Jasmonate ZIM-domain Repressors Research Articles

The transcriptional regulator JAZ8 interacts with the C2 protein from geminiviruses and limits the geminiviral infection in Arabidopsis.

Jasmonates (JAs) are phytohormones that finely regulate critical biological processes, including plant development and defense. JASMONATE ZIM-DOMAIN (JAZ) proteins are crucial transcriptional regulators that keep JA-responsive genes in a repressed state. In the presence of JA-Ile, JAZ repressors are ubiquitinated and targeted for degradation by the ubiquitin/proteasome system, allowing the activation of downstream transcription factors and, consequently, the induction of JA-responsive genes. A growing body of evidence has shown that JA signalling is crucial in defending against plant viruses and their insect vectors. Here, we describe the interaction of C2 proteins from two tomato-infecting geminiviruses from the genus Begomovirus, tomato yellow leaf curl virus (TYLCV) and tomato yellow curl Sardinia virus (TYLCSaV), with the transcriptional repressor JAZ8 from Arabidopsis thaliana and its closest orthologue in tomato, SlJAZ9. Both JAZ and C2 proteins colocalize in the nucleus, forming discrete nuclear speckles. Overexpression of JAZ8 did not lead to altered responses to TYLCV infection in Arabidopsis; however, knock-down of JAZ8 favours geminiviral infection. Low levels of JAZ8 likely affect the viral infection specifically, since JAZ8-silenced plants do not display obvious developmental phenotypes nor present differences in their interaction with the viral insect vector. In summary, our results show that the geminivirus-encoded C2 interacts with JAZ8 in the nucleus, and suggest that this plant protein exerts an anti-geminiviral effect. This article is protected by copyright. All rights reserved.

Auxin contributes to jasmonate-mediated regulation of abscisic acid signaling during seed germination in Arabidopsis.

Abscisic acid (ABA) represses seed germination and postgerminative growth in Arabidopsis thaliana. Auxin and jasmonic acid (JA) stimulate ABA function; however, the possible synergistic effects of auxin and JA on ABA signaling and the underlying molecular mechanisms remain elusive. Here, we show that exogenous auxin works synergistically with JA to enhance the ABA-induced delay of seed germination. Auxin biosynthesis, perception, and signaling are crucial for JA-promoted ABA responses. The auxin-dependent transcription factors AUXIN RESPONSE FACTOR10 (ARF10) and ARF16 interact with JASMONATE ZIM-DOMAIN (JAZ) repressors of JA signaling. ARF10 and ARF16 positively mediate JA-increased ABA responses, and overaccumulation of ARF16 partially restores the hyposensitive phenotype of JAZ-accumulating plants defective in JA signaling in response to combined ABA and JA treatment. Furthermore, ARF10 and ARF16 physically associate with ABSCISIC ACID INSENSITIVE5 (ABI5), a critical regulator of ABA signaling, and the ability of ARF16 to stimulate JA-mediated ABA responses is mainly dependent on ABI5. ARF10 and ARF16 activate the transcriptional function of ABI5, whereas JAZ repressors antagonize their effects. Collectively, our results demonstrate that auxin contributes to the synergetic modulation of JA on ABA signaling, and explain the mechanism by which ARF10/16 coordinate with JAZ and ABI5 to integrate the auxin, JA, and ABA signaling pathways.

CO interacts with JAZ repressors and bHLH subgroup IIId factors to negatively regulate jasmonate signaling in Arabidopsis seedlings.

CONSTANS (CO) is a master flowering-time regulator that integrates photoperiodic and circadian signals in Arabidopsis thaliana. CO is expressed in multiple tissues, including young leaves and seedling roots, but little is known about the roles and underlying mechanisms of CO in mediating physiological responses other than flowering. Here, we show that CO expression is responsive to jasmonate. CO negatively modulated jasmonate-imposed root-growth inhibition and anthocyanin accumulation. Seedlings from co mutants were more sensitive to jasmonate, whereas overexpression of CO resulted in plants with reduced sensitivity to jasmonate. Moreover, CO mediated the diurnal gating of several jasmonate-responsive genes under long-day conditions. We demonstrate that CO interacts with JASMONATE ZIM-DOMAIN (JAZ) repressors of jasmonate signaling. Genetic analyses indicated that CO functions in a CORONATINE INSENSITIVE1 (COI1)-dependent manner to modulate jasmonate responses. Furthermore, CO physically associated with the basic helix-loop-helix (bHLH) subgroup IIId transcription factors bHLH3 and bHLH17. CO acted cooperatively with bHLH17 in suppressing jasmonate signaling, but JAZ proteins interfered with their transcriptional functions and physical interaction. Collectively, our results reveal the crucial regulatory effects of CO on mediating jasmonate responses and explain the mechanism by which CO works together with JAZ and bHLH subgroup IIId factors to fine-tune jasmonate signaling.

