MicroRNA-directed regulation of Arabidopsis AUXIN RESPONSE FACTOR17 is essential for proper development and modulates expression of early auxin response genes

A prescient evolutionary model for genesis, duplication and differentiation of MIR160 homologs in Brassicaceae

MicroRNA160 is a class of nitrogen-starvation responsive genes which governs establishment of root system architecture by down-regulating AUXIN RESPONSE FACTOR genes (ARF10, ARF16 and ARF17) in plants. The high copy number of MIR160 variants discovered by us from land plants, especially polyploid crop Brassicas, posed questions regarding genesis, duplication, evolution and function. Absence of studies on impact of whole genome and segmental duplication on retention and evolution of MIR160 homologs in descendent plant lineages prompted us to undertake the current study. Herein, we describe ancestry and fate of MIR160 homologs in Brassicaceae in context of polyploidy driven genome re-organization, copy number and differentiation. Paralogy amongst Brassicaceae MIR160a, MIR160b and MIR160c was inferred using phylogenetic analysis of 468 MIR160 homologs from land plants. The evolutionarily distinct MIR160a was found to represent ancestral form and progenitor of MIR160b and MIR160c. Chronology of evolutionary events resulting in origin and diversification of genomic loci containing MIR160 homologs was delineated using derivatives of comparative synteny. A prescient model for causality of segmental duplications in establishment of paralogy in Brassicaceae MIR160, with whole genome duplication accentuating the copy number increase, is being posited in which post-segmental duplication events viz. differential gene fractionation, gene duplications and inversions are shown to drive divergence of chromosome segments. While mutations caused the diversification of MIR160a, MIR160b and MIR160c, duplicated segments containing these diversified genes suffered gene rearrangements via gene loss, duplications and inversions. Yet the topology of phylogenetic and phenetic trees were found congruent suggesting similar evolutionary trajectory. Over 80% of Brassicaceae genomes and subgenomes showed a preferential retention of single copy each of MIR160a, MIR160b and MIR160c suggesting functional relevance. Thus, our study provides a blue-print for reconstructing ancestry and phylogeny of MIRNA gene families at genomics level and analyzing the impact of polyploidy on organismal complexity. Such studies are critical for understanding the molecular basis of agronomic traits and deploying appropriate candidates for crop improvement.

Aux/IAA and ARF Gene Families in Salix suchowensis: Identification, Evolution, and Dynamic Transcriptome Profiling During the Plant Growth Process

The phytohormone auxin plays a pivotal role in the regulation of plant growth and development, including vascular differentiation and tree growth. The auxin/indole-3-acetic acid (Aux/IAA) and auxin response transcription factor (ARF) genes are key components of plant auxin signaling. To gain more insight into the regulation and functional features of Aux/IAA and ARF genes during these processes, we identified 38 AUX/IAA and 34 ARF genes in the genome of Salix suchowensis and characterized their gene structures, conserved domains, and encoded amino acid compositions. Phylogenetic analysis of some typical land plants showed that the Aux/IAA and ARF genes of Salicaceae originated from a common ancestor and were significantly amplified by the ancestral eudicot hexaploidization event and the "salicoid" duplication that occurred before the divergence of poplar and willow. By analyzing dynamic transcriptome profiling data, some Aux/IAA and ARF genes were found to be involved in the regulation of plant growth, especially in the initial plant growth process. Additionally, we found that the expression of several miR160/miR167-ARFs was in agreement with canonical miRNA-ARF interactions, suggesting that miRNAs were possibly involved in the regulation of the auxin signaling pathway and the plant growth process. In summary, this study comprehensively analyzed the sequence features, origin, and expansion of Aux/IAA and ARF genes, and the results provide useful information for further studies on the functional involvement of auxin signaling genes in the plant growth process.

Design, synthesis and mode of action of novel 3-chloro-6-pyrazolyl picolinate derivatives as herbicide candidates

BACKGROUND

Picolinate/picolinic acid compounds are an important class of synthetic auxin herbicides. To explore the herbicidal activity of 6-pyrazolyl picolinate compounds, a series of 3-chloro-6-pyrazolyl-picolinate derivatives was designed and synthesized.

RESULTS

Twenty-five 3-chloro-6-pyrazolyl-picolinate derivatives synthesized were tested for herbicidal activity and the IC₅₀ value of compound c5 to the growth of Arabidopsis thaliana root was 27 times lower than that of the commercial herbicide clopyralid. Compound c5 displayed better post-emergence herbicidal activity and broader (Picloram, Clopyralid, Aminopyralid) herbicidal spectrum at a dosage of 400 g ha^-1 in comparison with clopyralid; it also was safe to wheat and maize at this dosage. Arabidopsis thaliana phenotypes and expression of auxin-response genes demonstrated that compound c5 might be a novel auxin-type herbicide. Molecular docking analyses revealed that compound c5 had stronger binding ability to receptor AFB5 (auxin signaling F-box protein 5) than clopyralid.

CONCLUSION

These 6-pyrazolyl picolinate compounds could be used as potential lead structures for the discovery of a novel synthetic auxin herbicide. © 2021 Society of Chemical Industry.

Nitrogen Starvation-Responsive MicroRNAs Are Affected by Transgenerational Stress in Durum Wheat Seedlings

Stress events have transgenerational effects on plant growth and development. In Mediterranean regions, water-deficit and heat (WH) stress is a frequent issue that negatively affects crop yield and quality. Nitrogen (N) is an essential plant macronutrient and often a yield-limiting factor for crops. Here, the response of durum wheat seedlings to N starvation under the transgenerational effects of WH stress was investigated in two genotypes. Both genotypes showed a significant reduction in seedling height, leaf number, shoot and root weight (fresh and dry), primary root length, and chlorophyll content under N starvation stress. However, in the WH stress-tolerant genotype, the percentage reduction of most traits was lower in progeny from the stressed parents than progeny from the control parents. Small RNA sequencing identified 1534 microRNAs in different treatment groups. Differentially expressed microRNAs (DEMs) were characterized subject to N starvation, parental stress and genotype factors, with their target genes identified in silico. GO and KEGG enrichment analyses revealed the biological functions, associated with DEM-target modules in stress adaptation processes, that could contribute to the phenotypic differences observed between the two genotypes. The study provides the first evidence of the transgenerational effects of WH stress on the N starvation response in durum wheat.

smartPARE: An R Package for Efficient Identification of True mRNA Cleavage Sites

Degradome sequencing is commonly used to generate high-throughput information on mRNA cleavage sites mediated by small RNAs (sRNA). In our datasets of potato (Solanum tuberosum, St) and Phytophthora infestans (Pi), initial predictions generated high numbers of cleavage site predictions, which highlighted the need of improved analytic tools. Here, we present an R package based on a deep learning convolutional neural network (CNN) in a machine learning environment to optimize discrimination of false from true cleavage sites. When applying smartPARE to our datasets on potato during the infection process by the late blight pathogen, 7.3% of all cleavage windows represented true cleavages distributed on 214 sites in P. infestans and 444 sites in potato. The sRNA landscape of the two organisms is complex with uneven sRNA production and cleavage regions widespread in the two genomes. Multiple targets and several cases of complex regulatory cascades, particularly in potato, was revealed. We conclude that our new analytic approach is useful for anyone working on complex biological systems and with the interest of identifying cleavage sites particularly inferred by sRNA classes beyond miRNAs.

