miR390, Arabidopsis TAS3 tasiRNAs, and their AUXIN RESPONSE FACTOR targets define an autoregulatory network quantitatively regulating lateral root growth

Identification and characterization of microRNAs in phloem and xylem from ramie (Boehmeria nivea)

Ramie (Boehmeria nivea) is a widely cropped species in southern China due to its high economic value of natural fiber for industry. Development of phloem and xylem is key evidence for generating fiber. However, the MicroRNA (miRNA) profiles of phloem and xylem in ramie have not been reported yet. miRNA belong to a small RNA family which has been recognized as an important regulator for various biological processes. In the present study, we aimed to identify differently expressed miRNAs between phloem and xylem in adult ramie. The results showed that 137 and 122 unique conserved miRNAs were identified from phloem and xylem libraries, respectively. Meanwhile, 4 novel miRNAs were identified from ramie by miRDeep2. Of these miRNAs, 77 conserved miRNAs in ramie were differentially expressed. Among the differentially expressed miRNAs, 44 miRNAs and 33 miRNAs were up-regulated and down-regulated in phloem compared to that in xylem, respectively. The functions of differentially expressed miRNAs were associated with regulating the development and differentiation of phloem and xylem. The present study provides a glance of miRNA profiles for further understanding of miRNA role in ramie development.

Integrated mRNA and small RNA sequencing reveals microRNA regulatory network associated with internode elongation in sugarcane (Saccharum officinarum L.)

Background

Internode elongation is one of the most important traits in sugarcane because of its relation to crop productivity. Understanding the microRNA (miRNA) and mRNA expression profiles related to sugarcane internode elongation would help develop molecular improvement strategies but they are not yet well-investigated. To identify genes and miRNAs involved in internode elongation, the cDNA and small RNA libraries from the pre-elongation stage (EI), early elongation stage (EII) and rapid elongation stage (EIII) were sequenced and their expression were studied.

Results

Based on the sequencing results, 499,495,518 reads and 80,745 unigenes were identified from stem internodes of sugarcane. The comparisons of EI vs. EII, EI vs. EIII, and EII vs. EIII identified 493, 5035 and 3041 differentially expressed genes, respectively. Further analysis revealed that the differentially expressed genes were enriched in the GO terms oxidoreductase activity and tetrapyrrole binding. KEGG pathway annotation showed significant enrichment in “zeatin biosynthesis”, “nitrogen metabolism” and “plant hormone signal transduction”, which might be participating in internode elongation. miRNA identification showed 241 known miRNAs and 245 novel candidate miRNAs. By pairwise comparison, 11, 42 and 26 differentially expressed miRNAs were identified from EI and EII, EI and EIII, and EII and EIII comparisons, respectively. The target prediction revealed that the genes involved in “zeatin biosynthesis”, “nitrogen metabolism” and “plant hormone signal transduction” pathways are targets of the miRNAs. We found that the known miRNAs miR2592-y, miR1520-x, miR390-x, miR5658-x, miR6169-x and miR8154-x were likely regulators of genes with internode elongation in sugarcane.

Conclusions

The results of this study provided a global view of mRNA and miRNA regulation during sugarcane internode elongation. A genetic network of miRNA-mRNA was identified with miRNA-mediated gene expression as a mechanism in sugarcane internode elongation. Such evidence will be valuable for further investigations of the molecular regulatory mechanisms underpinning sugarcane growth and development.

Identification and Characterization of Stress-Responsive TAS3-Derived TasiRNAs in Melon

Small interfering RNAs (siRNA) are key regulators of gene expression that play essential roles in diverse biological processes. Trans-acting siRNAs (tasiRNAs) are a class of plant-endogenous siRNAs that lead the cleavage of nonidentical transcripts. TasiRNAs are usually involved in fine-tuning development. However, increasing evidence supports that tasiRNAs may be involved in stress response. Melon is a crop of great economic importance extensively cultivated in semiarid regions frequently exposed to changing environmental conditions that limit its productivity. However, knowledge of the precise role of siRNAs in general, and of tasiRNAs in particular, in regulating the response to adverse environmental conditions is limited. Here, we provide the first comprehensive analysis of computationally inferred melon-tasiRNAs responsive to two biotic (viroid-infection) and abiotic (cold treatment) stress conditions. We identify two TAS3-loci encoding to length (TAS3-L) and short (TAS3-S) transcripts. The TAS candidates predicted from small RNA-sequencing data were characterized according to their chromosome localization and expression pattern in response to stress. The functional activity of cmTAS genes was validated by transcript quantification and degradome assays of the tasiRNA precursors and their predicted targets. Finally, the functionality of a representative cmTAS3-derived tasiRNA (TAS3-S) was confirmed by transient assays showing the cleavage of ARF target transcripts.

Genome-Wide Analysis of Cotton miRNAs During Whitefly Infestation Offers New Insights into Plant-Herbivore Interaction

Although the regulatory function of miRNAs and their targets have been characterized in model plants, a possible underlying role in the cotton response to herbivore infestation has not been determined. To investigate this, we performed small RNA and degradome sequencing between resistant and susceptible cotton cultivar following infestation with the generalist herbivore whitefly. In total, the 260 miRNA families and 241 targets were identified. Quantitative-PCR analysis revealed that several miRNAs and their corresponding targets exhibited dynamic spatio-temporal expression patterns. Moreover, 17 miRNA precursors were generated from 29 long intergenic non-coding RNA (lincRNA) transcripts. The genome-wide analysis also led to the identification of 85 phased small interfering RNA (phasiRNA) loci. Among these, nine PHAS genes were triggered by miR167, miR390, miR482a, and two novel miRNAs, including those encoding a leucine-rich repeat (LRR) disease resistance protein, an auxin response factor (ARF) and MYB transcription factors. Through combined modeling and experimental data, we explored and expanded the miR390-tasiARF cascade during the cotton response to whitefly. Virus-induced gene silencing (VIGS) of ARF8 from miR390 target in whitefly-resistant cotton plants increased auxin and jasmonic acid (JA) accumulation, resulting in increased tolerance to whitefly infestation. These results highlight the provides a useful transcriptomic resource for plant-herbivore interaction.

Integrated Analysis of Small RNA, Transcriptome and Degradome Sequencing Provides New Insights into Floral Development and Abscission in Yellow Lupine (Lupinus luteus L.)

The floral development in an important legume crop yellow lupine (Lupinus luteus L., Taper cv.) is often affected by the abscission of flowers leading to significant economic losses. Small non-coding RNAs (sncRNAs), which have a proven effect on almost all developmental processes in other plants, might be of key players in a complex net of molecular interactions regulating flower development and abscission. This study represents the first comprehensive sncRNA identification and analysis of small RNA, transcriptome and degradome sequencing data in lupine flowers to elucidate their role in the regulation of lupine generative development. As shedding in lupine primarily concerns flowers formed at the upper part of the inflorescence, we analyzed samples from extreme parts of raceme separately and conducted an additional analysis of pedicels from abscising and non-abscising flowers where abscission zone forms. A total of 394 known and 28 novel miRNAs and 316 phased siRNAs were identified. In flowers at different stages of development 59 miRNAs displayed differential expression (DE) and 46 DE miRNAs were found while comparing the upper and lower flowers. Identified tasiR-ARFs were DE in developing flowers and were strongly expressed in flower pedicels. The DEmiR-targeted genes were preferentially enriched in the functional categories related to carbohydrate metabolism and plant hormone transduction pathways. This study not only contributes to the current understanding of how lupine flowers develop or undergo abscission but also holds potential for research aimed at crop improvement.

