and structural genes and splicing factors of rice undergo different types of stress-induced AS events, and the functional significance of some of them in stress tolerance has been defined. Both rice and its pathogens use this complex regulatory mechanism to devise strategies against each other. This review covers the current understanding and evidence for the involvement of AS in biotic and abiotic stress-responsive genes, and its relevance to rice growth and development. Furthermore, we discuss implications of AS for the virulence of different rice pathogens and highlight the areas of further research and potential future avenues to develop climate-smart and disease-resistant rice varieties.

Can omics deliver temperature resilient ready-to-grow crops?

Plants are extensively well-thought-out as the main source for nourishing natural life on earth. In the natural environment, plants have to face several stresses, mainly heat stress (HS), chilling stress (CS) and freezing stress (FS) due to adverse climate fluctuations. These stresses are considered as a major threat for sustainable agriculture by hindering plant growth and development, causing damage, ultimately leading to yield losses worldwide and counteracting to achieve the goal of "zero hunger" proposed by the Food and Agricultural Organization (FAO) of the United Nations. Notably, this is primarily because of the numerous inequities happening at the cellular, molecular and/or physiological levels, especially during plant developmental stages under temperature stress. Plants counter to temperature stress via a complex phenomenon including variations at different developmental stages that comprise modifications in physiological and biochemical processes, gene expression and differences in the levels of metabolites and proteins. During the last decade, omics approaches have revolutionized how plant biologists explore stress-responsive mechanisms and pathways, driven by current scientific developments. However, investigations are still required to explore numerous features of temperature stress responses in plants to create a complete idea in the arena of stress signaling. Therefore, this review highlights the recent advances in the utilization of omics approaches to understand stress adaptation and tolerance mechanisms. Additionally, how to overcome persisting knowledge gaps. Shortly, the combination of integrated omics, genome editing, and speed breeding can revolutionize modern agricultural production to feed millions worldwide in order to accomplish the goal of "zero hunger."

High temperature-mediated disturbance of carbohydrate metabolism and gene expressional regulation in rice: a review

Global warming has induced higher frequencies of excessively high-temperature weather episodes, which pose damage risk to rice growth and production. Past studies seldom specified how high temperature-induced carbohydrate metabolism disturbances from both source and sink affect rice fertilization and production. Here we discuss the mechanism of heat-triggered damage to rice quality and production through disturbance of carbohydrate generation and consumption under high temperatures. Furthermore, we provide strong evidence from past studies that rice varieties that maintain high photosynthesis and carbohydrate usage efficiencies under high temperatures will suffer less heat-induced damage during reproductive developmental stages. We also discuss the complexity of expressional regulation of rice genes in response to high temperatures, while highlighting the important roles of heat-inducible post-transcriptional regulations of gene expression. Lastly, we predict future directions in heat-tolerant rice breeding and also propose challenges that need to be conquered in the future.

Wheat 2-Cys peroxiredoxin plays a dual role in chlorophyll biosynthesis and adaptation to high temperature

The molecular mechanism of high-temperature stress (HTS) response, in plants, has so far been investigated using transcriptomics, while the dynamics of HTS-responsive proteome remain unexplored. We examined the adaptive responses of the resilient wheat cultivar 'Unnat Halna' and dissected the HTS-responsive proteome landscape. This led to the identification of 55 HTS-responsive proteins (HRPs), which are predominantly involved in metabolism and defense pathways. Interestingly, HRPs included a 2-cysteine peroxiredoxin (2CP), designated Ta2CP, presumably involved in stress perception and adaptation. Complementation of Ta2CP in yeast and heterologous expression in Arabidopsis demonstrated its role in thermotolerance. Both Ta2CP silencing and overexpression inferred the involvement of Ta2CP in plant growth and chlorophyll biosynthesis. We demonstrated that Ta2CP interacts with protochlorophyllide reductase b, TaPORB. Reduced TaPORB expression was found in Ta2cp-silenced plants, while upregulation was observed in Ta2CP-overexpressed plants. Furthermore, the downregulation of Ta2CP in Taporb-silenced plants and reduction of protochlorophyllide in Ta2cp-silenced plants suggested the key role of Ta2CP in chlorophyll metabolism. Additionally, the transcript levels of AGPase1 and starch were increased in Ta2cp-silenced plants. More significantly, HTS-treated Ta2cp-silenced plants showed adaptive responses despite increased reactive oxygen species and peroxide concentrations, which might help in rapid induction of high-temperature acclimation.

Heterologous expression of heat stress-responsive AtPLC9 confers heat tolerance in transgenic rice

BACKGROUND

As global warming becomes increasingly severe, it is urgent that we enhance the heat tolerance of crops. We previously reported that Arabidopsis thaliana PHOSPHOINOSITIDE-SPECIFIC PHOSPHOLIPASE C9 (AtPLC9) promotes heat tolerance.

RESULTS

In this study, we ectopically expressed AtPLC9 in rice to examine its potential to improve heat tolerance in this important crop. Whereas AtPLC9 did not improve rice tolerance to salt, drought or cold, transgenic rice did exhibit greater heat tolerance than the wild type. High-throughput RNA-seq revealed extensive and dynamic transcriptome reprofiling in transgenic plants after heat stress. Moreover, the expression of some transcription factors and calcium ion-related genes showed specific upregulation in transgenic rice after heat stress, which might contribute to the enhanced heat tolerance.

CONCLUSIONS

This study provides preliminary guidance for using AtPLC9 to improve heat tolerance in cereal crops and, more broadly, highlights that heterologous transformation can assist with molecular breeding.

