MicroRNA160 Modulates Plant Development and Heat Shock Protein Gene Expression to Mediate Heat Tolerance in Arabidopsis

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.

MiR160 and its target genes ARF10, ARF16 and ARF17 modulate hypocotyl elongation in a light, BRZ, or PAC-dependent manner in Arabidopsis: miR160 promotes hypocotyl elongation

Multiple hormonal and environmental signals participate in the regulation of plant hypocotyl elongation, which allow the plants to optimize their survival strategy from seed germination to seedling establishment. Auxin plays key roles in cell elongation via auxin signaling transduction and its interactions with other hormonal and environmental signals. However, the roles of auxin response factor (ARF) family in cross-talk between auxin and other hormonal or environmental signals during hypocotyl elongation are not fully understood. Here we show that miR160 and its target genes ARF10, ARF16 and ARF17 modulate hypocotyl elongation in a light, brassinazole (BRZ, a BR biosynthesis inhibitor), or paclobutrazol (PAC, a GA biosynthesis inhibitor)-dependent manner in Arabidopsis. miR160, ARF10, ARF16 and ARF17 have no effects on hypocotyl elongation in the dark. However, in the presence of either light, BRZ, or PAC, ARF10, ARF16 and ARF17 inhibit hypocotyl elongation, and miR160 promotes hypocotyl elongation via cleavage of their mRNA. miR160 and ARF10 are both expressed in the hypocotyl. ARF10 represses the expression of PACLOBUTRAZOL RESISTANCE1 (PRE1) and 35S::PRE1 could partly rescue the phenotype of mARF10 (a miR160-resistant form of ARF10), suggesting that PRE1 acts downstream of ARF10 in regulating hypocotyl elongation. In conclusion, our results indicate that miR160-ARF10/16/17 might serve as a molecular link in cross-talk of auxin, light, BR, and GA in hypocotyl elongation.

Root endophyte induced plant thermotolerance by constitutive chromatin modification at heat stress memory gene loci

Global warming has become a critical challenge to food security, causing severe yield losses of major crops worldwide. Conventional and transgenic breeding strategies to enhance plant thermotolerance are laborious and expensive. Therefore, the use of beneficial microbes could be an alternative approach. Here, we report that the root endophyte Enterobacter sp. SA187 induces thermotolerance in wheat in the laboratory as well as in open-field agriculture. To unravel the molecular mechanisms, we used Arabidopsis thaliana as model plant. SA187 reprogramed the Arabidopsis transcriptome via HSFA2-dependent enhancement of H3K4me3 levels at heat stress memory gene loci. Unlike thermopriming, SA187-induced thermotolerance is mediated by ethylene signaling via the transcription factor EIN3. In contrast to the transient chromatin modification by thermopriming, SA187 induces constitutive H3K4me3 modification of heat stress memory genes, generating robust thermotolerance in plants. Importantly, microbial community composition of wheat plants in open-field agriculture is not influenced by SA187, indicating that beneficial microbes can be a powerful tool to enhance thermotolerance of crops in a sustainable manner.

Ectopic expression of nucleolar DEAD-Box RNA helicase OsTOGR1 confers improved heat stress tolerance in transgenic Chinese cabbage

