A comparison of the ultrastructure and composition of fruits' cuticular wax from the wild-type 'Newhall' navel orange (Citrus sinensis [L.] Osbeck cv. Newhall) and its glossy mutant

1-MCP treatment improves the postharvest quality of Jinxiu yellow peach by regulating cuticular wax composition and gene expression during cold storage

This study aimed to analyze the effect of 1-methylcyclopropene (1-MCP) treatment on the postharvest quality, epidermal wax morphology, composition, and gene expression of Jinxiu yellow peach during cold storage. The results showed that 1-MCP treatment could maintain the postharvest quality of peach fruit as compared to control (CK) during cold storage. The wax crystals of peach fruit were better retained by 1-MCP, and they still existed in 0.6 and 0.9 µL/L 1-MCP treated fruit at 36 days. The total wax content in all the fruit increased first and then decreased during cold storage. Meanwhile, n-alkanes and primary alcohols were the main wax components. Compared to CK, 1-MCP treatment could delay the reduction of wax content during cold storage. The correlation analysis indicated that the postharvest quality of yellow peach was mainly affected by the contents of fatty acids and triterpenoids in cuticular wax. The transcriptomics results revealed PpaCER1, PpaKCS, PpaKCR1, PpaCYP86B1, PpaFAR, PpaSS2, and PpaSQE1 played the important roles in the formation of peach fruit wax. 1-MCP treatment upregulated PpaCER1 (18785414, 18786441, and 18787644), PpaKCS (18774919, 18789438, and 18793503), PpaKCR1 (18790432), and PpaCYP86B1 (18789815) to deposit more n-alkanes and fatty acids during cold storage. This study could provide a new perspective for regulating the postharvest quality of yellow peach in view of the application of cuticular wax. PRACTICAL APPLICATION: 'Jinxiu' yellow peach fruit is favorable among consumers because of its high commercial value. However, it ripens and deteriorates rapidly during storage, leading to serious economic loss and consumer disappointment. The effect of 1-methylcyclopropene (1-MCP) treatment on the postharvest quality, epidermal wax morphology, composition, and genes regulation of 'Jinxiu' yellow peach during cold storage was assessed. Compared to control, 1-MCP treatment could retain the storage quality of yellow peach by affecting cuticular wax composition and gene expression. This study could provide new perspective for regulating the postharvest quality of yellow peach in view of the application of cuticular wax.

Reveal the relationship between the quality and the cuticle composition of Satsuma mandarin (Citrus unshiu) by postharvest heat treatment

To investigate the influence of heat treatment (HT) on Satsuma mandarin fruit's postharvest quality and cuticle composition, we immersed the fruit for 3 min in hot water at 52°C and subsequently stored them at room temperature (25°C) for 28 days, and fruit quality parameters, such as good fruit rate, weight loss rate, firmness, total soluble solids, total titratable acidity, and ascorbic acid content, were monitored. Additionally, changes in the peel's cuticle composition were analyzed, and wax crystal morphologies on the fruit surface were examined using scanning electron microscopy (SEM). The findings revealed that appropriate HT effectively preserved fruit quality. The main compositions of wax and cutin on the fruit's surface remained consistent between the HT and the CK during storage. The total content of wax and cutin initially increased, peaking on the 14th day of storage, and then decreased, falling below the levels observed on day 0. Notably, the total amount of cutin in the HT group exceeded that of the control group. Specifically, ω-hydroxy fatty acids with mid-chain oxo groups and mid-oh-ω-hydroxy fatty acids constituted approximately 90% of the total cutin content. Moreover, the HT group exhibited higher (p < 0.05) total wax content in relation to the control. Fatty acids and alkanes were the predominant components, accounting for approximately 87.5% of the total wax. SEM analysis demonstrated that HT caused wax crystals to melt and redistribute, effectively filling wax gaps. It suggests that HT holds promising potential as a green, safe, and eco-friendly commercial treatment for preserving the postharvest quality of Satsuma mandarin. PRACTICAL APPLICATION: In this study, Satsuma citrus (Citrus unshiu) underwent heat treatment (HT) and was subsequently preserved at room temperature (25°C) for 28 days. The findings revealed that HT significantly improved fruit quality compared to the control group. These findings provide valuable insights into the advancement of eco-friendly and pollution-free citrus preservation methods, offering essential strategies and process parameters for their practical application.

