The Mediator Complex Subunits MED14, MED15, and MED16 Are Involved in Defense Signaling Crosstalk in Arabidopsis

MEDIATOR15 destabilizes DELLA protein to promote gibberellin-mediated plant development

Mediator, a transcriptional coactivator, regulates plant growth and development by interacting with various transcriptional regulators. MEDIATOR15 (MED15) is a subunit in the Mediator complex potentially involved in developmental control. To uncover molecular functions of Arabidopsis MED15 in development, we searched for its interactors. MED15 was found to interact with DELLA proteins, which negatively regulate gibberellic acid (GA) signaling and positively regulate GA biosynthesis. Mutants and overexpressors of MED15 exhibited multiple GA-related growth phenotypes, which resembled the phenotypes of the DELLA overexpressor and mutant, respectively. Consistent with this observation, DELLA protein levels were inversely correlated with MED15 protein levels, suggesting that MED15 activates GA signaling through DELLA degradation. MED15 was required not only for DELLA-mediated induction of GA-biosynthesis gene expression but also for GA-mediated degradation of DELLA. Therefore, MED15 facilitates DELLA destruction not only by promoting GA biosynthesis but also by accelerating DELLA turnover. Furthermore, MED15-mediated GA signaling was required for timely developmental responses to dark and warm conditions. Our results provide insight into developmental control by Mediator via precise regulation of DELLA stability. These findings are potentially useful for the generation of new crop cultivars with ideal body architecture.

A multiprotein regulatory module, MED16-MBR1&2, controls MED25 homeostasis during jasmonate signaling

Mediator25 (MED25) has been ascribed as a signal-processing and -integrating center that controls jasmonate (JA)-induced and MYC2-dependent transcriptional output. A better understanding of the regulation of MED25 stability will undoubtedly advance our knowledge of the precise regulation of JA signaling-related transcriptional output. Here, we report that Arabidopsis MED16 activates JA-responsive gene expression by promoting MED25 stability. Conversely, two homologous E3 ubiquitin ligases, MED25-BINDING RING-H2 PROTEIN1 (MBR1) and MBR2, negatively regulate JA-responsive gene expression by promoting MED25 degradation. MED16 competes with MBR1&2 to bind to the von Willebrand Factor A (vWF-A) domain of MED25, thereby antagonizing the MBR1&2-mediated degradation of MED25 in vivo. In addition, we show that MED16 promotes hormone-induced interactions between MYC2 and MED25, leading to the activation of JA-responsive gene expression. Collectively, our findings reveal a multiprotein regulatory module that robustly and tightly maintains MED25 homeostasis, which determines the strength of the transcriptional output of JA signaling.

Bridging the Knowledge Gap: Utilization of Mediator Subunits for Crop Improvement

The Mediator complex is a multisubunit transcription coregulator that transfers regulatory signals from different transcription factors to RNA polymerase II (Pol II) to control Pol II-dependent transcription in eukaryotes. Studies on Arabidopsis Mediator subunits have revealed their unique or overlapping functions in various aspects of plant growth, stress adaptation and metabolite homeostasis. Therefore, the utilization of the plant Mediator complex for crop improvement has been of great interest. Advances in genome editing and sequencing techniques have expedited the characterization of Mediator subunits in economically important crops such as tomato, rice, wheat, soybean, sugarcane, pea, chickpea, rapeseed and hop. In this review, we summarize recent progress in understanding the molecular mechanisms of how the Mediator complex regulates crop growth, development and adaptation to environmental stress. We also discuss the conserved and diverse functions of the Mediator complex in different plant species. In addition, we propose several future research directions to deepen our understanding of the important roles of Mediator subunits and their interacting proteins, which would provide promising targets for genetic modification to develop new cultivars with desirable agronomic traits.

Enhanced Stability and Brightness through Co-Substitution: Promoting Plant Growth with Green-Excited Deep Red Phosphor Ca1-zSrzLi1-xMg2xAl3-xN4:yEu2

The research utilized a strategy of chemical unit co-substitution, successfully developing a novel blue-green to green excited, deep red-emitting phosphor, Ca1-zSrzLi1-xMg2xAl3-xN4:yEu2+ (CLA-2xM-zS:yEu, 0≤x≤0.8, 0.003≤y≤0.01, 0≤z≤1), through the replacement of [Li-Al]4+ by [Mg-Mg]4+. This phosphor uniquely converts unusable green light to growth-enhancing deep red, optimizing it for outdoor agriculture. Doping with Sr creates traps, causing a redshift in emission peaks, as confirmed by 7Li nuclear magnetic resonance (NMR) spectra, indicating Li presence and lattice changes. Ca0.2Sr0.8Li0.5MgAl2.5N4:0.005Eu2+ (CLAM-0.8S) phosphor maintained high luminescence intensity under extreme conditions of 85 °C/85% RH, demonstrating excellent photoluminescence performance and chemical stability, compared with conventional SrLi0.5MgAl2.5N4:0.005Eu2+ (SLMA) and SrLiAl3N4:0.005Eu2+(SLA). Experimental results surprised that the unique Ca0.2Sr0.8Li0.8Mg0.4Al2.8N4:0.005Eu2+ (CLA-0.4M-0.8S) prepared light-converting film, which is mainly excited by green light, demonstrated a 20% increase in optical density of Chlorella compared to the PP film and a remarkable 97.5% increase compared to the control group without any film. These findings suggest that this film has significant potential for applications in outdoor agriculture and other fields.

