Inferring the regulatory network of the miRNA-mediated response to biotic and abiotic stress in melon

Role of mi RNA in Phytoremediation of Heavy Metals and Metal Induced Stress Alleviation

Anthropogenic activities have contributed hugely in enhancing various types of environmental toxicity. One of these is higher accumulation of toxic heavy metals in soil and plant tissues. Although many heavy metals act as essential component for the growth and development of plants when present in low concentrations but at higher concentrations it becomes cytotoxic. Several innate mechanisms have evolved in plants to cope with it. In recent years the mechanism of using miRNA to combat metal induced toxicity has come to fore front. The miRNA or the microRNA regulates different physiological processes and induces a negative control in expressing the complementary target genes. The cleavage formation by post-transcriptional method and the inhibition of targeted translational mRNA are the two main procedures by which plant miRNAs function. The heavy and enhanced metal accumulation in plants has increased the production of different kinds of free radicals like reactive nitrogen and oxygen which damage the plants oxidatively. Several plant miRNA are capable of targeting and reducing the expression of those genes which are responsible for higher metal accumulation and storage. This can reduce the metal load and hence its negative impact on plant can also be reduced. This review depicts the biogenesis, the mode of action of miRNA, and the control mechanisms of miRNA in metal induced stress response in plant. A detailed review on the role of plant miRNA in alleviation of metal induced stress is discussed in this present study.

Integrative time-scale and multi-omics analysis of host responses to viroid infection

Viroids are circular RNAs of minimal complexity compelled to subvert plant-regulatory networks to accomplish their infectious process. Studies focused on the response to viroid-infection have mostly addressed specific regulatory levels and considered specifics infection-times. Thus, much remains to be done to understand the temporal evolution and complex nature of viroid-host interactions. Here we present an integrative analysis of the temporal evolution of the genome-wide alterations in cucumber plants infected with hop stunt viroid (HSVd) by integrating differential host transcriptome, sRNAnome and methylome. Our results support that HSVd promotes the redesign of the cucumber regulatory-pathways predominantly affecting specific regulatory layers at different infection-phases. The initial response was characterised by a reconfiguration of the host-transcriptome by differential exon-usage, followed by a progressive transcriptional downregulation modulated by epigenetic changes. Regarding endogenous small RNAs, the alterations were limited and mainly occurred at the late stage. Significant host-alterations were predominantly related to the downregulation of transcripts involved in plant-defence mechanisms, the restriction of pathogen-movement and the systemic spreading of defence signals. We expect that these data constituting the first comprehensive temporal-map of the plant-regulatory alterations associated with HSVd infection could contribute to elucidate the molecular basis of the yet poorly known host-response to viroid-induced pathogenesis.

Comparative bioinformatics analysis and abiotic stress responses of expansin proteins in Cucurbitaceae members: watermelon and melon

Watermelon and melon are members of the Cucurbitaceae family including economically significant crops in the world. The expansin protein family, which is one of the members of the cell wall, breaks down the non-covalent bonds between cell wall polysaccharides, causing pressure-dependent cell expansion. Comparative bioinformatics and molecular characterization analysis of the expansin protein family were carried out in the watermelon (Citrullus lanatus) and melon (Cucumis melo) plants in the study. Gene expression levels of expansin family members were analyzed in leaf and root tissues of watermelon and melon under ABA, drought, heat, cold, and salt stress conditions by quantitative real-time PCR analysis. After comprehensive searches, 40 expansin proteins (22 ClaEXPA, 14 ClaEXPLA, and 4 ClaEXPB) in watermelon and 43 expansin proteins (19 CmEXPA, 15 CmEXPLA, 3 CmEXPB, and 6 CmEXPLB) in melon were identified. The greatest orthologous genes were identified with soybean expansin genes for watermelon and melon. However, the latest divergence time between orthologous genes was determined with poplar expansin genes for watermelon and melon expansin genes. ClaEXPA-04, ClaEXPA-09, ClaEXPB-01, ClaEXPB-03, and ClaEXPLA-13 genes in watermelon and CmEXPA-12, CmEXPA-10, and CmEXPLA-01 genes in melon can be involved in tissue development and abiotic stress response of the plant. The current study combining bioinformatics and experimental analysis can provide a detailed characterization of the expansin superfamily which has roles in growth and reaction to the stress of the plant. The study ensures detailed data for future studies examining gene functions including the roles in plant growth and stress conditions.

