Arabidopsis FORGETTER1 mediates stress-induced chromatin memory through nucleosome remodeling

An Epigenetic Alphabet of Crop Adaptation to Climate Change

Crop adaptation to climate change is in a part attributed to epigenetic mechanisms which are related to response to abiotic and biotic stresses. Although recent studies increased our knowledge on the nature of these mechanisms, epigenetics remains under-investigated and still poorly understood in many, especially non-model, plants, Epigenetic modifications are traditionally divided into two main groups, DNA methylation and histone modifications that lead to chromatin remodeling and the regulation of genome functioning. In this review, we outline the most recent and interesting findings on crop epigenetic responses to the environmental cues that are most relevant to climate change. In addition, we discuss a speculative point of view, in which we try to decipher the “epigenetic alphabet” that underlies crop adaptation mechanisms to climate change. The understanding of these mechanisms will pave the way to new strategies to design and implement the next generation of cultivars with a broad range of tolerance/resistance to stresses as well as balanced agronomic traits, with a limited loss of (epi)genetic variability.

A 'hot' cocktail: The multiple layers of thermomemory in plants

Heat stress (HS) caused by above-optimal temperatures adversely affects plants' growth and development and diminishes crop yields. In natural and agricultural environments, these stresses are often transient but recurrent and may progressively increase in severity over time. In addition to the inherent ability to cope with a single HS event, plants have evolved mechanisms that enhance their capacity to survive and reproduce under such conditions. This involves the establishment of a molecular 'thermomemory' after moderate HS that allows them to withstand a later - and possibly more extreme - HS event. Here, I summarize the current understanding of the molecular and biochemical mechanisms underlying thermomemory across multiple cellular levels and discuss aspects that require further attention.

Chlamydomonas reinhardtii exhibits stress memory in the accumulation of triacylglycerols induced by nitrogen deprivation

Stress memory is a phenomenon whereby exposure to initial stress event influences a response to subsequent stress exposures. Studying stress memory is important to understand the cellular behavior in dynamic environment, especially nowadays, in times with growing environmental instability. Stress memory has been characterized in vascular plants but its occurrence in nonvascular plant species has been rarely investigated. We hypothesized that stress memory occurs in nonvascular plants in relation to metabolic stress. We sought to test it using accumulation of lipids (triacylglycerols) in model green alga Chlamydomonas reinhardtii subjected to nitrogen deprivation stress as a model system. Here, we established stress memory protocol on C. reinhardtii cells. Using a blend of microscopy and gas chromatography methods, we showed that the cells exposed to recurrent stress show differential accumulation of triacylglycerols on the quantitative level without qualitative changes in lipid composition, comparing to single stress controls. Overall, our results suggest that metabolic stress memory does occur in nonvascular plant C. reinhardtii and provides a starting point to characterize mechanistic principles of metabolic stress memory. Due to the commercial potential of algae, our findings are relevant for basic science, as well as industrial production of algae-derived compounds.

Molecular mechanisms of plant tolerance to heat stress: current landscape and future perspectives

We summarize recent studies focusing on the molecular basis of plant heat stress response (HSR), how HSR leads to thermotolerance, and promote plant adaptation to recurring heat stress events. The global crop productivity is facing unprecedented threats due to climate change as high temperature negatively influences plant growth and metabolism. Owing to their sessile nature, plants have developed complex signaling networks which enable them to perceive changes in ambient temperature. This in turn activates a suite of molecular changes that promote plant survival and reproduction under adverse conditions. Deciphering these mechanisms is an important task, as this could facilitate development of molecular markers, which could be ultimately used to breed thermotolerant crop cultivars. In current article, we summarize mechanisms involve in plant heat stress acclimation with special emphasis on advances related to heat stress perception, heat-induced signaling, heat stress-responsive gene expression and thermomemory that promote plant adaptation to short- and long-term-recurring heat-stress events. In the end, we will discuss impact of emerging technologies that could facilitate the development of heat stress-tolerant crop cultivars.

Epigenetic regulation of salinity stress responses in cereals

Cereals are important crops and are exposed to various types of environmental stresses that affect the overall growth and yield. Among the various abiotic stresses, salt stress is a major environmental factor that influences the genetic, physiological, and biochemical responses of cereal crops. Epigenetic regulation which includes DNA methylation, histone modification, and chromatin remodelling plays an important role in salt stress tolerance. Recent studies in rice genomics have highlighted that the epigenetic changes are heritable and therefore can be considered as molecular signatures. An epigenetic mechanism under salinity induces phenotypic responses involving modulations in gene expression. Association between histone modification and altered DNA methylation patterns and differential gene expression has been evidenced for salt sensitivity in rice and other cereal crops. In addition, epigenetics also creates stress memory that helps the plant to better combat future stress exposure. In the present review, we have discussed epigenetic influences in stress tolerance, adaptation, and evolution processes. Understanding the epigenetic regulation of salinity could help for designing salt-tolerant varieties leading to improved crop productivity.

The Epigenetic Mechanisms Underlying Thermomorphogenesis and Heat Stress Responses in Arabidopsis

Integration of temperature cues is crucial for plant survival and adaptation. Global warming is a prevalent issue, especially in modern agriculture, since the global rise in average temperature is expected to impact crop productivity worldwide. Hence, better understanding of the mechanisms by which plants respond to warmer temperatures is very important. This review focuses on the epigenetic mechanisms implicated in plant responses to high temperature and distinguishes the different epigenetic events that occur at warmer average temperatures, leading to thermomorphogenic responses, or subjected to extreme warm temperatures, leading to heat stress.

Pre-mRNA alternative splicing as a modulator for heat stress response in plants

The molecular responses of plants to the important abiotic stress, heat stress (HS), have been extensively studied at the transcriptional level. Alternative splicing (AS) is a post-transcriptional regulatory process in which an intron-containing gene can generate more than one mRNA variant. The impact of HS on the pre-mRNA splicing process has been reported in various eukaryotes but seldom discussed in-depth, especially in plants. Here, we review AS regulation in response to HS in different plant species. We discuss potential molecular mechanisms controlling heat-inducible AS regulation in plants and hypothesize that AS regulation participates in heat-priming establishment and HS memory maintenance. We propose that the pre-mRNA splicing variation is an important regulator of plant HS responses (HSRs).

ISWI chromatin remodeling factors repress PAD4-mediated plant immune responses in Arabidopsis

Chromatin remodeling proteins modulate nucleosome dynamic to affect global gene expression and other cellular processes. Their roles in the regulation of plant growth and development have been widely reported, but their roles in plant stress resistance, especially disease resistance, have not been extensively investigated. Here, we show that the Arabidopsis Immunity Switch (ISWI) chromatin-remodeling factors CHR11 and CHR17, are negative regulators of plant disease resistance. The loss of both CHR11 and CHR17 function led to upregulation of a large set of defense response genes in the absence of pathogen infection. The chr11 single mutant showed enhanced resistance against a virulent pathogen Pseudomonas syringae pv. tomato DC3000 (Pst DC3000). Further analysis revealed that mutation of Phytoalexin Deficient4 (PAD4) reduced the upregulation of defense gene expression as well as resistance against Pst DC3000 in the chr11 chr17 double mutant. However, mutation of PAD4 does not rescue the growth defects of chr11 chr17. Together, our study revealed a function of ISWI in repressing defense response under non-pathogenic conditions and indicates distinct target genes of ISWI in regulating plant growth and plant immunity.