The Mediator Subunit OsMED16 Interacts with the WRKY Transcription Factor OsWRKY45 to Enhance Rice Resistance Against Magnaporthe oryzae

Rice blast, caused by Magnaporthe oryzae (M. oryzae), is one of the most common and damaging diseases of rice that limits rice yield and quality. The mediator complex plays a vital role in promoting transcription by bridging specific transcription factors and RNA polymerase II. Here, we show that the rice mediator subunit OsMED16 is essential for full induction of the diterpenoid phytoalexin biosynthesis genes and resistance to the ascomycetous fungus M. oryzae. Mutants of Osmed16 show reduced expression of the DP biosynthesis genes and are markedly more susceptible to M. oryzae, while transgenic plants overexpressing OsMED16 increased the expression of the DP biosynthesis genes and significantly enhanced resistance to M. oryzae. Interestingly, OsMED16 is physically associated with the WRKY family transcription factor OsWRKY45, which interacts with the phytoalexin synthesis key regulator transcription factor OsWRKY62. Further, OsMED16-OsWRKY45-OsWRKY62 complex could bind to the promoter regions of phytoalexin synthesis-related genes and activate their gene expression. Our results show that OsMED16 may enhance rice tolerance to M. oryzae via directly manipulating phytoalexin de novo biosynthesis.

Comparative phylogenetic analysis of the mediator complex subunit in asparagus bean (Vigna unguiculata ssp. sesquipedialis) and its expression profile under cold stress

BACKGROUND

The mediator complex subunits (MED) constitutes a multiprotein complex, with each subunit intricately involved in crucial aspects of plant growth, development, and responses to stress. Nevertheless, scant reports pertain to the VunMED gene within the context of asparagus bean (Vigna unguiculata ssp. sesquipedialis). Establishing the identification and exploring the responsiveness of VunMED to cold stress forms a robust foundation for the cultivation of cold-tolerant asparagus bean cultivars.

RESULTS

Within this study, a comprehensive genome-wide identification of VunMED genes was executed in the asparagus bean cultivar 'Ningjiang3', resulting in the discovery of 36 distinct VunMED genes. A phylogenetic analysis encompassing 232 MED genes from diverse species, including Arabidopsis, tomatoes, soybeans, mung beans, cowpeas, and asparagus beans, underscored the highly conserved nature of MED gene sequences. Throughout evolutionary processes, each VunMED gene underwent purification and neutral selection, with the exception of VunMED19a. Notably, VunMED9/10b/12/13/17/23 exhibited structural variations discernible across four cowpea species. Divergent patterns of temporal and spatial expression were evident among VunMED genes, with a prominent role attributed to most genes during early fruit development. Additionally, an analysis of promoter cis-acting elements was performed, followed by qRT-PCR assessments on roots, stems, and leaves to gauge relative expression after exposure to cold stress and subsequent recovery. Both treatments induced transcriptional alterations in VunMED genes, with particularly pronounced effects observed in root-based genes following cold stress. Elucidating the interrelationships between subunits involved a preliminary understanding facilitated by correlation and principal component analyses.

CONCLUSIONS

This study elucidates the pivotal contribution of VunMED genes to the growth, development, and response to cold stress in asparagus beans. Furthermore, it offers a valuable point of reference regarding the individual roles of MED subunits.

OsMED16, a tail subunit of Mediator complex, interacts with OsE2Fa to synergistically regulate rice leaf development and blast resistance

Mediator, a universal eukaryotic coactivator, is a multiprotein complex to transduce information from the DNA-bound transcription factors to the RNA polymerase II transcriptional machinery. In this study, the biofunctions of a rice mediator subunit OsMED16 in leaf development and blast resistance were characterized. OsMED16 encodes a putative protein of 1170 amino acids, which is 393 bp shorted than the version in Rice Genome Annotation Project databases. Overexpression of OsMED16 plants exhibited wider leaves with larger and more numerous cells in lateral axis, and enhanced resistance to M. oryzae with hyperaccumulated salicylic acid. Further analysis revealed that OsMED16 interacts with OsE2Fa in nuclei, and the complex could directly regulate the transcriptional levels of several genes involved in cell cycle regulation and SA mediated blast resistance, such as OsCC52A1, OsCDKA1, OsCDKB2;2, OsICS1 and OsWRKY45. Altogether, this study proved that OsMED16 is a positive regulator of rice leaf development and blast resistance, and providing new insights into the crosstalk between cell cycle regulation and immunity.