microRNAs: Key Players in Plant Response to Metal Toxicity

Environmental metal pollution is a common problem threatening sustainable and safe crop production. Heavy metals (HMs) cause toxicity by targeting key molecules and life processes in plant cells. Plants counteract excess metals in the environment by enhancing defense responses, such as metal chelation, isolation to vacuoles, regulating metal intake through transporters, and strengthening antioxidant mechanisms. In recent years, microRNAs (miRNAs), as a small non-coding RNA, have become the central regulator of a variety of abiotic stresses, including HMs. With the introduction of the latest technologies such as next-generation sequencing (NGS), more and more miRNAs have been widely recognized in several plants due to their diverse roles. Metal-regulated miRNAs and their target genes are part of a complex regulatory network. Known miRNAs coordinate plant responses to metal stress through antioxidant functions, root growth, hormone signals, transcription factors (TF), and metal transporters. This article reviews the research progress of miRNAs in the stress response of plants to the accumulation of HMs, such as Cu, Cd, Hg, Cr, and Al, and the toxicity of heavy metal ions.

Regulatory role of microRNAs (miRNAs) in the recent development of abiotic stress tolerance of plants

MicroRNAs (miRNAs) are a distinct groups of single-stranded non-coding, tiny regulatory RNAs approximately 20-24 nucleotides in length. miRNAs negatively influence gene expression at the post-transcriptional level and have evolved considerably in the development of abiotic stress tolerance in a number of model plants and economically important crop species. The present review aims to deliver the information on miRNA-mediated regulation of the expression of major genes or Transcription Factors (TFs), as well as genetic and regulatory pathways. Also, the information on adaptive mechanisms involved in plant abiotic stress responses, prediction, and validation of targets, computational tools, and databases available for plant miRNAs, specifically focus on their exploration for engineering abiotic stress tolerance in plants. The regulatory function of miRNAs in plant growth, development, and abiotic stresses consider in this review, which uses high-throughput sequencing (HTS) technologies to generate large-scale libraries of small RNAs (sRNAs) for conventional screening of known and novel abiotic stress-responsive miRNAs adds complexity to regulatory networks in plants. The discoveries of miRNA-mediated tolerance to multiple abiotic stresses, including salinity, drought, cold, heat stress, nutritional deficiency, UV-radiation, oxidative stress, hypoxia, and heavy metal toxicity, are highlighted and discussed in this review.

Differential microRNA expression, microRNA arm switching, and microRNA:long noncoding RNA interaction in response to salinity stress in soybean

BACKGROUND

Soybean is a major legume crop with high nutritional and environmental values suitable for sustainable agriculture. Noncoding RNAs (ncRNAs), including microRNAs (miRNAs) and long noncoding RNAs (lncRNAs), are important regulators of gene functions in eukaryotes. However, the interactions between these two types of ncRNAs in the context of plant physiology, especially in response to salinity stress, are poorly understood.

RESULTS

Here, we challenged a cultivated soybean accession (C08) and a wild one (W05) with salt treatment and obtained their small RNA transcriptomes at six time points from both root and leaf tissues. In addition to thoroughly analyzing the differentially expressed miRNAs, we also documented the first case of miRNA arm-switching (miR166m), the swapping of dominant miRNA arm expression, in soybean in different tissues. Two arms of miR166m target different genes related to salinity stress (chloroplastic beta-amylase 1 targeted by miR166m-5p and calcium-dependent protein kinase 1 targeted by miR166m-3p), suggesting arm-switching of miR166m play roles in soybean in response to salinity stress. Furthermore, two pairs of miRNA:lncRNA interacting partners (miR166i-5p and lncRNA Gmax_MSTRG.35921.1; and miR394a-3p and lncRNA Gmax_MSTRG.18616.1) were also discovered in reaction to salinity stress.

CONCLUSIONS

This study demonstrates how ncRNA involves in salinity stress responses in soybean by miRNA arm switching and miRNA:lncRNA interactions. The behaviors of ncRNAs revealed in this study will shed new light on molecular regulatory mechanisms of stress responses in plants, and hence provide potential new strategies for crop improvement.