Transcriptional memory and response to adverse temperatures in plants

Temperature is one of the major environmental signals controlling plant development, geographical distribution, and seasonal behavior. Plants perceive adverse temperatures, such as high, low, and freezing temperatures, as stressful signals that can cause physiological defects and even death. As sessile organisms, plants have evolved sophisticated mechanisms to adapt to recurring stressful environments through changing gene expression or transcriptional reprogramming. Transcriptional memory refers to the ability of primed plants to remember previously experienced stress and acquire enhanced tolerance to similar or different stresses. Epigenetic modifications mediate transcriptional memory and play a key role in adapting to adverse temperatures. Understanding the mechanisms of the formation, maintenance, and resetting of stress-induced transcriptional memory will not only enable us to understand why there is a trade-off between plant defense and growth, but also provide a theoretical basis for generating stress-tolerant crops optimized for future climate change. In this review, we summarize recent advances in dissecting the mechanisms of plant transcriptional memory in response to adverse temperatures, based mainly on studies of the model plant Arabidopsis thaliana. We also discuss remaining questions that are important for further understanding the mechanisms of transcriptional memory during the adverse temperature response.

Heat memory in plants: histone modifications, nucleosome positioning and miRNA accumulation alter heat memory gene expression

Plant adaptation to high temperature, often referred to as heat acclimation, is a process in which exposure to moderately high temperatures increases a plant's tolerance to subsequent (normally) lethal high temperatures. Plants store heat experience information (heat memory) obtained from previous exposure to high temperatures for several days and develop future temperature responsiveness. However, our understanding of heat acclimation is very limited. In the model plant Arabidopsis thaliana, changes in the expression patterns of heat memory genes play a central role in regulating plant survival and adaptation to recurring heat stress. Heat stress-related transcription factors and histone-modifying enzymes function in the sensitized expression of heat memory genes via the deposition and removal of histone modifications. Chromatin-remodeling complexes and miRNA accumulation also trigger the sustained expression of heat memory genes. In this review, I describe studies of heat acclimation that have provided important insights into the molecular mechanisms that lead to flexible and reversible gene expression upon heat stress in plants.

Complex Networks of Prion-Like Proteins Reveal Cross Talk Between Stress and Memory Pathways in Plants

Prions are often considered as molecular memory devices, generating reproducible memory of a conformational change. Prion-like proteins (PrLPs) have been widely demonstrated to be present in plants, but their role in plant stress and memory remains unexplored. In this work, we report the widespread presence of PrLPs in plants through a comprehensive meta-analysis of 39 genomes representing major taxonomic groups. We find diverse functional roles associated with these proteins in various species and term the full complement of PrLPs in a genome as its "prionome." In particular, we found the rice prionome being significantly enriched in transposons/retrotransposons (Ts/RTRs) and identified over 60 rice PrLPs that were differentially regulated in stress and developmental responses. This prompted us to explore whether and to what extent PrLPs may build stress memory. By integrating the available rice interactome, transcriptome, and regulome data sets, we could find links between stress and memory pathways that would not have otherwise been discernible. Regulatory inferences derived from the superimposition of these data sets revealed a complex network and cross talk between PrLPs, transcription factors (TFs), and the genes involved in stress priming. This integrative meta-analysis connects transient and transgenerational memory mechanisms in plants with PrLPs, suggesting that plant memory may rely upon protein-based signals in addition to chromatin-based epigenetic signals. Taken together, our work provides important insights into the anticipated role of prion-like candidates in stress and memory, paving the way for more focused studies for validating the role of the identified PrLPs in memory acclimation.

Delineating the epigenetic regulation of heat and drought response in plants

Being sessile in nature, plants cannot overlook the incursion of unfavorable environmental conditions, including heat and drought. Heat and drought severely affect plant growth, development, reproduction and therefore productivity which poses a severe threat to global food security. Plants respond to these hostile environmental circumstances by rearranging their genomic and molecular architecture. One such modification commonly known as epigenetic changes involves the perishable to inheritable changes in DNA or DNA-binding histone proteins leading to modified chromatin organization. Reversible epigenetic modifications include DNA methylation, exchange of histone variants, histone methylation, histone acetylation, ATP-dependent nucleosome remodeling, and others. These modifications are employed to regulate the spatial and temporal expression of genes in response to external stimuli or specific developmental requirements. Understanding the epigenetic regulation of stress-related gene expression in response to heat and drought would commence manifold avenues for crop improvement through molecular breeding or biotechnological approaches.

Coral environmental memory: causes, mechanisms, and consequences for future reefs

The apparent ability of corals to acquire and maintain enhanced stress tolerance through a dose-dependent environmental memory, which may persist for multiple years, has critical implications for coral reef conservation research. Such responses are variable across coral species and environmental stressors, with primed corals exhibiting a modified response to secondary stress exposures. While the mechanisms underlying coral memory responses are poorly understood, they likely involve both the coral host and microbiome. With advances in molecular technologies, it is now possible to investigate potential memory mechanisms in non-model organisms, including transcriptional regulation through epigenetic modifications. We integrate evidence of coral environmental memory and suggest future research directions to evaluate the potential for this process to enhance coral resilience under climate change.

H3K27me3 demethylases alter HSP22 and HSP17.6C expression in response to recurring heat in Arabidopsis

Acclimation to high temperature increases plants' tolerance of subsequent lethal high temperatures. Although epigenetic regulation of plant gene expression is well studied, how plants maintain a memory of environmental changes over time remains unclear. Here, we show that JUMONJI (JMJ) proteins, demethylases involved in histone H3 lysine 27 trimethylation (H3K27me3), are necessary for Arabidopsis thaliana heat acclimation. Acclimation induces sustained H3K27me3 demethylation at HEAT SHOCK PROTEIN22 (HSP22) and HSP17.6C loci by JMJs, poising the HSP genes for subsequent activation. Upon sensing heat after a 3-day interval, JMJs directly reactivate these HSP genes. Finally, jmj mutants fail to maintain heat memory under fluctuating field temperature conditions. Our findings of an epigenetic memory mechanism involving histone demethylases may have implications for environmental adaptation of field plants.

Heteromeric HSFA2/HSFA3 complexes drive transcriptional memory after heat stress in Arabidopsis

Adaptive plasticity in stress responses is a key element of plant survival strategies. For instance, moderate heat stress (HS) primes a plant to acquire thermotolerance, which allows subsequent survival of more severe HS conditions. Acquired thermotolerance is actively maintained over several days (HS memory) and involves the sustained induction of memory-related genes. Here we show that FORGETTER3/ HEAT SHOCK TRANSCRIPTION FACTOR A3 (FGT3/HSFA3) is specifically required for physiological HS memory and maintaining high memory-gene expression during the days following a HS exposure. HSFA3 mediates HS memory by direct transcriptional activation of memory-related genes after return to normal growth temperatures. HSFA3 binds HSFA2, and in vivo both proteins form heteromeric complexes with additional HSFs. Our results indicate that only complexes containing both HSFA2 and HSFA3 efficiently promote transcriptional memory by positively influencing histone H3 lysine 4 (H3K4) hyper-methylation. In summary, our work defines the major HSF complex controlling transcriptional memory and elucidates the in vivo dynamics of HSF complexes during somatic stress memory.