Key Genes in the JAZ Signaling Pathway Are Up-Regulated Faster and More Abundantly in Caterpillar-Resistant Maize

Jasmonic acid (JA) and its derivatives, collectively known as jasmonates (JAs), are important signaling hormones for plant responses against chewing herbivores. In JA signaling networks, jasmonate ZIM-domain (JAZ) proteins are transcriptional repressors that regulate JA-modulated downstream herbivore defenses. JAZ repressors are widely presented in land plants, however, there is only limited information about the regulation/function of JAZ proteins in maize. In this study, we performed a comprehensive expression analysis of ZmJAZ genes with other selected genes in the jasmonate pathway in response to feeding by fall armyworm (Spodoptera frugiperda, FAW), mechanical wounding, and exogenous hormone treatments in two maize genotypes differing in FAW resistance. Results showed that transcript levels of JAZ genes and several key genes in JA-signaling and biosynthesis pathways were rapidly and abundantly expressed in both genotypes in response to these various treatments. However, there were key differences between the two genotypes in the expression of ZmJAZ1 and ZmCOI1a, these two genes were expressed significantly rapidly and abundantly in the resistant line which was tightly regulated by endogenous JA level upon feeding. For instance, transcript levels of ZmJAZ1 increase dramatically within 30min of FAW-fed Mp708 but not Tx601, correlating with the JA accumulation. The results also demonstrated that wounding or JA treatment alone was not as effective as FAW feeding; this suggests that insect-derived factors are required for optimal defense responses.

OsJAZ11 regulates phosphate starvation responses in rice

Main conclusionOsJAZ11 regulates phosphate homeostasis by suppressing jasmonic acid signaling and biosynthesis in rice roots.Jasmonic Acid (JA) is a key plant signaling molecule which negatively regulates growth processes including root elongation. JAZ (JASMONATE ZIM-DOMAIN) proteins function as transcriptional repressors of JA signaling. Therefore, targeting JA signaling by deploying JAZ repressors may enhance root length in crops. In this study, we overexpressed JAZ repressor OsJAZ11 in rice to alleviate the root growth inhibitory action of JA. OsJAZ11 is a low phosphate (Pi) responsive gene which is transcriptionally regulated by OsPHR2. We report that OsJAZ11 overexpression promoted primary and seminal root elongation which enhanced Pi foraging. Expression studies revealed that overexpression of OsJAZ11 also reduced Pi starvation response (PSR) under Pi limiting conditions. Moreover, OsJAZ11 overexpression also suppressed JA signaling and biosynthesis as compared to wild type (WT). We further demonstrated that the C-terminal region of OsJAZ11 was crucial for stimulating root elongation in overexpression lines. Rice transgenics overexpressing truncated OsJAZ11ΔC transgene (i.e., missing C-terminal region) exhibited reduced root length and Pi uptake. Interestingly, OsJAZ11 also regulates Pi homeostasis via physical interaction with a key Pi sensing protein, OsSPX1. Our study highlights the functional connections between JA and Pi signaling and reveals JAZ repressors as a promising candidate for improving low Pi tolerance of elite rice genotypes.