The Mechanism of Metal Homeostasis in Plants: A New View on the Synergistic Regulation Pathway of Membrane Proteins, Lipids and Metal Ions

Heavy metal stress (HMS) is one of the most destructive abiotic stresses which seriously affects the growth and development of plants. Recent studies have shown significant progress in understanding the molecular mechanisms underlying plant tolerance to HMS. In general, three core signals are involved in plants' responses to HMS; these are mitogen-activated protein kinase (MAPK), calcium, and hormonal (abscisic acid) signals. In addition to these signal components, other regulatory factors, such as microRNAs and membrane proteins, also play an important role in regulating HMS responses in plants. Membrane proteins interact with the highly complex and heterogeneous lipids in the plant cell environment. The function of membrane proteins is affected by the interactions between lipids and lipid-membrane proteins. Our review findings also indicate the possibility of membrane protein-lipid-metal ion interactions in regulating metal homeostasis in plant cells. In this review, we investigated the role of membrane proteins with specific substrate recognition in regulating cell metal homeostasis. The understanding of the possible interaction networks and upstream and downstream pathways is developed. In addition, possible interactions between membrane proteins, metal ions, and lipids are discussed to provide new ideas for studying metal homeostasis in plant cells.

Combined Stress Conditions in Melon Induce Non-additive Effects in the Core miRNA Regulatory Network

Climate change has been associated with a higher incidence of combined adverse environmental conditions that can promote a significant decrease in crop productivity. However, knowledge on how a combination of stresses might affect plant development is still scarce. MicroRNAs (miRNAs) have been proposed as potential targets for improving crop productivity. Here, we have combined deep-sequencing, computational characterization of responsive miRNAs and validation of their regulatory role in a comprehensive analysis of response of melon to several combinations of four stresses (cold, salinity, short day, and infection with a fungus). Twenty-two miRNA families responding to double and/or triple stresses were identified. The regulatory role of the differentially expressed miRNAs was validated by quantitative measurements of the expression of the corresponding target genes. A high proportion (ca. 60%) of these families (mainly highly conserved miRNAs targeting transcription factors) showed a non-additive response to multiple stresses in comparison with that observed under each one of the stresses individually. Among those miRNAs showing non-additive response to stress combinations, most interactions were negative, suggesting the existence of functional convergence in the miRNA-mediated response to combined stresses. Taken together, our results provide compelling pieces of evidence that the response to combined stresses cannot be easily predicted from the study individual stresses.

The miR528-AO Module Confers Enhanced Salt Tolerance in Rice by Modulating the Ascorbic Acid and Abscisic Acid Metabolism and ROS Scavenging

The monocot lineage-specific miR528 was previously established as a multistress regulator. However, it remains largely unclear how miR528 participates in response to salinity stress in rice. Here, we show that miR528 positively regulates rice salt tolerance by down-regulating a gene encoding l-ascorbate oxidase (AO), thereby bolstering up the AO-mediated abscisic acid (ABA) synthesis and ROS scavenging. Overexpression of miR528 caused a substantial increase in ascorbic acid (AsA) and ABA contents but a significant reduction in ROS accumulation, resulting in the enhanced salt tolerance of rice plants. Conversely, knockdown of miR528 or overexpression of AO stimulated the expression of the AO gene, hence lowering the level of AsA, a critical antioxidant that promotes the ABA content but reduces the ROS level, and then compromising rice tolerance to salinity. Together, the findings reveal a novel mechanism of the miR528-AO module-mediated salt tolerance by modulating the processes of AsA and ABA metabolism as well as ROS detoxification, which adds a new regulatory role to the miR528-AO stress defense pathway in rice.

Symptom Severity, Infection Progression and Plant Responses in Solanum Plants Caused by Three Pospiviroids Vary with the Inoculation Procedure