Coupling VIGS with Short- and Long-Term Stress Exposure to Understand the Fiskeby III Iron Deficiency Stress Response

Yield loss due to abiotic stress is an increasing problem in agriculture. Soybean is a major crop for the upper Midwestern United States and calcareous soils exacerbate iron deficiency for growers, resulting in substantial yield losses. Fiskeby III is a soybean variety uniquely resistant to a variety of abiotic stresses, including iron deficiency. Previous studies identified a MATE transporter (Glyma.05G001700) associated with iron stress tolerance in Fiskeby III. To understand the function of this gene in the Fiskeby III response to iron deficiency, we coupled its silencing using virus-induced gene silencing with RNAseq analyses at two timepoints. Analyses of these data confirm a role for the MATE transporter in Fiskeby III iron stress responses. Further, they reveal that Fiskeby III induces transcriptional reprogramming within 24 h of iron deficiency stress, confirming that like other soybean varieties, Fiskeby III is able to quickly respond to stress. However, Fiskeby III utilizes novel genes and pathways in its iron deficiency response. Identifying and characterizing these genes and pathways in Fiskeby III provides novel targets for improving abiotic stress tolerance in elite soybean lines.

PHYTOCHROME-INTERACTING FACTOR 4/HEMERA-mediated thermosensory growth requires the Mediator subunit MED14

While moderately elevated ambient temperatures do not trigger stress responses in plants, they do substantially stimulate the growth of specific organs through a process known as thermomorphogenesis. The basic helix-loop-helix transcription factor PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) plays a central role in regulating thermomorphogenetic hypocotyl elongation in various plant species, including Arabidopsis (Arabidopsis thaliana). Although it is well known that PIF4 and its co-activator HEMERA (HMR) promote plant thermosensory growth by activating genes involved in the biosynthesis and signaling of the phytohormone auxin, the detailed molecular mechanism of such transcriptional activation is not clear. In this report, we investigated the role of the Mediator complex in the PIF4/HMR-mediated thermoresponsive gene expression. Through the characterization of various mutants of the Mediator complex, a tail subunit named MED14 was identified as an essential factor for thermomorphogenetic hypocotyl growth. MED14 was required for the thermal induction of PIF4 target genes but had a marginal effect on the levels of PIF4 and HMR. Further transcriptomic analyses confirmed that the expression of numerous PIF4/HMR-dependent, auxin-related genes required MED14 at warm temperatures. Moreover, PIF4 and HMR physically interacted with MED14 and both were indispensable for the association of MED14 with the promoters of these thermoresponsive genes. While PIF4 did not regulate MED14 levels, HMR was required for the transcript abundance of MED14. Taken together, these results unveil an important thermomorphogenetic mechanism, in which PIF4 and HMR recruit the Mediator complex to activate auxin-related growth-promoting genes when plants sense moderate increases in ambient temperature.

Pathogenic strategies and immune mechanisms to necrotrophs: Differences and similarities to biotrophs and hemibiotrophs

Pathogenesis in plant diseases is complex comprising diverse pathogen virulence and plant immune mechanisms. These pathogens cause damaging plant diseases by deploying specialized and generic virulence strategies that are countered by intricate resistance mechanisms. The significant challenges that necrotrophs pose to crop production are predicted to increase with climate change. Immunity to biotrophs and hemibiotrophs is dominated by intracellular receptors that recognize specific effectors and activate resistance. These mechanisms play only minor roles in resistance to necrotrophs. Pathogen- or host-derived conserved pattern molecules trigger immune responses that broadly contribute to plant immunity. However, certain pathogen or host-derived immune elicitors are enriched by the virulence activities of necrotrophs. Different plant hormones modulate systemic resistance and cell death that have differential impacts on resistance to pathogens of different lifestyles. Knowledge of mechanisms that contribute to resistance to necrotrophs has expanded. Besides toxins and cell wall degrading enzymes that dominate the pathogenesis of necrotrophs, other effectors with subtle contributions are being identified.

Coordinately regulated transcription factors EIN3/EIL1 and MYCs in ethylene and jasmonate signaling interact with the same domain of MED25

Ethylene (ET) and jasmonate (JA) are plant hormones that act synergistically to regulate plant development and defense against necrotrophic fungi infections, and antagonistically in response to wounds and apical hook formation. Previous studies revealed that the coordination of these responses is due to dynamic protein-protein interactions (PPI) between their master transcription factors (TFs) EIN3/EIL1 and MYC in ET and JA signaling, respectively. In addition, both TFs are activated via interactions with the same transcriptional mediator MED25, which upregulates downstream gene expression. Herein, we analyzed the PPI between EIN3/EIL1 and MED25, and as with the PPI between MYC3 and MED25, found that the short binding domain of MED25 (CMIDM) is also responsible for the interaction with EIN3/EIL1 - a finding which suggests that both TFs compete for binding with MED25. These results further inform our understanding of the coordination between the ET and JA regulatory systems.