Auxin Response Factor 2 (ARF2), ARF3, and ARF4 Mediate Both Lateral Root and Nitrogen Fixing Nodule Development in Medicago truncatula

Auxin Response Factors (ARFs) constitute a large family of transcription factors that mediate auxin-regulated developmental programs in plants. ARF2, ARF3, and ARF4 are post-transcriptionally regulated by the microRNA390 (miR390)/trans-acting small interference RNA 3 (TAS3) module through the action of TAS3-derived trans - acting small interfering RNAs (ta-siRNA). We have previously reported that constitutive activation of the miR390/TAS3 pathway promotes elongation of lateral roots but impairs nodule organogenesis and infection by rhizobia during the nitrogen-fixing symbiosis established between Medicago truncatula and its partner Sinorhizobium meliloti. However, the involvement of the targets of the miR390/TAS3 pathway, i.e., MtARF2, MtARF3, MtARF4a, and MtARF4b, in root development and establishment of the nitrogen-fixing symbiosis remained unexplored. Here, promoter:reporter fusions showed that expression of both MtARF3 and MtARF4a was associated with lateral root development; however, only the MtARF4a promoter was active in developing nodules. In addition, up-regulation of MtARF2, MtARF3, and MtARF4a/b in response to rhizobia depends on Nod Factor perception. We provide evidence that simultaneous knockdown of MtARF2, MtARF3, MtARF4a, and MtARF4b or mutation in MtARF4a impaired nodule formation, and reduced initiation and progression of infection events. Silencing of MtARF2, MtARF3, MtARF4a, and MtARF4b altered mRNA levels of the early nodulation gene nodulation signaling pathway 2 (MtNSP2). In addition, roots with reduced levels of MtARF2, MtARF3, MtARF4a, and MtARF4b, as well as arf4a mutant plants exhibited altered root architecture, causing a reduction in primary and lateral root length, but increasing lateral root density. Taken together, our results suggest that these ARF members are common key players of the morphogenetic programs that control root development and the formation of nitrogen-fixing nodules.

Differential miRNA profiles in South African cassava mosaic virus-infected cassava landraces reveal clues to susceptibility and tolerance to cassava mosaic disease

Specific miRNA families are involved in susceptibility or antiviral immunity in plants. Manihot esculenta Crantz (cassava) is a perennial plant that is an important food security crop in sub-Saharan Africa. Cassava is susceptible to several begomoviruses that cause cassava mosaic disease (CMD). In this study, we investigated the leaf miRNAome response in a tolerant (TME3) and susceptible (T200) cassava landrace challenged with South African cassava mosaic virus. RNAseq was performed on leaf samples at 12, 32 and 67 days post infection (dpi), representing early, symptomatic and late persistent stages of CMD infection. Significantly, distinct profiles of conserved miRNA family expression between the T200 and TME3 landraces at the three infection stages were observed. Notably at 12 days post SACMV infection, TME3 exhibited significant downregulation (log2fold<2.0) of 42 %, compared to 9% in T200, of the conserved miRNA families. This demonstrates an overall early response to SACMV in TME3 prior to symptom appearance not observed in T200, and expression of a large cohort of miRNA-regulated genes. Notably, at early infection, downregulation of mes-miR162 and 168 that target antiviral posttransriptional gene silencing (PTGS) regulators DCL1 and AGO1, respectively, was observed in TME3, and AGO1 and DCL1 expression was higher compared to T200 post infection. Early rapid responses prior to symptom development, including RNA silencing, may be key to establishing the tolerance/recovery phenotype exhibited by TME3 landrace later on at 67 dpi. At recovery, TME3 was hallmarked by a highly significant down-regulation of mes-miR167. MiR167 targets an auxin responsive factor which plays a role in auxin signaling and adaptive responses to stress, suggesting the importance of the auxin signaling in recovery of SACMV-induced symptoms. The gene targets of these miRNAs and their associated networks may provide clues to the molecular basis of CMD tolerance in perennial hosts such as cassava.

Auxin Biology in Bryophyta: A Simple Platform with Versatile Functions

Bryophytes, including liverworts, mosses, and hornworts, are gametophyte-dominant land plants that are derived from a common ancestor and underwent independent evolution from the sporophyte-dominant vascular plants since their divergence. The plant hormone auxin has been shown to play pleiotropic roles in the haploid bodies of bryophytes. Pharmacological and chemical studies identified conserved auxin molecules, their inactivated forms, and auxin transport in bryophyte tissues. Recent genomic and molecular biological studies show deep conservation of components and their functions in auxin biosynthesis, inactivation, transport, and signaling in land plants. Low genetic redundancy in model bryophytes enable unique assays, which are elucidating the design principles of the auxin signaling pathway. In this article, the physiological roles of auxin and regulatory mechanisms of gene expression and development by auxin in Bryophyta are reviewed.

GhARF16-1 modulates leaf development by transcriptionally regulating the GhKNOX2-1 gene in cotton

The leaf is a crucial organ evolved with remarkable morphological diversity to maximize plant photosynthesis. The leaf shape is a key trait that affects photosynthesis, flowering rates, disease resistance and yield. Although many genes regulating leaf development have been identified in the past years, the precise regulatory architecture underlying the generation of diverse leaf shapes remains to be elucidated. We used cotton as a reference model to probe the genetic framework underlying divergent leaf forms. Comparative transcriptome analysis revealed that the GhARF16-1 and GhKNOX2-1 genes might be potential regulators of leaf shape. We functionally characterized the auxin-responsive factor ARF16-1 acting upstream of GhKNOX2-1 to determine leaf morphology in cotton. The transcription of GhARF16-1 was significantly higher in lobed-leaved cotton than in smooth-leaved cotton. Furthermore, the overexpression of GhARF16-1 led to the up-regulation of GhKNOX2-1 and resulted in more and deeper serrations in cotton leaves, similar to the leaf shape of cotton plants overexpressing GhKNOX2-1. We found that GhARF16-1 specifically bound to the promoter of GhKNOX2-1 to induce its expression. The heterologous expression of GhARF16-1 and GhKNOX2-1 in Arabidopsis led to lobed and curly leaves, and a genetic analysis revealed that GhKNOX2-1 is epistatic to GhARF16-1 in Arabidopsis, suggesting that the GhARF16-1 and GhKNOX2-1 interaction paradigm also functions to regulate leaf shape in Arabidopsis. To our knowledge, our results uncover a novel mechanism by which auxin, through the key component ARF16-1 and its downstream-activated gene KNOX2-1, determines leaf morphology in eudicots.

Auxin and Flower Development: A Blossoming Field

The establishment of the species-specific floral organ body plan involves many coordinated spatiotemporal processes, which include the perception of positional information that specifies floral meristem and floral organ founder cells, coordinated organ outgrowth coupled with the generation and maintenance of inter-organ and inter-whorl boundaries, and the termination of meristem activity. Auxin is integrated within the gene regulatory networks that control these processes and plays instructive roles at the level of tissue-specific biosynthesis and polar transport to generate local maxima, perception, and signaling. Key features of auxin function in several floral contexts include cell nonautonomy, interaction with cytokinin gradients, and the central role of MONOPTEROS and ETTIN to regulate canonical and noncanonical auxin response pathways, respectively. Arabidopsis flowers are not representative of the enormous angiosperm floral diversity; therefore, comparative studies are required to understand how auxin underlies these developmental differences. It will be of great interest to compare the conservation of auxin pathways among flowering plants and to discuss the evolutionary role of auxin in floral development.