Infectious viroid clones consist of dimeric cDNAs used to generate transcripts which mimic the longer-than-unit replication intermediates. These transcripts can be either generated in vitro or produced in vivo by agro-inoculation. We have designed a new plasmid, which allows both inoculation methods, and we have compared them by infecting Solanum lycopersicum and Solanum melongena with clones of Citrus exocortis virod (CEVd), Tomato chlorotic dwarf viroid (TCDVd), and Potato spindle tuber viroid (PSTVd). Our results showed more uniform and severe symptoms in agro-inoculated plants. Viroid accumulation and the proportion of circular and linear forms were different depending on the host and the inoculation method and did not correlate with the symptoms, which correlated with an increase in PR1 induction, accumulation of the defensive signal molecules salicylic (SA) and gentisic (GA) acids, and ribosomal stress in tomato plants. The alteration in ribosome biogenesis was evidenced by both the upregulation of the tomato ribosomal stress marker SlNAC082 and the impairment in 18S rRNA processing, pointing out ribosomal stress as a novel signature of the pathogenesis of nuclear-replicating viroids. In conclusion, this updated binary vector has turned out to be an efficient and reproducible method that will facilitate the studies of viroid–host interactions.

Genome-Wide Identification of Key Candidate microRNAs and Target Genes Associated with Peanut Drought Tolerance

Peanut is an important crash crop worldwide, and it is often threatened by drought stress due to unexpected extreme weather events. In this work, NH5 and FH18 were selected as drought-tolerant and drought-sensitive varieties, respectively. Comparison of their physiological responses revealed that NH5 showed less wilting, higher relative water content and lower water loss rate of detached leaves, lower electrolyte leakage, and stronger antioxidant ability under drought stress than did FH18. Based on comparative transcriptomic analysis, 5376 differentially expressed mRNAs were commonly identified in the two varieties, and 2993 genes specifically changed in the drought-tolerant variety and were mainly enriched in photosynthesis-antenna proteins and photosynthetic pathways. Furthermore, 73 microRNAs (miRNAs) were differentially expressed in the drought tolerance variety specifically under drought stress; of these, two key candidate miRNAs, novel miR_416 and novel miR_73, were identified, and the majority of their target genes were enriched in phenylpropanoid biosynthesis, linoleic acid metabolism, and cutin, suberine, and wax biosynthesis. This study lays the foundation for the analysis of the molecular mechanism of drought tolerance and promotes the genetic improvement of peanut drought tolerance.

Identifying transcripts associated with efficient transport and accumulation of Fe and Zn in hexaploid wheat (T. aestivum L.)

Wheat (T. aestivum L.) is the second most important staple food crop consumed in the form of various end-use products across the world. However, it contains lower concentrations of Fe and Zn leading to micronutrient deficiency in human beings where wheat is the sole diet. Therefore, increasing grain Fe/Zn content in wheat has become priority in wheat breeding programmes across the world. Understanding the molecular mechanism of Fe/Zn transport and accumulation in grains is required to expedite the breeding process. For this purpose, whole seedling transcriptome analysis was conducted in four wheat genotypes (CRP 1660, Sonora 64, Vinata, : high, and DBW17: low) differing in grain Fe/Zn content under controlled and Fe/Zn deficient conditions. Twenty eight key transcripts involved in phytosiderophore biosynthesis, Fe/Zn uptake and transport were identified. Expression analysis of 12 of the transcripts using qPCR was conducted in seedling stage and flag leaf which exhibited greater differential accumulation in CRP 1660 followed by Vinata, Sonora 64 and DBW 17 in both flag leaf and seedling. However, there was significantly higher differential accumulation of the transcripts in flag leaf as compared to seedling. In CRP 1660, transcripts pertaining to phytosiderophore biosynthesis like DMAS1-B, NRAMP2 and NAAT2-D showed greater accumulation. Additionally, corresponding miRNAs were also identified for these 28 transcripts. The findings will help in better understanding of molecular basis of Fe/Zn transport and accumulation in grain and subsequent utilization in breeding to improve Fe/Zn content in wheat grain.

Identification of MicroRNAs and Their Targets That Respond to Powdery Mildew Infection in Cucumber by Small RNA and Degradome Sequencing