The Mediator Complex: A Central Coordinator of Plant Adaptive Responses to Environmental Stresses

As sessile organisms, plants are constantly exposed to a variety of environmental stresses and have evolved adaptive mechanisms, including transcriptional reprogramming, in order to survive or acclimate under adverse conditions. Over the past several decades, a large number of gene-specific transcription factors have been identified in the transcriptional regulation of plant adaptive responses. The Mediator complex plays a key role in transducing signals from gene-specific transcription factors to the transcription machinery to activate or repress target gene expression. Since its first purification about 15 years ago, plant Mediator complex has been extensively analyzed for its composition and biological functions. Mutants of many plant Mediator subunits are not lethal but are compromised in growth, development and response to biotic and abiotic stress, underscoring a particularly important role in plant adaptive responses. Plant Mediator subunits also interact with partners other than transcription factors and components of the transcription machinery, indicating the complexity of the regulation of gene expression by plant Mediator complex. Here, we present a comprehensive discussion of recent analyses of the structure and function of plant Mediator complex, with a particular focus on its roles in plant adaptive responses to a wide spectrum of environmental stresses and associated biological processes.

The Arabidopsis mediator complex subunit 8 regulates oxidative stress responses

Signaling events triggered by hydrogen peroxide (H2O2) regulate plant growth and defense by orchestrating a genome-wide transcriptional reprogramming. However, the specific mechanisms that govern H2O2-dependent gene expression are still poorly understood. Here, we identify the Arabidopsis Mediator complex subunit MED8 as a regulator of H2O2 responses. The introduction of the med8 mutation in a constitutive oxidative stress genetic background (catalase-deficient, cat2) was associated with enhanced activation of the salicylic acid pathway and accelerated cell death. Interestingly, med8 seedlings were more tolerant to oxidative stress generated by the herbicide methyl viologen (MV) and exhibited transcriptional hyperactivation of defense signaling, in particular salicylic acid- and jasmonic acid-related pathways. The med8-triggered tolerance to MV was manipulated by the introduction of secondary mutations in salicylic acid and jasmonic acid pathways. In addition, analysis of the Mediator interactome revealed interactions with components involved in mRNA processing and microRNA biogenesis, hence expanding the role of Mediator beyond transcription. Notably, MED8 interacted with the transcriptional regulator NEGATIVE ON TATA-LESS, NOT2, to control the expression of H2O2-inducible genes and stress responses. Our work establishes MED8 as a component regulating oxidative stress responses and demonstrates that it acts as a negative regulator of H2O2-driven activation of defense gene expression.

Chromatin dynamics during DNA damage and repair in plants: new roles for old players

The genome of plants is organized into chromatin. The chromatin structure regulates the rates of DNA metabolic processes such as replication, transcription, DNA recombination, and repair. Different aspects of plant growth and development are regulated by changes in chromatin status by the action of chromatin-remodeling activities. Recent data have also shown that many of these chromatin-associated proteins participate in different aspects of the DNA damage response, regulating DNA damage and repair, cell cycle progression, programmed cell death, and entry into the endocycle. In this review, we present different examples of proteins and chromatin-modifying enzymes with roles during DNA damage responses, demonstrating that rapid changes in chromatin structure are essential to maintain genome stability.

Differential requirement of MED14/17 recruitment for activation of heat inducible genes

The mechanism of heat stress response in plants has been studied, focusing on the function of transcription factors (TFs). Generally, TFs recruit coactivators, such as Mediator, are needed to assemble the transcriptional machinery. However, despite the close relationship with TFs, how coactivators are involved in transcriptional regulation under heat stress conditions is largely unclear. We found a severe thermosensitive phenotype of Arabidopsis mutants of MED14 and MED17. Transcriptomic analysis revealed that a quarter of the heat stress (HS)-inducible genes were commonly downregulated in these mutants. Furthermore, chromatin immunoprecipitation assay showed that the recruitment of Mediator by HsfA1s, the master regulators of heat stress response, is an important step for the expression of HS-inducible genes. There was a differential requirement of Mediator among genes; TF genes have a high requirement whereas heat shock proteins (HSPs) have a low requirement. Furthermore, artificial activation of HsfA1d mimicking perturbation of protein homeostasis induced HSP gene expression without MED14 recruitment but not TF gene expression. Considering the essential role of MED14 in Mediator function, other coactivators may play major roles in HSP activation depending on the cellular conditions. Our findings highlight the importance of differential recruitment of Mediator for the precise control of HS responses in plants.