The long noncoding RNA T5120 regulates nitrate response and assimilation in Arabidopsis

Long noncoding RNAs (lncRNAs) are crucial regulators in many plant biological processes. However, it remains unknown whether lncRNAs can respond to nitrate or function in nitrate regulation. We detected 695 lncRNAs, 480 known and 215 novel, in Arabidopsis seedling roots; six showed altered expression in response to nitrate treatment, among which T5120 showed the highest induction. Overexpression of T5120 in Arabidopsis promoted the response to nitrate, enhanced nitrate assimilation and improved biomass and root development. Biochemical and molecular analyses revealed that NLP7, a master nitrate regulatory transcription factor, directly bound to the nitrate-responsive cis-element (NRE)-like motif of the T5120 promoter and activated T5120 transcription. In addition, T5120 partially restored the nitrate signalling and assimilation phenotypes of nlp7 mutant, suggesting that T5120 is involved in NLP7-mediated nitrate regulation. Interestingly, the expression of T5120 was regulated by the nitrate sensor NRT1.1. Therefore, T5120 is modulated by NLP7 and NRT1.1 to regulate nitrate signalling. Our work reveals a new regulatory mechanism in which lncRNA T5120 functions in nitrate regulation, providing new insights into the nitrate signalling network. Importantly, lncRNA T5120 can promote nitrate assimilation and plant growth to improve nitrogen use efficiency.

Identification of miRNAs and Their Target Genes Involved in Cucumber Fruit Expansion Using Small RNA and Degradome Sequencing

Fruit expansion is an essential and very complex biological process. Regulatory roles of microRNAs (miRNAs) and miRNA-mRNA modules in the cucumber fruit expansion are not yet to be investigated. In this work, 1253 known and 1269 novel miRNAs were identified from nine cucumber fruit small RNA (sRNA) libraries through high-throughput sequencing. A total of 105 highly differentially expressed miRNAs were recognized in the fruit on five days post anthesis with pollination (EXP_5d) sRNA library. Further, expression patterns of 11 differentially expressed miRNAs were validated by quantitative real-time PCR (qRT-PCR). The expression patterns were similar to sRNAs sequencing data. Transcripts of 1155 sequences were predicted as target genes of differentially expressed miRNAs by degradome sequencing. Gene Ontology (GO) enrichment showed that these target genes were involved in 24 biological processes, 15 cell components and nine molecular functions. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis demonstrated that these target genes were significantly enriched in 19 pathways and the enriched KEGG pathways were associated with environmental adaptation, signal transduction and translation. Based on the functional prediction of miRNAs and target genes, our findings suggest that miRNAs have a potential regulatory role in cucumber fruit expansion by targeting their target genes, which provide important data for understanding the miRNA-mediated regulatory networks controlling fruit expansion in cucumber. Specific miRNAs could be selected for further functional research and molecular breeding in cucumber.

The barley miR393 has multiple roles in regulation of seedling growth, stomatal density, and drought stress tolerance

microRNA393 (miR393) and its target module have been implicated as comprising a conserved mechanism to regulate developmental processes and plant growth in response to environmental signals through the auxin signaling pathway. Our previous work identified miR393 and its two targets in barley. In this study, we further investigated the expression pattern of miR393 and its biological functions in seedling growth and drought tolerance. We showed that the miR393 overexpressing line (OE) exhibited increased stomatal density with decreased guard cell length, while the miR393 knockdown line (MIM) displayed the opposite phenotype, which might be due to the effects of miR393 on AUXIN RESPONSE FACTOR5 (ARF5) and three stomatal development-related genes, such as EPIDERMAL PATTERNING FACTOR1 (EPF1), SPEECHLESS (SPCH), and MUTE. In addition, the MIM line conferred enhanced drought tolerance, with alleviated leaf chlorosis and lipid peroxidation after 22 days drought treatment. In contrast, the OE line was more sensitive to drought stress and accumulated more malondialdehyde and hydrogen peroxide than the wild type. Furthermore, polyethylene glycol (PEG) treatment-induced abscisic acid (ABA) accumulation in leaves was suppressed in the OE line, indicating that miR393 might regulate drought stress response and tolerance through its interaction with ABA biosynthesis. Overall, these data suggest that miR393 might be a potential target for manipulation of stomatal density and improvement of drought tolerance in barley.

Genome-Wide Analysis of Long Intergenic Noncoding RNAs Responding to Low-Nutrient Conditions in Arabidopsis thaliana: Possible Involvement of Trans-Acting siRNA3 in Response to Low Nitrogen

Long intergenic noncoding RNAs (lincRNAs) play critical roles in transcriptional and post-transcriptional regulation of gene expression in a wide variety of organisms. Thousands of lincRNAs have been identified in plant genomes, although their functions remain mostly uncharacterized. Here, we report a genome-wide survey of lincRNAs involved in the response to low-nutrient conditions in Arabidopsis thaliana. We used RNA sequencing data derived from A. thaliana roots exposed to low levels of 12 different nutrients. Using bioinformatics approaches, 60 differentially expressed lincRNAs were identified that were significantly upregulated or downregulated under deficiency of at least one nutrient. To clarify their roles in nutrient response, correlations of expression patterns between lincRNAs and reference genes were examined across the 13 conditions (12 low-nutrient conditions and control). This analysis allowed us to identify lincRNA-RNA pairs with highly positive or negative correlations. In addition, calculating interaction energies of those pairs showed lincRNAs that may act as regulatory interactors; e.g. small interfering RNAs (siRNAs). Among them, trans-acting siRNA3 (TAS3), which is known to promote lateral root development by producing siRNA against Auxin response factor 2, 3, and 4, was revealed as a nitrogen (N)-responsive lincRNA. Furthermore, nitrate transporter 2 was identified as a potential target of TAS3-derived siRNA, suggesting that TAS3 participates in multiple pathways by regulating N transport and root development under low-N conditions. This study provides the first resource for candidate lincRNAs involved in multiple nutrient responses in plants.

Tackling Plant Phosphate Starvation by the Roots

Plant responses to phosphate deprivation encompass a wide range of strategies, varying from altering root system architecture, entering symbiotic interactions to excreting root exudates for phosphorous release, and recycling of internal phosphate. These processes are tightly controlled by a complex network of proteins that are specifically upregulated upon phosphate starvation. Although the different effects of phosphate starvation have been intensely studied, the full extent of its contribution to altered root system architecture remains unclear. In this review, we focus on the effect of phosphate starvation on the developmental processes that shape the plant root system and their underlying molecular pathways.

CRISPR/Cas9-mediated knockout of the RDR6 gene in Nicotiana benthamiana for efficient transient expression of recombinant proteins

MAIN CONCLUSION

RDR6 gene knockout Nicotiana benthamiana plant was successfully produced using CRISPR/Cas9 technology. The production of recombinant proteins in plants has many advantages, such as safety and reduced costs. However, there are several problems with this technology, especially low levels of protein production. The dysfunction of the RNA silencing mechanism in plant cells would be effective to improve recombinant protein production because the RNA silencing mechanism efficiently degrades transgene-derived mRNAs. Therefore, to overcome this problem, clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 technology was used to develop RNA silencing-related gene knockout transgenic Nicotiana benthamiana. We successfully produced RNA-dependent RNA polymerase 6 (RDR6), one of the most important components of the RNA silencing mechanism-knockout N. benthamiana (ΔRDR6 plants). The ΔRDR6 plants had abnormal flowers and were sterile, as with the Arabidopsis RDR6 mutants. However, a transient gene expression assay showed that the ΔRDR6 plants accumulated larger amounts of green fluorescent protein (GFP) and GFP mRNA than the wild-type (WT) plants. Small RNA sequencing analysis revealed that levels of small interfering RNA against the GFP gene were greatly reduced in the ΔRDR6 plants, as compared to that of the WT plants. These findings demonstrate that the ΔRDR6 plants can express larger amounts of recombinant proteins than WT plants and, therefore, would be useful for recombinant protein production and understanding the contributions of RDR6 to genetic and physiological events in plants.

