microRNA profiling of root tissues and root forming explant cultures in Medicago truncatula

Integrated analysis of transcriptome and miRNAome reveals the heat stress response of Pinellia ternata seedlings

Pinellia ternata (Thunb.) Briet., a valuable herb native to China, is susceptible to the "sprout tumble" phenomenon because of high temperatures, resulting in a significant yield reduction. However, the molecular regulatory mechanisms underlying the response of P. ternata to heat stress are not well understood. In this study, we integrated transcriptome and miRNAome sequencing to identify heat-response genes, microRNAs (miRNAs), and key miRNA-target pairs in P. ternata that differed between heat-stress and room-temperature conditions. Transcriptome analysis revealed extensive reprogramming of 4,960 genes across various categories, predominantly associated with cellular and metabolic processes, responses to stimuli, biological regulation, cell parts, organelles, membranes, and catalytic and binding activities. miRNAome sequencing identified 1,597 known/conserved miRNAs that were differentially expressed between the two test conditions. According to the analysis, genes and miRNAs associated with the regulation of transcription, DNA template, transcription factor activity, and sequence-specific DNA binding pathways may play a major role in the resistance to heat stress in P. ternata. Integrated analysis of the transcriptome and miRNAome expression data revealed 41 high-confidence miRNA-mRNA pairs, forming 25 modules. MYB-like proteins and calcium-responsive transcription coactivators may play an integral role in heat-stress resistance in P. ternata. Additionally, the candidate genes and miRNAs were subjected to quantitative real-time polymerase chain reaction to validate their expression patterns. These results offer a foundation for future studies exploring the mechanisms and critical genes involved in heat-stress resistance in P. ternata.

Identification and function of miRNA-mRNA interaction pairs during lateral root development of hemi-parasitic Santalum album L. seedlings

Sandalwood (Santalum album L.) is a hemi-parasitic tree species famous for its santalol and santalene, which are extracted from its heartwood and roots. The ability to understand root functionality within its branched root system would benefit the regulation of sandalwood growth and enhance the commercial value of sandalwood. Phenotypic and anatomical evidence in this study indicated that seed germination stage 4 (SG4) seemed pivotal for lateral root (LR) morphogenesis. Small RNA (sRNA) high-throughput sequencing of root tissues at three sub-stages of SG4 (lateral root primordia initiation (LRPI), lateral root primordia development (LRPD), and lateral root primordia emergence (LRPE)) was performed to identify microRNAs (miRNAs) associated with LR development. A total of 135 miRNAs, including 70 differentially expressed miRNAs (DEMs), were screened. Ten DEMs were selected to investigate transcript abundance in different organs or developmental stages. Among 100 negative DEM-mRNA interaction pairs, four targets (Sa-miR166m_2, 408d, 858a, and novel_Sa-miR8) were selected for studying cleavage sites by 5' RLM-RACE validation. The expression mode of the four miRNA-mRNA pairs was investigated after indole-3-acetic acid (IAA) treatment. IAA enhanced the abundance of homeobox-leucine-zipper protein 32 (HOX32), laccase 12 (LAC12), myeloblastosis86 (MYB86), and pectin methylesterase inhibitor6 (PMEI6) target transcripts by reducing the expression of Sa-miR166m_2, 408d, 858a, and novel_Sa-miR8 in the first 10 min. A schematic model of miRNA-regulated LR development is proposed for this hemi-parasitic species. This novel genetic information for improving sandalwood root growth and development may allow for the cultivation of fast-growing and high-yielding plantations.

