Tuning the Orchestra: miRNAs in Plant Immunity

Phosphorylation of AGO1a by MAP kinases is required for miRNA mediated resistance against Xanthomonas oryzae pv. oryzae infection in rice

Bacterial leaf blight is a devastating disease caused by Xanthomonas oryzae pv. oryzae (Xoo) which causes severe crop loss in rice. The molecular mechanism that initiates defense against such pathogens remains unexplored. Reports have suggested crucial role of several miRNAs in regulating immune responses in plants. Argonaute (AGO) proteins have been implicated in imparting immunity against pathogens by using small RNAs as guide molecules. Here, we show that phosphorylation of rice AGO1a by MAP kinases is required for miRNA expression regulation during Xoo infection. AGO1a is induced in response to pathogen infection and is under the control of SA signaling pathway. The pathogen responsive MAP kinases MPK3, MPK4 and MPK6, interact with AGO1a in planta and can phosphorylate the protein in vitro. Overexpression of AGO1a extends disease resistance against Xoo in rice and leads to a higher accumulation of miRNAs. Conversely, overexpression of a non phosphorylatable mutant protein aggravates disease susceptibility and remarkably suppresses the miRNA expression levels. At a molecular level, phosphorylation of AGO1a by MAP kinase is required for increased accumulation of miRNAs during pathogen challenge. Taken together, the data suggests that OsAGO1a is a direct phosphorylation target of MAP kinases and this phosphorylation is crucial for its role in imparting disease resistance.

Integrating omics reveals that miRNA-guided genetic regulation on plant hormone level and defense response pathways shape resistance to Cladosporium fulvum in the tomato Cf-10-gene-carrying line

Invasion of C. fulvum causes the most serious diseases affecting the reproduction of tomatoes. Cf-10-gene-carrying line showed remarkable resistance to Cladosporium fulvum. To exploit its defense response mechanism, we performed a multiple-omics profiling of Cf-10-gene-carrying line and a susceptible line without carrying any resistance genes at non-inoculation and 3 days post-inoculation (dpi) of C. fulvum. We detected 54 differentially expressed miRNAs (DE-miRNAs) between the non-inoculation and 3 dpi in the Cf-10-gene-carrying line, which potentially regulated plant-pathogen interaction pathways and hormone signaling pathways. We also revealed 3,016 differentially expressed genes (DEGs) between the non-inoculated and 3 dpi in the Cf-10-gene-carrying line whose functions enriched in pathways that were potentially regulated by the DE-miRNAs. Integrating DE-miRNAs, gene expression and plant-hormone metabolites indicated a regulation network where the downregulation of miRNAs at 3 dpi activated crucial resistance genes to trigger host hypersensitive cell death, improved hormone levels and upregulated the receptors/critical responsive transcription factors (TFs) of plant hormones, to shape immunity to the pathogen. Notably, our transcriptome, miRNA and hormone metabolites profiling and qPCR analysis suggested that that the downregulation of miR9472 potentially upregulated the expression of SAR Deficient 1 (SARD1), a key regulator for ICS1 (Isochorismate Synthase 1) induction and salicylic acid (SA) synthesis, to improve the level of SA in the Cf-10-gene-carrying line. Our results exploited potential regulatory network and new pathways underlying the resistance to C. fulvum in Cf-10-gene-carrying line, providing a more comprehensive genetic circuit and valuable gene targets for modulating resistance to the virus.

Intrinsically disordered proteins SAID1/2 condensate on SERRATE for dual inhibition of miRNA biogenesis in Arabidopsis

Intrinsically disordered proteins (IDPs) SAID1/2 are hypothetic dentin sialophosphoprotein-like proteins, but their true functions are unknown. Here, we identified SAID1/2 as negative regulators of SERRATE (SE), a core factor in miRNA biogenesis complex (microprocessor). Loss-of-function double mutants of said1; said2 caused pleiotropic developmental defects and thousands of differentially expressed genes that partially overlapped with those in se. said1; said2 also displayed increased assembly of microprocessor and elevated accumulation of microRNAs (miRNAs). Mechanistically, SAID1/2 promote pre-mRNA processing 4 kinase A-mediated phosphorylation of SE, causing its degradation in vivo. Unexpectedly, SAID1/2 have strong binding affinity to hairpin-structured pri-miRNAs and can sequester them from SE. Moreover, SAID1/2 directly inhibit pri-miRNA processing by microprocessor in vitro. Whereas SAID1/2 did not impact SE subcellular compartmentation, the proteins themselves exhibited liquid-liquid phase condensation that is nucleated on SE. Thus, we propose that SAID1/2 reduce miRNA production through hijacking pri-miRNAs to prevent microprocessor activity while promoting SE phosphorylation and its destabilization in Arabidopsis.

