Small RNAs in regulating temperature stress response in plants

The Expression Profile of Genes Related to Carotenoid Biosynthesis in Pepper Under Abiotic Stress Reveals a Positive Correlation with Plant Tolerance

In light of the increasingly adverse environmental conditions and the concomitant challenges to the survival of important crops, there is a pressing need to enhance the resilience of pepper seedlings to extreme weather. Carotenoid plays an important role in plants' resistance to abiotic stress. Nevertheless, the relationship between carotenoid biosynthesis and sweet pepper seedlings' resistance to different abiotic stresses remains uncertain. In this study, the carotenoid content in abiotic-stressed sweet pepper seedling roots was determined, revealing that carotenoid content was extremely significantly elevated by more than 16-fold under salt stress, followed by drought stress (8-fold), and slightly elevated by only about 1-fold under waterlogging stress. After that, serine/threonine-protein phosphatase 2A (PP2A) was found to be the suitable reference gene (RG) in sweet pepper seedling roots under different abiotic stresses by using RT-qPCR and RefFinder analysis. Subsequently, using PP2A as the RG, RT-qPCR analysis showed that the expression level of most genes associated with carotenoid biosynthesis was extremely significantly up-regulated in sweet pepper seedlings under salt and drought stress. Specifically, violoxanthin deepoxidase (VDE) was significantly up-regulated by more than 481- and 36-fold under salt and drought stress, respectively; lycopene epsilon cyclase (LCYE) was significantly up-regulated by more than 840- and 23-fold under salt and drought stress, respectively. This study contributes to a more comprehensive understanding of the carotenoid biosynthesis pathway serving as a major source of retrograde signals in pepper subjected to different abiotic stresses.

Suppression of SMXL4 and SMXL5 confers enhanced thermotolerance through promoting HSFA2 transcription in Arabidopsis

Identifying the essential factors and underlying mechanisms regulating plant heat stress (HS) responses is crucial for mitigating the threat posed by HS on plant growth, development, distribution, and productivity. In this study, we found that the Arabidopsis (Arabidopsis thaliana) super-killer2 (ski2) dicer-like4 (dcl4) mutant, characterized by RNA processing defects and the accumulation of abundant 22-nt small interfering RNAs derived from protein-coding transcripts, displayed significantly increased expression levels of HS-responsive genes and enhanced thermotolerance. These traits primarily resulted from the suppression of SMAX1-LIKE4 (SMXL4) and SMXL5, which encode 2 putative transcriptional regulators that belong to the SMXL protein family. While smxl4 and smxl5 single mutants were similar to wild type, the smxl4 smxl5 double mutant displayed substantially heightened seedling thermotolerance. Further investigation demonstrated that SMXL4 and SMXL5 repressed the transcription of HEAT-SHOCK TRANSCRIPTION FACTOR A2 (HSFA2), encoding a master regulator of thermotolerance, independently of ethylene-response factor-associated amphiphilic repression motifs. Moreover, SMXL4 and SMXL5 interacted with HSFA1d and HSFA1e, central regulators sensing and transducing HS stimuli, and antagonistically affected their transactivation activity. In addition, HSFA2 directly bound to the SMXL4 and SMXL5 promoters, inducing their expression during recovery from HS. Collectively, our findings elucidate the role of the SMXL4/SMXL5-HSFA2 regulatory module in orchestrating plant thermotolerance under HS.

Integrated analysis of transcriptome and small RNAome reveals regulatory network of rapid and long-term response to heat stress in Rhododendron moulmainense

MAIN CONCLUSION

The post-transcriptional gene regulatory pathway and small RNA pathway play important roles in regulating the rapid and long-term response of Rhododendron moulmainense to high-temperature stress. The Rhododendron plays an important role in maintaining ecological balance. However, it is difficult to domesticate for use in urban ecosystems due to their strict optimum growth temperature condition, and its evolution and adaptation are little known. Here, we combined transcriptome and small RNAome to reveal the rapid response and long-term adaptability regulation strategies in Rhododendron moulmainense under high-temperature stress. The post-transcriptional gene regulatory pathway plays important roles in stress response, in which the protein folding pathway is rapidly induced at 4 h after heat stress, and alternative splicing plays an important role in regulating gene expression at 7 days after heat stress. The chloroplasts oxidative damage is the main factor inhibiting photosynthesis efficiency. Through WGCNA analysis, we identified gene association patterns and potential key regulatory genes responsible for maintaining the ROS steady-state under heat stress. Finally, we found that the sRNA synthesis pathway is induced under heat stress. Combined with small RNAome, we found that more miRNAs are significantly changed under long-term heat stress. Furthermore, MYBs might play a central role in target gene interaction network of differentially expressed miRNAs in R. moulmainense under heat stress. MYBs are closely related to ABA, consistently, ABA synthesis and signaling pathways are significantly inhibited, and the change in stomatal aperture is not obvious under heat stress. Taken together, we gained valuable insights into the transplantation and long-term conservation domestication of Rhododendron, and provide genetic resources for genetic modification and molecular breeding to improve heat resistance in Rhododendron.

