MiR1918 enhances tomato sensitivity to Phytophthora infestans infection

The lncRNA20718-miR6022-RLPs module regulates tomato resistance to Phytophthora infestans

KEY MESSAGE

Sl-lncRNA20718 acts as an eTM of Sl-miR6022 regulating its expression thereby affecting SlRLP6/10 expression. SlRLP6/10 regulate PRs expression, ROS accumulation, and JA/ET content thereby affecting tomato resistance to P. infestans. Tomato (Solanum lycopersicum) is an important horticultural and cash crop whose yield and quality can be severely affected by Phytophthora infestans (P. infestans). Long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) are widely involved in plant defense responses against pathogens. The involvement of Sl-lncRNA20718 and Sl-miR6022 in tomato resistance to P. infestans as well as the targeting of Sl-miR6022 to receptor-like protein genes (RLPs) were predicted in our previous study. However, uncertainty exists regarding their potential interaction as well as the molecular processes regulating tomato resistance. Here, we found that Sl-lncRNA20718 and Sl-miR6022 are positive and negative regulators of tomato resistance to P. infestans by gain- and loss-of-function experiments, respectively. Overexpression of Sl-lncRNA20718 decreased the expression of Sl-miR6022, induced the expression of PRs, reduced the diameter of lesions (DOLs), thereby enhanced disease resistance. A six-point mutation in the binding region of Sl-lncRNA20718 to Sl-miR6022 disabled the interaction, indicating that Sl-lncRNA20718 acts as an endogenous target mimic (eTM) of Sl-miR6022. We demonstrated that Sl-miR6022 cleaves SlRLP6/10. Overexpression of Sl-miR6022 decreases the expression levels of SlRLP6/10, induces the accumulation of reactive oxygen species (ROS) and reduces the content of JA and ET, thus inhibiting tomato resistance to P. infestans. In conclusion, our study provides detailed information on the lncRNA20718-miR6022-RLPs module regulating tomato resistance to P. infestans by affecting the expression of disease resistance-related genes, the accumulation of ROS and the phytohormone levels, providing a new reference for tomato disease resistance breeding.

Identification of nitric oxide mediated defense signaling and its microRNA mediated regulation during Phytophthora capsici infection in black pepper

Nitric oxide plays a significant role in the defense signaling during pathogen interaction in plants. Quick wilt disease is a devastating disease of black pepper, and leads to sudden mortality of pepper vines in plantations. In this study, the role of nitric oxide was studied during Phytophthora capsici infection in black pepper variety Panniyur-1. Nitric oxide was detected from the different histological sections of P. capsici infected leaves. Furthermore, the genome-wide transcriptome analysis characterized typical domain architect and structural features of nitrate reductase (NR) and nitric oxide associated 1 (NOA1) gene that are involved in nitric oxide biosynthesis in black pepper. Despite the upregulation of nitrate reductase (Pn1_NR), a reduced expression of Pn1_NOA1 was detected in the P. capsici infected black pepper leaf. Subsequent sRNAome-assisted in silico analysis revealed possible microRNA mediated regulation of Pn1_NOA mRNAs. Furthermore, sRNA/miRNA mediated cleavage on Pn1_NOA1 mRNA was validated through modified 5' RLM RACE experiments. Several hormone-responsive cis-regulatory elements involved in stress response was detected from the promoter regions of Pn_NOA1, Pn_NR1 and Pn_NR2 genes. Our results revealed the role of nitric oxide during stress response of P. capsici infection in black pepper, and key genes involved in nitric oxide biosynthesis and their post-transcriptional regulatory mechanisms.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01414-z.

MicroRNA Expression Profiles in Response to Phytophthora infestans and Oidium neolycopersici and Functional Identification of sly-miR397 in Tomato

Late blight and powdery mildew are two widespread tomato diseases caused by Phytophthora infestans and Oidium neolycopersici, respectively, which reduce the quantity and quality of tomato. MicroRNAs (miRNAs) play critical roles in tomato resistance to various pathogens. Investigating the function of miRNAs is of great significance in controlling tomato diseases. To identify potential miRNAs involved in the interaction of tomato with P. infestans or O. neolycopersici, we analyzed the expression profiles of small RNAs in tomato leaves infected with these two pathogens using RNA-seq technology. A total of 330 and 288 miRNAs exhibited differences in expression levels after exposure to P. infestans and O. neolycopersici, respectively. One hundred and forty-six commonly differentially expressed (DE) miRNAs responsive to P. infestans and O. neolycopersici infestation were detected, including 10 commonly known conserved DE miRNAs and 136 novel miRNAs. Among these known DE miRNAs, sly-miR397 was strongly downregulated in response to P. infestans or O. neolycopersici infection. Silencing of sly-miR397 resulted in enhanced tolerance to the pathogens, whereas overexpression of sly-miR397 showed increased susceptibility. Furthermore, changes in sly-miR397 expression could also affect expression levels of pathogenesis-related genes and reactive oxygen species-scavenging genes, leading to altered necrotic cells and H₂O₂ levels. In addition, the number of lateral branches significantly changed in transgenic plants. Taken together, our results provide potential miRNA resources for further research of miRNA-disease associations and indicates that sly-miR397 acts as a negative regulator of disease resistance and influences lateral branch development in tomato.