MicroRNA-Mediated Responses to Cadmium Stress in Arabidopsis thaliana

In recent decades, the presence of cadmium (Cd) in the environment has increased significantly due to anthropogenic activities. Cd is taken up from the soil by plant roots for its subsequent translocation to shoots. However, Cd is a non-essential heavy metal and is therefore toxic to plants when it over-accumulates. MicroRNA (miRNA)-directed gene expression regulation is central to the response of a plant to Cd stress. Here, we document the miRNA-directed response of wild-type Arabidopsis thaliana (Arabidopsis) plants and the drb1, drb2 and drb4 mutant lines to Cd stress. Phenotypic and physiological analyses revealed the drb1 mutant to display the highest degree of tolerance to the imposed stress while the drb2 mutant was the most sensitive. RT-qPCR-based molecular profiling of miRNA abundance and miRNA target gene expression revealed DRB1 to be the primary double-stranded RNA binding (DRB) protein required for the production of six of the seven Cd-responsive miRNAs analyzed. However, DRB2, and not DRB1, was determined to be required for miR396 production. RT-qPCR further inferred that transcript cleavage was the RNA silencing mechanism directed by each assessed miRNA to control miRNA target gene expression. Taken together, the results presented here reveal the complexity of the miRNA-directed molecular response of Arabidopsis to Cd stress.

In Situ Detection of Mature miRNAs in Plants Using LNA-Modified DNA Probes

MicroRNAs (miRNAs) play important roles in development in plants, and some miRNAs show developmentally regulated organ- and tissue-specific expression patterns. Therefore, in situ detection of mature miRNAs is important for understanding the functions for both miRNAs and their targets. The construction of promoter-reporter fusions and examination of their in planta expression has been widely used and the results obtained thus far are rather informative; however, in some cases, the length of promoter that contains entire regulatory elements is difficult to determine. In addition, traditional in situ hybridization with the antisense RNA fragment as the probe usually fails to detect miRNAs, because the mature miRNAs are too short (~21-nucleotides) to exhibit stable hybridization signals. In recent years, the Locked nucleic acid (LNA) modified DNA probe has been successfully used in animals and plants to detect small RNAs. Here, we describe a modified protocol using LNA-modified DNA probes to detect mature miRNAs in plant tissues, including the design of LNA probes and detailed steps for the in situ hybridization experiment, using Arabidopsis miR165 as an example.

Integration of Small RNA and Transcriptome Sequencing Reveal the Roles of miR395 and ATP Sulfurylase in Developing Seeds of Chinese Kale

Seed development is closely related to plant production and reproduction, and MicroRNAs (miRNA) is widely involved in plant development including seed development. Chinese kale, as a Brassicaceae vegetable, mainly depends on seed for proper reproduction. In the present study, Chinese kale seed and silique at different stages were selected to establish small RNA (sRNA) libraries including silique wall sRNA libraries at torpedo-embryo stage (PC), silique wall sRNA libraries at cotyledonary-embryo stage (PD), seed sRNA libraries at torpedo-embryo stage (SC), and seed sRNA libraries at cotyledonary-embryo stage (SD). The results showed that miRNA expressed differentially in the seeds and corresponding siliques at different stages. To further clarify the functional mode of miRNA in the process of seed development, Kyoto encyclopedia of genes and genomes (KEGG) enrichment analysis was performed on target genes of the differentially expressed miRNAs, and these target genes were mainly enriched in plant hormone signal transduction, primary and secondary metabolic pathways. After joint analysis with the transcriptome change of the corresponding period, miR156-SPL10/SPL11, miR395-APS3, and miR397-LAC2/LAC11 modules were identified to be directly involved in the development of Chinese kale seeds. What's more, modified 5'RLM-RACE and Agrobacteria-mediated Chinese kale transient transformation suggest miR395b_2 is involved in sulfur metabolism during seed development by regulating its target gene APS3.

RNAi Mediated Hypoxia Stress Tolerance in Plants

Small RNAs regulate various biological process involved in genome stability, development, and adaptive responses to biotic or abiotic stresses. Small RNAs include microRNAs (miRNAs) and small interfering RNAs (siRNAs). MicroRNAs (miRNAs) are regulators of gene expression that affect the transcriptional and post-transcriptional regulation in plants and animals through RNA interference (RNAi). miRNAs are endogenous small RNAs that originate from the processing of non-coding primary miRNA transcripts folding into hairpin-like structures. The mature miRNAs are incorporated into the RNA-induced silencing complex (RISC) and drive the Argonaute (AGO) proteins towards their mRNA targets. siRNAs are generated from a double-stranded RNA (dsRNA) of cellular or exogenous origin. siRNAs are also involved in the adaptive response to biotic or abiotic stresses. The response of plants to hypoxia includes a genome-wide transcription reprogramming. However, little is known about the involvement of RNA signaling in gene regulation under low oxygen availability. Interestingly, miRNAs have been shown to play a role in the responses to hypoxia in animals, and recent evidence suggests that hypoxia modulates the expression of various miRNAs in plant systems. In this review, we describe recent discoveries on the impact of RNAi on plant responses to hypoxic stress in plants.

Characterization of the leaf rust responsive ARF genes in wheat (Triticum aestivum L.)

Genome-wide identification, classification, functional characterization and expression analysis of Auxin Responsive Factor (ARF) gene family in wheat reveal their attributes and role during leaf rust infection. Auxins are important plant growth regulators that also impact plant-pathogen interaction. Auxin responsive factors (ARF) are plant specific transcription factors that control responses to auxins. Whole genome investigation of ARF gene family is limited in allohexaploid wheat (Triticum aestivum L.). Comprehensive study of this gene family was carried out by employing the currently available reference genome sequence of wheat. In total, 27 ARF genes were identified and located on the wheat genome as well as were positioned on wheat chromosome arms. Additionally, examination of the predicted genes unveiled a decent degree of relatedness within and among the phylogenetic clades. Leaf rust, caused by the obligate biotrophic fungal pathogen Puccinia triticina, is responsible for drastic loss of wheat crop worldwide reducing grain yield by 10-90%. Expression profiling of ARF genes in retort to leaf rust infection indicated their differential regulation during this plant-pathogen interaction. Highest expression of ARF genes were observed at 12 hpi that was maintained up to 72 hpi during incompatible interaction, whereas the high expression levels receded at 48 hpi during compatible interactions. Few of the identified ARF genes were likely to be post-transcriptionally regulated by microRNAs. Many light and stress responsive elements were detected in the promoter regions of ARF genes. Microsynteny analysis showed the conservation of ARF genes within the members of the Poaceae family. This study provides fundamental details for understanding the different types of ARF genes in wheat and there putative roles during leaf rust-wheat interaction.

