Pseudomonas syringae Type III Effector HopBB1 Promotes Host Transcriptional Repressor Degradation to Regulate Phytohormone Responses and Virulence

Coronatine orchestrates ABI1-mediated stomatal opening to facilitate bacterial pathogen infection through importin β protein SAD2

Stomatal immunity plays an important role during bacterial pathogen invasion. Abscisic acid (ABA) induces plants to close their stomata and halt pathogen invasion, but many bacterial pathogens secrete phytotoxin coronatine (COR) to antagonize ABA signaling and reopen the stomata to promote infection at early stage of invasion. However, the underlining mechanism is not clear. SAD2 is an importin β family protein, and the sad2 mutant shows hypersensitivity to ABA. We discovered ABI1, which negatively regulated ABA signaling and reduced plant sensitivity to ABA, was accumulated in the plant nucleus after COR treatment. This event required SAD2 to import ABI1 to the plant nucleus. Abolition of SAD2 undermined ABI1 accumulation. Our study answers the long-standing question of how bacterial COR antagonizes ABA signaling and reopens plant stomata during pathogen invasion.

Phylogenomic analyses and comparative genomics of Pseudomonas syringae associated with almond (Prunus dulcis) in California

We sequenced and comprehensively analysed the genomic architecture of 98 fluorescent pseudomonads isolated from different symptomatic and asymptomatic tissues of almond and a few other Prunus spp. Phylogenomic analyses, genome mining, field pathogenicity tests, and in vitro ice nucleation and antibiotic sensitivity tests were integrated to improve knowledge of the biology and management of bacterial blast and bacterial canker of almond. We identified Pseudomonas syringae pv. syringae, P. cerasi, and P. viridiflava as almond canker pathogens. P. syringae pv. syringae caused both canker and foliar (blast) symptoms. In contrast, P. cerasi and P. viridiflava only caused cankers, and P. viridiflava appeared to be a weak pathogen of almond. Isolates belonging to P. syringae pv. syringae were the most frequently isolated among the pathogenic species/pathovars, composing 75% of all pathogenic isolates. P. cerasi and P. viridiflava isolates composed 8.3 and 16.7% of the pathogenic isolates, respectively. Laboratory leaf infiltration bioassays produced results distinct from experiments in the field with both P. cerasi and P. syringae pv. syringae, causing significant necrosis and browning of detached leaves, whereas P. viridiflava conferred moderate effects. Genome mining revealed the absence of key epiphytic fitness-related genes in P. cerasi and P. viridiflava genomic sequences, which could explain the contrasting field and laboratory bioassay results. P. syringae pv. syringae and P. cerasi isolates harboured the ice nucleation protein, which correlated with the ice nucleation phenotype. Results of sensitivity tests to copper and kasugamycin showed a strong linkage to putative resistance genes. Isolates harbouring the ctpV gene showed resistance to copper up to 600 μg/ml. In contrast, isolates without the ctpV gene could not grow on nutrient agar amended with 200 μg/ml copper, suggesting ctpV can be used to phenotype copper resistance. All isolates were sensitive to kasugamycin at the label-recommended rate of 100μg/ml.

Protein interaction mapping reveals widespread targeting of development-related host transcription factors by phytoplasma effectors

Phytoplasmas are pathogenic bacteria that reprogram plant host development for their own benefit. Previous studies have characterized a few different phytoplasma effector proteins that destabilize specific plant transcription factors. However, these are only a small fraction of the potential effectors used by phytoplasmas; therefore, the molecular mechanisms through which phytoplasmas modulate their hosts require further investigation. To obtain further insights into the phytoplasma infection mechanisms, we generated a protein-protein interaction network between a broad set of phytoplasma effectors and a large, unbiased collection of Arabidopsis thaliana transcription factors and transcriptional regulators. We found widespread, but specific, interactions between phytoplasma effectors and host transcription factors, especially those related to host developmental processes. In particular, many unrelated effectors target specific sets of TCP transcription factors, which regulate plant development and immunity. Comparison with other host-pathogen protein interaction networks shows that phytoplasma effectors have unusual targets, indicating that phytoplasmas have evolved a unique and unusual infection strategy. This study contributes a rich and solid data source that guides further investigations of the functions of individual effectors, as demonstrated for some herein. Moreover, the dataset provides insights into the underlying molecular mechanisms of phytoplasma infection.

Integrative transcriptomics reveals association of abscisic acid and lignin pathways with cassava whitefly resistance

BACKGROUND

Whiteflies are a global threat to crop yields, including the African subsistence crop cassava (Manihot esculenta). Outbreaks of superabundant whitefly populations throughout Eastern and Central Africa in recent years have dramatically increased the pressures of whitefly feeding and virus transmission on cassava. Whitefly-transmitted viral diseases threaten the food security of hundreds of millions of African farmers, highlighting the need for developing and deploying whitefly-resistant cassava. However, plant resistance to whiteflies remains largely poorly characterized at the genetic and molecular levels. Knowledge of cassava-defense programs also remains incomplete, limiting characterization of whitefly-resistance mechanisms. To better understand the genetic basis of whitefly resistance in cassava, we define the defense hormone- and Aleurotrachelus socialis (whitefly)-responsive transcriptome of whitefly-susceptible (COL2246) and whitefly-resistant (ECU72) cassava using RNA-seq. For broader comparison, hormone-responsive transcriptomes of Arabidopsis thaliana were also generated.

RESULTS

Whitefly infestation, salicylic acid (SA), jasmonic acid (JA), ethylene (ET), and abscisic acid (ABA) transcriptome responses of ECU72 and COL2246 were defined and analyzed. Strikingly, SA responses were largely reciprocal between the two cassava genotypes and we suggest candidate regulators. While susceptibility was associated with SA in COL2246, resistance to whitefly in ECU72 was associated with ABA, with SA-ABA antagonism observed. This was evidenced by expression of genes within the SA and ABA pathways and hormone levels during A. socialis infestation. Gene-enrichment analyses of whitefly- and hormone-responsive genes suggest the importance of fast-acting cell wall defenses (e.g., elicitor recognition, lignin biosynthesis) during early infestation stages in whitefly-resistant ECU72. A surge of ineffective immune and SA responses characterized the whitefly-susceptible COL2246's response to late-stage nymphs. Lastly, in comparison with the model plant Arabidopsis, cassava's hormone-responsive genes showed striking divergence in expression.

CONCLUSIONS

This study provides the first characterization of cassava's global transcriptome responses to whitefly infestation and defense hormone treatment. Our analyses of ECU72 and COL2246 uncovered possible whitefly resistance/susceptibility mechanisms in cassava. Comparative analysis of cassava and Arabidopsis demonstrated that defense programs in Arabidopsis may not always mirror those in crop species. More broadly, our hormone-responsive transcriptomes will also provide a baseline for the cassava community to better understand global responses to other yield-limiting pests/pathogens.

Purification and characterization of tomato arginine decarboxylase and its inhibition by the bacterial small molecule phevamine A

Polyamines play essential roles in plant growth and survival. Arginine decarboxylase (ADC), which converts arginine to agmatine, catalyzes the first step in polyamine biosynthesis from arginine. However, few biochemical studies with purified plant ADCs have been conducted to evaluate their catalytic efficiency. Tomato genome encodes two arginine decarboxylases: SlADC1 and SlADC2, which are critical for growth, development, and immune responses against bacterial pathogens. We expressed and purified soluble SlADC1 as a recombinant protein fused with maltose-binding protein tag from E. coli Rosetta 2(DE3) cells. Using the purified fusion protein, we characterized the biochemical properties of SlADC1 in vitro and explored it as a target of the bacterial small molecule phevamine A. We confirmed that the activity of SlADC1 depends on the cofactor pyridoxal 5'-phosphate. SlADC1 is specific toward l-arginine and its kinetic parameters were measured using a liquid chromatography-mass spectrometry method. Phevamine A is a competitive inhibitor of SlADC1 and reduces the activity of SlADC1 at high micromolar concentrations. Our purification and biochemical characterization of SlADC1 sets the stage for inhibition studies of this enzyme.

