The PHD-finger module of the Arabidopsis thaliana defense regulator EDM2 can recognize triply modified histone H3 peptides

The homeodomain (PHD) protein, PdbPHD3, confers salt tolerance by regulating squamosa promoter binding protein PdbSBP3 in Populus davidiana × P. bolleana

Plant homeodomain (PHD) proteins are a family of zinc finger transcription factors that play roles in abiotic stress tolerance. However, their mechanisms in conferring salt tolerance are largely unknown. In this study, we characterized a PHD gene, PdbPHD3, from Populus davidiana × P. bolleana (Shanxin poplar) in response to salt stress. PdbPHD3 is a nuclear protein that is strongly induced by salt and abscisic acid (ABA) treatments. Overexpression of PdbPHD3 conferred salt tolerance, while silencing of PdbPHD3 increased sensitivity to salt. PdbPHD3 could enhance the activities of superoxide dismutase and peroxidase to reduce the abundance of reactive oxygen species, and enhance the osmotic potential by increasing the proline content. Chromatin immunoprecipitation sequencing (ChIP-seq) revealed that PdbPHD3 could bind to various DNA motifs, including the G-box ("CACGTG"), PALBOXAPC ("GGACGG"), and POLLEN1LELAT52 ("TTTCTT"). ChIP-seq combined with RNA sequencing identified a transcription factor gene, squamosa promoter binding protein 3 (PdbSBP3), which is directly regulated by PdbPHD3. Overexpression and silencing of PdbSBP3 improved and decreased salt tolerance, respectively. PdbSBP3 could also regulate all the physiological changes associated with salt tolerance, similar to PdbPHD3. These results suggest that PdbPHD3 confers salt tolerance by regulating PdbSBP3 to reduce ROS accumulation and increase proline content. Therefore, the regulatory axis of PdbPHD3 and PdbSBP3 confers salt tolerance in Shanxin poplar.

ZmABF4-ZmVIL2/ZmFIP37 module enhances drought tolerance in maize seedlings

Drought, as a primary environmental factor, imposes significant constraints on developmental processes and productivity of plants. PHDs were identified as stress-responsive genes in a wide range of eukaryotes. However, the regulatory mechanisms governing PHD genes in maize under abiotic stress conditions are still largely unknown and require further investigation. Here, we identified a mutant, zmvil2, in the EMS mutant library with a C to T mutation in the exon of the Zm00001d053875 (VIN3-like protein 2, ZmVIL2), resulting in premature termination of protein coding. ZmVIL2 belongs to PHD protein family. Compared to WT, zmvil2 mutant exhibited increased sensitivity to drought stress. Consistently, overexpression of ZmVIL2 enhances drought resistance in maize. Y2H, BiFC, and Co-IP experiments revealed that ZmVIL2 directly interacts with ZmFIP37 (FKBP12-interacting protein of 37). zmfip37 knockout mutants also exhibit decreased drought tolerance. Interestingly, we demonstrated that ZmABF4 directly binds to the ZmVIL2 promoter to enhance its activity in yeast one hybrid (Y1H), electrophoretic mobility shift assay (EMSA) and dual luciferase reporter assays. Therefore, we uncovered a novel model ZmABF4-ZmVIL2/ZmFIP37 that promotes drought tolerance in maize. Overall, these findings have enriched the knowledge of the functions of PHD genes in maize and provides genetic resources for breeding stress-tolerant maize varieties.

Transcriptome wide identification, characterization and expression analysis of PHD gene family in Crocus sativus

Crocus sativus L., of the Iridaceae family, yields world's most prized spice, saffron. Saffron is well known for its distinctive aroma, odour and colour, which are imputed to the presence of some specific glycosylated apocarotenoids. Even though the main biosynthetic pathway and most of the enzymes leading to apocarotenoid production have been identified, the regulatory mechanisms that govern the developmental stage and tissue specific production of apocarotenoids in Crocus remain comparatively unravelled. Towards this, we report identification, and characterization of plant homeodomain (PHD) finger transcription factor family in Crocus sativus. We also report cloning and characterisation of CstPHD27 from C. sativus. CstPHD27 recorded highest expression in stigma throughout flower development. CstPHD27 exhibited expression pattern which corresponded to the apocarotenoid accumulation in Crocus stigmas. CstPHD27 is nuclear localized and transcriptionally active in yeast Y187 strain. Over-expression of CstPHD27 in Crocus stigmas enhanced apocarotenoid content by upregulating the biosynthetic pathway genes. This report on PHD finger transcription factor family from C. sativus may offer a basis for elucidating role of this gene family in this traditionally and industrially prized crop.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01410-3.