Interactions of JAZ Repressors with Anthocyanin Biosynthesis-Related Transcription Factors of Fragaria × ananassa

Strawberry fruits are rich in flavonoids like proanthocyanidins and anthocyanins. Their biosynthesis and accumulation are controlled by the MYB-bHLH-WD40 (MBW) transcriptional complex, which is mainly formed by basic helix-loop-helix (bHLH) and MYB transcription factors (TFs). In Arabidopsis thaliana both bHLH and MYB TFs are repressed by JASMONATE ZIM-DOMAIN (JAZ) proteins, the key repressors of the jasmonate-signaling pathway. The aim of this research was the characterization of the FaJAZ1/8.1/9/10 proteins and molecular targets of signaling components and anthocyanin biosynthesis-related TFs of Fragaria × ananassa by protein–protein interactions. For this, domain compositions were studied by multiple alignments and phylogenetic analyses, while interactions were analyzed by yeast two-hybrid (Y2H) assays. We detected high conservation of FaJAZ proteins and jasmonate-signaling components, as well as FabHLHs and FaMYB10 TFs. Moreover, we report the F. × ananassa YABBY1 (FaYAB1) TF, which is related to anthocyanin biosynthesis in Arabidopsis, showed high conservation of functional domains. We demonstrated that FaJAZ repressors interacted with F. × ananassa NOVEL INTERACTOR OF JAZ (FaNINJA), FaMYC2, and JASMONATE ASSOCIATED MYC2-LIKE (FaJAM) proteins. Besides, transcription factors of MBW-complex like FabHLH3, FabHLH33, and FaMYB10, together with FaYAB1, were molecular targets of FaJAZ repressors, exhibiting specificity or redundancy of interaction depending on particular FaJAZ protein. Overall, these results suggest that interactions of jasmonate-signaling components are fully conserved, and anthocyanin biosynthesis might be regulated by JAZ repressors in F. × ananassa.

Molecular Mechanism Underlying the Synergetic Effect of Jasmonate on Abscisic Acid Signaling during Seed Germination in Arabidopsis

Abscisic acid (ABA) is known to suppress seed germination and post-germinative growth of Arabidopsis (Arabidopsis thaliana), and jasmonate (JA) enhances ABA function. However, the molecular mechanism underlying the crosstalk between the ABA and JA signaling pathways remains largely elusive. Here, we show that exogenous coronatine, a JA analog structurally similar to the active conjugate jasmonate-isoleucine, significantly enhances the delayed seed germination response to ABA. Disruption of the JA receptor CORONATINE INSENSITIVE1 or accumulation of the JA signaling repressor JASMONATE ZIM-DOMAIN (JAZ) reduced ABA signaling, while jaz mutants enhanced ABA responses. Mechanistic investigations revealed that several JAZ repressors of JA signaling physically interact with ABSCISIC ACID INSENSITIVE3 (ABI3), a critical transcription factor that positively modulates ABA signaling, and that JAZ proteins repress the transcription of ABI3 and ABI5. Further genetic analyses showed that JA activates ABA signaling and requires functional ABI3 and ABI5. Overexpression of ABI3 and ABI5 simultaneously suppressed the ABA-insensitive phenotypes of the coi1-2 mutant and JAZ-accumulating (JAZ-ΔJas) plants. Together, our results reveal a previously uncharacterized signaling module in which JAZ repressors of the JA pathway regulate the ABA-responsive ABI3 and ABI5 transcription factors to integrate JA and ABA signals during seed germination and post-germinative growth.

A Phytochrome B-Independent Pathway Restricts Growth at High Levels of Jasmonate Defense.

The plant hormone jasmonate (JA) promotes resistance to biotic stress by stimulating the degradation of JASMONATE ZIM-DOMAIN (JAZ) proteins, which relieves repression on MYC transcription factors that execute defense programs. JA-triggered depletion of JAZ proteins in Arabidopsis (Arabidopsis thaliana) is also associated with reduced growth and seed production, but the mechanisms underlying these pleiotropic growth effects remain unclear. Here, we investigated this question using an Arabidopsis JAZ-deficient mutant (jazD; jaz1-jaz7, jaz9, jaz10, and jaz 13) that exhibits high levels of defense and strong growth inhibition. Genetic suppressor screens for mutations that uncouple growth-defense tradeoffs in the jazD mutant identified nine independent causal mutations in the red-light receptor phytochrome B (phyB). Unlike the ability of the phyB mutations to completely uncouple the mild growth-defense phenotypes in a jaz mutant (jazQ) defective in JAZ1, JAZ3, JAZ4, JAZ9, and JAZ10, phyB null alleles only weakly alleviated the growth and reproductive defects in the jazD mutant. phyB-independent growth restriction of the jazD mutant was tightly correlated with upregulation of the Trp biosynthetic pathway but not with changes in central carbon metabolism. Interestingly, jazD and jazD phyB plants were insensitive to a chemical inhibitor of Trp biosynthesis, which is a phenotype previously observed in plants expressing hyperactive MYC transcription factors that cannot bind JAZ repressors. These data provide evidence that the mechanisms underlying JA-mediated growth-defense balance depend on the level of defense, and they further establish an association between growth inhibition at high levels of defense and dysregulation of Trp biosynthesis.

