Characterization of the Tomato (Solanum lycopersicum) Pectin Methylesterases: Evolution, Activity of Isoforms and Expression During Fruit Ripening

Fruit ripening under heat stress: The intriguing role of ethylene-mediated signaling

Crop production is significantly influenced by climate, and even minor climate changes can have a substantial impact on crop yields. Rising temperature due to climate change can lead to heat stress (HS) in plants, which not only hinders plant growth and development but also result in significant losses in crop yields. To cope with the different stresses including HS, plants have evolved a variety of adaptive mechanisms. In response to these stresses, phytohormones play a crucial role by generating endogenous signals that regulate the plant's defensive response. Among these, Ethylene (ET), a key phytohormone, stands out as a major regulator of stress responses in plants and regulates many plant traits, which are critical for crop productivity and nutritional quality. ET is also known as a ripening hormone for decades in climacteric fruit and many studies are available deciphering the function of different ET biosynthesis and signaling components in the ripening process. Recent studies suggest that HS significantly affects fruit quality traits and perturbs fruit ripening by altering the regulation of many ethylene biosynthesis and signaling genes resulting in substantial loss of fruit yield, quality, and postharvest stability. Despite the significant progress in this field in recent years the interplay between ET, ripening, and HS is elusive. In this review, we summarized the recent advances and current understanding of ET in regulating the ripening process under HS and explored their crosstalk at physiological and molecular levels to shed light on intricate relationships.

Sequence, structure and functionality of pectin methylesterases and their use in sustainable carbohydrate bioproducts: A review

Pectin methylesterases (PMEs) are enzymes that play a critical role in modifying pectins, a class of complex polysaccharides in plant cell walls. These enzymes catalyze the removal of methyl ester groups from pectins, resulting in a change in the degree of esterification and consequently, the physicochemical properties of the polymers. PMEs are found in various plant tissues and organs, and their activity is tightly regulated in response to developmental and environmental factors. In addition to the biochemical modification of pectins, PMEs have been implicated in various biological processes, including fruit ripening, defense against pathogens, and cell wall remodelling. This review presents updated information on PMEs, including their sources, sequences and structural diversity, biochemical properties and function in plant development. The article also explores the mechanisms of PME action and the factors influencing enzyme activity. In addition, the review highlights the potential applications of PMEs in various industrial sectors related to biomass exploitation, food, and textile industries, with a focus on development of bioproducts based on eco-friendly and efficient industrial processes.

Behavior of enzymes under high pressure in food processing: mechanisms, applications, and developments

High pressure processing (HPP) offers the benefits of safety, uniformity, energy-efficient, and low waste, which is widely applied for microbial inactivation and shelf-life extension for foods. Over the past forty years, HPP has been extensively researched in the food industry, enabling the inactivation or activation of different enzymes in future food by altering their molecular structure and active site conformation. Such activation or inactivation of enzymes effectively hinders the spoilage of food and the production of beneficial substances, which is crucial for improving food quality. This paper reviews the mechanism in which high pressure affects the stability and activity of enzymes, concludes the roles of key enzymes in the future food processed using high pressure technologies. Moreover, we discuss the application of modified enzymes based on high pressure, providing insights into the future direction of enzyme evolution under complex food processing conditions (e.g. high temperature, high pressure, high shear, and multiple elements). Finally, we conclude with prospects of high pressure technology and research directions in the future. Although HPP has shown positive effects in improving the future food quality, there is still a pressing need to develop new and effective combined processing methods, upgrade processing modes, and promote sustainable lifestyles.

Silencing of a Pectin Acetylesterase (PAE) Gene Highly Expressed in Tobacco Pistils Negatively Affects Pollen Tube Growth

Successful plant reproduction and fruit formation depend on adequate pollen and pistil development, and pollen-pistil interactions. In Nicotiana tabacum, pollen tubes grow through the intercellular spaces of pistil-specialized tissues, stigmatic secretory zone, and stylar transmitting tissue (STT). These intercellular spaces are supposed to be formed by the modulation of cell wall pectin esterification. Previously we have identified a gene preferentially expressed in pistils encoding a putative pectin acetylesterase (PAE), named NtPAE1. Here, we characterized the NtPAE1 gene and performed genome-wide and phylogenetic analyses of PAEs. We identified 30 PAE sequences in the N. tabacum genome, distributed in four clades. The expression of NtPAE1 was assessed by RT-qPCR and in situ hybridization. We confirmed NtPAE1 preferential expression in stigmas/styles and ovaries and demonstrated its high expression in the STT. Structural predictions and comparisons between NtPAE1 and functional enzymes validated its identity as a PAE. Transgenic plants were produced, overexpressing and silencing the NtPAE1 gene. Overexpressed plants displayed smaller flowers while silencing plants exhibited collapsed pollen grains, which hardly germinate. NtPAE1 silencing plants do not produce fruits, due to impaired pollen tube growth in their STTs. Thus, NtPAE1 is an essential enzyme regulating pectin modifications in flowers and, ultimately, in plant reproduction.