The Arabidopsis RRM domain proteins EDM3 and IBM2 coordinate the floral transition and basal immune responses

The transition from vegetative to reproductive development (floral transition) is a costly process in annual plants requiring increased investments in metabolic resources. The Arabidopsis thaliana (Arabidopsis) PHD finger protein EDM2 and RRM domain proteins EDM3 and IBM2 are known to form chromatin-associated complexes controlling transcript processing. We are reporting that distinct splice isoforms of EDM3 and IBM2 cooperate in the coordination of the floral transition with basal immune responses. These cooperating splice isoforms, termed EDM3L and IBM2L, control the intensity of basal immunity and, via a separate pathway, the timing of the floral transition. During the developmental phase prior to the floral transition expression of EDM3L and IBM2L strongly and gradually increases, while these isoforms simultaneously down-regulate expression of the floral suppressor gene FLC and promote the transition to reproductive growth. At the same time these accumulating EDM3 and IBM2 splice isoforms gradually suppress basal immunity against the virulent Noco2 isolate of the pathogenic oomycete Hyaloperonospora arabidopsidis and down-regulate expression of a set of defense-associated genes and immune receptor genes. We are providing clear evidence for a functional link between the floral transition and basal immunity in the annual plant Arabidopsis. Coordination of these two biological processes, which compete for metabolic resources, is likely critical for plant survival and reproductive success.

Molecular basis of locus-specific H3K9 methylation catalyzed by SUVH6 in plants

Dimethylated histone H3 Lys9 (H3K9me2) is a conserved heterochromatic mark catalyzed by SUPPRESSOR OF VARIEGATION 3-9 HOMOLOG (SUVH) methyltransferases in plants. However, the mechanism underlying the locus specificity of SUVH enzymes has long been elusive. Here, we show that a conserved N-terminal motif is essential for SUVH6-mediated H3K9me2 deposition in planta. The SUVH6 N-terminal peptide can be recognized by the bromo-adjacent homology (BAH) domain of the RNA- and chromatin-binding protein ANTI-SILENCING 1 (ASI1), which has been shown to function in a complex to confer gene expression regulation. Structural data indicate that a classic aromatic cage of ASI1-BAH domain specifically recognizes an arginine residue of SUVH6 through extensive hydrogen bonding interactions. A classic aromatic cage of ASI1 specifically recognizes an arginine residue of SUVH6 through extensive cation-π interactions, playing a key role in recognition. The SUVH6-ASI1 module confers locus-specific H3K9me2 deposition at most SUVH6 target loci and gives rise to distinct regulation of gene expression depending on the target loci, either conferring transcriptional silencing or posttranscriptional processing of mRNA. More importantly, such mechanism is conserved in multiple plant species, indicating a coordinated evolutionary process between SUVH6 and ASI1. In summary, our findings uncover a conserved mechanism for the locus specificity of H3K9 methylation in planta. These findings provide mechanistic insights into the delicate regulation of H3K9 methylation homeostasis in plants.

Genome-wide distribution and functions of the AAE complex in epigenetic regulation in Arabidopsis

Heterochromatin is widespread in eukaryotic genomes and has diverse impacts depending on its genomic context. Previous studies have shown that a protein complex, the ASI1-AIPP1-EDM2 (AAE) complex, participates in polyadenylation regulation of several intronic heterochromatin-containing genes. However, the genome-wide functions of AAE are still unknown. Here, we show that the ASI1 and EDM2 mostly target the common genomic regions on a genome-wide level and preferentially interacts with genetic heterochromatin. Polyadenylation (poly(A) sequencing reveals that AAE complex has a substantial influence on poly(A) site usage of heterochromatin-containing genes, including not only intronic heterochromatin-containing genes but also the genes showing overlap with heterochromatin. Intriguingly, AAE is also involved in the alternative splicing regulation of a number of heterochromatin-overlapping genes, such as the disease resistance gene RPP4. We provided evidence that genic heterochromatin is indispensable for the recruitment of AAE in polyadenylation and splicing regulation. In addition to conferring RNA processing regulation at genic heterochromatin-containing genes, AAE also targets some transposable elements (TEs) outside of genes (including TEs sandwiched by genes and island TEs) for epigenetic silencing. Our results reveal new functions of AAE in RNA processing and epigenetic silencing, and thus represent important advances in epigenetic regulation.

