The C-terminal WD40 repeats on the TOPLESS co-repressor function as a protein-protein interaction surface

A function of TPL/TBL1-type corepressors is to nucleate the assembly of the preinitiation complex

The plant corepressor TPL is recruited to diverse chromatin contexts, yet its mechanism of repression remains unclear. Previously, we leveraged the fact that TPL retains its function in a synthetic transcriptional circuit in the yeast model Saccharomyces cerevisiae to localize repressive function to two distinct domains. Here, we employed two unbiased whole-genome approaches to map the physical and genetic interactions of TPL at a repressed locus. We identified SPT4, SPT5, and SPT6 as necessary for repression with SPT4 acting as a bridge connecting TPL to SPT5 and SPT6. We discovered the association of multiple additional constituents of the transcriptional preinitiation complex at TPL-repressed promoters, specifically those involved early in transcription initiation. These findings were validated in yeast and plants, including a novel method to analyze the conditional loss of function of essential genes in plants. Our findings support a model where TPL nucleates preassembly of the transcription activation machinery to facilitate the rapid onset of transcription once repression is relieved.

Tip of the iceberg? Three novel TOPLESS-interacting effectors of the gall-inducing fungus Ustilago maydis

Ustilago maydis is a biotrophic pathogen causing smut disease in maize. It secretes a cocktail of effector proteins, which target different host proteins during its biotrophic stages in the host plant. One such class of proteins we identified previously is TOPLESS (TPL) and TOPLESS-RELATED (TPR) transcriptional corepressors. Here, we screened 297 U. maydis effector candidates for their ability to interact with maize TPL protein RAMOSA 1 ENHANCER LOCUS 2 LIKE 2 (RELK2) and their ability to induce auxin signaling and thereby identified three novel TPL-interacting protein effectors (Tip6, Tip7, and Tip8). Structural modeling and mutational analysis allowed the identification of TPL-interaction motifs of Tip6 and Tip7. In planta interaction between Tip6 and Tip7 with RELK2 occurs mainly in nuclear compartments, whereas Tip8 colocalizes with RELK2 in a compartment outside the nucleus. Overexpression of Tip8 in nonhost plants leads to cell death, indicating recognition of the effector or its activity. By performing infection assays with single and multideletion mutants of U. maydis, we demonstrate a positive role of Tip6 and Tip7 in U. maydis virulence. Transcriptional profiling of maize leaves infected with Tip effector mutants in comparison with SG200 strain suggests Tip effector activities are not merely redundant.

Transcriptome-wide Identification of Nine Tandem Repeat Protein Families in Roselle (Hibiscus sabdariffa L.)

Plants are rich in tandem repeats-containing proteins. It is postulated that the occurrence of tandem repeat gene families facilitates the adaptation and survival of plants in adverse environmental conditions. This study intended to identify the tandem repeats in the transcriptome of a high potential tropical horticultural plant, roselle (Hibiscus sabdariffa L.). A total of 92,974 annotated de novo assembled transcripts were analysed using in silico approach, and 6,541 transcripts that encoded proteins containing tandem repeats with length of 20-60 amino acid residues were identified. Domain analysis revealed a total of nine tandem repeat protein families in the transcriptome of roselle, which are the Ankyrin repeats (ANK), Armadillo repeats (ARM), elongation factor-hand domain repeats (EF-hand), Huntingtin, elongation factor 3, protein phosphatase 2A, yeast kinase TOR1 repeats (HEAT), Kelch repeats (Kelch), leucine rich repeats (LRR), pentatricopeptide repeats (PPR), tetratricopeptide repeats (TPR) and WD40 repeats (WD40). Functional annotation analysis further matched 6,236 transcripts to 1,045 known proteins that contained tandem repeats including proteins implicated in plant development, protein-protein interaction, immunity and abiotic stress responses. The findings provide new insights into the occurrence of tandem repeats in the transcriptome and lay the foundation to elucidate the functional associations between tandem peptide repeats (TRs) and proteins in roselle and facilitate the identification of novel biotic and abiotic response related tandem repeats genes that may be useful in breeding improved varieties.

