Polycomb-group protein SlMSI1 represses the expression of fruit-ripening genes to prolong shelf life in tomato

Epigenetic regulation in tomato fruit ripening

Fruit ripening is a crucial stage in quality development, influenced by a diverse array of internal and external factors. Among these factors, epigenetic regulation holds significant importance and has garnered substantial research attention in recent years. Here, this review aims to discuss the breakthrough in epigenetic regulation of tomato (Solanum lycopersicum) fruit ripening, including DNA methylation, N6-Methyladenosine mRNA modification, histone demethylation/deacetylation, and non-coding RNA. Through this brief review, we seek to enhance our understanding of the regulatory mechanisms governing tomato fruit ripening, while providing fresh insights for the precise modulation of these mechanisms.

Current insights into posttranscriptional regulation of fleshy fruit ripening

Fruit ripening is a complicated process that is accompanied by the formation of fruit quality. It is not only regulated at the transcriptional level via transcription factors or DNA methylation but also fine-tuned after transcription occurs. Here, we review recent advances in our understanding of key regulatory mechanisms of fleshy fruit ripening after transcription. We mainly highlight the typical mechanisms by which fruit ripening is controlled, namely, alternative splicing, mRNA N6-methyladenosine RNA modification methylation, and noncoding RNAs at the posttranscriptional level; regulation of translation efficiency and upstream open reading frame-mediated translational repression at the translational level; and histone modifications, protein phosphorylation, and protein ubiquitination at the posttranslational level. Taken together, these posttranscriptional regulatory mechanisms, along with transcriptional regulation, constitute the molecular framework of fruit ripening. We also critically discuss the potential usage of some mechanisms to improve fruit traits.

Histone H3K4 methyltransferase DcATX1 promotes ethylene induced petal senescence in carnation

Petal senescence is controlled by a complex regulatory network. Epigenetic regulation like histone modification influences chromatin state and gene expression. However, the involvement of histone methylation in regulating petal senescence remains poorly understood. Here, we found that the trimethylation of histone H3 at Lysine 4 (H3K4me3) is increased during ethylene-induced petal senescence in carnation (Dianthus caryophyllus L.). H3K4me3 levels were positively associated with the expression of transcription factor DcWRKY75, ethylene biosynthetic genes 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (DcACS1), and ACC oxidase (DcACO1), and senescence associated genes (SAGs) DcSAG12 and DcSAG29. Further, we identified that carnation ARABIDOPSIS HOMOLOG OF TRITHORAX1 (DcATX1) encodes a histone lysine methyltransferase which can methylate H3K4. Knockdown of DcATX1 delayed ethylene-induced petal senescence in carnation, which was associated with the down-regulated expression of DcWRKY75, DcACO1, and DcSAG12, whereas overexpression of DcATX1 exhibited the opposite effects. DcATX1 promoted the transcription of DcWRKY75, DcACO1, and DcSAG12 by elevating the H3K4me3 levels within their promoters. Overall, our results demonstrate that DcATX1 is a H3K4 methyltransferase that promotes the expression of DcWRKY75, DcACO1, DcSAG12 and potentially other downstream target genes by regulating H3K4me3 levels, thereby accelerating ethylene-induced petal senescence in carnation. This study further indicates that epigenetic regulation is important for plant senescence processes.

Exploitation of epigenetic variation of crop wild relatives for crop improvement and agrobiodiversity preservation

Crop wild relatives (CWRs) are recognized as the best potential source of traits for crop improvement. However, successful crop improvement using CWR relies on identifying variation in genes controlling desired traits in plant germplasms and subsequently incorporating them into cultivars. Epigenetic diversity may provide an additional layer of variation within CWR and can contribute novel epialleles for key traits for crop improvement. There is emerging evidence that epigenetic variants of functional and/or agronomic importance exist in CWR gene pools. This provides a rationale for the conservation of epigenotypes of interest, thus contributing to agrobiodiversity preservation through conservation and (epi)genetic monitoring. Concepts and techniques of classical and modern breeding should consider integrating recent progress in epigenetics, initially by identifying their association with phenotypic variations and then by assessing their heritability and stability in subsequent generations. New tools available for epigenomic analysis offer the opportunity to capture epigenetic variation and integrate it into advanced (epi)breeding programmes. Advances in -omics have provided new insights into the sources and inheritance of epigenetic variation and enabled the efficient introduction of epi-traits from CWR into crops using epigenetic molecular markers, such as epiQTLs.