Overexpression of lncRNA08489 enhances tomato immunity against Phytophthora infestans by decoying miR482e-3p

LncRNAs are widely involved in various biological processes of plants. Recent evidences indicated that lncRNAs could act as competing endogenous RNAs (ceRNAs) to adsorb complementary miRNAs in a type of target mimicry, thereby indirectly regulating the target genes of miRNAs. In this study, a lncRNA, lncRNA08489 was identified to be the ceRNA of miR482e-3p in tomato plants. The expression patterns of lncRNA08489 and miR482e-3p showed opposite trends after tomato plants infected with Phytophthora infestans. In tomato leaves overexpressing lncRNA08489 (OE08489), the expression level of miR482e-3p decreased and its target gene, NBS-LRR increased. After infection with P. infestans, the resistance of OE08489 plants was stronger than that of the wild type, and the reactive oxygen species (ROS) scavenging ability of OE08489 plants was significantly improved. Taken together, these results indicated that lncRNA08489 acted as a ceRNA to decoy miR482e-3p and regulate the expression of NBS-LRR to enhance tomato resistance through ROS-scavenging system.

Identification and functional deciphering suggested the regulatory roles of long intergenic ncRNAs (lincRNAs) in increasing grafting pepper resistance to Phytophthora capsici

BACKGROUND

As a popular and valuable technique, grafting is widely used to protect against soil-borne diseases and nematodes in vegetable production. Growing evidences have revealed that long intergenic ncRNAs (lincRNAs) are strictly regulated and play essential roles in plants development and stress responses. Nevertheless, genome-wide identification and function deciphering of pepper lincRNAs, especially for their roles in improving grafting pepper resistance to Phytophthora capsici is largely unknown.

RESULTS

In this study, RNA-seq data of grafting and control pepper plants with or without P. capsici inoculation were used to identify lincRNAs. In total, 2,388 reliable lincRNAs were identified. They were relatively longer and contained few exons than protein-coding genes. Similar to coding genes, lincRNAs had higher densities in euchromatin regions; and longer chromosome transcribed more lincRNAs. Expression pattern profiling suggested that lincRNAs commonly had lower expression than mRNAs. Totally, 607 differentially expressed lincRNAs (DE-lincRANs) were identified, of which 172 were found between P. capsici resistance grafting pepper sample GR and susceptible sample LDS. The neighboring genes of DE-lincRNAs and miRNAs competitively sponged by DE-lincRNAs were identified. Subsequently, the expression level of DE-lincRNAs was further confirmed by qRT-PCR and regulation patterns between DE-lincRNAs and neighboring mRNAs were also validated. Function annotation revealed that DE-lincRNAs increased the resistance of grafting prepper to P. capsici by modulating the expression of disease-defense related genes through cis-regulating and/or lincRNA-miRNA-mRNA interaction networks.

CONCLUSIONS

This study identified pepper lincRNAs and suggested their potential roles in increasing the resistance level of grafting pepper to P. capsici.

