Advances in identification and validation of plant microRNAs and their target genes

PmiR-Select® - a computational approach to plant pre-miRNA identification in genomes

Precursors of microRNAs (pre-miRNAs) are less used in silico to mine miRNAs. This study developed PmiR-Select® based on covariance models (CMs) to identify new pre-miRNAs, detecting conserved secondary structural features across RNA sequences and eliminating the redundancy. The pipeline preceded PmiR-Select® filtered 20% plant pre-miRNAs (from 38589 to 8677) from miRBase. The second filter reduced pre-miRNAs by 7% (from 8677 to 8045) through length limit to pre-miRNAs (70-300 nt) and miRNAs (20-24 nt). The 80% redundancy threshold was statistically the best, eliminating 55% pre-miRNAs (from 8045 to 3608). Angiosperms retained the highest number of pre-miRNAs and their families (2981 and 2202), followed by gymnosperms (362 and 271), bryophytes (183 and 119), and algae (82 and 78). Thirty-seven conserved pre-miRNA families happened among plant land clades, but none with algae. The PmiR-Select® was applied to the rice genome, producing 8536 pre-miRNAs from 36 families. The 80% redundancy threshold retained 3% pre-miRNAs (n = 264) from 36 families, valuable experimental and computational research resources. 14% (n = 1216) of 8536 were new pre-miRNAs from 19 new families in rice. Only 16 new sequences from six families overlapped (39 to 54% identities) with rice pre-miRNAs and five species on miRBase. The validation against mature miRNAs identified 8086 pre-miRNAs from 13 families. Eleven ones have already been recorded, but two new and abundant pre-miRNAs [miR437 (n = 296) and miR1435 (n = 725)] scattered in all 12-rice chromosomes. PmiR-Select® identified pre-miRNAs, decreased the redundancy, and discovered new miRNAs. These findings pave the way to delineating benchtop and computational experiments.

MicroRNA and Degradome Profiling Uncover Defense Response of Fraxinus velutina Torr. to Salt Stress

Soil salinization is a major environmental problem that seriously threatens the sustainable development of regional ecosystems and local economies. Fraxinus velutina Torr. is an excellent salt-tolerant tree species, which is widely planted in the saline-alkaline soils in China. A growing body of evidence shows that microRNAs (miRNAs) play important roles in the defense response of plants to salt stress; however, how miRNAs in F. velutina exert anti-salt stress remains unclear. We previously identified two contrasting F. velutina cuttings clones, salt-tolerant (R7) and salt-sensitive (S4) and found that R7 exhibits higher salt tolerance than S4. To identify salt-responsive miRNAs and their target genes, the leaves and roots of R7 and S4 exposed to salt stress were subjected to miRNA and degradome sequencing analysis. The results showed that compared with S4, R7 showed 89 and 138 differentially expressed miRNAs in leaves and roots, respectively. Specifically, in R7 leaves, miR164d, miR171b/c, miR396a, and miR160g targeting NAC1, SCL22, GRF1, and ARF18, respectively, were involved in salt tolerance. In R7 roots, miR396a, miR156a/b, miR8175, miR319a/d, and miR393a targeting TGA2.3, SBP14, GR-RBP, TCP2/4, and TIR1, respectively, participated in salt stress responses. Taken together, the findings presented here revealed the key regulatory network of miRNAs in R7 responding to salt stress, thereby providing new insights into improving salt tolerance of F. velutina through miRNA manipulation.

Integrative genome, transcriptome, microRNA, and degradome analysis of water dropwort (Oenanthe javanica) in response to water stress