Identification of Auxin Response Factor-Encoding Genes Expressed in Distinct Phases of Leaf Vein Development and with Overlapping Functions in Leaf Formation

Based on mutant phenotypes the MONOPTEROS (MP)/Auxin Response Factor 5 (ARF5) gene acts in several developmental processes including leaf vein development. Since overlapping functions among ARF genes are common, we assessed the related ARF 3-8 and 19 genes for potential overlap in expression during vein development using in-situ hybridization. Like MP/ARF5, ARF3 was expressed in preprocambial and procambial cells. ARF7 was also expressed in procambial cells, close to and during vein differentiation. ARF19 was expressed in differentiating vessel elements. To assess if genes with vein expression have overlapping functions, double mutants were generated. While arf3, 5 and 7 mutants formed leaves normally, double mutant combinations of mp/arf5 with arf3 or arf7 resulted in a breakdown of leaf formation. Instead, novel structures not present in any of the single mutants formed. The results implicate ARF3 and ARF7 in rosette leaf formation and suggest that their functions overlap and act in parallel with MP/ARF5 in this process. The observed vascular expression patterns suggest unique functions (ARF7 and 19) and potentially overlapping functions (ARF3 and 5) in vein development. Since arf3 arf5 double mutants do not form leaves, assessment of their potential combined action in vein development will require the use of conditional mutants.

Expression Pattern of Plant miRNAs by Classical Transcriptional Fusion Constructs

microRNAs are noncoding RNAs of 20-24 nucleotides (nt) in length that act as repressors of genes and are important in key developmental processes in the entire life cycle of plants. To determine the function of a microRNA, the first step is to resolve its expression pattern; this can be achieved by in situ hybridization, RNA blot assays, or quantitative PCR. However, the study of the expression of a MIR gene is straightforward with the use of reporter proteins such as β-D-glucuronidase (GUS), GFP, or mCherry. To do this, it is necessary to clone the promoter region of the MIR gene and place it upstream of the reporter gene; in this way the activity of the promoter will be a direct reflection of the expression of the MIR gene. Here, we indicate step by step how to make transcriptional fusion constructs from the cloning of a promoter region of a MIR gene fused to the classical reporter proteins GUS and mCherry, the latter with codon optimization for better expression in Arabidopsis thaliana. This method is particularly useful to dissect the promoter region of a MIR gene and to find its expression pattern in a tissue and developmental specific manner.

In Situ Localization of Small RNAs in Plants

Small RNAs have vital roles in numerous aspects of plant biology. Deciphering their precise contributions requires knowledge of a small RNA's spatiotemporal pattern of accumulation. The in situ hybridization protocol described here takes advantage of locked nucleic acid (LNA) oligonucleotide probes to visualize small RNA expression at the cellular level with high sensitivity and specificity. The procedure is optimized for paraffin-embedded plant tissue sections, is applicable to a wide range of plants and tissues, and can be completed within 2-6 days.

Perspectives on microRNAs and Phased Small Interfering RNAs in Maize (Zea mays L.): Functions and Big Impact on Agronomic Traits Enhancement

Small RNA (sRNA) population in plants comprises of primarily micro RNAs (miRNAs) and small interfering RNAs (siRNAs). MiRNAs play important roles in plant growth and development. The miRNA-derived secondary siRNAs are usually known as phased siRNAs, including phasiRNAs and tasiRNAs. The miRNA and phased siRNA biogenesis mechanisms are highly conserved in plants. However, their functional conservation and diversification may differ in maize. In the past two decades, lots of miRNAs and phased siRNAs have been functionally identified for curbing important maize agronomic traits, such as those related to developmental timing, plant architecture, sex determination, reproductive development, leaf morphogenesis, root development and nutrition, kernel development and tolerance to abiotic stresses. In contrast to Arabidopsis and rice, studies on maize miRNA and phased siRNA biogenesis and functions are limited, which restricts the small RNA-based fundamental and applied studies in maize. This review updates the current status of maize miRNA and phased siRNA mechanisms and provides a survey of our knowledge on miRNA and phased siRNA functions in controlling agronomic traits. Furthermore, improvement of those traits through manipulating the expression of sRNAs or their targets is discussed.

Genome-Wide Analysis of the miRNA-mRNAs Network Involved in Cold Tolerance in Populus simonii × P. nigra

Background: Cold tolerance is important for plants’ geographical distribution and survival in extreme seasonal variations of climate. However, Populus simonii × P. nigra shows wide adaptability and strong cold resistance. Transcriptional and post-transcriptional regulation of cold-responsive genes is crucial for cold tolerance in plants. To understand the roles of regulatory RNAs under cold induction in Populus simonii × P. nigra, we constructed cDNA and small RNA libraries from leaf buds treated or not with −4 °C for 8 h for analysis. Results: Through high-throughput sequencing and differential expression analysis, 61 miRNAs and 1229 DEGs were identified under cold induction condition in Populus simonii × P. nigra. The result showed that miR167a, miR1450, miR319a, miR395b, miR393a-5p, miR408-5p, and miR168a-5p were downregulated, whereas transcription level of miR172a increased under the cold treatment. Thirty-one phased-siRNA were also obtained (reads ≥ 4) and some of them proceeded from TAS3 loci. Analysis of the differentially expressed genes (DEGs) showed that transcription factor genes such as Cluster-15451.2 (putative MYB), Cluster-16493.29872 (putative bZIP), Cluster-16493.29175 (putative SBP), and Cluster-1378.1 (putative ARF) were differentially expressed in cold treated and untreated plantlets of Populus simonii × P. nigra. Integrated analysis of miRNAs and transcriptome showed miR319, miR159, miR167, miR395, miR390, and miR172 and their target genes, including MYB, SBP, bZIP, ARF, LHW, and ATL, were predicted to be involved in ARF pathway, SPL pathway, DnaJ related photosystem II, and LRR receptor kinase, and many of them are known to resist chilling injury. Particularly, a sophisticated regulatory model including miRNAs, phasiRNAs, and targets of them was set up. Conclusions: Integrated analysis of miRNAs and transcriptome uncovered the complicated regulation of the tolerance of cold in Populus simonii × P. nigra. MiRNAs, phasiRNAs, and gene-encoded transcription factors were characterized at a whole genome level and their expression patterns were proved to be complementary. This work lays a foundation for further research of the pathway of sRNAs and regulatory factors involved in cold tolerance.

miRNAs associated with auxin signaling, stress response, and cellular activities mediate adventitious root formation in apple rootstocks

Adventitious root (AR) formation is essential for the vegetative propagation of apple rootstocks. miRNAs play a significant role in regulating AR development, however, large-scale transcriptomic data on miRNA mediated AR formation in apple rootstocks is lacking. Therefore, in order to identify the molecular mechanisms underlying AR formation in 'M9-T337' apple rootstocks, transcriptomic changes occurring during key time points of AR formation (0, 3, and 16 days) were analyzed using high-throughput sequencing with a focus on miRNAs. A total of 84 known miRNAs and 56 novel miRNAs have differentially expressed were identified. Additionally, a total of 88 target genes of known miRNAs and 76 target genes of novel miRNAs were identified by degradome sequencing. The expression levels of the miRNAs and target genes were quantified by RT-qPCR. Results indicate that miRNAs and their target genes are associated with auxin signal-related (miR160 and miR390), stress response-related (miR398, miR395 and miR408), cell fate transformation-, proliferation- and enlargement-related (miR171, miR156, miR166, miR319 and miR396). These all involve pathways that participate in AR formation in 'M9-T337' apple rootstock. In addition, hormones (AUX, CTK, GA3, BR, JA, and ABA) are also involved in regulating AR formation. The candidate genes belonging to pathways associated with AR formation exhibited significantly higher expression levels, providing evidence that they may be involved in the regulation of AR development. The collective results of the present study indicate that the developmental process associated with AR formation in apple rootstock is extremely complex. The known and novel miRNAs and target genes that were identified by high-throughput and degradome sequencing, respectively, provide a framework for the future analysis of miRNAs associated with AR development in apple rootstocks, and provide new information that can be used to better understand AR development in woody plants.

