The fruit cuticles of wild tomato species exhibit architectural and chemical diversity, providing a new model for studying the evolution of cuticle function

Enzymatic treatment to improve permeability and quality of cherry tomatoes for production of dried products

BACKGROUND

Cherry tomatoes are nutritious and favored by consumers. Processing them into dried cherry tomatoes can prolong their storage life and improve their flavor. The pretreatment of tomato pericarp is crucial for the subsequent processing. However, the traditional physical and chemical treatments of tomato pericarp generally cause nutrient loss and environmental pollution.

RESULTS

In this study, a novel enzymatic method for cherry tomatoes was performed using mixed enzymes containing cutinase, cellulase and pectinase. Results showed that the pericarp permeability of cherry tomatoes was effectively improved due to enzymatic treatment. Changes in the microscopic structure and composition of the cuticle were revealed. After treatment with different concentrations of enzymes, cherry tomatoes exhibited higher pericarp permeability and sensory quality to varying degrees. The lycopene content and total polyphenol content significantly increased 2.4- and 1.45-fold, respectively. In addition, the satisfactory effect of the six-time reuse of enzymes on cherry tomatoes could still reach the same level as the initial effect, which effectively reduced the cost of production.

CONCLUSIONS

This study revealed for the first time that a mixed enzymatic treatment consisting of cutinase, pectinase and cellulase could effectively degrade the cuticle, enhance the pericarp permeability and improve the quality of cherry tomatoes, with the advantages of being mildly controllable and environmentally friendly, providing a new strategy for the processing of dried cherry tomatoes. © 2023 Society of Chemical Industry.

NAC transcription factor SlNOR-like1 plays a dual regulatory role in tomato fruit cuticle formation

The plant cuticle is an important protective barrier on the plant surface, constructed mainly by polymerized cutin matrix and a complex wax mixture. Although the pathway of plant cuticle biosynthesis has been clarified, knowledge of the transcriptional regulation network underlying fruit cuticle formation remains limited. In the present work, we discovered that tomato fruits of the NAC transcription factor SlNOR-like1 knockout mutants (nor-like1) produced by CRISPR/Cas9 [clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9] displayed reduced cutin deposition and cuticle thickness, with a microcracking phenotype, while wax accumulation was promoted. Further research revealed that SlNOR-like1 promotes cutin deposition by binding to the promoters of glycerol-3-phosphate acyltransferase6 (SlGPAT6; a key gene for cutin monomer formation) and CUTIN DEFICIENT2 (SlCD2; a positive regulator of cutin production) to activate their expression. Meanwhile, SlNOR-like1 inhibits wax accumulation, acting as a transcriptional repressor by targeting wax biosynthesis, and transport-related genes 3-ketoacyl-CoA synthase1 (SlKCS1), ECERIFERUM 1-2 (SlCER1-2), SlWAX2, and glycosylphosphatidylinositol-anchored lipid transfer protein 1-like (SlLTPG1-like). In conclusion, SlNOR-like1 executes a dual regulatory effect on tomato fruit cuticle development. Our results provide a new model for the transcriptional regulation of fruit cuticle formation.

MdDEWAX decreases plant drought resistance by regulating wax biosynthesis

Apple epidermal wax protects plants from environmental stresses, determines fruit gloss and improves postharvest storage quality. However, the molecular mechanisms underlying the biosynthesis and regulation of apple epidermal waxes are not fully understood. In this study, we isolated a MdDEWAX gene from apple, which localized in the nucleus, expressed mainly in apple fruit, and induced by drought. We transformed the MdDEWAX gene into Arabidopsis, and found that heterologous expression of MdDEWAX reduced the accumulation of cuticular waxes in leaves and stems, increased epidermal permeability, the rate of water loss, and the rate of chlorophyll extraction of leaves and stems, altered the sensitivity to ABA, and reduced drought tolerance. Meanwhile, overexpression or silencing of the gene in the epidermis of apple fruits decreased or increased wax content, respectively. This study provides candidate genes for breeding apple cultivars and rootstocks with better drought tolerance.

Composition, metabolism and postharvest function and regulation of fruit cuticle: A review

The cuticle of plants, a hydrophobic membrane that covers their aerial organs, is crucial to their ability to withstand biotic and abiotic stressors. Fruit is the reproductive organ of plants, and an important dietary source that can offer a variety of nutrients for the human body, and fruit cuticle performs a crucial protective role in fruit development and postharvest quality. This review discusses the universality and diversity of the fruit cuticle composition, and systematically summarizes the metabolic process of fruit cuticle, including the biosynthesis, transport and regulatory factors (including transcription factors, phytohormones and environmental elements) of fruit cuticle. Additionally, we emphasize the postharvest functions and postharvest regulatory technologies of fruit cuticle, and propose future research directions for fruit cuticle.

13 CO2 labelling as a tool for elucidating the mechanism of cuticle development: a case of Clusia rosea

The plant cuticle is an important plant-atmosphere boundary, the synthesis and maintenance of which represents a significant metabolic cost. Only limited information regarding cuticle dynamics is available. We determined the composition and dynamics of Clusia rosea cuticular waxes and matrix using ¹³ CO₂ labelling, compound-specific and bulk isotope ratio mass spectrometry. Collodion was used for wax collection; gas exchange techniques to test for any collodion effects on living leaves. Cutin matrix (MX) area density did not vary between young and mature leaves and between leaf sides. Only young leaves incorporated new carbon into their MX. Collodion-based sampling discriminated between epicuticular (EW) and intracuticular wax (IW) effectively. Epicuticular differed in composition from IW. The newly synthetised wax was deposited in IW first and later in EW. Both young and mature leaves synthetised IW and EW. The faster dynamics in young leaves were due to lower wax coverage, not a faster synthesis rate. Longer-chain alkanes were deposited preferentially on the abaxial, stomatous leaf side, producing differences between leaf sides in wax composition. We introduce a new, sensitive isotope labelling method and demonstrate that cuticular wax is renewed during leaf ontogeny of C. rosea. We discuss the ecophysiological significance of the new insights.

The cutin polymer matrix undergoes a fine architectural tuning from early tomato fruit development to ripening

The cuticle is a complex polymer matrix that protects all aerial organs of plants, fulfills multiple roles in plant-environment interactions, and is critical for plant development. These functions are associated with the structural features of cuticles, and the architectural modeling of cuticles during plant development is crucial for understanding their physical properties and biological functions. In this work, the in-depth architecture of the cutin polymer matrix during fruit development was investigated. Using cherry tomato fruit (Solanum lycopersicum) as a model from the beginning of the cell expansion phase to the red ripe stage, we designed an experimental scheme combining sample pretreatment, Raman mapping, multivariate data analyses, and biochemical analyses. These approaches revealed clear chemical areas with different contributions of cutin, polysaccharides, and phenolics within the cutin polymer matrix. Besides, we demonstrated that these areas are finely tuned during fruit development, including compositional and macromolecular rearrangements. The specific spatiotemporal accumulation of phenolic compounds (p-coumaric acid and flavonoids) suggests that they fulfill distinct functions during fruit development. In addition, we highlighted an unexpected dynamic remodeling of the cutin-embedded polysaccharides pectin, cellulose, and hemicellulose. Such structural tuning enables consistent adaption of the cutin-polysaccharide continuum and the functional performance of the fruit cuticle at the different developmental stages. This study provides insights into the plant cuticle architecture and in particular into the organization of the epidermal cell wall-cuticle.

The PoLACS4 Gene May Participate in Drought Stress Resistance in Tree Peony (Paeonia ostii ‘Feng Dan Bai’)

The tree peony (Paeonia ostii ‘Feng Dan Bai’) has excellent drought tolerance. Although it has already been reported that the cuticle is an essential barrier against drought stress, the critical genes for cuticle resistance to drought remain unclear. However, the long-chain acyl-CoA synthetases (LACS) family of genes may be significant for the synthesis of cuticle wax. To test whether the LACS gene family is involved in cuticle response to drought stress in tree peony, we measure the thickness of cuticle stems and leaves alongside LACS enzyme activity. It is found that the cuticle thickens and the LACS enzyme increases with the maturation of stems and leaves, and there is a positive correlation between them. The LACS enzyme increases within 12 h under drought stress induced by polyethylene glycol (PEG). The transcriptome sequencing result (BioProject accession number PRJNA317164) is searched for, and a LACS gene with high expression is cloned. This gene has high homology and similarity with LACS4 from Arabidopsis thaliana. The gene is named PoLACS4. It is show to be highly expressed in mature leaves and peaks within 1 h under drought and salt stresses. All these results suggest that the LACS family of genes may be involved in cuticle response to drought stress and that PoLACS4 is a crucial gene which responds rapidly to drought in the tree peony.