Positional cloning and characterization reveal the role of a miRNA precursor gene ZmLRT in the regulation of lateral root number and drought tolerance in maize

Lateral roots play essential roles in drought tolerance in maize (Zea mays L.). However, the genetic basis for the variation in the number of lateral roots in maize remains elusive. Here, we identified a major quantitative trait locus (QTL), qLRT5-1, controlling lateral root number using a recombinant inbred population from a cross between the maize lines Zong3 (with many lateral roots) and 87-1 (with few lateral roots). Fine-mapping and functional analysis determined that the candidate gene for qLRT5-1, ZmLRT, expresses the primary transcript for the microRNA miR166a. ZmLRT was highly expressed in root tips and lateral root primordia, and knockout and overexpression of ZmLRT increased and decreased lateral root number, respectively. Compared with 87-1, the ZmLRT gene model of Zong3 lacked the second and third exons and contained a 14 bp deletion at the junction between the first exon and intron, which altered the splicing site. In addition, ZmLRT expression was significantly lower in Zong3 than in 87-1, which might be attributed to the insertions of a transposon and over large DNA fragments in the Zong3 ZmLRT promoter region. These mutations decreased the abundance of mature miR166a in Zong3, resulting in increased lateral roots at the seedling stage. Furthermore, miR166a post-transcriptionally repressed five development-related class-III homeodomain-leucine zipper genes. Moreover, knockout of ZmLRT enhanced drought tolerance of maize seedlings. Our study furthers our understanding of the genetic basis of lateral root number variation in maize and highlights ZmLRT as a target for improving drought tolerance in maize.

Identification of miRNAs Involving Potato-Phytophthora infestans Interaction

sRNAs (small RNAs) play an important role in regulation of plant immunity against a variety of pathogens. In this study, sRNA sequencing analysis was performed to identify miRNAs (microRNAs) during the interaction of potato and Phytophthora infestans. Totally, 171 potato miRNAs were identified, 43 of which were annotated in the miRNA database and 128 were assigned as novel miRNAs in this study. Those potato miRNAs may target 878 potato genes and half of them encode resistance proteins. Fifty-three potato miRNAs may target 194 P. infestans genes. Three potato miRNAs (novel 72, 133, and 140) were predicted to have targets only in the P. infestans genome. miRNAs transient expression and P. infestans inoculation assay showed that miR396, miR166, miR6149-5P, novel133, or novel140 promoted P. infestans colonization, while miR394 inhibited colonization on Nicotiana benthamiana leaves. An artificial miRNA target (amiRNA) degradation experiment demonstrated that miR394 could target both potato gene (PGSC0003DMG400034305) and P. infestans genes. miR396 targets the multicystatin gene (PGSC0003DMG400026899) and miR6149-5p could shear the galactose oxidase F-box protein gene CPR30 (PGSC0003DMG400021641). This study provides new information on the aspect of cross-kingdom immune regulation in potato-P. infestans interaction at the sRNAs regulation level.

What Do We Know about Barley miRNAs?

Plant miRNAs are powerful regulators of gene expression at the post-transcriptional level, which was repeatedly proved in several model plant species. miRNAs are considered to be key regulators of many developmental, homeostatic, and immune processes in plants. However, our understanding of plant miRNAs is still limited, despite the fact that an increasing number of studies have appeared. This systematic review aims to summarize our current knowledge about miRNAs in spring barley (Hordeum vulgare), which is an important agronomical crop worldwide and serves as a common monocot model for studying abiotic stress responses as well. This can help us to understand the connection between plant miRNAs and (not only) abiotic stresses in general. In the end, some future perspectives and open questions are summarized.