Insights into the expression of DNA (de)methylation genes responsive to nitric oxide signaling in potato resistance to late blight disease

Our previous study concerning the pathogen-induced biphasic pattern of nitric oxide (NO) burst revealed that the decline phase and a low level of NO, due to S-nitrosoglutathione reductase (GSNOR) activity, might be decisive in the upregulation of stress-sensitive genes via histone H3/H4 methylation in potato leaves inoculated with avr P. infestans. The present study refers to the NO-related impact on genes regulating DNA (de)methylation, being in dialog with histone methylation. The excessive amounts of NO after the pathogen or GSNO treatment forced the transient upregulation of histone SUVH4 methylation and DNA hypermethylation. Then the diminished NO bioavailability reduced the SUVH4-mediated suppressive H3K9me2 mark on the R3a gene promoter and enhanced its transcription. However, we found that the R3a gene is likely to be controlled by the RdDM methylation pathway. The data revealed the time-dependent downregulation of the DCL3, AGO4, and miR482e genes, exerting upregulation of the targeted R3a gene correlated with ROS1 overexpression. Based on these results, we postulate that the biphasic waves of NO burst in response to the pathogen appear crucial in establishing potato resistance to late blight through the RdDM pathway controlling R gene expression.

Integrated High-Throughput Sequencing, Microarray Hybridization and Degradome Analysis Uncovers MicroRNA-Mediated Resistance Responses of Maize to Pathogen Curvularia lunata

Curvularia lunata (Wakker) Boed, the causal agent of leaf spot in maize, is prone to mutation, making it difficult to control. RNAi technology has proven to be an important tool of genetic engineering and functional genomics aimed for crop improvement. MicroRNAs (miRNAs), which act as post-transcriptional regulators, often cause translational repression and gene silencing. In this article, four small RNA (sRNA) libraries were generated from two maize genotypes inoculated by C. lunata; among these, ltR1 and ltR2 were from the susceptible variety Huangzao 4 (HZ), ltR3 and ltR4, from the resistant variety Luyuan (LY), and 2286, 2145, 1556 and 2504 reads were annotated as miRNA in these four sRNA libraries, respectively. Through the combined analysis of high-throughput sequencing, microarray hybridization and degradome, 48 miRNAs were identified as being related to maize resistance to C. lunata. Among these, PC-732 and PC-169, two new maize miRNAs discovered, were predicted to cleave mRNAs of metacaspase 1 (AMC1) and thioredoxin family protein (Trx), respectively, possibly playing crucial roles in the resistance of maize to C. lunata. To further confirm the role of PC-732 in the interaction of maize and C. lunata, the miRNA was silenced through STTM (short tandem target mimic) technology, and we found that knocking down PC-732 decreased the susceptibility of maize to C. lunata. Precisely speaking, the target gene of PC-732 might inhibit the expression of disease resistance-related genes during the interaction between maize and C. lunata. Overall, the findings of this study indicated the existence of miRNAs involved in the resistance of maize to C. lunata and will contribute to rapidly clarify the resistant mechanism of maize to C. lunata.

Epigenomics as Potential Tools for Enhancing Magnitude of Breeding Approaches for Developing Climate Resilient Chickpea

Epigenomics has become a significant research interest at a time when rapid environmental changes are occurring. Epigenetic mechanisms mainly result from systems like DNA methylation, histone modification, and RNA interference. Epigenetic mechanisms are gaining importance in classical genetics, developmental biology, molecular biology, cancer biology, epidemiology, and evolution. Epigenetic mechanisms play important role in the action and interaction of plant genes during development, and also have an impact on classical plant breeding programs, inclusive of novel variation, single plant heritability, hybrid vigor, plant-environment interactions, stress tolerance, and performance stability. The epigenetics and epigenomics may be significant for crop adaptability and pliability to ambient alterations, directing to the creation of stout climate-resilient elegant crop cultivars. In this review, we have summarized recent progress made in understanding the epigenetic mechanisms in plant responses to biotic and abiotic stresses and have also tried to provide the ways for the efficient utilization of epigenomic mechanisms in developing climate-resilient crop cultivars, especially in chickpea, and other legume crops.