Transcriptomic analysis of effects of 1-methylcyclopropene (1-MCP) and ethylene treatment on kiwifruit (Actinidia chinensis) ripening

Ethylene (ET) is a gaseous phytohormone with a crucial role in the ripening of many fruits, including kiwifruit (Actinidia spp.). Meanwhile, treatment with 1-methylcyclopropene (1-MCP), an artificial ET inhibitor delays the ripening of kiwifruit. The objective of this study was to determine the effect of ET and 1-MCP application during time-course storage of kiwifruit. In addition, we aimed to elucidate the molecular details underlying ET-mediated ripening process in kiwifruit. For this purpose, we conducted a time-course transcriptomic analysis to determine target genes of the ET-mediated maturation process in kiwifruit during storage. Thousands of genes were identified to be dynamically changed during storage and clustered into 20 groups based on the similarity of their expression patterns. Gene ontology analysis using the list of differentially expressed genes (DEGs) in 1-MCP-treated kiwifruit revealed that the identified DEGs were significantly enriched in the processes of photosynthesis metabolism and cell wall composition throughout the ripening process. Meanwhile, ET treatment rapidly triggered secondary metabolisms related to the ripening process, phenylpropanoid (e.g. lignin) metabolism, and the biosynthesis of amino acids (e.g. Phe, Cys) in kiwifruit. It was demonstrated that ET biosynthesis and signaling genes were oppositely affected by ET and 1-MCP treatment during ripening. Furthermore, we identified a ET transcription factor, AcEIL (Acc32482) which is oppositely responsive by ET and 1-MCP treatment during early ripening, potentially one of key signaling factor of ET- or 1-MCP-mediated physiological changes. Therefore, this transcriptomic study unveiled the molecular targets of ET and its antagonist, 1-MCP, in kiwifruit during ripening. Our results provide a useful foundation for understanding the molecular details underlying the ripening process in kiwifruit.

Genome-wide characterization of the NLR gene family in tomato (Solanum lycopersicum) and their relatedness to disease resistance

Nucleotide-binding leucine-rich-repeat receptors (NLR), the largest group of genes associated with plant disease resistance (R), have attracted attention due to their crucial role in protecting plants from pathogens. Genome-wide studies of NLRs have revealed conserved domains in the annotated tomato genome. The 321 NLR genes identified in the tomato genome have been randomly mapped to 12 chromosomes. Phylogenetic analysis and classification of NLRs have revealed that 211 genes share full-length domains categorized into three major clades (CNL, TNL, and RNL); the remaining 110 NLRs share partial domains and are classified in CN, TN, and N according to their motifs and gene structures. The cis-regulatory elements of NLRs exhibit the maximum number of these elements and are involved in response to biotic and abiotic stresses, pathogen recognition, and resistance. Analysis of the phylogenetic relationship between tomato NLRs and orthologs in other species has shown conservation among Solanaceae members and variation with A. thaliana. Synteny and Ka/Ks analyses of Solanum lycopersicum and Solanum tuberosum orthologs have underscored the importance of NLR conservation and diversification from ancestral species millions of years ago. RNA-seq data and qPCR analysis of early and late blight diseases in tomatoes revealed consistent NLR expression patterns, including upregulation in infected compared to control plants (with some exceptions), suggesting the role of NLRs as key regulators in early blight resistance. Moreover, the expression levels of NLRs associated with late blight resistance (Solyc04g007060 [NRC4] and Solyc10g008240 [RIB12]) suggested that they regulate S. lycopersicum resistance to P. infestans. These findings provide important fundamental knowledge for understanding NLR evolution and diversity and will empower the broader characterization of disease resistance genes for pyramiding through speed cloning to develop disease-tolerant varieties.

Biochemical Characterization and Molecular Insights into Substrate Recognition of Pectin Methylesterase from Phytophthora Infestans

Pectin methylesterases (PMEs) are a class of carbohydrate-active enzymes that act on the O6-methyl ester groups of the homogalacturonan component of pectins, resulting in de-esterification of the substrate polymers and formation of pectate and methanol. PMEs occur in higher plants and microorganisms, including fungi, oomycetes, bacteria, and archaea. Microbial PMEs play a crucial role in pathogens’ invasion of plant tissues. Here, we have determined the structural and functional properties of Pi-PME, a PME from the oomycete plant pathogen Phytophthora infestans. This enzyme exhibits maximum activity at alkaline pH (8.5) and is active over a wide temperature range (25–50 °C). In silico determination of the structure of Pi-PME reveals that the protein consists essentially of three parallel β-sheets interconnected by loops that adopt an overall β-helix organization. The loop regions in the vicinity of the active site are extended compared to plant and fungal PMEs, but they are shorter than the corresponding bacterial and insect regions. Molecular dynamic simulations revealed that Pi-PME interacts most strongly with partially de-methylated homogalacturonans, suggesting that it preferentially uses this type of substrates. The results are compared and discussed with other known PMEs from different organisms, highlighting the specific features of Pi-PME.

