Co-incidence of Damage and Microbial Patterns Controls Localized Immune Responses in Roots.

Recognition of microbe-associated molecular patterns (MAMPs) is crucial for the plant's immune response. How this sophisticated perception system can be usefully deployed in roots, continuously exposed to microbes, remains a mystery. By analyzing MAMP receptor expression and response at cellular resolution in Arabidopsis, we observed that differentiated outer cell layers show low expression of pattern-recognition receptors (PRRs) and lack MAMP responsiveness. Yet, these cells can be gated to become responsive by neighbor cell damage. Laser ablation of small cell clusters strongly upregulates PRR expression in their vicinity, and elevated receptor expression is sufficient to induce responsiveness in non-responsive cells. Finally, localized damage also leads to immune responses to otherwise non-immunogenic, beneficial bacteria. Damage-gating is overridden by receptor overexpression, which antagonizes colonization. Our findings that cellular damage can "switch on" local immune responses helps to conceptualize how MAMP perception can be used despite the presence of microbial patterns in the soil.

Plant immunity suppression via PHR1-RALF-FERONIA shapes the root microbiome to alleviate phosphate starvation.

The microbiome plays an important role in shaping plant growth and immunity, but few plant genes and pathways impacting plant microbiome composition have been reported. In Arabidopsis thaliana, the phosphate starvation response (PSR) was recently found to modulate the root microbiome upon phosphate (Pi) starvation through the transcriptional regulator PHR1. Here, we report that A. thaliana PHR1 directly binds to the promoters of rapid alkalinization factor (RALF) genes, and activates their expression under phosphate-starvation conditions. RALFs in turn suppress complex formation of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) receptor through FERONIA, a previously-identified PTI modulator that increases resistance to certain detrimental microorganisms. Suppression of immunity via the PHR1-RALF-FERONIA axis allows colonization by specialized root microbiota that help to alleviate phosphate starvation by upregulating the expression of PSR genes. These findings provide a new paradigm for coordination of host-microbe homeostasis through modulating plant innate immunity after environmental perturbations.

An ancient cis-element targeted by Ralstonia solanacearum TALE-like effectors facilitates the development of a promoter trap that could confer broad-spectrum wilt resistance.

Ralstonia solanacearum, a species complex of bacterial plant pathogens that causes bacterial wilt, comprises four phylotypes that evolved when a founder population was split during the continental drift ~180 million years ago. Each phylotype contains strains with RipTAL proteins structurally related to transcription activator-like (TAL) effectors from the bacterial pathogen Xanthomonas. RipTALs have evolved in geographically separated phylotypes and therefore differ in sequence and potentially functionality. Earlier work has shown that phylotype I RipTAL Brg11 targets a 17-nucleotide effector binding element (EBE) and transcriptionally activates the downstream arginine decarboxylase (ADC) gene. The predicted DNA binding preferences of Brg11 and RipTALs from other phylotypes are similar, suggesting that most, if not all, RipTALs target the Brg11-EBE motif and activate downstream ADC genes. Here we show that not only phylotype I RipTAL Brg11 but also RipTALs from other phylotypes activate host genes when preceded by the Brg11-EBE motif. Furthermore, we show that Brg11 and RipTALs from other phylotypes induce the same quantitative changes of ADC-dependent plant metabolites, suggesting that most, if not all, RipTALs induce functionally equivalent changes in host cells. Finally, we report transgenic tobacco lines in which the RipTAL-binding motif Brg11-EBE mediates RipTAL-dependent transcription of the executor-type resistance (R) gene Bs4C from pepper, thereby conferring resistance to RipTAL-delivering R. solanacearum strains. Our results suggest that cell death-inducing executor-type R genes, preceded by the RipTAL-binding motif Brg11-EBE, could be used to genetically engineer broad-spectrum bacterial wilt resistance in crop plants without any apparent fitness penalty.

The functions of a 5' tRNA-Ala-derived fragment in gene expression.