Ubiquitin-Dependent and Independent Proteasomal Degradation in Host-Pathogen Interactions

Ubiquitin, a small protein, is well known for tagging target proteins through a cascade of enzymatic reactions that lead to protein degradation. The ubiquitin tag, apart from its signaling role, is paramount in destabilizing the modified protein. Here, we explore the complex role of ubiquitin-mediated protein destabilization in the intricate proteolysis process by the 26S proteasome. In addition, the significance of the so-called ubiquitin-independent pathway and the role of the 20S proteasome are considered. Next, we discuss the ubiquitin-proteasome system's interplay with pathogenic microorganisms and how the microorganisms manipulate this system to establish infection by a range of elaborate pathways to evade or counteract host responses. Finally, we focus on the mechanisms that rely either on (i) hijacking the host and on delivering pathogenic E3 ligases and deubiquitinases that promote the degradation of host proteins, or (ii) counteracting host responses through the stabilization of pathogenic effector proteins.

The Plant Ubiquitin-Proteasome System as a Target for Microbial Manipulation

The plant immune system perceives pathogens to trigger defense responses. In turn, pathogens secrete effector molecules to subvert these defense responses. The initiation and maintenance of defense responses involve not only de novo synthesis of regulatory proteins and enzymes but also their regulated degradation. The latter is achieved through protein degradation pathways such as the ubiquitin-proteasome system (UPS). The UPS regulates all stages of immunity, from the perception of the pathogen to the execution of the response, and, therefore, constitutes an ideal candidate for microbial manipulation of the host. Pathogen effector molecules interfere with the plant UPS through several mechanisms. This includes hijacking general UPS functions or perturbing its ability to degrade specific targets. In this review, we describe how the UPS regulates different immunity-related processes and how pathogens subvert this to promote disease.

Nuclear effectors of plant pathogens: Distinct strategies to be one step ahead

Nuclear effector proteins released by bacteria, oomycete, nematode, and fungi burden the global environment and crop yield. Microbial effectors are key weapons in the evolutionary arms race between plants and pathogens, vital in determining the success of pathogenic colonization. Secreted effectors undermine a multitude of host cellular processes depending on their target destination. Effectors are classified by their localization as either extracellular (apoplastic) or intracellular. Intracellular effectors can be further subclassified by their compartment such as the nucleus, cytoplasm or chloroplast. Nuclear effectors bring into question the role of the plant nucleus' intrinsic defence strategies and their vulnerability to effector-based manipulation. Nuclear effectors interfere with multiple nuclear processes including the epigenetic regulation of the host chromatin, the impairment of the trans-kingdom antifungal RNAi machinery, and diverse classes of immunity-associated host proteins. These effector-targeted pathways are widely conserved among different hosts and regulate a broad array of plant cellular processes. Thus, these nuclear sites constitute meaningful targets for effectors to subvert the plant defence system and acquire resources for pathogenic propagation. This review provides an extensive and comparative compilation of diverse plant microbe nuclear effector libraries, thereby highlighting the distinct and conserved mechanisms these effectors employ to modulate plant cellular processes for the pathogen's profit.

HexR Transcription Factor Contributes to Pseudomonas cannabina pv. alisalensis Virulence by Coordinating Type Three Secretion System Genes

Pseudomonas cannabina pv. alisalensis (Pcal) causes bacterial blight on cabbage. We previously conducted a screening for reduced virulence using Tn5 transposon mutants and identified one of the transcriptional factors, HexR, as a potential Pcal virulence factor. However, the role of HexR in plant pathogenic Pseudomonas virulence has not been investigated well. Here, we demonstrated that the Pcal hexR mutant showed reduced disease symptoms and bacterial populations on cabbage, indicating that HexR contributes to Pcal virulence. We used RNA-seq analysis to characterize the genes regulated by HexR. We found that several type three secretion system (T3SS)-related genes had lower expression of the Pcal hexR mutant. Five genes were related to T3SS machinery, two genes were related to type three helper proteins, and three genes encoded type three effectors (T3Es). We also confirmed that T3SS-related genes, including hrpL, avrPto, hopM1, and avrE1, were also down-regulated in the Pcal hexR mutant both in culture and in vivo by using RT-qPCR. T3SS functions to suppress plant defense in host plants and induce hypersensitive response (HR) cell death in non-host plants. Therefore, we investigated the expression profiles of cabbage defense-related genes, including PR1 and PR5, and found that the expressions of these genes were greater in the Pcal hexR mutant. We also demonstrated that the hexR mutant did not induce HR cell death in non-host plants, indicating that HexR contributes in causing HR in nonhost plants. Together, these results indicate that the mutation in hexR leads to a reduction in the T3SS-related gene expression and thus an impairment in plant defense suppression, reducing Pcal virulence.

Physiological Roles and Mechanisms of Action of Class I TCP Transcription Factors

TEOSINTE BRANCHED1, CYCLOIDEA, PROLIFERATING CELL FACTOR 1 and 2 (TCP) proteins constitute a plant-specific transcription factors family exerting effects on multiple aspects of plant development, such as germination, embryogenesis, leaf and flower morphogenesis, and pollen development, through the recruitment of other factors and the modulation of different hormonal pathways. They are divided into two main classes, I and II. This review focuses on the function and regulation of class I TCP proteins (TCPs). We describe the role of class I TCPs in cell growth and proliferation and summarize recent progresses in understanding the function of class I TCPs in diverse developmental processes, defense, and abiotic stress responses. In addition, their function in redox signaling and the interplay between class I TCPs and proteins involved in immunity and transcriptional and posttranslational regulation is discussed.

Distinct function of SPL genes in age-related resistance in Arabidopsis

In plants, age-related resistance (ARR) refers to a gain of disease resistance during shoot or organ maturation. ARR associated with vegetative phase change, a transition from juvenile to adult stage, is a widespread agronomic trait affecting resistance against multiple pathogens. How innate immunity in a plant is differentially regulated during successive stages of shoot maturation is unclear. In this work, we found that Arabidopsis thaliana showed ARR against its bacterial pathogen Pseudomonas syringae pv. tomato DC3000 during vegetative phase change. The timing of the ARR activation was associated with a temporal drop of miR156 level. The microRNA miR156 maintains juvenile phase by inhibiting the accumulation and translation of SPL transcripts. A systematic inspection of the loss- and gain-of-function mutants of 11 SPL genes revealed that a subset of SPL genes, notably SPL2, SPL10, and SPL11, activated ARR in adult stage. The immune function of SPL10 was independent of its role in morphogenesis. Furthermore, the SPL10 mediated an age-dependent augmentation of the salicylic acid (SA) pathway partially by direct activation of PAD4. Disrupting SA biosynthesis or signaling abolished the ARR against Pto DC3000. Our work demonstrated that the miR156-SPL10 module in Arabidopsis is deployed to operate immune outputs over developmental timing.

Endogenous salicylic acid suppresses de novo root regeneration from leaf explants

Plants can regenerate new organs from damaged or detached tissues. In the process of de novo root regeneration (DNRR), adventitious roots are frequently formed from the wound site on a detached leaf. Salicylic acid (SA) is a key phytohormone regulating plant defenses and stress responses. The role of SA and its acting mechanisms during de novo organogenesis is still unclear. Here, we found that endogenous SA inhibited the adventitious root formation after cutting. Free SA rapidly accumulated at the wound site, which was accompanied by an activation of SA response. SA receptors NPR3 and NPR4, but not NPR1, were required for DNRR. Wounding-elevated SA compromised the expression of AUX1, and subsequent transport of auxin to the wound site. A mutation in AUX1 abolished the enhanced DNRR in low SA mutants. Our work elucidates a role of SA in regulating DNRR and suggests a potential link between biotic stress and tissue regeneration.