Transcriptional regulation of fleshy fruit texture

Fleshy fruit texture is a critically important quality characteristic of ripe fruit. Softening is an irreversible process which operates in most fleshy fruits during ripening which, together with changes in color and taste, contributes to improvements in mouthfeel and general attractiveness. Softening results mainly from the expression of genes encoding enzymes responsible for cell wall modifications but starch degradation and high levels of flavonoids can also contribute to texture change. Some fleshy fruit undergo lignification during development and post-harvest, which negatively affects eating quality. Excessive softening can also lead to physical damage and infection, particularly during transport and storage which causes severe supply chain losses. Many transcription factors (TFs) that regulate fruit texture by controlling the expression of genes involved in cell wall and starch metabolism have been characterized. Some TFs directly regulate cell wall targets, while others act as part of a broader regulatory program governing several aspects of the ripening process. In this review, we focus on advances in our understanding of the transcriptional regulatory mechanisms governing fruit textural change during fruit development, ripening and post-harvest. Potential targets for breeding and future research directions for the control of texture and quality improvement are discussed.

Molecular and biochemical basis of softening in tomato

We review the latest information related to the control of fruit softening in tomato and where relevant compare the events with texture changes in other fleshy fruits. Development of an acceptable texture is essential for consumer acceptance, but also determines the postharvest life of fruits. The complex modern supply chain demands effective control of shelf life in tomato without compromising colour and flavour.The control of softening and ripening in tomato (Solanum lycopersicum) are discussed with respect to hormonal cues, epigenetic regulation and transcriptional modulation of cell wall structure-related genes. In the last section we focus on the biochemical changes closely linked with softening in tomato including key aspects of cell wall disassembly. Some important elements of the softening process have been identified, but our understanding of the mechanistic basis of the process in tomato and other fruits remains incomplete, especially the precise relationship between changes in cell wall structure and alterations in fruit texture.

Multi-omics data integration provides insights into the post-harvest biology of a long shelf-life tomato landrace

In this study we investigated the transcriptome and epigenome dynamics of the tomato fruit during post-harvest in a landrace belonging to a group of tomatoes (Solanum lycopersicum L.) collectively known as "Piennolo del Vesuvio", all characterized by a long shelf-life. Expression of protein-coding genes and microRNAs as well as DNA methylation patterns and histone modifications were analysed in distinct post-harvest phases. Multi-omics data integration contributed to the elucidation of the molecular mechanisms underlying processes leading to long shelf-life. We unveiled global changes in transcriptome and epigenome. DNA methylation increased and the repressive histone mark H3K27me3 was lost as the fruit progressed from red ripe to 150 days post-harvest. Thousands of genes were differentially expressed, about half of which were potentially epi-regulated as they were engaged in at least one epi-mark change in addition to being microRNA targets in ~5% of cases. Down-regulation of the ripening regulator MADS-RIN and of genes involved in ethylene response and cell wall degradation was consistent with the delayed fruit softening. Large-scale epigenome reprogramming that occurred in the fruit during post-harvest likely contributed to delayed fruit senescence.