The calcium-mediated homogalacturonan pectin complexation in cell walls contributes the firmness increase in loquat fruit during postharvest storage

INTRODUCTION

Postharvest textural changes in fruit are mainly divided into softening and lignification. Loquat fruit could have severe lignification with increased firmness during postharvest storage. Pectin is mainly associated with the postharvest softening of fruit, but some studies also found that pectin could be involved in strengthening the mechanical properties of the plant.

OBJECTIVES

This study focused on characterizing the dynamics of pectin and its complexation in the cell wall of lignified loquat fruit during postharvest storage, and how these changes could influence fruit firmness.

METHODS

The homogalacturonan (HG) pectin in the cell wall of loquat fruit was identified using monoclonal antibodies. An oligogalacturonide (OG) probe was used to label the egg-box structure formed by Ca²⁺ cross-linking with low-methylesterified HG. An exogenous injection was used to verify the role of egg-box structures in the firmness increase in loquat fruit.

RESULTS

The JIM5 antibody revealed that low-methylesterified HG accumulated in the tricellular junctions and middle lamella of loquat fruit that had severe lignification symptoms. The pectin methylesterase (PME) activity increased during the early stages of storage at 0°C, and the calcium-pectate content and flesh firmness constantly increased during storage. The OG probe demonstrated the accumulation of egg-box structures at the cellular level. The exogenous injection of PME and Ca²⁺ into the loquat flesh led to an increase in firmness with more low-methylesterified HG and egg-box structure signals.

CONCLUSION

PME-mediated demethylesterification generated large amounts of low-methylesterified HG in the cell wall. This low-methylesterified HG further cross-linked with Ca²⁺ to form egg-box structures. The pectin-involved complexations then contributed to the increased firmness in loquat fruit. Overall, besides being involved in fruit softening, pectin could also be involved in strengthening the mechanical properties of postharvest fruit. This study provides new ideas for obtaining a better texture of postharvest loquat fruits based on pectin regulation.

Characterization of pectin methylesterase gene family and its possible role in juice sac granulation in navel orange (Citrus sinensis Osbeck)

BACKGROUND

Citrus is one of the most important fresh fruit crops worldwide. Juice sac granulation is a physiological disorder, which leads to a reduction in soluble solid concentration, total sugar, and titratable acidity of citrus fruits. Pectin methylesterase (PME) catalyzes the de-methylesterification of homogalacturonans and plays crucial roles in cell wall modification during plant development and fruit ripening. Although PME family has been well investigated in various model plants, little is known regarding the evolutionary property and biological function of PME family genes in citrus.

RESULTS

In this study, 53 non-redundant PME genes were identified from Citrus sinensis genome, and these PME genes were divided into four clades based on the phylogenetic relationship. Subsequently, bioinformatics analyses of gene structure, conserved domain, chromosome localization, gene duplication, and collinearity were performed on CsPME genes, providing important clues for further research on the functions of CsPME genes. The expression profiles of CsPME genes in response to juice sac granulation and low-temperature stress revealed that CsPME genes were involved in the low temperature-induced juice sac granulation in navel orange fruits. Subcellular localization analysis suggested that CsPME genes were localized on the apoplast, endoplasmic reticulum, plasma membrane, and vacuole membrane. Moreover, yeast one-hybrid screening and dual luciferase activity assay revealed that the transcription factor CsRVE1 directly bound to the promoter of CsPME3 and activated its activity.

CONCLUSION

In summary, this study conducts a comprehensive analysis of the PME gene family in citrus, and provides a novel insight into the biological functions and regulation patterns of CsPME genes during juice sac granulation of citrus.