Prevention of Stomatal Entry as a Strategy for Plant Disease Control against Foliar Pathogenic Pseudomonas Species

The genus Pseudomonas includes some of the most problematic and studied foliar bacterial pathogens. Generally, in a successful disease cycle there is an initial epiphytic lifestyle on the leaf surface and a subsequent aggressive endophytic stage inside the leaf apoplast. Leaf-associated bacterial pathogens enter intercellular spaces and internal leaf tissues by natural surface opening sites, such as stomata. The stomatal crossing is complex and dynamic, and functional genomic studies have revealed several virulence factors required for plant entry. Currently, treatments with copper-containing compounds, where authorized and admitted, and antibiotics are commonly used against bacterial plant pathogens. However, strains resistant to these chemicals occur in the fields. Therefore, the demand for alternative control strategies has been increasing. This review summarizes efficient strategies to prevent bacterial entry. Virulence factors required for entering the leaf in plant-pathogenic Pseudomonas species are also discussed.

A highly accumulated secretory protein from cotton bollworm interacts with basic helix-loop-helix transcription factors to dampen plant defense

Caterpillar oral secretion (OS) contains active molecules that modulate plant defense signaling. We isolated an effector-like protein (Highly Accumulated Secretory Protein 1, HAS1) from cotton bollworm (Helicoverpa armigera) that is the most highly accumulated secretory protein of the nondigestive components in OS and belongs to venom R-like protein. Elimination of HAS1 by plant-mediated RNA interference reduced the suppression of OS on the defense response in plants. Plants expressing HAS1 are more susceptible to insect herbivory accompanied by the reduced expressions of multiple defense genes. HAS1 binds to the basic helix-loop-helix (bHLH) transcription factors, including GoPGF involved in pigmented gland formation and defense compounds biosynthesis in cotton and MYC3/MYC4 the main regulators in jasmonate (JA) signaling in Arabidopsis. The binding activity is required for HAS1 to inhibit the activation of bHLHs on plant defense gene expressions. Together with our previous study that another venom R-like protein HARP1 in cotton bollworm OS blocks JA signaling by interacting with JASMONATE-ZIM-domain repressors, we conclude that the venom R-like proteins in OS interfere with plant defense in a dual suppression manner. Considering the venom proteins in parasitic wasp assault the immune system of its host animal, our investigation reveals their conserved function in carnivorous and herbivorous insects.

Tomato receptor-like cytoplasmic kinase Fir1 interacts with a negative regulator of jasmonic acid signaling

Plant cells detect potential pathogens through plasma membrane-localized pattern recognition receptors (PRRs) that recognize microbe-associated molecular patterns (MAMPs) and activate pattern-triggered immunity (PTI). PRR-mediated MAMP perception is linked to PTI signaling by receptor-like cytoplasmic kinases (RLCKs). In tomato, Flagellin-sensing 2 (Fls2)/Fls3 interacting RLCK 1 (Fir1) is involved in PTI triggered by flagellin perception. Fir1 is necessary for regulation of jasmonic acid (JA) signaling and is involved in pre-invasion immunity. We show that Fir1 physically interacts with JASMONATE-ZIM-DOMAIN PROTEIN 3 (JAZ3), a negative regulator of JA signaling. This finding suggests that Fir1 modulates JA signaling by regulating JAZ3.

The Phytophthora effector Avh94 manipulates host jasmonic acid signaling to promote infection

The oomycete pathogen Phytophthora sojae is a causal agent of soybean root rot. Upon colonization of soybeans, P. sojae secretes various RXLR effectors to suppress host immune responses, supporting successful infection. Previous research has demonstrated that the RXLR effector Avh94 functions as a virulence effector, but the molecular mechanism underlying its role in virulence remains unknown. Here, we demonstrate that Avh94 overexpression in plants and pathogens promotes Phytophthora infection. Avh94 interacts with soybean JAZ1/2, which is a repressor of jasmonic acid (JA) signaling. Avh94 stabilizes JAZ1/2 to inhibit JA signaling and silencing of JAZ1/2 enhances soybean resistance against P. sojae. Moreover, P. sojae lines overexpressing Avh94 inhibit JA signaling. Furthermore, exogenous application of methyl jasmonate improves plant resistance to Phytophthora. Taken together, these findings suggest that P. sojae employs an RXLR effector to hijack JA signaling and thereby promote infection.

Transcriptomic and Metabolomic Analysis of a Pseudomonas-Resistant versus a Susceptible Arabidopsis Accession

Accessions of one plant species may show significantly different levels of susceptibility to stresses. The Arabidopsis thaliana accessions Col-0 and C24 differ significantly in their resistance to the pathogen Pseudomonas syringae pv. tomato (Pst). To help unravel the underlying mechanisms contributing to this naturally occurring variance in resistance to Pst, we analyzed changes in transcripts and compounds from primary and secondary metabolism of Col-0 and C24 at different time points after infection with Pst. Our results show that the differences in the resistance of Col-0 and C24 mainly involve mechanisms of salicylic-acid-dependent systemic acquired resistance, while responses of jasmonic-acid-dependent mechanisms are shared between the two accessions. In addition, arginine metabolism and differential activity of the biosynthesis pathways of aliphatic glucosinolates and indole glucosinolates may also contribute to the resistance. Thus, this study highlights the difference in the defense response strategies utilized by different genotypes.

Acidovorax citrulli Effector AopV Suppresses Plant Immunity and Interacts with Aromatic Dehydratase ADT6 in Watermelon

Bacterial fruit blotch (BFB) is a disease of cucurbit plants caused by Acidovorax citrulli. Although A. citrulli has great destructive potential, the molecular mechanisms of pathogenicity of A. citrulli are not clear, particularly with regard to its type III secreted effectors. In this study, we characterized the type III secreted effector protein, AopV, from A. citrulli strain Aac5. We show that AopV significantly inhibits reactive oxygen species and the expression of PTI marker genes, and helps the growth of Pseudomonas syringae D36E in Nicotiana benthamiana. In addition, we found that the aromatic dehydratase ADT6 from watermelon was a target of AopV. AopV interacts with ADT6 in vivo and in vitro. Subcellular localization indicated ADT6 and AopV were co-located at the cell membrane. Together, our results reveal that AopV suppresses plant immunity and targets ADT6 in the cell membrane. These findings provide an new characterization of the molecular interaction of A. citrulli effector protein AopV with host cells.

Class I TCP transcription factor AtTCP8 modulates key brassinosteroid-responsive genes

The plant-specific TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR (TCP) transcription factor family is most closely associated with regulating plant developmental programs. Recently, TCPs were also shown to mediate host immune signaling, both as targets of pathogen virulence factors and as regulators of plant defense genes. However, comprehensive characterization of TCP gene targets is still lacking. Loss of function of the class I TCP gene AtTCP8 attenuates early immune signaling and, when combined with mutations in AtTCP14 and AtTCP15, additional layers of defense signaling in Arabidopsis (Arabidopsis thaliana). Here, we focus on TCP8, the most poorly characterized of the three to date. We used chromatin immunoprecipitation and RNA sequencing to identify TCP8-bound gene promoters and differentially regulated genes in the tcp8 mutant; these datasets were heavily enriched in signaling components for multiple phytohormone pathways, including brassinosteroids (BRs), auxin, and jasmonic acid. Using BR signaling as a representative example, we showed that TCP8 directly binds and activates the promoters of the key BR transcriptional regulatory genes BRASSINAZOLE-RESISTANT1 (BZR1) and BRASSINAZOLE-RESISTANT2 (BZR2/BES1). Furthermore, tcp8 mutant seedlings exhibited altered BR-responsive growth patterns and complementary reductions in BZR2 transcript levels, while TCP8 protein demonstrated BR-responsive changes in subnuclear localization and transcriptional activity. We conclude that one explanation for the substantial targeting of TCP8 alongside other TCP family members by pathogen effectors may lie in its role as a modulator of BR and other plant hormone signaling pathways.