The dynamic changes of mango (Mangifera indica L.) epicuticular wax during fruit development and effect of epicuticular wax on Colletotrichum gloeosporioides invasion

Mango fruits are susceptible to diseases, such as anthracnose, during fruit development, leading to yield reduction. Epicuticular wax is closely related to resistance of plants to pathogenic bacterial invasion. In this study, the effect of mango fruit epicuticular wax on the invasion of Colletotrichum gloeosporioides was investigated, followed by to understand the changes of wax chemical composition and crystal morphology during mango fruit development using GC-MS and SEM. Results showed that the epicuticular wax of mango fruits can prevent the invasion of C. gloeosporioides, and 'Renong' showed the strongest resistance to C. gloeosporioides. The wax content of four mango varieties first increased and then decreased from 40 days after full bloom (DAFB) to 120 DAFB. In addition, 95 compounds were detected in the epicuticular wax of the four mango varieties at five developmental periods, in which primary alcohols, terpenoids and esters were the main wax chemical composition. Furthermore, the surface wax structure of mango fruit changed dynamically during fruit development, and irregular platelet-like crystals were the main wax structure. The present study showed the changes of wax content, chemical composition and crystal morphology during mango fruit development, and the special terpenoids (squalene, farnesyl acetate and farnesol) and dense crystal structure in the epicuticular wax of 'Renong' fruit may be the main reason for its stronger resistance to C. gloeosporioides than other varieties. Therefore, these results provide a reference for the follow-up study of mango fruit epicuticular wax synthesis mechanism and breeding.

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.

Identification of an epicuticular wax crystal deficiency gene Brwdm1 in Chinese cabbage (Brassica campestris L. ssp. pekinensis)

Introduction

The cuticle wax covering the plant surface is a whitish hydrophobic protective barrier in Chinese cabbage, and the epicuticular wax crystal deficiency normally has higher commodity value for a tender texture and glossy appearance. Herein, two allelic epicuticular wax crystal deficiency mutants, wdm1 and wdm7, were obtained from the EMS mutagenesis population of a Chinese cabbage DH line 'FT'.

Methods

The cuticle wax morphology was observed by Cryo-scanning electron microscopy (Cryo-SEM) and the composition of wax was determined by GC-MS. The candidate mutant gene was found by MutMap and validated by KASP. The function of candidate gene was verified by allelic variation.

Results

The mutants had fewer wax crystals and lower leaf primary alcohol and ester content. Genetic analysis revealed that the epicuticular wax crystal deficiency phenotype was controlled by a recessive nuclear gene, named Brwdm1. MutMap and KASP analyses indicated that BraA01g004350.3C, encoding an alcohol-forming fatty acyl-CoA reductase, was the candidate gene for Brwdm1. A SNP 2,113,772 (C to T) variation in the 6^th exon of Brwdm1 in wdm1 led to the 262^nd amino acid substitution from threonine (T) to isoleucine (I), which existed in a rather conserved site among the amino acid sequences from Brwdm1 and its homologs. Meanwhile, the substitution changed the three-dimensional structure of Brwdm1. The SNP 2,114,994 (G to A) in the 10^th exon of Brwdm1 in wdm7 resulted in the change of the 434^th amino acid from valine (V) to isoleucine (I), which occurred in the STERILE domain. KASP genotyping showed that SNP 2,114,994 was co-segregated with glossy phenotype. Compared with the wild type, the relative expression of Brwdm1 was significantly decreased in the leaves, flowers, buds and siliques of wdm1.

Discussion

These results indicated that Brwdm1 was indispensable for the wax crystals formation and its mutation resulted in glossy appearance in Chinese cabbage.

Effect of 1-methylcyclopropene on peel greasiness, yellowing, and related gene expression in postharvest 'Yuluxiang' pear

'Yuluxiang' pear (Pyrus sinkiangensis) commonly develop a greasy coating and yellowing during storage. In this study, 1.0 μL L^-1 1-methylcyclopropene (1-MCP) was applied to 'Yuluxiang' pear to investigate its effects on fruit quality, peel wax composition, greasiness index, chlorophyll content, and the expression pattern of related genes during storage at ambient temperature (25°C). The results showed that 1-MCP treatment maintained higher fruit firmness and chlorophyll content, decreased respiration rate, and postponed the peak of ethylene production rate, lowered the greasy index of the peel. The main wax components of peel accumulated during storage, the principal ones being alkenes (C23, C25, and C29), fatty acids (C16, C18:1, and C28), aldehydes (C24:1, C26:1, and C28:1), and esters (C22:1 fatty alcohol-C16 fatty acid, C22:1 fatty alcohol-C18:1 fatty acid, C22 fatty alcohol-C16 fatty acid, C22 fatty alcohol-C18:1 fatty acid, C24:1 fatty alcohol-C18:1 fatty acid, and C24 fatty alcohol-C18:1 fatty acid), and were reduced by 1-MCP. 1-MCP also decreased the expression of genes associated with ethylene biosynthesis and signal transduction (ACS1, ACO1, ERS1, ETR2, and ERF1), chlorophyll breakdown (NYC1, NOL, PAO, PPH, and SGR), and wax accumulation (LACS1, LACS6, KCS1, KCS2, KCS4, KCS10L, KCS11L, KCS20, FDH, CER10, KCR1, ABCG11L, ABCG12, ABCG21L, LTPG1, LTP4, CAC3, CAC3L, and DGAT1L). There were close relationships among wax components (alkanes, alkenes, fatty acids, esters, and aldehydes), chlorophyll content, greasiness index, and level of expression of genes associated with wax synthesis and chlorophyll breakdown. These results suggest that 1-MCP treatment decreased the wax content of 'Yuluxiang' pear and delayed the development of peel greasiness and yellowing by inhibiting the expression of genes related to the ethylene synthesis, signal transduction, wax synthesis, and chlorophyll degradation.

