High-Throughput Sequencing Identifies Novel and Conserved Cucumber (Cucumis sativus L.) microRNAs in Response to Cucumber Green Mottle Mosaic Virus Infection

Transcriptional and post-transcriptional regulation of RNAi-related gene expression during plant-virus interactions

As sessile organisms, plants encounter diverse invasions from pathogens including viruses. To survive and thrive, plants have evolved multilayered defense mechanisms to combat virus infection. RNAi, also known as RNA silencing, is an across-kingdom innate immunity and gene regulatory machinery. Molecular framework and crucial roles of RNAi in antiviral defense have been well-characterized. However, it is largely unknown that how RNAi is transcriptionally regulated to initiate, maintain and enhance cellular silencing under normal or stress conditions. Recently, insights into the transcriptional and post-transcriptional regulation of RNAi-related genes in different physiological processes have been emerging. In this review, we integrate these new findings to provide updated views on how plants modulate RNAi machinery at the (post-) transcriptional level to respond to virus infection.

Exogenous Application of dsRNA for the Control of Viruses in Cucurbits

The recurrent emergence of viral diseases in intensive horticultural crops requires alternative control strategies. The topical application of double-stranded RNA (dsRNA) molecules homologous to pathogens has been proposed as a tool for virus control in plants. These dsRNAs induce the silencing mechanism, the RNA interference (RNAi), that degrades homologous dsRNAs. Cucumber green mottle mosaic virus (CGMMV) represents a serious threat to cucurbit crops. Since genetic resistance to the virus is not yet available in commercial varieties, we aimed to control this virus by RNAi. For this purpose, we obtained constructions both for expressing dsRNA in bacteria to treat cucumber plants by topical application and for agroinoculation in experiments done in the growth chamber. Besides, greenhouse tests were performed in spring and in summer when plants were challenged with the virus, and differences in several parameters were investigated, including the severity of symptoms, dry weight, total height, virus accumulation, and virus-derived small interfering RNAs (vsiRNAs). Spraying of plants with dsRNA reduced significatively CGMMV symptoms in the plants in growth chamber tests. Agroinfiltration experiments done under identical conditions were also effective in limiting the progress of CGMMV disease. In the greenhouse assay performed in spring, symptoms were significantly reduced in dsRNA-sprayed plants, and the development of the plants improved with respect to non-treated plants. Virus titers and vsiRNAs were clearly reduced in dsRNA-treated plants. The effect of protection of the dsRNA was less evident in the greenhouse assay carried out in the summer. Besides, we investigated the mobility of long (ds)RNA derived from spraying or agroinfiltrated dsRNA and found that it could be detected in local, close distal, and far distal points from the site of application. VsiRNAs were also detected in local and distal points and the differences in accumulation were compared. In parallel, we investigated the capacity of dsRNAs derived from genes of tomato leaf curl New Delhi virus (ToLCNDV), another economically important virus in cucurbits, to limit the disease in zucchini, both by agroinfiltration or by direct spraying, but found no protective effect. In view of the results, the topical application of dsRNAs is postulated as a promising strategy for CGMMV control in the cucumber.

Genome and Phylogenetic Analysis of Cucumber Green Mottle Mosaic Virus Global Isolates and Validation of a Highly Sensitive RT-qPCR Assay

The last two decades have seen exponential growth in the international movement of seeds for annual food crops, from a gross U.S. import value of $349 million in 1999 to $1.05 billion in 2019. This has led to the proportionate growth of seedborne pathogens dispersed with seed stocks. One such viral pathogen is cucumber green mottle mosaic virus (CGMMV), a tobamovirus that infects cucurbit crops such as melon, watermelon, cucumber, pumpkin, and squash. The first CGMMV introduction to California occurred in 2013, with subsequent annual outbreaks or detections since then. Here, we describe the use of next-generation sequencing to characterize the full genomes of 25 CGMMV isolates collected between 2013 and 2020 in California, either from CGMMV field detections or seed lots identified as CGMMV positive. We sequenced an additional 31 CGMMV isolates collected in Europe, Israel, and southeast Asia that were provided by industry collaborators. We also performed an in silico nucleotide database search in GenBank for full genome CGMMV sequences to include in all in silico analyses. Based on conserved regions within the coat protein gene, we then developed a quantitative reverse-transcription PCR assay for the sensitive and specific detection of CGMMV in seed and plant samples. Finally, based on our sequence and phylogenetic analysis, our data support that CGMMV has been introduced multiple times into California.