Powdery mildew (PM) is a prevalent disease known to limit cucumber production worldwide. MicroRNAs (miRNAs) are single-stranded molecules that regulate host defense responses through posttranscriptional gene regulation. However, which specific miRNAs are involved and how they regulate cucumber PM resistance remain elusive. A PM-resistant single-segment substitution line, SSSL508-28, was developed previously using marker-assisted backcrossing of the PM-susceptible cucumber inbred D8 line. In this study, we applied small RNA and degradome sequencing to identify PM-responsive miRNAs and their target genes in the D8 and SSSL508-28 lines. The deep sequencing resulted in the identification of 156 known and 147 novel miRNAs. Among them, 32 and six differentially expressed miRNAs (DEMs) were detected in D8 and SSSL508-28, respectively. The positive correlation between DEMs measured by small RNA sequencing and stem-loop quantitative real-time reverse transcription-polymerase chain reaction confirmed the accuracy of the observed miRNA abundances. The 32 DEMs identified in the PM-susceptible D8 were all upregulated, whereas four of the six DEMs identified in the PM-resistant SSSL508-28 were downregulated. Using in silico and degradome sequencing approaches, 517 and 20 target genes were predicted for the D8 and SSSL508-28 DEMs, respectively. Comparison of the DEM expression profiles with the corresponding mRNA expression profiles obtained in a previous study with the same experimental design identified 60 and three target genes in D8 and SSSL508-28, respectively, which exhibited inverse expression patterns with their respective miRNAs. In particular, five DEMs were located in the substituted segment that contained two upregulated DEMs, Csa-miR172c-3p and Csa-miR395a-3p, in D8 and two downregulated DEMs, Csa-miR395d-3p and Csa-miR398b-3p, in SSSL508-28. One gene encoding L-aspartate oxidase, which was targeted by Csa-miR162a, was also located on the same segment and was specifically downregulated in PM-inoculated D8 leaves. Our results will facilitate the future use of miRNAs in breeding cucumber varieties with enhanced resistance to PM.

Empowering crop resilience to environmental multiple stress through the modulation of key response components

Crop plants have developed a multitude of defense and adaptation responses to protect themselves against invading pathogens and challenging environmental stresses, mostly operating jointly. The plant perception of overall stress induces a coordinated response mediated by complex signaling networks. Experimental evidences proved that plant response to combined biotic and abiotic stresses substantially diverge from the responses to individual stresses. Moreover, the cross-talk of signaling pathways involved in responding to biotic and abiotic stresses is pivoted on several converging elements able to simultaneously modulate the timing and amplitude of the overall plant response. Comprehensively, the interaction between biotic and abiotic stresses can dramatically changes the plant response to the individual stress and the phenotypical outcome of each stress factor. System biology and data mining can synergistically help biologists in finding out regulative mechanisms and key genes controlling the response to biotic and abiotic stresses. Deploying new genetic engineering solutions can rely on the modification of genes involved in resistance/tolerance processes and/or in the modulation of regulatory elements. Finally, a model of the engineered crop for enhanced tolerance to pressures resulting from invasive pathogens and abiotic constraints in semiarid and warm environment is discussed.

Emerging Roles of microRNAs in Plant Heavy Metal Tolerance and Homeostasis

Heavy metal stress is a major growth- and yield-limiting factor for plants. Heavy metals include essential metals (copper, iron, zinc, and manganese) and non-essential metals (cadmium, mercury, aluminum, arsenic, and lead). Plants use complex mechanisms of gene regulation under heavy metal stress. MicroRNAs are 21-nucleotide non-coding small RNAs as important modulators of gene expression post-transcriptionally. Recently, high-throughput sequencing has led to the identification of an increasing number of heavy-metal-responsive microRNAs in plants. Metal-regulated microRNAs and their target genes are part of a complex regulatory network that controls various biological processes, including heavy metal uptake and transport, protein folding and assembly, metal chelation, scavenging of reactive oxygen species, hormone signaling, and microRNA biogenesis. In this review, we summarize the recent molecular studies that identify heavy-metal-regulated microRNAs and their roles in the regulation of target genes as part of the microRNA-associated regulatory network in response to heavy metal stress in plants.

Dynamic architecture and regulatory implications of the miRNA network underlying the response to stress in melon