Specific tissue proteins 1 and 6 are involved in root biology during normal development and under symbiotic and pathogenic interactions in Medicago truncatula

ST1 and ST6 are possibly involved in primary and lateral root and symbiotic nodule development, but only ST6 participates in the interaction with hemibiotrophic fungi. Specific tissue (ST) proteins have been shown to be involved in several processes related to plant nutritional status, development, and responses to biotic agents. In particular, ST1 and ST6 are mainly expressed in roots throughout plant development. Here, we analyze where and how the expression of the genes encoding both proteins are modulated in the legume model plant Medicago truncatula in response to the plant developmental program, nodulation induced by a beneficial nitrogen-fixing bacterium (Sinorhizobium meliloti) and the defense response triggered by a pathogenic hemibiotrophic fungus (Fusarium oxysporum). Gene expression results show that ST1 and ST6 participate in the vasculature development of both primary and lateral roots, although only ST6 is related to meristem activity. ST1 and ST6 clearly display different roles in the biotic interactions analyzed, where ST1 is activated in response to a N₂-fixing bacterium and ST6 is up-regulated after inoculation with F. oxysporum. The role of ST1 and ST6 in the nodulation process may be related to nodule organogenesis rather than to the establishment of the interaction itself, and an increase in ST6 correlates with the activation of the salicylic acid signaling pathway during the infection and colonization processes. These results further support the role of ST6 in response to hemibiotrophic fungi. This research contributes to the understanding of the complex network that controls root biology and strengthens the idea that ST proteins are involved in several processes such as primary and lateral root development, nodule organogenesis, and the plant-microbe interaction.

Utilization of Transcriptome, Small RNA, and Degradome Sequencing to Provide Insights Into Drought Stress and Rewatering Treatment in Medicago ruthenica

Drought is a major limiting factor in foraging grass yield and quality. Medicago ruthenica (M. ruthenica) is a high-quality forage legume with drought resistance, cold tolerance, and strong adaptability. In this study, we integrated transcriptome, small RNA, and degradome sequencing in identifying drought response genes, microRNAs (miRNAs), and key miRNA-target pairs in M. ruthenica under drought and rewatering treatment conditions. A total of 3,905 genes and 50 miRNAs (45 conserved and 5 novel miRNAs) were significantly differentially expressed in three test conditions (CK: control, DS: plants under drought stress, and RW: plants rewatering after drought stress). The degradome sequencing (AllenScore < 4) analysis revealed that 104 miRNAs (11 novel and 93 conserved miRNAs) were identified with 263 target transcripts, forming 296 miRNA-target pairs in three libraries. There were 38 differentially expressed targets from 16 miRNAs in DS vs. CK, 31 from 11 miRNAs in DS vs. RW, and 6 from 3 miRNAs in RW vs. CK; 21, 18, and 3 miRNA-target gene pairs showed reverse expression patterns in DS vs. CK, DS vs. RW, and RW vs. CK comparison groups, respectively. These findings provide valuable information for further functional characterization of genes and miRNAs in response to abiotic stress, in general, and drought stress in M. ruthenica, and potentially contribute to drought resistance breeding of forage in the future.

Identification of Exogenous Nitric Oxide-Responsive miRNAs from Alfalfa (Medicago sativa L.) under Drought Stress by High-Throughput Sequencing

Alfalfa (Medicago sativa L.) is a high quality leguminous forage. Drought stress is one of the main factors that restrict the development of the alfalfa industry. High-throughput sequencing was used to analyze the microRNA (miRNA) profiles of alfalfa plants treated with CK (normal water), PEG (polyethylene glycol-6000; drought stress), and PEG + SNP (sodium nitroprusside; nitric oxide (NO) sprayed externally under drought stress). We identified 90 known miRNAs belonging to 46 families and predicted 177 new miRNAs. Real-time quantitative fluorescent PCR (qRT-PCR) was used to validate high-throughput expression analysis data. A total of 32 (14 known miRNAs and 18 new miRNAs) and 55 (24 known miRNAs and 31 new miRNAs) differentially expressed miRNAs were identified in PEG and PEG + SNP samples. This suggested that exogenous NO can induce more new miRNAs. The differentially expressed miRNA maturation sequences in the two treatment groups were targeted by 86 and 157 potential target genes, separately. The function of target genes was annotated by gene ontology (GO) enrichment and kyoto encyclopedia of genes and genomes (KEGG) analysis. The expression profiles of nine selected miRNAs and their target genes verified that their expression patterns were opposite. This study has documented that analysis of miRNA under PEG and PEG + SNP conditions provides important insights into the improvement of drought resistance of alfalfa by exogenous NO at the molecular level. This has important scientific value and practical significance for the improvement of plant drought resistance by exogenous NO.