Genome-Wide Identification and Analysis of the Polycomb Group Family in Medicago truncatula

Polycomb group (PcG) proteins, which are important epigenetic regulators, play essential roles in the regulatory networks involved in plant growth, development, and environmental stress responses. Currently, as far as we know, no comprehensive and systematic study has been carried out on the PcG family in Medicago truncatula. In the present study, we identified 64 PcG genes with distinct gene structures from the M. truncatula genome. All of the PcG genes were distributed unevenly over eight chromosomes, of which 26 genes underwent gene duplication. The prediction of protein interaction network indicated that 34 M. truncatula PcG proteins exhibited protein-protein interactions, and MtMSI1;4 and MtVRN2 had the largest number of protein-protein interactions. Based on phylogenetic analysis, we divided 375 PcG proteins from 27 species into three groups and nine subgroups. Group I and Group III were composed of five components from the PRC1 complex, and Group II was composed of four components from the PRC2 complex. Additionally, we found that seven PcG proteins in M. truncatula were closely related to the corresponding proteins of Cicer arietinum. Syntenic analysis revealed that PcG proteins had evolved more conservatively in dicots than in monocots. M. truncatula had the most collinearity relationships with Glycine max (36 genes), while collinearity with three monocots was rare (eight genes). The analysis of various types of expression data suggested that PcG genes were involved in the regulation and response process of M. truncatula in multiple developmental stages, in different tissues, and for various environmental stimuli. Meanwhile, many differentially expressed genes (DEGs) were identified in the RNA-seq data, which had potential research value in further studies on gene function verification. These findings provide novel and detailed information on the M. truncatula PcG family, and in the future it would be helpful to carry out related research on the PcG family in other legumes.

Perspectives for epigenetic editing in crops

Site-specific nucleases (SSNs) have drawn much attention in plant biotechnology due to their ability to drive precision mutagenesis, gene targeting or allele replacement. However, when devoid of its nuclease activity, the underlying DNA-binding activity of SSNs can be used to bring other protein functional domains close to specific genomic sites, thus expanding further the range of applications of the technology. In particular, the addition of functional domains encoding epigenetic effectors and chromatin modifiers to the CRISPR/Cas ribonucleoprotein complex opens the possibility to introduce targeted epigenomic modifications in plants in an easily programmable manner. Here we examine some of the most important agronomic traits known to be controlled epigenetically and review the best studied epigenetic catalytic effectors in plants, such as DNA methylases/demethylases or histone acetylases/deacetylases and their associated marks. We also review the most efficient strategies developed to date to functionalize Cas proteins with both catalytic and non-catalytic epigenetic effectors, and the ability of these domains to influence the expression of endogenous genes in a regulatable manner. Based on these new technical developments, we discuss the possibilities offered by epigenetic editing tools in plant biotechnology and their implications in crop breeding.

Overexpression of SlMYB75 enhances resistance to Botrytis cinerea and prolongs fruit storage life in tomato

SlMYB75 increased the accumulation of JA and improved the scavenging of excess H₂O₂ to resist B. cinerea. Overexpression of SlMYB75 greatly prolongs tomato fruit storage life. Botrytis cinerea (B. cinerea) is a major threat to the production and storage life of tomato (Solanum lycopersicum) fruit around the world. SlMYB75 is an R2R3MYB transcription factor associated with the biosynthesis of anthocyanidin, but little is known about its function in the resistance of tomato to B. cinerea. In this study, we found that the overexpression of SlMYB75 regulated the accumulation of jasmonic acid (JA) and promoted the JA-mediated signaling pathway to resist B. cinerea infection. Moreover, the activities of peroxidase and superoxide dismutase, which were activated to scavenge hydrogen peroxide produced as a result of the B. cinerea infection, were enhanced in the transgenic tomato plants. Scanning electron microscopy images showed that the wax on the fruit skin surface was significantly decreased in the transgenic tomatoes compared with the wild type. However, SlMYB75 prolonged fruit storage life by both enhancing resistance to B. cinerea and directly downregulating the fruit shelf life-related gene SlFSR. Collectively, this study provides a good candidate gene for breeding high-quality tomatoes with a long storage life and high disease resistance.