Genome-wide analysis of TOPLESS/TOPLESS-RELATED co-repressors and functional characterization of BnaA9.TPL regulating the embryogenesis and leaf morphology in rapeseed

TOPLESS/TOPLESS-RELATED (TPL/TPR) proteins belong to the Groucho (Gro)/Tup1 family co-repressors and act as broad co-repressors that modulate multiple phytohormone signalling pathways and various developmental processes in plant. However, TPL/TPR co-repressors so far are poorly understood in the rapeseed, one of the world-wide important oilseed crops. In this study, we comprehensively characterized eighteen TPL/TPR genes into five groups in the rapeseed genome. Members of TPL/TPR1/TPR4 and TPR2/TPR3 had close evolutionary relationship, respectively. All TPL/TPRs had similar expression patterns and encode conserved protein domain. In addition, we demonstrated that BnaA9.TPL interacted with all known plant repression domain (RD) sequences, which were distributed in non-redundant 24,238 (22.6 %) genes and significantly enriched in transcription factors in the rapeseed genome. These transcription factors were largely co-expressed with the TPL/TPR genes and involved in diverse pathway, including phytohormone signal transduction, protein kinases and circadian rhythm. Furthermore, BnaA9.TPL was revealed to regulate apical embryonic fate by interaction with Bna.IAA12 and suppression of PLETHORA1/2. BnaA9.TPL was also identified to regulate leaf morphology by interaction with Bna.AS1 (Asymmetric leaves 1) and suppression of KNOTTED-like homeobox genes and YABBY5. These data not only suggest the rapeseed TPL/TPRs play broad roles in different processes, but also provide useful information to uncover more TPL/TPR-mediated control of plant development in rapeseed.

Integrated Dual-Channel Retrograde Signaling Directs Stress Responses by Degrading the HAT1/TPL/IMPα-9 Suppressor Complex and Activating CAMTA3

The intricate communication between plastids and the nucleus, shaping stress-responsive gene expression, has long intrigued researchers. This study combines genetics, biochemical analysis, cellular biology, and protein modeling to uncover how the plastidial metabolite MEcPP activates the stress-response regulatory hub known as the Rapid Stress Response Element (RSRE). Specifically, we identify the HAT1/TPL/IMPα- 9 suppressor complex, where HAT1 directly binds to RSRE and its activator, CAMTA3, masking RSRE and sequestering the activator. Stress-induced MEcPP disrupts this complex, exposing RSRE and releasing CAMTA3, while enhancing Ca 2+ influx and raising nuclear Ca 2+ levels crucial for CAMTA3 activation and the initiation of RSRE- containing gene transcription. This coordinated breakdown of the suppressor complex and activation of the activator highlights the dual-channel role of MEcPP in plastid-to- nucleus signaling. It further signifies how this metabolite transcends its expected biochemical role, emerging as a crucial initiator of harmonious signaling cascades essential for maintaining cellular homeostasis under stress.

Summary

This study uncovers how the stress-induced signaling metabolite MEcPP disrupts the HAT1/TPL/IMPα-9 suppressor complex, liberating the activator CAMTA3 and enabling Ca 2+ influx essential for CAMTA3 activation, thus orchestrating stress responses via repressor degradation and activator induction.

Redox regulation of gene expression: proteomics reveals multiple previously undescribed redox-sensitive cysteines in transcription complexes and chromatin modifiers

Redox signalling is crucial for regulating plant development and adaptation to environmental changes. Proteins with redox-sensitive cysteines can sense oxidative stress and modulate their functions. Recent proteomics efforts have comprehensively mapped the proteins targeted by oxidative modifications. The nucleus, the epicentre of transcriptional reprogramming, contains a large number of proteins that control gene expression. Specific redox-sensitive transcription factors have long been recognized as key players in decoding redox signals in the nucleus and thus in regulating transcriptional responses. Consequently, the redox regulation of the nuclear transcription machinery and its cofactors has received less attention. In this review, we screened proteomic datasets for redox-sensitive cysteines on proteins of the core transcription complexes and chromatin modifiers in Arabidopsis thaliana. Our analysis indicates that redox regulation affects every step of gene transcription, from initiation to elongation and termination. We report previously undescribed redox-sensitive subunits in transcription complexes and discuss the emerging challenges in unravelling the landscape of redox-regulated processes involved in nuclear gene transcription.