Auxins in potato: molecular aspects and emerging roles in tuber formation and stress resistance

The study of the effects of auxins on potato tuberization corresponds to one of the oldest experimental systems in plant biology, which has remained relevant for over 70 years. However, only recently, in the postgenomic era, the role of auxin in tuber formation and other vital processes in potatoes has begun to emerge. This review describes the main results obtained over the entire period of auxin-potato research, including the effects of exogenous auxin; the content and dynamics of endogenous auxins; the effects of manipulating endogenous auxin content; the molecular mechanisms of auxin signaling, transport and inactivation; the role and position of auxin among other tuberigenic factors; the effects of auxin on tuber dormancy; the prospects for auxin use in potato biotechnology. Special attention is paid to recent insights into auxin function in potato tuberization and stress resistance. Taken together, the data discussed here leave no doubt on the important role of auxin in potato tuberization, particularly in the processes of tuber initiation, growth and sprouting. A new integrative model for the stage-dependent auxin action on tuberization is presented. In addition, auxin is shown to differentially affects the potato resistance to biotrophic and necrotrophic biopathogens. Thus, the modern auxin biology opens up new perspectives for further biotechnological improvement of potato crops.

Identification of miRNAs and Their Response to Cold Stress in Astragalus Membranaceus

Astragalus membranaceus is an important medicinal plant widely cultivated in East Asia. MicroRNAs (miRNAs) are endogenous regulatory molecules that play essential roles in plant growth, development, and the response to environmental stresses. Cold is one of the key environmental factors affecting the yield and quality of A. membranaceus, and miRNAs may mediate the gene regulation network under cold stress in A. membranaceus. To identify miRNAs and reveal their functions in cold stress response in A. membranaceus, small RNA sequencing was conducted followed by bioinformatics analysis, and quantitative real time PCR (qRT-PCR) analysis was performed to profile the expression of miRNAs under cold stress. A total of 168 conserved miRNAs belonging to 34 families and 14 putative non-conserved miRNAs were identified. Many miRNA targets were predicted and these targets were involved in diversified regulatory and metabolic pathways. By using qRT-PCR, 27 miRNAs were found to be responsive to cold stress, including 4 cold stress-induced and 17 cold-repressed conserved miRNAs, and 6 cold-induced non-conserved miRNAs. These cold-responsive miRNAs probably mediate the response to cold stress by regulating development, hormone signaling, defense, redox homeostasis, and secondary metabolism in A. membranaceus. These cold-corresponsive miRNAs may be used as the candidate genes in further molecular breeding for improving cold tolerance of A. membranaceus.

An Essential Role for miRNA167 in Maternal Control of Embryonic and Seed Development

Maternal cells play a critical role in ensuring the normal development of embryos, endosperms, and seeds. Mutations that disrupt the maternal control of embryogenesis and seed development are difficult to identify. Here, we completely deleted four MICRORNA167 (MIR167) genes in Arabidopsis (Arabidopsis thaliana) using a clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein9 (Cas9) genome-editing technology. We found that plants with a deletion of MIR167A phenocopied plants overexpressing miRNA167-resistant versions of Auxin Response Factor6 (ARF6) or ARF8, two miRNA167 targets. Both the mir167a mutant and the ARF overexpression lines were defective in anther dehiscence and ovule development. Serendipitously, we found that the mir167a (♀) × wild type (♂) crosses failed to produce normal embryos and endosperms, despite the findings that embryos with either mir167a^+/- or mir167a^-/- genotypes developed normally when mir167a^+/- plants were self-pollinated, revealing a central role of MIR167A in maternal control of seed development. The mir167a phenotype is 100% penetrant, providing a great genetic tool for studying the roles of miRNAs and auxin in maternal control. Moreover, we found that mir167a mutants flowered significantly later than wild-type plants, a phenotype that was not observed in the ARF overexpression lines. We show that the reproductive defects of mir167a mutants were suppressed by a decrease of activities of ARF6, ARF8, or both. Our results clearly demonstrate that MIR167A is the predominant MIR167 member in regulating Arabidopsis reproduction and that MIR167A acts as a maternal gene that functions largely through ARF6 and ARF8.

Gene Regulation Mediated by microRNA-Triggered Secondary Small RNAs in Plants

In plants, proper development and response to abiotic and biotic stimuli requires an orchestrated regulation of gene expression. Small RNAs (sRNAs) are key molecules involved in this process, leading to downregulation of their target genes. Two main classes of sRNAs exist, the small interfering RNAs (siRNAs) and microRNAs (miRNAs). The role of the latter class in plant development and physiology is well known, with many examples of how miRNAs directly impact the expression of genes in cells where they are produced, with dramatic consequences to the life of the plant. However, there is an aspect of miRNA biology that is still poorly understood. In some cases, miRNA targeting can lead to the production of secondary siRNAs from its target. These siRNAs, which display a characteristic phased production pattern, can act in cis, reinforcing the initial silencing signal set by the triggering miRNA, or in trans, affecting genes that are unrelated to the initial target. In this review, the mechanisms and implications of this process in the gene regulation mediated by miRNAs will be discussed. This work will also explore techniques for gene silencing in plants that are based on this unique pathway.

Expression of the high-affinity K+ transporter 1 (PpHKT1) gene from almond rootstock 'Nemaguard' improved salt tolerance of transgenic Arabidopsis

Soil salinity affects plant growth and development, which directly impact yield. Plants deploy many mechanisms to cope with, or mitigate, salt stress. One of such mechanism is to control movement of ions from root to shoot by regulating the loading of Na⁺ in the transpiration stream. The high-affinity K⁺ transporter 1 (HKT1) is known to play a role in the removal of Na⁺ from the xylem and bring it back to the root. As almond is a salt-sensitive crop, the rootstock plays an important role in successful almond cultivation in salt-affected regions. We currently lack knowledge on the molecular mechanisms involved in salt tolerance of almond rootstocks. In this study, we complemented the Arabidopsis athkt1 knockout mutant with HKT1 ortholog (PpHKT1) from the almond rootstock ‘Nemaguard’. Arabidopsis transgenic lines that were generated in athkt1 background with the constitutive promoter (PpHKT1OE2.2) and the native promoter (PpHKT1NP6) were subjected to different salt treatments. Both transgenic lines survived salt concentrations up to 120 mM NaCl, however, the mutant athkt1 died after 18 days under 120 mM NaCl. At 90 mM NaCl, the dry weight of athkt1 decreased significantly compared to the transgenic lines. Both transgenic lines showed significantly longer lateral roots compared to the athkt1 mutant at 80 mM NaCl treatment. The transgenic lines, PpHKT1OE2.2 and PpHKTNP6 had lower electrolyte leakage and higher relative water content compared to athkt1, suggesting that transgenic plants coped well with increased salt concentration by maintaining the integrity of the membranes. The expression analyses showed that PpHKT1 was induced in PpHKT1OE2.2 and PpHKTNP6 lines under salt treatment, which confirmed that both over-expression and native expression of PpHKT1 in the Arabidopsis mutant can complement salt tolerance function.