Structural and Functional Diversification of SEPALLATA Genes TM5 and RIN in Tomato Species (Section Lycopersicon)

New TOMATO MADS 5 (TM5) homologous genes were identified in evolutionarily recent, red-fruited and more ancient, wild green-fruited tomato species. It was shown that the identified TM5 homologs belong to the SEPALLATA3 clade; thus, the SEP subfamily diversification was characterized. For the first time, the TM5 and RIN co-expression pattern was determined in flowers, immature green fruits, and ripe fruits of Solanum lycopersicum and in five wild tomato species. It was shown that, regardless of the species, the level of TM5 transcription in flowers was higher than that of RIN, whereas in fruits it was lower than the level of RIN transcription. The data obtained suggest that TM5, together with other transcription factors RIN and SlCMB1, is involved in the regulation of fruit development and ripening.

Like Heterochromatin Protein 1b represses fruit ripening via regulating the H3K27me3 levels in ripening-related genes in tomato

Polycomb group (PcG) proteins play vital roles in plant development via epigenetically repressing the transcription of target genes. However, to date, their function in fruit ripening is largely unknown. Combining reverse genetic approaches, physiological methods, yeast two-hybrid, co-immunoprecipitation, and chromatin immunoprecipitation assays, we show that Like Heterochromatin Protein 1b (SlLHP1b), a tomato Polycomb Repressive Complex 1 (PRC1)-like protein with a ripening-related expression pattern, represses fruit ripening via colocalization with epigenetic mark H3K27me3. RNA interference (RNAi)-mediated downregulation of SlLHP1b advanced ripening initiation, climacteric ethylene production, and fruit softening, whereas SlLHP1b overexpression delayed these events. Ripening-related genes were significantly upregulated in SlLHP1b RNAi fruits and downregulated in overexpressing fruits compared with wild-type. Furthermore, SlLHP1b protein interacts with ripening regulator MSI1, a subunit of the PRC2 complex. Moreover, SlLHP1b also binds the epigenetic histone mark H3K27me3 in vivo and chromatin immunoprecipitation-quantitative PCR results showed binding occurs preferentially to regions of ripening-associated chromatin marked by histone H3K27me3. Furthermore, the H3K27me3 levels in chromatin of ripening-related genes is negatively correlated with accumulation of their transcripts in SlLHP1b down or upregulated fruits during ripening. Our findings reveal a novel regulatory function of SlLHP1b in fruit and provide new insights into the PcG-mediated epigenetic regulation of climacteric fruit ripening.

Comparative N-glycoproteome analysis provides novel insights into the regulation mechanism in tomato (solanum lycopersicum L.) During fruit ripening process

Protein N-glycosylation plays key roles in protein folding, stability, solubility, biogenesis, and enzyme activity. Tomato (Solanum lycopersicum L.) is an important vegetable crop with abundant nutritional value, and the formation of tomato fruit qualities primarily occurs in the fruit ripening process. However, a large number of N-glycosylation-mediated mechanisms in regulating tomato fruit ripening have not been elucidated to date. In this study, western blot assays showed that the extents of mature N-glycoproteins were differentially expressed in mature green fruits (fruit start ripening) and ripe fruits (fruit stop ripening). Next, through performing a comparative N-glycoproteome analysis strategy, a total of 553 N-glycosites from 363 N-glycoproteins were identified in mature green fruits compared with ripe fruits. Among them, 252 N-glycosites from 191 N-glycoproteins were differentially expressed in mature green fruits compared with ripe fruits. The differentially expressed N-glycoproteins were mainly located in the chloroplast (30 %) and cytoplasm (16 %). Gene Ontology (GO) analysis showed that these N-glycoproteins were involved in various biological processes, cellular components and molecular functions. These N-glycoproteins participate in biological processes, such as metabolic processes, cellular processes and single-organism processes. These N-glycoproteins are also cellular components in biological process cells, membranes and organelles and have different molecular functions, such as catalytic activity and binding. Notably, these N-glycoproteins were enriched in starch and sucrose metabolism and galactose metabolism by KEGG pathway analysis. This community resource regarding N-glycoproteins is the first large-scale N-glycoproteome during plant fruit ripening. This study will contribute to understanding the function of N-glycosylation in regulating plant fruit ripening.