CsCER6 and CsCER7 Influence Fruit Glossiness by Regulating Fruit Cuticular Wax Accumulation in Cucumber

Fruit glossiness is an important external fruit quality trait that greatly affects the marketability of fresh cucumber (Cucumis sativus) fruits. A few reports have suggested that the extent of cuticular wax loading influences the glossiness of the fruit surface. In the present study, we tested the wax contents of two inbred cucumber lines, comparing a line with waxy fruit (3401) and a line with glossy fruit (3413). Wax content analysis and dewaxing analysis demonstrate that fruit cuticular wax loads negatively correlate with fruit glossiness in cucumber. Identifying genes that were differentially expressed in fruit pericarps between 3401 and 3413 and genes induced by abscisic acid suggested that the wax biosynthesis gene CsCER6 (Cucumis sativus ECERIFERUM 6) and the regulatory gene CsCER7 may affect wax accumulation on cucumber fruit. Expression analysis via RT-qPCR, GUS-staining, and in situ hybridization revealed that CsCER6 and CsCER7 are abundantly expressed in the epidermis cells in cucumber fruits. Furthermore, the overexpression and RNAi lines of CsCER6 and CsCER7 showed dramatic effects on fruit cuticular wax contents, fruit glossiness, and cuticle permeability. Our results suggest that CsCER6 and CsCER7 positively regulate fruit cuticular wax accumulation and negatively influence fruit glossiness.

Comparative Transcriptome Analysis Reveals Gene Expression Differences in Eggplant (Solanum melongena L.) Fruits with Different Brightness

Fruit brightness is an important quality trait that affects the market value of eggplant. However, few studies have been conducted on eggplant brightness. In this study, we aimed to identify genes related to this trait in three varieties of eggplant with different fruit brightness between 14 and 22 days after pollination. Using RNA-Seq Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses, we found that wax- and cutin-related pathways and differentially expressed genes displayed significant differences among different development stages and varieties. Scanning electron microscopy revealed that the wax layer was thinner in '30-1' and 'QPCQ' than in '22-1'. Gas chromatography-mass spectrometry analysis revealed that wax content was significantly lower in '30-1' than in '22-1', which indicated that wax may be an important factor determining fruit brightness. We further identified and analyzed the KCS gene family, which encodes the rate-limiting enzyme of FA elongation in wax synthesis. The results provide an insight into the molecular mechanisms of fruit brightness in eggplants and further eggplant breeding programs.

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.

Function and transcriptional regulation of CsKCS20 in the elongation of very-long-chain fatty acids and wax biosynthesis in Citrus sinensis flavedo

Cuticular wax on plant aerial surfaces plays a vital role in the defense against various stresses, and the genes related to wax metabolism have been well documented in several model plants. However, there is very limited research on the key enzymes and transcription factors (TFs) associated with carbon chain distribution and wax biosynthesis in citrus fruit. In this study, an analysis of wax metabolites indicated that even carbon-chain (C24-C28) metabolites are the dominant wax components in citrus fruit, and a 3-ketoacyl-CoA synthase (KCS) family gene (CsKCS20) plays an important role in the carbon chain distribution during wax biosynthesis in a wax-deficient mutant (MT). Expression of CsKCS20 in yeast indicated that CsKCS20 can catalyze the biosynthesis of C22 and C24 very-long-chain fatty acids (VLCFAs). In addition, transcriptome and sequence analysis indicated that the differential expression of CsKCS20 between the wild-type (WT) and MT fruit can be partly attributed to the regulation of CsMYB96, which was further confirmed by yeast one-hybrid (Y1H) assays, electrophoretic mobility shift assays (EMSAs) and dual luciferase assays. The functions of CsMYB96 and CsKCS20 in wax biosynthesis were further validated by heterologous expression in Arabidopsis. In summary, this study elucidates the important roles of CsKCS20 and CsMYB96 in regulating VLCFA elongation and cuticular wax biosynthesis, which provides new directions for the improvement of citrus fruit wax quality in genetic breeding programs.