Water dropwort (Liyang Baiqin, Oenanthe javanica (BI.) DC.) is an aquatic perennial plant from the Apiaceae family with abundant protein, dietary fiber, vitamins, and minerals. It usually grows in wet soils and can even grow in water. Here, whole-genome sequencing of O. javanica via HiSeq 2000 sequencing technology was reported for the first time. The genome size was 1.28 Gb, including 42,270 genes, of which 93.92% could be functionally annotated. An online database of the whole-genome sequences of water dropwort, Water dropwortDB, was established to share the results and facilitate further research on O. javanica (database homepage: http://apiaceae.njau.edu.cn/waterdropwortdb ). Water dropwortDB offers whole-genome and transcriptome sequences and a Basic Local Alignment Search Tool. Comparative analysis with other species showed that the evolutionary relationship between O. javanica and Daucus carota was the closest. Twenty-five gene families of O. javanica were found to be expanded, and some genetic factors (such as genes and miRNAs) related to phenotypic and anatomic differentiation in O. javanica under different water conditions were further investigated. Two miRNA and target gene pairs (miR408 and Oja15472, miR171 and Oja47040) were remarkably regulated by water stress. The obtained reference genome of O. javanica provides important information for future work, thus making in-depth genetic breeding and gene editing possible. The present study also provides a foundation for the understanding of the O. javanica response to water stress, including morphological, anatomical, and genetic differentiation.

Characterization of the regulation mechanism of grapevine microRNA172 family members during flower development

BACKGROUND

Grapevine (Vitis vinifera L.), which has important nutritional values and health benefits, is one of the most economically important fruit crops cultivated worldwide. Several studies showed a large number of microRNAs (VvmiRNAs) involved in the modulation of grape growth and development, and many VvmiRNA families have multiple members. However, the way by which various members from the same miRNA family work is unclear, particularly in grapes.

RESULTS

In this study, an important conserved VvmiR172 family (VvmiR172s) and their targets were set as a good example for elucidating the interaction degree, mechanism, and spatio-temporal traits of diverse members from the same miRNA family. miR-RACE and Stem-loop RT-PCR were used to identify the spatio-temporal expressions of various members of VvmiR172s; together with RLM-RACE, PPM-RACE, Western blot, transgenic technologies, their interaction degree, and regulation mechanism were further validated. The expression of VvmiR172c was significantly higher than that of VvmiR172a, b, and d and showed a positive correlation with the abundance of VvAP2 cleavage products. These findings indicated that VvmiR172c might be one of the main action factors of the VvmiR172 family in flower development. The ability of VvmiR172c to cleave target genes differed due to divergence in complementary degree with VvAP2 and expression levels of various members. In VvmiR172 transgenic lines, we observed that 35S::VvmiR172c resulted in the earliest and abundant flowering, indicating the strong function of VvmiR172c. In contrast, the non-significant phenotypic changes were detected in the VvAP2 transgenic lines. The qRT-PCR and Western bolt results demonstrated that VvmiR172c plays a major role in targeting VvAP2.

CONCLUSIONS

VvmiR172 up-regulated the expression of NtFT and decreased the expression of NtFLC. The up/down regulation of VvmiR172c was the most pronounced. The functions of four VvmiR172 members in grape differed, and miR172c had the strongest regulation on AP2.

Regulation of micoRNA2111 and its target IbFBK in sweet potato on wounding

Plant microRNAs (miRNAs) are non-coding RNAs, which are composed of 20-24 nucleotides. MiRNAs play important roles in plant growth and responses to biotic and abiotic stresses. Wounding is one of the most serious stresses for plants; however, the regulation of miRNAs in plants upon wounding is not well studied. In this study, miR2111, a wound-repressed miRNA, identified previously in sweet potato (Ipomoea batatas cv Tainung 57) by small RNA deep sequencing was chosen for further analysis. Based on sweet potato transcriptome database, F-box/kelch repeat protein (IbFBK), a target gene of miR2111, was identified. IbFBK is a wound-inducible gene, and the miR2111-induced cleavage site in IbFBK mRNA is between the 10th and 11th nucleotides of miR2111. IbFBK is a component of the E3 ligase SCF (SKP1-Cullin-F-box) complex participating in protein ubiquitination and degradation. The results of yeast two-hybrid and bimolecular fluorescence complementation assays demonstrate that IbFBK was conjugated with IbSKP1 through the F-box domain in IbFBK N-terminus to form SCF complex, and interacted with IbCNR8 through the kelch-repeat domain in IbFBK C-terminus. The interaction of IbFBK and IbCNR8 may lead to the ubiquitination and degradation of IbCNR8. In conclusion, the suppression of miR2111 resulted in the increase of IbFBK, and may regulate protein degradation of IbCNR8 in sweet potato responding to wounding.