The Arabidopsis RRM domain protein EDM3 mediates race-specific disease resistance by controlling H3K9me2-dependent alternative polyadenylation of RPP7 immune receptor transcripts

The NLR-receptor RPP7 mediates race-specific immunity in Arabidopsis. Previous screens for enhanced downy mildew (edm) mutants identified the co-chaperone SGT1b (EDM1) and the PHD-finger protein EDM2 as critical regulators of RPP7. Here, we describe a third edm mutant compromised in RPP7 immunity, edm3. EDM3 encodes a nuclear-localized protein featuring an RNA-recognition motif. Like EDM2, EDM3 promotes histone H3 lysine 9 dimethylation (H3K9me2) at RPP7. Global profiling of H3K9me2 showed EDM3 to affect this silencing mark at a large set of loci. Importantly, both EDM3 and EDM2 co-associate in vivo with H3K9me2-marked chromatin and transcripts at a critical proximal polyadenylation site of RPP7, where they suppress proximal transcript polyadeylation/termination. Our results highlight the complexity of plant NLR gene regulation, and establish a functional and physical link between a histone mark and NLR-transcript processing.

Transcript-level expression control of plant NLR genes

Plant NLR genes encode sensitive immune receptors that can mediate the specific recognition of pathogen avirulence effectors and activate a strong defence response, termed effector-triggered immunity. The expression of NLRs requires strict regulation, as their ability to trigger immunity is dependent on their dose, and overexpression of NLRs results in autoimmunity and massive fitness costs. An elaborate interplay of different mechanisms controlling NLR transcript levels allows plants to maximize their defence capacity, whilst limiting negative impact on their fitness. Global suppression of NLR transcripts may be a prerequisite for the fast evolution of new NLR variants and the expansion of this gene family. Here, we summarize recent progress made towards a comprehensive understanding of NLR transcript-level expression control. Multiple mechanistic steps, including transcription as well as co-/post-transcriptional processing and transcript turn-over, contribute to balanced base levels of NLR transcripts and allow for dynamic adjustments to defence situations.

Contribution of transposable elements in the plant's genome

Plants maintain extensive growth flexibility under different environmental conditions, allowing them to continuously and rapidly adapt to alterations in their environment. A large portion of many plant genomes consists of transposable elements (TEs) that create new genetic variations within plant species. Different types of mutations may be created by TEs in plants. Many TEs can avoid the host's defense mechanisms and survive alterations in transposition activity, internal sequence and target site. Thus, plant genomes are expected to utilize a variety of mechanisms to tolerate TEs that are near or within genes. TEs affect the expression of not only nearby genes but also unlinked inserted genes. TEs can create new promoters, leading to novel expression patterns or alternative coding regions to generate alternate transcripts in plant species. TEs can also provide novel cis-acting regulatory elements that act as enhancers or inserts within original enhancers that are required for transcription. Thus, the regulation of plant gene expression is strongly managed by the insertion of TEs into nearby genes. TEs can also lead to chromatin modifications and thereby affect gene expression in plants. TEs are able to generate new genes and modify existing gene structures by duplicating, mobilizing and recombining gene fragments. They can also facilitate cellular functions by sharing their transposase-coding regions. Hence, TE insertions can not only act as simple mutagens but can also alter the elementary functions of the plant genome. Here, we review recent discoveries concerning the contribution of TEs to gene expression in plant genomes and discuss the different mechanisms by which TEs can affect plant gene expression and reduce host defense mechanisms.

Mechanisms to Mitigate the Trade-Off between Growth and Defense

Plants have evolved an array of defenses against pathogens. However, mounting a defense response frequently comes with the cost of a reduction in growth and reproduction, carrying critical implications for natural and agricultural populations. This review focuses on how costs are generated and whether and how they can be mitigated. Most well-characterized growth-defense trade-offs stem from antagonistic crosstalk among hormones rather than an identified metabolic expenditure. A primary way plants mitigate such costs is through restricted expression of resistance; this can be achieved through inducible expression of defense genes or by the concentration of defense to particular times or tissues. Defense pathways can be primed for more effective induction, and primed states can be transmitted to offspring. We examine the resistance (R) genes as a case study of how the toll of defense can be generated and ameliorated. The fine-scale regulation of R genes is critical to alleviate the burden of their expression, and the genomic organization of R genes into coregulatory modules reduces costs. Plants can also recruit protection from other species. Exciting new evidence indicates that a plant's genotype influences the microbiome composition, lending credence to the hypothesis that plants shape their microbiome to enhance defense.

Epigenetic Regulation of Intronic Transgenes in Arabidopsis

Defense mechanisms of plant genomes can epigenetically inactivate repetitive sequences and exogenous transgenes. Loss of mutant phenotypes in intronic T-DNA insertion lines by interaction with another T-DNA locus, termed T-DNA suppression, has been observed in Arabidopsis thaliana, although the molecular basis of establishment and maintenance of T-DNA suppression is poorly understood. Here we show that maintenance of T-DNA suppression requires heterochromatinisation of T-DNA sequences and the nuclear proteins, INCREASED IN BONSAI METHYLATION 2 (IBM2) and ENHANCED DOWNY MILDEW 2 (EDM2), which prevent ectopic 3′ end processing of mRNA in atypically long introns containing T-DNA sequences. Initiation of T-DNA suppression is mediated by the canonical RdDM pathway after hybridisation of two T-DNA strains, accompanied by DNA hypermethylation of T-DNA sequences in the F1 generation. Our results reveal the presence of a genome surveillance mechanism through genome hybridisation that masks repetitive DNAs intruding into transcription units.