Multifunctional Contribution of the Inflated Fruiting Calyx: Implication for Cuticular Barrier Profiles of the Solanaceous Genera Physalis, Alkekengi, and Nicandra

Pivotal barrier properties of the hydrophobic plant cuticle covering aerial plant surfaces depend on its physicochemical composition. Among plant species and organs, compounds of this boundary layer between the plant interior and the environment vary considerably but cuticle-related studies comparing different organs from the same plant species are still scarce. Thus, this study focused on the cuticle profiles of Physalis peruviana, Physalis ixocarpa, Alkekengi officinarum, and Nicandra physalodes species. Inflated fruiting calyces enveloping fruits make Physalis, Alkekengi, and Nicandra highly recognizable genera among the Solanoideae subfamily. Although the inflation of fruiting calyces is well discussed in the literature still little is known about their post-floral functionalities. Cuticular composition, surface structure, and barrier function were examined and compared in fully expanded amphistomatous leaves, ripe astomatous fruits, and fully inflated hypostomatous fruiting calyces. Species- and organ-specific abundances of non-glandular and glandular trichomes revealed high structural diversity, covering not only abaxial and adaxial leaf surfaces but also fruiting calyx surfaces, whereas fruits were glabrous. Cuticular waxes, which limit non-stomatal transpiration, ranged from <1 μg cm^-2 on P. peruviana fruiting calyces and N. physalodes fruits to 22 μg cm^-2 on P. peruviana fruits. Very-long-chain aliphatic compounds, notably n-alkanes, iso-, and anteiso-branched alkanes, alkanols, alkanoic acids, and alkyl esters, dominated the cuticular wax coverages (≥86%). Diversity of cuticular wax patterns rose from leaves to fruiting calyces and peaked in fruits. The polymeric cutin matrix providing the structural framework for cuticular waxes was determined to range from 81 μg cm^-2 for N. physalodes to 571 μg cm^-2 for A. officinarum fruits. Cuticular transpiration barriers were highly efficient, with water permeabilities being ≤5 × 10^-5 m s^-1. Only the cuticular water permeability of N. physalodes fruits was 10 × 10^-5 m s^-1 leading to their early desiccation and fruits that easily split, whereas P. peruviana, P. ixocarpa, and A. officinarum bore fleshy fruits for extended periods after maturation. Regarding the functional significance, fruiting calyces establish a physicochemical shield that reduces water loss and enables fruit maturation within a protective microclimate, and promotes different seed dispersal strategies among plant species investigated.

Integration of morphological, physiological and multi-omics analysis reveals a comprehensive mechanism for cuticular wax during development of greasiness in postharvest apples

Skin greasiness is a common postharvest disorder of apple (Malus × domestica). However, the molecular mechanism of skin greasiness is unclear. In this study, fruits of 'Golden Delicious' (GD), 'Granny Smith', and 'Fuji' with distinct characteristics of greasiness were used for greasiness scoring, wax morphology, wax metabolite, and RNA-seq analyses. Additionally, GD fruit were treated with 1-methylcyclopropene (1-MCP), which repressed greasiness. A partial least squares discriminant analysis (PLS-DA) revealed that wax esters were the critical wax fraction for skin greasiness. Among these wax esters, liquid linoleate esters of short-chain alcohols (C4-C6) and farnesol showed increased contents with increasing greasiness. Their concentrations were significantly correlated with greasiness score. To identify the genes encoding key enzymes for the synthesis of liquid linoleate esters, a weighted gene co-expression network analysis was conducted. MdDCR1, encoding an acyltransferase (defective in cuticular ridges, DCR), was selected as a candidate gene. MdDCR1 was significantly upregulated in greasy skin, and significantly suppressed by 1-MCP treatment. MdDCR1 silencing suppressed the accumulation of liquid linoleate esters of short-chain alcohols, including butyl linoleate, pentyl linoleate, and hexyl linoleate, in GD skin. These results provide insights into the molecular mechanisms of cuticular wax metabolism related to skin greasiness in apple. Our results show that transcriptional regulation of MdDCR1, encoding an acyltransferase that catalyzes the biosynthesis of liquid linoleate esters of short-chain alcohols (C4-C6) via an independent side branch of the C18:2 CoA pathway, regulates the formation of greasiness.

Molecular Biology, Composition and Physiological Functions of Cuticle Lipids in Fleshy Fruits

Fleshy fruits represent a valuable resource of economic and nutritional relevance for humanity. The plant cuticle is the external lipid layer covering the nonwoody aerial organs of land plants, and it is the first contact between fruits and the environment. It has been hypothesized that the cuticle plays a role in the development, ripening, quality, resistance to pathogen attack and postharvest shelf life of fleshy fruits. The cuticle's structure and composition change in response to the fruit's developmental stage, fruit physiology and different postharvest treatments. This review summarizes current information on the physiology and molecular mechanism of cuticle biosynthesis and composition changes during the development, ripening and postharvest stages of fleshy fruits. A discussion and analysis of studies regarding the relationship between cuticle composition, water loss reduction and maintaining fleshy fruits' postharvest quality are presented. An overview of the molecular mechanism of cuticle biosynthesis and efforts to elucidate it in fleshy fruits is included. Enhancing our knowledge about cuticle biosynthesis mechanisms and identifying specific transcripts, proteins and lipids related to quality traits in fleshy fruits could contribute to the design of biotechnological strategies to improve the quality and postharvest shelf life of these important fruit crops.

Structural analysis of mixed α- and β-amyrin samples

Little is known about the structure and molecular arrangement of α- and β-amyrin, a class of triterpenoids found within the cuticle of higher plants. Blends of both amyrin isomers with different ratios have been studied taking into consideration a combined methodology of density functional theory (DFT) calculations with experimental data from scanning electron microscopy, differential scanning calorimetry and Raman vibrational spectroscopy. Results indicate that trigonal trimeric aggregations of isomer mixtures are more stable, especially in the 1 : 2 (α : β) ratio. A combination of Raman spectroscopy and DFT calculations has allowed to develop an equation to determine the amount of β-amyrin in a mixed sample.

Tomato SlCER1-1 catalyzes the synthesis of wax alkanes which increases the drought tolerance and fruit storability

Very-long-chain (VLC) alkanes are the main wax compounds of tomato fruit and leaf. ECERIFERUM1 (CER1) and ECERIFERUM3 (CER3) are the two key genes involved in VLC alkane biosynthesis in Arabidopsis thaliana. However, the CER1 and CER3 homologous genes in tomato have not been investigated and their exact biological function remains unknown. We analyzed the wax profiles in tomato leaves and fruits at different growth stages, and characterized the CER1 and CER3 homologous genes. VLC alkanes were the predominant wax compounds both in the leaf and fruit at all developmental stages. We identified five CER1 homologs and two CER3 homologs in tomato, which were designated as SlCER1-1 to SlCER1-5 and SlCER3-1 and SlCER3-2 respectively. The genes exhibited tissue- and organ-dependent expression patterns and were induced by abiotic stresses. SlCER1-1 was localized to the endoplasmic reticulum (ER), which is also the main site of wax biosynthesis. Silencing the SlCER1-1 gene in tomato significantly reduced the amounts of n-Alkanes and branched alkanes, whereas its overexpression in Arabidopsis had the opposite effect. Under drought stress, both n-Alkanes and branched alkanes increased significantly in wild-type but not the SlCER1-1 RNAi tomato plants. Furthermore, SlCER1-1 silencing also increased the cuticular permeabilities of the leaves and fruits. In conclusion, SlCER1-1 is involved in wax alkane biosynthesis in tomato and plays an important role in the drought tolerance and fruit storability.