Comparative Analysis of Cuticular Wax in Various Grape Cultivars During Berry Development and After Storage

Cuticular wax covering the surface of fleshy fruit is closely related to fruit glossiness, development, and post-harvest storage quality. However, the information about formation characteristics and molecular mechanisms of cuticular wax in grape berry is limited. In this study, crystal morphology, chemical composition, and gene expression of cuticular wax in grape berry were comprehensively investigated. Morphological analysis revealed high density of irregular lamellar crystal structures, which were correlated with the glaucous appearances of grape berry. Compositional analysis showed that the dominant wax compounds were triterpenoids, while the most diverse were alkanes. The amounts of triterpenoids declined sharply after véraison, while those of other compounds maintained nearly constant throughout the berry development. The amounts of each wax compounds varied among different cultivars and showed no correlation with berry skin colors. Moreover, the expression profiles of related genes were in accordance with the accumulation of wax compounds. Further investigation revealed the contribution of cuticular wax to the water preservation capacity during storage. These findings not only facilitate a better understanding of the characteristics of cuticular wax, but also shed light on the molecular basis of wax biosynthesis in grape.

Ectopic Overexpression of CsECR From Navel Orange Increases Cuticular Wax Accumulation in Tomato and Enhances Its Tolerance to Drought Stress

Drought stress often occurred in citrus to limit its growth, distribution, and fruit quality. Cuticular waxes play an important role in regulating plant tolerance to drought stress. Plant enoyl-CoA reductase (ECR) is involved in the biosynthesis of cuticular waxes and catalyzes the last step of very long-chain fatty acids (VLCFAs) elongation. In this study, a putative ECR gene, named CsECR, was cloned from "Newhall" navel orange. CsECR protein has high identities with other plant ECR proteins and contained a conserved NADP/NAD-binding motif and three conserved functional sites. The highest expression of CsECR was observed in leaves, followed by stems, flavedos, ovaries, juice sacs, stigmas, stamens, albedos, and petals. Besides, the expression of CsECR was significantly induced by PEG6000 and ABA treatments. Ectopic overexpression of CsECR increased the contents of total waxes and aliphatic wax fractions (n-fatty acids, unsaturated fatty acids, n-alkanes, alkenes, iso-, and anteiso-alkanes) in the leaves and fruits of the transgenic tomato. Furthermore, ectopic overexpression of CsECR reduced the cuticle permeability in the leaves and fruits of the transgenic tomato and increased its tolerance to drought stress. Taken together, our results revealed that CsECR plays an important role in plant response to drought stresses by regulating cuticular wax biosynthesis.

Interferences of Waxes on Enzymatic Saccharification and Ethanol Production from Lignocellulose Biomass

Wax is an organic compound found on the surface of lignocellulose biomass to protect plants from physical and biological stresses in nature. With its small mass fraction in biomass, wax has been neglected from inclusion in the design of the biorefinery process. This study investigated the interfering effect of wax in three types of lignocellulosic biomass, including rice straw (RS), Napier grass (NG), and sugarcane bagasse (SB). In this study, although small fractions of wax were extracted from RS, NG, and SB at 0.57%, 0.61%, and 1.69%, respectively, dewaxing causes changes in the plant compositions and their functional groups and promotes dissociations of lignocellulose fibrils. Additionally, dewaxing of biomass samples increased reducing sugar by 1.17-, 1.04-, and 1.35-fold in RS, NG, and SB, respectively. The ethanol yield increased by 1.11-, 1.05-, and 1.23-fold after wax removal from RS, NG, and SB, respectively. The chemical composition profiles of the waxes obtained from RS, NG, and SB showed FAME, alcohol, and alkane as the major groups. According to the conversion rate of the dewaxing process and ethanol fermentation, the wax outputs of RS, NG, and SB are 5.64, 17.00, and 6.00 kg/ton, respectively. The current gasoline price is around USD 0.903 per liter, making ethanol more expensive than gasoline. Therefore, in order to reduce the cost of ethanol in the biorefinery industry, other valuable products (such as wax) should be considered for commercialization. The cost of natural wax ranges from USD 2 to 22 per kilogram, depending on the source of the extracted wax. The wax yields obtained from RS, SB, and NG have the potential to increase profits in the biorefining process and could provide an opportunity for application in a wider range of downstream industries than just biofuels.

Comparative analysis of the cuticular waxes and related gene expression between 'Newhall' and 'Ganqi 3' navel orange during long-term cold storage

Previously, we obtained a wax-deficient mutant 'Ganqi 3' (MT) from 'Newhall' navel orange (Citrus sinensis [L.] Osbeck cv. Newhall, WT). The weight loss and postharvest decay in MT fruit were much higher than those in WT fruit after long-term cold storage. To understand the underlying mechanism, the changes in the morphology, chemical composition and gene expression of cuticular waxes between WT and MT fruit were compared during 150 days of storage at 4 °C. The density of epicuticular wax crystals and the contents of most of the aliphatic wax fractions in MT fruit were much lower than those in WT fruit over 90 days of storage. Further research revealed that the differences in the morphology and chemical composition of cuticular waxes might be important causes for the differences of postharvest weight loss and decay rates between WT and MT fruit. Notably, the expression profiles of 16 wax-related genes in WT and MT fruit were consistent with the change trends of corresponding cuticular wax components during cold storage. These results suggest that the morphology and chemical composition of cuticular waxes may be regulated by wax-related genes and play an important role in regulating the postharvest weight loss and the tolerances to postharvest decay in navel orange.