The DEAD-Box RNA helicase OsTOGR1 positively regulates heat stress tolerance in Chinese cabbage. Non-heading Chinese cabbage (Brassica rapa L. ssp. chinensis) is primarily cultivated vegetable crop in Asian countries. Heat stress is one of the major threats for its growth and yield. Numerous regulatory genes in various crops have shown to contribute thermotolerance. Among them, Thermotolerant growth required 1 (TOGR1) is an important DEAD-box RNA helicase. To examine whether its role is conserved in other crops, we constructed pCAMBIA1300-pHSP:OsTOGR1 expression vector driven by the rice small heat shock protein promoter (pHSP17.9) and successfully produced transgenic non-heading Chinese cabbage plants expressing OsTOGR1 gene via Agrobacterium-mediated vacuum infiltration transformation. In total, we generated three independent transgenic cabbage lines expressing TOGR1 gene. Expression and integration of TOGR1 was confirmed by PCR, RT-PCR and qPCR in T₁ and T₂ generations. The relative leaf electrical conductivity of transgenic seedlings was reduced subjected to high temperature (38 °C) compared to heat shock treatment (46 °C). In addition, hypocotyl length of transgenic seedlings increased compared to wild-type plants under high temperature and heat shock treatment. Furthermore, the transgenic plants exhibited higher chlorophyll content than wild-type plants under high temperature and heat shock treatment. The transgenic seeds displayed better germination under heat shock treatment. Tested heat stress-responsive genes were also up-regulated in the transgenic plants subjected to high temperature or heat shock treatment. To the best of our knowledge, this is the first report on describing the role of DAED-Box RNA helicases in improving heat stress tolerance of transgenic plants.

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.

Transcriptome and proteome analyses reveal the regulatory networks and metabolite biosynthesis pathways during the development of Tolypocladium guangdongense

Tolypocladium guangdongense has a similar metabolite profile to Ophiocordyceps sinensis, a highly regarded fungus used for traditional Chinese medicine with high nutritional and medicinal value. Although the genome sequence of T. guangdongense has been reported, relatively little is known about the regulatory networks for fruiting body development and about the metabolite biosynthesis pathways. In order to address this, an analysis of transcriptome and proteome at differential developmental stages of T. guangdongense was performed. In total, 9076 genes were found to be expressed and 2040 proteins were identified. There were a large number of genes that were significantly differentially expressed between the mycelial stage and the stages. Interestingly, the correlation between the transcriptomic and proteomic data was low, suggesting the importance of the post-transcriptional processes in the growth and development of T. guangdongense. Among the genes/proteins that were both differentially expressed during the developmental process, there were numerous heat shock proteins and transcription factors. In addition, there were numerous proteins involved in terpenoid, ergosterol, adenosine and polysaccharide biosynthesis that also showed significant downregulation in their expression levels during the developmental process. Furthermore, both tryptophan and tryptamine were present at higher levels in the primordium stage. However, indole-3-acetic acid (IAA) levels continuously decreased as development proceeded, and the enzymes involved in IAA biosynthesis were also clearly differentially downregulated. These data could be meaningful in studying the molecular mechanisms of fungal development, and for the industrial and medicinal application of macro-fungi.

Genome-Wide Identification of RNA Silencing-Related Genes and Their Expressional Analysis in Response to Heat Stress in Barley (Hordeum vulgare L.)

Barley (Hordeum vulgare L.) is an economically important crop cultivated in temperate climates all over the world. Adverse environmental factors negatively affect its survival and productivity. RNA silencing is a conserved pathway involved in the regulation of growth, development and stress responses. The key components of RNA silencing are the Dicer-like proteins (DCLs), Argonautes (AGOs) and RNA-dependent RNA polymerases (RDRs). Despite its economic importance, there is no available comprehensive report on barley RNA silencing machinery and its regulation. In this study, we in silico identified five DCL (HvDCL), eleven AGO (HvAGO) and seven RDR (HvRDR) genes in the barley genome. Genomic localization, phylogenetic analysis, domain organization and functional/catalytic motif identification were also performed. To understand the regulation of RNA silencing, we experimentally analysed the transcriptional changes in response to moderate, persistent or gradient heat stress treatments: transcriptional accumulation of siRNA- but not miRNA-based silencing factor was consistently detected. These results suggest that RNA silencing is dynamically regulated and may be involved in the coordination of development and environmental adaptation in barley. In summary, our work provides information about barley RNA silencing components and will be a ground for the selection of candidate factors and in-depth functional/mechanistic analyses.