MicroRNA-mediated host defense mechanisms against pathogens and herbivores in rice: balancing gains from genetic resistance with trade-offs to productivity potential

BACKGROUND

Rice (Oryza sativa L.) is the major source of daily caloric intake for more than 30% of the human population. However, the sustained productivity of this staple food crop is continuously threatened by various pathogens and herbivores. Breeding has been successful in utilizing various mechanisms of defense by gene pyramiding in elite cultivars, but the continuous resurgence of highly resistant races of pathogens and herbivores often overcomes the inherent capacity of host plant immunity. MicroRNAs (miRNAs) are endogenous, short, single-stranded, non-coding RNA molecules that regulate gene expression by sequence-specific cleavage of target mRNA or suppressing target mRNA translation. While miRNAs function as upstream regulators of plant growth, development, and host immunity, their direct effects on growth and development in the context of balancing defenses with agronomic potential have not been extensively discussed and explored as a more viable strategy in breeding for disease and pest resistant cultivars of rice with optimal agronomic potentials.

RESULTS

Using the available knowledge in rice and other model plants, this review examines the important roles of miRNAs in regulating host responses to various fungal, bacterial, and viral pathogens, and insect pests, in the context of gains and trade-offs to crop yield. Gains from R-gene-mediated resistance deployed in modern rice cultivars are often undermined by the rapid breakdown of resistance, negative pleiotropic effects, and linkage drags with undesirable traits. In stark contrast, several classes of miRNAs are known to efficiently balance the positive gains from host immunity without significant costs in terms of losses in agronomic potentials (i.e., yield penalty) in rice. Defense-related miRNAs such as Osa-miR156, Osa-miR159, Osa-miR162, Osa-miR396, Osa-530, Osa-miR1432, Osa-miR1871, and Osa-miR1873 are critical in fine-tuning and integrating immune responses with physiological processes that are necessary to the maintenance of grain yield. Recent research has shown that many defense-related miRNAs regulate complex and agronomically important traits.

CONCLUSIONS

Identification of novel immune-responsive miRNAs that orchestrate physiological processes critical to the full expression of agronomic potential will facilitate the stacking of optimal combinations of miRNA-encoding genes to develop high-yielding cultivars with durable resistance to disease and insect pests with minimal penalties to yield.

In silico identification of sugarcane (Saccharum officinarum L.) genome encoded microRNAs targeting sugarcane bacilliform virus

Sugarcane bacilliform virus (SCBV) is considered one of the most economically damaging pathogens for sugarcane production worldwide. Three open reading frames (ORFs) are characterized in the circular, ds-DNA genome of the SCBV; these encode for a hypothetical protein (ORF1), a DNA binding protein (ORF2), and a polyprotein (ORF3). A comprehensive evaluation of sugarcane (Saccharum officinarum L.) miRNAs for the silencing of the SCBV genome using in silico algorithms were carried out in the present study using mature sugarcane miRNAs. miRNAs of sugarcane are retrieved from the miRBase database and assessed in terms of hybridization with the SCBV genome. A total of 14 potential candidate miRNAs from sugarcane were screened out by all used algorithms used for the silencing of SCBV. The consensus of three algorithms predicted the hybridization site of sof-miR159e at common locus 5534. miRNA-mRNA interactions were estimated by computing the free-energy of the miRNA-mRNA duplex using the RNAcofold algorithm. A regulatory network of predicted candidate miRNAs of sugarcane with SCBV-ORFs, generated using Circos-is used to identify novel targets. The predicted data provide useful information for the development of SCBV-resistant sugarcane plants.

A Novel DCL2-Dependent Micro-Like RNA Vm-PC-3p-92107_6 Affects Pathogenicity by Regulating the Expression of Vm-VPS10 in Valsa mali