Submergence and Waterlogging Stress in Plants: A Review Highlighting Research Opportunities and Understudied Aspects

Soil flooding creates composite and complex stress in plants known as either submergence or waterlogging stress depending on the depth of the water table. In nature, these stresses are important factors dictating the species composition of the ecosystem. On agricultural land, they cause economic damage associated with long-term social consequences. The understanding of the plant molecular responses to these two stresses has benefited from research studying individual components of the stress, in particular low-oxygen stress. To a lesser extent, other associated stresses and plant responses have been incorporated into the molecular framework, such as ion and ROS signaling, pathogen susceptibility, and organ-specific expression and development. In this review, we aim to highlight known or suspected components of submergence/waterlogging stress that have not yet been thoroughly studied at the molecular level in this context, such as miRNA and retrotransposon expression, the influence of light/dark cycles, protein isoforms, root architecture, sugar sensing and signaling, post-stress molecular events, heavy-metal and salinity stress, and mRNA dynamics (splicing, sequestering, and ribosome loading). Finally, we explore biotechnological strategies that have applied this molecular knowledge to develop cultivars resistant to flooding or to offer alternative uses of flooding-prone soils, like bioethanol and biomass production.

Identification and analysis of oxygen responsive microRNAs in the root of wild tomato (S. habrochaites)

BACKGROUND

MicroRNA (miRNA) are key players in regulating expression of target genes at post-transcriptional level. A number of miRNAs are implicated in modulating tolerance to various abiotic stresses. Waterlogging is an abiotic stress that deters plant growth and productivity by hypoxia. Dozens of reports mention about the miRNAs expressed in response to waterlogging and hypoxia. Despite the fact that tomato is a model vegetable but waterlogging sensitive crop, the role of miRNAs in hypoxia tolerance is poorly understood in tomato.

RESULTS

In this study, we investigated the differentially expressed miRNAs between hypoxia-treated and untreated wild tomato root by using high-throughput sequencing technology. A total of 33 known miRNAs were lowly expressed, whereas only 3 miRNAs showed higher expression in hypoxia-treated wild tomato root compared with untreated wild tomato root. Then two conserved and lowly expressed miRNAs, miR171 and miR390, were deactivated by Short Tandem Target Mimic (STTM) technology in Arabidopsis. As the results, the number and length of lateral roots were more in STTM171 and STTM390 transgenic lines compared with that of wild type plant, which partly phenocopy the increase root number and shortening the root length in hypoxia-treated wild tomato root.

CONCLUSIONS

The differentially expressed miRNAs between hypoxia-treated wild tomato and control root, which contribute to the auxin homeostasis, morphologic change, and stress response, might result in reduction in the biomass and length of the root in hypoxiated conditions.

Homeostasis in the soybean miRNA396-GRF network is essential for productive soybean cyst nematode infections

Heterodera glycines, the soybean cyst nematode, penetrates soybean roots and migrates to the vascular cylinder where it forms a feeding site called the syncytium. MiRNA396 (miR396) targets growth-regulating factor (GRF) genes, and the miR396-GRF1/3 module is a master regulator of syncytium development in model cyst nematode H. schachtii infection of Arabidopsis. Here, we investigated whether this regulatory system operates similarly in soybean roots and is likewise important for H. glycines infection. We found that a network involving nine MIR396 and 23 GRF genes is important for normal development of soybean roots and that GRF function is specified in the root apical meristem by miR396. All MIR396 genes are down-regulated in the syncytium during its formation phase while, specifically, 11 different GRF genes are up-regulated. The switch to the syncytium maintenance phase coincides with up-regulation of MIR396 and down-regulation of the 11 GRF genes specifically via post-transcriptional regulation by miR396. Furthermore, interference with the miR396-GRF6/8-13/15-17/19 regulatory network, through either overexpression or knockdown experiments, does not affect the number of H. glycines juveniles that enter the vascular cylinder to initiate syncytia, but specifically inhibits efficient H. glycines development to adult females. Therefore, homeostasis in the miR396-GRF6/8-13/15-17/19 regulatory network is essential for productive H. glycines infections.

Translating auxin responses into ovules, seeds and yield: Insight from Arabidopsis and the cereals

Grain production in cereal crops depends on the stable formation of male and female gametes in the flower. In most angiosperms, the female gamete is produced from a germline located deep within the ovary, protected by several layers of maternal tissue, including the ovary wall, ovule integuments and nucellus. In the field, germline formation and floret fertility are major determinants of yield potential, contributing to traits such as seed number, weight and size. As such, stimuli affecting the timing and duration of reproductive phases, as well as the viability, size and number of cells within reproductive organs can significantly impact yield. One key stimulant is the phytohormone auxin, which influences growth and morphogenesis of female tissues during gynoecium development, gametophyte formation, and endosperm cellularization. In this review we consider the role of the auxin signaling pathway during ovule and seed development, first in the context of Arabidopsis and then in the cereals. We summarize the gene families involved and highlight distinct expression patterns that suggest a range of roles in reproductive cell specification and fate. This is discussed in terms of seed production and how targeted modification of different tissues might facilitate improvements.

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

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

PILS6 is a temperature-sensitive regulator of nuclear auxin input and organ growth in Arabidopsis thaliana

Temperature modulates growth and development throughout the entire lifecycle of a plant. High temperature (HT) triggers the auxin biosynthesis-dependent growth in aerial tissues. On the other hand, the contribution of auxin to HT-induced root growth is currently under debate. Here we show that the putative intracellular auxin carrier PIN-LIKES 6 (PILS6) is a negative regulator of organ growth and that its abundance is highly sensitive to HT. PILS6 localizes to the endoplasmic reticulum and limits the nuclear availability of auxin, consequently reducing the auxin signaling output. HT represses the PILS6 protein abundance, which impacts on PILS6-dependent auxin signaling in roots and root expansion. Accordingly, we hypothesize that PILS6 is part of an alternative mechanism linking HT to auxin responses in roots.

Auxin Response Factors promote organogenesis by chromatin-mediated repression of the pluripotency gene SHOOTMERISTEMLESS

Specification of new organs from transit amplifying cells is critical for higher eukaryote development. In plants, a central stem cell pool maintained by the pluripotency factor SHOOTMERISTEMLESS (STM), is surrounded by transit amplifying cells competent to respond to auxin hormone maxima by giving rise to new organs. Auxin triggers flower initiation through Auxin Response Factor (ARF) MONOPTEROS (MP) and recruitment of chromatin remodelers to activate genes promoting floral fate. The contribution of gene repression to reproductive primordium initiation is poorly understood. Here we show that downregulation of the STM pluripotency gene promotes initiation of flowers and uncover the mechanism for STM silencing. The ARFs ETTIN (ETT) and ARF4 promote organogenesis at the reproductive shoot apex in parallel with MP via histone-deacetylation mediated transcriptional silencing of STM. ETT and ARF4 directly repress STM, while MP acts indirectly, through its target FILAMENTOUS FLOWER (FIL). Our data suggest that - as in animals- downregulation of the pluripotency program is important for organogenesis in plants.