Exploring the Effect of Methyl Jasmonate on the Expression of microRNAs Involved in Biosynthesis of Active Compounds of Rosemary Cell Suspension Cultures through RNA-Sequencing

Our aim in the experiment was to study the effects of methyl jasmonates (MeJA) on the active compounds of rosemary suspension cells, the metabolites' change of contents under different concentrations of MeJA, including 0 (CK), 10 (M10), 50 (M50) and 100 μM MeJA (M100). The results demonstrated that MeJA treatments promoted the accumulation of rosmarinic acid (RA), carnosic acid (CA), flavonoids, jasmonate (JA), gibberellin (GA), and auxin (IAA); but reduced the accumulations of abscisic acid (ABA), salicylic acid (SA), and aspartate (Asp). In addition, 50 and 100 μM MeJA promoted the accumulation of alanine (Ala) and glutamate (Glu), and 50 μM MeJA promoted the accumulation of linoleic acid and alpha-linolenic acid in rosemary suspension cells. Comparative RNA-sequencing analysis of different concentrations of MeJA showed that a total of 30, 61, and 39 miRNAs were differentially expressed in the comparisons of CKvsM10, CKvsM50, CKvsM100, respectively. The analysis of the target genes of the differentially expressed miRNAs showed that plant hormone signal transduction, linoleic acid, and alpha-linolenic acid metabolism-related genes were significantly enriched. In addition, we found that miR160a-5p target ARF, miR171d_1 and miR171f_3 target DELLA, miR171b-3p target ETR, and miR156a target BRI1, which played a key role in rosemary suspension cells under MeJA treatments. qRT-PCR of 12 differentially expressed miRNAs and their target genes showed a high correlation between the RNA-seq and the qRT-PCR result. Amplification culture of rosemary suspension cells in a 5 L stirred bioreactor showed that cell biomass accumulation in the bioreactor was less than that in the shake flask under the same conditions, and the whole cultivation period was extended to 14 d. Taken together, MeJA promoted the synthesis of the active compounds in rosemary suspension cells in a wide concentration range via concentration-dependent differential expression patterns. This study provided an overall view of the miRNAs responding to MeJA in rosemary.

New early phenotypic markers for cucumber green mottle mosaic virus disease in cucumbers exposed to fluctuating extreme temperatures

Studies of early stages of cucumber green mottle mosaic virus (CGMMV) disease have been recently focused on plant molecular responses. However, extreme diurnal environmental temperatures, characteristic of global climate changes, could affect plant susceptibility and disease phenotype progression. Our studies of CGMMV disease progression, under simulated extreme temperature waves, have revealed two new disease initiation phenotypes that developed gradually, preceding severe symptom manifestations of post-recovery CGMMV systemic infections. 'Early post-recovery stage' bright yellow islands (BYIs) with defined boundaries amid asymptomatic leaf blades were first emerging followed by 'late post-recovery stage' BYIs with diffused boundaries. A deduced CGMMV disease progression scheme, postulating BYI symptom occurrence time-windows, revealed BYIs in field grown cucumber plants exposed to extreme diurnal temperatures. Profiling ontology of cucumber differentially expressed genes in BYIs vs the associated dark-green surrounding tissues disclosed activation of jasmonic acid (JA) pathway in 'early post-recovery stage' BYIs. JA signaling was inactivated in 'late post-recovery stage' BYIs concomitant with increasing expressions of JA signaling inhibitors and downregulation of JA responsive phenylpropanoid pathway. Our results disclosed a new phenotypic description of CGMMV disease initiation, characteristic of cucumbers grown under extreme environmental temperature fluctuations. The BYI phenotypes could define a time-window for CGMMV disease management applications.