Integrated analysis of co-expression, conserved genes and gene families reveal core regulatory network of heat stress response in Cleistogenes songorica, a xerophyte perennial desert plant

BACKGROUND

As global warming continues, heat stress (HS) is becoming an increasingly significant factor limiting plant growth and reproduction, especially for cool-season grass species. The objective of this study was to determine the transcriptional regulatory network of Cleistogenes songorica under HS via transcriptome profiling, identify of gene families and comparative analysis across major Poaceae species.

RESULTS

Physiological analysis revealed significantly decreased leaf relative water content (RWC) but increased proline (Pro) content in C. songorica under 24 h of HS. Transcriptome profiling indicated that 16,028 and 14,645 genes were differentially expressed in the shoots and roots of C. songorica under HS, respectively. Two subgenomes of C. songorica provide equal contribution under HS on the basis of the distribution and expression of differentially expressed genes (DEGs). Furthermore, 216 DEGs were identified as key evolutionarily conserved genes involved in the response to HS in C. songorica via comparative analysis with genes of four Poaceae species; these genes were involved in the 'response to heat' and 'heat acclimation'. Notably, most of the conserved DEGs belonged to the heat-shock protein (HSP) superfamily. Similar results were also obtained from co-expression analysis. Interestingly, hub-genes of co-expression analysis were found to overlap with conserved genes, especially heat-shock protein (HSP). In C. songorica, 84 HSP and 32 heat-shock transcription factor (HSF) genes were identified in the allotetraploid C. songorica genome, and might have undergone purifying selection during evolutionary history based on syntenic and phylogenetic analysis. By analysing the expression patterns of the CsHSPs and CsHSFs, we found that the transcript abundance of 72.7% of the CsHSP genes and of 62.5% of the CsHSF genes changed under heat stress in both the shoots and roots. Finally, a core regulatory network of HS was constructed on the basis of the CsHSP, CsHSF and other responsive genes in C. songorica.

CONCLUSIONS

Regulatory network and key genes were comprehensively analysed and identified in C. songorica under HS. This study improves our knowledge of thermotolerance mechanisms in native grasses, and also provides candidate genes for potential applications in the genetic improvement of grasses.

Transcriptomic Analysis Reveals Important Roles of Lignin and Flavonoid Biosynthetic Pathways in Rice Thermotolerance During Reproductive Stage

Rice is one of the major staple cereals in the world, but heat stress is increasingly threatening its yield. Analyzing the thermotolerance mechanism from new thermotolerant germplasms is very important for rice improvement. Here, physiological and transcriptome analyses were used to characterize the difference between two germplasms, heat-sensitive MH101 and heat-tolerant SDWG005. Two genotypes exhibited diverse heat responses in pollen viability, pollination characteristics, and antioxidant enzymatic activity in leaves and spikelets. Through cluster analysis, the global transcriptomic changes indicated that the ability of SDWG005 to maintain a steady-state balance of metabolic processes played an important role in thermotolerance. After analyses of gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment, we found that the thermotolerance mechanism in SDWG00 was associated with reprogramming the cellular activities, such as response to abiotic stress and metabolic reorganization. In contrast, the down-regulated genes in MH101 that appeared to be involved in DNA replication and DNA repair proofreading, could cause serious injury to reproductive development when exposed to high temperature during meiosis. Furthermore, we identified 77 and 11 differentially expressed genes (DEGs) involved in lignin and flavonoids biosynthetic pathways, respectively. Moreover, we found that more lignin deposition and flavonoids accumulation happened in SDWG005 than in MH101 under heat stress. The results indicated that lignin and flavonoid biosynthetic pathways might play important roles in rice heat resistance during meiosis.

Reproductive tissues-specific meta-QTLs and candidate genes for development of heat-tolerant rice cultivars

By integrating genetics and genomics data, reproductive tissues-specific and heat stress responsive 35 meta-QTLs and 45 candidate genes were identified, which could be exploited through marker-assisted breeding for fast-track development of heat-tolerant rice cultivars. Rice holds the key to future food security. In rice-growing areas, temperature has already reached an optimum level for growth, hence, any further increase due to global climate change could significantly reduce rice yield. Several mapping studies have identified a plethora of reproductive tissue-specific and heat stress associated inconsistent quantitative trait loci (QTL), which could be exploited for improvement of heat tolerance. In this study, we performed a meta-analysis on previously reported QTLs and identified 35 most consistent meta-QTLs (MQTLs) across diverse genetic backgrounds and environments. Genetic and physical intervals of nearly 66% MQTLs were narrower than 5 cM and 2 Mb respectively, indicating hotspot genomic regions for heat tolerance. Comparative analyses of MQTLs underlying genes with microarray and RNA-seq based transcriptomic data sets revealed a core set of 45 heat-responsive genes, among which 24 were reproductive tissue-specific and have not been studied in detail before. Remarkably, all these genes corresponded to various stress associated functions, ranging from abiotic stress sensing to regulating plant stress responses, and included heat-shock genes (OsBiP2, OsMed37_1), transcription factors (OsNAS3, OsTEF1, OsWRKY10, OsWRKY21), transmembrane transporters (OsAAP7A, OsAMT2;1), sugar metabolizing (OsSUS4, α-Gal III) and abiotic stress (OsRCI2-7, SRWD1) genes. Functional data evidences from Arabidopsis heat-shock genes also suggest that OsBIP2 may be associated with thermotolerance of pollen tubes under heat stress conditions. Furthermore, promoters of identified genes were enriched with heat, dehydration, pollen and sugar responsive cis-acting regulatory elements, proposing a common regulatory mechanism might exist in rice for mitigating reproductive stage heat stress. These findings strongly support our results and provide new candidate genes for fast-track development of heat-tolerant rice cultivars.