Sugar Signaling and Post-transcriptional Regulation in Plants: An Overlooked or an Emerging Topic?

Plants are autotrophic organisms that self-produce sugars through photosynthesis. These sugars serve as an energy source, carbon skeletons, and signaling entities throughout plants' life. Post-transcriptional regulation of gene expression plays an important role in various sugar-related processes. In cells, it is regulated by many factors, such as RNA-binding proteins (RBPs), microRNAs, the spliceosome, etc. To date, most of the investigations into sugar-related gene expression have been focused on the transcriptional level in plants, while only a few studies have been conducted on post-transcriptional mechanisms. The present review provides an overview of the relationships between sugar and post-transcriptional regulation in plants. It addresses the relationships between sugar signaling and RBPs, microRNAs, and mRNA stability. These new items insights will help to reach a comprehensive understanding of the diversity of sugar signaling regulatory networks, and open onto new investigations into the relevance of these regulations for plant growth and development.

Genome-Wide Transcript and Small RNA Profiling Reveals Transcriptomic Responses to Heat Stress

Because of climate change, crops will experience increasing heat stress. However, the ways in which heat stress affects crop growth and yield at the molecular level remain poorly understood. We generated spatiotemporal mRNA and small RNA transcriptome data, spanning seven tissues at three time points, to investigate the effects of heat stress on vegetative and reproductive development in maize (Zea mays). Among the small RNAs significantly induced by heat stress was a plastid-derived 19-nucleotide small RNA, which is possibly the residual footprint of a pentatricopeptide repeat protein. This suggests that heat stress induces the turnover of certain plastid transcripts. Consistently, genes responsible for photosynthesis in chloroplasts were repressed after heat stress. Analysis also revealed that the abundance of 24-nucletide small interfering RNAs from transposable elements was conspicuously reduced by heat stress in tassels and roots; nearby genes showed a similar expression trend. Finally, specific microRNA and passenger microRNA species were identified, which in other plant species have not before been reported as responsive to heat stress. This study generated an atlas of genome-wide transcriptomic responses to heat stress, revealing several key regulators as potential targets for thermotolerance improvement in maize.

Methyl-CpG-binding domain 9 (MBD9) is required for H2A.Z incorporation into chromatin at a subset of H2A.Z-enriched regions in the Arabidopsis genome

The SWR1 chromatin remodeling complex, which deposits the histone variant H2A.Z into nucleosomes, has been well characterized in yeast and animals, but its composition in plants has remained uncertain. We used the conserved SWR1 subunit ACTIN RELATED PROTEIN 6 (ARP6) as bait in tandem affinity purification experiments to isolate associated proteins from Arabidopsis thaliana. We identified all 11 subunits found in yeast SWR1 and the homologous mammalian SRCAP complexes, demonstrating that this complex is conserved in plants. We also identified several additional proteins not previously associated with SWR1, including Methyl-CpG-BINDING DOMAIN 9 (MBD9) and three members of the Alfin1-like protein family, all of which have been shown to bind modified histone tails. Since mbd9 mutant plants were phenotypically similar to arp6 mutants, we explored a potential role for MBD9 in H2A.Z deposition. We found that MBD9 is required for proper H2A.Z incorporation at thousands of discrete sites, which represent a subset of the genomic regions normally enriched with H2A.Z. We also discovered that MBD9 preferentially interacts with acetylated histone H4 peptides, as well as those carrying mono- or dimethylated H3 lysine 4, or dimethylated H3 arginine 2 or 8. Considering that MBD9-dependent H2A.Z sites show a distinct histone modification profile, we propose that MBD9 recognizes particular nucleosome modifications via its PHD- and Bromo-domains and thereby guides SWR1 to these sites for H2A.Z deposition. Our data establish the SWR1 complex as being conserved across eukaryotes and suggest that MBD9 may be involved in targeting the complex to specific genomic sites through nucleosomal interactions. The finding that MBD9 does not appear to be a core subunit of the Arabidopsis SWR1 complex, along with the synergistic phenotype of arp6;mbd9 double mutants, suggests that MBD9 also has important roles beyond H2A.Z deposition.