Multiple levels of crosstalk in hormone networks regulating plant defense

Plant hormones are essential for regulating the interactions between plants and their complex biotic and abiotic environments. Each hormone initiates a specific molecular pathway and these different hormone pathways are integrated in a complex network of synergistic, antagonistic and additive interactions. This inter-pathway communication is called hormone crosstalk. By influencing the immune network topology, hormone crosstalk is essential for tailoring plant responses to diverse microbes and insects in diverse environmental and internal contexts. Crosstalk provides robustness to the immune system but also drives specificity of induced defense responses against the plethora of biotic interactors. Recent advances in dry-lab and wet-lab techniques have greatly enhanced our understanding of the broad-scale effects of hormone crosstalk on immune network functioning and have revealed underlying principles of crosstalk mechanisms. Molecular studies have demonstrated that hormone crosstalk is modulated at multiple levels of regulation, such as by affecting protein stability, gene transcription and hormone homeostasis. These new insights into hormone crosstalk regulation of plant defense are reviewed here, with a focus on crosstalk acting on the jasmonic acid pathway in Arabidopsis thaliana, highlighting the transcription factors MYC2 and ORA59 as major targets for modulation by other hormones.

Dual-color 3D-dSTORM colocalization and quantification of ROXY1 and RNAPII variants throughout the transcription cycle in root meristem nuclei

To unravel the function of a protein of interest, it is crucial to asses to what extent it associates via direct interactions or by overlapping expression with other proteins. ROXY1, a land plant-specific glutaredoxin, exerts a function in Arabidopsis flower development and interacts with TGA transcription factors in the nucleus. We detected a novel ROXY1 function in the root meristem. Root cells that lack chlorophyll reducing plant-specific background problems that can hamper colocalization 3D microscopy. Thus far, a super-resolution three-dimensional stochastic optical reconstruction microscopy (3D-dSTORM) approach has mainly been applied in animal studies. We established 3D-dSTORM using the roxy1 mutant complemented with green fluorescence protein-ROXY1 and investigated its colocalization with three distinct RNAPII isoforms. To quantify the colocalization results, 3D-dSTORM was coupled with the coordinate-based colocalization method. Interestingly, ROXY1 proteins colocalize with different RNA polymerase II (RNAPII) isoforms that are active at distinct transcription cycle steps. Our colocalization data provide new insights on nuclear glutaredoxin activities suggesting that ROXY1 is not only required in early transcription initiation events via interaction with transcription factors but likely also participates throughout further transcription processes until late termination steps. Furthermore, we showed the applicability of the combined approaches to detect and quantify responses to altered growth conditions, exemplified by analysis of H₂ O₂ treatment, causing a dissociation of ROXY1 and RNAPII isoforms. We envisage that the powerful dual-color 3D-dSTORM/coordinate-based colocalization combination offers plant cell biologists the opportunity to colocalize and quantify root meristem proteins at an increased, unprecedented resolution level <50 nm, which will enable the detection of novel subcellular protein associations and functions.

Reprogramming and remodeling: transcriptional and epigenetic regulation of salicylic acid-mediated plant defense

As a plant hormone, salicylic acid (SA) plays essential roles in plant defense against biotrophic and hemibiotrophic pathogens. Significant progress has been made in understanding the SA biosynthesis pathways and SA-mediated defense signaling networks in the past two decades. Plant defense responses involve rapid and massive transcriptional reprogramming upon the recognition of pathogens. Plant transcription factors and their co-regulators are critical players in establishing a transcription regulatory network and boosting plant immunity. A multitude of transcription factors and epigenetic regulators have been discovered, and their roles in SA-mediated defense responses have been reported. However, our understanding of plant transcriptional networks is still limited. As such, novel genomic tools and bioinformatic techniques will be necessary if we are to fully understand the mechanisms behind plant immunity. Here, we discuss current knowledge, provide an update on the SA biosynthesis pathway, and describe the transcriptional and epigenetic regulation of SA-mediated plant immune responses.

Mediator subunit OsMED14_1 plays an important role in rice development

Mediator, a multisubunit co-activator complex, regulates transcription in eukaryotes and is involved in diverse processes in Arabidopsis through its different subunits. Here, we have explored developmental aspects of one of the rice Mediator subunit gene OsMED14_1. We analyzed its expression pattern through RNA in situ hybridization and pOsMED14_1:GUS transgenics that showed its expression in roots, leaves, anthers and seeds prominently at younger stages, indicating possible involvement of this subunit in multiple aspects of rice development. To understand the developmental roles of OsMED14_1 in rice, we generated and studied RNAi-based knockdown rice plants that showed multiple effects including less height, narrower leaves and culms with reduced vasculature, lesser lateral root branching, defective microspore development, reduced panicle branching and seed set, and smaller seeds. Histological analyses showed that slender organs were caused by reduction in both cell number and cell size in OsMED14_1 knockdown plants. Flow cytometric analyses and expression analyses of cell cycle-related genes revealed that defective cell-cycle progression led to these defects. Expression analyses of auxin-related genes and indole-3-acetic acid (IAA) immunolocalization study indicated altered auxin level in these knockdown plants. Reduction of lateral root branching in knockdown plants was corrected by exogenous IAA supplement. OsMED14_1 physically interacts with transcription factors YABBY5, TAPETUM DEGENERATION RETARDATION (TDR) and MADS29, possibly regulating auxin homeostasis and ultimately leading to lateral organ/leaf, microspore and seed development.

