m6A RNA demethylase AtALKBH9B promotes mobilization of a heat-activated long terminal repeat retrotransposon in Arabidopsis

RNA m6A modification meets plant hormones

Plant hormones are essential signalling molecules that control and coordinate diverse physiological processes in plant development and adaptation to ever-fluctuating environments. This hormonal regulation of plant development and environmental responses has recently been shown to extensively involve the most widespread RNA modification, N6-methyladenosine (m6A). Here we discuss the current understanding of the crosstalk between m6A and plant hormones, focusing on their reciprocal regulation, where hormonal signals induce m6A reprogramming and m6A affects hormone biosynthesis and signalling cascades. We also highlight new insights into how m6A contributes to the hormonal control of plant development and stress responses. Furthermore, we discuss future prospects for unveiling the regulatory networks that orchestrate epitranscriptome-hormone interactions and harnessing the related knowledge accrued to enhance crop productivity and resilience in changing environments.

Genome-wide identification and characterization of ALKB homolog gene family in wheat (Triticum aestivum L.)

Introduction

N6-methyladenosine (m6A) is the most prevalent posttranscriptional modification in eukaryotic mRNAs. AlkB homologs (ALKBHs) are involved in plant responses to stress by modulating m6A methylation. However, homologous genes in wheat remain largely uncharacterized.

Methods and results

In this study, 30 ALKBH genes were identified in wheat, and analyzed their physicochemical properties. The phylogenetic analysis allowed the classification of these genes into seven distinct subfamilies. Additionally, their conserved domains, motif compositions, gene structures, chromosomal localization, and synteny, and the predicted cis-acting elements within their promoters were examined. Expression analysis revealed that TaALKBH9B-5 exhibited the highest expression and its demethylase activity was investigated. Furthermore, TaALKBH9B-5 was significantly upregulated in response to abscisic acid treatment and cold stress, indicating a positive regulatory trend.

Discussion

In conclusion, this study provides a comprehensive genomic assessment of the TaALKBH gene family and offers a theoretical framework for understanding the role of TaALKBH9B in the response to abiotic stress in wheat.

mRNA m6A regulates gene expression via H3K4me3 shift in 5' UTR

BACKGROUND

N6-methyladenosine (m6A) is a prevalent and conserved RNA modification in eukaryotes. While its roles in the 3' untranslated regions (3' UTR) are well-studied, its role in the 5' UTR and its relationship with histone modifications remain underexplored.

RESULTS

We demonstrate that m6A methylation in the 5' UTR of mRNA triggers a downstream shift in H3K4me3 modification. This regulatory mechanism is conserved in Arabidopsis, rice, and chrysanthemum. The observed shift in H3K4me3 is genetically controlled by m6A modifiers and influences gene expression. MTA, the m6A methylase, preferentially binds to phosphorylated serine 5 (Ser5P)-CTD of RNA Pol II during transcription, leading to the displacement of ATX1, the H3K4me3 methylase. This dynamic binding of MTA and ATX1 to RNA Pol II ultimately results in the shift of H3K4me3 modification. Genetic evidence demonstrates that m6A in the 5' UTR controls H3K4me3 shift, thereby affecting SEDOHEPTULOSE-BISPHOSPHATASE expression and leaf senescence.

CONCLUSIONS

Our study provides new insights into the roles of m6A modification and its crosstalk with histone modification in 5' UTRs, shedding light on the mechanism of m6A-mediated gene expression regulation.

Molecular aspects of heat stress sensing in land plants

Heat stress impacts all aspects of life, from evolution to global food security. Therefore, it becomes essential to understand how plants respond to heat stress, especially in the context of climate change. The heat stress response (HSR) involves three main components: sensing, signal transduction, and cellular reprogramming. Here, I focus on the heat stress sensing component. How can cells detect heat stress if it is not a signalling particle? To answer this question, I have looked at the molecular definition of heat stress. It can be defined as any particular rise in the optimum growth temperature that leads to higher-than-normal levels of reactive molecular species and macromolecular damage to biological membranes, proteins, and nucleic acid polymers (DNA and RNA). It is precisely these stress-specific alterations that are detected by heat stress sensors, upon which they would immediately trigger the appropriate level of the HSR. In addition, the work towards thermotolerance is complemented by a second type of response, here called the cellular homeostasis response (CHR). Upon mild and extreme temperature changes, the CHR is triggered by plant thermosensors, which are responsible for monitoring temperature information. Heat stress sensors and thermosensors are distinct types of molecules, each with unique modes of activation and functions. While many recent reviews provide a comprehensive overview of plant thermosensors, there remains a notable gap in the review literature regarding an in-depth analysis of plant heat stress sensors. Here, I attempt to summarise our current knowledge of the cellular sensors involved in triggering the plant HSR.