L-Cysteine desulfhydrase-dependent hydrogen sulfide is required for methane-induced lateral root formation

Methane-triggered lateral root formation is not only a universal event, but also dependent on L-cysteine desulfhydrase-dependent hydrogen sulfide signaling. Whether or how methane (CH₄) triggers lateral root (LR) formation has not been elucidated. In this report, CH₄ induction of lateral rooting and the role of hydrogen sulfide (H₂S) were dissected in tomato and Arabidopsis by using physiological, anatomical, molecular, and genetic approaches. First, we discovered that CH₄ induction of lateral rooting is a universal event. Exogenously applied CH₄ not only triggered tomato lateral rooting, but also increased activities of L-cysteine desulfhydrase (DES; a major synthetic enzyme of H₂S) and induced endogenous H₂S production, and contrasting responses were observed in the presence of hypotaurine (HT; a scavenger of H₂S) or DL-propargylglycine (PAG; an inhibitor of DES) alone. CH₄-triggered lateral rooting were sensitive to the inhibition of endogenous H₂S with HT or PAG. The changes in the transcripts of representative cell cycle regulatory genes, miRNA and its target genes were matched with above phenotypes. In the presence of CH₄, Arabidopsis mutant Atdes1 exhibited defects in lateral rooting, compared with the wild-type. Molecular evidence showed that the transcriptional profiles of representative target genes modulated by CH₄ in wild-type plants were impaired in Atdes1 mutant. Overall, our data demonstrate the main branch of the DES-dependent H₂S signaling cascade in CH₄-triggered LR formation.

ARF5/MONOPTEROS directly regulates miR390 expression in the Arabidopsis thaliana primary root meristem

The root meristem is organized around a quiescent center (QC) surrounded by stem cells that generate all cell types of the root. In the transit-amplifying compartment, progeny of stem cells further divides prior to differentiation. Auxin controls the size of this transit-amplifying compartment via auxin response factors (ARFs) that interact with auxin response elements (AuxREs) in the promoter of their targets. The microRNA miR390 regulates abundance of ARF2, ARF3, and ARF4 by triggering the production of trans-acting (ta)-siRNA from the TAS3 precursor. This miR390/TAS3/ARF regulatory module confers sensitivity and robustness to auxin responses in diverse developmental contexts and organisms. Here, we show that miR390 is expressed in the transit-amplifying compartment of the root meristem where it modulates response to exogenous auxin. We show that a single AuxRE located in miR390 promoter is necessary for miR390 expression in this compartment and identify that ARF5/MONOPTEROS (MP) binds miR390 promoter via the AuxRE. We show that interfering with ARF5/MP-dependent auxin signaling attenuates miR390 expression in the transit-amplifying compartment of the root meristem. Our results show that ARF5/MP regulates directly the expression of miR390 in the root meristem. We propose that ARF5, miR390, and the ta-siRNAs-regulated ARFs modulate the response of the transit-amplifying region of the meristem to exogenous auxin.

Functional dissection of the ARGONAUTE7 promoter

ARGONAUTES are the central effector proteins of RNA silencing which bind target transcripts in a small RNA-guided manner. Arabidopsis thaliana has 10 ARGONAUTE (AGO) genes, with specialized roles in RNA-directed DNA methylation, post-transcriptional gene silencing, and antiviral defense. To better understand specialization among AGO genes at the level of transcriptional regulation we tested a library of 1497 transcription factors for binding to the promoters of AGO1,AGO10, and AGO7 using yeast 1-hybrid assays. A ranked list of candidate DNA-binding TFs revealed binding of the AGO7 promoter by a number of proteins in two families: the miR156-regulated SPL family and the miR319-regulated TCP family, both of which have roles in developmental timing and leaf morphology. Possible functions for SPL and TCP binding are unclear: we showed that these binding sites are not required for the polar expression pattern of AGO7, nor for the function of AGO7 in leaf shape. Normal AGO7 transcription levels and function appear to depend instead on an adjacent 124-bp region. Progress in understanding the structure of this promoter may aid efforts to understand how the conserved AGO7-triggered TAS3 pathway functions in timing and polarity.

A role for ALF4 during gall and giant cell development in the biotic interaction between Arabidopsis and Meloidogyne spp

Root-knot nematodes (RKNs; Meloidogyne spp.) are a major pest for the agriculture worldwide. RKNs induce specialized feeding cells (giant cells, GCs) inside galls which are de novo formed pseudo-organs in the roots that share similarities with other developmental processes as lateral root (LR) and callus formation or grafting involving new vascular development or pericycle proliferation. Hence, it is pertinent to study the molecular mechanisms directing the plant-nematode interaction. In this respect, ALF4 is a key gene during LR formation, vascular vessels reconnection in grafting, hormone-induced callus formation or de novo root organogenesis from leaf explants. Our results show that ALF4 is also induced in galls at early infection stages in an auxin-independent way. Furthermore, ALF4 activity is necessary for the formation of proper galls and GCs, as the mutant alf4-1 presents aberrant galls and GCs with severe structural abnormalities leading to a dramatic reduction in the nematode egg production. However, a low-reproduction rate is maintained, that might be explained by the local auxin maximum build by the nematodes in galls, partially rescuing alf4-1 phenotype. This would be similar to the partial rescue described for LR formation with exogenous auxins and also agrees with the LR emergence from alf4-1 galls but not from uninfected roots. In addition, ALF4 is also induced in syncytia formed by cyst nematodes. All these data support a pivotal role for ALF4 during de novo organogenesis processes induced by endoparasitic nematodes, in addition to its role in LR formation, callus development or vessel reconnection during grafting.

MicroRNAs, New Players in the Plant-Nematode Interaction

Plant-parasitic root-knot and cyst nematodes are microscopic worms that cause severe damage to crops and induce major agricultural losses worldwide. These parasites penetrate into host roots and induce the formation of specialized feeding structures, which supply the resources required for nematode development. Root-knot nematodes induce the redifferentiation of five to seven root cells into giant multinucleate feeding cells, whereas cyst nematodes induce the formation of a multinucleate syncytium by targeting a single root cell. Transcriptomic analyses have shown that the induction of these feeding cells by nematodes involves an extensive reprogramming of gene expression within the targeted root cells. MicroRNAs are small noncoding RNAs that act as key regulators of gene expression in eukaryotes by inducing the posttranscriptional silencing of protein coding genes, including many genes encoding transcription factors. A number of microRNAs (miRNAs) displaying changes in expression in root cells in response to nematode infection have recently been identified in various plant species. Modules consisting of miRNAs and the transcription factors they target were recently shown to be required for correct feeding site formation. Examples include miR396 and GRF in soybean syncytia and miR159 and MYB33 in Arabidopsis giant cells. Moreover, some conserved miRNA/target modules seem to have similar functions in feeding site formation in different plant species. These miRNAs may be master regulators of the reprogramming of expression occurring during feeding site formation. This review summarizes current knowledge about the role of these plant miRNAs in plant-nematode interactions.

The Involvement of Long Noncoding RNAs in Response to Plant Stress

Plant growth and productivity are greatly impacted by environmental stresses. Therefore, plants have evolved mechanisms which allow them to adapt to abiotic stresses through alterations in gene expression and metabolism. In recent years, studies have investigated the role of long noncoding RNA (lncRNA) in regulating gene expression in plants and characterized their involvement in various biological functions through their regulation of DNA methylation, DNA structural modifications, histone modifications, and RNA-RNA interactions. Genome-wide transcriptome analyses have identified various types of noncoding RNAs (ncRNAs) that respond to abiotic stress. These ncRNAs are in addition to the well-known housekeeping ncRNAs, such as rRNAs, tRNAs, snoRNAs, and snRNAs. In this review, recent research pertaining to the role of lncRNAs in the response of plants to abiotic stress is summarized and discussed.