Transfer RNA (tRNA) can produce smaller RNA fragments called tRNA-derived fragments (tRFs). tRFs play critical roles in multiple cellular programs, although the functional mechanisms of tRFs remain largely unknown in plants. In this study, we examined the phenotype associated with 5' tRF-Ala (tRF-Ala, produced from tRNA-Ala) overexpression and knockdown lines (tDR-Ala-OE and tDR-Ala-kd, respectively) and the mechanisms by which tRF-Ala affects mRNA levels in Arabidopsis (Arabidopsis thaliana). We investigated the candidate proteins associated with tRF-Ala by quantitative proteomics and confirmed the direct interaction between tRF-Ala and the splicing factor SERINE-ARGININE RICH PROTEIN 34 (SR34). A transcriptome sequencing analysis showed that 318 genes among all the genes (786) with substantial alternative splicing (AS) variance in tDR-Ala-OE lines are targets of SR34. tRF-Ala diminished the binding affinity between SR34 and its targets by direct competition for interaction with SR34. These findings reveal the critical roles of tRF-Ala in regulating mRNA levels and splicing.

A screening of inhibitors targeting the receptor kinase FERONIA reveals small molecules that enhance plant root immunity.

Receptor-like kinases (RLKs) constitute the largest receptor family involved in the regulation of plant immunity and growth, but small-molecule inhibitors that target RLKs to improve agronomic traits remain unexplored. The RLK member FERONIA (FER) negatively regulates plant resistance to certain soil-borne diseases that are difficult to control and cause huge losses in crop yields and economy. Here, we identified 33 highly effective FER kinase inhibitors from 1494 small molecules by monitoring FER autophosphorylation in vitro. Four representative inhibitors (reversine, cenisertib, staurosporine and lavendustin A) inhibited the kinase activity of FER and its homologues in several crops by targeting the conserved ATP pocket in the kinase structure. FER contributes to the physiological impact of representative inhibitors in plants. The treatment of roots with reversine, staurosporine and lavendustin A enhanced innate immunity in plant roots and thus alleviated soil-borne diseases in tobacco, tomato and rice without growth penalties. Consistently, RNA sequencing assays showed that lavendustin A and reversine exert profound impacts on immunity-related gene expression. Our results will set a new milestone in the development of the plant RLK kinase regulation theory and provide a novel strategy for the prevention and control of plant soil-borne diseases without growth penalties.

CAR modulates plasma membrane nano-organization and immune signaling downstream of RALF1-FERONIA signaling pathway.

In Arabidopsis, the receptor-like kinase (RLK) FERONIA (FER) senses peptide ligands in the plasma membrane (PM), modulates plant growth and development, and integrates biotic and abiotic stress signaling for downstream adaptive responses. However, the molecular interplay of these diverse processes is largely unknown. Here, we show that FER, the receptor of Rapid Alkalinization Factor 1 (RALF1), physically interacts with C2 domain ABA-related (CAR) proteins to control the nano-organization of the PM. During this process, the RALF1-FER pathway upregulates CAR protein translation, and then more CAR proteins are recruited to the PM. This acts as a rapid feedforward loop that stabilizes the PM liquid-ordered phase. FER interacts with and phosphorylates CARs, thereby reducing their lipid-binding ability and breaking the feedback regulation at later time points. The formation of the flg22-induced FLS2-BAK1 immune complex, which depends on the integrity of FER-containing nanodomains, is impaired in fer and pentuple car14569 mutant. Together, we propose that the FER-CAR module controls the formation of PM nano-organization during RALF signaling through a self-contained amplifying loop including both positive and negative feedback.

Metabolomics of tomato xylem sap during bacterial wilt reveals Ralstonia solanacearum produces abundant putrescine, a metabolite that accelerates wilt disease.