A conserved function of corepressors is to nucleate assembly of the transcriptional preinitiation complex

The plant corepressor TPL is recruited to diverse chromatin contexts, yet its mechanism of repression remains unclear. Previously, we have leveraged the fact that TPL retains its function in a synthetic transcriptional circuit in the yeast model Saccharomyces cerevisiae to localize repressive function to two distinct domains. Here, we employed two unbiased whole genome approaches to map the physical and genetic interactions of TPL at a repressed locus. We identified SPT4, SPT5 and SPT6 as necessary for repression with the SPT4 subunit acting as a bridge connecting TPL to SPT5 and SPT6. We also discovered the association of multiple additional constituents of the transcriptional preinitiation complex at TPL-repressed promoters, specifically those involved in early transcription initiation events. These findings were validated in yeast and plants through multiple assays, including a novel method to analyze conditional loss of function of essential genes in plants. Our findings support a model where TPL nucleates preassembly of the transcription activation machinery to facilitate rapid onset of transcription once repression is relieved.

TOPLESS Corepressors as an Emerging Hub of Plant Pathogen Effectors

Transcriptional corepressors form an ancient and essential layer of gene expression control in eukaryotes. TOPLESS and TOPLESS-RELATED (TPL/TPR) proteins constitute a conserved family of Groucho (Gro)/thymidine uptake 1 (Tup1)-type transcriptional corepressors and control diverse growth, developmental, and stress signaling responses in plants. Because of their central and versatile regulatory roles, they act as a signaling hub to integrate various input signaling pathways in the transcriptional responses. Recently, increasing pieces of evidence indicate the roles of TPL/TPR family proteins in the modulation of plant immunity. This is supported by studies on effectors of distantly related pathogens that target TPL/TPR proteins in planta. In this short review, we will summarize the latest findings concerning pathogens targeting plant TPL/TPR proteins to manipulate plant signaling responses for the successful invasion of their hosts. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Specific members of the TOPLESS family are susceptibility genes for Fusarium wilt in tomato and Arabidopsis

Vascular wilt diseases caused by Fusarium oxysporum are a major threat to many agriculturally important crops. Genetic resistance is rare and inevitably overcome by the emergence of new races. To identify potentially durable and non-race-specific genetic resistance against Fusarium wilt diseases, we set out to identify effector targets in tomato that mediate susceptibility to the fungus. For this purpose, we used the SIX8 effector protein, an important and conserved virulence factor present in many pathogenic F. oxysporum isolates. Using protein pull-downs and yeast two-hybrid assays, SIX8 was found to interact specifically with two members of the tomato TOPLESS family: TPL1 and TPL2. Loss-of-function mutations in TPL1 strongly reduced disease susceptibility to Fusarium wilt and a tpl1;tpl2 double mutant exerted an even higher level of resistance. Similarly, Arabidopsis tpl;tpr1 mutants became significantly less diseased upon F. oxysporum inoculation as compared to wildtype plants. We conclude that TPLs encode susceptibility genes whose mutation can confer resistance to F. oxysporum.

Canonical and Alternative Auxin Signaling Systems in Mono-, Di-, and Tetraploid Potatoes

It has long been known that the phytohormone auxin plays a promoting role in tuber formation and stress tolerance in potatoes. Our study aimed to identify and characterize the complete sets of auxin-related genes that presumably constitute the entire auxin signaling system in potato (Solanum tuberosum L.). The corresponding genes were retrieved from sequenced genomes of the doubled monoploid S. tuberosum DM1-3-516-R44 (DM) of the Phureja group, the heterozygous diploid line RH89-039-16 (RH), and the autotetraploid cultivar Otava. Both canonical and noncanonical auxin signaling pathways were considered. Phylogenetic and domain analyses of deduced proteins were supplemented by expression profiling and 3D molecular modeling. The canonical and ABP1-mediated pathways of auxin signaling appeared to be well conserved. The total number of potato genes/proteins presumably involved in canonical auxin signaling is 46 and 108 in monoploid DM and tetraploid Otava, respectively. Among the studied potatoes, spectra of expressed genes obviously associated with auxin signaling were partly cultivar-specific and quite different from analogous spectrum in Arabidopsis. Most of the noncanonical pathways found in Arabidopsis appeared to have low probability in potato. This was equally true for all cultivars used irrespective of their ploidy. Thus, some important features of the (noncanonical) auxin signaling pathways may be variable and species-specific.