The miR156b-GmSPL2b module mediates male fertility regulation of cytoplasmic male sterility-based restorer line under high-temperature stress in soybean

High-temperature (HT) stress at flowering stage causes significant damage to soybean, including pollen abortion and fertilization failure, but few genes involved in male fertility regulation under HT stress in soybean have been characterized. Here, we demonstrated that miR156b-GmSPL2b module involved in male fertility regulation of soybean cytoplasmic male sterility (CMS)-based restorer line under HT stress. Overexpression of miR156b decreased male fertility in soybean CMS-based restorer line and its hybrid F₁ with CMS line under HT stress. RNA-seq analysis found that miR156b mediated male fertility regulation in soybean under HT stress by regulating the expression of pollen development and HT response related genes. Metabolomic analysis of miR156bOE revealed reduction in flavonoid content under HT stress. Integrated transcriptomic and metabolomic analysis showed that the overexpression of miR156b caused flavonoid metabolism disorder in soybean flower bud under HT stress. Knockout of GmSPL2b also decreased the thermotolerance of soybean CMS-based restorer line during flowering. Moreover, GmSPL2b turned out to be directly bounded to the promoter of GmHSFA6b. Further verification indicated that GmHSFA6b overexpression enhanced HT tolerance in Arabidopsis during flowering. Substance content and gene expression analysis revealed that miR156b-GmSPL2b may mediate reactive oxygen species clearance by regulating flavonoid metabolism, thus participating in the regulation of male fertility in soybean under HT stress. This study not only provided important progress for understanding the molecular mechanism of miR156b-GmSPL2b regulating the male fertility of soybean CMS-based restorer line under HT stress, but also provided genetic resources and theoretical basis for creating HT-tolerant strong restorer lines.

Identification of microRNA158 from Anthurium andraeanum and Its Function in Cold Stress Tolerance

Anthurium andraeanum is a tropical flower with high ornamental and economic value. Cold stress is one of the major abiotic stresses affecting the quality and value of A. andraeanum; thus, improving the cold tolerance of this species is an important breeding objective. MicroRNAs (miRNAs) have a critical role in plant abiotic stress responses, but their specific molecular regulatory mechanisms are largely unknown, including those related to the cold stress response in A. andraeanum. Here, we identified and cloned the precursor of miR158 from A. andraeanum (Aa-miR158). Both Aa-miR158 and its target gene (c48247) had higher expression levels in strong leaves than in other tissues or organs. Further study revealed that the transcript level of Aa-miR158 was increased by cold stress. Heterologous overexpression of Aa-miR158 improved cold stress tolerance in Arabidopsis, which was associated with decreases in the malondialdehyde (MDA) concentration and relative electrical conductivity (REC) as well as increases in peroxidase (POD) and catalase (CAT) activity. Moreover, overexpressing Aa-miR158 significantly increased the expression of endogenous genes related to cold stress tolerance and reactive oxygen species (ROS) levels in transgenic Arabidopsis under cold stress. Overall, our results demonstrate that Aa-miR158 is significantly involved in the cold stress response and provide a new strategy for cold tolerance breeding of A. andraeanum.

A chloroplast-localized pentatricopeptide repeat protein involved in RNA editing and splicing and its effects on chloroplast development in rice

BACKGROUND

The chloroplast is the organelle responsible for photosynthesis in higher plants. The generation of functional chloroplasts depends on the precise coordination of gene expression in the nucleus and chloroplasts and is essential for the development of plants. However, little is known about nuclear-plastid regulatory mechanisms at the early stage of chloroplast generation in rice.

RESULTS

In this study, we identified a rice (Oryza sativa) mutant that exhibited albino and seedling-lethal phenotypes and named it ssa1(seedling stage albino1). Transmission electron microscopy (TEM) analysis indicated that the chloroplasts of ssa1 did not have organized thylakoid lamellae and that the chloroplast structure was destroyed. Genetic analysis revealed that the albino phenotypes of ssa1 were controlled by a pair of recessive nuclear genes. Map-based cloning experiments found that SSA1 encoded a pentapeptide repeat (PPR) protein that was allelic to OSOTP51,which was previously reported to participate in Photosystem I (PSI) assembly. The albino phenotype was reversed to the wild type (WT) phenotype when the normal SSA1 sequence was expressed in ssa1 under the drive of the actin promoter. Knockout experiments further created mutants ssa1-2/1-9, which had a phenotype similar to that of ssa1. SSA1 consisted of 7 pentatricopeptide repeat domains and two C-terminal LAGLIDADG tandem sequence motifs and was located in the chloroplast. GUS staining and qRT-PCR analysis showed that SSA1 was mainly expressed in young leaves and stems. In the ssa1 mutants, plastid genes transcribed by plastid-encoded RNA polymerase decreased, while those transcribed by nuclear-encoded RNA polymerase increased at the mRNA level. Loss-of-function SSA1 destroys RNA editing of ndhB-737 and intron splicing of atpF and ycf3-2 in the plastid genome. Yeast two-hybrid and BiFC assays revealed that SSA1 physically interacted with two new RNA editing partners, OsMORF8 and OsTRXz, which have potential functions in RNA editing and chloroplast biogenesis.