Stem rust resistance in wheat is suppressed by a subunit of the mediator complex

Stem rust is an important disease of wheat that can be controlled using resistance genes. The gene SuSr-D1 identified in cultivar ‘Canthatch’ suppresses stem rust resistance. SuSr-D1 mutants are resistant to several races of stem rust that are virulent on wild-type plants. Here we identify SuSr-D1 by sequencing flow-sorted chromosomes, mutagenesis, and map-based cloning. The gene encodes Med15, a subunit of the Mediator Complex, a conserved protein complex in eukaryotes that regulates expression of protein-coding genes. Nonsense mutations in Med15b.D result in expression of stem rust resistance. Time-course RNAseq analysis show a significant reduction or complete loss of differential gene expression at 24 h post inoculation in med15b.D mutants, suggesting that transcriptional reprogramming at this time point is not required for immunity to stem rust. Suppression is a common phenomenon and this study provides novel insight into suppression of rust resistance in wheat.

Stem rust is an important disease of wheat and resistance present in some cultivars can be suppressed by the SuSr-D1 locus. Here the authors show that SuSr-D1 encodes a subunit of the Mediator Complex and that nonsense mutations are sufficient to abolish suppression and confer stem rust resistance.

Genome-Wide Analysis Reveals the Role of Mediator Complex in the Soybean-Phytophthora sojae Interaction

The mediator complex is an essential link between transcription factors and RNA polymerase II, and mainly functions in the transduction of diverse signals to genes involved in different pathways. Limited information is available on the role of soybean mediator subunits in growth and development, and their participation in defense response regulation. Here, we performed genome-wide identification of the 95 soybean mediator subunits, which were unevenly localized on the 20 chromosomes and only segmental duplication events were detected. We focused on GmMED16-1, which is highly expressed in the roots, for further functional analysis. Transcription of GmMED16-1 was induced in response to Phytophthora sojae infection. Agrobacterium rhizogenes mediated soybean hairy root transformation was performed for the silencing of the GmMED16-1 gene. Silencing of GmMED16-1 led to an enhanced susceptibility phenotype and increased accumulation of P. sojae biomass in hairy roots of transformants. The transcript levels of NPR1, PR1a, and PR5 in the salicylic acid defense pathway in roots of GmMED16-1-silenced transformants were lower than those of empty-vector transformants. The results provide evidence that GmMED16-1 may participate in the soybean-P. sojae interaction via a salicylic acid-dependent process.

NPR1 Promotes Its Own and Target Gene Expression in Plant Defense by Recruiting CDK8

NPR1 (NONEXPRESSER OF PR GENES1) functions as a master regulator of the plant hormone salicylic acid (SA) signaling and plays an essential role in plant immunity. In the nucleus, NPR1 interacts with transcription factors to induce the expression of PR (PATHOGENESIS-RELATED) genes and thereby promote defense responses. However, the underlying molecular mechanism of PR gene activation is poorly understood. Furthermore, despite the importance of NPR1 in plant immunity, the regulation of NPR1 expression has not been extensively studied. Here, we show that SA promotes the interaction of NPR1 with both CDK8 (CYCLIN-DEPENDENT KINASE8) and WRKY18 (WRKY DNA-BINDING PROTEIN18) in Arabidopsis (Arabidopsis thaliana). NPR1 recruits CDK8 and WRKY18 to the NPR1 promoter, facilitating its own expression. Intriguingly, CDK8 and its associated Mediator subunits positively regulate NPR1 and PR1 expression and play a pivotal role in local and systemic immunity. Moreover, CDK8 interacts with WRKY6, WRKY18, and TGA transcription factors and brings RNA polymerase II to NPR1 and PR1 promoters and coding regions to facilitate their expression. Our studies reveal a mechanism in which NPR1 recruits CDK8, WRKY18, and TGA transcription factors along with RNA polymerase II in the presence of SA and thereby facilitates its own and target gene expression for the establishment of plant immunity.

The plant Mediator complex and its role in jasmonate signaling

The Mediator complex is an essential, multisubunit transcriptional coactivator that is highly conserved in eukaryotes. Mediator interacts with gene-specific transcription factors, the RNA polymerase II transcriptional machinery, as well as several other factors involved in transcription, and acts as an integral hub to regulate various aspects of transcription. Recent studies of the plant Mediator complex have established that it functions in diverse aspects of plant development and fitness. Jasmonate (JA) is an oxylipin-derived plant hormone that regulates plant immunity and development. The basic helix-loop-helix transcription factor MYC2, which is a master regulator of JA signaling, orchestrates genome-wide transcriptional reprogramming of plant cells to coordinate defense- and growth-related processes. Here, we review the function of the plant Mediator complex in regulating JA signaling. We focus on the multifunctional Mediator subunit MED25, which emerges as an integrative hub for the transcriptional regulation of jasmonate signaling.