DNA Methylation and Demethylation in Plant Immunity

Detection of plant and animal pathogens triggers a massive transcriptional reprogramming, which is directed by chromatin-based processes, and ultimately results in antimicrobial immunity. Although the implication of histone modifications in orchestrating biotic stress-induced transcriptional reprogramming has been well characterized, very little was known, until recently, about the role of DNA methylation and demethylation in this process. In this review, we summarize recent findings on the dynamics and biological relevance of DNA methylation and demethylation in plant immunity against nonviral pathogens. In particular, we report the implications of these epigenetic regulatory processes in the transcriptional and co-transcriptional control of immune-responsive genes and discuss their relevance in fine-tuning antimicrobial immune responses. Finally, we discuss the possible yet elusive role of DNA methylation and demethylation in systemic immune responses, transgenerational immune priming, and de novo epiallelism, which could be adaptive.

Structure and function of histone methylation-binding proteins in plants

Post-translational modifications of histones play important roles in modulating many essential biological processes in both animals and plants. These covalent modifications, including methylation, acetylation, phosphorylation, ubiquitination, SUMOylation and so on, are laid out and erased by histone-modifying enzymes and read out by effector proteins. Recent studies have revealed that a number of developmental processes in plants are under the control of histone post-translational modifications, such as floral transition, seed germination, organogenesis and morphogenesis. Therefore, it is critical to identify those protein domains, which could specifically recognize these post-translational modifications to modulate chromatin structure and regulate gene expression. In the present review, we discuss the recent progress in understanding the structure and function of the histone methylation readers in plants, by focusing on Arabidopsis thaliana proteins.

Transposable element influences on gene expression in plants

Transposable elements (TEs) comprise a major portion of many plant genomes and bursts of TE movements cause novel genomic variation within species. In order to maintain proper gene function, plant genomes have evolved a variety of mechanisms to tolerate the presence of TEs within or near genes. Here, we review our understanding of the interactions between TEs and gene expression in plants by assessing three ways that transposons can influence gene expression. First, there is growing evidence that TE insertions within introns or untranslated regions of genes are often tolerated and have minimal impact on expression level or splicing. However, there are examples in which TE insertions within genes can result in aberrant or novel transcripts. Second, TEs can provide novel alternative promoters, which can lead to new expression patterns or original coding potential of an alternate transcript. Third, TE insertions near genes can influence regulation of gene expression through a variety of mechanisms. For example, TEs may provide novel cis-acting regulatory sites behaving as enhancers or insert within existing enhancers to influence transcript production. Alternatively, TEs may change chromatin modifications in regions near genes, which in turn can influence gene expression levels. Together, the interactions of genes and TEs provide abundant evidence for the role of TEs in changing basic functions within plant genomes beyond acting as latent genomic elements or as simple insertional mutagens. This article is part of a Special Issue entitled: Plant Gene Regulatory Mechanisms and Networks, edited by Dr. Erich Grotewold and Dr. Nathan Springer.

Epigenetic Control of Defense Signaling and Priming in Plants

Immune recognition of pathogen-associated molecular patterns or effectors leads to defense activation at the pathogen challenged sites. This is followed by systemic defense activation at distant non-challenged sites, termed systemic acquired resistance (SAR). These inducible defenses are accompanied by extensive transcriptional reprogramming of defense-related genes. SAR is associated with priming, in which a subset of these genes is kept at a poised state to facilitate subsequent transcriptional regulation. Transgenerational inheritance of defense-related priming in plants indicates the stability of such primed states. Recent studies have revealed the importance and dynamic engagement of epigenetic mechanisms, such as DNA methylation and histone modifications that are closely linked to chromatin reconfiguration, in plant adaptation to different biotic stresses. Herein we review current knowledge regarding the biological significance and underlying mechanisms of epigenetic control for immune responses in plants. We also argue for the importance of host transposable elements as critical regulators of interactions in the evolutionary "arms race" between plants and pathogens.

DNA Methylation within Transcribed Regions

DNA methylation within transcribed genes is commonly found in diverse animals and plants. Here, we provide an overview of recent advances and the remaining mystery regarding intragenic DNA methylation.