JMJ Histone Demethylases Balance H3K27me3 and H3K4me3 Levels at the HSP21 Locus during Heat Acclimation in Arabidopsis

Exposure to moderately high temperature enables plants to acquire thermotolerance to high temperatures that might otherwise be lethal. In Arabidopsis thaliana, histone H3 lysine 27 trimethylation (H3K27me3) at the heat shock protein 17.6C (HSP17.6C) and HSP22 loci is removed by Jumonji C domain-containing protein (JMJ) histone demethylases, thus allowing the plant to ‘remember’ the heat experience. Other heat memory genes, such as HSP21, are downregulated in acclimatized jmj quadruple mutants compared to the wild type, but how those genes are regulated remains uncharacterized. Here, we show that histone H3 lysine 4 trimethylation (H3K4me3) at HSP21 was maintained at high levels for at least three days in response to heat. This heat-dependent H3K4me3 accumulation was compromised in the acclimatized jmj quadruple mutant as compared to the acclimatized wild type. JMJ30 directly bound to the HSP21 locus in response to heat and coordinated H3K27me3 and H3K4me3 levels under standard and fluctuating conditions. Our results suggest that JMJs mediate the balance between H3K27me3 and H3K4me3 at the HSP21 locus through proper maintenance of H3K27me3 removal during heat acclimation.

Epigenetic regulation of abiotic stress memory: maintaining the good things while they last

As sessile organisms, plants have evolved sophisticated ways to constantly gauge and adapt to changing environmental conditions including extremes that may be harmful to their growth and development and are thus perceived as stress. In nature, stressful events are often chronic or recurring and thus an initial stress may prime a plant to respond more efficiently to a subsequent stress event. An epigenetic basis of such stress memory was long postulated and in recent years it has been shown that this is indeed the case. High temperature stress has proven an excellent system to unpick the molecular basis of somatic stress memory, which includes histone modifications and nucleosome occupancy. This review discusses recent findings and pinpoints open questions in the field.

Seed 'primeomics': plants memorize their germination under stress

Seed priming is a pre-germination treatment administered through various chemical, physical and biological agents, which induce mild stress during the early phases of germination. Priming facilitates synchronized seed germination, better seedling establishment, improved plant growth and enhanced yield, especially in stressful environments. In parallel, the phenomenon of 'stress memory' in which exposure to a sub-lethal stress leads to better responses to future or recurring lethal stresses has gained widespread attention in recent years. The versatility and realistic yield gains associated with seed priming and its connection with stress memory make a critical examination useful for the design of robust approaches for maximizing future yield gains. Herein, a literature review identified selenium, salicylic acid, poly-ethylene glycol, CaCl₂ and thiourea as the seed priming agents (SPRs) for which the most studies have been carried out. The average priming duration for SPRs generally ranged from 2 to 48 h, i.e. during phase I/II of germination. The major signalling events for regulating early seed germination, including the DOG1 (delay of germination 1)-abscisic acid (ABA)-heme regulatory module, ABA-gibberellic acid antagonism and nucleus-organelle communication are detailed. We propose that both seed priming and stress memory invoke a 'bet-hedging' strategy in plants, wherein their growth under optimal conditions is compromised in exchange for better growth under stressful conditions. The molecular basis of stress memory is explained at the level of chromatin reorganization, alternative transcript splicing, metabolite accumulation and autophagy. This provides a useful framework to study similar mechanisms operating during seed priming. In addition, we highlight the potential for merging findings on seed priming with those of stress memory, with the dual benefit of advancing fundamental research and boosting crop productivity. Finally, a roadmap for future work, entailing identification of SPR-responsive varieties and the development of dual/multiple-benefit SPRs, is proposed for enhancing SPR-mediated agricultural productivity worldwide.

CSN5A Subunit of COP9 Signalosome Is Required for Resetting Transcriptional Stress Memory after Recurrent Heat Stress in Arabidopsis

In nature, plants are exposed to several environmental stresses that can be continuous or recurring. Continuous stress can be lethal, but stress after priming can increase the tolerance of a plant to better prepare for future stresses. Reports have suggested that transcription factors are involved in stress memory after recurrent stress; however, less is known about the factors that regulate the resetting of stress memory. Here, we uncovered a role for Constitutive Photomorphogenesis 5A (CSN5A) in the regulation of stress memory for resetting transcriptional memory genes (APX2 and HSP22) and H3K4me3 following recurrent heat stress. Furthermore, CSN5A is also required for the deposition of H3K4me3 following recurrent heat stress. Thus, CSN5A plays an important role in the regulation of histone methylation and transcriptional stress memory after recurrent heat stress.

Histone H3K4 methyltransferases SDG25 and ATX1 maintain heat-stress gene expression during recovery in Arabidopsis

Plants have short-term stress memory that enables them to maintain the expression state of a substantial subset of heat-inducible genes during stress recovery after heat stress. Little is known about the molecular mechanisms controlling stress-responsive gene expression at the recovery stage in plants, however. In this article, we demonstrate that histone H3K4 methyltransferases SDG25 and ATX1 are required for heat-stress tolerance in Arabidopsis. SDG25 and ATX1 are not only important for stress-responsive gene expression during heat stress, but also for maintaining stress-responsive gene expression during stress recovery. A combination of whole-genome bisulfite sequencing, RNA-sequencing and ChIP-qPCR demonstrated that mutations of SDG25 and ATX1 decrease histone H3K4me3 levels, increase DNA cytosine methylation and inhibit the expression of a subset of heat stress-responsive genes during stress recovery in Arabidopsis. ChIP-qPCR results confirm that ATX1 binds to chromatins associated with these target genes. Our results reveal that histone H3K4me3 affects DNA methylation at regions in the loci associated with heat stress-responsive gene expression during stress recovery, providing insights into heat-stress transcriptional memory in plants.

Chromatin remodeling factors regulate environmental stress responses in plants

Environmental stress from climate change and agricultural activity threatens global plant biodiversity as well as crop yield and quality. As sessile organisms, plants must maintain the integrity of their genomes and adjust gene expression to adapt to various environmental changes. In eukaryotes, nucleosomes are the basic unit of chromatin around which genomic DNA is packaged by condensation. To enable dynamic access to packaged DNA, eukaryotes have evolved Snf2 (sucrose nonfermenting 2) family proteins as chromatin remodeling factors (CHRs) that modulate the position of nucleosomes on chromatin. During plant stress responses, CHRs are recruited to specific genomic loci, where they regulate the distribution or composition of nucleosomes, which in turn alters the accessibility of these loci to general transcription or DNA damage repair machinery. Moreover, CHRs interplay with other epigenetic mechanisms, including DNA methylation, histone modifications, and deposition of histone variants. CHRs are also involved in RNA processing at the post-transcriptional level. In this review, we discuss major advances in our understanding of the mechanisms by which CHRs function during plants' response to environmental stress.