Aux/IAA14 Regulates microRNA-Mediated Cold Stress Response in Arabidopsis Roots

The phytohormone auxin and microRNA-mediated regulation of gene expressions are key regulators of plant growth and development at both optimal and under low-temperature stress conditions. However, the mechanistic link between microRNA and auxin in regulating plant cold stress response remains elusive. To better understand the role of microRNA (miR) in the crosstalk between auxin and cold stress responses, we took advantage of the mutants of Arabidopsis thaliana with altered response to auxin transport and signal. Screening of the mutants for root growth recovery after cold stress at 4 °C revealed that the auxin signaling mutant, solitary root 1 (slr1; mutation in Aux/IAA14), shows a hypersensitive response to cold stress. Genome-wide expression analysis of miRs in the wild-type and slr1 mutant roots using next-generation sequencing revealed 180 known and 71 novel cold-responsive microRNAs. Cold stress also increased the abundance of 26-31 nt small RNA population in slr1 compared with wild type. Comparative analysis of microRNA expression shows significant differential expression of 13 known and 7 novel miRs in slr1 at 4 °C compared with wild type. Target gene expression analysis of the members from one potential candidate miR, miR169, revealed the possible involvement of miR169/NF-YA module in the Aux/IAA14-mediated cold stress response. Taken together, these results indicate that SLR/IAA14, a transcriptional repressor of auxin signaling, plays a crucial role in integrating miRs in auxin and cold responses.

Genome-Wide Differential DNA Methylation and miRNA Expression Profiling Reveals Epigenetic Regulatory Mechanisms Underlying Nitrogen-Limitation-Triggered Adaptation and Use Efficiency Enhancement in Allotetraploid Rapeseed

Improving crop nitrogen (N) limitation adaptation (NLA) is a core approach to enhance N use efficiency (NUE) and reduce N fertilizer application. Rapeseed has a high demand for N nutrients for optimal plant growth and seed production, but it exhibits low NUE. Epigenetic modification, such as DNA methylation and modification from small RNAs, is key to plant adaptive responses to various stresses. However, epigenetic regulatory mechanisms underlying NLA and NUE remain elusive in allotetraploid B. napus. In this study, we identified overaccumulated carbohydrate, and improved primary and lateral roots in rapeseed plants under N limitation, which resulted in decreased plant nitrate concentrations, enhanced root-to-shoot N translocation, and increased NUE. Transcriptomics and RT-qPCR assays revealed that N limitation induced the expression of NRT1.1, NRT1.5, NRT1.7, NRT2.1/NAR2.1, and Gln1;1, and repressed the transcriptional levels of CLCa, NRT1.8, and NIA1. High-resolution whole genome bisulfite sequencing characterized 5094 differentially methylated genes involving ubiquitin-mediated proteolysis, N recycling, and phytohormone metabolism under N limitation. Hypermethylation/hypomethylation in promoter regions or gene bodies of some key N-metabolism genes might be involved in their transcriptional regulation by N limitation. Genome-wide miRNA sequencing identified 224 N limitation-responsive differentially expressed miRNAs regulating leaf development, amino acid metabolism, and plant hormone signal transduction. Furthermore, degradome sequencing and RT-qPCR assays revealed the miR827-NLA pathway regulating limited N-induced leaf senescence as well as the miR171-SCL6 and miR160-ARF17 pathways regulating root growth under N deficiency. Our study provides a comprehensive insight into the epigenetic regulatory mechanisms underlying rapeseed NLA, and it will be helpful for genetic engineering of NUE in crop species through epigenetic modification of some N metabolism-associated genes.

miRNAs as key regulators via targeting the phytohormone signaling pathways during somatic embryogenesis of plants

Somatic embryogenesis is the regeneration of embryos from the somatic cell via dedifferentiation and redifferentiation without the occurrence of fertilization. A complex network of genes regulates the somatic embryogenesis process. Especially, microRNAs (miRNAs) have emerged as key regulators by affecting phytohormone biosynthesis, transport and signal transduction pathways. miRNAs are small, non-coding small RNA regulatory molecules involved in various developmental processes including somatic embryogenesis. Several types of miRNAs such as miR156, miR157, miR 159, miR 160, miR165, miR166, miR167, miR390, miR393 and miR396 have been reported to intricate in regulating somatic embryogenesis via targeting the phytohormone signaling pathways. Here we review current research progress on the miRNA-mediated regulation involved in somatic embryogenesis via regulating auxin, ethylene, abscisic acid and cytokinin signaling pathways. Further, we also discussed the possible role of other phytohormone signaling pathways such as gibberellins, jasmonates, nitric oxide, polyamines and brassinosteroids. Finally, we conclude by discussing the expression of miRNAs and their targets involved in somatic embryogenesis and possible regulatory mechanisms cross talk with phytohormones during somatic embryogenesis.

Strontium stress disrupts miRNA biogenesis by reducing HYL1 protein levels in Arabidopsis

Strontium (Sr) is an emerging environmental pollutant that has become a major global concern after the nuclear accident at the Fukushima Daiichi Nuclear Power Plant in 2011. Although many studies have demonstrated the harmful effects of Sr on plant growth and development at the physiological level, knowledge regarding how plants sense and respond to Sr stress at the molecular level is limited. Recent studies have suggested that microRNAs (miRNAs) function as key regulators of plant growth and development as well as in the responses of plants to environmental stresses, including salinity, drought, cold, nutrient starvation, and heavy metals. In this study, we examined the global expression profile of miRNAs under Sr stress using small RNA sequencing analysis in Arabidopsis to better understand the molecular basis of plant responses to Sr stress. To identify specific Sr-responsive miRNAs, we performed comparative miRNA expression profiling analysis using control, CaCl₂-, and SrCl₂-treated seedlings. Compared to the control treatment, the expressions of most miRNAs were considerably decreased in the Sr-treated seedlings. However, under Sr stress, the expressions of primary miRNAs (pri-miRNAs) and their target genes were significantly increased; the protein levels of HYPONASTIC LEAVES 1 (HYL1), one of the core components of the microprocessor complex, were strongly reduced despite the increased HYL1 mRNA expression. In addition, hyl1-2 mutant plants were shown to be more sensitive to Sr stress than wild-type plants. Collectively, our results strongly suggested that Sr stress may be associated with the disruption of miRNA biogenesis by reducing the protein level of HYL1, which is required to maintain proper growth and development for plants. Our findings further indicated that some miRNAs may play important roles in plant responses to Sr stress.

OsmiR167a-targeted auxin response factors modulate tiller angle via fine-tuning auxin distribution in rice

Rice tiller angle determines plant growth density and further contributes grain production. Although a few genes have been characterized to regulate tiller angle in rice, the molecular mechanism underlying the control of tiller angle via microRNA is poorly understood. Here, we report that rice tiller angle is controlled by OsmiR167a-targeted auxin response factors OsARF12, OsARF17 and OsARF25. In the overexpression of OsMIR167a plants, the expression of OsARF12, OsARF17 and OsARF25 was severely repressed and displayed larger tiller angle as well as the osarf12/osarf17 and osarf12/ osarf25 plants. In addition, those plants showed compromised abnormal auxin distribution and less sensitive to gravity. We also demonstrate that OsARF12, OsARF17 and OsARF25 function redundantly and might be involved in HSFA2D and LAZY1-dependent asymmetric auxin distribution pathway to control rice tiller angle. Our results reveal that OsmiR167a represses its targets, OsARF12, OsARF17 and OsARF25, to control rice tiller angle by fine-tuning auxin asymmetric distribution in shoots.