Analysis of composition, morphology, and biosynthesis of cuticular wax in wild type bilberry (Vaccinium myrtillus L.) and its glossy mutant

In this study, cuticular wax load, its chemical composition, and biosynthesis, was studied during development of wild type (WT) bilberry fruit and its natural glossy type (GT) mutant. GT fruit cuticular wax load was comparable with WT fruits. In both, the proportion of triterpenoids decreased during fruit development concomitant with increasing proportions of total aliphatic compounds. In GT fruit, a higher proportion of triterpenoids in cuticular wax was accompanied by a lower proportion of fatty acids and ketones compared to WT fruit as well as lower density of crystalloid structures on berry surfaces. Our results suggest that the glossy phenotype could be caused by the absence of rod-like structures in GT fruit associated with reduction in proportions of ketones and fatty acids in the cuticular wax. Especially CER26-like, FAR2, CER3-like, LTP, MIXTA, and BAS genes showed fruit skin preferential expression patterns indicating their role in cuticular wax biosynthesis and secretion.

Metabolomic insights into the browning of the peel of bagging 'Rui Xue' apple fruit

BACKGROUND

Bagging is one of the most important techniques for producting high-quality fruits. In the actual of cultivating, we found a new kind of browning in peel of apple fruit that occurs before harvest and worsen during storage period. There are many studies on metabonomic analysis of browning about storage fruits, but few studies on the mechanism of browning before harvest.

RESULTS

In this study, five-year-old trees of 'Rui Xue' (CNA20151469.1) were used as materials. Bagging fruits without browning (BFW) and bagging fruits with browning (BFB) were set as the experimental groups, non-bagging fruits (NBF) were set as control. After partial least squares discriminant analysis (PLS-DA), 50 kinds of metabolites were important with predictive VIP > 1 and p-value < 0.05. The most important differential metabolites include flavonoids and lipids molecules, 11 flavonoids and 6 lipids molecules were significantly decreased in the BFW compared with NBF. After browning, 11 flavonoids and 7 lipids were further decreased in BFB compared with BFW. Meanwhile, the significantly enriched metabolic pathways include galactose metabolism, ABC membrane transporter protein, flavonoid biosynthesis and linoleic acid metabolism pathways et al. Physiological indicators show that, compared with NBF, the content of malondialdehyde (MDA), hydrogen peroxide (H₂O₂)_, superoxide anion (O₂^-) in peel of BFW and BFB were significantly increased, and the difference of BFB was more significant. Meanwhile, the antioxidant enzyme activities of BFW and BFB were inhibited, which accelerated the destruction of cell structure. In addition, the metabolome and physiological data showed that the significantly decrease of flavonoid was positively correlated with peel browning. So, we analyzed the expression of flavonoid related genes and found that, compared with NBF, the flavonoid synthesis genes MdLAR and MdANR were significantly up-regulated in BFW and BFB, but, the downstream flavonoids-related polymeric genes MdLAC7 and MdLAC14 were also significantly expressed.

CONCLUSIONS

Our findings demonstrated that the microenvironment of fruit was changed by bagging, the destruction of cell structure, the decrease of flavonoids and the increase of triterpenoids were the main reasons for the browning of peel.

Analysis of composition, morphology, and biosynthesis of cuticular wax in wild type bilberry (Vaccinium myrtillus L.) and its glossy mutant

In this study, cuticular wax load, its chemical composition, and biosynthesis, was studied during development of wild type (WT) bilberry fruit and its natural glossy type (GT) mutant. GT fruit cuticular wax load was comparable with WT fruits. In both, the proportion of triterpenoids decreased during fruit development concomitant with increasing proportions of total aliphatic compounds. In GT fruit, a higher proportion of triterpenoids in cuticular wax was accompanied by a lower proportion of fatty acids and ketones compared to WT fruit as well as lower density of crystalloid structures on berry surfaces. Our results suggest that the glossy phenotype could be caused by the absence of rod-like structures in GT fruit associated with reduction in proportions of ketones and fatty acids in the cuticular wax. Especially CER26-like, FAR2, CER3-like, LTP, MIXTA, and BAS genes showed fruit skin preferential expression patterns indicating their role in cuticular wax biosynthesis and secretion.