Differential Expression of MicroRNAs During Root Formation in Taxus Chinensis Var. mairei Cultivars

MicroRNAs (miRNAs) have been shown to play key roles in the regulation of plant growth and development by modifying the expression of their target genes. However, the influence of miRNAs on root formation and development in woody plants, such as Taxus chinensis, remains largely unknown. In the current study, we explored the phytohormone-response and nutrition-response miRNA expression profiles during T. chinensis rooting by quantitative real-time PCR (qPCR). We identified six phytohormone-response miRNAs, namely, miR164a, miR165, miR167a, miR171b, miR319, and miR391, and eight nutrition-response miRNAs, namely, miR169b, miR395a, miR399c, miR408, miR826, miR827, miR857, and miR2111a, that were differentially expressed at different rooting phases of T. chinensis. Using northern blot analysis of the putative target genes of these miRNAs, we detected the relative gene expression changes of the target genes. Taken together, our results suggest that miRNAs are involved in root formation of T. chinensis and that miRNAs may play important regulatory roles in primary root, crown root, and root hair formation by targeting phytohormone and/or nutrition response genes in T. chinensis. For the first time, these results expand our understanding of the molecular mechanisms of plant root formation and development in a conifer species.

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.

Identification of miRNAs and their targets in regulating tuberous root development in radish using small RNA and degradome analyses

High-throughput small RNA sequencing and degradome analysis were used in this study to thoroughly investigate the role of miRNA-mediated regulatory network in tuberous root development of radish. Samples from the early seedling stage (RE) and the cortex splitting stage (RL) were used for the construction of six small RNA libraries and one degradome library. A total of 518 known and 976 novel miRNAs were identified, of which, 338 known and 18 novel miRNAs were expressed in all six libraries, respectively. A total of 52 known and 57 novel miRNAs were identified to be significantly differentially expressed between RE and RL, and 195 mRNAs were verified to be the targets of 194 miRNAs by degradome sequencing. According to the degradome analysis, 11 differentially expressed miRNAs had miRNA-mRNA targets, and 13 targets were identified for these 11 miRNAs. Of the 13 miRNA-mRNA targets, 4 genes (RSG11079.t1, RSG11844.t1, RSG16775.t1, and RSG42419.t1) were involved in hormone-mediated signaling pathway, 2 gens (RSG11079.t1 and RSG16775.t1) were related to post-embryonic root development, and 1 gene (RSG23799.t1) was involved in anatomical structure morphogenesis, according to the GO function analysis for biological process. Target Genes participated in these processes are important candidates for further studies. This study provides valuable information for a better understanding of the molecular mechanisms involved in radish tuberous root formation and development.

Root Development in Medicago truncatula: Lessons from Genetics to Functional Genomics

This decade introduced "omics" approaches, such as genomics, transcriptomics, proteomics, and metabolomics in association with reverse and forward genetic approaches, developed earlier, to try to identify molecular pathways involved in the development or in the response to environmental conditions as well as in animals and plants. This review summarizes studies that utilized "omics" strategies to unravel the root development in the model legume Medicago truncatula and how external factors such as soil mineral status or the presence of bacteria and fungi affect root system architecture in this species. We also compare these "omics" data to the knowledges concerning the Arabidopsis thaliana root development, nowadays considered as the model of allorhiz root systems. However, unlike legumes, this species is unable to interact with soil nitrogen-fixing rhizobia and arbuscular-mycorrhizal (AM) fungi to develop novel root-derived symbiotic structures. Differences in root organization, development, and regulatory pathways between these two model species have been highlighted.