Function identification of miR394 in tomato resistance to Phytophthora infestans

MiR394 plays a negative role in tomato resistance to late blight. The lncRNA40787 severing as an eTM for miR394 to regulate LCR and exerting functions in tomato resistance. Tomato (Solanum lycopersicum), which was used as model species for studying the mechanism of plant disease defense, is susceptible to multiple pathogens. Non-coding RNA (ncRNA) has a pivotal role in plants response to biological stresses. It has previously been observed that the expression level of miR394 changed significantly after the infection of various pathogens. However, there has been no detailed investigation of the accumulated or suppressed mechanism of miR394. Our previous study predicted three lncRNAs (lncRNA40787, lncRNA27177, and lncRNA42566) that contain miR394 endogenous target mimics (eTM), which may exist as the competitive endogenous RNAs (ceRNAs) of miR394. In our study, the transcription levels of these three lncRNAs were strongly up-regulated in tomato upon infection with P. infestans. In contrast with the three lncRNAs, the accumulation of miR394 was significantly suppressed. Based on the expression pattern, and value of minimum free energy (mfes) that represents the binding ability between lncRNA and miRNA, lncRNA40787 was chosen for further investigation. Results showed that overexpression of lncRNA40787 reduced the expression of miR394 along with decreased lesion area and enhanced disease resistance. Overexpression of miR394, however, decreased the expression of its target gene Leaf Curling Responsiveness (LCR), and suppressed the synthesis components genes of jasmonic acid (JA), depressing the resistance of tomato to P. infestans infection. Taken together, our findings indicated that miR394 can be decoyed by lncRNA40787, and negatively regulated the expression of LCR to enhance tomato susceptibility under P. infestans infection. Our study provided detailed information on the lncRNA40787-miR394-LCR regulatory network and serves as a reference for future research.

Late blight in tomato: insights into the pathogenesis of the aggressive pathogen Phytophthora infestans and future research priorities

This review provides insights into the molecular interactions between Phytophthora infestans and tomato and highlights research gaps that need further attention. Late blight in tomato is caused by the oomycota hemibiotroph Phytophthora infestans, and this disease represents a global threat to tomato farming. The pathogen is cumbersome to control because of its fast-evolving nature, ability to overcome host resistance and inefficient natural resistance obtained from the available tomato germplasm. To achieve successful control over this pathogen, the molecular pathogenicity of P. infestans and key points of vulnerability in the host plant immune system must be understood. This review primarily focuses on efforts to better understand the molecular interaction between host pathogens from both perspectives, as well as the resistance genes, metabolomic changes, quantitative trait loci with potential for improvement in disease resistance and host genome manipulation via transgenic approaches, and it further identifies research gaps and provides suggestions for future research priorities.

Bacillus subtilis SL18r Induces Tomato Resistance Against Botrytis cinerea, Involving Activation of Long Non-coding RNA, MSTRG18363, to Decoy miR1918

Mounting evidence has indicated that beneficial rhizobacteria can suppress foliar pathogen invasion via elicitation of induced systemic resistance (ISR). However, it remains elusive whether long non-coding RNAs (lncRNAs) are involved in the mediation of the rhizobacteria-primed ISR processes in plants. Herein, we demonstrated the ability of the rhizobacterial strain Bacillus subtilis SL18r to trigger ISR in tomato plants against the foliar pathogen Botrytis cinerea. Comparative transcriptome analysis was conducted to screen differentially expressed lncRNAs (DELs) between the non-inoculated and SL18r-inoculated plants. Among these DELs, four variants of MSTRG18363 possessed conserved binding sites for miR1918, which negatively regulates immune systems in tomato plants. The expression of MSTRG18363 in tomato leaves was significantly induced by SL18r inoculation. The transcription of MSTRG18363 was negatively correlated with the expression of miR1918, but displayed a positive correlation with the transcription of the RING-H2 finger gene SlATL20 (a target gene of miR1918). Moreover, MSTRG18363-overexpressing plants exhibited the enhanced disease resistance, reduction of miR1918 transcripts, and marked increases of SlATL20 expression. However, the SL18r-induced disease resistance was largely impaired in the MSTRG18363-silenced plants. VIGS-mediated SlATL20 silencing also greatly weakened the SL18r-induced disease resistance. Collectively, our results suggested that induction of MSTRG18363 expression in tomato plants by SL18r was conducive to promoting the decoy of miR1918 and regulating the expression of SlATL20, thereby provoking the ISR responses against foliar pathogen infection.

Novel targets for engineering Physostegia chlorotic mottle and tomato brown rugose fruit virus-resistant tomatoes: in silico prediction of tomato microRNA targets

Background

Physostegia chlorotic mottle virus (PhCMoV; genus: Alphanucleorhabdovirus, family: Rhabdoviridae) and tomato brown rugose fruit virus (ToBRFV; genus: Tobamovirus, family: Virgaviridae) are newly emerging plant viruses that have a dramatic effect on tomato production. Among various known virus-control strategies, RNAi-mediated defence has shown the potential to protect plants against various pathogens including viral infections. Micro(mi)RNAs play a major role in RNAi-mediated defence.

Methods

Using in silico analyses, we investigated the possibility of tomato-encoded miRNAs (TomiRNA) to target PhCMoV and ToBRFV genomes using five different algorithms, i.e., miRanda, RNAhybrid, RNA22, Tapirhybrid and psRNATarget.