Dicer proteins are mainly responsible for generating small RNAs (sRNAs), which are involved in gene silencing in most eukaryotes. In previous research, two DCL proteins in Valsa mali, the pathogenic fungus causing apple tree Valsa canker, were found associated with both the pathogenicity and generation of sRNAs. In this study, the differential expression of small interfering RNAs (siRNAs) and miRNA-like RNAs (milRNAs) was analyzed based on the deep sequencing of the wild type and Vm-DCL2 mutant, respectively. Overall, the generation of 40 siRNAs and 18 milRNAs was evidently associated with Vm-DCL2. The target genes of milRNAs were then identified using degradome sequencing; according to the prediction results, most candidate targets are related to pathogenicity. Further, expression of Vm-PC-3p-92107_6 was confirmed in the wild type but not in the Vm-DCL2 mutant. Moreover, the pathogenicity of Vm-PC-3p-92107_6 deletion mutants (ΔVm-PC-3p-92107_6) and the over-expression transformants (Vm-PC-3p-92107_6-OE) was significantly increased and decreased, respectively. Based on those degradome results, vacuolar protein sorting 10 (Vm-VPS10) was identified as the target of Vm-PC-3p-92107_6. Co-expression analysis in tobacco leaves further confirmed that Vm-PC-3p-92107_6 could suppress the expression of Vm-VPS10. Meanwhile, the expression levels of Vm-PC-3p-92107_6 and Vm-VPS10 displayed divergent trends in ΔVm-PC-3p-92107_6 and Vm-PC-3p-92107_6-OE, respectively. Perhaps most importantly, ΔVm-VPS10 featured a significant reduction in pathogenicity. Taken together, our results indicate that a DCL2-dependent milRNA Vm-PC-3p-92107_6 plays roles in pathogenicity by regulating the expression of Vm-VPS10. This study lays a foundation for the comprehensive analysis of pathogenic mechanisms of V. mali and deepens our understanding of the generation and function of fungal sRNA.

Roles of small RNAs in crop disease resistance

Small RNAs (sRNAs) are a class of short, non-coding regulatory RNAs that have emerged as critical components of defense regulatory networks across plant kingdoms. Many sRNA-based technologies, such as host-induced gene silencing (HIGS), spray-induced gene silencing (SIGS), virus-induced gene silencing (VIGS), artificial microRNA (amiRNA) and synthetic trans-acting siRNA (syn-tasiRNA)-mediated RNA interference (RNAi), have been developed as disease control strategies in both monocot and dicot plants, particularly in crops. This review aims to highlight our current understanding of the roles of sRNAs including miRNAs, heterochromatic siRNAs (hc-siRNAs), phased, secondary siRNAs (phasiRNAs) and natural antisense siRNAs (nat-siRNAs) in disease resistance, and sRNAs-mediated trade-offs between defense and growth in crops. In particular, we focus on the diverse functions of sRNAs in defense responses to bacterial and fungal pathogens, oomycete and virus in crops. Further, we highlight the application of sRNA-based technologies in protecting crops from pathogens. Further research perspectives are proposed to develop new sRNAs-based efficient strategies to breed non-genetically modified (GMO), disease-tolerant crops for sustainable agriculture.

Dynamics of miRNA mediated regulation of legume symbiosis

Symbiotic nitrogen fixation in legume nodules is important in soils with low nitrogen availability. The initiation and sustainability of symbiosis require cellular reprogramming that involves the miRNA-mediated inhibition or activation of specific nodulation genes. The high-throughput sequencing of small RNA libraries has identified miRNAs and their targets, which are the major players in the post-transcriptional gene regulation (PTGS) of the different stages of legume-rhizobia symbiosis ranging from bacterial colonization and organogenesis to symbiotic nitrogen fixation. Here, we present an overview of information obtained from the miRNA libraries from nodulating tissues that have been sequenced to date. The functional analysis of miRNAs has revealed roles in phytohormone homeostasis and spatio-temporal regulation, as well as the mobility of miRNAs and their functions in shoot to root signalling that affects diverse functions, including bacterial entry, meristem division and differentiation, nitrogen fixation and senescence. Furthermore, small RNA fragments of rhizobial origin repress complementary plant mRNAs. We also consider the roles of miRNAs in determinate or indeterminate nodules. Taken together, this overview confirms that miRNAs are master regulators of the legume-rhizobia symbiosis.

High-Throughput Sequencing-Based Identification of Arabidopsis miRNAs Induced by Phytophthora capsici Infection

MicroRNAs (miRNAs) are a group of small non-coding endogenous RNAs. In plants, miRNAs play vital functions in regulating growth, development, and stress response. However, the role of miRNAs in Arabidopsis-Phytophthora capsici (P. capsici) model pathosystem is poorly understood. Here, we used a high-throughput sequencing approach to identify pathogen-responsive miRNAs using 15 small RNA (sRNA) libraries prepared from Arabidopsis thaliana leaves collected at 0, 3, 6, 12, and 24 h post-inoculation with P. capsici. A total of 293 known miRNAs and 6 potential novel sRNAs (miRNAs or siRNAs) were identified, of which 33 miRNAs were differentially expressed at four different infection stages. To verify the reliability of the sRNA-seq results, we investigated the expression of five sRNAs upregulated throughout the four infection stages and their potential target genes using northern blot analysis and/or stem-loop quantitative real-time polymerase chain reaction (qRT-PCR). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses revealed that the potential target genes of the differentially expressed miRNAs, both conserved and novel, were enriched in pathways such as starch and sugar metabolism, spliceosome, and plant-pathogen interaction, indicating that the splicing machinery and pathogenesis-related (PR) proteins play important roles in the response to P. capsici infection. Taken together, these results provide novel insights into the molecular mechanisms of pathogenesis by P. capsici. Additionally, these results will serve as a strong foundation for further in-depth analysis of miRNAs involved in the resistance to Phytophthora species in other crops.