Tomatoes: An Extensive Review of the Associated Health Impacts of Tomatoes and Factors That Can Affect Their Cultivation

Simple Summary

The research outlined in this review paper discusses potential health benefits associated with a diet enriched with tomatoes and tomato products. This includes details of previous studies investigating the anticancer properties of tomatoes, protection against cardiovascular and neurodegenerative diseases and diabetes, maintenance of a healthy gut microbiome, and improved skin health, fertility, immune response, and exercise recovery. The specific parts of a tomato fruit that contribute these health benefits are also outlined. The potential disadvantages to a tomato-rich diet are detailed, especially the consumption of supplements that contain compounds found in tomatoes, such as lycopene. This review also discusses how the cultivation of tomato plants can affect the nutritional value of the fruit harvested. Different environmental growing conditions such as light intensity, growing media, and temperature are explained in terms of the impact they have on the quality of fruit, its nutrient content, and hence the potential health benefits acquired from eating the fruit.

Abstract

This review outlines the health benefits associated with the regular consumption of tomatoes and tomato products. The first section provides a detailed account of the horticultural techniques that can impact the quality of the fruit and its nutritional properties, including water availability, light intensity, temperature, and growing media. The next section provides information on the components of tomato that are likely to contribute to its health effects. The review then details some of the health benefits associated with tomato consumption, including anticancer properties, cardiovascular and neurodegenerative diseases and skin health. This review also discusses the impact tomatoes can have on the gut microbiome and associated health benefits, including reducing the risk of inflammatory bowel diseases. Other health benefits of eating tomatoes are also discussed in relation to effects on diabetes, the immune response, exercise recovery, and fertility. Finally, this review also addresses the negative effects that can occur as a result of overconsumption of tomato products and lycopene supplements.

Cutin extraction and composition determined under differing depolymerisation conditions in cork oak leaves

INTRODUCTION

Cutin is a biopolyester involved in waterproofing aerial plant organs, including leaves. Cutin quantification and compositional profiling require depolymerisation, namely by methanolysis, but specific protocols are not available.

OBJECTIVES

Investigate how different methanolysis conditions regarding catalyst concentration effect cutin depolymerisation and monomer release, to better define protocols for cutin content determination and composition profiling.

MATERIAL AND METHODS

Cork oak (Quercus suber) dewaxed leaves were reacted with five sodium methoxide (NaOMe) concentrations. Extracts were analysed: glycerol by high-performance liquid chromatography (HPLC) and long-chain lipids by gas chromatography-mass spectrometry (GC-MS).

RESULTS

Cutin was completely removed by 3% NaOMe (8.4% of dewaxed leaves), while mild 0.1% and 0.01% NaOMe methanolysis only depolymerised 14% of total cutin. Reactivity of cutin ester bonds is not homogeneous and glyceridic ester bonds are more easily cleaved, releasing the existing glycerol already under the mildest conditions (0.53% with 0.01% NaOMe and 0.41% with 3% NaOMe). The composition of cutin extracts varies with depolymerisation extent, with easier release of alkanoic acids and alkanols, respectively, 34.9% and 8.8% of total monomers at 0.1% NaOMe, while ω-hydroxyacids (49.3% of total monomers) and α,ω-diacids (9.0% of the monomers) are solubilised under more intensive reactive conditions.

CONCLUSION

Cutin of Quercus suber leaves is confirmed as a glyceridic polyester of ω-hydroxyacids and alkanoic acids, with minor content of α,ω-diacids, and including coumarate moieties. The protocol for the determination of cutin content and compositional profiling was established regarding catalyst concentration. The molar composition of cutin suggests a macromolecular assembly based on glycerol linked to lipid oligomeric chains with moderate cross-linking.

Analysis of Wheat Wax Regulation Mechanism by Liposome and Transcriptome

As a barrier for plants to contact with the outside world, epidermal wax plays an important role in resisting biotic and abiotic stresses. In this study, we analyzed the effect of wax content on leaf permeability by measuring the wax loss rate in the leaf. To further clarify the wax composition of the wheat epidermis and its molecular regulation mechanism, we applied untargeted lipidomic and transcriptome analysis on the leaf epidermis wax of Jimai 22 low-wax mutant (waxless) and multi-wax mutant (waxy). Our research showed that the mutant waxy has a slow loss rate, which can maintain higher leaf water content. 31 lipid subclasses and 1,367 lipid molecules were identified. By analyzing the wax differences of the two mutants, we found that the main lipid components of leaf epidermis wax in Jimai 22 were WE (C19-C50), DG (C27-C53), MG (C31-C35), and OAHFA (C31-C52). Carbon chain length analysis showed that, in wheat epidermis wax, WE was dominated by C44 molecules, DG was mainly concentrated in C47, C45, C37, and C31 molecules, C48 played a leading role in OAHFA, and C35 and C31 played a major role in MG. Among them, DG, MG, and OAHFA were detected in wheat leaf wax for the first time, and they were closely related to stress resistance. Compared with the waxy, 6,840 DEGs were detected in the mutant waxless, 3,181 DEGs were upregulated, and 3,659 DEGs were downregulated. The metabolic pattern of main waxy components in the wheat epidermis was constructed according to KEGG metabolic pathway and 46 related genes were screened, including KSC, TER, FAR, WSD1, CER1, MAH1, ALDH7A1, CYP704B1, ACOT1_2_4, CYP86, MGLL, GPAT, ALDH, DPP1, dgkA, plsC, and E2.3.1.158 related genes. The screened wax-related genes were confirmed to be highly reliable by qRT-PCR. In addition, we found TER gene TraesCS6B03G1132900LC in wheat mutant waxless leaves for the first time, which inhibited the synthesis of long-chain acyl-CoA (n+2) by downregulating its expression. These results provide valuable reference information for further study of wheat epidermis wax heredity and molecular regulation.

Unraveling Cuticle Formation, Structure, and Properties by Using Tomato Genetic Diversity

The tomato (Solanum lycopersicum) fruit has a thick, astomatous cuticle that has become a model for the study of cuticle formation, structure, and properties in plants. Tomato is also a major horticultural crop and a long-standing model for research in genetics, fruit development, and disease resistance. As a result, a wealth of genetic resources and genomic tools have been established, including collections of natural and artificially induced genetic diversity, introgression lines of genome fragments from wild relatives, high-quality genome sequences, phenotype and gene expression databases, and efficient methods for genetic transformation and editing of target genes. This mini-review reports the considerable progresses made in recent years in our understanding of cuticle by using and generating genetic diversity for cuticle-associated traits in tomato. These include the synthesis of the main cuticle components (cutin and waxes), their role in the structure and properties of the cuticle, their interaction with other cell wall polymers as well as the regulation of cuticle formation. It also addresses the opportunities offered by the untapped germplasm diversity available in tomato and the current strategies available to exploit them.

Understanding colour retention in red chilli pepper fruit using a metabolite profiling approach

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Image analysis can be used to speed up identification of high retention phenotypes.

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High and low retention phenotypes can be identified by their volatomes.

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Low retention lines have increased unsaturated fatty acids.

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Post-harvest colour loss in chilli fruit involves lipid peroxidation.

Carotenoids are the pigments responsible for conferring the characteristic deep red colour to chilli pepper. The post-harvest retention of this colour is a key trait that governs the price of the produce. Determining colour retention and the associated underlying biochemical mechanisms are important issues that require investigation. In this present study, the ability of image analysis to determine colour change in ground chilli fruit was evaluated. This method enabled differentiation of extreme retention phenotypes whilst also reducing the duration of storage required to make accurate determinations. The analysis of volatiles indicated different levels of lipid and carotenoid derived volatiles in lines with different retention properties. Metabolite profiling of intermediary metabolism supported these findings, with increased levels of unsaturated fatty acids present in lines with low retention properties. Collectively, these data have led us to propose that in chilli fruit lipid peroxidation is one of the progenitors of carotenoid degradation.