Results

The results revealed that 14 loci on PhCMoV and 10 loci on ToBRFV can be targeted by the TomiRNAs based on the prediction of at least three algorithms. Interestingly, one TomiRNA, miR6026, can target open reading frames from both viruses, i.e., the phosphoprotein encoding gene of PhCMoV, and the two replicase components of ToBRFV. There are currently no commercially available PhCMoV- or ToBRFV-resistant tomato varieties, therefore the predicted data provide useful information for the development of PhCMoV- and ToBFRV-resistant tomato plants.

Negligible transcriptome and metabolome alterations in RNAi insecticidal maize against Monolepta hieroglyphica

RNAi-based genetically modified maize resistant to Monolepta hieroglyphica (Motschulsky) was demonstrated with negligible transcriptome and metabolome alterations compared to its unmodified equivalent. As one of the most prevalent insect pests afflicting various crops, Monolepta hieroglyphica (Motschulsky) causes severe loss of agricultural and economic productivity for many years in China. In an effort to reduce damages, in this study, an RNA interference (RNAi)-based genetically modified (GM) maize was developed. It was engineered to produce MhSnf7 double-stranded RNAs (dsRNAs), which can suppress the Snf7 gene expression and then lead M. hieroglyphica to death. Field trail analysis confirmed the robustly insecticidal ability of the MhSnf7 GM maize to resist damages by M. hieroglyphica. RNA sequencing analysis identified that only one gene was differentially expressed in the MhSnf7 GM maize compared to non-GM maize, indicating that the transcriptome in MhSnf7 GM maize is principally unaffected by the introduction of the MhSnf7 dsRNA expression vector. Likewise, metabolomics analysis identified that only 8 out of 5787 metabolites were significantly changed. Hence, the integration of transcriptomics and metabolomics demonstrates that there are negligible differences between MhSnf7 GM maize and its unmodified equivalent. This study not only presents a comprehensive assessment of cellular alteration in terms of gene transcription and metabolite abundance in RNAi-based GM maize, but also could be used as a reference for evaluating the unintended effect of GM crops.

Transcriptomic and functional analyses uncover the regulatory role of lncRNA000170 in tomato multicellular trichome formation

Trichomes are universal specific structures originating from nearly all terrestrial plants. Although quantities of long non-coding RNAs (lncRNAs) have been identified in many plant species, the role of lncRNAs in trichome formation still remains unknown. Here, we identified a total of 1303 lncRNAs in the young stems of woolly mutant LA3560 (Wo) and its non-woolly segregants (WT). Out of these lncRNAs, 86 lncRNAs were obviously upregulated in Wo and 110 lncRNAs were downregulated. We determined that seven lncRNAs were highly expressed in stem trichomes compared to trichome-free stems and several other tissues of LA3560 by a quantitative reverse transcriptase-polymerase chain reaction, including lncRNA000746, lncRNA000170, lncRNA000277, lncRNA000774, lncRNA000756, lncRNA000100, and lncRNA000898. Transgenic experiments revealed that overexpression of lncRNA000170 inhibited type I trichome formation on the lower stems of the adult transgenic plants. We further determined that lncRNA000170 was transcribed from the complementary strand of Solyc10g006360, for which expression can be induced by lncRNA000170 in its overexpression lines and woolly mutants. Solyc10g006360 overexpression also caused type I trichome decrease. In addition, several trichome regulators, such as Wo, H, SlCycB2, and SlCycB3, were markedly downregulated in lncRNA000170 overexpression lines. These findings demonstrate that lncRNA000170 may be involved in the regulatory pathway mediated by these trichome regulators.

Sl-lncRNA15492 interacts with Sl-miR482a and affects Solanum lycopersicum immunity against Phytophthora infestans