PendoTMBase: A Database for Plant Endogenous Target Mimics

With fast-evolving next-generation sequencing technology, a great amount of plant genome and transcriptome data are becoming available. Due to the availability of mature microRNA (miRNA) sequence information from the miRBase (release 21) database, it is possible to predict endogenous target mimics (eTMs) in plant by searching seed-matched target sites. We identified a total of 2669 non-redundant eTM records in 43 plant species to create a specialized web-based database platform. The platform is named PendoTMBase, which can provide details of the eTMs. Predicted pairing structure between eTMs and their target miRNA, expression levels of eTMs pairs and associated GO annotations are also stored in the database. With evaluations performed in silico, we have found that the eTMs are representative and fit for genetic studies and high value-added plants breeding. The platform also provides a function of predicting novel eTMs based on the miRNA sequences submitted by users.

Characterization of microRNA genes from Pigeonpea (Cajanus cajan L.) and understanding their involvement in drought stress

MicroRNAs (miRNA) are non-coding 20-24 nucleotide long RNAs regulating gene expression. In this study, we have characterized and analysed expression of miRNAs in Pigeonpea by using bioinformatics and experimental tools. We identified 116 miRNAs belonging to 32 phylogenetic families. Further, transcription start sites of miRNA genes revealed abundance and unique arrangement of adenine at +1 and thymine at -1 position. Promoter analysis exhibited presence of 19 most prevalent motifs which comprises majorly of TATA box and MYC domains. In total, 252 miRNA-targets were identified and found to be involved in various developmental processes and stress responses. Moreover, genome-wide localization studies demonstrated clustering of cca-miRNA 395 and 169 genes. The tandem and segmental duplication events were observed suggesting miRNA genes have been originated parallelly with protein coding genes. The expression analysis revealed induction of cca-miR169a, 398a and 408 miRNAs under drought stress highlighting their involvement. Conversely, down-regulation of their putative targets (NFYA, SOD, and UCLA, respectively) confirmed regulatory role of miRNAs in their expression emphasising the negative relationship between these miRNAs and targets in Pigeonpea. This study reports vast repertoire of miRNA genes which further can be experimentally characterized to elucidate their functions in various biological processes and can be recommended for Pigeonpea improvement programs.

Genetic dissection of the auxin response network

The expansion of gene families during evolution, which can generate functional overlap or specialization among their members, is a characteristic feature of signalling pathways in complex organisms. For example, families of transcriptional activators and repressors mediate responses to the plant hormone auxin. Although these regulators were identified more than 20 years ago, their overlapping functions and compensating negative feedbacks have hampered their functional analyses. Studies using loss-of-function approaches in basal land plants and gain-of-function approaches in angiosperms have in part overcome these issues but have still left an incomplete understanding. Here, we propose that renewed emphasis on genetic analysis of multiple mutants and species will shed light on the role of gene families in auxin response. Combining loss-of-function mutations in auxin-response activators and repressors can unravel complex outputs enabled by expanded gene families, such as fine-tuned developmental outcomes and robustness. Similar approaches and concepts may help to analyse other regulatory pathways whose components are also encoded by large gene families.

Emerging Connections between Small RNAs and Phytohormones

Small RNAs (sRNAs), mainly including miRNAs and siRNAs, are ubiquitous in eukaryotes. sRNAs mostly negatively regulate gene expression via (post-)transcriptional gene silencing through DNA methylation, mRNA cleavage, or translation inhibition. The mechanisms of sRNA biogenesis and function in diverse biological processes, as well as the interactions between sRNAs and environmental factors, like (a)biotic stress, have been deeply explored. Phytohormones are central in the plant's response to stress, and multiple recent studies highlight an emerging role for sRNAs in the direct response to, or the regulation of, plant hormonal pathways. In this review, we discuss recent progress on the unraveling of crossregulation between sRNAs and nine plant hormones.

Integrated Analysis of Small RNA, Transcriptome, and Degradome Sequencing Reveals the Water-Deficit and Heat Stress Response Network in Durum Wheat

Water-deficit and heat stress negatively impact crop production. Mechanisms underlying the response of durum wheat to such stresses are not well understood. With the new durum wheat genome assembly, we conducted the first multi-omics analysis with next-generation sequencing, providing a comprehensive description of the durum wheat small RNAome (sRNAome), mRNA transcriptome, and degradome. Single and combined water-deficit and heat stress were applied to stress-tolerant and -sensitive Australian genotypes to study their response at multiple time-points during reproduction. Analysis of 120 sRNA libraries identified 523 microRNAs (miRNAs), of which 55 were novel. Differentially expressed miRNAs (DEMs) were identified that had significantly altered expression subject to stress type, genotype, and time-point. Transcriptome sequencing identified 49,436 genes, with differentially expressed genes (DEGs) linked to processes associated with hormone homeostasis, photosynthesis, and signaling. With the first durum wheat degradome report, over 100,000 transcript target sites were characterized, and new miRNA-mRNA regulatory pairs were discovered. Integrated omics analysis identified key miRNA-mRNA modules (particularly, novel pairs of miRNAs and transcription factors) with antagonistic regulatory patterns subject to different stresses. GO (Gene Ontology) and KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analysis revealed significant roles in plant growth and stress adaptation. Our research provides novel and fundamental knowledge, at the whole-genome level, for transcriptional and post-transcriptional stress regulation in durum wheat.

Impact of potassium deficiency on cotton growth, development and potential microRNA-mediated mechanism

The goal of this study was to investigate the impact of potassium deficiency on cotton seedling growth and development at the individual, physiological, biochemical, and molecular levels. Potassium is an important plant nutrient; our results show that potassium deficiency significantly affected cotton seedling growth and development, evidenced by reduced plant height, and total areas of the leaves and roots as well as further reduced both fresh and dry biomass of the entire plants. Potassium deficiency also significantly inhibited root and leaf respiration and leaf photosynthesis. Compared with the controls, potassium deficiency significantly inhibited root elongation and total root surface areas that further inhibited cotton seedlings to uptake nutrients from the medium. Potassium deficiency induced aberrant expression of both microRNAs (miRNAs) and their protein-coding targets. These miRNAs regulate plant root development as well as response to abiotic stresses. Potassium deficiency altered the expression of miRNAs that regulate the expression of protein-coding genes controlling root development and response to potassium deficiency. miRNAs regulate root development and further control plant development in cotton seedlings under potassium deficiency. In summary, potassium deficiency significantly affected the cotton seedling photosynthesis and respiration that resulted in inhibition of cotton seedling growth and development potentially due to the miRNA-mediated mechanism.