Immediate transcriptional responses of Arabidopsis leaves to heat shock

Plants have evolved efficient mechanisms for adapting to temperature fluctuations, known as heat stress response and heat stress memory. Although the transcriptional regulatory network of plant heat stress response has been established, little is known about the genome-wide transcriptional changes occurring within the first several minutes after heat shock. Here, we investigated the nascent RNA and mature messenger RNA (mRNA) from plant leaf tissues exposed to 5 min of heat shock treatment using global run-on sequencing and RNA sequencing methods. Only a small group of genes were up- or downregulated at both the nascent RNA and mRNA levels. Primed plants that were already exposed to mild heat stress exhibited a more drastic alteration at multiple transcriptional steps than naïve plants that had not experienced heat stress. Upon heat shock, we also observed the following: (i) engaged RNA polymerase II accumulated downstream of transcription start sites; (ii) 5' pausing release was a rate-limiting step for the induction of some heat shock protein genes; (iii) numerous genes switched transcription modes; (iv) pervasive read-through was induced at terminators; and (v) heat stress memory occurs at multiple steps of the transcription cycle, such as at Pol II recruitment, 5' pausing, elongation, and termination.

Multifaceted Chromatin Structure and Transcription Changes in Plant Stress Response

Sessile plants are exposed throughout their existence to environmental abiotic and biotic stress factors, such as cold, heat, salinity, drought, dehydration, submergence, waterlogging, and pathogen infection. Chromatin organization affects genome stability, and its dynamics are crucial in plant stress responses. Chromatin dynamics are epigenetically regulated and are required for stress-induced transcriptional regulation or reprogramming. Epigenetic regulators facilitate the phenotypic plasticity of development and the survival and reproduction of plants in unfavorable environments, and they are highly diversified, including histone and DNA modifiers, histone variants, chromatin remodelers, and regulatory non-coding RNAs. They contribute to chromatin modifications, remodeling and dynamics, and constitute a multilayered and multifaceted circuitry for sophisticated and robust epigenetic regulation of plant stress responses. However, this complicated epigenetic regulatory circuitry creates challenges for elucidating the common or differential roles of chromatin modifications for transcriptional regulation or reprogramming in different plant stress responses. Particularly, interacting chromatin modifications and heritable stress memories are difficult to identify in the aspect of chromatin-based epigenetic regulation of transcriptional reprogramming and memory. Therefore, this review discusses the recent updates from the three perspectives—stress specificity or dependence of transcriptional reprogramming, the interplay of chromatin modifications, and transcriptional stress memory in plants. This helps solidify our knowledge on chromatin-based transcriptional reprogramming for plant stress response and memory.

Root endophyte induced plant thermotolerance by constitutive chromatin modification at heat stress memory gene loci

Global warming has become a critical challenge to food security, causing severe yield losses of major crops worldwide. Conventional and transgenic breeding strategies to enhance plant thermotolerance are laborious and expensive. Therefore, the use of beneficial microbes could be an alternative approach. Here, we report that the root endophyte Enterobacter sp. SA187 induces thermotolerance in wheat in the laboratory as well as in open-field agriculture. To unravel the molecular mechanisms, we used Arabidopsis thaliana as model plant. SA187 reprogramed the Arabidopsis transcriptome via HSFA2-dependent enhancement of H3K4me3 levels at heat stress memory gene loci. Unlike thermopriming, SA187-induced thermotolerance is mediated by ethylene signaling via the transcription factor EIN3. In contrast to the transient chromatin modification by thermopriming, SA187 induces constitutive H3K4me3 modification of heat stress memory genes, generating robust thermotolerance in plants. Importantly, microbial community composition of wheat plants in open-field agriculture is not influenced by SA187, indicating that beneficial microbes can be a powerful tool to enhance thermotolerance of crops in a sustainable manner.

Recent advances in plant thermomemory

This review summarizes the process of thermal acquired tolerance in plants and the knowledge gap compared to systemic acquired resistance that a plant shows after pathogen inoculation. Plants are continuously challenged by several biotic stresses such as pests and pathogens, or abiotic stresses like high light, UV radiation, drought, salt, and very high or low temperature. Interestingly, for most stresses, prior exposure makes plants more tolerant during the subsequent exposures, which is often referred to as acclimatization. Research of the last two decades reveals that the memory of most of the stresses is associated with epigenetic changes. Heat stress causes damage to membrane proteins, denaturation and inactivation of various enzymes, and accumulation of reactive oxygen species leading to cell injury and death. Plants are equipped with thermosensors that can recognize certain specific changes and activate protection machinery. Phytochrome and calcium signaling play critical roles in sensing sudden changes in temperature and activate cascades of signaling, leading to the production of heat shock proteins (HSPs) that keep protein-unfolding under control. Heat shock factors (HSFs) are the transcription factors that read the activation of thermosensors and induce the expression of HSPs. Epigenetic modifications of HSFs are likely to be the key component of thermal acquired tolerance (TAT). Despite the advances in understanding the process of thermomemory generation, it is not known whether plants are equipped with systemic activation thermal protection, as happens in the form of systemic acquired resistance (SAR) upon pathogen infection. This review describes the recent advances in the understanding of thermomemory development in plants and the knowledge gap in comparison with SAR.

Two Arabidopsis Homologs of Human Lysine-Specific Demethylase Function in Epigenetic Regulation of Plant Defense Responses

Epigenetic marks such as covalent histone modification and DNA methylation are crucial for mitotically and meiotically inherited cellular memory-based plant immunity. However, the roles of individual players in the epigenetic regulation of plant immunity are not fully understood. Here we reveal the functions of two Arabidopsis thaliana homologs of human lysine-specific demethylase1-like1, LDL1 and LDL2, in the maintenance of methyl groups at lysine 4 of histone H3 and in plant immunity to Pseudomonas syringae infection. The growth of virulent P. syringae strains was reduced in ldl1 and ldl2 single mutants compared to wild-type plants. Local and systemic disease resistance responses, which coincided with the rapid, robust transcription of defense-related genes, were more stably expressed in ldl1 ldl2 double mutants than in the single mutants. At the nucleosome level, mono-methylated histone H3K4 accumulated in ldl1 ldl2 plants genome-wide and in the mainly promoter regions of the defense-related genes examined in this study. Furthermore, in silico comparative analysis of RNA-sequencing and chromatin immunoprecipitation data suggested that several WRKY transcription factors, e.g., WRKY22/40/70, might be partly responsible for the enhanced immunity of ldl1 ldl2. These findings suggest that LDL1 and LDL2 control the transcriptional sensitivity of a group of defense-related genes to establish a primed defense response in Arabidopsis.