Ralstonia solanacearum thrives in plant xylem vessels and causes bacterial wilt disease despite the low nutrient content of xylem sap. We found that R. solanacearum manipulates its host to increase nutrients in tomato xylem sap, enabling it to grow better in sap from infected plants than in sap from healthy plants. Untargeted GC/MS metabolomics identified 22 metabolites enriched in R. solanacearum-infected sap. Eight of these could serve as sole carbon or nitrogen sources for R. solanacearum. Putrescine, a polyamine that is not a sole carbon or nitrogen source for R. solanacearum, was enriched 76-fold to 37 µM in R. solanacearum-infected sap. R. solanacearum synthesized putrescine via a SpeC ornithine decarboxylase. A ΔspeC mutant required ≥ 15 µM exogenous putrescine to grow and could not grow alone in xylem even when plants were treated with putrescine. However, co-inoculation with wildtype rescued ΔspeC growth, indicating R. solanacearum produced and exported putrescine to xylem sap. Intriguingly, treating plants with putrescine before inoculation accelerated wilt symptom development and R. solanacearum growth and systemic spread. Xylem putrescine concentration was unchanged in putrescine-treated plants, so the exogenous putrescine likely accelerated disease indirectly by affecting host physiology. These results indicate that putrescine is a pathogen-produced virulence metabolite.

New Insights into the Antibacterial Activity of Hydroxycoumarins against Ralstonia solanacearum.

Coumarins are important plant-derived natural products with wide-ranging bioactivities and extensive applications. In this study, we evaluated for the first time the antibacterial activity and mechanisms of action of coumarins against the phytopathogen Ralstonia solanacearum, and investigated the effect of functional group substitution. We first tested the antibacterial activity of 18 plant-derived coumarins with different substitution patterns, and found that daphnetin, esculetin, xanthotol, and umbelliferone significantly inhibited the growth of R. solanacearum. Daphnetin showed the strongest antibacterial activity, followed by esculetin and umbelliferone, with MICs of 64, 192, and 256 mg/L, respectively, better than the archetypal coumarin with 384 mg/L. We further demonstrated that the hydroxylation of coumarins at the C-6, C-7 or C-8 position significantly enhanced the antibacterial activity against R. solanacearum. Transmission electron microscope (TEM) and fluorescence microscopy images showed that hydroxycoumarins may interact with the pathogen by mechanically destroying the cell membrane and inhibiting biofilm formation. The antibiofilm effect of hydroxycoumarins may relate to the repression of flagellar genes fliA and flhC. These physiological changes in R. solanacearum caused by hydroxycoumarins can provide information for integral pathogen control. The present findings demonstrated that hydroxycoumarins have superior antibacterial activity against the phytopathogen R. solanacearum, and thus have the potential to be applied for controlling plant bacterial wilt.

Manipulation of plant metabolism by pathogen effectors: more than just food.

To successfully infect plants, pathogens secrete effector proteins to the plant apoplast or inside plant cells, where they suppress plant immunity or interfere with other cellular processes to facilitate infection. Plant metabolism is crucial for most cellular processes and plays a key role in defense against pathogens, making it a major target for pathogen effectors. Effector proteins manipulate host metabolism to provide the pathogen with nutrients or to indirectly suppress plant chemical defense responses. Recent studies have shown that pathogens also utilize effectors to shape the microbiota composition by altering the concentration of certain plant metabolites. Here, we summarize current knowledge on the manipulation of plant metabolism by pathogen effectors. We also discuss what remains unknown regarding the manipulation of host metabolism by pathogen effectors.

A screening identifies harmine as a novel antibacterial compound against Ralstonia solanacearum.

Ralstonia solanacearum, the causal agent of bacterial wilt, is a devastating plant pathogenic bacterium that infects more than 450 plant species. Until now, there has been no efficient control strategy against bacterial wilt. In this study, we screened a library of 100 plant-derived compounds for their antibacterial activity against R. solanacearum. Twelve compounds, including harmine, harmine hydrochloride, citral, vanillin, and vincamine, suppressed bacterial growth of R. solanacearum in liquid medium with an inhibition rate higher than 50%. Further focus on harmine revealed that the minimum inhibitory concentration of this compound is 120 mg/L. Treatment with 120 mg/L of harmine for 1 and 2 h killed more than 90% of bacteria. Harmine treatment suppressed the expression of the virulence-associated gene xpsR. Harmine also significantly inhibited biofilm formation by R. solanacearum at concentrations ranging from 20 mg/L to 60 mg/L. Furthermore, application of harmine effectively reduced bacterial wilt disease development in both tobacco and tomato plants. Collectively, our results demonstrate the great potential of plant-derived compounds as antibacterial agents against R. solanacearum, providing alternative ways for the efficient control of bacterial wilt.