Regulatory networks controlling the development of the root system and the formation of lateral roots: a comparative analysis of the roles of pericycle and vascular cambium

Background

The production of a new lateral root from parental root primary tissues has been investigated extensively, and the most important regulatory mechanisms are now well known. A first regulatory mechanism is based on the synthesis of small peptides which interact ectopically with membrane receptors to elicit a modulation of transcription factor target genes. A second mechanism involves a complex cross-talk between plant hormones. It is known that lateral roots are formed even in parental root portions characterized by the presence of secondary tissues, but there is not yet agreement about the putative tissue source providing the cells competent to become founder cells of a new root primordium.

Scope

We suggest models of possible regulatory mechanisms for inducing specific root vascular cambium (VC) stem cells to abandon their activity in the production of xylem and phloem elements and to start instead the construction of a new lateral root primordium. Considering the ontogenic nature of the VC, the models which we suggest are the result of a comparative review of mechanisms known to control the activity of stem cells in the root apical meristem, procambium and VC. Stem cells in the root meristems can inherit various competences to play different roles, and their fate could be decided in response to cross-talk between endogenous and exogenous signals.

Conclusions

We have found a high degree of relatedness among the regulatory mechanisms controlling the various root meristems. This fact suggests that competence to form new lateral roots can be inherited by some stem cells of the VC lineage. This kind of competence could be represented by a sensitivity of specific stem cells to factors such as those presented in our models.

MiR393 and miR390 synergistically regulate lateral root growth in rice under different conditions

BACKGROUND

Plants have evolved excellent ability of flexibly regulating the growth of organs to adapt to changing environment, for example, the modulation of lateral root development in response to environmental stresses. Despite of fundamental discovery that some microRNAs are involved in this process, the molecular mechanisms of how these microRNAs work together are still largely unknown.

RESULTS

Here we show that miR390 induced by auxin promotes lateral root growth in rice. However, this promotion can be suppressed by miR393, which is induced by various stresses and ABA (Abscisic Acid). Results that miR393 responded to ABA stronger and earlier than other stresses implied that ABA likely is authentic factor for inducing miR393. The transgenic lines respectively over-expressing miR393 and OsTAS3a (Oryza sativa Trans-Acting Short RNA precursor 3a) displayed opposite phenotypes in lateral root growth. MiR390 was found to be dominantly expressed at lateral root primordia and roots tips while miR393 mainly expressed in the base part of roots at very low level. When miR393 was up-regulated by various stresses, miR390 expression level fell down. However, the risen expression level of miR390 induced by auxin didn't affect the expression of miR393 and its target OsTIR1 (Transport Inhibitor Response 1). Together with analysis of the two transgenic lines, we provide a model of how the growth of lateral roots in rice is regulated distinctively by the 2 microRNAs.

CONCLUSION

We propose that miR390 induced by auxin triggers the lateral root growth under normal growth conditions, meanwhile miR393 just lurks as a potentially regulative role; Once plants suffer from stresses, miR393 will be induced to negatively regulate miR390-mediated growth of lateral roots in rice.

Tissue-specific transcriptomic profiling of Plantago major provides insights for the involvement of vasculature in phosphate deficiency responses

The vasculature of higher plants is important with transport of both nutrient and information molecules. To understand the correspondence of this tissue in molecular responses under phosphate (Pi) deficiency, Plantago major, a model plant for vasculature biology study, was chosen in our analysis. After RNA-Seq and de novo transcriptome assembly of 24 libraries prepared from the vasculature of P. major, 37,309 unigenes with a mean length of 1571 base pairs were obtained. Upon 24 h of Pi deficiency, 237 genes were shown to be differentially expressed in the vasculature of P. major. Among these genes, only 27 have been previously identified to be specifically expressed in the vasculature tissues in other plant species. Temporal expression of several marker genes associated with Pi deficiency showed that the time period of first 24 h is at the beginning stage of more dynamic expression patterns. In this study, we found several physiological processes, e.g., "phosphate metabolism and remobilization", "sucrose metabolism, loading and synthesis", "plant hormone metabolism and signal transduction", "transcription factors", and "metabolism of other minerals", were mainly involved in early responses to Pi deficiency in the vasculature. A number of vasculature genes with promising roles in Pi deficiency adaptation have been identified and deserve further functional characterization. This study clearly demonstrated that plant vasculature is actively involved in Pi deficiency responses and understanding of this process may help to create plants proficient to offset Pi deficiency.

Molecular Mechanisms of Leaf Morphogenesis

Plants maintain the ability to form lateral appendages throughout their life cycle and form leaves as the principal lateral appendages of the stem. Leaves initiate at the peripheral zone of the shoot apical meristem and then develop into flattened structures. In most plants, the leaf functions as a solar panel, where photosynthesis converts carbon dioxide and water into carbohydrates and oxygen. To produce structures that can optimally fulfill this function, plants precisely control the initiation, shape, and polarity of leaves. Moreover, leaf development is highly flexible but follows common themes with conserved regulatory mechanisms. Leaves may have evolved from lateral branches that are converted into determinate, flattened structures. Many other plant parts, such as floral organs, are considered specialized leaves, and thus leaf development underlies their morphogenesis. Here, we review recent advances in the understanding of how three-dimensional leaf forms are established. We focus on how genes, phytohormones, and mechanical properties modulate leaf development, and discuss these factors in the context of leaf initiation, polarity establishment and maintenance, leaf flattening, and intercalary growth.

Plant small RNAs: advancement in the understanding of biogenesis and role in plant development

Present review addresses the advances made in the understanding of biogenesis of plant small RNAs and their role in plant development. We discuss the elaborate role of microRNAs (miRNAs) and trans-acting small interfering RNAs (ta-siRNAs) in various aspects of plant growth and development and highlight relevance of small RNA mobility. Small non-coding RNAs regulate various aspects of plant development. Small RNAs (sRNAs) of 21-24 nucleotide length are derived from double-stranded RNAs through the combined activity of several biogenesis and processing components. These sRNAs function by negatively regulating the expression of target genes. miRNAs and ta-siRNAs constitute two important classes of endogenous small RNAs in plants, which play important roles in plant growth and developmental processes like embryogenesis, organ formation and patterning, shoot and root growth, and reproductive development. Biogenesis of miRNAs is a multistep process which includes transcription, processing and modification, and their loading onto RNA-induced silencing complex (RISC). RISC-loaded miRNAs carry out post-transcriptional silencing of their target(s). Recent studies identified orthologues of different biogenesis components of novel and conserved small RNAs from different model plants. Although many small RNAs have been identified from diverse plant species, only a handful of them have been functionally characterized. In this review, we discuss the advances made in understanding the biogenesis, functional conservation/divergence in miRNA-mediated gene regulation, and the developmental role of small RNAs in different plant species.

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.