Broad and Complex Roles of NBR1-Mediated Selective Autophagy in Plant Stress Responses

Selective autophagy is a highly regulated degradation pathway for the removal of specific damaged or unwanted cellular components and organelles such as protein aggregates. Cargo selectivity in selective autophagy relies on the action of cargo receptors and adaptors. In mammalian cells, two structurally related proteins p62 and NBR1 act as cargo receptors for selective autophagy of ubiquitinated proteins including aggregation-prone proteins in aggrephagy. Plant NBR1 is the structural and functional homolog of mammalian p62 and NBR1. Since its first reports almost ten years ago, plant NBR1 has been well established to function as a cargo receptor for selective autophagy of stress-induced protein aggregates and play an important role in plant responses to a broad spectrum of stress conditions including heat, salt and drought. Over the past several years, important progress has been made in the discovery of specific cargo proteins of plant NBR1 and their roles in the regulation of plant heat stress memory, plant-viral interaction and special protein secretion. There is also new evidence for a possible role of NBR1 in stress-induced pexophagy, sulfur nutrient responses and abscisic acid signaling. In this review, we summarize these progresses and discuss the potential significance of NBR1-mediated selective autophagy in broad plant responses to both biotic and abiotic stresses.

MSH1-induced heritable enhanced growth vigor through grafting is associated with the RdDM pathway in plants

Plants transmit signals long distances, as evidenced in grafting experiments that create distinct rootstock-scion junctions. Noncoding small RNA is a signaling molecule that is graft transmissible, participating in RNA-directed DNA methylation; but the meiotic transmissibility of graft-mediated epigenetic changes remains unclear. Here, we exploit the MSH1 system in Arabidopsis and tomato to introduce rootstock epigenetic variation to grafting experiments. Introducing mutations dcl2, dcl3 and dcl4 to the msh1 rootstock disrupts siRNA production and reveals RdDM targets of methylation repatterning. Progeny from grafting experiments show enhanced growth vigor relative to controls. This heritable enhancement-through-grafting phenotype is RdDM-dependent, involving 1380 differentially methylated genes, many within auxin-related gene pathways. Growth vigor is associated with robust root growth of msh1 graft progeny, a phenotype associated with auxin transport based on inhibitor assays. Large-scale field experiments show msh1 grafting effects on tomato plant performance, heritable over five generations, demonstrating the agricultural potential of epigenetic variation.

The meiotic transmissibility and progeny phenotypic influence of graft-mediated epigenetic changes remain unclear. Here, the authors use the msh1 mutant in the rootstock to trigger heritable enhanced growth vigor in Arabidopsis and tomato, and show it is associated with the RNA-directed DNA methylation pathway.

Selective autophagy regulates heat stress memory in Arabidopsis by NBR1-mediated targeting of HSP90 and ROF1

In nature, plants are constantly exposed to many transient, but recurring, stresses. Thus, to complete their life cycles, plants require a dynamic balance between capacities to recover following cessation of stress and maintenance of stress memory. Recently, we uncovered a new functional role for macroautophagy/autophagy in regulating recovery from heat stress (HS) and resetting cellular memory of HS in Arabidopsis thaliana. Here, we demonstrated that NBR1 (next to BRCA1 gene 1) plays a crucial role as a receptor for selective autophagy during recovery from HS. Immunoblot analysis and confocal microscopy revealed that levels of the NBR1 protein, NBR1-labeled puncta, and NBR1 activity are all higher during the HS recovery phase than before. Co-immunoprecipitation analysis of proteins interacting with NBR1 and comparative proteomic analysis of an nbr1-null mutant and wild-type plants identified 58 proteins as potential novel targets of NBR1. Cellular, biochemical and functional genetic studies confirmed that NBR1 interacts with HSP90.1 (heat shock protein 90.1) and ROF1 (rotamase FKBP 1), a member of the FKBP family, and mediates their degradation by autophagy, which represses the response to HS by attenuating the expression of HSP genes regulated by the HSFA2 transcription factor. Accordingly, loss-of-function mutation of NBR1 resulted in a stronger HS memory phenotype. Together, our results provide new insights into the mechanistic principles by which autophagy regulates plant response to recurrent HS.Abbreviations: AIM: Atg8-interacting motif; ATG: autophagy-related; BiFC: bimolecular fluorescence complementation; ConA: concanamycinA; CoIP: co-immunoprecipitation; DMSO: dimethyl sulfoxide; FKBP: FK506-binding protein; FBPASE: fructose 1,6-bisphosphatase; GFP: green fluorescent protein; HS: heat stress; HSF: heat shock factor; HSFA2: heat shock factor A2; HSP: heat shock protein; HSP90: heat shock protein 90; LC-MS/MS: Liquid chromatography-tandem mass spectrometry; 3-MA: 3-methyladenine; NBR1: next-to-BRCA1; PQC: protein quality control; RFP: red fluorescent protein; ROF1: rotamase FKBP1; TF: transcription factor; TUB: tubulin; UBA: ubiquitin-associated; YFP: yellow fluorescent protein.

FORGETTER2 protein phosphatase and phospholipase D modulate heat stress memory in Arabidopsis

Plants can mitigate environmental stress conditions through acclimation. In the case of fluctuating stress conditions such as high temperatures, maintaining a stress memory enables a more efficient response upon recurring stress. In a genetic screen for Arabidopsis thaliana mutants impaired in the memory of heat stress (HS) we have isolated the FORGETTER2 (FGT2) gene, which encodes a type 2C protein phosphatase (PP2C) of the D-clade. Fgt2 mutants acquire thermotolerance normally; however, they are defective in the memory of HS. FGT2 interacts with phospholipase D α2 (PLDα2), which is involved in the metabolism of membrane phospholipids and is also required for HS memory. In summary, we have uncovered a previously unknown component of HS memory and identified the FGT2 protein phosphatase and PLDα2 as crucial players, suggesting that phosphatidic acid-dependent signaling or membrane composition dynamics underlie HS memory.

Chromatin regulation of somatic abiotic stress memory

In nature, plants are often subjected to periods of recurrent environmental stress that can strongly affect their development and productivity. To cope with these conditions, plants can remember a previous stress, which allows them to respond more efficiently to a subsequent stress, a phenomenon known as priming. This ability can be maintained at the somatic level for a few days or weeks after the stress is perceived, suggesting that plants can store information of a past stress during this recovery phase. While the immediate responses to a single stress event have been extensively studied, knowledge on priming effects and how stress memory is stored is still scarce. At the molecular level, memory of a past condition often involves changes in chromatin structure and organization, which may be maintained independently from transcription. In this review, we will summarize the most recent developments in the field and discuss how different levels of chromatin regulation contribute to priming and plant abiotic stress memory.

Epigenetics: possible applications in climate-smart crop breeding

To better adapt transiently or lastingly to stimuli from the surrounding environment, the chromatin states in plant cells vary to allow the cells to fine-tune their transcriptional profiles. Modifications of chromatin states involve a wide range of post-transcriptional histone modifications, histone variants, DNA methylation, and activity of non-coding RNAs, which can epigenetically determine specific transcriptional outputs. Recent advances in the area of '-omics' of major crops have facilitated identification of epigenetic marks and their effect on plant response to environmental stresses. As most epigenetic mechanisms are known from studies in model plants, we summarize in this review recent epigenetic studies that may be important for improvement of crop adaptation and resilience to environmental changes, ultimately leading to the generation of stable climate-smart crops. This has paved the way for exploitation of epigenetic variation in crop breeding.