Rhizospheric microbial consortium of Lilium lancifolium Thunb. causes lily root rot under continuous cropping system.

Tiger lily (Lilium lancifolium Thunb.) is a cash crop with a long history of cultivation in China. Its roots have long been used as a valuable component of Chinese medicine. Continuous cropping, the conventional planting approach for tiger lily, often leads to severe root rot disease, but it is not yet clear how this planting method leads to root rot. In this study, we analyzed the rhizosphere microbiome and predicted microbial protein function in tiger lily planted with the continuous cropping method in three different geological types of soil. In order to explore the specific rhizosphere microbiota triggering root rot disease, tiger lily was compared to maize grown in a similar system, which showed no disease development. An analysis of the chemical elements in the soil revealed that the Pseudomonas and Streptomyces genera, with pathogenic functions, were dominant in the tiger lily rhizosphere. The lower soil pH of tiger lily compared to maize supports the accumulation of pathogenic bacteria in the tiger lily rhizosphere. Meanwhile, we discovered that bacteria of the Flavobacterium genus, with their predicted phosphate transport function, specifically accumulated in the maize rhizosphere. Our findings suggest that Pseudomonas and Streptomyces bacteria may result in continuous cropping-induced root rot disease in tiger lily and that Flavobacterium could serve to protect maize from pathogenic bacteria.

The RALF1-FERONIA complex interacts with and activates TOR signaling in response to low nutrients.

Target of rapamycin (TOR) kinase is an evolutionarily conserved major regulator of nutrient metabolism and organismal growth in eukaryotes. In plants, nutrients are remobilized and reallocated between shoots and roots under low-nutrient conditions, and nitrogen and nitrogen-related nutrients (e.g., amino acids) are key upstream signals leading to TOR activation in shoots under low-nutrient conditions. However, how these forms of nitrogen can be sensed to activate TOR in plants is still poorly understood. Here we report that the Arabidopsis receptor kinase FERONIA (FER) interacts with the TOR pathway to regulate nutrient (nitrogen and amino acid) signaling under low-nutrient conditions and exerts similar metabolic effects in response to nitrogen deficiency. We found that FER and its partner, RPM1-induced protein kinase (RIPK), interact with the TOR/RAPTOR complex to positively modulate TOR signaling activity. During this process, the receptor complex FER/RIPK phosphorylates the TOR complex component RAPTOR1B. The RALF1 peptide, a ligand of the FER/RIPK receptor complex, increases TOR activation in the young leaf by enhancing FER-TOR interactions, leading to promotion of true leaf growth in Arabidopsis under low-nutrient conditions. Furthermore, we showed that specific amino acids (e.g., Gln, Asp, and Gly) promote true leaf growth under nitrogen-deficient conditions via the FER-TOR axis. Collectively, our study reveals a mechanism by which the RALF1-FER pathway activates TOR in the plant adaptive response to low nutrients and suggests that plants prioritize nutritional stress response over RALF1-mediated inhibition of cell growth under low-nutrient conditions.

Systematic Analysis of Tobacco CrRLK1L Family Genes and Functional Identification of NtCrRLK1L47 in Environmental Stresses.