Arabidopsis JAZ Proteins Interact with and Suppress RHD6 Transcription Factor to Regulate Jasmonate-Stimulated Root Hair Development.

Root hairs arise from trichoblasts and are crucial for plant anchorage, nutrient acquisition, and environmental interactions. The phytohormone jasmonate is known to regulate root hair development in Arabidopsis (Arabidopsis thaliana), but little is known about the molecular mechanism underlying jasmonate modulation in this process. Here, we show that the application of exogenous jasmonate significantly stimulated root hair elongation, but, on the contrary, blocking the perception or signaling of jasmonate resulted in defective root hairs. Jasmonate consistently elevated the expression levels of several crucial genes positively involved in root hair growth. Mechanistic investigation revealed that JASMONATE ZIM-DOMAIN (JAZ) proteins, critical repressors of jasmonate signaling, physically interacted with ROOT HAIR DEFECTIVE 6 (RHD6) and RHD6 LIKE1 (RSL1), two transcription factors that are essential for root hair development. JAZ proteins inhibited the transcriptional function of RHD6 and interfered with the interaction of RHD6 with RSL1. Genetic analysis indicated that jasmonate promoted root hair growth in a RHD6/RSL1-dependent manner. Moreover, overexpression of RHD6 largely rescued the root hair defects of JAZ-accumulating plants. Collectively, our study reveals a key signaling module in which JAZ repressors of the jasmonate pathway directly modulate RHD6 and RSL1 transcription factors to integrate jasmonate signaling and the root hair developmental process.

Comprehensive Identification of PTI Suppressors in Type III Effector Repertoire Reveals that Ralstonia solanacearum Activates Jasmonate Signaling at Two Different Steps.

Ralstonia solanacearum is the causative agent of bacterial wilt in many plants. To identify R. solanacearum effectors that suppress pattern-triggered immunity (PTI) in plants, we transiently expressed R. solanacearum RS1000 effectors in Nicotiana benthamiana leaves and evaluated their ability to suppress the production of reactive oxygen species (ROS) triggered by flg22. Out of the 61 effectors tested, 11 strongly and five moderately suppressed the flg22-triggered ROS burst. Among them, RipE1 shared homology with the Pseudomonas syringae cysteine protease effector HopX1. By yeast two-hybrid screening, we identified jasmonate-ZIM-domain (JAZ) proteins, which are transcriptional repressors of the jasmonic acid (JA) signaling pathway in plants, as RipE1 interactors. RipE1 promoted the degradation of JAZ repressors and induced the expressions of JA-responsive genes in a cysteine–protease-activity-dependent manner. Simultaneously, RipE1, similarly to the previously identified JA-producing effector RipAL, decreased the expression level of the salicylic acid synthesis gene that is required for the defense responses against R. solanacearum. The undecuple mutant that lacks 11 effectors with a strong PTI suppression activity showed reduced growth of R. solanacearum in Nicotiana plants. These results indicate that R. solanacearum subverts plant PTI responses using multiple effectors and manipulates JA signaling at two different steps to promote infection.

Jasmonates: signal transduction components and their roles in environmental stress responses.

Jasmonates, oxylipin-type plant hormones, are implicated in diverse aspects of plant growth development and interaction with the environment. Following diverse developmental and environmental cues, jasmonate is produced, conjugated to the amino acid isoleucine and perceived by a co-receptor complex composed of the Jasmonate ZIM-domain (JAZ) repressor proteins and an E3 ubiquitin ligase complex containing the F-box CORONATINE INSENSITIVE 1 (COI1). This event triggers the degradation of the JAZ proteins and the release of numerous transcription factors, including MYC2 and its homologues, which are otherwise bound and inhibited by the JAZ repressors. Here, we will review the role of the COI1, JAZ and MYC2 proteins in the interaction of the plant with its environment, illustrating the significance of jasmonate signalling, and of the proteins involved, for responses to both biotic stresses caused by insects and numerous microbial pathogens and abiotic stresses caused by adverse climatic conditions. It has also become evident that crosstalk with other hormone signals, as well as light and clock signals, plays an important role in the control and fine-tuning of these stress responses. Finally, we will discuss how several pathogens exploit the jasmonate perception and early signalling machinery to decoy the plants defence systems.