RNA modifications in plant adaptation to abiotic stresses

Epitranscriptomic chemical modifications of RNAs have emerged as potent regulatory mechanisms in the process of plant stress adaptation. Currently, over 170 distinct chemical modifications have been identified in mRNAs, tRNAs, rRNAs, microRNAs (miRNAs), and long noncoding RNAs (lncRNAs). Genetic and molecular studies have identified the genes responsible for addition and removal of chemical modifications from RNA molecules, which are known as "writers" and "erasers," respectively. N6-methyladenosine (m6A) is the most prevalent chemical modification identified in eukaryotic mRNAs. Recent studies have identified m6A writers and erasers across different plant species, including Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), cotton (Gossypium hirsutum), and tomato (Solanum lycopersicum). Accumulating discoveries have improved our understanding of the functions of RNA modifications in plant stress responses. This review highlights the latest research on RNA modification, emphasizing the biological and cellular roles of diverse chemical modifications of mRNAs, tRNAs, rRNAs, miRNAs, and lncRNAs in plant responses to environmental and hormonal signals. We also propose and discuss critical questions and future challenges for enhancing our understanding of the cellular and mechanistic roles of RNA modifications in plant stress responses. Integrating molecular insights into the regulatory roles of RNA modifications in stress responses with novel genome- and RNA-editing technologies will facilitate the breeding of stress-tolerant crops through precise engineering of RNA modifications.

Epigenetic control of plant abiotic stress responses

On top of genetic information, organisms have evolved complex and sophisticated epigenetic regulation to adjust gene expression in response to developmental and environmental signals. Key epigenetic mechanisms include DNA methylation, histone modifications and variants, chromatin remodeling, and chemical modifications of RNAs. Epigenetic control of environmental responses is particularly important for plants, which are sessile and unable to move away from adverse environments. Besides enabling plants to rapidly respond to environmental stresses, some stress-induced epigenetic changes can be maintained, providing plants with a pre-adapted state to recurring stresses. Understanding these epigenetic mechanisms offers valuable insights for developing crop varieties with enhanced stress tolerance. Here, we focus on abiotic stresses and summarize recent progress in characterizing stress-induced epigenetic changes and their regulatory mechanisms and roles in plant abiotic stress resistance.

The FvABF3-FvALKBH10B-FvSEP3 cascade regulates fruit ripening in strawberry

Fruit ripening is a highly-orchestrated process that requires the fine-tuning and precise control of gene expression, which is mainly governed by phytohormones, epigenetic modifiers, and transcription factors. How these intrinsic regulators coordinately modulate the ripening remains elusive. Here we report the identification and characterization of FvALKBH10B as an N6-methyladenosine (m6A) RNA demethylase necessary for the normal ripening of strawberry (Fragaria vesca) fruit. FvALKBH10B is induced by phytohormone abscisic acid (ABA), and ABA-Responsive Element Binding Factor 3 (FvABF3), a master regulator in ABA signaling, is responsible for this activation. FvALKBH10B mutation leads to a delay in fruit ripening and causes global m6A hypermethylation of 1859 genes. Further analyses show that FvALKBH10B positively modulates the mRNA stability of SEPALLATA3 (FvSEP3) encoding a transcription factor via m6A demethylation. In turn, FvSEP3 targets numerous ripening-related genes including those associated with biosynthesis of ABA and anthocyanin and regulates their expression. Our findings uncover an FvABF3-FvALKBH10B-FvSEP3 cascade in controlling fruit ripening in strawberry and provide insights into the complex regulatory networks involved in this process.

Epitranscriptomic regulation through phase separation in plants

Epitranscriptomic regulation has emerged as a crucial layer of gene control where RNA modifications, particularly N6-methyladenosine (m6A), introduce complexity and versatility to gene regulation. Increasing evidence suggests that epitranscriptomic regulation through phase separation plays critical roles in mediating RNA metabolism during plant development and stress responses. m6A-associated biomolecular condensates formed via phase separation act as dynamic cellular hotspots where m6A effectors, RNAs, and other regulatory proteins coalesce to facilitate RNA regulation. Moreover, m6A modulates condensate assembly. Herein, I summarize the current understanding of how m6A- and m6A effector-mediated formation of biomolecular condensates mediates plant development and stress adaptation. I also discuss several working models for m6A-associated biomolecular condensates and highlight the prospects for future research on epitranscriptomic regulation through phase separation.