Maintenance of tRNA and elongation factors supports T3SS proteins translational elongations in pathogenic bacteria during nutrient starvation

Background

Sufficient nutrition contributes to rapid translational elongation and protein synthesis in eukaryotic cells and prokaryotic bacteria. Fast synthesis and accumulation of type III secretion system (T3SS) proteins conduce to the invasion of pathogenic bacteria into the host cells. However, the translational elongation patterns of T3SS proteins in pathogenic bacteria under T3SS-inducing conditions remain unclear. Here, we report a mechanism of translational elongation of T3SS regulators, effectors and structural protein in four model pathogenic bacteria (Pseudomonas syringae, Pseudomonas aeruginosa, Xanthomonas oryzae and Ralstonia solanacearum) and a clinical isolate (Pseudomonas aeruginosa UCBPP-PA14) under nutrient-limiting conditions. We proposed a luminescence reporter system to quantitatively determine the translational elongation rates (ERs) of T3SS regulators, effectors and structural protein under different nutrient-limiting conditions and culture durations.

Results

The translational ERs of T3SS regulators, effectors and structural protein in these pathogenic bacteria were negatively regulated by the nutrient concentration and culture duration. The translational ERs in 0.5× T3SS-inducing medium were the highest of all tested media. In 1× T3SS-inducing medium, the translational ERs were highest at 0 min and then rapidly decreased. The translational ERs of T3SS regulators, effectors and structural protein were inhibited by tRNA degradation and by reduced levels of elongation factors (EFs).

Conclusions

Rapid translational ER and synthesis of T3SS protein need adequate tRNAs and EFs in nutrient-limiting conditions. Numeric presentation of T3SS translation visually indicates the invasion of bacteria and provides new insights into T3SS expression that can be applied to other pathogenic bacteria.

Inter-organismal phytohormone networks in plant-microbe interactions

Phytohormones are produced by plants and play central roles in interactions with pathogenic and beneficial microbes as well as plant growth and development. Each phytohormone pathway consists of its biosynthesis, transport, perception, and signaling and is intertwined with each other at various levels to form phytohormone networks in plants. Different kinds of microbes also produce phytohormones that exert physiological roles within microbes and manipulate phytohormone networks in plants by using phytohormones, their mimics, and proteinaceous effectors. In turn, plant-derived phytohormones can directly or indirectly through plant signaling networks affect microbial metabolism and community assembly. Therefore, phytohormone networks in plants and microbes are connected through plant and microbial phytohormones and other molecules to form inter-organismal phytohormone networks. In this review, we summarize recent progress on molecular mechanisms of inter-organismal phytohormone networks and discuss future steps necessary for advancing our understanding of phytohormone networks.

Beyond Destabilizing Activity of SAP11-like Effector of Candidatus Phytoplasma mali Strain PM19

It was shown that the SAP11 effector of different Candidatus Phytoplasma can destabilize some TEOSINE BRANCHES/CYCLOIDEA/PROLIFERATING CELL FACTORs (TCPs), resulting in plant phenotypes such as witches’ broom and crinkled leaves. Some SAP11 exclusively localize in the nucleus, while the others localize in the cytoplasm and the nucleus. The SAP11-like effector of Candidatus Phytoplasma mali strain PM19 (SAP11_PM19) localizes in both compartments of plant cells. We show here that SAP11_PM19 can destabilize TCPs in both the nucleus and the cytoplasm. However, expression of SAP11_PM19 exclusively in the nucleus resulted in the disappearance of leaf phenotypes while still showing the witches’ broom phenotype. Moreover, we show that SAP11_PM19 can not only destabilize TCPs but also relocalizes these proteins in the nucleus. Interestingly, three different transgenic Nicotiana species expressing SAP11_PM19 show all the same witches’ broom phenotype but different leaf phenotypes. A possible mechanism of SAP11-TCP interaction is discussed.

Transcriptional regulation of plant innate immunity

Transcriptional reprogramming is an integral part of plant immunity. Tight regulation of the immune transcriptome is essential for a proper response of plants to different types of pathogens. Consequently, transcriptional regulators are proven targets of pathogens to enhance their virulence. The plant immune transcriptome is regulated by many different, interconnected mechanisms that can determine the rate at which genes are transcribed. These include intracellular calcium signaling, modulation of the redox state, post-translational modifications of transcriptional regulators, histone modifications, DNA methylation, modulation of RNA polymerases, alternative transcription inititation, the Mediator complex and regulation by non-coding RNAs. In addition, on their journey from transcription to translation, mRNAs are further modulated through mechanisms such as nuclear RNA retention, storage of mRNA in stress granules and P-bodies, and post-transcriptional gene silencing. In this review, we highlight the latest insights into these mechanisms. Furthermore, we discuss some emerging technologies that promise to greatly enhance our understanding of the regulation of the plant immune transcriptome in the future.

Action Mechanisms of Effectors in Plant-Pathogen Interaction

Plant pathogens are one of the main factors hindering the breeding of cash crops. Pathogens, including oomycetes, fungus, and bacteria, secrete effectors as invasion weapons to successfully invade and propagate in host plants. Here, we review recent advances made in the field of plant-pathogen interaction models and the action mechanisms of phytopathogenic effectors. The review illustrates how effectors from different species use similar and distinct strategies to infect host plants. We classify the main action mechanisms of effectors in plant-pathogen interactions according to the infestation process: targeting physical barriers for disruption, creating conditions conducive to infestation, protecting or masking themselves, interfering with host cell physiological activity, and manipulating plant downstream immune responses. The investigation of the functioning of plant pathogen effectors contributes to improved understanding of the molecular mechanisms of plant-pathogen interactions. This understanding has important theoretical value and is of practical significance in plant pathology and disease resistance genetics and breeding.

Salicylic acid and jasmonic acid crosstalk in plant immunity

The phytohormones salicylic acid (SA) and jasmonic acid (JA) are major players in plant immunity. Numerous studies have provided evidence that SA- and JA-mediated signaling interact with each other (SA-JA crosstalk) to orchestrate plant immune responses against pathogens. At the same time, SA-JA crosstalk is often exploited by pathogens to promote their virulence. In this review, we summarize our current knowledge of molecular mechanisms for and modulations of SA-JA crosstalk during pathogen infection.

Comparative Genomics of Prunus-Associated Members of the Pseudomonas syringae Species Complex Reveals Traits Supporting Co-evolution and Host Adaptation

Members of the Pseudomonas syringae species complex cause symptoms that are ranging from leaf spots to cankers on a multitude of plant species, including some of the genus Prunus. To date, a total of two species of the P. syringae species complex and six different pathovars have been associated with diseases on Prunus spp., which were shown to belong to different phylogenetic units (phylogroups, PG) based on sequence similarity of housekeeping genes or whole genomes, suggesting that virulence to Prunus spp. may be the result of convergent pathoadaptation. In this study, a comparative genomics approach was used to determine genes significantly associated with strains isolated from Prunus spp. across a phylogeny of 97 strains belonging to the P. syringae species complex. Our study revealed the presence of a set of orthologous proteins which were significantly associated with strains isolated from Prunus spp. than in strains isolated from other hosts or from non-agricultural environments. Among them, the type III effector HopAY predicted to encode for a C58 cysteine protease was found to be highly associated with strains isolated from Prunus spp. and revealed patterns supporting co-evolution and host adaptation.