Effects of Host-Adaptive Mutations on Hop Stunt Viroid Pathogenicity and Small RNA Biogenesis

Accidental transmission of hop stunt viroid (HSVd) from grapevine to hop has led to several epidemics of hop stunt disease with convergent evolution of HSVd-g(rape) into HSVd-h(op) containing five mutations. However, the biological function of these five mutations remains unknown. In this study, we compare the biological property of HSVd-g and HSVd-h by bioassay and analyze HSVd-specific small RNA (HSVd-sRNA) using high-throughput sequencing. The bioassay indicated an association of these five mutations with differences in infectivity, replication capacity, and pathogenicity between HSVd-g and HSVd-h, e.g., HSVd-g induced more severe symptoms than HSVd-h in cucumber. Site-directed mutagenesis of HSVd-g showed that the mutation at position 54 increased pathogenicity. HSVd-sRNA analysis of cucumber and hop plants infected with different HSVd variants showed that several sRNA species containing adaptive nucleotides were specifically down-regulated in plants infected with HSVd-h. Several HSVd-sRNAs containing adaptive mutations were predicted to target cucumber genes, but changes in the levels of these genes were not directly correlated with changes in symptom expression. Furthermore, expression levels of two other cucumber genes targeted by HSVd-RNAs, encoding ethylene-responsive transcription factor ERF011, and trihelix transcription factor GTL2, were altered by HSVd infection. The possible relationship between these two genes to HSVd pathogenicity is discussed.

Identification of microRNAs regulated by tobacco curly shoot virus co-infection with its betasatellite in Nicotiana benthamiana

BACKGROUND

MicroRNAs (miRNAs) are a class of 21-24 nucleotide endogenous non-coding small RNAs that play important roles in plant development and defense responses to biotic and abiotic stresses. Tobacco curly shoot virus (TbCSV) is a monopartite begomovirus, cause leaf curling and plant stunting symptoms in many Solanaceae plants. The betasatellite of TbCSV (TbCSB) induces more severe symptoms and enhances virus accumulation when co-infect the plants with TbCSV.

METHODS

In this study, miRNAs regulated by TbCSV and TbCSB co-infection in Nicotiana benthamiana were characterized using high-throughput sequencing technology.

RESULTS

Small RNA sequencing analysis revealed that a total of 13 known miRNAs and 42 novel miRNAs were differentially expressed in TbCSV and TbCSB co-infected N. benthamiana plants. Several potential miRNA-targeted genes were identified through data mining and were involved in both catalytic and metabolic processes, in addition to plant defense mechanisms against virus infections according to Gene Ontology (GO) analyses. In addition, the expressions of several differentially expressed miRNAs and their miRNA-targeted gene were validated through quantitative real time polymerase chain reaction (qRT-PCR) approach.

CONCLUSIONS

A large number of miRNAs are identified, and their target genes, functional annotations also have been explored. Our results provide the information on N. benthamiana miRNAs and would be useful to further understand miRNA regulatory mechanisms after TbCSV and TbCSB co-infection.

DNA Methylation Analysis of the Citrullus lanatus Response to Cucumber Green Mottle Mosaic Virus Infection by Whole-Genome Bisulfite Sequencing

DNA methylation is an important epigenetic mark associated with plant immunity, butlittle is known about its roles in viral infection of watermelon. We carried out whole-genomebisulfite sequencing of watermelon leaves at 0 h (ck), 48 h, and 25 days post-inoculation withCucumber green mottle mosaic virus (CGMMV). The number of differentially methylated regions(DMRs) increased during CGMMV infection and 2788 DMR-associated genes (DMGs) werescreened out among three libraries. Most DMRs and DMGs were obtained under the CHH context.These DMGs were significantly enriched in the Kyoto Encyclopedia of Genes and Genomes (KEGG)pathways of secondary biosynthesis and metabolism, plant-pathogen interactions, Toll-likereceptor signaling, and ABC transporters. Additionally, DMGs encoding PR1a, CaMs, calciumbindingprotein, RIN4, BAK1, WRKYs, RBOHs, STKs, and RLPs/RLKs were involved in thewatermelon-CGMMV interaction and signaling. The association between DNA methylation andgene expression was analyzed by RNA-seq and no clear relationship was detected. Moreover,downregulation of genes in the RdDM pathway suggested the reduced RdDM-directed CHHmethylation plays an important role in antiviral defense in watermelon. Our findings providegenome-wide DNA methylation profiles of watermelon and will aid in revealing the molecularmechanism in response to CGMMV infection at the methylation level.