CONCLUSIONS

Rice SSA1 encodes a pentatricopeptide repeat protein, which is targeted to the chloroplast. SSA1 regulates early chloroplast development and plays a critical role in RNA editing and intron splicing in rice. These data will facilitate efforts to further elucidate the molecular mechanism of chloroplast biogenesis.

Epigenetic marks for mitigating abiotic stresses in plants

Abiotic stressors are one of the major factors affecting agricultural output. Plants have evolved adaptive systems to respond appropriately to various environmental cues. These responses can be accomplished by modulating or fine-tuning genetic and epigenetic regulatory mechanisms. Understanding the response of plants' molecular features to abiotic stress is a priority in the current period of continued environmental changes. Epigenetic modifications are necessary that control gene expression by changing chromatin status and recruiting various transcription regulators. The present study summarized the current knowledge on epigenetic modifications concerning plant responses to various environmental stressors. The functional relevance of epigenetic marks in regulating stress tolerance has been revealed, and epigenetic changes impact the effector genes. This study looks at the epigenetic mechanisms that govern plant abiotic stress responses, especially DNA methylation, histone methylation/acetylation, chromatin remodeling, and various metabolites. Plant breeders will benefit from a thorough understanding of these processes to create alternative crop improvement approaches. Genome editing with clustered regularly interspaced short palindromic repeat/CRISPR-associated proteins (CRISPR/Cas) provides genetic tools to make agricultural genetic engineering more sustainable and publicly acceptable.

Integrated sRNA-seq and RNA-seq Analyses Reveal a microRNA Regulation Network Involved in Cold Response in Pisum sativum L

(1) Background: Cold stress affects growth and development in plants and is a major environmental factor that decreases productivity. Over the past two decades, the advent of next generation sequencing (NGS) technologies has opened new opportunities to understand the molecular bases of stress resistance by enabling the detection of weakly expressed transcripts and the identification of regulatory RNAs of gene expression, including microRNAs (miRNAs). (2) Methods: In this study, we performed time series sRNA and mRNA sequencing experiments on two pea (Pisum sativum L., Ps) lines, Champagne frost-tolerant and Térèse frost-sensitive, during a low temperature treatment versus a control condition. (3) Results: An integrative analysis led to the identification of 136 miRNAs and a regulation network composed of 39 miRNA/mRNA target pairs with discordant expression patterns. (4) Conclusions: Our findings indicate that the cold response in pea involves 11 miRNA families as well as their target genes related to antioxidative and multi-stress defense mechanisms and cell wall biosynthesis.

miR2119, a Novel Transcriptional Regulator, Plays a Positive Role in Woody Plant Drought Tolerance by Mediating the Degradation of the CkBI-1 Gene Associated with Apoptosis

Caragana korshinskii, an important vegetation restoration species with economic and ecological benefits in the arid region of northwest China, is characterized by significant drought tolerance. However, the underlying molecular mechanisms by which miRNAs confer this trait in C. korshinskii are unclear. Here, we investigate the effect of CkmiR2119 on drought tolerance and identified its target gene, CkBI-1. A negative correlation of CkmiR2119 and CkBI-1 in both stems and leaves in a drought gradient treatment followed by target gene validation suggest that CkmiR2119 might negatively regulate CkBI-1. Consistently, a decrease in the expression of the CkBI-1 gene was observed after both transient transformation and stable transformation of CkamiR2119 in tobacco (Nicotiana tabacum). Moreover, the physiological analysis of CkamiR2119 and CkBI-1 transgenic plants further indicate that CkmiR2119 can enhance the drought tolerance of C. korshinskii in two aspects: (i) downregulating CkBI-1 expression to accelerate vessel maturation in stems; (ii) contributing to a higher level of CkBI-1 in mesophyll cells to inhibit programmed cell death (PCD). This work reveals that CkmiR2119 can increase plants' drought tolerance by downregulating the expression of CkBI-1, providing a theoretical basis to improve plants' ability to withstand stress tolerance by manipulating miRNAs.

Molecular insights into sensing, regulation and improving of heat tolerance in plants

Climate-change-mediated increase in temperature extremes has become a threat to plant productivity. Heat stress-induced changes in growth pattern, sensitivity to pests, plant phonologies, flowering, shrinkage of maturity period, grain filling, and increased senescence result in significant yield losses. Heat stress triggers multitude of cellular, physiological and molecular responses in plants beginning from the early sensing followed by signal transduction, osmolyte synthesis, antioxidant defense, and heat stress-associated gene expression. Several genes and metabolites involved in heat perception and in the adaptation response have been isolated and characterized in plants. Heat stress responses are also regulated by the heat stress transcription factors (HSFs), miRNAs and transcriptional factors which together form another layer of regulatory circuit. With the availability of functionally validated candidate genes, transgenic approaches have been applied for developing heat-tolerant transgenic maize, tobacco and sweet potato. In this review, we present an account of molecular mechanisms of heat tolerance and discuss the current developments in genetic manipulation for heat tolerant crops for future sustainable agriculture.