MicroRNA profiling provides insights into post-transcriptional regulation of gene expression in chickpea root apex under salinity and water deficiency

Activity of root apical meristem (RAM) at the root apex is critical for stress-mediated modulation of root-architecture. Chickpea, like other legumes, possesses a basic open root meristem. Deep sequencing was used to perform microRNA expression profiling in root apex of chickpea (Cicer arietinum L.) in order to investigate post-transcriptional regulation of gene expression in this tissue in response to salinity and water deficit. Five small RNA libraries prepared from chickpea root apices at different stages of stress treatments were sequenced to obtain 284 unique miRNA sequences including 60 novel miRNAs belonging to total 255 families. Two hundred and fiftynine miRNAs were differentially expressed in stress. Six hundred and nine mRNA targets involved in diverse cellular processes were predicted for 244 miRNAs. Stress-responsive expression patterns of selected miRNAs, inverse expression patterns of their target genes and the target-cleavage sites were validated. Three candidate miRNA-target gene relationships were validated in transient expression system in chickpea. The miRNA expression profiling under salinity and water deficiency in a legume root apex and the reported function of their target genes suggested important roles of miRNA-mediated post-transcriptional regulation of gene expression involved in re-patterning of root hair cells, lateral root formation and high-affinity K+-uptake under these stresses.

Integrated mRNA and microRNA analysis identifies genes and small miRNA molecules associated with transcriptional and post-transcriptional-level responses to both drought stress and re-watering treatment in tobacco

BACKGROUND

Drought stress is one of the most severe problem limited agricultural productivity worldwide. It has been reported that plants response to drought-stress by sophisticated mechanisms at both transcriptional and post-transcriptional levels. However, the precise molecular mechanisms governing the responses of tobacco leaves to drought stress and water status are not well understood. To identify genes and miRNAs involved in drought-stress responses in tobacco, we performed both mRNA and small RNA sequencing on tobacco leaf samples from the following three treatments: untreated-control (CL), drought stress (DL), and re-watering (WL).

RESULTS

In total, we identified 798 differentially expressed genes (DEGs) between the DL and CL (DL vs. CL) treatments and identified 571 DEGs between the WL and DL (WL vs. DL) treatments. Further analysis revealed 443 overlapping DEGs between the DL vs. CL and WL vs. DL comparisons, and, strikingly, all of these genes exhibited opposing expression trends between these two comparisons, strongly suggesting that these overlapping DEGs are somehow involved in the responses of tobacco leaves to drought stress. Functional annotation analysis showed significant up-regulation of genes annotated to be involved in responses to stimulus and stress, (e.g., late embryogenesis abundant proteins and heat-shock proteins) antioxidant defense (e.g., peroxidases and glutathione S-transferases), down regulation of genes related to the cell cycle pathway, and photosynthesis processes. We also found 69 and 56 transcription factors (TFs) among the DEGs in, respectively, the DL vs. CL and the WL vs. DL comparisons. In addition, small RNA sequencing revealed 63 known microRNAs (miRNA) from 32 families and 368 novel miRNA candidates in tobacco. We also found that five known miRNA families (miR398, miR390, miR162, miR166, and miR168) showed differential regulation under drought conditions. Analysis to identify negative correlations between the differentially expressed miRNAs (DEMs) and DEGs revealed 92 mRNA-miRNA interactions between CL and DL plants, and 32 mRNA-miRNA interactions between DL and WL plants.

CONCLUSIONS

This study provides a global view of the transcriptional and the post-transcriptional responses of tobacco under drought stress and re-watering conditions. Our results establish an empirical foundation that should prove valuable for further investigations into the molecular mechanisms through which tobacco, and plants more generally, respond to drought stress at multiple molecular genetic levels.