The RING E3 Ligase KEEP ON GOING Modulates JASMONATE ZIM-DOMAIN12 Stability.

Jasmonate (JA) signaling in plants is mediated by the JASMONATE ZIM-DOMAIN (JAZ) proteins that repress the activity of several transcription factors regulating JA-inducible gene expression. The hormone JA-isoleucine triggers the interaction of JAZ repressor proteins with the F-box protein CORONATINE INSENSITIVE1 (COI1), part of an S-phase kinase-associated protein1/Cullin1/F-box protein COI1 (SCF(COI1)) E3 ubiquitin ligase complex, and their degradation by the 26S proteasome. In Arabidopsis (Arabidopsis thaliana), the JAZ family consists of 13 members. The level of redundancy or specificity among these members is currently not well understood. Here, we characterized JAZ12, encoded by a highly expressed JAZ gene. JAZ12 interacted with the transcription factors MYC2, MYC3, and MYC4 in vivo and repressed MYC2 activity. Using tandem affinity purification, we found JAZ12 to interact with SCF(COI1) components, matching with observed in vivo ubiquitination and with rapid degradation after treatment with JA. In contrast to the other JAZ proteins, JAZ12 also interacted directly with the E3 RING ligase KEEP ON GOING (KEG), a known repressor of the ABSCISIC ACID INSENSITIVE5 transcription factor in abscisic acid signaling. To study the functional role of this interaction, we circumvented the lethality of keg loss-of-function mutants by silencing KEG using an artificial microRNA approach. Abscisic acid treatment promoted JAZ12 degradation, and KEG knockdown led to a decrease in JAZ12 protein levels. Correspondingly, KEG overexpression was capable of partially inhibiting COI1-mediated JAZ12 degradation. Our results provide additional evidence for KEG as an important factor in plant hormone signaling and a positive regulator of JAZ12 stability.

Structural basis of JAZ repression of MYC transcription factors in jasmonate signalling.

The plant hormone jasmonate (JA) plays crucial roles in regulating plant responses to herbivorous insects and microbial pathogens and is an important regulator of plant growth and development1–7. Key mediators of JA signaling include MYC transcription factors, which are repressed by JAZ transcriptional repressors at the resting state. In the presence of active JA, JAZ proteins function as JA co-receptors by forming a hormone-dependent complex with COI1, the F-box subunit of an SCF-type ubiquitin E3 ligase8–11. The hormone-dependent formation of the COI1–JAZ co-receptor complex leads to ubiquitination and proteasome-dependent degradation of JAZ repressors and release of MYC proteins from transcriptional repression3,10,12. The mechanism by which JAZ proteins repress MYC transcription factors and how JAZ proteins switch between the repressor function in the absence of hormone and the co-receptor function in the presence of hormone remain enigmatic. Here we show that Arabidopsis MYC3 undergoes pronounced conformational changes when bound to the conserved Jas motif of the JAZ9 repressor. The Jas motif, previously shown to bind to hormone as a partially unwound helix, forms a complete α-helix that displaces the N-terminal helix of MYC3 and becomes an integral part of the MYC N-terminal fold. In this position, the Jas helix competitively inhibits MYC3 interaction with the MED25 subunit of the transcriptional Mediator complex. Our study elucidates a novel molecular switch mechanism that governs the repression and activation of a major plant hormone pathway.

Repression of jasmonate signaling by a non-TIFY JAZ protein in Arabidopsis.