Genome-wide QTL analysis of tomato fruit cuticle deposition and composition

Genetics of traits related to fruit cuticle deposition and composition was studied in two red-fruited tomato species. Two mapping populations derived from the cross between the cultivated tomato (Solanum lycopersicum L.) and its closest relative wild species Solanum pimpinellifolium L. were employed to conduct a QTL analysis. A combination of fruit cuticle deposition, components and anatomical traits were investigated and the individual effect of each QTL evaluated. A total of 70 QTLs were identified, indicating that all the cuticle traits analyzed have a complex polygenic nature. A combination of additive and epistatic interactions was observed for all the traits, with positive contribution of both parental lines to most of them. Colocalization of QTLs for various traits uncovered novel genomic regions producing extensive changes in the cuticle. Cuticle density emerges as an important trait since it can modulate cuticle thickness and invagination thus providing a strategy for sustaining mechanical strength without compromising palatability. Two genomic regions, located in chromosomes 1 and 12, are responsible for the negative interaction between cuticle waxes and phenolics identified in tomato fruit. Several candidate genes, including transcription factors and structural genes, are postulated and their expression analyzed throughout development.

Biosurfactants Produced by Phyllosphere-Colonizing Pseudomonads Impact Diesel Degradation but Not Colonization of Leaves of Gnotobiotic Arabidopsis thaliana

Biosurfactant production is a common trait in leaf surface-colonizing bacteria that has been associated with increased survival and movement on leaves. At the same time, the ability to degrade aliphatics is common in biosurfactant-producing leaf colonizers. Pseudomonads are common leaf colonizers and have been recognized for their ability to produce biosurfactants and degrade aliphatic compounds. In this study, we investigated the role of biosurfactants in four non-plant-pathogenic Pseudomonas strains by performing a series of experiments to characterize their surfactant properties and their role during leaf colonization and diesel degradation. The biosurfactants produced were identified using mass spectrometry. Two strains produced viscosin-like biosurfactants, and the other two produced massetolide A-like biosurfactants, which aligned with the phylogenetic relatedness between the strains. To further investigate the role of surfactant production, random Tn5 transposon mutagenesis was performed to generate knockout mutants. The knockout mutants were compared to their respective wild types with regard to their ability to colonize gnotobiotic Arabidopsis thaliana and to degrade diesel or dodecane. It was not possible to detect negative effects during plant colonization in direct competition or individual colonization experiments. When grown on diesel, knockout mutants grew significantly slower than their respective wild types. When grown on dodecane, knockout mutants were less impacted than during growth on diesel. By adding isolated wild-type biosurfactants, it was possible to complement the growth of the knockout mutants.IMPORTANCE Many leaf-colonizing bacteria produce surfactants and are able to degrade aliphatic compounds; however, whether surfactant production provides a competitive advantage during leaf colonization is unclear. Furthermore, it is unclear if leaf colonizers take advantage of the aliphatic compounds that constitute the leaf cuticle and cuticular waxes. Here, we tested the effect of surfactant production on leaf colonization, and we demonstrate that the lack of surfactant production decreases the ability to degrade aliphatic compounds. This indicates that leaf surface-dwelling, surfactant-producing bacteria contribute to degradation of environmental hydrocarbons and may be able to utilize leaf surface waxes. This has implications for plant-microbe interactions and future studies.

Application of Genome Editing in Tomato Breeding: Mechanisms, Advances, and Prospects

Plants regularly face the changing climatic conditions that cause biotic and abiotic stress responses. The abiotic stresses are the primary constraints affecting crop yield and nutritional quality in many crop plants. The advances in genome sequencing and high-throughput approaches have enabled the researchers to use genome editing tools for the functional characterization of many genes useful for crop improvement. The present review focuses on the genome editing tools for improving many traits such as disease resistance, abiotic stress tolerance, yield, quality, and nutritional aspects of tomato. Many candidate genes conferring tolerance to abiotic stresses such as heat, cold, drought, and salinity stress have been successfully manipulated by gene modification and editing techniques such as RNA interference, insertional mutagenesis, and clustered regularly interspaced short palindromic repeat (CRISPR/Cas9). In this regard, the genome editing tools such as CRISPR/Cas9, which is a fast and efficient technology that can be exploited to explore the genetic resources for the improvement of tomato and other crop plants in terms of stress tolerance and nutritional quality. The review presents examples of gene editing responsible for conferring both biotic and abiotic stresses in tomato simultaneously. The literature on using this powerful technology to improve fruit quality, yield, and nutritional aspects in tomato is highlighted. Finally, the prospects and challenges of genome editing, public and political acceptance in tomato are discussed.

Chemical composition of leaf cutin in six Quercus suber provenances

The cutin content and composition of cork oak (Quercus suber) leaves was determined in six provenances with different seed geographical origin spreading across the species natural distribution. The cutin layer on the leaf surface was on average 518 μg/cm² of leaf area and represented 6.7% of the leaf dry weight, with no significant differences among provenances. Cutin depolymerisation was carried out by transesterification on whole leaves. The cutin composition of cork oak leaves is presented here for the first time. It is essentially composed of long-chain aliphatic ω-hydroxy fatty acids (44.4% of the total monomers), mostly with mid-chain hydroxyl and epoxy groups, fatty acids (20.7%), and a smaller proportion of α,ω-dicarboxylic acids (6.5%). The predominant compounds are 10,16-dihydroxy hexadecanoic acid (17.7-25.2%) and 9,10,18-trihydroxyoctadecanoic acid (15.6-18.0%). Alkanols represent 2.8% and aromatic compounds 12.8%, mainly coumarates. Isolation of cuticles from Q. suber leaves was performed using an enzymatic separation procedure and the fragments were analysed. Cuticle isolation is difficult and direct depolymerisation applied to whole leaves proved a suitable method to study cutin monomeric composition, which did not differ substantially to that of the isolated cuticles. No differences between provenances were found regarding cutin content and composition, thereby ruling out a significant genetic determination of these traits, but rather a highly adaptive phenotypic plasticity of cork oak. Although overall similar in their chemical nature, cutin and suberin in cork oak differ in the proportion of the major chemical families, i.e. ω-hydroxy acids, α,ω-diacids, and fatty acids.

Functional divergence of principal alcohol o-acyltransferase for biosynthesis of volatile acetate esters among tomato wild species (Solanum Sect. Lycopersicon)

Volatile esters are the chemicals that have multiple physiological functions including plant defense responses and reproduction. From a human perspective, the esters largely contribute to the fruity aroma of freshy fruits. Composition of volatile esters show a significant diversity among the wild tomato species (Solanum sect. Lycopersicon). To address the basis for this divergence, here we conducted functional analysis of a gene encoding major alcohol o-acyltransferase (AAT1) that catalyzes volatile ester formation. Although AAT1 transcripts were highly expressed in the ripe fruits of all the wild species examined, their enzymatic properties significantly differed due to amino acid sequence variations. Notably, AAT1s from S. pennellii showed the highest ability to produce acetate esters whereas AAT1s from S. neorickii, S. chmielewskii and S. habrochaites had the lowest activities. Further, screenings using domain-swapped or point-mutated AAT1s allowed us to identify Met/Thr352 as one of the critical residues related to the transferase activity with acetyl-CoA. This finding is potentially applied to aroma engineering in which a site-directed mutagenesis at this position in alcohol o-acyltransferases could enable to manipulate volatile ester levels in ripe fruits.

Unexplored nutritive potential of tomato to combat global malnutrition

Tomato, a widely consumed vegetable crop, offers a real potential to combat human nutritional deficiencies. Tomatoes are rich in micronutrients and other bioactive compounds (including vitamins, carotenoids, and minerals) that are known to be essential or beneficial for human health. This review highlights the current state of the art in the molecular understanding of the nutritional aspects, conventional and molecular breeding efforts, and biofortification studies undertaken to improve the nutritional content and quality of tomato. Transcriptomics and metabolomics studies, which offer a deeper understanding of the molecular regulation of the tomato's nutrients, are discussed. The potential uses of the wastes from the tomato processing industry (i.e., the peels and seed extracts) that are particularly rich in oils and proteins are also discussed. Recent advancements with CRISPR/Cas mediated gene-editing technology provide enormous opportunities to enhance the nutritional content of agricultural produces, including tomatoes. In this regard, genome editing efforts with respect to biofortification in the tomato plant are also discussed. The recent technological advancements and knowledge gaps described herein aim to help explore the unexplored nutritional potential of the tomato.