Memory of 5-min heat stress in Arabidopsis thaliana

An ability of plants memorizing past heat exposure to modulate the expression of stress response transcripts during recovery is essential for efficient acquired thermotolerance. In this study, we demonstrated that expression of heat response transcripts spiked at 30 min or 1 h, but dramatically declined at 3 h during recoveries following exposure to 5-min heat stress in Arabidopsis. In contrast, expression of transcripts up-regulated by 45-min heat stress was sustained for 30 min or 1 h then declined during recovery. These results suggest that heat memory can be differently modulated depending on the duration of heat exposure, and indicate that plants can memorize even 5-min heat stress to regulate acclimatory responses during recovery. Later hypothesis can be supported by the finding that accumulation of heat response proteins was also modulated during recovery following 5-min heat stress. In addition, 5-min heat stress followed by 3 h recovery was efficient to activate acquired thermotolerance of plants, although spike of transcript expression was observed at 1 h during recovery. These results suggest that plants possess the ability to quickly memorize heat stress and reset cellular states during recovery to adapt to subsequent severe heat stress.

BRAHMA-interacting proteins BRIP1 and BRIP2 are core subunits of Arabidopsis SWI/SNF complexes

Switch defective/sucrose non-fermentable (SWI/SNF) chromatin remodelling complexes are multi-protein machineries that control gene expression by regulating chromatin structure in eukaryotes. However, the full subunit composition of SWI/SNF complexes in plants remains unclear. Here we report that in Arabidopsis thaliana, two homologous glioma tumour suppressor candidate region domain-containing proteins, named BRAHMA-interacting proteins 1 (BRIP1) and BRIP2, are core subunits of plant SWI/SNF complexes. brip1 brip2 double mutants exhibit developmental phenotypes and a transcriptome remarkably similar to those of BRAHMA (BRM) mutants. Genetic interaction tests indicated that BRIP1 and BRIP2 act together with BRM to regulate gene expression. Furthermore, BRIP1 and BRIP2 physically interact with BRM-containing SWI/SNF complexes and extensively co-localize with BRM on chromatin. Simultaneous mutation of BRIP1 and BRIP2 results in decreased BRM occupancy at almost all BRM target loci and substantially reduced abundance of the SWI/SNF assemblies. Together, our work identifies new core subunits of BRM-containing SWI/SNF complexes in plants and uncovers the essential role of these subunits in maintaining the abundance of SWI/SNF complexes in plants.

Nuclear pore complex-mediated gene expression in Arabidopsis thaliana

Nuclear pore complexes (NPCs) are large multi-protein complexes that control bidirectional trafficking of macromolecules between the nucleus and cytoplasm. This trafficking is highly regulated and participates in a considerably broader range of cellular activities, including defense responses against pathogens in plants. Recently, NPC is emerging as a platform to physically associate the underlying chromatin with the nuclear periphery, thus regulating chromatin structure and gene expression. For instance, NPC components have been shown to promote the formation of specific genomics loops, which is linked to transcriptional memory for rapid reactivation of genes. With newly developed techniques and tools, our insight in this area has been substantially advanced. This review summarizes recent works on the molecular function of NPC machinery as hubs for transcriptional regulation and compares systems between plant and non-plant organisms.

Unique and contrasting effects of light and temperature cues on plant transcriptional programs

Plants have adapted to tolerate and survive constantly changing environmental conditions by reprogramming gene expression in response to stress or to drive developmental transitions. Among the many signals that plants perceive, light and temperature are of particular interest due to their intensely fluctuating nature which is combined with a long-term seasonal trend. Whereas specific receptors are key in the light-sensing mechanism, the identity of plant thermosensors for high and low temperatures remains far from fully addressed. This review aims at discussing common as well as divergent characteristics of gene expression regulation in plants, controlled by light and temperature. Light and temperature signaling control the abundance of specific transcription factors, as well as the dynamics of co-transcriptional processes such as RNA polymerase elongation rate and alternative splicing patterns. Additionally, sensing both types of cues modulates gene expression by altering the chromatin landscape and through the induction of long non-coding RNAs (lncRNAs). However, while light sensing is channeled through dedicated receptors, temperature can broadly affect chemical reactions inside plant cells. Thus, direct thermal modifications of the transcriptional machinery add another level of complexity to plant transcriptional regulation. Besides the rapid transcriptome changes that follow perception of environmental signals, plant developmental transitions and acquisition of stress tolerance depend on long-term maintenance of transcriptional states (active or silenced genes). Thus, the rapid transcriptional response to the signal (Phase I) can be distinguished from the long-term memory of the acquired transcriptional state (Phase II - remembering the signal). In this review we discuss recent advances in light and temperature signal perception, integration and memory in Arabidopsis thaliana, focusing on transcriptional regulation and highlighting the contrasting and unique features of each type of cue in the process.

Segregation of an MSH1 RNAi transgene produces heritable non-genetic memory in association with methylome reprogramming

MSH1 is a plant-specific protein. RNAi suppression of MSH1 results in phenotype variability for developmental and stress response pathways. Segregation of the RNAi transgene produces non-genetic msh1 ‘memory’ with multi-generational inheritance. First-generation memory versus non-memory comparison, and six-generation inheritance studies, identifies gene-associated, heritable methylation repatterning. Genome-wide methylome analysis integrated with RNAseq and network-based enrichment studies identifies altered circadian clock networks, and phytohormone and stress response pathways that intersect with circadian control. A total of 373 differentially methylated loci comprising these networks are sufficient to discriminate memory from nonmemory full sibs. Methylation inhibitor 5-azacytidine diminishes the differences between memory and wild type for growth, gene expression and methylation patterning. The msh1 reprogramming is dependent on functional HISTONE DEACETYLASE 6 and methyltransferase MET1, and transition to memory requires the RNA-directed DNA methylation pathway. This system of phenotypic plasticity may serve as a potent model for defining accelerated plant adaptation during environmental change.

Segregation of an MSH1 RNAi transgene produces non-genetic memory that displays transgenerational inheritance in Arabidopsis. Here, the authors compare memory and non-memory full-sib progenies to show the involvement of DNA methylation reprogramming, involving the RdDM pathway, in transition to a heritable memory state.

Interactions between metabolism and chromatin in plant models

BACKGROUND

One of the fascinating aspects of epigenetic regulation is that it provides means to rapidly adapt to environmental change. This is particularly relevant in the plant kingdom, where most species are sessile and exposed to increasing habitat fluctuations due to global warming. Although the inheritance of epigenetically controlled traits acquired through environmental impact is a matter of debate, it is well documented that environmental cues lead to epigenetic changes, including chromatin modifications, that affect cell differentiation or are associated with plant acclimation and defense priming. Still, in most cases, the mechanisms involved are poorly understood. An emerging topic that promises to reveal new insights is the interaction between epigenetics and metabolism.

SCOPE OF REVIEW

This study reviews the links between metabolism and chromatin modification, in particular histone acetylation, histone methylation, and DNA methylation, in plants and compares them to examples from the mammalian field, where the relationship to human diseases has already generated a larger body of literature. This study particularly focuses on the role of reactive oxygen species (ROS) and nitric oxide (NO) in modulating metabolic pathways and gene activities that are involved in these chromatin modifications. As ROS and NO are hallmarks of stress responses, we predict that they are also pivotal in mediating chromatin dynamics during environmental responses.