A phytoplasma effector acts as a ubiquitin-like mediator between floral MADS-box proteins and proteasome shuttle proteins

Plant pathogenic bacteria have developed effectors to manipulate host cell functions to facilitate infection. A certain number of effectors use the conserved ubiquitin-proteasome system in eukaryotic to proteolyze targets. The proteasome utilization mechanism is mainly mediated by ubiquitin interaction with target proteins destined for degradation. Phyllogens are a family of protein effectors produced by pathogenic phytoplasmas that transform flowers into leaves in diverse plants. Here, we present a noncanonical mechanism for phyllogen action that involves the proteasome and is ubiquitin-independent. Phyllogens induce proteasomal degradation of floral MADS-box transcription factors (MTFs) in the presence of RADIATION-SENSITIVE23 (RAD23) shuttle proteins, which recruit ubiquitinated proteins to the proteasome. Intracellular localization analysis revealed that phyllogen induced colocalization of MTF with RAD23. The MTF/phyllogen/RAD23 ternary protein complex was detected not only in planta but also in vitro in the absence of ubiquitin, showing that phyllogen directly mediates interaction between MTF and RAD23. A Lys-less nonubiquitinated phyllogen mutant induced degradation of MTF or a Lys-less mutant of MTF. Furthermore, the method of sequential formation of the MTF/phyllogen/RAD23 protein complex was elucidated, first by MTF/phyllogen interaction and then RAD23 recruitment. Phyllogen recognized both the evolutionarily conserved tetramerization region of MTF and the ubiquitin-associated domain of RAD23. Our findings indicate that phyllogen functionally mimics ubiquitin as a mediator between MTF and RAD23.

Regulation and integration of plant jasmonate signaling: a comparative view of monocot and dicot

The phytohormone jasmonate plays a pivotal role in various aspects of plant life, including developmental programs and defense against pests and pathogens. A large body of knowledge on jasmonate biosynthesis, signal transduction as well as its functions in diverse plant processes has been gained in the past two decades. In addition, there exists extensive crosstalk between jasmonate pathway and other phytohormone pathways, such as salicylic acid (SA) and gibberellin (GA), in co-regulation of plant immune status, fine-tuning the balance of plant growth and defense, and so on, which were mostly learned from studies in the dicotyledonous model plants Arabidopsis thaliana and tomato but much less in monocot. Interestingly, existing evidence suggests both conservation and functional divergence in terms of core components of jasmonate pathway, its biological functions and signal integration with other phytohormones, between monocot and dicot. In this review, we summarize the current understanding on JA signal initiation, perception and regulation, and highlight the distinctive characteristics in different lineages of plants.

Oomycete intracellular effectors: specialised weapons targeting strategic plant processes

Oomycete phytopathogens have adapted to colonise plants using effectors as their molecular weapons. Intracellular effectors, mostly proteins but also small ribonucleic acids, are delivered by the pathogens into the host cell cytoplasm where they interfere with normal plant physiology. The diverse host processes emerging as 'victims' of these 'specialised bullets' include gene transcription and RNA-mediated silencing, cell death, protein stability, protein secretion and autophagy. Some effector targets are directly involved in defence execution, while others participate in fundamental metabolisms whose alteration collaterally affects defences. Other effector targets are susceptibility factors (SFs), that is host components that make plants vulnerable to pathogens. SFs are mostly negative regulators of immunity, but some seem necessary to sustain or promote pathogen colonisation.

Pseudomonas syringae activates ZAT18 to inhibit salicylic acid accumulation by repressing EDS1 transcription for bacterial infection

Phytopathogens can manipulate plant hormone signaling to counteract immune responses; however, the underlying mechanism is mostly unclear. Here, we report that Pseudomonas syringae pv tomato (Pst) DC3000 induces expression of C2H2 zinc finger transcription factor ZAT18 in a jasmonic acid (JA)-signaling-dependent manner. Biochemical assays further confirmed that ZAT18 is a direct target of MYC2, which is a very important regulator in JA signaling. CRISPR/Cas9-generated zat18-cr mutants exhibited enhanced resistance to Pst DC3000, while overexpression of ZAT18 resulted in impaired disease resistance. Genetic characterization of ZAT18 mutants demonstrated that ZAT18 represses defense responses by inhibiting the accumulation of the key plant immune signaling molecule salicylic acid (SA), which is dependent on its EAR motif. ZAT18 exerted this inhibitory effect by directly repressing the transcription of Enhanced Disease Susceptibility 1 (EDS1), which is the key signaling component of pathogen-induced SA accumulation. Overexpression of ZAT18 resulted in decreased SA content, while loss of function of ZAT18 showed enhanced SA accumulation upon pathogen infection. Furthermore, enhanced resistance and SA content in zat18-cr mutants was abolished by the mutation in EDS1. Our data indicate that pathogens induce ZAT18 expression to repress the transcription of EDS1, further antagonising SA accumulation for bacterial infection.

Multiomic Analysis Reveals Core Regulatory Mechanisms underlying Steroidal Glycoalkaloid Metabolism in Potato Tubers

Steroidal glycoalkaloids (SGAs) present in germinated potato tubers are toxic; however, the mechanisms underlying SGA metabolism are poorly understood. Therefore, integrated transcriptome, metabolome, and hormone analyses were performed in this study to identify and characterize the key regulatory genes, metabolites, and phytohormones related to glycoalkaloid regulation. Based on transcriptome sequencing of bud eyes of germinated and dormant potato tubers, a total of 6260 differentially expressed genes were identified, which were mainly responsible for phytohormone signal transduction, carbohydrate metabolism, and secondary metabolite biosynthesis. Two TCP14 genes were identified as the core transcription factors that potentially regulate SGA synthesis. Metabolite analysis indicated that 149 significantly different metabolites were detected, and they were enriched in metabolic and biosynthetic pathways of secondary metabolites. In these pathways, the α-solanine content was increased and the expression of genes related to glycoalkaloid biosynthesis was upregulated. Levels of gibberellin and jasmonic acid were increased, whereas that of abscisic acid was decreased. This study lays a foundation for investigating the biosynthesis and regulation of SGAs and provides the reference for the production and consumption of potato tubers.

Pseudomonas syringae pv. actinidiae Effector HopAU1 Interacts with Calcium-Sensing Receptor to Activate Plant Immunity

Kiwifruit canker, caused by Pseudomonas syringae pv. actinidiae (Psa), is a destructive pathogen that globally threatens the kiwifruit industry. Understanding the molecular mechanism of plant-pathogen interaction can accelerate applying resistance breeding and controlling plant diseases. All known effectors secreted by pathogens play an important role in plant-pathogen interaction. However, the effectors in Psa and their function mechanism remain largely unclear. Here, we successfully identified a T3SS effector HopAU1 which had no virulence contribution to Psa, but could, however, induce cell death and activate a series of immune responses by agroinfiltration in Nicotiana benthamiana, including elevated transcripts of immune-related genes, accumulation of reactive oxygen species (ROS), and callose deposition. We found that HopAU1 interacted with a calcium sensing receptor in N. benthamiana (NbCaS) as well as its close homologue in kiwifruit (AcCaS). More importantly, silencing CaS by RNAi in N. benthamiana greatly attenuated HopAU1-triggered cell death, suggesting CaS is a crucial component for HopAU1 detection. Further researches showed that overexpression of NbCaS in N. benthamiana significantly enhanced plant resistance against Sclerotinia sclerotiorum and Phytophthora capsici, indicating that CaS serves as a promising resistance-related gene for disease resistance breeding. We concluded that HopAU1 is an immune elicitor that targets CaS to trigger plant immunity.