Exploiting Natural Variation in Tomato to Define Pathway Structure and Metabolic Regulation of Fruit Polyphenolics in the Lycopersicum Complex

While the structures of plant primary metabolic pathways are generally well defined and highly conserved across species, those defining specialized metabolism are less well characterized and more highly variable across species. In this study, we investigated polyphenolic metabolism in the lycopersicum complex by characterizing the underlying biosynthetic and decorative reactions that constitute the metabolic network of polyphenols across eight different species of tomato. For this purpose, GC-MS- and LC-MS-based metabolomics of different tissues of Solanum lycopersicum and wild tomato species were carried out, in concert with the evaluation of cross-hybridized microarray data for MapMan-based transcriptomic analysis, and publicly available RNA-sequencing data for annotation of biosynthetic genes. The combined data were used to compile species-specific metabolic networks of polyphenolic metabolism, allowing the establishment of an entire pan-species biosynthetic framework as well as annotation of the functions of decoration enzymes involved in the formation of metabolic diversity of the flavonoid pathway. The combined results are discussed in the context of the current understanding of tomato flavonol biosynthesis as well as a global view of metabolic shifts during fruit ripening. Our results provide an example as to how large-scale biology approaches can be used for the definition and refinement of large specialized metabolism pathways.

Autofluorescence changes of tomato surface tissues during overripening

We investigated the autofluorescence of tomato surface tissues during overripening at 25 °C for 13 days. Microscopic images and fluorescence spectra of tissues, including the epidermis and cuticle, were examined (excitation at 360 nm), revealing that the autofluorescence changes were related to the epidermis, particularly the fluorophores in the cuticle.

A Proposed Method for Simultaneous Measurement of Cuticular Transpiration From Different Leaf Surfaces in Camellia sinensis

The plant cuticle is the major barrier that limits unrestricted water loss and hence plays a critical role in plant drought tolerance. Due to the presence of stomata on the leaf abaxial surface, it is technically challenging to measure abaxial cuticular transpiration. Most of the existing reports were only focused on leaf astomatous adaxial surface, and few data are available regarding abaxial cuticular transpiration. Developing a method that can measure cuticular transpiration from both leaf surfaces simultaneously will improve our understanding about leaf transpiration barrier organization. Here, we developed a new method that enabled the simultaneous measurement of cuticular transpiration rates from the adaxial and abaxial surfaces. The proposed method combined multi-step leaf pretreatments including water equilibration under dark and ABA treatment to close stomata, as well as gum arabic or vaseline application to remove or seal the epicuticular wax layer. Mathematical formulas were established and used to calculate the transpiration rates of individual leaf surfaces from observed experimental data. This method facilitates the simultaneous quantification of cuticular transpiration from adaxial and abaxial leaf surfaces. By applying this method, we demonstrated that the adaxial intracuticular waxes and the abaxial epicuticular waxes constitute the major transpiration barriers in Camellia sinensis. Wax analysis indicated that adaxial intracuticular waxes had higher coverage of very long chain fatty acids, 1-alkanol esters, and glycols, which may be attributed to its higher transpiration barrier than that of the abaxial intracuticular waxes.

Genome-Wide Association Study for Maize Leaf Cuticular Conductance Identifies Candidate Genes Involved in the Regulation of Cuticle Development

The cuticle, a hydrophobic layer of cutin and waxes synthesized by plant epidermal cells, is the major barrier to water loss when stomata are closed at night and under water-limited conditions. Elucidating the genetic architecture of natural variation for leaf cuticular conductance (g_c) is important for identifying genes relevant to improving crop productivity in drought-prone environments. To this end, we conducted a genome-wide association study of g_c of adult leaves in a maize inbred association panel that was evaluated in four environments (Maricopa, AZ, and San Diego, CA, in 2016 and 2017). Five genomic regions significantly associated with g_c were resolved to seven plausible candidate genes (ISTL1, two SEC14 homologs, cyclase-associated protein, a CER7 homolog, GDSL lipase, and β-D-XYLOSIDASE 4). These candidates are potentially involved in cuticle biosynthesis, trafficking and deposition of cuticle lipids, cutin polymerization, and cell wall modification. Laser microdissection RNA sequencing revealed that all these candidate genes, with the exception of the CER7 homolog, were expressed in the zone of the expanding adult maize leaf where cuticle maturation occurs. With direct application to genetic improvement, moderately high average predictive abilities were observed for whole-genome prediction of g_c in locations (0.46 and 0.45) and across all environments (0.52). The findings of this study provide novel insights into the genetic control of g_c and have the potential to help breeders more effectively develop drought-tolerant maize for target environments.

Manipulation of β-carotene levels in tomato fruits results in increased ABA content and extended shelf life

Tomato fruit ripening is controlled by the hormone ethylene and by a group of transcription factors, acting upstream of ethylene. During ripening, the linear carotene lycopene accumulates at the expense of cyclic carotenoids. Fruit-specific overexpression of LYCOPENE β-CYCLASE (LCYb) resulted in increased β-carotene (provitamin A) content. Unexpectedly, LCYb-overexpressing fruits also exhibited a diverse array of ripening phenotypes, including delayed softening and extended shelf life. These phenotypes were accompanied, at the biochemical level, by an increase in abscisic acid (ABA) content, decreased ethylene production, increased density of cell wall material containing linear pectins with a low degree of methylation, and a thicker cuticle with a higher content of cutin monomers and triterpenoids. The levels of several primary metabolites and phenylpropanoid compounds were also altered in the transgenic fruits, which could be attributed to delayed fruit ripening and/or to ABA. Network correlation analysis and pharmacological experiments with the ABA biosynthesis inhibitor, abamine, indicated that altered ABA levels were a direct effect of the increased β-carotene content and were in turn responsible for the extended shelf life phenotype. Thus, manipulation of β-carotene levels results in an improvement not only of the nutritional value of tomato fruits, but also of their shelf life.

Network Analysis Provides Insight into Tomato Lipid Metabolism

Metabolic correlation networks have been used in several instances to obtain a deeper insight into the complexity of plant metabolism as a whole. In tomato (Solanum lycopersicum), metabolites have a major influence on taste and overall fruit quality traits. Previously a broad spectrum of metabolic and phenotypic traits has been described using a Solanum pennellii introgression-lines (ILs) population. To obtain insights into tomato fruit metabolism, we performed metabolic network analysis from existing data, covering a wide range of metabolic traits, including lipophilic and volatile compounds, for the first time. We provide a comprehensive fruit correlation network and show how primary, secondary, lipophilic, and volatile compounds connect to each other and how the individual metabolic classes are linked to yield-related phenotypic traits. Results revealed a high connectivity within and between different classes of lipophilic compounds, as well as between lipophilic and secondary metabolites. We focused on lipid metabolism and generated a gene-expression network with lipophilic metabolites to identify new putative lipid-related genes. Metabolite–transcript correlation analysis revealed key putative genes involved in lipid biosynthesis pathways. The overall results will help to deepen our understanding of tomato metabolism and provide candidate genes for transgenic approaches toward improving nutritional qualities in tomato.

Characterizing the involvement of FaMADS9 in the regulation of strawberry fruit receptacle development

FaMADS9 is the strawberry (Fragaria x ananassa) gene that exhibits the highest homology to the tomato (Solanum lycopersicum) RIN gene. Transgenic lines were obtained in which FaMADS9 was silenced. The fruits of these lines did not show differences in basic parameters, such as fruit firmness or colour, but exhibited lower Brix values in three of the four independent lines. The gene ontology MapMan category that was most enriched among the differentially expressed genes in the receptacles at the white stage corresponded to the regulation of transcription, including a high percentage of transcription factors and regulatory proteins associated with auxin action. In contrast, the most enriched categories at the red stage were transport, lipid metabolism and cell wall. Metabolomic analysis of the receptacles of the transformed fruits identified significant changes in the content of maltose, galactonic acid-1,4-lactone, proanthocyanidins and flavonols at the green/white stage, while isomaltose, anthocyanins and cuticular wax metabolism were the most affected at the red stage. Among the regulatory genes that were differentially expressed in the transgenic receptacles were several genes previously linked to flavonoid metabolism, such as MYB10, DIV, ZFN1, ZFN2, GT2, and GT5, or associated with the action of hormones, such as abscisic acid, SHP, ASR, GTE7 and SnRK2.7. The inference of a gene regulatory network, based on a dynamic Bayesian approach, among the genes differentially expressed in the transgenic receptacles at the white and red stages, identified the genes KAN1, DIV, ZFN2 and GTE7 as putative targets of FaMADS9. A MADS9-specific CArG box was identified in the promoters of these genes.