Papillae formation on Arabidopsis leaf trichomes requires the function of Mediator tail subunits 2, 14, 15a, 16, and 25

Arabidopsis Mediator subunits 2, 14, 15a, 16, and 25 are required for papillae development on the trichome cell wall surface. Arabidopsis leaf hairs exhibit raised protrusions, termed papillae, on their cell wall surfaces. Here, we show that the glassy hair mutant, glh2, exhibits trichomes with an approximate 11-fold decrease in papillae density on their surfaces in comparison to wild type. This phenotype was found to be the result of mutations in Arabidopsis Mediator subunit 16. MED16 is localized to the nucleus of trichomes, consistent with Mediator's role in transcription. The expression patterns of the trichome development reporters, ETR2pro::GUS and GL2pro::GUS, as well as GL2 transcript levels were not altered in the glh2 mutant. Screening of available T-DNA insertion lines in other subunits of the Mediator tail module revealed glassy trichome phenotypes in med2, med14, and med15a mutants. The data suggest that the Mediator complex is required for expression of genes involved in trichome papillae development.

WRINKLED1 Is Subject to Evolutionary Conserved Negative Autoregulation

High accumulation of storage compounds such as oil and starch are economically important traits of most agricultural crops. The genetic network determining storage compounds composition in crops has been the target of many biotechnological endeavors. Especially WRINKLED1 (WRI1), a well-known key transcription factor involved in the allocation of carbon into oil, has attracted much interest. Here we investigate the presence of an autoregulatory system involving WRI1 through transient expression in Nicotiana benthamiana leaves. Different lengths of the Arabidopsis WRI1 promotor region were coupled to a GUS reporter gene and the activity was measured when combined with constitutive expression of different WRI1 homologs from Arabidopsis thaliana, oat (Avena sativa L.), yellow nutsedge (Cyperus esculentus L.), and potato (Solanum tuberosum L.). We could show that increasing levels of each WRI1 homolog reduced the transcriptional activity of the Arabidopsis WRI1 upstream region. Through structural analysis and domain swapping between oat and Arabidopsis WRI1, we were able to determine that the negative autoregulation was clearly dependent on the DNA-binding AP2-domains. A DNA/protein interaction assay showed that AtWRI1 is unable to bind to its corresponding upstream region indicating non-direct interaction in vivo. Taken together, our results demonstrate a negative feedback loop of WRI1 expression and that it is an indirect interaction most likely caused by downstream targets of WRI1. We also show that it is possible to release WRI1 expression from this autoregulation by creating semi-synthetic WRI1 homologs increasing the potential use of WRI1 in biotechnological applications.

A role for MED14 and UVH6 in heterochromatin transcription upon destabilization of silencing

The TFIIH component UVH6 and the mediator subunit MED14 are differentially required for the release of heterochromatin silencing, and MED14 regulates non-CG DNA methylation in Arabidopsis.

Constitutive heterochromatin is associated with repressive epigenetic modifications of histones and DNA which silence transcription. Yet, particular mutations or environmental changes can destabilize heterochromatin-associated silencing without noticeable changes in repressive epigenetic marks. Factors allowing transcription in this nonpermissive chromatin context remain poorly known. Here, we show that the transcription factor IIH component UVH6 and the mediator subunit MED14 are both required for heat stress–induced transcriptional changes and release of heterochromatin transcriptional silencing in Arabidopsis thaliana. We find that MED14, but not UVH6, is required for transcription when heterochromatin silencing is destabilized in the absence of stress through mutating the MOM1 silencing factor. In this case, our results raise the possibility that transcription dependency over MED14 might require intact patterns of repressive epigenetic marks. We also uncover that MED14 regulates DNA methylation in non-CG contexts at a subset of RNA-directed DNA methylation target loci. These findings provide insight into the control of heterochromatin transcription upon silencing destabilization and identify MED14 as a regulator of DNA methylation.

Transcriptome Analysis of Four Arabidopsis thaliana Mediator Tail Mutants Reveals Overlapping and Unique Functions in Gene Regulation

The Mediator complex is a central component of transcriptional regulation in Eukaryotes. The complex is structurally divided into four modules known as the head, middle, tail and kinase modules, and in Arabidopsis thaliana, comprises 28-34 subunits. Here, we explore the functions of four Arabidopsis Mediator tail subunits, MED2, MED5a/b, MED16, and MED23, by comparing the impact of mutations in each on the Arabidopsis transcriptome. We find that these subunits affect both unique and overlapping sets of genes, providing insight into the functional and structural relationships between them. The mutants primarily exhibit changes in the expression of genes related to biotic and abiotic stress. We find evidence for a tissue specific role for MED23, as well as in the production of alternative transcripts. Together, our data help disentangle the individual contributions of these MED subunits to global gene expression and suggest new avenues for future research into their functions.