In vitro neuroprotective potential of terpenes from industrial orange juice by-products

Citrus sinensis (orange) by-products represent one of the most abundant citric residues from orange juice industrial production, and are a promising source of health-promoting compounds like terpenes. In this work, different extraction solvents have been employed to increase terpene extraction yield and selectivity from this orange juice by-product. A set of bioactivity assays including enzymatic (acetylcholinesterase (AChE), butylcholinesterase (BChE) and lipoxygenase (LOX)) as well as antioxidant (ABTS, reactive oxygen species (ROS) and reactive nitrogen species (RNS)) activity tests have been applied to investigate the neuroprotective potential of these compounds. New fluorescence-based methodologies were developed for AChE and BChE assays to overcome the drawbacks of these tests when used in vitro to determine the anticholinergic activity of colored extracts. Comprehensive phytochemical profiling based on gas chromatography coupled to quadrupole time of flight mass spectrometry (GC-qTOF-MS) analysis showed ahigh content of mono- and sesquiterpenes in the extracts obtained with ethyl acetate, whereas n-heptane extracts exhibited a large amount of triterpenes and carotenoids. From a neuroprotective activity point of view, ethyl acetate extract is the most promising due to its anticholinergic activity and antioxidant capacity. Finally, a multivariate data analysis revealed a good correlation between some monoterpenes (e.g. nerol or limonene) and the antioxidant capacity of the natural extract, while a group of sesquiterpenes (e.g.δ-Cadinene or nootkatone) showed correlation with the observed AChE, BChE and LOX inhibition capacity. Hydrocarbons mono- and sesquiterpenoids reveal high capacity in vitro to cross the blood-brain barrier (BBB).

Cuticular Wax Composition of Wild and Cultivated Northern Berries

The outer-most layer of plant surface, the cuticle, consists of epi- and intra-cuticular wax. It protects the plant from dehydration, extreme temperatures and UV radiation, as well as attacks from pests such as molds and bacteria. Berry cuticular waxes are studied to understand the metabolism character (factors affecting wax layer composition in different berry species) and increase the microbial resistance and shelf life of berries. The aim of this study was analysis of the surface wax composition of nine species of wild and cultivated berries from Northern Europe. Cuticular wax analysis were done using gas chromatography-mass spectrometry. A total of 59 different compounds were identified belonging to nine groups of compounds, namely, alkanes, phytosterols, alcohols, fatty acids, phenolic acids, ketones, aldehydes, esters and tocopherols. The analyzed blueberries had the highest amount of wax present on their surface (0.9 mg berry-1), triterpenoids were the main wax constituent in these berries, with up to 62% wax composition. Berry species and varieties were compared based on their surface wax composition-similarities were found between different blueberry varieties; however, other berries showed differences based on concentration and composition of cuticular wax.

Lipidomic and transcriptomic analysis reveals reallocation of carbon flux from cuticular wax into plastid membrane lipids in a glossy "Newhall" navel orange mutant

Both cuticle and membrane lipids play essential roles in quality maintenance and disease resistance in fresh fruits. Many reports have indicated the modification of alternative branch pathways in epicuticular wax mutants; however, the specific alterations concerning lipids have not been clarified thus far. Here, we conducted a comprehensive, time-resolved lipidomic, and transcriptomic analysis on the "Newhall" navel orange (WT) and its glossy mutant (MT) "Gannan No. 1". The results revealed severely suppressed wax formation accompanied by significantly elevated production of 36-carbon plastid lipids with increasing fruit maturation in MT. Transcriptomics analysis further identified a series of key functional enzymes and transcription factors putatively involved in the biosynthesis pathways of wax and membrane lipids. Moreover, the high accumulation of jasmonic acid (JA) in MT was possibly due to the need to maintain plastid lipid homeostasis, as the expression levels of two significantly upregulated lipases (CsDAD1 and CsDALL2) were positively correlated with plastid lipids and characterized to hydrolyze plastid lipids to increase the JA content. Our results will provide new insights into the molecular mechanisms underlying the natural variation of plant lipids to lay a foundation for the quality improvement of citrus fruit.

Changes in Cuticle Components and Morphology of 'Satsuma' Mandarin (Citrus unshiu) during Ambient Storage and Their Potential Role on Penicillium digitatum Infection

To elucidate the role of fruit cuticle in fungal infection, changes in cuticle composition and morphology of 'Satsuma' mandarin during ambient (at 25 °C) storage and their role in Penicillium digitatum infection were investigated. Results showed that the epicuticular wax yield increased from 1.11 μg cm-2 to 4.21 μg cm-2 during storage for 20 days and then decreased to 1.35 μg cm-2 as storage time prolonged to 40 days. Intracuticular wax content of fruits stored for 20 days showed a peak value that was 1.7-fold higher than that of fruits stored for 40 days. The contents of cutin monomers of fruits showed a decreased trend during storage, while their proportions in the cutin stayed stable. Acids were identified as the most abundant components in epicuticular wax independently of the storage time, followed by alkanes and terpenoids. Terpenoids were found as the predominant components in intracuticular wax during the whole storage, followed by alkanes and acids. The flattened platelets crystals of fruits at harvest changed into small granule-like wax ones after 10 days of storage then gradually distributed across the surface of the fruits as stored for 40 days. Results of in vitro tests showed that mycelial growth of Penicillium digitatum could be promoted by epicuticular wax and conidial germination could be inhibited by cutin at different storage stages. These results shed new light on the chemical basis for cuticle involvement in fungal infection.