Identification of microRNAs and Their Target Genes Explores miRNA-Mediated Regulatory Network of Cytoplasmic Male Sterility Occurrence during Anther Development in Radish (Raphanus sativus L.)

MicroRNAs (miRNAs) are a type of endogenous non-coding small RNAs that play critical roles in plant growth and developmental processes. Cytoplasmic male sterility (CMS) is typically a maternally inherited trait and widely used in plant heterosis utilization. However, the miRNA-mediated regulatory network of CMS occurrence during anther development remains largely unknown in radish. In this study, a comparative small RNAome sequencing was conducted in floral buds of CMS line 'WA' and its maintainer line 'WB' by high-throughput sequencing. A total of 162 known miRNAs belonging to 25 conserved and 24 non-conserved miRNA families were isolated and 27 potential novel miRNA families were identified for the first time in floral buds of radish. Of these miRNAs, 28 known and 14 potential novel miRNAs were differentially expressed during anther development. Several target genes for CMS occurrence-related miRNAs encode important transcription factors and functional proteins, which might be involved in multiple biological processes including auxin signaling pathways, signal transduction, miRNA target silencing, floral organ development, and organellar gene expression. Moreover, the expression patterns of several CMS occurrence-related miRNAs and their targets during three stages of anther development were validated by qRT-PCR. In addition, a potential miRNA-mediated regulatory network of CMS occurrence during anther development was firstly proposed in radish. These findings could contribute new insights into complex miRNA-mediated genetic regulatory network of CMS occurrence and advance our understanding of the roles of miRNAs during CMS occurrence and microspore formation in radish and other crops.

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.

Small RNA and degradome sequencing reveals microRNAs and their targets involved in tomato pedicel abscission

We constructed small RNA and degradome sequencing libraries to identify miRNAs and targets involved in tomato pedicel abscission, and confirmed their roles via quantitative real-time PCR. MicroRNAs (miRNAs) are endogenous small RNAs which play crucial negatively regulatory roles at both the transcriptional and post-transcriptional levels in plants; however, limited knowledge is available on the expression profiles of miRNAs and their target genes during tomato pedicel abscission. Taking advantage of small RNA (sRNA) and degradome sequencing technology, a total of 56 known and 11 novel candidate miRNAs targeting 223 mRNA genes were confirmed during pedicel abscission. Gene ontology annotation and KEGG pathway analysis showed that these target genes were significantly enriched in intracellular, membrane-bounded organelle-related biological processes as well as in metabolic, plant-pathogen interaction and hormone signaling pathways. We screened 17 miRNA/target pairs for further analysis and performed quantitative real-time PCR to identify the roles. Cluster analysis of selected miRNAs revealed that the expression profiles of miRNAs varied in different stages of abscission and could be impacted by ethylene treatment. In the present study, the correlations between miRNAs and targets suggested a complex regulatory network of miRNA-mediated target interaction during pedicel abscission. Additionally, the expression profiles of miRNAs and their targets changed by ethylene might be a considerable reason why ethylene promotes pedicel abscission. Our study provides new insights into the expression and regulatory profiles of miRNAs during tomato pedicel abscission.

Identification of fruit related microRNAs in cucumber (Cucumis sativus L.) using high-throughput sequencing technology

MicroRNAs (miRNAs) are approximately 21 nt noncoding RNAs that influence the phenotypes of different species through the post-transcriptional regulation of gene expression. Although many miRNAs have been identified in a few model plants, less is known about miRNAs specific to cucumber (Cucumis sativus L.). In this study, two libraries of cucumber RNA, one based on fruit samples and another based on mixed samples from leaves, stems, and roots, were prepared for deep-sequencing. A total of 110 sequences were matched to known miRNAs in 47 families, while 56 sequences in 46 families are newly identified in cucumber. Of these, 77 known and 44 new miRNAs were differentially expressed, with a fold-change of at least 2 and p-value < 0.05. In addition, we predicted the potential targets of known and new miRNAs. The identification and characterization of known and new miRNAs will enable us to better understand the role of these miRNAs in the formation of cucumber fruit.

