Epitranscriptome engineering in crop improvement

Direct RNA sequencing in plants: Practical applications and future perspectives

The transcriptome serves as a bridge that links genomic variation to phenotypic diversity. A vast number of studies using next-generation RNA sequencing (RNA-seq) over the last 2 decades have emphasized the essential roles of the plant transcriptome in response to developmental and environmental conditions, providing numerous insights into the dynamic changes, evolutionary traces, and elaborate regulation of the plant transcriptome. With substantial improvement in accuracy and throughput, direct RNA sequencing (DRS) has emerged as a new and powerful sequencing platform for precise detection of native and full-length transcripts, overcoming many limitations such as read length and PCR bias that are inherent to short-read RNA-seq. Here, we review recent advances in dissecting the complexity and diversity of plant transcriptomes using DRS as the main technological approach, covering many aspects of RNA metabolism, including novel isoforms, poly(A) tails, and RNA modification, and we propose a comprehensive workflow for processing of plant DRS data. Many challenges to the application of DRS in plants, such as the need for machine learning tools tailored to plant transcriptomes, remain to be overcome, and together we outline future biological questions that can be addressed by DRS, such as allele-specific RNA modification. This technology provides convenient support on which the connection of distinct RNA features is tightly built, sustainably refining our understanding of the biological functions of the plant transcriptome.

N6-methyladenosine-mediated feedback regulation of abscisic acid perception via phase-separated ECT8 condensates in Arabidopsis

N6-methyladenosine (m6A) is the most abundant internal modification in eukaryotic mRNAs, yet how plants recognize this chemical modification to swiftly adjust developmental plasticity under environmental stresses remains unclear. Here we show that m6A mRNA modification and its reader protein EVOLUTIONARILY CONSERVED C-TERMINAL REGION 8 (ECT8) act together as a key checkpoint for negative feedback regulation of abscisic acid (ABA) signalling by sequestering the m6A-modified ABA receptor gene PYRABACTIN RESISTANCE 1-LIKE 7 (PYL7) via phase-separated ECT8 condensates in stress granules in response to ABA. This partially depletes PYL7 mRNA from its translation in the cytoplasm, thus reducing PYL7 protein levels and compromising ABA perception. The loss of ECT8 results in defective sequestration of m6A-modified PYL7 in stress granules and permits more PYL7 transcripts for translation. This causes overactivation of ABA-responsive genes and the consequent ABA-hypersensitive phenotypes, including drought tolerance. Overall, our findings reveal that m6A-mediated sequestration of PYL7 by ECT8 in stress granules negatively regulates ABA perception, thereby enabling prompt feedback regulation of ABA signalling to prevent plant cell overreaction to environmental stresses.

Current technical advancements in plant epitranscriptomic studies

The growth and development of plants are the result of the interplay between the internal developmental programming and plant-environment interactions. Gene expression regulations in plants are made up of multi-level networks. In the past few years, many studies were carried out on co- and post-transcriptional RNA modifications, which, together with the RNA community, are collectively known as the "epitranscriptome." The epitranscriptomic machineries were identified and their functional impacts characterized in a broad range of physiological processes in diverse plant species. There is mounting evidence to suggest that the epitranscriptome provides an additional layer in the gene regulatory network for plant development and stress responses. In the present review, we summarized the epitranscriptomic modifications found so far in plants, including chemical modifications, RNA editing, and transcript isoforms. The various approaches to RNA modification detection were described, with special emphasis on the recent development and application potential of third-generation sequencing. The roles of epitranscriptomic changes in gene regulation during plant-environment interactions were discussed in case studies. This review aims to highlight the importance of epitranscriptomics in the study of gene regulatory networks in plants and to encourage multi-omics investigations using the recent technical advancements.

Epigenetic control of plant senescence and cell death and its application in crop improvement

Plant senescence is the last stage of plant development and a type of programmed cell death, occurring at a predictable time and cell. It involves the functional conversion from nutrient assimilation to nutrient remobilization, which substantially impacts plant architecture and plant biomass, crop quality, and horticultural ornamental traits. In past two decades, DNA damage was believed to be a main reason for cell senescence. Increasing evidence suggests that the alteration of epigenetic information is a contributing factor to cell senescence in organisms. In this review, we summarize the current research progresses of epigenetic and epitranscriptional mechanism involved in cell senescence of plant, at the regulatory level of DNA methylation, histone methylation and acetylation, chromatin remodeling, non-coding RNAs and RNA methylation. Furthermore, we discuss their molecular genetic manipulation and potential application in agriculture for crop improvement. Finally we point out the prospects of future research topics.

Recent advances in the plant epitranscriptome

Chemical modifications of RNAs, known as the epitranscriptome, are emerging as widespread regulatory mechanisms underlying gene regulation. The field of epitranscriptomics advances recently due to improved transcriptome-wide sequencing strategies for mapping RNA modifications and intensive characterization of writers, erasers, and readers that deposit, remove, and recognize RNA modifications, respectively. Herein, we review recent advances in characterizing plant epitranscriptome and its regulatory mechanisms in post-transcriptional gene regulation and diverse physiological processes, with main emphasis on N⁶-methyladenosine (m⁶A) and 5-methylcytosine (m⁵C). We also discuss the potential and challenges for utilization of epitranscriptome editing in crop improvement.