Transcriptional Coactivators: Driving Force of Plant Immunity

Salicylic acid (SA) is a plant defense signal that mediates local and systemic immune responses against pathogen invasion. However, the underlying mechanism of SA-mediated defense is very complex due to the involvement of various positive and negative regulators to fine-tune its signaling in diverse pathosystems. Upon pathogen infections, elevated level of SA promotes massive transcriptional reprogramming in which Non-expresser of PR genes 1 (NPR1) acts as a central hub and transcriptional coactivator in defense responses. Recent findings show that Enhanced Disease Susceptibility 1 (EDS1) also functions as a transcriptional coactivator and stimulates the expression of PR1 in the presence of NPR1 and SA. Furthermore, EDS1 stabilizes NPR1 protein level, while NPR1 sustains EDS1 expression during pathogenic infection. The interaction of NPR1 and EDS1 coactivators initiates transcriptional reprogramming by recruiting cyclin-dependent kinase 8 in the Mediator complex to control immune responses. In this review, we highlight the recent breakthroughs that considerably advance our understanding on how transcriptional coactivators interact with their functional partners to trigger distinct pathways to facilitate immune responses, and how SA accumulation induces dynamic changes in NPR1 structure for transcriptional reprogramming. In addition, the functions of different Mediator subunits in SA-mediated plant immunity are also discussed in light of recent discoveries. Taken together, the available evidence suggests that transcriptional coactivators are essential and potent regulators of plant defense pathways and play crucial roles in coordinating plant immune responses during plant-pathogen interactions.

Cotton miR319b-Targeted TCP4-Like Enhances Plant Defense Against Verticillium dahliae by Activating GhICS1 Transcription Expression

Teosinte branched1/Cincinnata/proliferating cell factor (TCP) transcription factors play important roles in plant growth and defense. However, the molecular mechanisms of TCPs participating in plant defense remain unclear. Here, we characterized a cotton TCP4-like fine-tuned by miR319b, which could interact with NON-EXPRESSER OF PATHOGEN-RELATED GENES 1 (NPR1) to directly activate isochorismate synthase 1 (ICS1) expression, facilitating plant resistance against Verticillium dahliae. mRNA degradome data and GUS-fused assay showed that GhTCP4-like mRNA was directedly cleaved by ghr-miR319b. Knockdown of ghr-miR319b increased plant resistance to V. dahliae, whereas silencing GhTCP4-like increased plant susceptibility by the virus-induced gene silencing (VIGS) method, suggesting that GhTCP4-like is a positive regulator of plant defense. According to the electrophoretic mobility shift assay and GUS reporter analysis, GhTCP4-like could transcriptionally activate GhICS1 expression, resulting in increased salicylic acid (SA) accumulation. Yeast two-hybrid and luciferase complementation image analyses demonstrated that GhTCP4-like interacts with GhNPR1, which can promote GhTCP4-like transcriptional activation in GhICS1 expression according to the GUS reporter assay. Together, these results revealed that GhTCP4-like interacts with GhNPR1 to promote GhICS1 expression through fine-tuning of ghr-miR319b, leading to SA accumulation, which is percepted by NPR1 to increase plant defense against V. dahliae. Therefore, GhTCP4-like participates in a positive feedback regulation loop of SA biosynthesis via NPR1, increasing plant defenses against fungal infection.

An angiosperm NLR Atlas reveals that NLR gene reduction is associated with ecological specialization and signal transduction component deletion

Nucleotide-binding leucine-rich-repeat (NLR) genes comprise the largest family of plant disease-resistance genes. Angiosperm NLR genes are phylogenetically divided into the TNL, CNL, and RNL subclasses. NLR copy numbers and subclass composition vary tremendously across angiosperm genomes. However, the evolutionary associations between genomic NLR content and ecological adaptation, or between NLR content and signal transduction components, are poorly characterized because of limited genome availability. In this study, we established an angiosperm NLR atlas (ANNA, https://biobigdata.nju.edu.cn/ANNA/) that includes NLR genes from over 300 angiosperm genomes. Using ANNA, we revealed that NLR copy numbers differ up to 66-fold among closely related species owing to rapid gene loss and gain. Interestingly, NLR contraction was associated with adaptations to aquatic, parasitic, and carnivorous lifestyles. The convergent NLR reduction in aquatic plants resembles the lack of NLR expansion during the long-term evolution of green algae before the colonization of land. A co-evolutionary pattern between NLR subclasses and plant immune pathway components was also identified, suggesting that immune pathway deficiencies may drive TNL loss. Finally, we identified a conserved TNL lineage that may function independently of the EDS1-SAG101-NRG1 module. Collectively, these findings provide new insights into the evolution of NLR genes in the context of ecological adaptation and genome content variation.

Overexpression of watermelon m6A methyltransferase ClMTB enhances drought tolerance in tobacco by mitigating oxidative stress and photosynthesis inhibition and modulating stress-responsive gene expression

N6-methyladenosine (m6A) in RNA is a very important post-transcriptional modification mechanism in eukaryotes. It has been reported to have important regulatory roles in some stress responses in model plants, but there has been no research regarding m6A modifications in watermelon. In this study, we cloned and characterized m6A methyltransferase, ClMTB (mRNA adenosine methylase B, METTL14 human homolog protein) in watermelon. ClMTB expression could be weakly induced by drought stress as determined by the quantitative real-time PCR (qRT-PCR) and Promoter::GUS analyses. ClMTB over-expressed in tobacco plants increased drought tolerance via enhancing reactive oxygen species (ROS) scavenging system and alleviating photosynthesis inhibition under drought. Transcriptome profiles indicated the multiple hormone and stress-responsive genes were specifically induced in over-expressed ClMTB plants under drought conditions. These results suggest that ClMTB-mediated m6A modification serves as a positive regulatory factor of drought tolerance. This study is the first one to provide an understanding of the specific roles of ClMTB in watermelon adaptation to drought stress, and may also provide important insights into the signaling pathway mediated by m6A modification in response to stress conditions.

A class of independently evolved transcriptional repressors in plant RNA viruses facilitates viral infection and vector feeding

Plant viruses employ diverse virulence strategies to achieve successful infection, but there are few known general strategies of viral pathogenicity and transmission used by widely different plant viruses. Here, we report a class of independently evolved virulence factors in different plant RNA viruses which possess active transcriptional repressor activity. Rice viruses in the genera Fijivirus, Tenuivirus, and Cytorhabdovirus all have transcriptional repressors that interact in plants with the key components of jasmonic acid (JA) signaling, namely mediator subunit OsMED25, OsJAZ proteins, and OsMYC transcription factors. These transcriptional repressors can directly disassociate the OsMED25-OsMYC complex, inhibit the transcriptional activation of OsMYC, and then combine with OsJAZ proteins to cooperatively attenuate the JA pathway in a way that benefits viral infection. At the same time, these transcriptional repressors efficiently enhanced feeding by the virus insect vectors by repressing JA signaling. Our findings reveal a common strategy in unrelated plant viruses in which viral transcriptional repressors hijack and repress the JA pathway in favor of both viral pathogenicity and vector transmission.