Integrated Metabolomic and Transcriptomic Analysis to Characterize Cutin Biosynthesis between Low- and High-Cutin Genotypes of Capsicum chinense Jacq

Habanero peppers constantly face biotic and abiotic stresses such as pathogen/pest infections, extreme temperature, drought and UV radiation. In addition, the fruit cutin lipid composition plays an important role in post-harvest water loss rates, which in turn causes shriveling and reduced fruit quality and storage. In this study, we integrated metabolome and transcriptome profiling pertaining to cutin in two habanero genotypes: PI 224448 and PI 257145. The fruits were selected by the waxy or glossy phenotype on their surfaces. Metabolomics analysis showed a significant variation in cutin composition, with about 6-fold higher cutin in PI 257145 than PI 224448. It also revealed that 10,16-dihydroxy hexadecanoic acid is the most abundant monomer in PI 257145. Transcriptomic analysis of high-cutin PI 257145 and low-cutin PI 224448 resulted in the identification of 2703 statistically significant differentially expressed genes, including 1693 genes upregulated and 1010 downregulated in high-cutin PI 257145. Genes and transcription factors such as GDSL lipase, glycerol-3 phosphate acyltransferase 6, long-chain acyltransferase 2, cytochrome P450 86A/77A, SHN1, ANL2 and HDG1 highly contributed to the high cutin content in PI 257145. We predicted a putative cutin biosynthetic pathway for habanero peppers based on deep transcriptome analysis. This is the first study of the transcriptome and metabolome pertaining to cutin in habanero peppers. These analyses improve our knowledge of the molecular mechanisms regulating the accumulation of cutin in habanero pepper fruits. These resources can be built on for developing cultivars with high cutin content that show resistance to biotic and abiotic stresses with superior postharvest appearance.

Low temperature upregulates cwp expression and modifies alternative splicing patterns, increasing the severity of cwp-induced tomato fruit cuticular microfissures

The cwp (cuticular water permeability) gene controls the development of cuticular microfissuring and subsequent fruit dehydration in tomato. The gene underwent silencing in the evolution of the fleshy cultivated tomato but is expressed in the primitive wild tomato relatives. The introgression of the expressed allele from the wild S. habrochaites (cwp ^h ) into the cultivated tomato (Solanum lycopersicum) leads to the phenotype of fruit water loss during and following ripening. In this report, we show that low temperature impacts on the severity of the cuticular microfissure phenotype via a combination of effects on both expression and alternative splicing of cwp ^h . The cwp gene, comprising four exons and three introns, undergoes post-transcriptional alternative splicing processes, leading to seven alternative transcripts that differ in reading-frame lengths. Transgenic plants expressing each of the alternative transcripts identified the longest reading frame (VAR1) as the functional splice variant. Low temperature led to a strong upregulation of cwp ^h expression, compounded by an increase in the relative proportion of the functional VAR1 transcript, leading to increased severity of microfissuring of the cuticle. In summary, we demonstrate the molecular mechanism behind the horticultural phenomenon of the low-temperature effect on cuticular microfissures in the dehydrating tomato.

Shelf Life Potential and the Fruit Cuticle: The Unexpected Player

The plant cuticle is an extracellular barrier that protects the aerial, non-lignified parts of plants from the surrounding environment, and furthermore plays important functions in organ growth and development. The role of the cuticle in post-harvest quality of fruits is a topic currently driving a lot of interest since an increasing bulk of research data show its modulating influence on a number of important traits determining shelf life and storage potential, including water transpiration and fruit dehydration, susceptibility to rots, pests and disorders, and even firmness. Moreover, the properties of fruit cuticles keep evolving after harvest, and have also been shown to be highly responsive to the external conditions surrounding the fruit. Indeed, common post-harvest treatments will have an impact on cuticle integrity and performance that needs to be evaluated for a deeper understanding of changes in post-harvest quality. In this review, chemical and biophysical properties of fruit cuticles are summarized. An overview is also provided of post-harvest changes in cuticles and the effects thereupon of some post-harvest procedures, with the purpose of offering a comprehensive summary of currently available information. Identification of natural sources of variability in relevant quality traits would allow breeding for the improvement of post-harvest life of fruit commodities.

Developmental and Environmental Regulation of Cuticular Wax Biosynthesis in Fleshy Fruits

The aerial parts of land plants are covered by a hydrophobic layer called cuticle that limits non-stomatal water loss and provides protection against external biotic and abiotic stresses. The cuticle is composed of polymer cutin and wax comprising a mixture of very-long-chain fatty acids and their derivatives, while also bioactive secondary metabolites such as triterpenoids are present. Fleshy fruits are also covered by the cuticle, which has an important protective role during the fruit development and ripening. Research related to the biosynthesis and composition of cuticles on vegetative plant parts has largely promoted the research on cuticular waxes in fruits. The chemical composition of the cuticular wax varies greatly between fruit species and is modified by developmental and environmental cues affecting the protective properties of the wax. This review focuses on the current knowledge of the cuticular wax biosynthesis during fleshy fruits development, and on the effect of environmental factors in regulation of the biosynthesis. Bioactive properties of fruit cuticular waxes are also briefly discussed, as well as the potential for recycling of industrial fruit residues as a valuable raw material for natural wax to be used in food, cosmetics and medicine.

Identification of quantitative trait loci and candidate genes for primary metabolite content in strawberry fruit

Improvement of nutritional and organoleptic quality of fruits is a key goal in current strawberry breeding programs. The ratio of sugars to acids is a determinant factor contributing to fruit liking, although different sugars and acids contribute in varying degrees to this complex trait. A segregating F₁ population of 95 individuals, previously characterized for several fruit quality characters, was used to map during 2 years quantitative trait loci (QTL) for 50 primary metabolites, l-ascorbic acid (L-AA) and other related traits such as soluble solid content (SSC), titratable acidity (TA), and pH. A total of 133 mQTL were detected above the established thresholds for 44 traits. Only 12.9% of QTL were detected in the 2 years, suggesting a large environmental influence on primary metabolite content. An objective of this study was the identification of key metabolites that were associated to the overall variation in SSC and acidity. As it was observed in previous studies, a number of QTL controlling several metabolites and traits were co-located in homoeology group V (HG V). mQTL controlling a large variance in raffinose, sucrose, succinic acid, and L-AA were detected in approximate the same chromosomal regions of different homoeologous linkage groups belonging to HG V. Candidate genes for selected mQTL are proposed based on their co-localization, on the predicted function, and their differential gene expression among contrasting F₁ progeny lines. RNA-seq analysis from progeny lines contrasting in L-AA content detected 826 differentially expressed genes and identified Mannose-6-phosphate isomerase, FaM6PI1, as a candidate gene contributing to natural variation in ascorbic acid in strawberry fruit.

Maintenance of grafting-induced epigenetic variations in the asexual progeny of Brassica oleracea and B. juncea chimera

Grafting-induced variations have been observed in many plant species, but the heritability of variation in progeny is not well understood. In our study, adventitious shoots from the C cell lineage of shoot apical meristem (SAM) grafting chimera TCC (where the origin of the outmost, middle and innermost cell layers, respectively, of SAM is designated by 'T' for tuber mustard and 'C' for red cabbage) were induced and identified as r-CCC (r = regenerated). To investigate the maintenance of grafting variations during cell propagation and regeneration, different generations of asexual progeny (r-CCCn, n = generation) were established through successive regeneration of axillary shoots from r-CCC. The fourth generation of r-CCC (r-CCC4) was selected to perform whole genome bisulfite sequencing for comparative analysis of hetero-grafting-induced global methylation changes relative to r-s-CCC4 (s = self-grafting). Increased CHH methylation levels and proportions were observed in r-CCC4, with substantial changes occurring in the repeat elements. Small RNA sequencing revealed 1135 specific small interfering RNA (siRNA) tags that were typically expressed in r-CCC, r-CCC2 and r-CCC4. Notably, 65% of these specific siRNAs were associated with repeat elements, termed RE siRNAs. Subsequent analysis revealed that the CHH methylation of RE siRNA-overlapping regions was mainly hypermethylation in r-CCC4, indicating that they were responsible for directing and maintaining grafting-induced CHH methylation. Moreover, the expression of 13 differentially methylated genes (DMGs) correlated with the phenotypic variation, showing differential expression levels between r-CCC4 and r-s-CCC4. These DMGs were predominantly CG hypermethylated, their methylation modifications corresponded to the transcription of relative methyltransferase.