Cis-regulatory elements and transcription factors related to auxin signaling in the streptophyte algae Klebsormidium nitens

The phytohormone auxin affects numerous processes in land plants. The central auxin signaling machinery, called the nuclear auxin pathway, is mediated by its pivotal receptor named TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX (TIR1/AFB). The nuclear auxin pathway is widely conserved in land plants, but auxin also accumulates in various algae. Although auxin affects the growth of several algae, the components that mediate auxin signaling have not been identified. We previously reported that exogenous auxin suppresses cell proliferation in the Klebsormidium nitens that is a member of streptophyte algae, a paraphyletic group sharing the common ancestor with land plants. Although K. nitens lacks TIR1/AFB, auxin affects the expression of numerous genes. Thus, elucidation of the mechanism of auxin-inducible gene expression in K. nitens would provide important insights into the evolution of auxin signaling. Here, we show that some motifs are enriched in the promoter sequences of auxin-inducible genes in K. nitens. We also found that the transcription factor KnRAV activates several auxin-inducible genes and directly binds the promoter of KnLBD1, a representative auxin-inducible gene. We propose that KnRAV has the potential to regulate auxin-responsive gene expression in K. nitens.

Head-to-tail polymerization by VEL proteins underpins cold-induced Polycomb silencing in flowering control

Transcriptional silencing through the Polycomb silencing machinery utilizes a "read-write" mechanism involving histone tail modifications. However, nucleation of silencing and long-term stable transmission of the silenced state also requires P-olycomb Repressive Complex 2 (PRC2) accessory proteins, whose molecular role is poorly understood. The Arabidopsis VEL proteins are accessory proteins that interact with PRC2 to nucleate and propagate silencing at the FLOWERING LOCUS C (FLC) locus, enabling early flowering in spring. Here, we report that VEL proteins contain a domain related to an atypical four-helix bundle that engages in spontaneous concentration-dependent head-to-tail polymerization to assemble dynamic biomolecular condensates. Mutations blocking polymerization of this VEL domain prevent Polycomb silencing at FLC. Plant VEL proteins thus facilitate assembly of dynamic multivalent Polycomb complexes required for inheritance of the silenced state.

Many ways to TOPLESS - manipulation of plant auxin signalling by a cluster of fungal effectors

Plant biotrophic pathogens employ secreted molecules, called effectors, to suppress the host immune system and redirect the host's metabolism and development in their favour. Putative effectors of the gall-inducing maize pathogenic fungus Ustilago maydis were analysed for their ability to induce auxin signalling in plants. Using genetic, biochemical, cell-biological, and bioinformatic approaches we functionally elucidate a set of five, genetically linked effectors, called Topless (TPL) interacting protein (Tips) effectors that induce auxin signalling. We show that Tips induce auxin signalling by interfering with central corepressors of the TPL family. CRISPR-Cas9 mutants and deletion strain analysis indicate that the auxin signalling inducing subcluster effectors plays a redundant role in virulence. Although none of the Tips seem to have a conserved interaction motif, four of them bind solely to the N-terminal TPL domain and, for Tip1 and Tip4, we demonstrate direct competition with auxin/indole-3-acetic acid transcriptional repressors for their binding to TPL class of corepressors. Our findings reveal that TPL proteins, key regulators of growth-defence antagonism, are a major target of the U. maydis effectome.

TOPLESS in the regulation of plant immunity

This review presents the multiple ways how topless and topless-related proteins regulate defense activation in plants and help in optimizing the defense-growth tradeoff. Eukaryotic gene expression is tightly regulated at various levels by hormones, transcription regulators, post-translational modifications, and transcriptional coregulators. TOPLESS (TPL)/TOPLESS-related (TPR) corepressors regulate gene expression by interacting with other transcription factors. TPRs regulate auxin, gibberellins, jasmonic acid, strigolactone, and brassinosteroid signaling in plants. In general, except for GA, TPLs suppress these signaling pathways to prevent unwanted activation of hormone signaling. The association of TPL/TPRs in these hormonal signaling reflects a wide role of this class of corepressors in plants' normal and stress physiology. The involvement of TPL in immune responses was first demonstrated a decade ago as a repressor of DND1 and DND2 that are negative regulators of plant immune response. Over the last decade, several research groups have established a larger role of TPL/TPRs in plant immunity during both pattern- and effector-triggered immunity. Very recent research unraveled the significant involvement of TPRs in balancing the growth and defense trade-off. TPRs, along with proteasomal degradation complex, miRNA, and phasiRNA, suppress the activation of autoimmunity in plants under normal conditions and promote defense under pathogen attack.