HPE1, an Effector from Zebra Chip Pathogen Interacts with Tomato Proteins and Perturbs Ubiquitinated Protein Accumulation

The gram-negative bacterial genus Liberibacter includes economically important pathogens, such as 'Candidatus Liberibacter asiaticus' that cause citrus greening disease (or Huanglongbing, HLB) and 'Ca. Liberibacter solanacearum' (Lso) that cause zebra chip disease in potato. Liberibacter pathogens are fastidious bacteria transmitted by psyllids. Pathogen manipulation of the host' and vector's immune system for successful colonization is hypothesized to be achieved by Sec translocon-dependent effectors (SDE). In previous work, we identified hypothetical protein effector 1 (HPE1), an SDE from Lso, that acts as a suppressor of the plant's effector-triggered immunity (ETI)-like response. In this study, using a yeast two-hybrid system, we identify binding interactions between tomato RAD23 proteins and HPE1. We further show that HPE1 interacts with RAD23 in both nuclear and cytoplasmic compartments in planta. Immunoblot assays show that HPE1 is not ubiquitinated in the plant cell, but rather the expression of HPE1 induced the accumulation of other ubiquitinated proteins. A similar accumulation of ubiquitinated proteins is also observed in Lso infected tomato plants. Finally, earlier colonization and symptom development following Lso haplotype B infection are observed in HPE1 overexpressing plants compared to wild-type plants. Overall, our results suggest that HPE1 plays a role in virulence in Lso pathogenesis, possibly by perturbing the ubiquitin-proteasome system via direct interaction with the ubiquitin-like domain of RAD23 proteins.

Pathogen Effectors: Exploiting the Promiscuity of Plant Signaling Hubs

Pathogens produce effectors to overcome plant immunity, thereby threatening crop yields and global food security. Large-scale interactomic studies have revealed that pathogens from different kingdoms of life target common plant proteins during infection, the so-called effector hubs. These hubs often play central roles in numerous plant processes through their ability to interact with multiple plant proteins. This ability arises partly from the presence of intrinsically disordered domains (IDDs) in their structure. Here, we highlight the role of the TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR (TCP) and JASMONATE-ZIM DOMAIN (JAZ) transcription regulator families as plant signaling and effector hubs. We consider different evolutionary hypotheses to rationalize the existence of diverse effectors sharing common targets and the possible role of IDDs in this interaction.

Regulation of the Pseudomonas syringae Type III Secretion System by Host Environment Signals

Pseudomonas syringae are Gram-negative, plant pathogenic bacteria that use a type III secretion system (T3SS) to disarm host immune responses and promote bacterial growth within plant tissues. Despite the critical role for type III secretion in promoting virulence, T3SS-encoding genes are not constitutively expressed by P. syringae and must instead be induced during infection. While it has been known for many years that culturing P. syringae in synthetic minimal media can induce the T3SS, relatively little is known about host signals that regulate the deployment of the T3SS during infection. The recent identification of specific plant-derived amino acids and organic acids that induce T3SS-inducing genes in P. syringae has provided new insights into host sensing mechanisms. This review summarizes current knowledge of the regulatory machinery governing T3SS deployment in P. syringae, including master regulators HrpRS and HrpL encoded within the T3SS pathogenicity island, and the environmental factors that modulate the abundance and/or activity of these key regulators. We highlight putative receptors and regulatory networks involved in linking the perception of host signals to the regulation of the core HrpRS–HrpL pathway. Positive and negative regulation of T3SS deployment is also discussed within the context of P. syringae infection, where contributions from distinct host signals and regulatory networks likely enable the fine-tuning of T3SS deployment within host tissues. Last, we propose future research directions necessary to construct a comprehensive model that (a) links the perception of host metabolite signals to T3SS deployment and (b) places these host–pathogen signaling events in the overall context of P. syringae infection.

What the Wild Things Do: Mechanisms of Plant Host Manipulation by Bacterial Type III-Secreted Effector Proteins

Phytopathogenic bacteria possess an arsenal of effector proteins that enable them to subvert host recognition and manipulate the host to promote pathogen fitness. The type III secretion system (T3SS) delivers type III-secreted effector proteins (T3SEs) from bacterial pathogens such as Pseudomonas syringae, Ralstonia solanacearum, and various Xanthomonas species. These T3SEs interact with and modify a range of intracellular host targets to alter their activity and thereby attenuate host immune signaling. Pathogens have evolved T3SEs with diverse biochemical activities, which can be difficult to predict in the absence of structural data. Interestingly, several T3SEs are activated following injection into the host cell. Here, we review T3SEs with documented enzymatic activities, as well as T3SEs that facilitate virulence-promoting processes either indirectly or through non-enzymatic mechanisms. We discuss the mechanisms by which T3SEs are activated in the cell, as well as how T3SEs modify host targets to promote virulence or trigger immunity. These mechanisms may suggest common enzymatic activities and convergent targets that could be manipulated to protect crop plants from infection.

Identification of a Putative DNA-Binding Protein in Arabidopsis That Acts as a Susceptibility Hub and Interacts With Multiple Pseudomonas syringae Effectors

Phytopathogens use secreted effector proteins to suppress host immunity and promote pathogen virulence, and there is increasing evidence that the host-pathogen interactome comprises a complex network. To identify novel interactors of the Pseudomonas syringae effector HopZ1a, we performed a yeast two-hybrid screen that identified a previously uncharacterized Arabidopsis protein that we designate HopZ1a interactor 1 (ZIN1). Additional analyses in yeast and in planta revealed that ZIN1 also interacts with several other P. syringae effectors. We show that an Arabidopsis loss-of-function zin1 mutant is less susceptible to infection by certain strains of P. syringae, while overexpression of ZIN1 results in enhanced susceptibility. Functionally, ZIN1 exhibits topoisomerase-like activity in vitro. Transcriptional profiling of wild-type and zin1 Arabidopsis plants inoculated with P. syringae indicated that while ZIN1 regulates a wide range of pathogen-responsive biological processes, the list of genes more highly expressed in zin1 versus wild-type plants is particularly enriched for ribosomal protein genes. Altogether, these data illuminate ZIN1 as a potential susceptibility hub that interacts with multiple effectors to influence the outcome of plant-microbe interactions.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Chloroplast: The Emerging Battlefield in Plant-Microbe Interactions

Higher plants and some algae convert the absorbed light into chemical energy through one of the most important organelles, chloroplast, for photosynthesis and store it in the form of organic compounds to supply their life activities. However, more and more studies have shown that the role of chloroplasts is more than a factory for photosynthesis. In the process of light conversion to chemical energy, any damage to the components of chloroplast may affect the photosynthesis efficiency and promote the production of by-products, reactive oxygen species, that are mainly produced in the chloroplasts. Substantial evidence show that chloroplasts are also involved in the battle of plants and microbes. Chloroplasts are important in integrating a variety of external environmental stimuli and regulate plant immune responses by transmitting signals to the nucleus and other cell compartments through retrograde signaling pathways. Besides, chloroplasts can also regulate the biosynthesis and signal transduction of phytohormones, including salicylic acid and jasmonic acid, to affect the interaction between the plants and microbes. Since chloroplasts play such an important role in plant immunity, correspondingly, chloroplasts have become the target of pathogens. Different microbial pathogens target the chloroplast and affect its functions to promote their colonization in the host plants.

Ustilago maydis effector Jsi1 interacts with Topless corepressor, hijacking plant jasmonate/ethylene signaling

Ustilago maydis is the causal agent of maize smut disease. During the colonization process, the fungus secretes effector proteins that suppress immune responses and redirect the host metabolism in favor of the pathogen. As effectors play a critical role during plant colonization, their identification and functional characterization are essential to understanding biotrophy and disease. Using biochemical, molecular, and transcriptomic techniques, we performed a functional characterization of the U. maydis effector Jasmonate/Ethylene signaling inducer 1 (Jsi1). Jsi1 interacts with several members of the plant corepressor family Topless/Topless related (TPL/TPR). Jsi1 expression in Zea mays and Arabidopsis thaliana leads to transcriptional induction of the ethylene response factor (ERF) branch of the jasmonate/ethylene (JA/ET) signaling pathway. In A. thaliana, activation of the ERF branch leads to biotrophic susceptibility. Jsi1 likely activates the ERF branch via an EAR (ET-responsive element binding-factor-associated amphiphilic repression) motif, which resembles EAR motifs from plant ERF transcription factors, that interacts with TPL/TPR proteins. EAR-motif-containing effector candidates were identified from different fungal species, including Magnaporthe oryzae, Sporisorium scitamineum, and Sporisorium reilianum. Interaction between plant TPL proteins and these effector candidates from biotrophic and hemibiotrophic fungi indicates the convergent evolution of effectors modulating the TPL/TPR corepressor hub.