Genome-wide identification of microRNAs associated with taproot development in radish (Raphanus sativus L.)

MicroRNAs (miRNAs) are small, endogenous, non-coding RNAs that play vital regulatory roles in plant growth and development. To identify the miRNAs associated with taproot development at the whole genome level, we sequenced five RNA libraries constructed from radish taproots at different developmental stages and generated a total of 148M clean reads. Using an integrative bioinformatics analysis, 494 known miRNAs belonging to 434 families and 220 putative novel miRNAs were identified. Combining the differential expression analysis and target prediction, we found that 77 miRNAs were potentially associated with taproot development. Target transcripts generated significant GO terms relating to cell proliferation, root development and hormone-mediated signaling. The KEGG analyses revealed that plant hormone signal transduction, zeatin biosynthesis, biosynthesis of secondary metabolites, cell cycle, MAPK signaling and p53 signaling were closely associated with taproot development. These findings will provide valuable information for further functional verification of miRNAs and their targets in radish taproot development.

Application of microRNA gene resources in the improvement of agronomic traits in rice

microRNAs (miRNAs) are important nonprotein-coding genes that are involved in almost all biological processes, including cell differentiation and fate determination, developmental regulation, and immune responses. Investigations have shown that some miRNAs can highly affect plant agricultural traits, including virus resistance, nematode resistance, drought and salinity tolerance, heavy metal detoxification, biomass yield, grain yield, fruit development and flower development. Therefore, these miRNAs are considered a newly identified gene resource for the genetic improvement of crops. In this review, we will summarize the recent findings of the rice miRNA-directed regulatory network, which controls agronomic traits such as yield, quality and stress tolerance, and explore the outlook for the uses of these miRNA-associated traits in rice biotechnology.

Transcriptome profiling of root microRNAs reveals novel insights into taproot thickening in radish (Raphanus sativus L.)

BACKGROUND

Radish (Raphanus sativus L.) is an economically important root vegetable crop, and the taproot-thickening process is the most critical period for the final productivity and quality formation. MicroRNAs (miRNAs) are a family of non-coding small RNAs that play an important regulatory function in plant growth and development. However, the characterization of miRNAs and their roles in regulating radish taproot growth and thickening remain largely unexplored. A Solexa high-throughput sequencing technology was used to identify key miRNAs involved in taproot thickening in radish.

RESULTS

Three small RNA libraries from 'NAU-YH' taproot collected at pre-cortex splitting stage, cortex splitting stage and expanding stage were constructed. In all, 175 known and 107 potential novel miRNAs were discovered, from which 85 known and 13 novel miRNAs were found to be significantly differentially expressed during taproot thickening. Furthermore, totally 191 target genes were identified for the differentially expressed miRNAs. These target genes were annotated as transcription factors and other functional proteins, which were involved in various biological functions including plant growth and development, metabolism, cell organization and biogenesis, signal sensing and transduction, and plant defense response. RT-qPCR analysis validated miRNA expression patterns for five miRNAs and their corresponding target genes.

CONCLUSIONS

The small RNA populations of radish taproot at different thickening stages were firstly identified by Solexa sequencing. Totally 98 differentially expressed miRNAs identified from three taproot libraries might play important regulatory roles in taproot thickening. Their targets encoding transcription factors and other functional proteins including NF-YA2, ILR1, bHLH74, XTH16, CEL41 and EXPA9 were involved in radish taproot thickening. These results could provide new insights into the regulatory roles of miRNAs during the taproot thickening and facilitate genetic improvement of taproot in radish.