Identification of cucumber circular RNAs responsive to salt stress

BACKGROUND

Circular RNAs (circRNAs) are 3'-5' head-to-tail covalently closed non-coding RNA that have been proved to play essential roles in many cellular and developmental processes. However, no information relate to cucumber circRNAs is available currently, especially under salt stress condition.

RESULTS

In this study, we sequenced circRNAs in cucumber and a total of 2787 were identified, with 1934 in root and 44 in leaf being differentially regulated under salt stress. Characteristics analysis of these circRNAs revealed following features: most of them are exon circRNAs (79.51%) and they prefer to arise from middle exon(s) of parent genes (2035/2516); moreover, most of circularization events (88.3%) use non-canonical-GT/AG splicing signals; last but not least, pairing-driven circularization is not the major way to generate cucumber circRNAs since very few circRNAs (18) contain sufficient flanking complementary sequences. Annotation and enrichment analysis of both parental genes and target mRNAs were launched to uncover the functions of differentially expressed circRNAs induced by salt stress. The results showed that circRNAs may be paly roles in salt stress response by mediating transcription, signal transcription, cell cycle, metabolism adaptation, and ion homeostasis related pathways. Moreover, circRNAs may function to regulate proline metabolisms through regulating associated biosynthesis and degradation genes.

CONCLUSIONS

The present study identified large number of cucumber circRNAs and function annotation revealed their possible biological roles in response to salt stress. Our findings will lay a solid foundation for further structure and function studies of cucumber circRNAs.

Expression profiling and regulatory network of cucumber microRNAs and their putative target genes in response to cucumber green mottle mosaic virus infection

Cucumber green mottle mosaic virus (CGMMV) is an important pathogen of cucumber (Cucumis sativus). The molecular mechanisms mediating host-pathogen interactions are likely to be strongly influenced by microRNAs (miRNAs), which are known to regulate gene expression during the disease cycle. This study focused on 14 miRNAs (miR159, miR169, miR172, miR838, miR854, miR5658, csa-miRn1-3p, csa-miRn2-3p, csa-miRn3-3p, csa-miRn4-5p, csa-miRn5-5p, csa-miRn6-3p, csa-miRn7-5p and csa-miRn8-3p) and their target genes. The data collected was used to construct a regulatory network of miRNAs and target genes associated with cucumber-CGMMV interactions, which identified 608 potential target genes associated with all of the miRNAs except csa-miRn7-5p. Five of the miRNAs (miR159, miR838, miR854, miR5658 and csa-miRn6-3p) were found to be mutually linked by target genes, while another eight (miR169, miR172, csa-miRn1-3p, csa-miRn2-3p, csa-miRn3-3p, csa-miRn4-5p, csa-miRn5-5p and csa-miRn8-3p) formed subnetworks that did not display any connectivity with other miRNAs or their target genes. Reverse transcription quantitative real-time PCR (RT-qPCR) was used to analyze the expression levels of the different miRNAs and their putative target genes in leaf, stem and root samples of cucumber over a 42-day period after inoculation with CGMMV. A positive correlation was found between some of the miRNAs and their respective target genes, although for most, the response varied greatly depending on the time point, indicating that additional factors are likely to be involved in the interaction between cucumber miRNAs and their target genes. Several miRNAs, including miR159 and csa-miRn6-3p, were linked to target genes that have been associated with plant responses to disease. A model linking miRNAs, their targets and downstream biological processes is proposed to indicate the roles of these miRNAs in the cucumber-CGMMV pathosystem.