The histone H2A variant, H2A.Z, is found in all known eukaryotes and plays important roles in transcriptional regulation. H2A.Z is selectively incorporated into nucleosomes within many genes by the activity of a conserved ATP-dependent chromatin remodeling complex in yeast, insects, and mammals. Whether this complex exists in the same form in plants, and how the complex is targeted to specific genomic locations have remained open questions. In this study we demonstrate that plants do indeed utilize a complex analogous to those of fungi and animals to deposit H2A.Z, and we also identify several new proteins that interact with this complex. We found that one such interactor, Methyl-CpG-BINDING DOMAIN 9 (MBD9), is required for H2A.Z incorporation at thousands of genomic sites that share a distinct histone modification profile. The histone binding properties of MBD9 suggest that it may guide H2A.Z deposition to specific sites by interacting with modified nucleosomes and with the H2A.Z deposition complex. We hypothesize that this represents a general paradigm for the targeting of H2A.Z to specific sites.

Priming of Plant Resistance to Heat Stress and Tomato Yellow Leaf Curl Thailand Virus With Plant-Derived Materials

Plants are often simultaneously exposed to diverse environmental stresses, and can tune suitable responses to them through hormones. Salicylic acid (SA) and jasmonic acid (JA) signaling pathways are known to enhance resistance against heat stress and tomato yellow leaf curl Thailand virus (TYLCTHV) infection. However, there is limited information regarding alternative natural priming agents against heat stress and viruses. In this study, two plant-derived priming agents, eugenol and anise oil, were tested for their roles in conferring thermotolerance and virus resistance in tomato plants. Under heat stress, the survival rates and average fresh weight were higher in plants treated with eugenol or anise oil than in control plants. These two priming agents were further tested for antiviral activities. After TYLCTHV infection, the disease incidence and relative abundance of TYLCTHV were lower in anise oil- and eugenol-treated plants than in control plants. Further analyses revealed that a few SA, JA, and RNA silencing genes were enhanced in the former. Moreover, SA, JA, and H2O2 contents increased considerably after eugenol and anise oil treatments. Our findings imply that anise oil and eugenol initiated SA- and JA-mediated defenses to promote thermotolerance and antiviral responses of tomato plants.

Hydrogen peroxide is involved in methane-induced tomato lateral root formation

Pharmacological and molecular evidence reveals a novel role of methane (CH₄) gas in root organogenesis, the induction of lateral root (LR) formation, and this response might require hydrogen peroxide (H₂O₂) synthesis. Although plants can produce CH₄ and release this to atmosphere, the beneficial role(s) of CH₄ are not fully elucidated. In this study, the fumigation with CH₄ not only increased NADPH oxidase activity and H₂O₂ production, but also induced tomato lateral root primordial formation and thereafter LR development. However, exogenously applied argon and nitrogen failed to influence LR formation. Above responses triggered by CH₄ were sensitive to the removal of endogenous H₂O₂ with dimethylthiourea (DMTU; a membrane-permeable scavenger of H₂O₂), suggesting the hypothesis that CH₄'s effect on LR formation could be mediated by endogenous H₂O₂. Diphenylene iodonium (DPI) inhibition of the H₂O₂ generating enzyme NADPH oxidase attenuated H₂O₂ synthesis and impaired LR formation in response to CH₄, confirming the requirement of NADPH oxidase-dependent H₂O₂. Meanwhile, the alterations of endogenous H₂O₂ concentrations failed to influence CH₄ production in tomato seedlings. Molecular evidence revealed that CH₄-induced SlCDKA1, SlCYCA2;1, and SlCYCA3;1 transcripts, and -decreased SlKRP2 mRNA were impaired by DMTU or DPI. Contrasting changes in LR formation-related miR390a and miR160 transcripts and their target genes, including SlARF4 and SlARF16, were observed. Together, our pharmacological and molecular evidence suggested the requirement of H₂O₂ synthesis in CH₄-triggered tomato LR formation, partially via the regulation of cell cycle regulatory genes, miRNA-, and tasiRNA-modulated gene expression.