VvmiR160s/VvARFs interaction and their spatio-temporal expression/cleavage products during GA-induced grape parthenocarpy

BACKGROUND

Grape (Vitis vinifera) is highly sensitive to gibberellin (GA), which effectively induce grape parthenocarpy. Studies showed that miR160s and their target AUXIN RESPONSIVE FACTOR (ARF) responding hormones are indispensable for various aspects of plant growth and development, but their functions in GA-induced grape parthenocarpy remain elusive.

RESULTS

In this study, the morphological changes during flower development in response to GA treatments were examined in the 'Rosario Bianco' cultivar. The precise sequences of VvmiR160a/b/c/d/e and their VvARF10/16/17 target genes were cloned, sequenced and characterized. The phylogenetic relationship and intron-exon structure of VvARFs and other ARF family members derived from different species were investigated. All VvmiR160s (except VvmiR160b) and VvARF10/16/17 had the common cis-elements responsive to GA, which support their function in GA-mediated grape parthenocarpy. The cleavage role of VvmiR160s-mediated VvARF10/16/17 was verified in grape flowers. Moreover, spatio-temporal expression analysis demonstrated that among VvmiR160 family, VvmiR160a/b/c highly expressed at late stage of flower/berry development, while VvARF10/16/17showed a reverse expression trend. Interestingly, GA exhibited a long-term effect through inducing the expression of VvmiR160a/b/c/e to increase their cleavage product accumulations from 5 to 9 days after treatment, but GA enhanced the expressions of VvARF10/16/17 only at short term. Pearson correlation analysis based on expression data revealed a negative correlation between VvmiR160a/b/c and VvARF10/16/17 in flowers not berries during GA-induced grape parthenocarpy.

CONCLUSIONS

This work demonstrated that the negative regulation of VvARF10/16/17 expression by VvmiR160a/b/c as key regulatory factors is critical for GA-mediated grape parthenocarpy, and provide significant implications for molecular breeding of high-quality seedless berry.

Root precursors of microRNAs in wild emmer and modern wheats show major differences in response to drought stress

MicroRNAs, small regulatory molecules with significant impacts on the transcriptional network of all living organisms, have been the focus of several studies conducted mostly on modern wheat cultivars. In this study, we investigated miRNA repertoires of modern durum wheat and its wild relatives, with differing degrees of drought tolerance, to identify miRNA candidates and their targets involved in drought stress response. Root transcriptomes of Triticum turgidum ssp. durum variety Kızıltan and two Triticum turgidum ssp. dicoccoides genotypes TR39477 and TTD-22 under control and drought conditions were assembled from individual RNA-Seq reads and used for in silico identification of miRNAs. A total of 66 miRNAs were identified from all species, across all conditions, of which 46 and 38 of the miRNAs identified from modern durum wheat and wild genotypes, respectively, had not been previously reported. Genotype- and/or stress-specific miRNAs provide insights into our understanding of the complex drought response. Particularly, miR1435, miR5024, and miR7714, identified only from drought-stress roots of drought-tolerant genotype TR39477, can be candidates for future studies to explore and exploit the drought response to develop tolerant varieties.

De novo transcriptome assembly, gene annotation, marker development, and miRNA potential target genes validation under abiotic stresses in Oenanthe javanica

Oenanthe javanica is an aquatic perennial herb with known medicinal properties and an edible vegetable with high vitamin and mineral content. The understanding of the biology of O. javanica is limited by the absence of information on its genome, transcriptome, and small RNA. In this study, transcriptome sequencing and small RNA sequencing were performed to annotate function genes, develop SSR markers and analyze potential target genes of miRNAs in O. javanica. All reads with total nucleotides number of 1,440,321,408 bp were assembled into 58,072 transcripts and 40,208 unigenes. A total of 1,233 SSRs were identified from O. javanica. Generated unigenes were aligned against seven databases and annotated with functions. A total of 29 potential targets were predicted. Expression of 10 miRNAs and their corresponding target genes under abiotic stresses (heat, cold, salinity, and drought) was validated. All ten miRNAs were confirmed to response to abiotic stresses. A pair of miRNA and its target gene was found. This study can serve as a valuable resource for future studies on O. javanica, which may focus on novel gene discovery, SSR development, gene mapping, and miRNA-affected processes and pathways. This can promote the development of the useful medicinal properties of O. javanica in medical science.