Transcriptome Analysis of Watermelon Leaves Reveals Candidate Genes Responsive to Cucumber green mottle mosaic virus Infection

Cucumber green mottle mosaic virus (CGMMV) is a member of the genus Tobamovirus, which cause diseases in cucurbits, especially watermelon. In watermelon, symptoms develop on the whole plant, including leaves, stems, peduncles, and fruit. To better understand the molecular mechanisms of watermelon early responses to CGMMV infection, a comparative transcriptome analysis of 24 h CGMMV-infected and mock-inoculated watermelon leaves was performed. A total of 1641 differently expressed genes (DEGs) were identified, with 886 DEGs upregulated and 755 DEGs downregulated after CGMMV infection. A functional analysis indicated that the DEGs were involved in photosynthesis, plant⁻pathogen interactions, secondary metabolism, and plant hormone signal transduction. In addition, a few transcription factor families, including WRKY, MYB, HLH, bZIP and NAC, were responsive to the CGMMV-induced stress. To confirm the high-throughput sequencing results, 15 DEGs were validated by qRT-PCR analysis. The results provide insights into the identification of candidate genes or pathways involved in the responses of watermelon leaves to CGMMV infection.

High-throughput sequencing analysis of microRNAs in gills of red swamp crayfish, Procambarus clarkii infected with white spot syndrome virus

MicroRNAs (miRNAs) are important posttranscriptional regulators. They play an important role in the antiviral innate immunity of invertebrates. In the present study, high-throughput small RNAs Illumina sequencing systems were carried out to identify differentially expressed miRNAs (DEMs) in the gills of Procambarus clarkii, which was challenged with white spot syndrome virus (WSSV). Our results identified 11,617 known and 6 novel miRNAs in normal group (NG) and WSSV-challenged group (WG) small RNA libraries. Additionally, 27 DEMs were shown to participate in the antiviral innate immunity of P. clarkii and were significantly upregulated or downregulated. In addition, the results of the KEGG pathway prediction of the DEMs target genes showed that putative target genes of these 27 DEMs were related mainly to the RNA transport pathway, tight junction pathway, mRNA surveillance pathway, regulation actin cytoskeleton pathway, focal adhesion pathway, and MAPK signaling pathway. These results provide important information for future studies about the antiviral innate immunity of crustaceans.

High-throughput sequencing of small RNAs revealed the diversified cold-responsive pathways during cold stress in the wild banana (Musa itinerans)

BACKGROUND

Cold stress is one of the most severe abiotic stresses affecting the banana production. Although some miRNAs have been identified, little is known about the role of miRNAs in response to cold stress in banana, and up to date, there is no report about the role of miRNAs in the response to cold stress in the plants of the cultivated or wild bananas.

RESULT

Here, a cold-resistant line wild banana (Musa itinerans) from China was used to profile the cold-responsive miRNAs by RNA-seq during cold stress. Totally, 265 known mature miRNAs and 41 novel miRNAs were obtained. Cluster analysis of differentially expressed (DE) miRNAs indicated that some miRNAs were specific for chilling or 0 °C treated responses, and most of them were reported to be cold-responsive; however, some were seldom reported to be cold-responsive in response to cold stress, e.g., miR395, miR408, miR172, suggesting that they maybe play key roles in response to cold stress. The GO and KEGG pathway enrichment analysis of DE miRNAs targets indicated that there existed diversified cold-responsive pathways, and miR172 was found likely to play a central coordinating role in response to cold stress, especially in the regulation of CK2 and the circadian rhythm. Finally, qPCR assays indicated the related targets were negatively regulated by the tested DE miRNAs during cold stress in the wild banana.

CONCLUSIONS

In this study, the profiling of miRNAs by RNA-seq in response to cold stress in the plants of the wild banana (Musa itinerans) was reported for the first time. The results showed that there existed diversified cold-responsive pathways, which provided insight into the roles of miRNAs during cold stress, and would be helpful for alleviating cold stress and cold-resistant breeding in bananas.

Suppression of nbe-miR166h-p5 attenuates leaf yellowing symptoms of potato virus X on Nicotiana benthamiana and reduces virus accumulation