miR824/AGAMOUS-LIKE16 Module Integrates Recurring Environmental Heat Stress Changes to Fine-Tune Poststress Development

Plant development is continually fine-tuned based on environmental factors. How environmental perturbations are integrated into the developmental programs and how poststress adaptation is regulated remains an important topic to dissect. Vegetative to reproductive phase change is a very important developmental transition that is complexly regulated based on endogenous and exogenous cues. Proper timing of flowering is vital for reproductive success. It has been shown previously that AGAMOUS LIKE 16 (AGL16), a MADS-box transcription factor negatively regulates flowering time transition through FLOWERING LOCUS T (FT), a central downstream floral integrator. AGL16 itself is negatively regulated by the microRNA miR824. Here we present a comprehensive molecular analysis of miR824/AGL16 module changes in response to mild and recurring heat stress. We show that miR824 accumulates gradually in response to heat due to the combination of transient transcriptional induction and posttranscriptional stability. miR824 induction requires heat shock cis-elements and activity of the HSFA1 family and HSFA2 transcription factors. Parallel to miR824 induction, its target AGL16 is decreased, implying direct causality. AGL16 posttranscriptional repression during heat stress, however, is more complex, comprising of a miRNA-independent, and a miR824-dependent pathway. We also show that AGL16 expression is leaf vein-specific and overlaps with miR824 (and FT) expression. AGL16 downregulation in response to heat leads to a mild derepression of FT. Finally, we present evidence showing that heat stress regulation of miR824/AGL16 is conserved within Brassicaceae. In conclusion, due to the enhanced post-transcriptional stability of miR824, stable repression of AGL16 is achieved following heat stress. This may serve to fine-tune FT levels and alter flowering time transition. Stress-induced miR824, therefore, can act as a "posttranscriptional memory factor" to extend the acute impact of environmental fluctuations in the poststress period.

miRNomes involved in imparting thermotolerance to crop plants

Thermal stress is one of the challenges to crop plants that negatively impacts crop yield. To overcome this ever-growing problem, utilization of regulatory mechanisms, especially microRNAs (miRNAs), that provide efficient and precise regulation in a targeted manner have been found to play determining roles. Besides their roles in plant growth and development, many recent studies have shown differential regulation of several miRNAs during abiotic stresses including heat stress (HS). Thus, understanding the underlying mechanism of miRNA-mediated gene expression during HS will enable researchers to exploit this regulatory mechanism to address HS responses. This review focuses on the miRNAs and regulatory networks that were involved in physiological, metabolic and morphological adaptations during HS in plant, specifically in crops. Illustrated examples including, the miR156-SPL, miR169-NF-YA5, miR395-APS/AST, miR396-WRKY, etc., have been discussed in specific relation to the crop plants. Further, we have also discussed the available plant miRNA databases and bioinformatics tools useful for miRNA identification and study of their regulatory role in response to HS. Finally, we have briefly discussed the future prospects about the miRNA-related mechanisms of HS for improving thermotolerance in crop plants.