Identification of candidate miRNAs related in storage root development of sweet potato by high throughput sequencing

Sweet potato (Ipomoea batatas L.) is a food consumed worldwide, an industrial raw material and new energy crop. The storage root is the most economical part of the crop. However, the mechanism of storage root initiation and development is still unclear. In this study, conserved and novel miRNAs during storage root development were identified by high-throughput sequencing technology by constructing small RNA libraries from sweet potato fibrous roots (F) and storage roots at four different developmental stages (storage roots with different diameters: 1 cm, D1; 3 cm, D3; 5 cm, D5 and 10 cm, D10). A total of 61 known miRNAs and 471 novel miRNAs were identified. In addition, 145 differentially expressed miRNAs were identified in the F library compared with the four storage root libraries, with 30 known miRNAs and 115 novel miRNAs. Moreover, the targets of the differentially expressed miRNAs were predicted and their network was further investigated by GO analysis using our previous transcriptome data. The GO analysis revealed that antioxidant activity and binding process were the most enriched terms of the target genes. The secondary structure and expression of six candidate miRNAs including three conserved miRNAs and three novel miRNAs were investigated and their predicted targets were validated by qRT-PCR. The results showed that the expression levels of the miRNAs were all consistent with the sequencing data. Most of the miRNAs and their corresponding targets had obvious negative correlations. This study contributed to elucidating the potential miRNA mediated regulatory mechanism of storage root development in sweet potato. The specific differentially expressed miRNAs in sweet potato storage roots can be used to breed high-yield sweet potatoes and other tuberous root crops.

TIAs pathway genes and associated miRNA identification in Vinca minor: supporting aspidosperma and eburnamine alkaloids linkage via transcriptomic analysis

V. minor contains monomeric eburnamine-type of indole alkaloids having utilization as a neuro-medicinal plant. The biosynthetic pathway studies using miRNAs has been the focal point for plant genomic research in recent years and this technique is utilized to get an insight into a possible pathway level study in V. minor as understanding of genes in this prized medicinal plant is meagrely understood. The de novo transcriptomic analysis using Illumina Next gen sequencing has been performed in glasshouse shifted plant and transformed roots to elucidate the possible non confirmed steps of terpenoid indole alkaloids (TIAs) pathway in V. minor. A putative TIA pathway is elucidated in the study including twelve possible TIAs biosynthetic genes. The specific miRNA associated with TIAs pathway were identified and their roles were discussed for the first time in V. minor. The comparative analysis of transcriptomic data of glasshouse shifted plant and transformed roots showed that the raw reads of transformed roots were higher (83,740,316) compared to glasshouse shifted plant (67,733,538). The EST-SSR prediction showed the maximum common repeats among glasshouse shifted plant and transformed roots, although small variation was found in trinucleotide repeats restricted to glasshouse shifted plant. The study reveals overall 37 miRNAs which were observed to be true and can have a role in pathway as they can regulate the growth and alkaloid production. The identification of putative pathway genes plays an important role in establishing linkage between Aspidosperma and Eburnamine alkaloids.

Identification of microRNAs and their targets in inflorescences of an Ogura-type cytoplasmic male-sterile line and its maintainer fertile line of turnip (Brassica rapa ssp. rapifera) via high-throughput sequencing and degradome analysis

Cytoplasmic male sterility (CMS) is a widely used trait in angiosperms caused by perturbations in nucleus-mitochondrion interactions that suppress the production of functional pollen. MicroRNAs (miRNAs) are small non-coding RNAs that act as regulatory molecules of transcriptional or post-transcriptional gene silencing in plants. The discovery of miRNAs and their possible implications in CMS induction provides clues for the intricacies and complexity of this phenomenon. Previously, we characterized an Ogura-CMS line of turnip (Brassica rapa ssp. rapifera) that displays distinct impaired anther development with defective microspore production and premature tapetum degeneration. In the present study, high-throughput sequencing was employed for a genome-wide investigation of miRNAs. Six small RNA libraries of inflorescences collected from the Ogura-CMS line and its maintainer fertile (MF) line of turnip were constructed. A total of 120 pre-miRNAs corresponding to 89 mature miRNAs were identified, including 87 conversed miRNAs and 33 novel miRNAs. Among these miRNAs, the expression of 10 differentially expressed mature miRNAs originating from 12 pre-miRNAs was shown to have changed by more than two-fold between inflorescences of the Ogura-CMS line and inflorescences of the MF line, including 8 down- and 2 up-regulated miRNAs. The expression profiles of the differentially expressed miRNAs were confirmed by stem-loop quantitative real-time PCR. In addition, to identify the targets of the identified miRNAs, a degradome analysis was performed. A total of 22 targets of 25 miRNAs and 17 targets of 28 miRNAs were identified as being involved in the reproductive development for Ogura-CMS and MF lines of turnip, respectively. Negative correlations of expression patterns between partial miRNAs and their targets were detected. Some of these identified targets, such as squamosa promoter-binding-like transcription factor family proteins, auxin response factors and pentatricopeptide repeat-containing proteins, were previously reported to be involved in reproductive development in plants. Taken together, our results can help improve the understanding of miRNA-mediated regulatory pathways that might be involved in CMS occurrence in turnip.

Molecular and Hormonal Regulation of Leaf Morphogenesis in Arabidopsis

Shoot apical meristems (SAM) are tissues that function as a site of continuous organogenesis, which indicates that a small pool of pluripotent stem cells replenishes into lateral organs. The coordination of intercellular and intracellular networks is essential for maintaining SAM structure and size and also leads to patterning and formation of lateral organs. Leaves initiate from the flanks of SAM and then develop into a flattened structure with variable sizes and forms. This process is mainly regulated by the transcriptional regulators and mechanical properties that modulate leaf development. Leaf initiation along with proper orientation is necessary for photosynthesis and thus vital for plant survival. Leaf development is controlled by different components such as hormones, transcription factors, miRNAs, small peptides, and epigenetic marks. Moreover, the adaxial/abaxial cell fate, lamina growth, and shape of margins are determined by certain regulatory mechanisms. The over-expression and repression of various factors responsible for leaf initiation, development, and shape have been previously studied in several mutants. However, in this review, we collectively discuss how these factors modulate leaf development in the context of leaf initiation, polarity establishment, leaf flattening and shape.

Research Tools for the Functional Genomics of Plant miRNAs During Zygotic and Somatic Embryogenesis

During early plant embryogenesis, some of the most fundamental decisions on fate and identity are taken making it a fascinating process to study. It is no surprise that higher plant embryogenesis was intensively analysed during the last century, while somatic embryogenesis is probably the most studied regeneration model. Encoded by the MIRNA, short, single-stranded, non-coding miRNAs, are commonly present in all Eukaryotic genomes and are involved in the regulation of the gene expression during the essential developmental processes such as plant morphogenesis, hormone signaling, and developmental phase transition. During the last few years dedicated to miRNAs, analytical methods and tools have been developed, which have afforded new opportunities in functional analyses of plant miRNAs, including (i) databases for in silico analysis; (ii) miRNAs detection and expression approaches; (iii) reporter and sensor lines for a spatio-temporal analysis of the miRNA-target interactions; (iv) in situ hybridisation protocols; (v) artificial miRNAs; (vi) MIM and STTM lines to inhibit miRNA activity, and (vii) the target genes resistant to miRNA. Here, we attempted to summarise the toolbox for functional analysis of miRNAs during plant embryogenesis. In addition to characterising the described tools/methods, examples of the applications have been presented.

miRNAs involved in transcriptome remodeling during pollen development and heat stress response in Solanum lycopersicum