Quantitative Trait Loci Analysis Identifies a Prominent Gene Involved in the Production of Fatty Acid-Derived Flavor Volatiles in Tomato

To gain insight into the genetic regulation of lipid metabolism in tomato, we conducted metabolic trait loci (mQTL) analysis following the lipidomic profiling of fruit pericarp and leaf tissue of the Solanum pennellii introgression lines (IL). To enhance mapping resolution for selected fruit-specific mQTL, we profiled the lipids in a subset of independently derived S. pennellii backcross inbred lines, as well as in a near-isogenic sub-IL population. We identified a putative lecithin:cholesterol acyltransferase that controls the levels of several lipids, and two members of the class III lipase family, LIP1 and LIP2, that were associated with decreased levels of diacylglycerols (DAGs) and triacylglycerols (TAGs). Lipases of this class cleave fatty acids from the glycerol backbone of acylglycerols. The released fatty acids serve as precursors of flavor volatiles. We show that LIP1 expression correlates with fatty acid-derived volatile levels. We further confirm the function of LIP1 in TAG and DAG breakdown and volatile synthesis using transgenic plants. Taken together, our study extensively characterized the genetic architecture of lipophilic compounds in tomato and demonstrated at molecular level that release of free fatty acids from the glycerol backbone can have a major impact on downstream volatile synthesis.

Breeding for cuticle-associated traits in crop species: traits, targets, and strategies

Improving crop productivity and quality while promoting sustainable agriculture have become major goals in plant breeding. The cuticle is a natural film covering the aerial organs of plants and consists of lipid polyesters covered and embedded with wax. The cuticle protects plants against water loss and pathogens and affects traits with strong impacts on crop quality such as, for horticultural crops, fruit brightness, cracking, russeting, netting, and shelf life. Here we provide an overview of the most important cuticle-associated traits that can be targeted for crop improvement. To date, most studies on cuticle-associated traits aimed at crop breeding have been done on fleshy fruits. Less information is available for staple crops such as rice, wheat or maize. Here we present new insights into cuticle formation and properties resulting from the study of genetic resources available for the various crop species. Our review also covers the current strategies and tools aimed at exploiting available natural and artificially induced genetic diversity and the technologies used to transfer the beneficial alleles affecting cuticle-associated traits to commercial varieties.

Cutin from agro-waste as a raw material for the production of bioplastics

Cutin is the main component of plant cuticles constituting the framework that supports the rest of the cuticle components. This biopolymer is composed of esterified bi- and trifunctional fatty acids. Despite its ubiquity in terrestrial plants, it has been underutilized as raw material due to its insolubility and lack of melting point. However, in recent years, a few technologies have been developed to obtain cutin monomers from several agro-wastes at an industrial scale. This review is focused on the description of cutin properties, biodegradability, chemical composition, processability, abundance, and the state of art of the fabrication of cutin-based materials in order to evaluate whether this biopolymer can be considered a source for the production of renewable materials.

Assimilation of 'omics' strategies to study the cuticle layer and suberin lamellae in plants

The assembly of the lipophilic cuticle layer and suberin lamellae, approximately 450 million years ago, was a major evolutionary development that enabled plants to colonize terrestrial habitats. The cuticle layer is composed of cutin polyester and embedded cuticular waxes, whereas the suberin lamellae consist of very long chain fatty acid derivatives, glycerol, and phenolics cross-linked with alkyl ferulate-embedded waxes. Due to their substantial biological roles in plant life, the mechanisms underlying the assembly of these structures have been extensively investigated. In the last decade, the introduction of 'omics' approaches, including genomics, transcriptomics, proteomics, and metabolomics, have been key in the identification of novel genetic and chemical elements involved in the formation and function of the cuticle layer and suberin lamellae. This review summarizes contemporary studies that utilized various large-scale, 'omics' strategies in combination with novel technologies to unravel how building blocks and polymers of these lipophilic barriers are made, and moreover linking structure to function along developmental programs and stress responses. We anticipate that the studies discussed here will inspire scientists studying lipophilic barriers to integrate complementary 'omics' approaches in their efforts to tackle as yet unresolved questions and engage the main challenges of the field to date.

Postharvest changes in LIN5-down-regulated plants suggest a role for sugar deficiency in cuticle metabolism during ripening

The cell wall invertase gene (LIN5) was reported to be a key enzyme influencing sugar uptake of tomato (Solanum lycopersicum) fruit. It was additionally revealed to be a key regulator of total soluble solids content in fruit as well as for reproductive development, being mainly involved in flower development, early fruit and seed development but also in ripening. Here, we demonstrate that silencing of the LIN5 gene promotes changes affecting fruit cuticle development which has a direct effect on postharvest properties. Transformants were characterized by reduced transpirational water loss in mature fruits accompanied by several other changes in the cuticle. Quantitative chemical composition, coupled with microscopy of isolated cuticle fruits revealed that the cuticle of the transformants were characterized by an increase of the thickness as well as significant increase in the content of cuticle components (cutin, phenolic compounds, and waxes). Furthermore, detailed analysis of the waxes revealed that the transformants displayed changes in waxes composition, showing higher levels of n-alkanes and triterpenoids which can shift the proportion of crystalline and amorphous waxes and change the water flux through the cuticle. Expression of the genes involved in cuticle biosynthesis indicated that LIN5 influences the biosynthesis of components of the cuticle, indicating that this process is coupled to sugar uploading via a mechanism which links carbon supply with the capacity for fruit expansion.

Uncovering tomato quantitative trait loci and candidate genes for fruit cuticular lipid composition using the Solanum pennellii introgression line population

The cuticle is a specialized cell wall layer that covers the outermost surface of the epidermal cells and has important implications for fruit permeability and pathogen susceptibility. In order to decipher the genetic control of tomato fruit cuticle composition, an introgression line (IL) population derived from a biparental cross between Solanum pennellii (LA0716) and the Solanum lycopersicum cultivar M82 was used to build a first map of associated quantitative trait loci (QTLs). A total of 24 cuticular waxes and 26 cutin monomers were determined. They showed changes associated with 18 genomic regions distributed in nine chromosomes affecting 19 ILs. Out of the five main fruit cuticular components described for the wild species S. pennellii, three of them were associated with IL3.4, IL12.1, and IL7.4.1, causing an increase in n-alkanes (≥C30), a decrease in amyrin content, and a decrease in cuticle thickness of ~50%, respectively. Moreover, we also found a QTL associated with increased levels of amyrins in IL3.4. In addition, we propose some candidate genes on the basis of their differential gene expression and single nucleotide polymorphism variability between the introgressed and the recurrent alleles, which will be the subjects of further investigation.

RNA-seq reveals mechanisms of SlMX1 for enhanced carotenoids and terpenoids accumulation along with stress resistance in tomato

Improving nutritional fruit quality and impacts important agro-traits such as biotic or abiotic stresses are extremely important for human civilization. Our previous study reported that manipulation of SlMX1 gene enhanced carotenoids accumulation and drought resistance in tomato. Here, RNA-Seq analysis proved to be a very useful tool to provide insights into the regulatory mechanisms of SlMX1 involved in stress resistance and enhanced secondary metabolites. Physiological analysis showed that over-expression of SlMX1 results in substantially increased broad-spectrum tolerance to a wide-range of abiotic and biotic (fungus, bacteria, virus and insects) stresses in tomato. This research appears to be of remarkable interest because enhanced terpenoids content has been achieved by increasing trichome density. In addition, we reported two types of trichome which seems to be aberrant types in tomato. This study unravels the mechanism of regulation of SlMX1, which simultaneously modulates resistance and metabolic processes through regulating key structural and regulatory genes of the corresponding pathways.