MicroRNAs (miRNAs) play essential roles in plant development. There is increasing evidence that changed expression of miRNAs in virus-infected plants contributes to the development of viral symptoms. Here, we analysed the altered expression of miRNAs of Nicotiana benthamiana in response to Potato virus X (PVX) by Illumina Solexa sequencing. One of the 21 miRNAs significantly affected, nbe-miR166h-p5, was closely associated with viral symptoms. Using the Tobacco rattle virus-based miRNA suppression (VbMS) system, we found that the suppression of nbe-miR166h-p5 in plants caused leaves to turn dark green with increased chlorophyll. When PVX was inoculated on nbe-miR166h-p5-suppressed plants, the leaf yellowing symptom of PVX was largely attenuated with less reduction in chlorophyll content, and the accumulation of PVX was decreased. nbe-miR166h-p5 was also up-regulated in plants infected by Turnip mosaic virus (TuMV), and its suppression attenuated the leaf yellowing symptom of TuMV and decreased viral accumulation. Three potential targets of nbe-miR166h-p5 were identified. The results indicate the association of nbe-miR166h-p5 with symptoms of PVX and also with those of TuMV, providing useful information on the relationship between miRNA and viral infection.

Genome-wide identification of new reference genes for RT-qPCR normalization in CGMMV-infected Lagenaria siceraria

Lagenaria siceraria is an economically important cucurbitaceous crop, but suitable reference genes (RGs) to use when the plants are infected by cucumber green mottle mosaic virus (CGMMV) have not been determined. Sixteen candidate RGs of both leaf and fruit and 18 candidate RGs mostly from separate RNA-Seq datasets of bottle gourd leaf or fruit were screened and assessed by RT-qPCR. The expression stability of these genes was determined and ranked using geNorm, NormFinder, BestKeeper and RefFinder. Comprehensive analysis resulted in the selection of LsCYP, LsH3, and LsTBP as the optimal RGs for bottle gourd leaves, and LsP4H, LsADP, and LsTBP for fruits. LsWD, LsGAPDH, and LsH3 were optimal for use in both leaves and fruits under the infection of CGMMV. Isopentenyl transferase (IPT) and DNA-directed RNA polymerase (DdRP) were used to validate the applicability of the most stable identified RGs from bottle gourd in response to CGMMV. All the candidate RGs performed in RT-qPCR consistently with the data from the transcriptome database. The results demonstrated that LsWD, LsGAPDH and LsH3 were the most suitable internal RGs for the leaf, and LsH3, LsGAPDH, LsP4H and LsCYP for the fruit.

Down-regulation of genes coding for core RNAi components and disease resistance proteins via corresponding microRNAs might be correlated with successful Soybean mosaic virus infection in soybean

Plants protect themselves from virus infections by several different defence mechanisms. RNA interference (RNAi) is one prominent antiviral mechanism, which requires the participation of AGO (Argonaute) and Dicer/DCL (Dicer-like) proteins. Effector-triggered immunity (ETI) is an antiviral mechanism mediated by resistance (R) genes, most of which encode nucleotide-binding site-leucine-rich repeat (NBS-LRR) family proteins. MicroRNAs (miRNAs) play important regulatory roles in plants, including the regulation of host defences. Soybean mosaic virus (SMV) is the most common virus in soybean and, in this work, we identified dozens of SMV-responsive miRNAs by microarray analysis in an SMV-susceptible soybean line. Amongst the up-regulated miRNAs, miR168a, miR403a, miR162b and miR1515a predictively regulate the expression of AGO1, AGO2, DCL1 and DCL2, respectively, and miR1507a, miR1507c and miR482a putatively regulate the expression of several NBS-LRR family disease resistance genes. The regulation of target gene expression by these seven miRNAs was validated by both transient expression assays and RNA ligase-mediated rapid amplification of cDNA ends (RLM-RACE) experiments. Transcript levels for AGO1, DCL1, DCL2 and five NBS-LRR family genes were repressed at different time points after SMV infection, whereas the corresponding miRNA levels were up-regulated at these same time points. Furthermore, inhibition of miR1507a, miR1507c, miR482a, miR168a and miR1515a by short tandem target mimic (STTM) technology compromised SMV infection efficiency in soybean. Our results imply that SMV can counteract soybean defence responses by the down-regulation of several RNAi pathway genes and NBS-LRR family resistance genes via the induction of the accumulation of their corresponding miRNA levels.