JAsmonate ZIM-domain (JAZ) proteins repress the activity of transcription factors that execute responses to the plant hormone jasmonoyl-L-isoleucine (JA-Ile). The ZIM protein domain recruits the co-repressors NINJA and TOPLESS to JAZ-bound transcription factors, and contains a highly conserved TIF[F/Y]XG motif that defines the larger family of TIFY proteins to which JAZs belong. Here, we report that diverse plant species contain genes encoding putative non-TIFY JAZ proteins, including a previously unrecognized JAZ repressor in Arabidopsis (JAZ13, encoded by At3g22275). JAZ13 is most closely related to JAZ8 and includes divergent EAR, TIFY/ZIM, and Jas motifs. Unlike JAZ8, however, JAZ13 contains a Ser-rich C-terminal tail that is a site for phosphorylation. Overexpression of JAZ13 resulted in reduced sensitivity to JA, attenuation of wound-induced expression of JA-response genes, and decreased resistance to insect herbivory. JAZ13 interacts with the bHLH transcription factor MYC2 and the co-repressor TOPLESS but, consistent with the absence of a TIFY motif, neither NINJA nor other JAZs. Analysis of single and higher-order T-DNA insertion jaz null mutants provided further evidence that JAZ13 is a repressor JA signaling. Our results demonstrate that proteins outside the TIFY family are functional JAZ repressors and further suggest that this expansion of the JAZ family allows fine-tuning of JA-mediated transcriptional responses.

Different shades of JAZ during plant growth and defense.

Ever since their discovery as key regulators of the jasmonate (JA) signaling pathway, repressor proteins of the JASMONATE ZIM-domain (JAZ) family are rising stars in research on hormonal regulation of plant growth and defense. In plant cells, JAZ repressor proteins interact with an E3 ubiquitin ligase complex (SCFCOI1) that together functions as a JA receptor. In uninduced cells, JAZs block the activity of transcriptional regulators of JA responses by physically binding to them. Upon perception of bioactive JAs, JAZ proteins are rapidly degraded via the ubiquitin/26S proteasome-dependent proteolytic pathway. This releases the JAZ-bound transcription factors, resulting in the activation of downstream JA responses. JAs play a dominant role in regulating defense responses against herbivorous insects and necrotrophic pathogens, and in adaptive responses to beneficial soil-borne microbes. In addition, JAs have a signal function in a myriad of other processes, including abiotic stress reactions and plant growth responses to environmental cues. The JA pathway functions in the context of a complex network of hormone-regulated signaling pathways that, depending on the environmental or developmental condition, can act antagonistically or synergistically on each other to finely balance resource allocation between growth and defense and minimize fitness tradeoffs. In the process of balancing plant growth and defense, gibberellins (GAs) emerged as dominant antagonists of the JA signaling output. GAs regulate different aspects of plant growth via DELLA repressor proteins that block the activity of transcriptional regulators of GA responses by physically binding to them. Analogous to the role of JAZs in the JA pathway, DELLAs are degraded upon perception of GAs, resulting in the activation of downstream growth responses. Interestingly, DELLAs also interact with JAZs, thereby mutually limiting the cellular binding capacity to their cognate transcription factors. Consequently, GA-mediated degradation of DELLAs enhances the cellular binding capacity of JAZs to their cognate transcription factors, thus reducing the potential JA signaling output. This GA-mediated antagonistic effect on the JA pathway becomes apparent during the shade-avoidance response of plants that grow in dense vegetation stands. Shade-intolerant plant species respond to competition for light by increasing apical dominance and accelerating stem and petiole elongation. These growth and developmental responses occur in response to a drop in the red:far-red (R:FR) light ratio that is sensed by the phytochrome photoreceptors, predominantly phyB, and are GA-dependent through GA-mediated degradation of DELLA proteins. This allows them to outgrow neighboring plants, but at the cost of a reduced defensive capacity against necrotrophic pathogens and insect herbivores. In this issue of New Phytologist, Leone and co-workers zoomed in on the specific role of the Arabidopsis thaliana (Arabidopsis) JAZ10 protein in this process and shed light on the involvement of DELLAs in GA-JA crosstalk during the shade avoidance response. Besides growth-related hormones, also effector proteins of pathogens and plant growth-promoting mycorrhizal fungi have recently been shown to target JAZ repressor proteins, thereby changing the defense-related signaling circuitry for their own benefit. Hence, JAZ repressor proteins are emerging as central targets in the rewiring of the hormone-regulated signaling circuitry that regulates growth and defense.