Defining context-dependent m6A RNA methylomes in Arabidopsis

N6-Methyladenosine (m6A) prevalently occurs on cellular RNA across almost all kingdoms of life. It governs RNA fate and is essential for development and stress responses. However, the dynamic, context-dependent m6A methylomes across tissues and in response to various stimuli remain largely unknown in multicellular organisms. Here, we generate a comprehensive census that identifies m6A methylomes in 100 samples during development or following exposure to various external conditions in Arabidopsis thaliana. We demonstrate that m6A is a suitable biomarker to reflect the developmental lineage, and that various stimuli rapidly affect m6A methylomes that constitute the regulatory network required for an effective response to the stimuli. Integrative analyses of the census and its correlation with m6A regulators identify multiple layers of regulation on highly context-dependent m6A modification in response to diverse developmental and environmental stimuli, providing insights into m6A modification dynamics in the myriad contexts of multicellular organisms.

Sequestration of DBR1 to stress granules promotes lariat intronic RNAs accumulation for heat-stress tolerance

Heat stress (HS) poses a significant challenge to plant survival, necessitating sophisticated molecular mechanisms to maintain cellular homeostasis. Here, we identify SICKLE (SIC) as a key modulator of HS responses in Arabidopsis (Arabidopsis thaliana). SIC is required for the sequestration of RNA DEBRANCHING ENZYME 1 (DBR1), a rate-limiting enzyme of lariat intronic RNA (lariRNA) decay, into stress granules (SGs). The sequestration of DBR1 by SIC enhances the accumulation of lariRNAs, branched circular RNAs derived from excised introns during pre-mRNA splicing, which in turn promote the transcription of their parental genes. Our findings further demonstrate that SIC-mediated DBR1 sequestration in SGs is crucial for plant HS tolerance, as deletion of the N-terminus of SIC (SIC1-244) impairs DBR1 sequestration and compromises plant response to HS. Overall, our study unveils a mechanism of transcriptional regulation in the HS response, where lariRNAs are enriched through DBR1 sequestration, ultimately promoting the transcription of heat stress tolerance genes.

OsALKBH9-mediated m6A demethylation regulates tapetal PCD and pollen exine accumulation in rice

The N6-methyladenosine (m6A) mRNA modification is crucial for plant development and stress responses. In rice, the male sterility resulting from the deficiency of OsFIP37, a core component of m6A methyltransferase complex, emphasizes the significant role of m6A in male fertility. m6A is reversible and can be removed by m6A demethylases. However, whether mRNA m6A demethylase regulates male fertility in rice has remained unknown. Here, we identify the mRNA m6A demethylase OsALKBH9 and demonstrate its involvement in male fertility regulation. Knockout of OsALKBH9 causes male sterility, dependent on its m6A demethylation activity. Cytological analysis reveals defective tapetal programmed cell death (PCD) and excessive accumulation of microspores exine in Osalkbh9-1. Transcriptome analysis of anthers shows up-regulation of genes involved in tapetum development, sporopollenin synthesis, and transport pathways in Osalkbh9-1. Additionally, we demonstrate that OsALKBH9 demethylates the m6A modification in TDR and GAMYB transcripts, which affects the stability of these mRNAs and ultimately leads to excessive accumulation of pollen exine. Our findings highlight the precise control of mRNA m6A modification and reveal the pivotal roles played by OsALKBH9-mediated m6A demethylation in tapetal PCD and pollen exine accumulation in rice.

Principles, functions, and biological implications of m6A in plants

Over the past decade, N 6-methyladenosine (m6A) has emerged as a prevalent and dynamically regulated modification across the transcriptome; it has been reversibly installed, removed, and interpreted by specific binding proteins, and has played crucial roles in molecular and biological processes. Within this scope, we consolidate recent advancements of m6A research in plants regarding gene expression regulation, diverse physiologic and pathogenic processes, as well as crop trial implications, to guide discussions on challenges associated with and leveraging epitranscriptome editing for crop improvement.

Plant RNA-binding proteins: Phase separation dynamics and functional mechanisms underlying plant development and stress responses

RNA-binding proteins (RBPs) accompany RNA from synthesis to decay, mediating every aspect of RNA metabolism and impacting diverse cellular and developmental processes in eukaryotes. Many RBPs undergo phase separation along with their bound RNA to form and function in dynamic membraneless biomolecular condensates for spatiotemporal coordination or regulation of RNA metabolism. Increasing evidence suggests that phase-separating RBPs with RNA-binding domains and intrinsically disordered regions play important roles in plant development and stress adaptation. Here, we summarize the current knowledge about how dynamic partitioning of RBPs into condensates controls plant development and enables sensing of experimental changes to confer growth plasticity under stress conditions, with a focus on the dynamics and functional mechanisms of RBP-rich nuclear condensates and cytoplasmic granules in mediating RNA metabolism. We also discuss roles of multiple factors, such as environmental signals, protein modifications, and N6-methyladenosine RNA methylation, in modulating the phase separation behaviors of RBPs, and highlight the prospects and challenges for future research on phase-separating RBPs in crops.