Advances in 5-Aminolevulinic Acid Priming to Enhance Plant Tolerance to Abiotic Stress

Priming is an adaptive strategy that improves plant defenses against biotic and abiotic stresses. Stimuli from chemicals, abiotic cues, and pathogens can trigger the establishment of priming state. Priming with 5-aminolevulinic acid (ALA), a potential plant growth regulator, can enhance plant tolerance to the subsequent abiotic stresses, including salinity, drought, heat, cold, and UV-B. However, the molecular mechanisms underlying the remarkable effects of ALA priming on plant physiology remain to be elucidated. Here, we summarize recent progress made in the stress tolerance conferred by ALA priming in plants and provide the underlying molecular and physiology mechanisms of this phenomenon. Priming with ALA results in changes at the physiological, transcriptional, metabolic, and epigenetic levels, and enhances photosynthesis and antioxidant capacity, as well as nitrogen assimilation, which in turn increases the resistance of abiotic stresses. However, the signaling pathway of ALA, including receptors as well as key components, is currently unknown, which hinders the deeper understanding of the defense priming caused by ALA. In the future, there is an urgent need to reveal the molecular mechanisms by which ALA regulates plant development and enhances plant defense with the help of forward genetics, multi-omics technologies, as well as genome editing technology.

A small RNA of miR2119b from soybean CMS line acts as a negative regulator of male fertility in transgenic Arabidopsis

The miR2119 is involved in the growth, development and abiotic stress response of some legumes, including Medicago truncatula, Phaseolus vulgaris and soybean (Glycine max (L.) Merr.). Our previous small RNA sequencing analysis showed that miR2119b was up-regulated in the flower buds of soybean cytoplasmic male sterile (CMS) line compared with its maintainer line, but the role and mechanism of miR2119b in the regulation of soybean male fertility are still unclear. In this study, the gma-miR2119b and its target gene alcohol dehydrogenase 1.3b (ADH1.3b) were characterized and found to be highly expressed in the flowers of soybean CMS line and its maintainer. Transgenic Arabidopsis plants overexpressing gma-miR2119b exhibit male fertility abnormalities, including pollen fertility and germination rate decreased. Enzyme activity detection found the ADH and catalase (CAT) enzyme activities in inflorescence of gma-miR2119b overexpressed plants were lower than those of wild-type. Bioinformatics and gene expression analysis showed that gma-miR2119b/GmADH1.3b module was responsive to high temperature (HT) stress during flowering. After HT stress, the gma-miR2119b overexpressed plants showed male sterility, including shorter filament, sterile pollen, indehiscent anther and non seed. Moreover, some key genes involved in HT response and reactive oxygen species (ROS) signal regulation pathway, including heat shock protein70, galactinol synthase 1 and CAT, showed down-regulated expression in transgenic plants under HT stress, suggesting that gma-miR2119b regulates male fertility via HT-ROS signaling pathway under HT stress. It was speculated that the gma-miR2119b acted as a negative regulator of male fertility in plants by regulating ADH1, HT-induced and ROS scavenging genes expression.

Small RNAs: The Essential Regulators in Plant Thermotolerance

Small RNAs (sRNAs) are a class of non-coding RNAs that consist of 21-24 nucleotides. They have been extensively investigated as critical regulators in a variety of biological processes in plants. sRNAs include two major classes: microRNAs (miRNAs) and small interfering RNAs (siRNAs), which differ in their biogenesis and functional pathways. Due to global warming, high-temperature stress has become one of the primary causes for crop loss worldwide. Recent studies have shown that sRNAs are involved in heat stress responses in plants and play essential roles in high-temperature acclimation. Genome-wide studies for heat-responsive sRNAs have been conducted in many plant species using high-throughput sequencing. The roles for these sRNAs in heat stress response were also unraveled subsequently in model plants and crops. Exploring how sRNAs regulate gene expression and their regulatory mechanisms will broaden our understanding of sRNAs in thermal stress responses of plant. Here, we highlight the roles of currently known miRNAs and siRNAs in heat stress responses and acclimation of plants. We also discuss the regulatory mechanisms of sRNAs and their targets that are responsive to heat stress, which will provide powerful molecular biological resources for engineering crops with improved thermotolerance.