Long non-coding RNAs (lncRNAs) are involved in the resistance of plants to infection by pathogens via interactions with microRNAs (miRNAs). Long non-coding RNAs are cleaved by miRNAs to produce phased small interfering RNAs (phasiRNAs), which, as competing endogenous RNAs (ceRNAs), function as decoys for mature miRNAs, thus inhibiting their expression, and contain pre-miRNA sequences to produce mature miRNAs. However, whether lncRNAs and miRNAs mediate other molecular mechanisms during plant resistance to pathogens is unknown. In this study, as a positive regulator, Sl-lncRNA15492 from tomato (Solanum lycopersicum Zaofen No. 2) plants affected tomato resistance to Phytophthora infestans. Gain- and loss-of-function experiments and RNA ligase-mediated 5'-amplification of cDNA ends (RLM-5' RACE) also revealed that Sl-miR482a was negatively involved in tomato resistance by targeting Sl-NBS-LRR genes and that silencing of Sl-NBS-LRR1 decreased tomato resistance. Sl-lncRNA15492 inhibited the expression of mature Sl-miR482a, whose precursor was located within the antisense sequence of Sl-lncRNA15492. Further degradome analysis and additional RLM-5' RACE experiments verified that mature Sl-miR482a could also cleave Sl-lncRNA15492. These results provide a mechanism by which lncRNAs might inhibit precursor miRNA expression through antisense strands of lncRNAs, and demonstrate that Sl-lncRNA15492 and Sl-miR482a mutually inhibit the maintenance of Sl-NBS-LRR1 homeostasis during tomato resistance to P. infestans.

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.

Genome-Wide Identification of lncRNAs and Analysis of ceRNA Networks During Tomato Resistance to Phytophthora infestans

Our previous studies have revealed the function of long noncoding RNAs (lncRNAs) and microRNAs (miRNAs) in tomato in response to Phytophthora infestans infection. However, the interaction relationships between lncRNAs and miRNAs during tomato resistance to P. infestans infection are unknown. In this study, 9,011 lncRNAs were identified from tomato plants, including 115 upregulated and 81 downregulated lncRNAs. Among these, 148 were found to be differentially expressed and might affect the expression of 771 genes, which are composed of 887 matched lncRNA-mRNA pairs. In total, 88 lncRNAs were identified as endogenous RNAs (ceRNAs) and predicted to decoy 46 miRNAs. Degradome sequencing revealed that 11 miRNAs that were decoyed by 20 lncRNAs could target 30 genes. These lncRNAs, miRNAs, and target genes were predicted to form 10 regulatory modules. Among them, lncRNA42705/lncRNA08711, lncRNA39896, and lncRNA11265/lncRNA15816 might modulate MYB, HD-Zip, and NAC transcription factors by decoying miR159, miR166b, and miR164a-5p, respectively. Upon P. infestans infection, the expression levels of lncRNA42705 and lncRNA08711 displayed a negative correlation with the expression level of miR159 and a positive correlation with the expression levels of MYB genes. Tomato plants in which lncRNA42705 and lncRNA08711 were silenced displayed increased levels of miR159 and decreased levels of MYB, respectively. The result demonstrated that lncRNAs might function as ceRNAs to decoy miRNAs and affect their target genes in tomato plants, increasing resistance to disease.

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.

Identification of Appropriate Reference Genes for Normalizing miRNA Expression in Citrus Infected by Xanthomonas citri subsp. citri

MicroRNAs (miRNAs) are short noncoding RNA molecules that regulate gene expression at the posttranscriptional level. Reverse transcription-quantitative PCR (RT-qPCR) is one of the most common methods used for quantification of miRNA expression, and the levels of expression are normalized by comparing with reference genes. Thus, the selection of reference genes is critically important for accurate quantification. The present study was intended to identify appropriate miRNA reference genes for normalizing the level of miRNA expression in Citrus sinensis L. Osbeck and Citrus reticulata Blanco infected by Xanthomonas citri subsp. citri, which caused citrus canker disease. Five algorithms (Delta Ct, geNorm, NormFinder, BestKeeper and RefFinder) were used for screening reference genes, and two quantification approaches, poly(A) extension RT-qPCR and stem-loop RT-qPCR, were used to determine the most appropriate method for detecting expression patterns of miRNA. An overall comprehensive ranking output derived from the multi-algorithms showed that poly(A)-tailed miR162-3p/miR472 were the best reference gene combination for miRNA RT-qPCR normalization in citrus canker research. Candidate reference gene expression profiles determined by poly(A) RT-qPCR were more consistent in the two citrus species. To the best of our knowledge, this is the first systematic comparison of two miRNA quantification methods for evaluating reference genes. These results highlight the importance of rigorously assessing candidate reference genes and clarify some contradictory results in miRNA research on citrus.