The Catharanthus roseus RLK1-like (CrRLK1L) family is involved in the regulation of plant reproduction, growth and development, cell wall integrity sensing, as well as responses to both biotic and abiotic stress conditions. Extraordinary progress has been made in elucidating the CrRLK1L family receptor kinases-mediated signaling pathway, while limited research addressed the functions of CrRLK1L proteins in tobacco. In this study, we identified and analyzed 48 NtCrRLK1L members from the tobacco genome. The newly identified NtCrRLK1L members were divided into seven groups together with the Arabidopsis CrRLK1L members. The syntenic analysis revealed that four pairs of NtCrRLK1L genes were predicted to have arisen from segmental duplication events. Expression profiling showed that the NtCrRLK1L genes were expressed in various tissues, and most NtCrRLK1L genes were induced by salt and drought stress conditions. Notably, NtCrRLK1L47 was upregulated under drought and salinity stresses, and the NtCrRLK1L47-GFP fusion protein was located in the cell membrane. Furthermore, overexpression of the NtCrRLK1L47 gene enhanced the salt tolerance in tobacco seedlings.

Emerging roles of pathogen-secreted host mimics in plant disease development.

Plant pathogens and parasites use multiple virulence factors to successfully infect plants. While most plant-pathogen interaction studies focus on pathogen effectors and their functions in suppressing plant immunity or interfering with normal cellular processes, other virulence factors likely also contribute. Here we highlight another important strategy used by pathogens to promote virulence: secretion of mimics of host molecules, including peptides, phytohormones, and small RNAs, which play diverse roles in plant development and stress responses. Pathogen-secreted mimics hijack the host endogenous signaling pathways, thereby modulating host cellular functions to the benefit of the pathogen and promoting infection. Understanding the mechanisms of pathogen-secreted host mimics will expand our knowledge of host-pathogen coevolution and interactions, while providing new targets for plant disease control.

Nematode RALF-Like 1 Targets Soybean Malectin-Like Receptor Kinase to Facilitate Parasitism.

Soybean [Glycine max (L.) Merr. ] is one of the most strategical oilseed crops that provides sustainable source of protein and oil worldwide. Cultivation of soybean is severely affected by root-knot nematode (RKN). However, the mechanism of RKN parasitism to soybeans is largely unknown. In this study, we identify GmLMM1, which encodes a homolog of FERONIA-like receptor kinase in soybean, as a susceptible gene toward nematode. Mutations of GmLMM1 exhibit enhanced resistance against the RKN Meloidogyne incognita. RNA-sequencing (RNA-seq) analysis reveals a similar differential expression pattern for genes regulated by GmLMM1 (Gmlmm1 vs. wild-type) and M. incognita (M. incognita vs. mock), supporting the role of GmLMM1 in M. incognita infection. Unlike FERONIA in Arabidopsis, GmLMM1 specifically binds to MiRALF1 and AtRALF23 that suppress plant immunity, but not MiRALF3 and AtRALF1. Moreover, we found that the single-nucleotide polymorphism (SNP) in GmLMM1 leads to the natural resistance against RKNs in soybeans. Collectively, these findings uncover GmLMM1 as a susceptible target of nematode RALF-like 1 and provide new genetic resource for nematode resistant breeding.

RALF-FERONIA Signaling: Linking Plant Immune Response with Cell Growth.

Plants perceive various external and internal signals to self-modulate biological processes through members of the receptor-like kinase (RLK) family, among which Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) proteins with their ligands, rapid alkalinization factor (RALF) peptides, have attracted considerable interest. FERONIA (FER), a CrRLK1L member, was initially reported to act as a major plant cell growth modulator in distinct tissues. Subsequently, the RALF-FER pathway was confirmed to function as an essential regulator of plant stress responses, including but not limited to immune responses. Furthermore, the RALF-FER pathway modulates immune responses and cell growth in a context-specific manner, and the vital roles of this pathway are beginning to be appreciated in crop species. The recent remarkable advances in understanding the functions and molecular mechanisms of the RALF-FER pathway have also raised many interesting questions that need to be answered in the future. This review mainly focuses on the roles of FER and other CrRLK1L members in modulating immune responses in the context of cell growth in response to their RALF peptide ligands and presents a brief outlook for future research.