Defense response changes in roots of oil palm (Elaeis guineensis Jacq.) seedlings after internal symptoms of Ganoderma boninense Pat. infection

BACKGROUND

The development of basal stem rot (BSR) disease in oil palm is associated with lignin during vegetative growth and salicylic acid (SA) biosynthesis. The increase in the lignin content, SA accumulation, growth, and root biomass could indicate the resistance of oil palm seedlings to BSR disease. Therefore, although there are many studies on the interactions between the Ganoderma boninense and oil palm, research on evaluation of physiological processes, biochemistry, and molecules occurring during early internal symptoms of BSR in roots of oil palm (Elaeis guineensis Jacq.) are essential.

RESULTS

Ganoderma boninense inoculation indicated that C01, C02, and C05 seedlings were susceptible, while the other three seedlings, C03, C07, and C08, were resistant based on Ganoderma Disease Index (GDI). Infection by G. boninense in the most susceptible seedlings C05 reduced fresh weight of roots (FW) by 9.0%, and lignin content by 10.9%. The most resistant seedlings C08 were reduced by only 8.4%, and 0.2% regarding their fresh weight and lignin content, respectively. BSR disease induced SA accumulation in the most susceptible C08 and decreased peroxidase (PRX) enzyme (EC 1.11.1.7) activities in root tissues of oil palm seedlings except C07 and C08 where PRX activities remained high in the 4 months after planting. Infection with G. boninense also increased glutathione S-transferase U19-like (EgGSTU19) gene expression in the root tissues of susceptible seedlings, while laccase-24 (EgLCC24) gene expression was associated with resistance against BSR disease. Based on the relative expression of twelve genes, two genes are categorized as receptors (EgWAKL5, EgMIK1), two genes as biosynthesis signal transduction compound (EgOPR5, EgACO1), five genes as defense responses (EgROMT, EgSOT12, EgLCC24, EgGLT3, EgGSTU19), and one gene as trans-resveratrol di-O-methyltransferase-like (EgRNaseIII) predicted related to BSR infection. While two other genes remain unknown (EgUnk1, EgUnk2).

CONCLUSIONS

Ganoderma infection-induced SA accumulation and lignification in resistant accessions promote the seedlings root biomass. Oil palm seedlings have a synergistic physical, biochemical, and molecular defense mechanism to the BSR disease. The utilization of nucleotide-based molecular markers using EgLCC24 gene is able to detect resistant oil palm seedlings to G. boninense.

Creativity comes from interactions: modules of protein interactions in plants

Protein interactions are the foundation of cell biology. For robust signal transduction to occur, proteins interact selectively and modulate their behavior to direct specific biological outcomes. Frequently, modular protein interaction domains are central to these processes. Some of these domains bind proteins bearing post-translational modifications, such as phosphorylation, whereas other domains recognize and bind to specific amino acid motifs. Other modules act as diverse protein interaction scaffolds or can be multifunctional, forming head-to-head homodimers and binding specific peptide sequences or membrane phospholipids. Additionally, the so-called head-to-tail oligomerization domains (SAM, DIX, and PB1) can form extended polymers to regulate diverse aspects of biology. Although the mechanism and structures of these domains are diverse, they are united by their modularity. Together, these domains are versatile and facilitate the evolution of complex protein interaction networks. In this review, we will highlight the role of select modular protein interaction domains in various aspects of plant biology.

Genome-Wide Identification, Characterization and Expression Analysis of the JAZ Gene Family in Resistance to Gray Leaf Spots in Tomato

The plant disease resistance system involves a very complex regulatory network in which jasmonates play a key role in response to external biotic or abiotic stresses. As inhibitors of the jasmonic acid (JA) signaling pathway, JASMONATE ZIM domain (JAZ) proteins have been identified in many plant species, and their functions are gradually being clarified. In this study, 26 JAZ genes were identified in tomato. The physical and chemical properties, predicted subcellular localization, gene structure, cis-acting elements, and interspecies collinearity of 26 SlJAZ genes were subsequently analyzed. RNA-seq data combined with qRT-PCR analysis data showed that the expression of most SlJAZ genes were induced in response to Stemphylium lycopersici, methyl jasmonate (MeJA) and salicylic acid (SA). Tobacco rattle virus RNA2-based VIGS vector (TRV2)-SlJAZ25 plants were more resistant to tomato gray leaf spots than TRV2-00 plants. Therefore, we speculated that SlJAZ25 played a negative regulatory role in tomato resistance to gray leaf spots. Based on combining the results of previous studies and those of our experiments, we speculated that SlJAZ25 might be closely related to JA and SA hormone regulation. SlJAZ25 interacted with SlJAR1, SlCOI1, SlMYC2, and other resistance-related genes to form a regulatory network, and these genes played an important role in the regulation of tomato gray leaf spots. The subcellular localization results showed that the SlJAZ25 gene was located in the nucleus. Overall, this study is the first to identify and analyze JAZ family genes in tomato via bioinformatics approaches, clarifying the regulatory role of SlJAZ25 genes in tomato resistance to gray leaf spots and providing new ideas for improving plant disease resistance.