PcG Proteins MSI1 and BMI1 Function Upstream of miR156 to Regulate Aerial Tuber Formation in Potato

Polycomb Repressive Complexes (PRC1 and PRC2) regulate developmental transitions in plants. AtBMI1, a PRC1 member, represses micro RNA156 (miR156) to trigger the onset of adult phase in Arabidopsis (Arabidopsis thaliana). miR156 overexpression (OE) reduces below-ground tuber yield, but stimulates aerial tubers in potato (Solanum tuberosum ssp andigena) under short-day (SD) photoperiodic conditions. Whether PRC members could govern tuber development through photoperiod-mediated regulation of miR156 is unknown. Here, we investigated the role of two PRC proteins, StMSI1 (PRC2 member) and StBMI1-1, in potato development. In wild-type andigena plants, StMSI1 and miR156 levels increased in stolon, whereas StBMI1-1 decreased under SD conditions. StMSI1-OE and StBMI1-1-antisense (AS) lines produced pleiotropic effects, including altered leaf architecture/compounding and reduced below-ground tuber yield. Notably, these lines showed enhanced miR156 accumulation accompanied by aerial stolons and tubers from axillary nodes, similar to miR156-OE lines. Further, grafting of StMSI1-OE or StBMI1-1-AS on wild-type stock resulted in reduced root biomass and showed increased accumulation of miR156a/b and -c precursors in the roots of wild-type stocks. RNA-sequencing of axillary nodes from StMSI1-OE and StBMI1-1-AS lines revealed downregulation of auxin and brassinosteroid genes, and upregulation of cytokinin transport/signaling genes, from 1,023 differentially expressed genes shared between the two lines. Moreover, we observed downregulation of genes encoding H2A-ubiquitin ligase and StBMI1-1/3, and upregulation of Trithorax group H3K4-methyl-transferases in StMSI1-OE Chromatin immunoprecipitation-quantitative PCR confirmed H3K27me3-mediated suppression of StBMI1-1/3, and H3K4me3-mediated activation of miR156 in StMSI1-OE plants. In summary, we show that cross talk between histone modifiers regulates miR156 and alters hormonal response during aerial tuber formation in potato under SD conditions.

A critical evaluation of the role of ethylene and MADS transcription factors in the network controlling fleshy fruit ripening

Contents Summary 1724 I. Introduction 1725 II. Ripening genes 1725 III. The importance of ethylene in controlling ripening 1727 IV. The importance of MADS-RIN in controlling ripening 1729 V. Interactions between components of the ripening regulatory network 1734 VI. Conclusions 1736 Acknowledgements 1738 Author contributions 1738 References 1738 SUMMARY: Understanding the regulation of fleshy fruit ripening is biologically important and provides insights and opportunities for controlling fruit quality, enhancing nutritional value for animals and humans, and improving storage and waste reduction. The ripening regulatory network involves master and downstream transcription factors (TFs) and hormones. Tomato is a model for ripening regulation, which requires ethylene and master TFs including NAC-NOR and the MADS-box protein MADS-RIN. Recent functional characterization showed that the classical RIN-MC gene fusion, previously believed to be a loss-of-function mutation, is an active TF with repressor activity. This, and other evidence, has highlighted the possibility that MADS-RIN itself is not important for ripening initiation but is required for full ripening. In this review, we discuss the diversity of components in the control network, their targets, and how they interact to control initiation and progression of ripening. Both hormones and individual TFs affect the status and activity of other network participants, which changes overall network signaling and ripening outcomes. MADS-RIN, NAC-NOR and ethylene play critical roles but there are still unanswered questions about these and other TFs. Further attention should be paid to relationships between ethylene, MADS-RIN and NACs in ripening control.