A Plant Pathogen Type III Effector Protein Subverts Translational Regulation to Boost Host Polyamine Levels.

Pathogenic bacteria inject effector proteins into host cells to manipulate cellular processes and facilitate the infection. Transcription-activator-like effectors (TALEs), an effector class in plant pathogenic bacteria, transcriptionally activate host genes to promote disease. We identify arginine decarboxylase (ADC) genes as the host targets of Brg11, a TALE-like effector from the plant pathogen Ralstonia solanacearum. Brg11 targets a 17-bp sequence that was found to be part of a conserved 50-bp motif, termed the ADC-box, upstream of ADC genes involved in polyamine biosynthesis. The transcribed ADC-box attenuates translation from native ADC mRNAs; however, Brg11 induces truncated ADC mRNAs lacking the ADC-box, thus bypassing this translational control. As a result, Brg11 induces elevated polyamine levels that trigger a defense reaction and likely inhibits bacterial niche competitors but not R. solanacearum. Our findings suggest that Brg11 may give R. solanacearum a competitive advantage and uncover a role for bacterial effectors in regulating ternary microbe-host-microbe interactions.

A modular toolbox for Golden-Gate-based plasmid assembly streamlines the generation of Ralstonia solanacearum species complex knockout strains and multi-cassette complementation constructs.

Members of the Ralstonia solanacearum species complex (Rssc) cause bacterial wilt, a devastating plant disease that affects numerous economically important crops. Like other bacterial pests, Rssc injects a cocktail of effector proteins via the bacterial type III secretion system into host cells that collectively promote disease. Given their functional relevance in disease, the identification of Rssc effectors and the investigation of their in planta function are likely to provide clues on how to generate pest-resistant crop plants. Accordingly, molecular analysis of effector function is a focus of Rssc research. The elucidation of effector function requires corresponding gene knockout strains or strains that express the desired effector variants. The cloning of DNA constructs that facilitate the generation of such strains has hindered the investigation of Rssc effectors. To overcome these limitations, we have designed, generated and functionally validated a toolkit consisting of DNA modules that can be assembled via Golden-Gate (GG) cloning into either desired gene knockout constructs or multi-cassette expression constructs. The Ralstonia-GG-kit is compatible with a previously established toolkit that facilitates the generation of DNA constructs for in planta expression. Accordingly, cloned modules, encoding effectors of interest, can be transferred to vectors for expression in Rssc strains and plant cells. As many effector genes have been cloned in the past as GATEWAY entry vectors, we have also established a conversion vector that allows the implementation of GATEWAY entry vectors into the Ralstonia-GG-kit. In summary, the Ralstonia-GG-kit provides a valuable tool for the genetic investigation of genes encoding effectors and other Rssc genes.

RNA-Seq Analysis Reveals Candidate Targets for Curcumin against Tetranychus cinnabarinus.

Tetranychus cinnabarinus is an important agricultural pest with a broad host range. We previously identified curcumin as a promising acaricidal compound against T. cinnabarinus. However, the acaricidal mechanism of curcumin remains unknown. In this study, RNA-seq was employed to analyze the transcriptome changes in T. cinnabarinus treated with curcumin or the solvent. A total of 105,706,297 clean sequence reads were generated by sequencing, with more than 90% of the reads successfully mapped to the reference sequence. The RNA-seq identified 111 and 96 differentially expressed genes between curcumin- and solvent-treated mites at 24 and 48 h after treatment, respectively. GO enrichment analysis of differentially expressed genes showed that the cellular process was the dominant group at both time points. Finally, we screened 23 differentially expressed genes that were functionally identical or similar to the targets of common insecticide/acaricides or genes that were associated with mite detoxification and metabolism. Calmodulin, phospholipase A2, and phospholipase C were activated upon curcumin treatment suggesting that the calcium channel related genes might play important roles in mite's response to curcumin. Overall our results revealed the global transcriptional changes in T. cinnabarinus after curcumin treatment to enable further identification of the targets of curcumin in mites.