Solanum lycopersicum microRNA1916 targets multiple target genes and negatively regulates the immune response in tomato

MicroRNA1916 (miR1916) is one of the nonconserved miRNAs that respond to various stresses in plants, but little has been known at present about its mechanisms in biotic stresses. In this study, the expression of Solanum lycopersicum (sly)-miR1916 in tomato was found to be down-regulated after infection with Phytophthora infestans or Botrytis cinerea. Tomato plants that overexpressed sly-miR1916 displayed significant enhancement in susceptibility to P. infestans and B. cinerea infection, as well as increased tendency to produce reactive oxygen species. Silencing of sly-miR1916 by short tandem target mimic and artificial microRNA strategies caused the tomato plants to become more tolerant to adverse conditions. In addition, lower sly-miR1916 expression could up-regulate the expression of strictosidine synthase (STR-2), UDP-glycosyltransferases (UGTs), late blight resistance protein homolog R1B-16, disease resistance protein RPP13-like, and MYB transcription factor (MYB12), which ultimately resulted in the accumulation of α-tomatine and anthocyanins via STR-2, UGT, and MYB12. Furthermore, ectopic expression of sly-miR1916/STR-2 significantly changed the tolerance of tobacco to B. cinerea. Taken together, the results demonstrated that sly-miR1916 might regulate the expression of STR-2, UGT, and MYB12 in tomato plant, conferring sensitivity to biotic stress via modulating α-tomatine and anthocyanins.

Identification of novel miRNAs and their target genes from Populus szechuanica infected with Melampsora larici-populina

Two novel miRNAs were selected from a pre-constructed RNA library of Populus szechuanica infected with the foliar rust fungus Melampsora larici-populina in order to detect the genes regulated as targets of the miRNAs novel_mir_11 and novel_mir_357. The novel miRNAs were identified from P. szechuanica using stem-loop methods and their precursors were able to fold into a complete stem loop structure. The predicted target genes of the novel miRNAs were verified with RNA ligase-mediated 5' rapid amplification of cDNA ends (RLM-5'RACE). The full-length sequences of target genes, RPM1 and RPS2/5, in P. szechuanica were obtained through rapid amplification of cDNA ends (RACE) and officially named PsRPM1 and PsRPS2/5. These genes contain nucleotide binding site-leucine-rich repeats (NBS-LRR) domains typical of resistance genes. The expression levels of miRNAs and their target genes in different periods post infection were analysed with quantitative real-time PCR (qRT-PCR). After infection with the foliar rust fungus, the expression levels of the novel miRNAs and their target genes were dynamic. Both novel_mir_11 and novel_mir_357 negatively regulated the expression of their target genes. In this study, the regulatory effects of two novel miRNAs through their target genes were characterized to provide further mechanistic information regarding the interaction between Populus and a foliar rust fungus. Results of this study improve our understanding of the defence response mechanisms of Populus and will stimulate future work to characterize strategies to prevent and control Populus diseases.

Comparative transcriptome analysis shows the defense response networks regulated by miR482b

The transcriptomic profile in the leaves of miR482b-overexpressing tomato plants revealed that miR482b may suppress alpha-linolenic acid metabolism, cysteine and methionine metabolism, plant-pathogen interaction, and the MAPK pathway to reduce resistance to Phytophthora infestans. Our previous study showed that tomato miR482b acted as a negative regulator during tomato resistance to Phytophthora infestans by silencing NBS-LRR genes. To investigate pathways related to miR482b, the transcriptomic profile of tomato plants that overexpressed miR482b was constructed. A total of 47,124,670 raw sequence reads from the leaves of miR482b-overexpressing tomato plants were generated by Illumina sequencing. A total of 746 genes in miR482b-overexpressing tomato plants were found to show significantly differential expression relative to those in wild-type tomato plants, including 132 up-regulated genes and 614 down-regulated genes. GO and KEGG enrichment analyses showed that plant-pathogen interaction, the MAPK pathway, and the pathways related to JA and ET biosynthesis were affected by miR482b in tomato. qRT-PCR results showed that all the enriched genes in these pathways were down-regulated in tomato plants that overexpressed miR482b and up-regulated in tomato plants that overexpressed an NBS-LRR gene (Soly02g036270.2, the target gene of miR482b). After P. infestans infection, the expression of the enriched genes showed a time-dependent response, and the genes played different roles between resistant tomato (Solanum pimpinellifolium L3708) and tomato susceptible to P. infestans (S. lycopersicum Zaofen No. 2). Our results have, therefore, demonstrated that miR482b is an important component of defense response network. This will also help to identify candidate genes involved in plant-pathogen interaction.