EAR domain-containing transcription factors trigger PRC2-mediated chromatin marking in Arabidopsis

Polycomb group (PcG) complexes ensure that every cell in an organism expresses the genes needed at a particular stage, time, or condition. However, it is still not fully understood how PcG complexes PcG-repressive complex 1 (PRC1) and PRC2 are recruited to target genes in plants. Recent findings in Arabidopsis thaliana support the notion that PRC2 recruitment is mediated by different transcription factors (TFs). However, it is unclear how all these TFs interact with PRC2 and whether they also recruit PRC1 activity. Here, by using a system to bind selected TFs to a synthetic promoter lacking the complexity of PcG target promoters in vivo, we show that while binding of the TF VIVIPAROUS1/ABSCISIC ACID-INSENSITIVE3-LIKE1 recapitulates PRC1 and PRC2 marking, the binding of other TFs only renders PRC2 marking. Interestingly, all these TFs contain an Ethylene-responsive element binding factor-associated Amphiphilic Repression (EAR) domain that triggers both HISTONE DEACETYLASE COMPLEX and PRC2 activities, connecting two different repressive mechanisms. Furthermore, we show that different TFs can have an additive effect on PRC2 activity, which may be required to maintain long-term repression of gene expression.

TEM1 combinatorially binds to FLOWERING LOCUS T and recruits a Polycomb factor to repress the floral transition in Arabidopsis

Arabidopsis TEMPRANILLO 1 (TEM1) is a transcriptional repressor that participates in multiple flowering pathways and negatively regulates the juvenile-to-adult transition and the flowering transition. To understand the molecular basis for the site-specific regulation of FLOWERING LOCUS T (FT) by TEM1, we determined the structures of the two plant-specific DNA-binding domains in TEM1, AP2 and B3, in complex with their target DNA sequences from the FT gene 5'-untranslated region (5'-UTR), revealing the molecular basis for TEM1 specificity for its DNA targets. In vitro binding assays revealed that the combination of the AP2 and B3 binding sites greatly enhanced the overall binding of TEM1 to the FT 5'-UTR, indicating TEM1 combinatorically recognizes the FT gene 5'-UTR. We further showed that TEM1 recruits the Polycomb repressive complex 2 (PRC2) to the FT 5'-UTR. The simultaneous binding of the TEM1 AP2 and B3 domains to FT is necessary for deposition of H3K27me3 at the FT 5'-UTR and for the flowering repressor function of TEM1. Overall, our data suggest that the combinatorial recognition of FT 5'-UTR by TEM1 ensures H3K27me3 deposition to precisely regulate the floral transition.

Repressor for hire! The vital roles of TOPLESS-mediated transcriptional repression in plants

Transcriptional corepressors play important roles in establishing the appropriate levels of gene expression during growth and development. The TOPLESS (TPL) family of corepressors are critical for all plant life. TPLs are involved in numerous developmental processes and in the response to extrinsic challenges. As such these proteins have been the focus of intense study since Long and colleagues first described the TPL corepressor in 2006. In this review we will explore the evolutionary history of these essential plant-specific proteins, their mechanism of action based on recent structural analyses, and the myriad of pathways in which they function. We speculate how relatively minor changes in the peptide sequence of transcriptional regulators allowed them to recruit TPL into new processes, driving innovation and resulting in TPL becoming vital for plant development.

Repression by the Arabidopsis TOPLESS corepressor requires association with the core mediator complex

The plant corepressor TOPLESS (TPL) is recruited to a large number of loci that are selectively induced in response to developmental or environmental cues, yet the mechanisms by which it inhibits expression in the absence of these stimuli are poorly understood. Previously, we had used the N-terminus of Arabidopsis thaliana TPL to enable repression of a synthetic auxin response circuit in Saccharomyces cerevisiae (yeast). Here, we leveraged the yeast system to interrogate the relationship between TPL structure and function, specifically scanning for repression domains. We identified a potent repression domain in Helix 8 located within the CRA domain, which directly interacted with the Mediator middle module subunits Med21 and Med10. Interactions between TPL and Mediator were required to fully repress transcription in both yeast and plants. In contrast, we found that multimer formation, a conserved feature of many corepressors, had minimal influence on the repression strength of TPL.