The mode of action of remorin1 in regulating fruit ripening at transcriptional and post-transcriptional levels

Remorins are plant-specific and plasma membrane-associated proteins that display a variety of functions in plant growth, development, biotic and abiotic stresses, and signal transduction. However, little information is available for understanding their role in fruit ripening. Here, remorin 1 (SlREM1) is cloned from tomato and its localization is examined by co-localization analysis and immunoblotting. Functions of SlREM1 in fruit ripening are characterized based on gene expression, co-immunoprecipitation coupled with mass spectroscopy and split luciferase complementation imaging assays in SlREM1 overexpression and RNA interference (RNAi) lines. The results indicate that SlREM1 is localized at the plasma membrane. Overexpression of SlREM1 in tomato stimulates fruit ripening with an increase in ethylene production and lycopene accumulation as compared to the wild-type. Consistently, these genes involved in ethylene and lycopene biosynthesis and ripening regulators also are upregulated in SlREM1 overexpression lines. SlREM1 can interact with ethylene biosynthesis proteins SAM1, ACO1 and ACS2 and is degraded by ubiquitin-mediated proteolysis. Our findings reveal that SlREM1 serves as a positive regulator of fruit ripening and provide novel cues for understanding of the molecular regulation network of fruit ripening.

Evaluation of multiple approaches to identify genome-wide polymorphisms in closely related genotypes of sweet cherry (Prunus avium L.)

Identification of genetic polymorphisms and subsequent development of molecular markers is important for marker assisted breeding of superior cultivars of economically important species. Sweet cherry (Prunus avium L.) is an economically important non-climacteric tree fruit crop in the Rosaceae family and has undergone a genetic bottleneck due to breeding, resulting in limited genetic diversity in the germplasm that is utilized for breeding new cultivars. Therefore, it is critical to recognize the best platforms for identifying genome-wide polymorphisms that can help identify, and consequently preserve, the diversity in a genetically constrained species. For the identification of polymorphisms in five closely related genotypes of sweet cherry, a gel-based approach (TRAP), reduced representation sequencing (TRAPseq), a 6k cherry SNParray, and whole genome sequencing (WGS) approaches were evaluated in the identification of genome-wide polymorphisms in sweet cherry cultivars. All platforms facilitated detection of polymorphisms among the genotypes with variable efficiency. In assessing multiple SNP detection platforms, this study has demonstrated that a combination of appropriate approaches is necessary for efficient polymorphism identification, especially between closely related cultivars of a species. The information generated in this study provides a valuable resource for future genetic and genomic studies in sweet cherry, and the insights gained from the evaluation of multiple approaches can be utilized for other closely related species with limited genetic diversity in the breeding germplasm.

Rosaceae Fruit Development, Ripening and Post-harvest: An Epigenetic Perspective

Rosaceae is a family with an extraordinary spectrum of fruit types, including fleshy peach, apple, and strawberry that provide unique contributions to a healthy diet for consumers, and represent an excellent model for studying fruit patterning and development. In recent years, many efforts have been made to unravel regulatory mechanism underlying the hormonal, transcriptomic, proteomic and metabolomic changes occurring during Rosaceae fruit development. More recently, several studies on fleshy (tomato) and dry (Arabidopsis) fruit model have contributed to a better understanding of epigenetic mechanisms underlying important heritable crop traits, such as ripening and stress response. In this context and summing up the results obtained so far, this review aims to collect the available information on epigenetic mechanisms that may provide an additional level in gene transcription regulation, thus influencing and driving the entire Rosaceae fruit developmental process. The whole body of information suggests that Rosaceae fruit could become also a model for studying the epigenetic basis of economically important phenotypes, allowing for their more efficient exploitation in plant breeding.