Tomato MYB49 enhances resistance to Phytophthora infestans and tolerance to water deficit and salt stress

MYB49-overexpressing tomato plants showed significant resistance to Phytophthora infestans and tolerance to drought and salt stresses. This finding reveals the potential application of tomato MYB49 in future molecular breeding. Biotic and abiotic stresses severely reduce the productivity of tomato worldwide. Therefore, it is necessary to find key genes to simultaneously improve plant resistance to pathogens and tolerance to various abiotic stresses. In this study, based on homologous relationships with Arabidopsis R2R3-MYBs (AtMYBs) involved in responses to biotic and abiotic stresses, we identified a total of 24 R2R3-MYB transcription factors in the tomato genome. Among these tomato R2R3-MYBs, MYB49 (Solyc10g008700.1) was clustered into subgroup 11 by phylogenetic analysis, and its expression level was significantly induced after treatment with P. infestans, NaCl and PEG6000. Overexpression of MYB49 in tomato significantly enhanced the resistance of tomato to P. infestans, as evidenced by decreases in the number of necrotic cells, sizes of lesion, abundance of P. infestans, and disease index. Likewise, MYB49-overexpressing transgenic tomato plants also displayed increased tolerance to drought and salt stresses. Compared to WT plants, the accumulation of reactive oxygen species (ROS), malonaldehyde content, and relative electrolyte leakage was decreased, and peroxidase activity, superoxide dismutase activity, chlorophyll content, and photosynthetic rate were increased in MYB49-overexpressing tomato plants under P. infestans, salt or drought stress. These results suggested that tomato MYB49, as a positive regulator, could enhance the capacity to scavenge ROS, inhibit cell membrane damage and cell death, and protect chloroplasts, resulting in an improvement in resistance to P. infestans and tolerance to salt and drought stresses, and they provide a candidate gene for tomato breeding to enhance biotic stress resistance and abiotic stress tolerance.

A Tomato Nucleotide Binding Sites-Leucine-Rich Repeat Gene Is Positively Involved in Plant Resistance to Phytophthora infestans

The nucleotide binding sites-leucine-rich repeat (NBS-LRR) genes are key regulatory components of plant to pathogens. Phytophthora infestans-inducible coding sequence encoding an NBS-LRR (SpNBS-LRR) protein in tomato (Solanum pimpinellifolium L3708) was cloned and characterized based on our RNA-Seq data and tomato genome. After sequence analysis, SpNBS-LRR was identified as a hydrophilic protein with no transmembrane topological structure and no signal peptide. SpNBS-LRR had a close genetic relationship to RPS2 of Arabidopsis thaliana by phylogenetic analysis. In addition, SpNBS-LRR gene was mainly expressed in root, with low expression observed in leaf and stem. To further investigate the role of SpNBS-LRR in tomato-P. infestans interaction, SpNBS-LRR was introduced in susceptible tomatoes and three transgenic lines with higher expression level of SpNBS-LRR were selected. These transgenic tomato plants that overexpressed SpNBS-LRR displayed greater resistance than wild-type tomato plants after infection with P. infestans, as shown by decreased disease index, lesion diameters, number of necrotic cells, P. infestans abundance, and higher expression levels of the defense-related genes. This information provides insight into SpNBS-LRR involved in the resistance of tomato to P. infestans infection and candidate for breeding to enhance biotic stress-resistance in tomato.

Transcriptome signatures of tomato leaf induced by Phytophthora infestans and functional identification of transcription factor SpWRKY3

SpWRKY3 was identified as a resistance gene to Phytophthora infestans from Solanum pimpinellifolium L3708 and its transgenic tomato showed a significant resistance to P. infestans. This finding reveals the potential application of SpWRKY3 in future molecular breeding. Transcription factors (TFs) play crucial roles in the plant response to various pathogens. In this present study, we used comparative transcriptome analysis of tomatoes inoculated with and without Phytophthora infestans to identify 1103 differentially expressed genes. Seven enrichment GO terms (level 4) associated with the plant resistance to pathogens were identified. It was found that thirty-five selected TF genes from GO enriched term, sequence-specific DNA binding transcription factor activity (GO: 0003700), were induced by P. infestans. Of these TFs, the accumulation of a homologous gene of WRKY (SpWRKY3) was significantly changed after P. infestans induction, and it was also isolated form P. infestans-resistant tomato, Solanum pimpinellifolium L3708. Overexpression of SpWRKY3 in tomato positively modulated P. infestans defense response as shown by decreased number of necrotic cells, lesion sizes and disease index, while the resistance was impaired after SpWRKY3 silencing. After P. infestans infection, the expression levels of PR genes in transgenic tomato plants overexpressed SpWRKY3 were significantly higher than those in WT, while the number of necrotic cells and the reactive oxygen species (ROS) accumulation were fewer and lower. These results suggest that SpWRKY3 induces PR gene expression and reduces the ROS accumulation to protect against cell membrane injury, leading to enhanced resistance to P. infestans. Our results provide insight into SpWRKY3 as a positive regulator involved in tomato-P. infestans interaction, and its function may enhance tomato resistance to P. infestans.