Bacterial Effector Activates Jasmonate Signaling by Directly Targeting JAZ Transcriptional Repressors

Gram-negative bacterial pathogens deliver a variety of virulence proteins through the type III secretion system (T3SS) directly into the host cytoplasm. These type III secreted effectors (T3SEs) play an essential role in bacterial infection, mainly by targeting host immunity. However, the molecular basis of their functionalities remains largely enigmatic. Here, we show that the Pseudomonas syringae T3SE HopZ1a, a member of the widely distributed YopJ effector family, directly interacts with jasmonate ZIM-domain (JAZ) proteins through the conserved Jas domain in plant hosts. JAZs are transcription repressors of jasmonate (JA)-responsive genes and major components of the jasmonate receptor complex. Upon interaction, JAZs can be acetylated by HopZ1a through a putative acetyltransferase activity. Importantly, P. syringae producing the wild-type, but not a catalytic mutant of HopZ1a, promotes the degradation of HopZ1-interacting JAZs and activates JA signaling during bacterial infection. Furthermore, HopZ1a could partially rescue the virulence defect of a P. syringae mutant that lacks the production of coronatine, a JA-mimicking phytotoxin produced by a few P. syringae strains. These results highlight a novel example by which a bacterial effector directly manipulates the core regulators of phytohormone signaling to facilitate infection. The targeting of JAZ repressors by both coronatine toxin and HopZ1 effector suggests that the JA receptor complex is potentially a major hub of host targets for bacterial pathogens.

The genomics of poplar-rust interactions to improve tree resistance against fungal disease

With their long life cycle, trees must have accurate mechanisms to perceive microbial invasion and elaborate signalling networks in order to activate the appropriate defense response through transcriptional reprogramming. Transcriptional activators and repressors participate in the tight regulation of the stress response, which is key to minimise the fitness costs associated with an activated response. With the availability of its whole genome sequence, ease of growth and clonal propagation, and routine transformation, poplar (Populus spp.) is considered a model tree species for genomics research and also a good system in forest pathology [1]. Moreover, genomes of a cortege of associated microorganisms are being sequenced including a tree pathogen, Melampsora poplar rust. We pursued various approaches to identify poplar genes involved in the interaction with the biotrophic Melampsora rust pathogen. Recent transcriptome analyses from our lab have shown that the expression of genes encoding several transcription factors are up-regulated during infection by Melampsora rust. Similarly we have also shown that mitogen activated protein kinases (MAPKs) are associated with poplar disease resistance against Melampsora rust [2]. New data obtained from various experimental approaches have directed our focus on two important families of transcription factors; the jasmonate ZIM-domain (JAZ) and Cys2/His2 zinc-finger protein (ZFP) families. We have identified a novel MAPK-interacting partner, PtiZFP1, which belongs to the C2H2 ZFP family of transcriptional EAR repressors.The JAZ family of transcriptional repressors were recently identified as key negative regulators of jasmonate (JA) responses.Transcript analyses show that some ZFP and JAZ members exhibit hormone-related expression profiles and up-regulation by rust infection. This up-regulation of JAZ and ZFP transcripts after rust infection strongly suggests that a hormonal response including JA is a key component of the poplar defence response against Melampsora. Late and sustained kinetics of PtiZFP1 and specificJAZ up-regulation suggest that the corresponding proteins may be required for late regulation of defense mechanisms. Several recent data have shownthat proteolytic cleavage of transcriptional repressors is a general mechanism used by plants to activate gene induction. This mechanism is now well documented for jasmonate signaling, which depends on proteasome-mediated degradation of the JAZ repressors [3,4]. In the present work, we also obtain clear evidence that MAPKs promote degradation of PtiZFP1 through the 26S proteasome.Our work suggests that PtiZFP1 and JAZs are part of Melampsora specific hormone-related responses, and by correlation as putative transcriptional repressors, participate in the regulation of the transcriptional responses downstream of these stress hormones.We hypothesize that the observed gene induction play a negative feedback loop needed to replace PtiZFP1 and JAZ proteins that were degraded following through the 26S proteasome. Newly synthesized repressor proteins could contribute to defense attenuation and therefore prevent a runaway response. Based on our data, we propose a model (Figure ​(Figure1)1) where two important families of transcription factors play a predominant role in poplar defense response to the biotrophic fungus Melampsora. The activation of poplar MAPKs would lead toPtiZFP1 degradation and as a result to transcriptional activation of defense-related genes. In parallel we also observed that several genes involved in the JA response (including JAZ) are induced following Melampsora infection. This model highlights the importance of the 26S proteasome in regulating protein pools of transcriptional repressor. For future studies we intend to uncover which cis elements and target genes are recognized by PtiZFP1 and perform functional approaches to uncover JAZ interacting proteins in poplar. Figure 1 Proposed model of PtiZFP1 function in the plant defense response. During normal conditions, direct transcriptional repressors such as PtiZFP1 inhibit the expression of stress-related genes. Following stress perception, a MAPK signaling cascade is activated leading to phosphorylation of PtiZFP1. Phosphorylation targets PtiZFP1 for degradation via the 26S proteasome (P26S), thus relieving repression of defense genes. Defense signaling in turn activates PtiZFP1 gene transcription in order to replenish normal PtiZFP1 protein levels and complete a regulatory cycle necessary to attenuate the defense response. A similar regulatory cycle exists for jasmonic acid (JA) signaling. JA biosynthesis is induced by stress which then promotes proteasome-mediated degradation of JAZ proteins. These indirect transcriptional repressors sequester the bHLH MYC2, thus inhibiting expression of defense genes under non-stressed conditions. Upon release, MYC2 ensures both positive and negative feedback loops by activating LOX3 and JAZ genes respectively. Confirmed and hypothetical pathways are presented in bold and dashed lines respectively.