Heat Shock Signaling in Land Plants: From Plasma Membrane Sensing to the Transcription of Small Heat Shock Proteins

Heat stress events are major factors limiting crop productivity. During summer days, land plants must anticipate in a timely manner upcoming mild and severe temperature. They respond by accumulating protective heat-shock proteins (HSPs), conferring acquired thermotolerance. All organisms synthetize HSPs; many of which are members of the conserved chaperones families. This review describes recent advances in plant temperature sensing, signaling, and response. We highlight the pathway from heat perception by the plasma membrane through calcium channels, such as cyclic nucleotide-gated channels, to the activation of the heat-shock transcription factors (HSFs). An unclear cellular signal activates HSFs, which act as essential regulators. In particular, the HSFA subfamily can bind heat shock elements in HSP promoters and could mediate the dissociation of bound histones, leading to HSPs transcription. Although plants can modulate their transcriptome, proteome, and metabolome to protect the cellular machinery, HSP chaperones prevent, use, and revert the formation of misfolded proteins, thereby avoiding heat-induced cell death. Remarkably, the HSP20 family is mostly tightly repressed at low temperature, suggesting that a costly mechanism can become detrimental under unnecessary conditions. Here, the role of HSP20s in response to HS and their possible deleterious expression at non-HS temperatures is discussed.

Differential miRNA profiles in South African cassava mosaic virus-infected cassava landraces reveal clues to susceptibility and tolerance to cassava mosaic disease

Specific miRNA families are involved in susceptibility or antiviral immunity in plants. Manihot esculenta Crantz (cassava) is a perennial plant that is an important food security crop in sub-Saharan Africa. Cassava is susceptible to several begomoviruses that cause cassava mosaic disease (CMD). In this study, we investigated the leaf miRNAome response in a tolerant (TME3) and susceptible (T200) cassava landrace challenged with South African cassava mosaic virus. RNAseq was performed on leaf samples at 12, 32 and 67 days post infection (dpi), representing early, symptomatic and late persistent stages of CMD infection. Significantly, distinct profiles of conserved miRNA family expression between the T200 and TME3 landraces at the three infection stages were observed. Notably at 12 days post SACMV infection, TME3 exhibited significant downregulation (log2fold<2.0) of 42 %, compared to 9% in T200, of the conserved miRNA families. This demonstrates an overall early response to SACMV in TME3 prior to symptom appearance not observed in T200, and expression of a large cohort of miRNA-regulated genes. Notably, at early infection, downregulation of mes-miR162 and 168 that target antiviral posttransriptional gene silencing (PTGS) regulators DCL1 and AGO1, respectively, was observed in TME3, and AGO1 and DCL1 expression was higher compared to T200 post infection. Early rapid responses prior to symptom development, including RNA silencing, may be key to establishing the tolerance/recovery phenotype exhibited by TME3 landrace later on at 67 dpi. At recovery, TME3 was hallmarked by a highly significant down-regulation of mes-miR167. MiR167 targets an auxin responsive factor which plays a role in auxin signaling and adaptive responses to stress, suggesting the importance of the auxin signaling in recovery of SACMV-induced symptoms. The gene targets of these miRNAs and their associated networks may provide clues to the molecular basis of CMD tolerance in perennial hosts such as cassava.

Heat-Responsive miRNAs Participate in the Regulation of Male Fertility Stability in Soybean CMS-Based F1 under High Temperature Stress

MicroRNAs (miRNAs), a class of noncoding small RNAs (sRNAs), are widely involved in the response to high temperature (HT) stress at both the seedling and flowering stages. To dissect the roles of miRNAs in regulating male fertility in soybean cytoplasmic male sterility (CMS)-based F1 under HT, sRNA sequencing was performed using flower buds from HT-tolerant and HT-sensitive CMS-based F1 combinations (NF1 and YF1, respectively). A total of 554 known miRNAs, 59 new members of known miRNAs, 712 novel miRNAs, and 1145 target genes of 580 differentially expressed miRNAs (DEMs) were identified under normal temperature and HT conditions. Further integrated analysis of sRNA and transcriptome sequencing found that 21 DEMs and 15 differentially expressed target genes, such as gma-miR397a/Laccase 2, gma-miR399a/Inorganic phosphate transporter 1-4, and gma-miR4413a/PPR proteins, mitochondrial-like, were negatively regulated under HT stress. Furthermore, all members of the gma-miR156 family were suppressed by HT stress in both NF1 and YF1, but were highly expressed in YF1 under HT condition. The negative correlation between gma-miR156b and its target gene squamosa promoter-binding protein-like 2b was confirmed by expression analysis, and overexpression of gma-miR156b in Arabidopsis led to male sterility under HT stress. With these results, we proposed that miRNAs play an important role in the regulation of male fertility stability in soybean CMS-based F1 under HT stress.