Modifying Anthocyanins Biosynthesis in Tomato Hairy Roots: A Test Bed for Plant Resistance to Ionizing Radiation and Antioxidant Properties in Space

Gene expression manipulation of specific metabolic pathways can be used to obtain bioaccumulation of valuable molecules and desired quality traits in plants. A single-gene approach to impact different traits would be greatly desirable in agrospace applications, where several aspects of plant physiology can be affected, influencing growth. In this work, MicroTom hairy root cultures expressing a MYB-like transcription factor that regulates the biosynthesis of anthocyanins in Petunia hybrida (PhAN4), were considered as a testbed for bio-fortified tomato whole plants aimed at agrospace applications. Ectopic expression of PhAN4 promoted biosynthesis of anthocyanins, allowing to profile 5 major derivatives of delphinidin and petunidin together with pelargonidin and malvidin-based anthocyanins, unusual in tomato. Consistent with PhAN4 features, transcriptomic profiling indicated upregulation of genes correlated to anthocyanin biosynthesis. Interestingly, a transcriptome reprogramming oriented to positive regulation of cell response to biotic, abiotic, and redox stimuli was evidenced. PhAN4 hairy root cultures showed the significant capability to counteract reactive oxygen species (ROS) accumulation and protein misfolding upon high-dose gamma irradiation, which is among the most potent pro-oxidant stress that can be encountered in space. These results may have significance in the engineering of whole tomato plants that can benefit space agriculture.

The metabolic changes that effect fruit quality during tomato fruit ripening

As the most valuable organ of tomato plants, fruit has attracted considerable attention which most focus on its quality formation during the ripening process. A considerable amount of research has reported that fruit quality is affected by metabolic shifts which are under the coordinated regulation of both structural genes and transcriptional regulators. In recent years, with the development of the next generation sequencing, molecular and genetic analysis methods, lots of genes which are involved in the chlorophyll, carotenoid, cell wall, central and secondary metabolism have been identified and confirmed to regulate pigment contents, fruit softening and other aspects of fruit flavor quality. Here, both research concerning the dissection of fruit quality related metabolic changes, the transcriptional and post-translational regulation of these metabolic pathways are reviewed. Furthermore, a weighted gene correlation network analysis of representative genes of fruit quality has been carried out and the potential of the combined application of the gene correlation network analysis, fine-mapping strategies and next generation sequencing to identify novel candidate genes determinants of fruit quality is discussed.

Genome-wide identification of PME genes, evolution and expression analyses in soybean (Glycine max L.)

BACKGROUND

Pectin methylesterase (PME) is one of pectin-modifying enzyme that affects the pectin homeostasis in cell wall and regulates plant growth and diverse biological processes. The PME genes have been well explored and characterized in different plants. Nevertheless, systematic research on the soybean (Glycine max L.) PME genes remain lacking.

RESULTS

We identified 127 Glycine max PME genes (GmPME) from the soybean Wm82.a2.v1 genome, which unevenly distributed on 20 soybean chromosomes. Phylogenetic analysis classified the GmPME genes into four clades (Group I, Group II, Group III and Group IV). GmPME gene members in the same clades displayed similar gene structures and motif patterns. The gene family expansion analysis demonstrated that segmental duplication was the major driving force to acquire novel GmPME genes compared to the tandem duplication events. Further synteny and evolution analyses showed that the GmPME gene family experienced strong purifying selective pressures during evolution. The cis-element analyses together with the expression patterns of the GmPME genes in various tissues suggested that the GmPME genes broadly participate in distinct biological processes and regulate soybean developments. Importantly, based on the transcriptome data and quantitative RT-PCR validations, we examined the potential roles of the GmPME genes in regulating soybean flower bud development and seed germination.

CONCLUSION

In conclusion, we provided a comprehensive characterization of the PME genes in soybean, and our work laid a foundation for the functional study of GmPME genes in the future.

SlBES1 promotes tomato fruit softening through transcriptional inhibition of PMEU1

Fruit softening indicated by firmness determines the texture, transportability, and shelf life of tomato products. However, the regulatory mechanism underlying firmness formation in tomato fruit is poorly understood. Here, we report the regulatory role of SlBES1, an essential component of brassinosteroid hormone signaling, in tomato fruit softening. We found that SlBES1 promotes fruit softening during tomato fruit ripening and postharvest storage. RNA-seq analysis suggested that PMEU1, which encodes a pectin methylesterase, might participate in SlBES1-mediated softening. Biochemical and immunofluorescence assays indicated that SlBES1 inhibited PMEU1-related pectin de-methylesterification. Further molecular and genetic evidence verified that SlBES1 directly binds to the E-box of PMEU1 to repress its expression, leading to fruits softening. Loss-of-function SlBES1 mutant generated by CRISPR-Cas9 showed firmer fruits and longer shelf life during postharvest storage without other quality alteration. Collectively, our results indicated the potential of manipulating SlBES1 to regulate firmness without negative consequence on visual and nutrition quality.

•

SlBES1 promotes tomato fruit softening without affecting nutritional quality

•

SlBES1 inhibits PMEU1-related fruit pectin de-methylesterification

•

SlBES1 represses PMEU1 expression through directly binding to the E-box

•

Knockout of SlBES1 by CRISPR-Cas9 enhances fruit firmness and extends shelf life