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.

Analysis of Small RNAs of Barley Genotypes Associated with Resistance to Barley Yellow Dwarf Virus

Barley yellow dwarf virus (BYDV) causes an often-devastating disease of cereals that is most effectively controlled by using plant genotypes that are resistant or tolerant to the virus. New barley lines Vir8:3 and Vir13:8, with pyramided resistance genes against different pathogens and resistance gene Ryd2 against BYDV, are currently being tested. Because microRNAs (miRNAs) are associated with antiviral plant defense, here we compared the miRNA profiles in these lines and in cultivar Wysor (carrying one resistance gene, Ryd2), with and without BYDV infection and after feeding by virus-free aphids, to determine whether the miRNA profile in the resistant variety bear similarities with the newly developed lines. The BYDV titer for each group was also determined and compared to the titer in sensitive cultivar Graciosa. Among 746 miRNAs identified in barley, 66 were known miRNAs, and 680 were novel. The expression of 73 miRNAs differed significantly after BYDV infection, including the strong, specific upregulation of novel miRNA10778 that was conserved across all the barley genotypes. This miRNA belongs to the H box and ACA box (H/ACA) snoR14 family of RNAs (Rf01280) and is associated with pseudourydilation. The expression of 48 miRNAs also differed depending on the barley genotype. The profile of miRNAs expressed in Vir8:3 and Vir13:8 in response to BYDV was similar and differed from that of Wysor. Insights into the expression patterns of miRNAs in response to BYDV in barley provided here will benefit further studies toward understanding the resistance mechanisms and developing novel strategies against virus infections.

High-Throughput Sequencing-Based Identification of Arabidopsis miRNAs Induced by Phytophthora capsici Infection

MicroRNAs (miRNAs) are a group of small non-coding endogenous RNAs. In plants, miRNAs play vital functions in regulating growth, development, and stress response. However, the role of miRNAs in Arabidopsis-Phytophthora capsici (P. capsici) model pathosystem is poorly understood. Here, we used a high-throughput sequencing approach to identify pathogen-responsive miRNAs using 15 small RNA (sRNA) libraries prepared from Arabidopsis thaliana leaves collected at 0, 3, 6, 12, and 24 h post-inoculation with P. capsici. A total of 293 known miRNAs and 6 potential novel sRNAs (miRNAs or siRNAs) were identified, of which 33 miRNAs were differentially expressed at four different infection stages. To verify the reliability of the sRNA-seq results, we investigated the expression of five sRNAs upregulated throughout the four infection stages and their potential target genes using northern blot analysis and/or stem-loop quantitative real-time polymerase chain reaction (qRT-PCR). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses revealed that the potential target genes of the differentially expressed miRNAs, both conserved and novel, were enriched in pathways such as starch and sugar metabolism, spliceosome, and plant-pathogen interaction, indicating that the splicing machinery and pathogenesis-related (PR) proteins play important roles in the response to P. capsici infection. Taken together, these results provide novel insights into the molecular mechanisms of pathogenesis by P. capsici. Additionally, these results will serve as a strong foundation for further in-depth analysis of miRNAs involved in the resistance to Phytophthora species in other crops.

The Role of MicroRNAs in Genome Response to Plant-Lepidoptera Interaction

RNA interference is a known phenomenon of plant immune responses, involving the regulation of gene expression. The key components triggering the silencing of targeted sequences are double-stranded RNA molecules. The regulation of host-pathogen interactions is controlled by miRNA molecules, which regulate the expression of host resistance genes or the genes of the pathogen. The review focused on basic principles of RNA interference as a gene-silencing-based defense mechanism and the role of miRNA molecules in insect genomes. RNA interference as a tool for plant protection management is discussed. The review summarizes current miRNA-based biotechnology approaches for plant protection management.