Aux/IAA14 Regulates microRNA-Mediated Cold Stress Response in Arabidopsis Roots

The phytohormone auxin and microRNA-mediated regulation of gene expressions are key regulators of plant growth and development at both optimal and under low-temperature stress conditions. However, the mechanistic link between microRNA and auxin in regulating plant cold stress response remains elusive. To better understand the role of microRNA (miR) in the crosstalk between auxin and cold stress responses, we took advantage of the mutants of Arabidopsis thaliana with altered response to auxin transport and signal. Screening of the mutants for root growth recovery after cold stress at 4 °C revealed that the auxin signaling mutant, solitary root 1 (slr1; mutation in Aux/IAA14), shows a hypersensitive response to cold stress. Genome-wide expression analysis of miRs in the wild-type and slr1 mutant roots using next-generation sequencing revealed 180 known and 71 novel cold-responsive microRNAs. Cold stress also increased the abundance of 26-31 nt small RNA population in slr1 compared with wild type. Comparative analysis of microRNA expression shows significant differential expression of 13 known and 7 novel miRs in slr1 at 4 °C compared with wild type. Target gene expression analysis of the members from one potential candidate miR, miR169, revealed the possible involvement of miR169/NF-YA module in the Aux/IAA14-mediated cold stress response. Taken together, these results indicate that SLR/IAA14, a transcriptional repressor of auxin signaling, plays a crucial role in integrating miRs in auxin and cold responses.

MicroRNAs as Important Regulators of Heat Stress Responses in Plants

Heat stress is a major abiotic stress that significantly affects plant growth and productivity. Plants have, however, evolved complex adaptive mechanisms to cope with heat stress. MicroRNAs (miRNAs) are important molecules that regulate gene expression through the post-transcriptional degradation of target mRNA molecules or by repressing translation. Plant miRNAs play essential roles in development and a variety of stress responses. Recent advances in high-throughput sequencing technologies have enabled the identification and characterization of an increasing number of heat-responsive miRNAs in diverse plant species. Heat-regulated miRNAs combined with their target genes constitute large regulatory networks that control various metabolic pathways, including protein refolding, antioxidant defense, maintenance of photosynthetic systems, protection of reproductive tissues, regulation of flowering time, and miRNA biogenesis. In this review, we summarize the information acquired to date about the significance of plant miRNAs and their target genes in heat stress tolerance, thereby helping to identify the regulatory mechanisms that underlie heat stress responses in plants.

Molecular and genetic bases of heat stress responses in crop plants and breeding for increased resilience and productivity

To ensure the food security of future generations and to address the challenge of the 'no hunger zone' proposed by the FAO (Food and Agriculture Organization), crop production must be doubled by 2050, but environmental stresses are counteracting this goal. Heat stress in particular is affecting agricultural crops more frequently and more severely. Since the discovery of the physiological, molecular, and genetic bases of heat stress responses, cultivated plants have become the subject of intense research on how they may avoid or tolerate heat stress by either using natural genetic variation or creating new variation with DNA technologies, mutational breeding, or genome editing. This review reports current understanding of the genetic and molecular bases of heat stress in crops together with recent approaches to creating heat-tolerant varieties. Research is close to a breakthrough of global relevance, breeding plants fitter to face the biggest challenge of our time.

Differential cell persistence is observed in the Arabidopsis female gametophyte during heat stress

The central cell withstands heat stress better than the egg and antipodal cells. Insilco analysis of transcriptomic data identified several heat responsive genes which are central cell specific. Crop damage due to heat stress (HS) is a major cause of yield lost. Plants are particularly susceptible to negative effects of HS during gametophyte development and fertilization. Extensive studies have been performed on the male gametophyte under HS, but how the female gametophyte copes with HS is largely unknown. To learn how the different cell types of the female gametophyte reacts to HS, we studied unfertilized CDC123::H2B:YFP ovules. We found that the YFP-specific florescent signal persisted in the central cell during HS significantly more than the egg cell. We also found that the fluorescent signal persistence was the lowest in the antipodal cells. This finding suggests that the reaction of the female gametophyte to HS is rather unique and differentially mediated according to the cell's identity. In addition, mining through published transcriptomic datasets we found that several important heat stress responsive genes which are extremely upregulated during HS (more than 64-fold) are specifically expressed in the CC but not in the EC. Further research such as comparative transcriptomics and cell biology will likely shed more light on the phenomena reported here and increase our basic understandings about the ways sexual reproduction processes are affected by heat stress.

Root Growth Adaptation to Climate Change in Crops

Climate change is threatening crop productivity worldwide and new solutions to adapt crops to these environmental changes are urgently needed. Elevated temperatures driven by climate change affect developmental and physiological plant processes that, ultimately, impact on crop yield and quality. Plant roots are responsible for water and nutrients uptake, but changes in soil temperatures alters this process limiting crop growth. With the predicted variable climatic forecast, the development of an efficient root system better adapted to changing soil and environmental conditions is crucial for enhancing crop productivity. Root traits associated with improved adaptation to rising temperatures are increasingly being analyzed to obtain more suitable crop varieties. In this review, we will summarize the current knowledge about the effect of increasing temperatures on root growth and their impact on crop yield. First, we will describe the main alterations in root architecture that different crops undergo in response to warmer soils. Then, we will outline the main coordinated physiological and metabolic changes taking place in roots and aerial parts that modulate the global response of the plant to increased temperatures. We will discuss on some of the main regulatory mechanisms controlling root adaptation to warmer soils, including the activation of heat and oxidative pathways to prevent damage of root cells and disruption of root growth; the interplay between hormonal regulatory pathways and the global changes on gene expression and protein homeostasis. We will also consider that in the field, increasing temperatures are usually associated with other abiotic and biotic stresses such as drought, salinity, nutrient deficiencies, and pathogen infections. We will present recent advances on how the root system is able to integrate and respond to complex and different stimuli in order to adapt to an increasingly changing environment. Finally, we will discuss the new prospects and challenges in this field as well as the more promising pathways for future research.