Expanding frontiers in plant transcriptomics in aid of functional genomics and molecular breeding

The transcript pool of a plant part, under any given condition, is a collection of mRNAs that will pave the way for a biochemical reaction of the plant to stimuli. Over the past decades, transcriptome study has advanced from Northern blotting to RNA sequencing (RNA-seq), through other techniques, of which real-time quantitative polymerase chain reaction (PCR) and microarray are the most significant ones. The questions being addressed by such studies have also matured from a solitary process to expression atlas and marker-assisted genetic enhancement. Not only genes and their networks involved in various developmental processes of plant parts have been elucidated, but also stress tolerant genes have been highlighted. The transcriptome of a plant with altered expression of a target gene has given information about the downstream genes. Marker information has been used for breeding improved varieties. Fortunately, the data generated by transcriptome analysis has been made freely available for ample utilization and comparison. The review discusses this wide variety of transcriptome data being generated in plants, which includes developmental stages, abiotic and biotic stress, effect of altered gene expression, as well as comparative transcriptomics, with a special emphasis on microarray and RNA-seq. Such data can be used to determine the regulatory gene networks, which can subsequently be utilized for generating improved plant varieties.

The small RNA diversity from Medicago truncatula roots under biotic interactions evidences the environmental plasticity of the miRNAome

BACKGROUND

Legume roots show a remarkable plasticity to adapt their architecture to biotic and abiotic constraints, including symbiotic interactions. However, global analysis of miRNA regulation in roots is limited, and a global view of the evolution of miRNA-mediated diversification in different ecotypes is lacking.

RESULTS

In the model legume Medicago truncatula, we analyze the small RNA transcriptome of roots submitted to symbiotic and pathogenic interactions. Genome mapping and a computational pipeline identify 416 miRNA candidates, including known and novel variants of 78 miRNA families present in miRBase. Stringent criteria of pre-miRNA prediction yield 52 new mtr-miRNAs, including 27 miRtrons. Analyzing miRNA precursor polymorphisms in 26 M. truncatula ecotypes identifies higher sequence polymorphism in conserved rather than Medicago-specific miRNA precursors. An average of 19 targets, mainly involved in environmental responses and signalling, is predicted per novel miRNA. We identify miRNAs responsive to bacterial and fungal pathogens or symbionts as well as their related Nod and Myc-LCO symbiotic signals. Network analyses reveal modules of new and conserved co-expressed miRNAs that regulate distinct sets of targets, highlighting potential miRNA-regulated biological pathways relevant to pathogenic and symbiotic interactions.

CONCLUSIONS

We identify 52 novel genuine miRNAs and large plasticity of the root miRNAome in response to the environment, and also in response to purified Myc/Nod signaling molecules. The new miRNAs identified and their sequence variation across M. truncatula ecotypes may be crucial to understand the adaptation of root growth to the soil environment, notably in the agriculturally important legume crops.

Transcriptome profiling of peanut (Arachis hypogaea) gynophores in gravitropic response

Peanut (Arachis hypogaea L.) produces flowers aerially, but the fruit develops underground. This process is mediated by the gynophore, which always grows vertically downwards. The genetic basis underlying gravitropic bending of gynophores is not well understood. To identify genes related to gynophore gravitropism, gene expression profiles of gynophores cultured in vitro with tip pointing upward (gravitropic stimulation sample) and downward (control) at both 6 and 12h were compared through a high-density peanut microarray. After gravitropic stimulation, there were 174 differentially expressed genes, including 91 upregulated and 83 downregulated genes at 6h, and 491 differentially expressed genes including 129 upregulated and 362 downregulated genes at 12h. The differentially expressed genes identified were assigned to 24 functional categories. Twenty pathways including carbon fixation, aminoacyl-tRNA biosynthesis, pentose phosphate pathway, starch and sucrose metabolism were identified. The quantitative real-time PCR analysis was performed for validation of microarray results. Our study paves the way to better understand the molecular mechanisms underlying the peanut gynophore gravitropism.