MAJOR CONCLUSIONS

Due to conservation of chromatin-modifying mechanisms, mammals and plants share a common dependence on metabolic intermediates that serve as cofactors for chromatin modifications. In addition, plant-specific non-CG methylation pathways are particularly sensitive to changes in folate-mediated one-carbon metabolism. Finally, reactive oxygen and nitrogen species may fine-tune epigenetic processes and include similar signaling mechanisms involved in environmental stress responses in plants as well as animals.

Acclimation, priming and memory in the response of Arabidopsis thaliana seedlings to cold stress

Because stress experiences are often recurrent plants have developed strategies to remember a first so-called priming stress to eventually respond more effectively to a second triggering stress. Here, we have studied the impact of discontinuous or sustained cold stress (4 °C) on in vitro grown Arabidopsis thaliana seedlings of different age and their ability to get primed and respond differently to a later triggering stress. Cold treatment of 7-d-old seedlings induced the expression of cold response genes but did not cause a significantly enhanced freezing resistance. The competence to increase the freezing resistance in response to cold was associated with the formation of true leaves. Discontinuous exposure to cold only during the night led to a stepwise modest increase in freezing tolerance provided that the intermittent phase at ambient temperature was less than 32 h. Seedlings exposed to sustained cold treatment developed a higher freezing tolerance which was further increased in response to a triggering stress during three days after the priming treatment had ended indicating cold memory. Interestingly, in all scenarios the primed state was lost as soon as the freezing tolerance had reached the level of naïve plants indicating that an effective memory was associated with an altered physiological state. Known mutants of the cold stress response (cbfs, erf105) and heat stress memory (fgt1) did not show an altered behaviour indicating that their roles do not extend to memory of cold stress in Arabidopsis seedlings.

Unwinding BRAHMA Functions in Plants

The ATP-dependent Switch/Sucrose non-fermenting (SWI/SNF) chromatin remodeling complex (CRC) regulates the transcription of many genes by destabilizing interactions between DNA and histones. In plants, BRAHMA (BRM), one of the two catalytic ATPase subunits of the complex, is the closest homolog of the yeast and animal SWI2/SNF2 ATPases. We summarize here the advances describing the roles of BRM in plant development as well as its recently reported chromatin-independent role in pri-miRNA processing in vitro and in vivo. We also enlighten the roles of plant-specific partners that physically interact with BRM. Three main types of partners can be distinguished: (i) DNA-binding proteins such as transcription factors which mostly cooperate with BRM in developmental processes, (ii) enzymes such as kinases or proteasome-related proteins that use BRM as substrate and are often involved in response to abiotic stress, and (iii) an RNA-binding protein which is involved with BRM in chromatin-independent pri-miRNA processing. This overview contributes to the understanding of the central position occupied by BRM within regulatory networks controlling fundamental biological processes in plants.

Plant growth-regulating molecules as thermoprotectants: functional relevance and prospects for improving heat tolerance in food crops

Among various abiotic stresses, heat stress is one of the most damaging, threatening plant productivity and survival all over the world. Warmer temperatures due to climatic anomalies above optimum growing temperatures have detrimental impacts on crop yield potential as well as plant distribution patterns. Heat stress affects overall plant metabolism in terms of physiology, biochemistry, and gene expression. Membrane damage, protein degradation, enzyme inactivation, and the accumulation of reactive oxygen species are some of the harmful effects of heat stress that cause injury to various cellular compartments. Although plants are equipped with various defense strategies to counteract these adversities, their defensive means are not sufficient to defend against the ever-rising temperatures. Hence, substantial yield losses have been observed in all crop species under heat stress. Here, we describe the involvement of various plant growth-regulators (PGRs) (hormones, polyamines, osmoprotectants, antioxidants, and other signaling molecules) in thermotolerance, through diverse cellular mechanisms that protect cells under heat stress. Several studies involving the exogenous application of PGRs to heat-stressed plants have demonstrated their role in imparting tolerance, suggesting the strong potential of these molecules in improving the performance of food crops grown under high temperature.

Organellar protein multi-functionality and phenotypic plasticity in plants

With the increasing impact of climate instability on agricultural and ecological systems has come a heightened sense of urgency to understand plant adaptation mechanisms in more detail. Plant species have a remarkable ability to disperse their progeny to a wide range of environments, demonstrating extraordinary resiliency mechanisms that incorporate epigenetics and transgenerational stability. Surprisingly, some of the underlying versatility of plants to adapt to abiotic and biotic stress emerges from the neofunctionalization of organelles and organellar proteins. We describe evidence of possible plastid specialization and multi-functional organellar protein features that serve to enhance plant phenotypic plasticity. These features appear to rely on, for example, spatio-temporal regulation of plastid composition, and unusual interorganellar protein targeting and retrograde signalling features that facilitate multi-functionalization. Although we report in detail on three such specializations, involving MSH1, WHIRLY1 and CUE1 proteins in Arabidopsis, there is ample reason to believe that these represent only a fraction of what is yet to be discovered as we begin to elaborate cross-species diversity. Recent observations suggest that plant proteins previously defined in one context may soon be rediscovered in new roles and that much more detailed investigation of proteins that show subcellular multi-targeting may be warranted.

This article is part of the theme issue ‘Linking the mitochondrial genotype to phenotype: a complex endeavour’.

Arabidopsis SWR1-associated protein methyl-CpG-binding domain 9 is required for histone H2A.Z deposition

Deposition of the histone variant H2A.Z by the SWI2/SNF2-Related 1 chromatin remodeling complex (SWR1-C) is important for gene regulation in eukaryotes, but the composition of the Arabidopsis SWR1-C has not been thoroughly characterized. Here, we aim to identify interacting partners of a conserved Arabidopsis SWR1 subunit ACTIN-RELATED PROTEIN 6 (ARP6). We isolate nine predicted components and identify additional interactors implicated in histone acetylation and chromatin biology. One of the interacting partners, methyl-CpG-binding domain 9 (MBD9), also strongly interacts with the Imitation SWItch (ISWI) chromatin remodeling complex. MBD9 is required for deposition of H2A.Z at a distinct subset of ARP6-dependent loci. MBD9 is preferentially bound to nucleosome-depleted regions at the 5’ ends of genes containing high levels of activating histone marks. These data suggest that MBD9 is a SWR1-C interacting protein required for H2A.Z deposition at a subset of actively transcribing genes.

The SWI2/SNF2-Related 1 chromatin remodeling complex (SWR1-C) is important for gene regulation, but its composition remains largely uncharacterized in plants. Here, the authors report that methyl-CpG-binding domain 9 (MBD9) is a SWR1-C interacting protein required for histone H2A.Z deposition in Arabidopsis.