Transgenic Breeding Approaches for Improving Abiotic Stress Tolerance: Recent Progress and Future Perspectives

The recent rapid climate changes and increasing global population have led to an increased incidence of abiotic stress and decreased crop productivity. Environmental stresses, such as temperature, drought, nutrient deficiency, salinity, and heavy metal stresses, are major challenges for agriculture, and they lead to a significant reduction in crop growth and productivity. Abiotic stress is a very complex phenomenon, involving a variety of physiological and biochemical changes in plant cells. Plants exposed to abiotic stress exhibit enhanced levels of ROS (reactive oxygen species), which are highly reactive and toxic and affect the biosynthesis of chlorophyll, photosynthetic capacity, and carbohydrate, protein, lipid, and antioxidant enzyme activities. Transgenic breeding offers a suitable alternative to conventional breeding to achieve plant genetic improvements. Over the last two decades, genetic engineering/transgenic breeding techniques demonstrated remarkable developments in manipulations of the genes for the induction of desired characteristics into transgenic plants. Transgenic approaches provide us with access to identify the candidate genes, miRNAs, and transcription factors (TFs) that are involved in specific plant processes, thus enabling an integrated knowledge of the molecular and physiological mechanisms influencing the plant tolerance and productivity. The accuracy and precision of this phenomenon assures great success in the future of plant improvements. Hence, transgenic breeding has proven to be a promising tool for abiotic stress improvement in crops. This review focuses on the potential and successful applications, recent progress, and future perspectives of transgenic breeding for improving abiotic stress tolerance and productivity in plants.

Environmental temperature and human epigenetic modifications: A systematic review

The knowledge about the effects of environmental temperature on human epigenome is a potential key to understand the health impacts of temperature and to guide acclimation under climate change. We performed a systematic review on the epidemiological studies that have evaluated the association between environmental temperature and human epigenetic modifications. We identified seven original articles on this topic published between 2009 and 2019, including six cohort studies and one cross-sectional study. They focused on DNA methylation in elderly people (blood sample) or infants (placenta sample), with sample size ranging from 306 to 1798. These studies were conducted in relatively low temperature setting (median/mean temperature: 0.8-13 °C), and linear models were used to evaluate temperature-DNA methylation association over short period (≤28 days). It has been reported that short-term ambient temperature could affect global human DNA methylation. A total of 15 candidate genes (ICAM-1, CRAT, F3, TLR-2, iNOS, ZKSCAN4, ZNF227, ZNF595, ZNF597, ZNF668, CACNA1H, AIRE, MYEOV2, NKX1-2 and CCDC15) with methylation status associated with ambient temperature have been identified. DNA methylation on ZKSCAN4, ICAM-1 partly mediated the effect of short-term cold temperature on high blood pressure and ICAM-1 protein (related to cardiovascular events), respectively. In summary, epidemiological evidence about the impacts of environment temperature on human epigenetics remains scarce and limited to short-term linear effect of cold temperature on DNA methylation in elderly people and infants. More studies are needed to broaden our understanding of temperature related epigenetic changes, especially under a changing climate.

MicroRNAs transcriptionally regulate promoter activity in Arabidopsis thaliana

MicroRNAs (miRNAs) are vital regulators that repress gene expression in the cytoplasm in two main ways: mRNA degradation and translational inhibition. Several animal studies have shown that miRNAs also target promoters, thereby activating expression. Whether this miRNA action also occurs in plants is unknown. In this study, we demonstrated that several miRNAs regulate target promoters in Arabidopsis thaliana. For example, miR5658 was predominantly present in the nucleus and activated the expression of AT3G25290 directly by binding to its promoter. Our observations suggest that this mode of action may be a general feature of plant miRNAs, and thus provide insight into the vital roles of plant miRNAs in the nucleus.

Small RNA and degradome deep sequencing reveal respective roles of cold-related microRNAs across Chinese wild grapevine and cultivated grapevine

Background

Chinese wild grapevine (Vitis amurensis) has remarkable cold stress tolerance, exceeding that of the common cultivated grapevine (Vitis vinifera L.).