Cellular transitions during development and stress response depend on coordinated transcriptomic and proteomic alterations. Pollen is particular because its development is a complex process that includes meiotic and mitotic divisions which causes a high heat sensitivity of these cells. Development and stress response are accompanied by a reprogramming of the transcriptome, e.g. by post-transcriptional regulation via miRNAs. We identified known and potentially novel miRNAs in the transcriptome of developing and heat-stressed pollen of Solanum lycopersicum (tomato). The prediction of target mRNAs yielded an equal number of predicted target-sites in CDS and 3'UTR regions of target mRNAs. The result enabled the postulation of a possible link between miRNAs and a fine-tuning of transcription factor abundance during pollen development. miRNAs seem to play a role in the pollen heat stress response as well. We identified several heat stress transcription factors and heat shock proteins as putative targets of miRNAs in response to heat stress, thereby placing these miRNAs as important elements of thermotolerance. Moreover, for members of the AP2, SBP and ARF family members we could predict a miRNA-mediated regulation during development via the miR172, mir156 and mir160-family strengthening the current concept of a cross-connection between development and stress response in plants.

Genome-wide identification of the ARF (auxin response factor) gene family in peach and their expression analysis

Auxin response factors (ARFs) are important transcription factors to relay auxin signaling. From the Genome Database for Rosaceae (GDR), we identified 17 peach ARF genes (PpARFs) encoding the proteins with three conserved domains. Their gene structure and functional domains were analyzed. Their transcriptional response to exogenous auxin treatment was tested and confirmed. We also expressed PpARF-GFP fusion reporters in tobacco leaves and observed their nuclear localization by fluorescence microscopy. It has been known that ARFs are widely involved in fruit development. We compared the expression pattern of all PpARFs in different tissues including the fruits at different developmental stages of two peach cultivars, “melting” and “stony hard”. We found eight PpARFs were more highly expressed in the “melting” peaches compared to “stony hard” peaches, while three PpARFs were more highly expressed in “stony hard” peaches. Among them, the expression difference of PpARF4, PpARF7 and PpARF12 was large, and their function in regulating fruit development and fruit quality was discussed. Our work provides a basis for further exploring the mechanisms underlying auxin regulated peach fruit ripening.

Small RNA Sequencing Analysis of miRNA Expression Reveals Novel Insihts into Root Formation under Root Restriction Cultivation in Grapevine (Vitis vinifera L.)

Root restriction cultivation (RRC) can influence plant root architecture, but its root phenotypic changes and molecular mechanisms are still unknown. In this study, phenotype observations of grapevine root under RRC and control cultivation (nRC) at 12 time points were conducted, and the root phenotype showed an increase of adventitious and lateral root numbers and root tip degeneration after RRC cultivation from 70 days after planting (DAP). The 70 and 125 DAP sampling of two different cultivations, named nR70, RR70, nR125, and RR125, were selected for small RNA sequencing. A total of 153 known miRNAs and 119 predicted novel miRNAs were obtained. Furthermore, BLAST was used to predict the novel miRNAs with miRBase databases using the default parameters; 96 of the 119 predicted novel miRNAs were similar to other species, and the remaining 23 grapevine-specific novel miRNAs were obtained. There were 26, 33, 26, and 32 miRNAs that were differentially expressed in different comparison groups (RR70 vs. nR70, RR125 vs. nR125, nR125 vs. nR70 and RR125 vs. RR70). Target genes prediction of differentially expressed miRNAs was annotated on a variety of biological processes, and 24 participated in root development. Moreover, multiple miRNAs were found to jointly regulate lateral root development under root restriction conditions. The miRNA expression pattern comparison between RRC and nRC may provide a framework for the future analysis of miRNAs associated with root development in grapevine.

MicroRNA160 regulates leaf curvature in potato (Solanum tuberosum L. cv. Désirée)

Leaf development is a complex process and factors such as size, shape, curvature, compounding, and texture determine the final leaf morphology. MicroRNA160 is one of the crucial players that has been shown to regulate lamina formation and compounding in tomato. In this study, we show that miR160 also regulates leaf curvature in potato. miR160 targets a group of Auxin Response Factors - StARF10, StARF16, and StARF17 - that are proposed to function majorly as repressors of auxin signaling. We observed that overexpression of miR160 (miR160-OE) results in decrease in the levels of these ARFs along with hypersensitivity to exogenous auxin treatment, whereas knockdown of miR160 (miR160-KD) causes increased ARF levels and auxin hyposensitivity. The leaves of miR160-OE plants have a high positive curvature, but of miR160-KD plants are flattened compared to wildtype. A prolonged activation of cell cycle - as indicated by increased levels of StCYCLIND3;2 - in the center region of miR160-OE leaves appears to have caused this positive curvature. However, a comparable StTCP4 activity at both center and margin regions of miR160-KD leaves could be the cause for its flattened leaf phenotype. In summary, we show that miR160 plays an important role in regulating leaf curvature in potato plants.

Indole-3-acetic acid improves drought tolerance of white clover via activating auxin, abscisic acid and jasmonic acid related genes and inhibiting senescence genes

BACKGROUND

Auxin may have a positive effect on plants under drought stress. White clover is widely cultivated and often prone to water shortages. In the present study, we investigated the effects of exogenous indole - 3-acetic acid (IAA) on growth and physiological changes of white clover under drought stress condition. The contents of endogenous IAA and other hormones including ABA, CTK, JA, GA, IAA, and SA were assayed. Moreover, expressions of auxin-responsive genes, drought-responsive genes and leaf senescence-associated genes were detected in response to exogenous IAA.

RESULTS

Compared to control, drought stress alone significantly diminished stem dry weigh, relative water content (RWC) and total chlorophyll content (Chl). Exogenous IAA treatment significantly increased RWC and Chl, whereas L-AOPP treatment drastically decreased stem dry weight, RWC and Chl under drought stress condition. Additionally, exogenous IAA treatment significantly increased ABA content and JA content, up-regulated expression of auxin responsive genes (GH3.1, GH3.9, IAA8), drought stress responsive genes (bZIP11, DREB2, MYB14, MYB48, WRKY2, WRKY56, WRKY108715 and RD22), and down-regulated expressions of auxin-responding genes (GH3.3, GH3.6, IAA27) and leaf senescence genes (SAG101 and SAG102) in the presence of PEG. Contrarily, L-AOPP treatment significantly reduced contents of ABA, GA3 and JA, down-regulated expressions of GH3.1, GH3.9, IAA8, bZIP11, DREB2, MYB14, MYB48, WRKY2, WRKY56, WRKY108715, ERD and RD22, and up-regulated SAG101 and SAG102.

CONCLUSIONS

Exogenous IAA improved drought tolerance of white clover possibly due to endogenous plant hormone concentration changes and modulation of genes involving in drought stress response and leaf senescence. These results provided useful information to understand mechanisms of IAA improved drought tolerance in white clover.