miRNAs are small RNAs that regulate mRNAs at both transcriptional and posttranscriptional level. In plants, miRNAs are involved in the regulation of different processes including development and stress-response. Elucidating how stress-responsive miRNAs are regulated is key to understand the global response to stress but also to develop efficient biotechnological tools that could help to cope with stress. Here, we describe a computational approach based on sRNA sequencing, transcript quantification and degradome data to analyse the accumulation, function and structural organization of melon miRNAs reactivated under seven biotic and abiotic stress conditions at two and four days post-treatment. Our pipeline allowed us to identify fourteen stress-responsive miRNAs (including evolutionary conserved such as miR156, miR166, miR172, miR319, miR398, miR399, miR894 and miR408) at both analysed times. According to our analysis miRNAs were categorized in three groups showing a broad-, intermediate- or narrow- response range. miRNAs reactive to a broad range of environmental cues appear as central components in the stress-response network. The strictly coordinated response of miR398 and miR408 (broad response-range) to the seven stress treatments during the period analysed here reinforces this notion. Although both, the amplitude and diversity of the miRNA-related response to stress changes during the exposition time, the architecture of the miRNA-network is conserved. This organization of miRNA response to stress is also conserved in rice and soybean supporting the conservation of miRNA-network organization in other crops. Overall, our work sheds light into how miRNA networks in plants organize and function during stress.

Identification and Characterization of Stress-Responsive TAS3-Derived TasiRNAs in Melon

Small interfering RNAs (siRNA) are key regulators of gene expression that play essential roles in diverse biological processes. Trans-acting siRNAs (tasiRNAs) are a class of plant-endogenous siRNAs that lead the cleavage of nonidentical transcripts. TasiRNAs are usually involved in fine-tuning development. However, increasing evidence supports that tasiRNAs may be involved in stress response. Melon is a crop of great economic importance extensively cultivated in semiarid regions frequently exposed to changing environmental conditions that limit its productivity. However, knowledge of the precise role of siRNAs in general, and of tasiRNAs in particular, in regulating the response to adverse environmental conditions is limited. Here, we provide the first comprehensive analysis of computationally inferred melon-tasiRNAs responsive to two biotic (viroid-infection) and abiotic (cold treatment) stress conditions. We identify two TAS3-loci encoding to length (TAS3-L) and short (TAS3-S) transcripts. The TAS candidates predicted from small RNA-sequencing data were characterized according to their chromosome localization and expression pattern in response to stress. The functional activity of cmTAS genes was validated by transcript quantification and degradome assays of the tasiRNA precursors and their predicted targets. Finally, the functionality of a representative cmTAS3-derived tasiRNA (TAS3-S) was confirmed by transient assays showing the cleavage of ARF target transcripts.

The Use of Nitrogen and Its Regulation in Cereals: Structural Genes, Transcription Factors, and the Role of miRNAs

Cereals and, especially, rice, maize, and wheat, are essential commodities, on which human nutrition is based. Expanding population and food demand have required higher production which has been achieved by increasing fertilization, and especially nitrogen supply to cereal crops. In fact, nitrogen is a crucial nutrient for the plant, but excessive use poses serious environmental and health issues. Therefore, increasing nitrogen use efficiency in cereals is of pivotal importance for sustainable agriculture. The main steps in the use of nitrogen are uptake and transport, reduction and assimilation, and translocation and remobilization. Many studies have been carried out on the genes involved in these phases, and on transcription factors regulating these genes. Lately, increasing attention has been paid to miRNAs responding to abiotic stress, including nutrient deficiency. Many miRNAs have been found to regulate transcription factors acting on the expression of specific genes for nitrogen uptake or remobilization. Recent studies on gene regulatory networks have also demonstrated that miRNAs can interact with several nodes in the network, functioning as key regulators in nitrogen metabolism.

In silico determination of transposon-derived miRNAs and targets in Aegilops species

Transposable elements (TEs) are found almost in all living organism, shaping organisms' genomes. miRNAs are noncoding RNA types which are especially important in gene expression regulations. Many previously determined plant miRNAs are identical/homologous to transposons (TE-MIR). The aim of this study was computational characterization of novel TE-related miRNAs and their targets in Aegilops genome by using stringent criteria. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses were performed by BLAST2GO. Seventeen novel TE-related miRNAs in Aegilops genome were identified for the first time. GO analyses indicated that 40 targets played different roles in biological processes, cellular components and molecular functions. Moreover, these genes were involved in 10 metabolic pathways such as purine metabolism, nitrogen metabolism, oxidative phosphorylation, etc. as a result of KEGG analyses. Identification of miRNAs and their targets are significant to understand miRNA-TEs relationships and even how TEs affect plant growth and development. Obtaining results of this study are expected to provide possible new insight into Aegilops and its related species, wheat, with respect to miRNAs evolution and domestication.Communicated by Ramaswamy H. Sarma.