Pectin Methylesterases: Cell Wall Remodeling Proteins Are Required for Plant Response to Heat Stress

Heat stress (HS) is expected to be of increasing worldwide concern in the near future, especially with regard to crop yield and quality as a consequence of rising or varying temperatures as a result of global climate change. HS response (HSR) is a highly conserved mechanism among different organisms but shows remarkable complexity and unique features in plants. The transcriptional regulation of HSR is controlled by HS transcription factors (HSFs) which allow the activation of HS-responsive genes, among which HS proteins (HSPs) are best characterized. Cell wall remodeling constitutes an important component of plant responses to HS to maintain overall function and growth; however, little is known about the connection between cell wall remodeling and HSR. Pectin controls cell wall porosity and has been shown to exhibit structural variation during plant growth and in response to HS. Pectin methylesterases (PMEs) are present in multigene families and encode isoforms with different action patterns by removal of methyl esters to influencing the properties of cell wall. We aimed to elucidate how plant cell walls respond to certain environmental cues through cell wall-modifying proteins in connection with modifications in cell wall machinery. An overview of recent findings shed light on PMEs contribute to a change in cell-wall composition/structure. The fine-scale modulation of apoplastic calcium ions (Ca2+) content could be mediated by PMEs in response to abiotic stress for both the assembly and disassembly of the pectic network. In particular, this modulation is prevalent in guard cell walls for regulating cell wall plasticity as well as stromal aperture size, which comprise critical determinants of plant adaptation to HS. These insights provide a foundation for further research to reveal details of the cell wall machinery and stress-responsive factors to provide targets and strategies to facilitate plant adaptation.

Understanding microRNAs regulation in heat shock response in the sea cucumber Apostichopus japonicus

The sea cucumber Apostichopus japonicus is a valuable species in China. The extreme high temperature in the summer often results in high mortality. MicroRNAs (miRNAs) play important post-transcriptional regulatory roles in gene expression and can influence heat shock response (HSR) greatly. In this study, we determined the expression profiles of miRNAs under heat stress (HS) in A. japonicus by using high-throughput sequencing technique. Among the differential expression miRNAs, we highlighted 41 differentially expressed miRNAs, many of which were involved in immunity process and disease regulation. Gene ontology and pathway analyses of putative target genes were also carried out. Cell-substrate adherens junction and cell-substrate junction were significantly enriched in GO analysis. Moreover, we made a correlation analysis between remarkable miRNAs and the differentially expressed genes (DEGs) in sea cucumbers under HS. We identified 17 key miRNA-target pairs potentially regulated HSR of sea cucumbers. These results will provide new insights about miRNAs regulation and molecular adaptive mechanisms in sea cucumbers under HS.

Plant IsomiR Atlas: Large Scale Detection, Profiling, and Target Repertoire of IsomiRs in Plants

microRNAs (miRNAs) play an important role as key regulators controlling the post-transcriptional events in plants across development, abiotic and biotic stress, tissue polarity and also in defining the evolutionary basis of the origin of the post-transcriptional machinery. Identifying patterns of regulated and co-regulated small RNAs, in particular miRNAs and their sequence variants with the availability of next generation sequencing approaches has widely demonstrated the role of miRNAs and their temporal regulation in maintaining plant development and their response to stress conditions. Although the role of canonical miRNAs has been widely explored and functional diversity is revealed, those works for isomiRs are still limited and urgent to be carried out across plants. This relative lack of information with respect to isomiRs might be attributed to the non-availability of large-scale detection of isomiRs across wide plant species. In the present research, we addressed this by developing Plant isomiR Atlas, which provides large-scale detection of isomiRs across 23 plant species utilizing 677 smallRNAs datasets and reveals a total of 98,374 templated and non-templated isomiRs from 6,167 precursors. Plant isomiR Atlas provides several visualization features such as species specific isomiRs, isomiRs and canonical miRNAs overlap, terminal modification classifications, target identification using psRNATarget and TargetFinder and also canonical miRNAs:target interactions. Plant isomiR Atlas will play a key role in understanding the regulatory nature of miRNAome and will accelerate to understand the functional role of isomiRs. Plant isomiR Atlas is available at www.mcr.org.in/isomir. One Sentence Summary  Plant isomiR Atlas will play a key role in understanding the regulatory nature of miRNAome and will accelerate the understanding and diversity of functional targets of plants isomiRs.