Identifying optimal reference genes for the normalization of microRNA expression in cucumber under viral stress

Cucumber green mottle mosaic virus (CGMMV) is an economically important pathogen and causes significant reduction of both yield and quality of cucumber (Cucumis sativus). Currently, there were no satisfied strategies for controlling the disease. A better understanding of microRNA (miRNA) expression related to the regulation of plant-virus interactions and virus resistance would be of great assistance when developing control strategies for CGMMV. However, accurate expression analysis is highly dependent on robust and reliable reference gene used as an internal control for normalization of miRNA expression. Most commonly used reference genes involved in CGMMV-infected cucumber are not universally expressed depending on tissue types and stages of plant development. It is therefore crucial to identify suitable reference genes in investigating the role of miRNA expression. In this study, seven reference genes, including Actin, Tubulin, EF-1α, 18S rRNA, Ubiquitin, GAPDH and Cyclophilin, were evaluated for the most accurate results in analyses using reverse transcription-quantitative polymerase chain reaction (RT-qPCR). Gene expression was assayed on cucumber leaves, stems and roots that were collected at different days post inoculation with CGMMV. The expression data were analyzed using algorithms including delta-Ct, geNorm, NormFinder, and BestKeeper as well as the comparative tool RefFinder. The reference genes were subsequently validated using miR159. The results showed that EF-1α and GAPDH were the most reliable reference genes for normalizing miRNA expression in leaf, root and stem samples, while Ubiquitin and EF-1α were the most suitable combination overall.

Comparative analysis of miRNA and mRNA abundance in determinate cucumber by high-throughput sequencing

Determinate cucumber is a type of short vines, fewer nodes, and terminal flowers, it is suitable for high-density planting and available harvesting in field cultivation, whereas the indeterminate cucumber is preferred to cultivate in greenhouses. However, many biotic or abiotic stresses could lead indeterminate cucumber to be determinate in greenhouse cultivation, which may decrease yield and fruit quality. Therefore, it is urgent and essential to investigate the key factors forming determinate and terminal flowering in cucumber. In this study, two close background inbred lines were selected and conducted the miRNA and mRNA high throughput sequencing. Interestingly, ethylene-associated miRNAs and mRNAs were intensively obtained, indicating that the plant hormone ethylene is a key factor impacting determinate and terminal flowering in cucumber. The ethylene metabolites analysis showed that significant higher ethylene was observed in determinate line than that in the indeterminate line. The RT-qPCR validation of ethylene related miRNAs Cas-miR172, Cas-miR396, and Cas-miR414 and their target mRNAs showed a significant negative correlation. These data suggested that ethylene-associated miRNAs might affect determinate and terminal flower phenotypes by regulating their target genes expression. This study not only provides a potential molecular mechanism for determinate formation in cucumber but also establishes a method to demonstrate important physiological processes through the comprehensive association of miRNA and mRNA high-throughput sequencing.

Transcriptome analysis of watermelon (Citrullus lanatus) fruits in response to Cucumber green mottle mosaic virus (CGMMV) infection

Cucumber green mottle mosaic virus (CGMMV) belongs to the Tobamovirus genus and is a major global plant virus on cucurbit plants. It causes severe disease symptoms on infected watermelon plants (Citrullus lanatus), particularly inducing fruit decay. However, little is known about the molecular mechanism of CGMMV-induced watermelon fruit decay. For this study, comparative analysis of transcriptome profiles of CGMMV-inoculated and mock-inoculated watermelon fruits were conducted via RNA-Seq. A total of 1,621 differently expressed genes (DEGs) were identified in CGMMV-inoculated watermelon, among which 1,052 were up-regulated and 569 were down-regulated. Functional annotation analysis showed that several DEGs were involved in carbohydrate metabolism, hormone biosynthesis and signaling transduction, secondary metabolites biosynthesis, and plant-pathogen interactions. We furthermore found that some DEGs were related to cell wall components and photosynthesis, which may directly be involve in the development of the symptoms associated with diseased watermelons. To confirm the RNA-Seq data, 15 DEGs were selected for gene expression analysis by qRT-PCR. The results showed a strong correlation between these two sets of data. Our study identified many candidate genes for further functional studies during CGMMV-watermelon interactions, and will furthermore help to clarify the understanding of pathogenic mechanism underlying CGMMV infection in cucurbit plants.