Exploring epitranscriptomics for crop improvement and environmental stress tolerance

Climate change and stressful environmental conditions severely hamper crop growth, development and yield. Plants respond to environmental perturbations, through their plasticity provided by key-genes, governed at post-/transcriptional levels. Gene-regulation in plants is a multilevel process controlled by diverse cellular entities that includes transcription factors (TF), epigenetic regulators and non-coding RNAs beside others. There are successful studies confirming the role of epigenetic modifications (DNA-methylation/histone-modifications) in gene expression. Recent years have witnessed emergence of a highly specialized field the "Epitranscriptomics". Epitranscriptomics deals with investigating post-transcriptional RNA chemical-modifications present across the life forms that change structural, functional and biological characters of RNA. However, deeper insights on of epitranscriptomic modifications, with >140 types known so far, are to be understood fully. Researchers have identified epitranscriptome marks (writers, erasers and readers) and mapped the site-specific RNA modifications (m6A, m⁵C, 3' uridylation, etc.) responsible for fine-tuning gene expression in plants. Simultaneous advancement in sequencing platforms, upgraded bioinformatic tools and pipelines along with conventional labelled techniques have further given a statistical picture of these epitranscriptomic modifications leading to their potential applicability in crop improvement and developing climate-smart crops. We present herein the insights on epitranscriptomic machinery in plants and how epitranscriptome and epitranscriptomic modifications underlying plant growth, development and environmental stress responses/adaptations. Third-generation sequencing technology, advanced bioinformatics tools and databases being used in plant epitranscriptomics are also discussed. Emphasis is given on potential exploration of epitranscriptome engineering for crop-improvement and developing environmental stress tolerant plants covering current status, challenges and future directions.

The plant epitranscriptome: revisiting pseudouridine and 2'-O-methyl RNA modifications

There is growing evidence that post-transcriptional RNA modifications are highly dynamic and can be used to improve crop production. Although more than 172 unique types of RNA modifications have been identified throughout the kingdom of life, we are yet to leverage upon the understanding to optimize RNA modifications in crops to improve productivity. The contributions of internal mRNA modifications such as N6-methyladenosine (m⁶ A) and 5-methylcytosine (m⁵ C) methylations to embryonic development, root development, leaf morphogenesis, flowering, fruit ripening and stress response are sufficiently known, but the roles of the two most abundant RNA modifications, pseudouridine (Ψ) and 2'-O-methylation (Nm), in the cell remain unclear due to insufficient advances in high-throughput technologies in plant development. Therefore, in this review, we discuss the latest methods and insights gained in mapping internal Ψ and Nm and their unique properties in plants and other organisms. In addition, we discuss the limitations that remain in high-throughput technologies for qualitative and quantitative mapping of these RNA modifications and highlight future challenges in regulating the plant epitranscriptome.

FIONA1-Mediated m6 A Modification Regulates the Floral Transition in Arabidopsis

N⁶ -methyladenosine (m⁶ A) mRNA modification represents the most widespread form of internal modifications in eukaryotic mRNAs. In the model plant Arabidopsis thaliana, those known methyltransferases mainly deposit m⁶ A at their target transcripts near the stop codon or in the 3' untranslated region. Here, it is reported that FIONA1 (FIO1), a human METTL16 ortholog, acts as a hitherto unknown m⁶ A methyltransferase that determines m⁶ A modifications at over 2000 Arabidopsis transcripts predominantly in the coding region. Mutants of FIO1 show a decrease in global m⁶ A mRNA methylation levels and an early-flowering phenotype. Nanopore direct RNA sequencing reveals that FIO1 is required for establishing appropriate levels of m⁶ A preferentially in the coding sequences of a subset of protein-coding transcripts, which is associated with changes in transcript abundance and alternative polyadenylation. It is further demonstrated that FIO1-mediated m⁶ A methylation determines the mRNA abundance of a central flowering integrator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and its upstream regulators, thus preventing premature flowering. The findings reveal that FIO1 acts as a unique m⁶ A methyltransferase that mainly modifies the coding regions of transcripts, which underlies the key developmental transition from vegetative to reproductive growth in plants.

Epigenome and Epitranscriptome: Potential Resources for Crop Improvement

Crop breeding faces the challenge of increasing food demand, especially under climatic changes. Conventional breeding has relied on genetic diversity by combining alleles to obtain desired traits. In recent years, research on epigenetics and epitranscriptomics has shown that epigenetic and epitranscriptomic diversity provides additional sources for crop breeding and harnessing epigenetic and epitranscriptomic regulation through biotechnologies has great potential for crop improvement. Here, we review epigenome and epitranscriptome variations during plant development and in response to environmental stress as well as the available sources for epiallele formation. We also discuss the possible strategies for applying epialleles and epitranscriptome engineering in crop breeding.