Investigation of the role of AcTPR2 in kiwifruit and its response to Botrytis cinerea infection

BACKGROUND

Elucidation of the regulatory mechanism of kiwifruit response to gray mold disease caused by Botrytis cinerea can provide the basis for its molecular breeding to impart resistance against this disease. In this study, 'Hongyang' kiwifruit served as the experimental material; the TOPLESS/TOPLESS-RELATED (TPL/TPR) co-repressor gene AcTPR2 was cloned into a pTRV2 vector (AcTPR2-TRV) and the virus-induced gene silencing technique was used to establish the functions of the AcTPR2 gene in kiwifruit resistance to Botrytis cinerea.

RESULTS

Virus-induced silencing of AcTPR2 enhanced the susceptibility of kiwifruit to Botrytis cinerea. Defensive enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and phenylalanine ammonia-lyase (PAL) and endogenous phytohormones such as indole acetic acid (IAA), gibberellin (GA3), abscisic acid (ABA), and salicylic acid (SA) were detected. Kiwifruit activated these enzymes and endogenous phytohormones in response to pathogen-induced stress and injury. The expression levels of the IAA signaling genes-AcNIT, AcARF1, and AcARF2-were higher in the AcTPR2-TRV treatment group than in the control. The IAA levels were higher and the rot phenotype was more severe in AcTPR2-TRV kiwifruits than that in the control. These results suggested that AcTPR2 downregulation promotes expression of IAA and IAA signaling genes and accelerates postharvest kiwifruit senescence. Further, Botrytis cinerea dramatically upregulated AcTPR2, indicating that AcTPR2 augments kiwifruit defense against pathogens by downregulating the IAA and IAA signaling genes.

CONCLUSIONS

The results of the present study could help clarify the regulatory mechanisms of disease resistance in kiwifruit and furnish genetic resources for molecular breeding of kiwifruit disease resistance.

Evolution of the Auxin Response Factors from charophyte ancestors

Auxin is a major developmental regulator in plants and the acquisition of a transcriptional response to auxin likely contributed to developmental innovations at the time of water-to-land transition. Auxin Response Factors (ARFs) Transcription Factors (TFs) that mediate auxin-dependent transcriptional changes are divided into A, B and C evolutive classes in land plants. The origin and nature of the first ARF proteins in algae is still debated. Here, we identify the most ‘ancient’ ARF homologue to date in the early divergent charophyte algae Chlorokybus atmophyticus, CaARF. Structural modelling combined with biochemical studies showed that CaARF already shares many features with modern ARFs: it is capable of oligomerization, interacts with the TOPLESS co-repressor and specifically binds Auxin Response Elements as dimer. In addition, CaARF possesses a DNA-binding specificity that differs from class A and B ARFs and that was maintained in class C ARF along plants evolution. Phylogenetic evidence together with CaARF biochemical properties indicate that the different classes of ARFs likely arose from an ancestral proto-ARF protein with class C-like features. The foundation of auxin signalling would have thus happened from a pre-existing hormone-independent transcriptional regulation together with the emergence of a functional hormone perception complex.

Plants transition from water to land was determining for the history of our planet, since it led to atmospheric and soil condition changes that promoted the appearance of other life forms. This transition initiated around 1 billion years ago from a Charophyte algae lineage that acquired features allowing it to adapt to the very different terrestrial conditions. Land plants coordinate their development with external stimuli through signalling mechanisms triggered by plant hormones. Therefore, evolution of these molecules and their signalling pathways likely played an important role in the aquatic to terrestrial move. In this manuscript we study the origin of auxin signalling, a plant hormone implicated in all plant developmental steps. Our studies suggest that out of the three families of proteins originally proposed to trigger auxin signalling in land plants, only one existed in Charophyte ancestors as a likely transcriptional repressor independent of auxin. We show that despite millions of years of evolution, this family of proteins has conserved its biochemical and structural properties that are found today in land plants. The results presented here provide an insight on how hormone signalling pathways could have evolved by co-opting a pre-existing hormone-independent transcriptional regulatory mechanism.