Arabidopsis Abscisic Acid Repressor 1 is a susceptibility hub that interacts with multiple Pseudomonas syringae effectors

Pathogens secrete effector proteins into host cells to suppress host immunity and promote pathogen virulence, although many features at the molecular interface of host-pathogen interactions remain to be characterized. In a yeast two-hybrid assay, we found that the Pseudomonas syringae effector HopZ1a interacts with the Arabidopsis transcriptional regulator Abscisic Acid Repressor 1 (ABR1). Further analysis revealed that ABR1 interacts with multiple P. syringae effectors, suggesting that it may be targeted as a susceptibility hub. Indeed, loss-of-function abr1 mutants exhibit reduced susceptibility to a number of P. syringae strains. The ABR1 protein comprises a conserved APETALA2 (AP2) domain flanked by long regions of predicted structural disorder. We verified the DNA-binding activity of the AP2 domain and demonstrated that the disordered domains act redundantly to enhance DNA binding and to facilitate transcriptional activation by ABR1. Finally, we compared gene expression profiles from wild-type and abr1 plants following inoculation with P. syringae, which suggested that the reduced susceptibility of abr1 mutants is due to the loss of a virulence target rather than an enhanced immune response. These data highlight ABR1 as a functionally important component at the host-pathogen interface.

A Conserved Oomycete CRN Effector Targets Tomato TCP14-2 to Enhance Virulence

Phytophthora spp. secrete vast arrays of effector molecules during infection to aid in host colonization. The crinkling and necrosis (CRN) protein family forms an extensive repertoire of candidate effectors that accumulate in the host nucleus to perturb processes required for immunity. Here, we show that CRN12_997 from Phytophthora capsici binds a TCP transcription factor, SlTCP14-2, to inhibit its immunity-associated activity against Phytophthora spp. Coimmunoprecipitation and bimolecular fluorescence complementation studies confirm a specific CRN12_997-SlTCP14-2 interaction in vivo. Coexpression of CRN12_997 specifically counteracts the TCP14-enhanced immunity phenotype, suggesting that CRN mediated perturbation of SlTCP14-2 function. We show that SlTCP14-2 associates with nuclear chromatin and that CRN12_997 diminishes SlTCP14-2 DNA binding. Collectively, our data support a model in which SlTCP14-2 associates with chromatin to enhance immunity. The interaction between CRN12_997 and SlTCP14-2 reduces DNA binding of the immune regulator. We propose that the modulation of SlTCP14-2 chromatin affinity, caused by CRN12-997, enhances susceptibility to P. capsici.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Dissecting Contrasts in Cell Death, Hormone, and Defense Signaling in Response to Botrytis cinerea and Reactive Oxygen Species

Plants require interaction between signaling pathways to differentiate and integrate stress responses and deploy appropriate defenses. The hormones ethylene, salicylic acid (SA), and jasmonic acid (JA) are important regulators of plant defenses. Numerous interactions between these signaling pathways are the cornerstone of robust plant immunity. Additionally, during the early response to pathogens, reactive oxygen species (ROS) act as signaling molecules. Here, we examined the extent of signal interaction in the early stages of Botrytis cinerea infection. To enable a comparison between B. cinerea infection with ROS signaling, we subjected plants to ozone treatment, which stimulates an apoplastic ROS burst. We used a collection of single, double, and triple signaling mutants defective in hormone signaling and biosynthesis and subjected them to B. cinerea infection and ozone treatment at different timepoints. We examined lesion size, cell death, and gene expression (both quantitatively and spatially). The two treatments shared many similarities, especially in JA-insensitive mutants, which were sensitive to both treatments. Unexpectedly, a B. cinerea-susceptible JA-insensitive mutant (coi1), became tolerant when both SA biosynthesis and signaling was impaired (coi1 npr1 sid2), demonstrating that JA responses may be under the control of SA. Extensive marker gene analysis indicated JA as the main regulator of both B. cinerea and ozone defenses. In addition, we identified the transcription factor SR1 as a crucial regulator of PLANT DEFENSIN expression and cell-death regulation, which contributes to resistance to B. cinerea. Overall, our work further defines the context of ROS in plant defense signaling.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Bacterial nucleomodulins: A coevolutionary adaptation to the eukaryotic command center

Through long-term interactions with their hosts, bacterial pathogens have evolved unique arsenals of effector proteins that interact with specific host targets and reprogram the host cell into a permissive niche for pathogen proliferation. The targeting of effector proteins into the host cell nucleus for modulation of nuclear processes is an emerging theme among bacterial pathogens. These unique pathogen effector proteins have been termed in recent years as "nucleomodulins." The first nucleomodulins were discovered in the phytopathogens Agrobacterium and Xanthomonas, where their nucleomodulins functioned as eukaryotic transcription factors or integrated themselves into host cell DNA to promote tumor induction, respectively. Numerous nucleomodulins were recently identified in mammalian pathogens. Bacterial nucleomodulins are an emerging family of pathogen effector proteins that evolved to target specific components of the host cell command center through various mechanisms. These mechanisms include: chromatin dynamics, histone modification, DNA methylation, RNA splicing, DNA replication, cell cycle, and cell signaling pathways. Nucleomodulins may induce short- or long-term epigenetic modifications of the host cell. In this extensive review, we discuss the current knowledge of nucleomodulins from plant and mammalian pathogens. While many nucleomodulins are already identified, continued research is instrumental in understanding their mechanisms of action and the role they play during the progression of pathogenesis. The continued study of nucleomodulins will enhance our knowledge of their effects on nuclear chromatin dynamics, protein homeostasis, transcriptional landscapes, and the overall host cell epigenome.

A Meta-Analysis Reveals Opposite Effects of Biotic and Abiotic Stresses on Transcript Levels of Arabidopsis Intracellular Immune Receptor Genes

Plant intracellular immune receptor NLR (nucleotide-binding leucine-rich repeat) proteins sense the presence of pathogens and trigger strong and robust immune responses. NLR genes are known to be tightly controlled at the protein level, but little is known about their dynamics at the transcript level. In this study, we presented a meta-analysis of transcript dynamics of all 207 NLR genes in the Col-0 accession of Arabidopsis thaliana under various biotic and abiotic stresses based on 88 publicly available RNA sequencing datasets from 27 independent studies. We find that about two thirds of the NLR genes are generally induced by pathogens, immune elicitors, or salicylic acid (SA), suggesting that transcriptional induction of NLR genes might be an important mechanism in plant immunity regulation. By contrast, NLR genes induced by biotic stresses are often repressed by abscisic acid, high temperature and drought, suggesting that transcriptional regulation of NLR genes might be important for interaction between abiotic and biotic stress responses. In addition, pathogen-induced expression of some NLR genes are dependent on SA induction. Interestingly, a small group of NLR genes are repressed under certain biotic stress treatments, suggesting an unconventional function of this group of NLRs. This meta-analysis thus reveals the transcript dynamics of NLR genes under biotic and abiotic stress conditions and suggests a contribution of NLR transcript regulation to plant immunity as well as interactions between abiotic and biotic stress responses.