Identification of protoplast-isolation responsive microRNAs in Citrus reticulata Blanco by high-throughput sequencing

Protoplast isolation is a stress-inducing process, during which a variety of physiological and molecular alterations take place. Such stress response affects the expression of totipotency of cultured protoplasts. MicroRNAs (miRNAs) play important roles in plant growth, development and stress responses. However, the underlying mechanism of miRNAs involved in the protoplast totipotency remains unclear. In this study, high-throughput sequencing technology was used to sequence two populations of small RNA from calli and callus-derived protoplasts in Citrus reticulata Blanco. A total of 67 known miRNAs from 35 families and 277 novel miRNAs were identified. Among these miRNAs, 18 known miRNAs and 64 novel miRNAs were identified by differentially expressed miRNAs (DEMs) analysis. The expression patterns of the eight DEMs were verified by qRT-PCR. Target prediction showed most targets of the miRNAs were transcription factors. The expression levels of half targets showed a negative correlation to those of the miRNAs. Furthermore, the physiological analysis showed high levels of antioxidant activities in isolated protoplasts. In short, our results indicated that miRNAs may play important roles in protoplast-isolation response.

Prediction of Host-Derived miRNAs with the Potential to Target PVY in Potato Plants

Potato virus Y has emerged as a threatening problem in all potato growing areas around the globe. PVY reduces the yield and quality of potato cultivars. During the last 30 years, significant genetic changes in PVY strains have been observed with an increased incidence associated with crop damage. In the current study, computational approaches were applied to predict Potato derived miRNA targets in the PVY genome. The PVY genome is approximately 9 thousand nucleotides, which transcribes the following 6 genes:CI, NIa, NIb-Pro, HC-Pro, CP, and VPg. A total of 343 mature miRNAs were retrieved from the miRBase database and were examined for their target sequences in PVY genes using the minimum free energy (mfe), minimum folding energy, sequence complementarity and mRNA-miRNA hybridization approaches. The identified potato miRNAs against viral mRNA targets have antiviral activities, leading to translational inhibition by mRNA cleavage and/or mRNA blockage. We found 86 miRNAs targeting the PVY genome at 151 different sites. Moreover, only 36 miRNAs potentially targeted the PVY genome at 101 loci. The CI gene of the PVY genome was targeted by 32 miRNAs followed by the complementarity of 26, 19, 18, 16, and 13 miRNAs. Most importantly, we found 5 miRNAs (miR160a-5p, miR7997b, miR166c-3p, miR399h, and miR5303d) that could target the CI, NIa, NIb-Pro, HC-Pro, CP, and VPg genes of PVY. The predicted miRNAs can be used for the development of PVY-resistant potato crops in the future.

Identification of miRNAs involved in fruit ripening in Cavendish bananas by deep sequencing

BACKGROUND

MicroRNAs (miRNAs) are a family of non-coding small RNAs that play an important regulatory role in various biological processes. Previous studies have reported that miRNAs are closely related to the ripening process in model plants. However, the miRNAs that are closely involved in the banana fruit ripening process remain unknown.

METHODS

Here, we investigated the miRNA populations from banana fruits in response to ethylene or 1-MCP treatment using a deep sequencing approach and bioinformatics analysis combined with quantitative RT-PCR validation.

RESULTS

A total of 125 known miRNAs and 26 novel miRNAs were identified from three libraries. MiRNA profiling of bananas in response to ethylene treatment compared with 1-MCP treatment showed differential expression of 82 miRNAs. Furthermore, the differentially expressed miRNAs were predicted to target a total of 815 target genes. Interestingly, some targets were annotated as transcription factors and other functional proteins closely involved in the development and the ripening process in other plant species. Analysis by qRT-PCR validated the contrasting expression patterns between several miRNAs and their target genes.

CONCLUSIONS

The miRNAome of the banana fruit in response to ethylene or 1-MCP treatment were identified by high-throughput sequencing. A total of 82 differentially expressed miRNAs were found to be closely associated with the ripening process. The miRNA target genes encode transcription factors and other functional proteins, including SPL, APETALA2, EIN3, E3 ubiquitin ligase, β-galactosidase, and β-glucosidase. These findings provide valuable information for further functional research of the miRNAs involved in banana fruit ripening.