Chitin and chitin-related compounds in plant-fungal interactions

Chitin is the second abundant polysaccharide in the world after cellulose. It is a vital structural component of the fungal cell wall but not for plants. In plants, fungi are recognised through the perception of conserved microbe-associated molecular patterns (MAMPs) to induce MAMP-triggered immunity (MTI). Chitin polymers and their modified form, chitosan, induce host defence responses in both monocotyledons and dicotyledons. The plants' response to chitin, chitosan, and derived oligosaccharides depends on the acetylation degree of these compounds which indicates possible biocontrol regulation of plant immune system. There has also been a considerable amount of recent research aimed at elucidating the roles of chitin hydrolases in fungi and plants as chitinase production in plants is not considered solely as an antifungal resistance mechanism. We discuss the importance of chitin forms and chitinases in the plant-fungal interactions and their role in persistent and possible biocontrol. Abbreviations ET, ethylene; GAP, GTPase-activating protein; GEF, GDP/GTP exchange factor; JA, jasmonic acid; LysM, lysin motif; MAMP, microbe-associated molecular pattern; MTI, MAMP-triggered immunity; NBS, nucleotide-binding site; NBS-LRR, nucleotide-binding site leucine-rich repeats; PM, powdery mildew; PR, pathogenesis-related; RBOH, respiratory burst oxidase homolog; RLK, receptor-like kinase; RLP, receptor-like protein; SA, salicylic acid; TF, transcription factor.

A Cotton Cyclin-Dependent Kinase E Confers Resistance to Verticillium dahliae Mediated by Jasmonate-Responsive Pathway

Many subunits of the Mediator transcriptional co-activator complex are multifunctional proteins that regulate plant immunity in Arabidopsis. Cotton cyclin-dependent kinase E (GhCDKE), which is a subunit of the cotton (Gossypium hirsutum) Mediator complex, has been annotated, but the biological functions of this gene associated with regulating disease resistance have not been characterized. Here, we cloned GhCDKE from cotton and confirmed that GhCDKE belonged to the E-type CDK family in the phylogenetic tree, and, as in other eukaryotes, we found that GhCDKE interacted with C-type cyclin (GhCycC) by yeast two-hybrid and luciferase complementation imaging assays. Expression of GhCDKE in cotton was induced by Verticillium dahliae infection and MeJA treatment, and silencing of GhCDKE expression in cotton led to enhanced susceptibility to V. dahliae, while overexpression of GhCDKE in Arabidopsis thaliana enhanced resistance to this pathogen. Transgenic expression assay demonstrated that the transcriptional activity of GhPDF1.2_pro:LUC in GhCDKE-silenced cotton was dramatically inhibited. In addition, the expression of jasmonic acid (JA)-regulated pathogen-responsive genes was dramatically upregulated in GhCDKE-overexpressed plants after inoculation with V. dahliae, and the roots of GhCDKE-overexpressed A. thaliana were more susceptible to JA treatment. These results indicated that GhCDKE regulates resistance against V. dahliae and that this resistance is mediated by JA response pathway.

Network Analysis Reveals a Common Host-Pathogen Interaction Pattern in Arabidopsis Immune Responses

Many plant pathogens secrete virulence effectors into host cells to target important proteins in host cellular network. However, the dynamic interactions between effectors and host cellular network have not been fully understood. Here, an integrative network analysis was conducted by combining Arabidopsis thaliana protein-protein interaction network, known targets of Pseudomonas syringae and Hyaloperonospora arabidopsidis effectors, and gene expression profiles in the immune response. In particular, we focused on the characteristic network topology of the effector targets and differentially expressed genes (DEGs). We found that effectors tended to manipulate key network positions with higher betweenness centrality. The effector targets, especially those that are common targets of an individual effector, tended to be clustered together in the network. Moreover, the distances between the effector targets and DEGs increased over time during infection. In line with this observation, pathogen-susceptible mutants tended to have more DEGs surrounding the effector targets compared with resistant mutants. Our results suggest a common plant-pathogen interaction pattern at the cellular network level, where pathogens employ potent local impact mode to interfere with key positions in the host network, and plant organizes an in-depth defense by sequentially activating genes distal to the effector targets.

The Multitalented MEDIATOR25

The multi-subunit Mediator complex, which links DNA-bound transcription factors to RNA Pol II during transcription, is an essential regulator of gene expression in all eukaryotes. Individual subunits of the Mediator complex integrate numerous endogenous and exogenous signals. In this paper, diverse regulatory functions performed by MEDIATOR25 (MED25), one of the subunits of the plant Mediator complex are reviewed. MED25 was first identified as a regulator of flowering time and named PHYTOCHROME AND FLOWERING TIME1 (PFT1). Since then, MED25 has been implicated in a range of other plant functions that vary from hormone signaling (JA, ABA, ethylene, and IAA) to biotic and abiotic stress tolerance and plant development. MED25 physically interacts with transcriptional activators (e.g., AP2/ERFs, MYCs, and ARFs), repressors (e.g., JAZs and Aux/IAAs), and other Mediator subunits (MED13 and MED16). In addition, various genetic and epigenetic interactions involving MED25 have been reported. These features make MED25 one of the most multifunctional Mediator subunits and provide new insights into the transcriptional control of gene expression in plants.