Plant hormonal changes and differential expression profiling reveal seed dormancy removal process in double dormant plant-herbaceous peony

Herbaceous peony (Paeonia lactiflora Pall.) is a popular ornamental and medicinal plant. Taking approximately six to seven months, the seeds germination under natural conditions experiences dual dormancies, which seriously affects horticultural cultivation. Few studies have been conducted on exploring both biological and molecular mechanism that regulates dormancy removal process in hypocotyls double dormant plants. Here, we first measured ABA and GA₃ content changes at four key dormancy break stages, and then performed transcriptomic analyses to identify the differentially expressed genes (DEGs) using RNA-seq. We subsequently carried out Quantitative real-time PCR (qRT-PCR) to validate RNA-seq data. ABA content decreased during the whole dormancy removal process and GA₃ content exhibited decreasing slightly and then increasing trend. RNA sequencing de novo assembly generated a total of 99,577 unigenes. 20,344 unigenes were differentially expressed in the whole dormancy release process. The qPCR results of 54 selected unigenes were consistent with the FPKM values obtained from RNA-seq. Our results summarize a valuable collection of gene expression profiles characterizing the dormancy release process. The DEGs are candidates for functional analyses of genes affecting the dormancy release, which is a precious resource for the on-going physiological and molecular investigation of seeds dormancy removal in other perennial plants.

Expression of rice siR109944 in Arabidopsis affects plant immunity to multiple fungal pathogens

Plant small RNA (sRNA)-mediated gene expression has a conserved role in regulating plant growth, development, and immunity. Heterologous expression of sRNA contributes to determining whether the function of sRNA is conservative or independent. We recently characterized the Tourist-miniature inverted-repeat transposable element (MITE)-derived siR109944 had a conserved function that enhanced susceptibility to Rhizoctonia solani infection by affecting auxin homeostasis in rice and Arabidopsis. To ascertain whether the function of rice siR109944 has a broad-spectrum immunity in Arabidopsis, we infected Arabidopsis with a variety of fungal pathogens. Overexpression of siR109944 in Arabidopsis increased susceptibility to Botrytis cinerea, Sclerotinia sclerotium, and Verticillium dahliae infection. Further studies found that Arabidopsis auxin-related miRNAs were suppressed in siR109944 OE. Our results demonstrated that overexpression of rice siR109944 in Arabidopsis affected immune responses to multiple pathogens by inhibiting auxin-related miRNA expression in planta.

Mapping the Salt Stress-Induced Changes in the Root miRNome in Pokkali Rice

A plant's response to stress conditions is governed by intricately coordinated gene expression. The microRNAs (miRs) have emerged as relatively new players in the genetic network, regulating gene expression at the transcriptional and post-transcriptional level. In this study, we performed comprehensive profiling of miRs in roots of the naturally salt-tolerant Pokkali rice variety to understand their role in regulating plant physiology in the presence of salt. For comparisons, root miR profiles of the salt-sensitive rice variety Pusa Basmati were generated. It was seen that the expression levels of 65 miRs were similar for roots of Pokkali grown in the absence of salt (PKNR) and Pusa Basmati grown in the presence of salt (PBSR). The salt-induced dis-regulations in expression profiles of miRs showed controlled changes in the roots of Pokkali (PKSR) as compared to larger variations seen in the roots of Pusa Basmati. Target analysis of salt-deregulated miRs identified key transcription factors, ion-transporters, and signaling molecules that act to maintain cellular Ca2+ homeostasis and limit ROS production. These miR:mRNA nodes were mapped to the Quantitative trait loci (QTLs) to identify the correlated root traits for understanding their significance in plant physiology. The results obtained indicate that the adaptability of Pokkali to excess salt may be due to the genetic regulation of different cellular components by a variety of miRs.

The Critical Role of miRNAs in Regulation of Flowering Time and Flower Development

Flowering is an important biological process for plants that ensures reproductive success. The onset of flowering needs to be coordinated with an appropriate time of year, which requires tight control of gene expression acting in concert to form a regulatory network. MicroRNAs (miRNAs) are non-coding RNAs known as master modulators of gene expression at the post-transcriptional level. Many different miRNA families are involved in flowering-related processes such as the induction of floral competence, floral patterning, and the development of floral organs. This review highlights the diverse roles of miRNAs in controlling the flowering process and flower development, in combination with potential biotechnological applications for miRNAs implicated in flower regulation.

The role of microRNAs in the legume-Rhizobium nitrogen-fixing symbiosis

Under nitrogen starvation, most legume plants form a nitrogen-fixing symbiosis with Rhizobium bacteria. The bacteria induce the formation of a novel organ called the nodule in which rhizobia reside as intracellular symbionts and convert atmospheric nitrogen into ammonia. During this symbiosis, miRNAs are essential for coordinating the various plant processes required for nodule formation and function. miRNAs are non-coding, endogenous RNA molecules, typically 20-24 nucleotides long, that negatively regulate the expression of their target mRNAs. Some miRNAs can move systemically within plant tissues through the vascular system, which mediates, for example, communication between the stem/leaf tissues and the roots. In this review, we summarize the growing number of miRNAs that function during legume nodulation focusing on two model legumes, Lotus japonicus and Medicago truncatula, and two important legume crops, soybean (Glycine max) and common bean (Phaseolus vulgaris). This regulation impacts a variety of physiological processes including hormone signaling and spatial regulation of gene expression. The role of mobile miRNAs in regulating legume nodule number is also highlighted.

Identification of microRNAs in developing wheat grain that are potentially involved in regulating grain characteristics and the response to nitrogen levels

BACKGROUND

MicroRNAs (miRNAs) play crucial roles in the regulation of plant development and growth, but little information is available concerning their roles during grain development under different nitrogen (N) application levels. Our objective was to identify miRNAs related to the regulation of grain characteristics and the response to different N fertilizer conditions.

RESULTS

A total of 79 miRNAs (46 known and 33 novel miRNAs) were identified that showed significant differential expression during grain development under both high nitrogen (HN) and low nitrogen (LN) treatments. The miRNAs that were significantly upregulated early in grain development target genes involved mainly in cell differentiation, auxin-activated signaling, and transcription, which may be associated with grain size; miRNAs abundant in the middle and later stages target genes mainly involved in carbohydrate and nitrogen metabolism, transport, and kinase activity and may be associated with grain filling. Additionally, we identified 50 miRNAs (22 known and 28 novel miRNAs), of which 11, 9, and 39 were differentially expressed between the HN and LN libraries at 7, 17, and 27 days after anthesis (DAA). The miRNAs that were differentially expressed in response to nitrogen conditions target genes involved mainly in carbohydrate and nitrogen metabolism, the defense response, and transport as well as genes that encode ubiquitin ligase. Only one novel miRNA (PC-5p-2614_215) was significantly upregulated in response to LN treatment at all three stages, and 21 miRNAs showed significant differential expression between HN and LN conditions only at 27 DAA. We therefore propose a model for target gene regulation by miRNAs during grain development with N-responsive patterns.

CONCLUSIONS

The potential targets of the identified miRNAs are related to various biological processes, such as carbohydrate/nitrogen metabolism, transcription, cellular differentiation, transport, and defense. Our results indicate that miRNA-mediated networks, via posttranscriptional regulation, play crucial roles in grain development and the N response, which determine wheat grain weight and quality. Our study provides useful information for future research of regulatory mechanisms that focus on improving grain yield and quality.

Emerging Roles of microRNAs in Plant Heavy Metal Tolerance and Homeostasis

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