Ectopic Expression of CsKCS6 From Navel Orange Promotes the Production of Very-Long-Chain Fatty Acids (VLCFAs) and Increases the Abiotic Stress Tolerance of Arabidopsis thaliana

Cuticular wax is closely related to plant resistance to abiotic stress. 3-Ketoacyl-CoA synthase (KCS) catalyzes the biosynthesis of very-long-chain fatty acid (VLCFA) wax precursors. In this study, a novel KCS family gene was isolated from Newhall navel orange and subsequently named CsKCS6. The CsKCS6 protein has two main domains that belong to the thiolase-like superfamily, the FAE1-CUT1-RppA and ACP_syn_III_C domains, which exist at amino acid positions 80-368 and 384-466, respectively. CsKCS6 was expressed in all tissues, with the highest expression detected in the stigma; in addition, the transcription of CsKCS6 was changed in response to drought stress, salt stress and abscisic acid (ABA) treatment. Heterologous expression of CsKCS6 in Arabidopsis significantly increased the amount of VLCFAs in the cuticular wax on the stems and leaves, but there were no significant changes in total wax content. Compared with that of the wild-type (WT) plants, the leaf permeability of the transgenic plants was lower. Further research showed that, compared with the WT plants, the transgenic lines experienced less water loss and ion leakage after dehydration stress, displayed increased survival under drought stress treatment and presented significantly longer root lengths and survival under salt stress treatment. Our results indicate that CsKCS6 not only plays an important role in the synthesis of fatty acid precursors involved in wax synthesis but also enhances the tolerance of transgenic Arabidopsis plants to abiotic stress. Thus, the identification of CsKSC6 could help to increase the abiotic stress tolerance of Citrus in future breeding programs.

Compositional and morphological analyses of wax in northern wild berry species

Aerial surfaces of plants are covered by a waxy cuticle protecting plants from excessive water loss and UV light. In the present study, composition and morphology of cuticular waxes of northern wild berry species bilberry (Vaccinium myrtillus L.), lingonberry (V. vitis-idaea L.), bog bilberry (V. uliginosum L.) and crowberry (Empetrum nigrum L.) were investigated. Scanning electron microscopy (SEM) revealed differences in epicuticular wax morphology, and gas chromatography-mass spectrometry (GC-MS) analysis confirmed variation in chemical composition of cuticular waxes between the berry species. The dominant compounds in bilberry and lingonberry cuticular waxes were triterpenoids, while fatty acids and alkanes were the dominant ones in bog bilberry and crowberry, respectively. Wax extracted by supercritical fluid extraction (SFE) from industrial press cakes of bilberry and lingonberry contained linoleic acid and γ-linolenic acid as the dominant compounds. Furthermore, in vitro sun protection factor (SPF) of berry waxes depicted good UV-B absorbing capacities.

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.

Changes in Cuticular Wax Composition of Two Blueberry Cultivars during Fruit Ripening and Postharvest Cold Storage

Cuticular wax plays an important role for the quality of blueberry fruits. In this study, the cuticular wax composition of two blueberry cultivars, 'Legacy' ( Vaccinium corymbosum) and 'Brightwell' ( Vaccinium ashei), was examined during fruit ripening and postharvest cold storage. The results showed that wax was gradually deposited on the epidermis of blueberry fruits and the content of major wax compounds, except that for diketones, increased significantly during fruit ripening. The total wax content was 2-fold greater in 'Brightwell' blueberries than that in 'Legacy' blueberries during fruit ripening. The total wax content of both cultivars decreased during 30 days of storage at 4 °C, and the variation of cuticular wax composition was cultivar-dependent. The content of diketones decreased significantly in 'Legacy' blueberries, while the content of triterpenoids and aliphatic compounds showed different fold changes in 'Brightwell' blueberries after 30 days of storage at 4 °C. Overall, our study provided a quantitative and qualitative overview of cuticular wax compounds of blueberry fruits during ripening and postharvest cold storage.

Mechanical properties of cuticles and their primary determinants

Cuticles envelope primary surfaces of the above-ground portion of plants. They function as barriers to water movement and to gas exchange, and in pathogen defense. To serve as a barrier on growing organs, cuticles must remain intact but at the same time must accommodate ongoing growth. Minimizing cuticle failure has stimulated significant research on the cuticle's mechanical properties. The objective here is to review the literature on the mechanical properties of isolated fruit and leaf cuticles. Cuticles are viscoelastic polymers. Viscoelasticity results mainly from the cutin matrix. Impregnation by waxes, flavonoids, and cutan increases stiffness and strength but decreases extensibility. On the inner side, the cutin matrix is impregnated by cell wall polysaccharides, which are responsible for its elastic behavior. Across species, the maximum forces sustainable by hydrated cuticles in uniaxial tensile tests averaged 0.82 N (range 0.15-1.63 N), the maximum stresses averaged 13.2 MPa (range 2.0-29.0 MPa), the maximum strains averaged 8.8% (range 1.6-28.0%), and the moduli of elasticity averaged 224 MPa (range 60-730 MPa). Among the environmental factors, high temperature and hydration both decreased stiffness. Therefore, the mechanical properties of cuticles in vivo depend largely on the relative proportions of their constituents. These proportions change during development and are also affected by environmental factors such as temperature.