The histone variant H2A.Z and chromatin remodeler BRAHMA act coordinately and antagonistically to regulate transcription and nucleosome dynamics in Arabidopsis

Plants adapt to environmental changes by regulating transcription and chromatin organization. The histone H2A variant H2A.Z and the SWI2/SNF2 ATPase BRAHMA (BRM) have overlapping roles in positively and negatively regulating environmentally responsive genes in Arabidopsis, but the extent of this overlap was uncharacterized. Both factors have been associated with various changes in nucleosome positioning and stability in different contexts, but their specific roles in transcriptional regulation and chromatin organization need further characterization. We show that H2A.Z and BRM co-localize at thousands of sites, where they interact both cooperatively and antagonistically in transcriptional repression and activation of genes involved in development and responses to environmental stimuli. We identified eight classes of genes that show distinct relationships between H2A.Z and BRM with respect to their roles in transcription. These include activating and silencing transcription both redundantly and antagonistically. We found that H2A.Z contributes to a range of different nucleosome properties, while BRM stabilizes nucleosomes where it binds and destabilizes or repositions flanking nucleosomes. We also found that, at many genes regulated by both BRM and H2A.Z, both factors overlap with binding sites of the light-regulated transcription factor FAR1-Related Sequence 9 (FRS9) and that a subset of these FRS9 binding sites are dependent on H2A.Z and BRM for accessibility. Collectively, we comprehensively characterized the antagonistic and cooperative contributions of H2A.Z and BRM to transcriptional regulation, and illuminated several interrelated roles in chromatin organization. The variability observed in their individual functions implies that both BRM and H2A.Z have more context-dependent roles than previously assumed.

Perspective on Alternative Splicing and Proteome Complexity in Plants

Alternative splicing (AS) generates multiple transcripts from the same gene, however, AS contribution to proteome complexity remains elusive in plants. AS is prevalent under stress conditions in plants, but it is counterintuitive why plants would invest in protein synthesis under declining energy supply. We propose that plants employ AS not only to potentially increasing proteomic complexity, but also to buffer against the stress-responsive transcriptome to reduce the metabolic cost of translating all AS transcripts. To maximise efficiency under stress, plants may make fewer proteins with disordered domains via AS to diversify substrate specificity and maintain sufficient regulatory capacity. Furthermore, we suggest that chromatin state-dependent AS engenders short/long-term stress memory to mediate reproducible transcriptional response in the future.

Glucose-Regulated HLP1 Acts as a Key Molecule in Governing Thermomemory

Induction of heat shock proteins (HSPs) in response to heat stress (HS) is indispensable for conferring thermotolerance. Glc, a fundamental signaling and metabolic molecule, provides energy to stressed seedlings to combat stress. The recovery of stressed plants from detrimental HS in response to Glc is largely mediated by HSPs, but the mechanistic basis of this thermotolerance is not well defined. In this study, we show that Glc has a prominent role in providing thermotolerance. Glc-mediated thermotolerance involves HSP induction via the TARGET OF RAPAMYCIN (TOR)-E2Fa signaling module. Apart from HSPs, TOR-E2Fa also regulates the Arabidopsis (Arabidopsis thaliana) ortholog of human Hikeshi, named HIKESHI-LIKE PROTEIN1 (HLP1). Expression of proHLP1::GUS in the shoot apical meristem (SAM) after HS coincides with TOR-E2Fa expression, substantiating a role for TOR-E2Fa-HLP1 in providing thermotolerance. We also demonstrate that Glc along with heat could induce proliferation activity in the SAM after HS recovery, which was arrested by the TOR inhibitor AZD-8055. Molecular and physiological studies suggest that HS-activated heat stress transcription factor A1s also positively regulate HLP1 transcription, suggesting convergence of the Glc and HS signaling pathways. Loss of functional HLP1 causes HS hypersensitivity, whereas HLP1 overexpressors display increased thermotolerance. HLP1 binds to the promoters of Glc-regulated HS-responsive genes and promotes chromatin acetylation. In addition, Glc modifies the chromatin landscape at thermomemory-related loci by promoting H3K4 trimethylation (H3K4me3). Glc-primed accumulation of H3K4me3 at thermomemory-associated loci is mediated through HLP1. These findings reveal the novel function of Glc-regulated HLP1 in mediating thermotolerance/thermomemory response.

Identification and Expression Analysis of Snf2 Family Proteins in Tomato (Solanum lycopersicum)

As part of chromatin-remodeling complexes (CRCs), sucrose nonfermenting 2 (Snf2) family proteins alter chromatin structure and nucleosome position by utilizing the energy of ATP, which allows other regulatory proteins to access DNA. Plant genomes encode a large number of Snf2 proteins, and some of them have been shown to be the key regulators at different developmental stages in Arabidopsis. Yet, little is known about the functions of Snf2 proteins in tomato (Solanum lycopersicum). In this study, 45 Snf2s were identified by the homologous search using representative sequences from yeast (S. cerevisiae), fruit fly (D. melanogaster), and Arabidopsis (A. thaliana) against the tomato genome annotation dataset. Tomato Snf2 proteins (also named SlCHRs) could be clustered into 6 groups and distributed on 11 chromosomes. All SlCHRs contained a helicase-C domain with about 80 amino acid residues and a SNF2-N domain with more variable amino acid residues. In addition, other conserved motifs were also identified in SlCHRs by using the MEME program. Expression profile analysis indicated that tomato Snf2 family genes displayed a wide range of expressions in different tissues and some of them were regulated by the environmental stimuli such as salicylic acid, abscisic acid, salt, and cold. Taken together, these results provide insights into the functions of SlCHRs in tomato.

Chromatin-based mechanisms of temperature memory in plants

For successful growth and development, plants constantly have to gauge their environment. Plants are capable to monitor their current environmental conditions, and they are also able to integrate environmental conditions over time and store the information induced by the cues. In a developmental context, such an environmental memory is used to align developmental transitions with favourable environmental conditions. One temperature-related example of this is the transition to flowering after experiencing winter conditions, that is, vernalization. In the context of adaptation to stress, such an environmental memory is used to improve stress adaptation even when the stress cues are intermittent. A somatic stress memory has now been described for various stresses, including extreme temperatures, drought, and pathogen infection. At the molecular level, such a memory of the environment is often mediated by epigenetic and chromatin modifications. Histone modifications in particular play an important role. In this review, we will discuss and compare different types of temperature memory and the histone modifications, as well as the reader, writer, and eraser proteins involved.

BRUSHY1/TONSOKU/MGOUN3 is required for heat stress memory

Plants encounter biotic and abiotic stresses many times during their life cycle and this limits their productivity. Moderate heat stress (HS) primes a plant to survive higher temperatures that are lethal in the naïve state. Once temperature stress subsides, the memory of the priming event is actively retained for several days preparing the plant to better cope with recurring HS. Recently, chromatin regulation at different levels has been implicated in HS memory. Here, we report that the chromatin protein BRUSHY1 (BRU1)/TONSOKU/MGOUN3 plays a role in the HS memory in Arabidopsis thaliana. BRU1 is also involved in transcriptional gene silencing and DNA damage repair. This corresponds with the functions of its mammalian orthologue TONSOKU-LIKE/NFΚBIL2. During HS memory, BRU1 is required to maintain sustained induction of HS memory-associated genes, whereas it is dispensable for the acquisition of thermotolerance. In summary, we report that BRU1 is required for HS memory in A. thaliana, and propose a model where BRU1 mediates the epigenetic inheritance of chromatin states across DNA replication and cell division.