The MicroRNA390/TRANS-ACTING SHORT INTERFERING RNA3 Module Mediates Lateral Root Growth under Salt Stress via the Auxin Pathway

Salt-induced developmental plasticity in a plant root system strongly depends on auxin signaling. However, the molecular events underlying this process are poorly understood. MicroRNA390 (miR390), trans-actin small interfering RNAs (tasiRNAs), and AUXIN RESPONSE FACTORs (ARFs) form a regulatory module involved in controlling lateral root (LR) growth. Here, we found that miR390 expression was strongly induced by exposure to salt during LR formation in poplar (Populus spp.) plants. miR390 overexpression stimulated LR development and increased salt tolerance, whereas miR390 knockdown caused by a short tandem target mimic repressed LR growth and compromised salt resistance. ARF3.1, ARF3.2, and ARF4 expression was inhibited significantly by the presence of salt, and transcript abundance was decreased dramatically in the miR390-overexpressing line but increased in the miR390-knockdown line. Constitutive expression of ARF4m harboring mutated trans-acting small interfering ARF-binding sites removed the salt resistance of the miR390 overexpressors. miR390 positively regulated auxin signaling in LRs subjected to salt, but ARF4 inhibited auxin signaling. Salinity stabilized the poplar Aux/IAA repressor INDOLE-3-ACETIC ACID17.1, and overexpression of an auxin/salt-resistant form of this repressor suppressed LR growth in miR390-overexpressing and ARF4-RNA interfering lines in the presence of salt. Thus, the miR390/TAS3/ARFs module is a key regulator, via modulating the auxin pathway, of LR growth in poplar subjected to salt stress.

Biogenesis and regulatory hierarchy of phased small interfering RNAs in plants

Several varieties of small RNAs including microRNAs (miRNAs) and small interfering RNAs (siRNAs) are generated in plants to regulate development, genome stability and response to adverse environments. Phased siRNA (phasiRNA) is a type of secondary siRNA that is processed from a miRNA-mediated cleavage of RNA transcripts, increasing silencing efficiency or simultaneously suppressing multiple target genes. Trans-acting siRNAs (ta-siRNAs) are a particular class of phasiRNA produced from noncoding transcripts that silence targets in trans. It was originally thought that 'one-hit' and 'two-hit' models were essential for processing distinct TAS precursors; however, a single hit event was recently shown to be sufficient at triggering all types of ta-siRNAs. This review discusses the findings about biogenesis, targeting modes and regulatory networks of plant ta-siRNAs. We also summarize recent advances in the generation of other phasiRNAs and their possible biological benefits to plants.

Transcription factors PRE3 and WOX11 are involved in the formation of new lateral roots from secondary growth taproot in A. thaliana

The spatial deployment of lateral roots determines the ability of a plant to interact with the surrounding environment for nutrition and anchorage. This paper shows that besides the pericycle, the vascular cambium becomes active in Arabidopsis thaliana taproot at a later stage of development and is also able to form new lateral roots. To demonstrate the above, we implemented a two-step approach in which the first step leads to development of a secondary structure in A. thaliana taproot, and the second applies a mechanical stress on the vascular cambium to initiate formation of a new lateral root primordium. GUS staining showed PRE3, DR5 and WOX11 signals in the cambial zone of the root during new lateral root formation. An advanced level of wood formation, characterized by the presence of medullar rays, was achieved. Preliminary investigations suggest the involvement of auxin and two transcription factors (PRE3/ATBS1/bHLH135/TMO7 and WOX11) in the transition of some vascular cambium initials from a role as producers of xylem/phloem mother cells to founder cells of a new lateral root primordium.

Small but powerful: function of microRNAs in plant development

MicroRNAs (miRNAs) are a group of endogenous noncoding small RNAs frequently 21 nucleotides long. miRNAs act as negative regulators of their target genes through sequence-specific mRNA cleavage, translational repression, or chromatin modifications. Alterations of the expression of a miRNA or its targets often result in a variety of morphological and physiological abnormalities, suggesting the strong impact of miRNAs on plant development. Here, we review the recent advances on the functional studies of plant miRNAs. We will summarize the regulatory networks of miRNAs in a series of developmental processes, including meristem development, establishment of lateral organ polarity and boundaries, vegetative and reproductive organ growth, etc. We will also conclude the conserved and species-specific roles of plant miRNAs in evolution and discuss the strategies for further elucidating the functional mechanisms of miRNAs during plant development.

Tomato AUXIN RESPONSE FACTOR 5 regulates fruit set and development via the mediation of auxin and gibberellin signaling

Auxin response factors (ARFs) encode transcriptional factors that function in the regulation of plant development processes. A tomato ARF gene, SlARF5, was observed to be expressed at high levels in emasculated ovaries but maintained low expression levels in pollinated ovaries. The amiRNA SlARF5 lines exhibited ovary growth and formed seedless fruits following emasculation. These parthenocarpic fruits developed fewer locular tissues, and the fruit size and weight were decreased in transgenic lines compared to those of wild-type fruits. Gene expression analysis demonstrated that several genes involved in the auxin-signaling pathway were downregulated, whereas some genes involved in the gibberellin-signaling pathway were enhanced by the decreased SlARF5 mRNA levels in transgenic plants, indicating that SlARF5 may play an important role in regulating both the auxin- and gibberellin-signaling pathways during fruit set and development.

Beyond the genetic code in leaf senescence

Leaf senescence is not only genetically programmed but also induced by exogenous stress to ensure completion of the plant life cycle, successful reproduction and environmental adaptability. Genetic reprogramming is a major aspect of leaf senescence, and the senescence signaling that follows is controlled by a complex regulatory network. Recent studies suggest that the activity of transcription factors together with epigenetic mechanisms ensures the robustness of this network, with the latter including chromatin remodeling, DNA modification, and RNA-mediated control of transcription factors and other senescence-associated genes. In this review, we provide an overview of the relevant epigenetic mechanisms and summarize recent findings of epigenetic regulators of plant leaf senescence involved in DNA methylation and histone modification along with the functions of small RNAs in this process.

Expression dynamics of miRNAs and their targets in seed germination conditions reveals miRNA-ta-siRNA crosstalk as regulator of seed germination

Seed germination paves the way for the dormant embryo to establish itself as a new plant marking the first critical step in postembryonic plant growth and development. Germination starts with the uptake of water (imbibition), followed by induction of transcription, translation, energy metabolism, and cell division processes. Although small RNAs have been implicated in many developmental processes, their role during seed germination stages and conditions remained elusive. Here we show that seed germination conditions, like imbibition and temperature, dynamically regulate the expression of many developmentally important miRNAs and their targets. We have identified 58 miRNAs belonging to 30 different families at different seed germination conditions. Amongst these, 15 miRNAs and their targets were significantly differentially expressed in Arabidopsis seeds in dry and 12 h, 24 h and 48 h of imbibition. Interestingly, differential expression of miR390, which targets trans-acting siRNA locus (TAS3) derived transcripts, resulted in alteration of tasiR-ARF mediated regulation of expression of target AUXIN RESPONSE FACTORs (ARF2/3/4). Our results suggest that the dynamic expression of several miRNAs, their targets, and a crosstalk between miRNA and ta-siRNA pathways contribute to the regulation of seed germination in Arabidopsis thaliana.

Integration of multiple signaling pathways shapes the auxin response

The phytohormone auxin is a pivotal signaling molecule that functions throughout the plant lifecycle. Proper regulation of the auxin response is critical for optimizing plant growth under ever-changing environmental conditions. Recent studies have demonstrated that the signaling components that modulate auxin sensitivity and responses are functionally and mechanically diverse. In addition to auxin itself, various environmental and hormonal signals are integrated to modulate the auxin response through directly controlling auxin signaling components. This review explores the non-canonical mechanisms that modulate auxin signaling components, including transcriptional, translational, and post-translational regulation. All of these contribute to the wide range in sensitivity and complexity in auxin responses to various signaling cues.