Function identification of miR482b, a negative regulator during tomato resistance to Phytophthora infestans

Tomato is an important horticultural and economic crop cultivated worldwide. As Phytophthora infestans becomes a huge threat to tomato production, it is necessary to study the resistance mechanisms of tomato against P. infestans. Our previous research has found that miR482 might be involved in tomato-P. infestans interaction. In this study, miR482b precursor was cloned from Solanum pimpinellifolium "L3708" and miR482b was shown to decrease in abundance in tomato following P. infestans infection. Compared to wild-type tomato plants, tomato plants that overexpressed miR482b displayed more serious disease symptoms after P. infestans infection, with more necrotic cells, longer lesion diameters, and increased P. infestans abundance. Meanwhile, silencing of miR482b was performed by short tandem target mimic (STTM), resulting in enhancement of tomato resistance to P. infestans. Using miRNA and degradome data sets, NBS-LRR disease-resistance genes targeted by miR482b were validated. Negative correlation between the expression of miR482b and its target genes was found in all miR482b-overexpressing and -silencing tomato plants. Our results provide insight into tomato miR482b involved in the response to P. infestans infection, and demonstrate that miR482b-NBS-LRR is an important component in the network of tomato-P. infestans interaction.

MicroRNA396a-5p and -3p induce tomato disease susceptibility by suppressing target genes and upregulating salicylic acid

Plants have evolved a variety of mechanisms to perceive and resist the assault of pathogens. The biotrophs, necrotrophs and hemibiotrophs are types of plant pathogens that activate diverse salicylic acid (SA) and jasmonic acid (JA) signaling pathways. In this study we showed that the expressions of miR396a-5p and -3p in Solanum lycopersicum (S. lycopersicum) were both down-regulated after infection by hemibiotroph Phytophthora infestans (P. infestans) and necrotroph Botrytis cinerea (B. cinerea) infection. Overexpression of miR396a-5p and -3p in transgenic tomato enhanced the susceptibility of S. lycopersicum to P. infestans and B. cinerea infection and the tendency to produce reactive oxygen species (ROS) under pathogen-related biotic stress. Additionally, miR396a regulated growth-regulating factor1 (GRF1), salicylic acid carboxyl methyltransferase (SAMT), glycosyl hydrolases (GH) and nucleotide-binding site-leucine-rich repeat (NBS-LRR) and down-regulated their levels. This ultimately led to inhibition of the expression of pathogenesis-related 1 (PR1), TGA transcription factors1 and 2 (TGA1 and TGA2) and JA-dependent proteinase inhibitors I and II (PI I and II), but enhanced the endogenous SA content and nonexpressor of pathogenesis-related genes 1 (NPR1) expression. Taken together, our results showed that negative regulation of target genes and their downstream genes expressions by miR396a-5p and -3p are critical for tomato abiotic stresses via affecting SA or JA signaling pathways.

Resistance Genes in Piper colubrinum: In Silico Survey From Leaf Transcriptome and Expression Studies Upon Challenge Inoculation with Phytophthora capsici

The oomycetes, Phytophthora capsici, cause foot rot disease in black pepper. Piper colubrinum Link, a distant relative of cultivated black pepper, is highly resistant to this destructive pathogen. Identification of resistance (R) genes in P. colubrinum and the study of its expression profile during interaction with the pathogen can help in understanding the resistance mechanism involved. In the present study, 1289 R gene-related transcripts were mined from P. colubrinum transcriptome, clustered, and classified according to the conserved motifs and domains. Transcripts belonging to four major R gene classes were identified in P. colubrinum, but TIR-NBS-LRR-type R genes were absent. The relative expression of 12 selected R genes was studied using two virulent isolates of P. capsici, and these were found to be upregulated in the initial hours of plant pathogen interaction. The R genes studied were expressed even in aseptically maintained tissue-cultured plants and uninoculated greenhouse-grown plants at basal level suggesting that the plants are geared up with the R gene all the time and are under continuous surveillance for the pathogen and basal level of R gene expression do not require a pathogen trigger. ACT, ATUB, and EIF3E were identified as the most stable reference genes that can be used for real-time PCR study. The present study identified promising R genes in P. colubrinum which can be used in developing Phytophthora-resistant black pepper.