Efficient Identification of Causal Mutations through Sequencing of Bulked F 2 from Two Allelic Bloomless Mutants of Sorghum bicolor

Sorghum (Sorghum bicolor Moench, L.) plant accumulates copious layers of epi-cuticular wax (EW) on its aerial surfaces, to a greater extent than most other crops. EW provides a vapor barrier that reduces water loss, and is therefore considered to be a major determinant of sorghum's drought tolerance. However, little is known about the genes responsible for wax accumulation in sorghum. We isolated two allelic mutants, bloomless40-1 (bm40-1) and bm40-2, from a mutant library constructed from ethyl methane sulfonate (EMS) treated seeds of an inbred, BTx623. Both mutants were nearly devoid of the EW layer. Each bm mutant was crossed to the un-mutated BTx623 to generated F₂ populations that segregated for the bm phenotype. Genomic DNA from 20 bm F₂ plants from each population was bulked for whole genome sequencing. A single gene, Sobic.001G228100, encoding a GDSL-like lipase/acylhydrolase, had unique homozygous mutations in each bulked F₂ population. Mutant bm40-1 harbored a missense mutation in the gene, whereas bm40-2 had a splice donor site mutation. Our findings thus provide strong evidence that mutation in this GDSL-like lipase gene causes the bm phenotype, and further demonstrate that this approach of sequencing two independent allelic mutant populations is an efficient method for identifying causal mutations. Combined with allelic mutants, MutMap provides powerful method to identify all causal genes for the large collection of bm mutants in sorghum, which will provide insight into how sorghum plants accumulate such abundant EW on their aerial surface. This knowledge may facilitate the development of tools for engineering drought-tolerant crops with reduced water loss.

Differential Lipid Composition and Gene Expression in the Semi-Russeted "Cox Orange Pippin" Apple Variety

Russeting is characterized by a particular rough and brown phenotype, which is mainly due to the accumulation of suberin in the inner part of the epidermal cell walls. In our previous bulk transcriptomic analysis, comparing fully russeted, and waxy apple varieties, showed, in apple fruit skin, a massive decreased expression of cutin, wax and some pentacyclic triterpene biosynthesis genes in the russeted varieties, with an expected concomitant enhanced expression of the suberin biosynthetic genes. In the present work, we performed a deep investigation of the aliphatic composition of the cutin, suberin, waxes, and triterpenes in the waxy and russeted patches of the semi-russeted apple variety "Cox Orange Pippin." A targeted gene expression profiling was performed to validate candidate genes which were identified in our previous work and might be involved in the respective metabolic pathways. Our results showed that a decrease of cuticular waxes, ursolic acid and oleanolic acid, accompanied by an accumulation of alkyl-hydroxycinamates and betulinic acid, occurs in the russeted patches. The suberin monomer composition is characterized by specific occurrence of 20, 22, and 24 carbon aliphatic chains, whereas cutin is mainly represented by common C16 and C18 aliphatic chains. This work depicts, for the first time in apple, the complex composition of suberin, cutin, waxes and triterpenes, and confirms the strong interplay between these epidermal polymers in apple fruit skin.

The moss Funaria hygrometrica has cuticular wax similar to vascular plants, with distinct composition on leafy gametophyte, calyptra and sporophyte capsule surfaces

BACKGROUND AND AIMS

Aerial surfaces of land plants are covered with a waxy cuticle to protect against water loss. The amount and composition of cuticular waxes on moss surfaces had rarely been investigated. Accordingly, the degree of similarity between moss and vascular plant waxes, and between maternal and offspring moss structure waxes is unknown. To resolve these issues, this study aimed at providing a comprehensive analysis of the waxes on the leafy gametophyte, gametophyte calyptra and sporophyte capsule of the moss Funaria hygrometrica

METHODS

Waxes were extracted from the surfaces of leafy gametophytes, gametophyte calyptrae and sporophyte capsules, separated by gas chromatography, identified qualitatively with mass spectrometry, and quantified with flame ionization detection. Diagnostic mass spectral peaks were used to determine the isomer composition of wax esters.

KEY RESULTS

The surfaces of the leafy gametophyte, calyptra and sporophyte capsule of F. hygrometrica were covered with 0·94, 2·0 and 0·44 μg cm(-2) wax, respectively. While each wax mixture was composed of mainly fatty acid alkyl esters, the waxes from maternal and offspring structures had unique compositional markers. β-Hydroxy fatty acid alkyl esters were limited to the leafy gametophyte and calyptra, while alkanes, aldehydes and diol esters were restricted to the sporophyte capsule. Ubiquitous fatty acids, alcohols, fatty acid alkyl esters, aldehydes and alkanes were all found on at least one surface.

CONCLUSIONS

This is the first study to determine wax coverage (μg cm(-2)) on a moss surface, enabling direct comparisons with vascular plants, which were shown to have an equal amount or more wax than F. hygrometrica Wax ester biosynthesis is of particular importance in this species, and the ester-forming enzyme(s) in different parts of the moss may have different substrate preferences. Furthermore, the alkane-forming wax biosynthesis pathway, found widely in vascular plants, is active in the sporophyte capsule, but not in the leafy gametophyte or calyptra. Overall, wax composition and coverage on F. hygrometrica were similar to those reported for some vascular plant species, suggesting that the underlying biosynthetic processes in plants of both lineages were inherited from a common ancestor.

Solanum pennellii backcross inbred lines (BILs) link small genomic bins with tomato traits

We present a resource for fine mapping of traits derived from the wild tomato species Solanum pennellii (LA0716). The population of backcross inbred lines (BILs) is composed of 446 lines derived after a few generations of backcrosses of the wild species with cultivated tomato (cultivar M82; LA3475), followed by more than seven generations of self-pollination. The BILs were genotyped using the 10K SOL-CAP single nucleotide polymorphism (SNP) -Chip, and 3700 polymorphic markers were used to map recombination break points relative to the physical map of Solanum lycopersicum. The BILs carry, on average, 2.7 introgressions per line, with a mean introgression length of 11.7 Mbp. Whereas the classic 76 introgression lines (ILs) partitioned the genome into 106 mapping bins, the BILs generated 633 bins, thereby enhancing the mapping resolution of traits derived from the wild species. We demonstrate the power of the BILs for rapid fine mapping of simple and complex traits derived from the wild tomato species.

The Glycerol-3-Phosphate Acyltransferase GPAT6 from Tomato Plays a Central Role in Fruit Cutin Biosynthesis

The thick cuticle covering and embedding the epidermal cells of tomato (Solanum lycopersicum) fruit acts not only as a protective barrier against pathogens and water loss but also influences quality traits such as brightness and postharvest shelf-life. In a recent study, we screened a mutant collection of the miniature tomato cultivar Micro-Tom and isolated several glossy fruit mutants in which the abundance of cutin, the polyester component of the cuticle, was strongly reduced. We employed a newly developed mapping-by-sequencing strategy to identify the causal mutation underlying the cutin deficiency in a mutant thereafter named gpat6-a (for glycerol-3-phosphate acyltransferase6). To this end, a backcross population (BC1F2) segregating for the glossy trait was phenotyped. Individuals displaying either a wild-type or a glossy fruit trait were then pooled into bulked populations and submitted to whole-genome sequencing prior to mutation frequency analysis. This revealed that the causal point mutation in the gpat6-a mutant introduces a charged amino acid adjacent to the active site of a GPAT6 enzyme. We further showed that this mutation completely abolished the GPAT activity of the recombinant protein. The gpat6-a mutant showed perturbed pollen formation but, unlike a gpat6 mutant of Arabidopsis (Arabidopsis thaliana), was not male sterile. The most striking phenotype was observed in the mutant fruit, where cuticle thickness, composition, and properties were altered. RNA sequencing analysis highlighted the main processes and pathways that were affected by the mutation at the transcriptional level, which included those associated with lipid, secondary metabolite, and cell wall biosynthesis.