The ZIM domain mediates homo‐ and heteromeric interactions between Arabidopsis JAZ proteins

Discovery of the jasmonate ZIM-domain (JAZ) repressors defined the core jasmonate (JA) signalling module as COI1-JAZ-MYC2, and allowed a full view of the JA signalling pathway from hormone perception to transcriptional reprogramming. JAZ proteins are repressors of MYC2 and targets of SCF(COI1), which is the likely jasmonate receptor. Upon hormone perception, JAZ repressors are degraded by the proteasome releasing MYC2 and allowing the activation of JA responses. All members of the JAZ family share two conserved domains, the Jas motif, required for JAZ interactions with MYC2 and COI1, and the ZIM domain, the function of which is so far unknown. Here, we show that the ZIM domain acts as a protein-protein interaction domain mediating homo- and heteromeric interactions between JAZ proteins. These JAZ-JAZ interactions are independent of the presence of the hormone. The observation that only a few members of the JAZ family form homo- and heteromers may suggest the relevance of these proteins in the regulation of JA signalling. Interestingly, the JAZ3DeltaJas protein interacts with several JAZ proteins, providing new clues to understanding the dominant JA insensitivity promoted by truncated JAZDeltaJas proteins. We also provide evidence that the Jas motif mediates the hormone-dependent interaction between Arabidopsis JAZ3 and COI1, and further confirm that the Jas motif is required and sufficient for Arabidopsis JAZ3-MYC2 interaction. Finally, we show that interaction with MYC2 is a common feature of the JAZ family, as most JAZ proteins can bind MYC2 in pull-down and yeast two-hybrid assays.

JAZ repressor proteins are targets of the SCFCOI1 complex during jasmonate signalling

Jasmonate and related signalling compounds have a crucial role in both host immunity and development in plants, but the molecular details of the signalling mechanism are poorly understood. Here we identify members of the jasmonate ZIM-domain (JAZ) protein family as key regulators of jasmonate signalling. JAZ1 protein acts to repress transcription of jasmonate-responsive genes. Jasmonate treatment causes JAZ1 degradation and this degradation is dependent on activities of the SCF(COI1) ubiquitin ligase and the 26S proteasome. Furthermore, the jasmonoyl-isoleucine (JA-Ile) conjugate, but not other jasmonate-derivatives such as jasmonate, 12-oxo-phytodienoic acid, or methyl-jasmonate, promotes physical interaction between COI1 and JAZ1 proteins in the absence of other plant proteins. Our results suggest a model in which jasmonate ligands promote the binding of the SCF(COI1) ubiquitin ligase to and subsequent degradation of the JAZ1 repressor protein, and implicate the SCF(COI1)-JAZ1 protein complex as a site of perception of the plant hormone JA-Ile.