Result

Here, we surveyed the expression dynamics of microRNAs (miRNAs) across Chinese wild grapevine (cv. Beibinghong) and cultivated grapevine (cv. Cabernet Sauvignon) under cold stress using high-throughput sequencing. We identified 186 known miRNAs in cultivated grape and 427 known miRNAs in Beibinghong. Of the identified miRNAs, 59 are conserved miRNAs orthologous in Cabernet Sauvignon and Beibinghong. In addition, 105 and 129 novel miRNAs were identified in Cabernet Sauvignon and Beibinghong, respectively. The expression of some miRNAs was related to cold stress both in Cabernet Sauvignon and Beibinghong. Many cold-related miRNAs in Cabernet Sauvignon and Beibinghong were predicted to target stress response-related genes such as MYB, WRKY, bHLH transcription factor genes, and heat shock protein genes. However, the expression tendency under cold treatment of many of these miRNAs was different between Cabernet Sauvignon and Beibinghong. Different modes of expression of cultivated and Chinese wild grape miRNAs were indicated in key pathways under cold stress by degradome, target prediction, GO, and KEGG analyses.

Conclusion

Our study indicated three likely reasons that led to the different cold stress tolerance levels of Cabernet Sauvignon and Beibinghong. Specifically, there may be (1) differential expression of orthologous miRNAs between cultivated grapevine and Chinese wild grape; (2) species-specific miRNAs or target genes; or (3) different regulatory models of miRNAs in cultivated and Chinese wild grape in some key pathways.

Heat Stress in Legume Seed Setting: Effects, Causes, and Future Prospects

Grain legumes provide a rich resource of plant nutrition to human diets and are vital for food security and sustainable cropping. Heat stress during flowering has a detrimental effect on legume seed yield, mainly due to irreversible loss of seed number. To start with, we provide an overview of the developmental and physiological basis of controlling seed setting in response to heat stress. It is shown that every single process of seed setting including male and female gametophyte development, fertilization, and early seed/fruit development is sensitive to heat stress, in particular male reproductive development in legume crops is especially susceptible. A series of physiochemical processes including heat shock proteins, antioxidants, metabolites, and hormones centered with sugar starvation are proposed to play a key role in regulating legume seed setting in response to heat stress. The exploration of the molecular mechanisms underlying reproductive heat tolerance is in its infancy. Medicago truncatula, with a small diploid genome, and well-established transformation system and molecular platforms, has become a valuable model for testing gene function that can be applied to advance the physiological and molecular understanding of legume reproductive heat tolerance.

Identification of microRNAs responding to cold stress in Dongxiang common wild rice

Low temperature is a vital effector of rice at different growth stages. MicroRNAs (miRNAs) play important roles in responding to abiotic and biotic stresses. Here, we confirmed the cold tolerance of Dongxiang common wild rice and explored the miRNAs differentially expressed under cold stress using genome-wide small RNA sequencing. In total, 16 miRNAs, nine upregulated and seven downregulated by cold stress, were characterized in Dongxiang common wild rice, and their target genes were predicted. Additionally, an AgriGO analysis of the target genes revealed that they were enriched in several terms related to cold-stress tolerance, suggesting a complex response mechanism, involving miRNAs, to cold stress in Dongxiang common wild rice.

Sensing of Abiotic Stress and Ionic Stress Responses in Plants

Plants need to cope with complex environments throughout their life cycle. Abiotic stresses, including drought, cold, salt and heat, can cause a reduction in plant growth and loss of crop yield. Plants sensing stress signals and adapting to adverse environments are fundamental biological problems. We review the stress sensors in stress sensing and the responses, and then discuss ionic stress signaling and the responses. During ionic stress, the calcineurin B-like proteins (CBL) and CBL-interacting protein kinases (CBL-CIPK) complex is identified as a primary element of the calcium sensor for perceiving environmental signals. The CBL-CIPK system shows specificity and variety in its response to different stresses. Obtaining a deeper understanding of stress signaling and the responses will mitigate or solve crop yield crises in extreme environments with fast-growing populations.

Small RNA and degradome profiling involved in seed development and oil synthesis of Brassica napus

MicroRNAs (miRNAs) play a prominent role in post-transcriptional gene expression regulation and have been involved in various biological and metabolic processes to regulate gene expression. For Brassica napus, improving seed-weight and oil-content is the main breeding goal. In order to better understand the regulation mechanism of miRNAs during seed-weight formation and oil-content accumulation in B. napus, in this study, a high-throughput sequencing technology was used to profile miRNAs expression of Brassica napus immature seeds from one to six weeks after flowering. A total of 1,276 miRNAs, including 1,248 novel and 28 known miRNAs, were obtained from both the high-seed-weight with low-oil-content RNA pool (S03) and the low-seed-weight with high-oil-content RNA pool (S04). Analysis of their expression profiles disclosed that 300 novel and two known miRNAs were differentially expressed between S03 and S04. For degradome analysis, 57 genes with 64 degradation sites were predicted to be targeted for degradation by these miRNAs. Further bioinformatics analysis indicated that these differentially expressed miRNAs might participate in regulation of myriad cellular and molecular processes, during seed development and oil synthesis. Finally, 6 target genes with potential roles in regulation of seed development and 9 other targets in seed oil synthesis, were further confirmed as candidate genes from small RNA and degradome sequencing.