Husk to caryopsis adhesion in barley is influenced by pre- and post-anthesis temperatures through changes in a cuticular cementing layer on the caryopsis

BACKGROUND

At ripeness, the outer husk of "covered" barley grains firmly adheres to the underlying caryopsis. A cuticular cementing layer on the caryopsis is required for husk adhesion, however the quality of adhesion varies significantly among cultivars which produce the cementing layer, resulting in the economically important malting defect, grain skinning. The composition of the cementing layer, and grain organ development have been hypothesised to influence the quality of husk adhesion. Plants of Hordeum vulgare 'Concerto' were grown at different temperatures pre- and post-anthesis to effect changes in the development of the husk, caryopsis and cuticular cementing layer, to determine how these variables influence the quality of husk-to-caryopsis adhesion.

RESULTS

Warm conditions pre-anthesis decreased the quality of husk adhesion, and consequently increased the incidence of grain skinning. Cool post-anthesis conditions further decreased the quality of husk adhesion. The composition of the cementing layer, rather than its structure, differed with respect to husk adhesion quality. This cementing layer was produced at the late milk stage, occurring between nine and 29 days post-anthesis, conditional on the temperature-dependent growth rate. The compounds octadecanol, tritriacontane, campesterol and β-sitosterol were most abundant in caryopses with high-quality husk adhesion. The differences in adhesion quality were not due to incompatible husk and caryopsis dimensions affecting organ contact.

CONCLUSIONS

This study shows that husk-to-caryopsis adhesion is dependent on cementing layer composition, and implies that this composition is regulated by temperature before, and during grain development. Understanding this regulation will be key to improving husk-to-caryopsis adhesion.

Analysis of cuticular wax constituents and genes that contribute to the formation of 'glossy Newhall', a spontaneous bud mutant from the wild-type 'Newhall' navel orange

Navel orange (Citrus sinensis [L.] Osbeck) fruit surfaces contain substantial quantities of cuticular waxes, which have important eco-physiological roles, such as water retention and pathogen defense. The wax constituents of ripe navel orange have been studied in various reports, while the wax changes occurring during fruit development and the molecular mechanism underlying their biosynthesis/export have not been investigated. Recently, we reported a spontaneous bud mutant from the wild-type (WT) 'Newhall' Navel orange. This mutant displayed unusual glossy fruit peels and was named 'glossy Newhall' (MT). In this study, we compared the developmental profiles of the epicuticular and intracuticular waxes on the WT and MT fruit surfaces. The formation of epicuticular wax crystals on the navel orange surface was shown to be dependent on the accumulation of high amounts of aliphatic wax components with trace amounts of terpenoids. In sharp contrast, the underlying intracuticular wax layers have relatively low concentrations of aliphatic wax components but high concentrations of cyclic wax compounds, especially terpenoids at the late fruit developmental stages. Our work also showed that many genes that are involved in wax biosynthesis and export pathways were down-regulated in MT fruit peels, leading to a decrease in aliphatic wax component amounts and the loss of epicuticular wax crystals, ultimately causing the glossy phenotype of MT fruits.

A focus on the biosynthesis and composition of cuticle in fruits

Cuticles are plant structures, composed mostly by lipidic layers, synthesized by nonwoody aerial plant organs and deposited on the surface of outer epidermal cell walls. Although its significance has been often disregarded, cuticle deposition modifies organ chemistry, influences mechanical properties, and plays a central role in sensing and interacting with the surrounding environment. Even though some research has been undertaken addressing cuticle biosynthesis and composition in vegetative plant tissues, comparatively less information is available regarding cuticle composition in the epidermis of fruits. However, recent work points to a role for cuticles in the modulation of fruit quality and postharvest performance, indicating that current models for the investigation of fruit development, metabolism, and quality need to integrate a comprehensive knowledge of the cuticle layer. This paper provides an overview of recent findings and observations regarding cuticle biosynthesis and composition in fruits from species of agronomic and economic relevance. Important, but often neglected differences in cuticle composition and biosynthesis patterns among diverse fruit species are described herein to generate an atlas of what is currently known about fruit cuticles and to highlight what remains to be explored. Emphasis is placed on the need to investigate each genetic background considering its own specificities, to permit correlations with the particular physiology of each species considered. Both specific composition and changes during maturation and ripening are reviewed.

Characterization of cuticle composition after cold storage of "Celeste" and "Somerset" sweet cherry fruit

Cuticle composition and structure may be relevant factors affecting the storage potential of fruits, but very few studies have analyzed fruit cuticle composition from a postharvest perspective. In this work, the chemical composition of waxes and cutin (major cuticular components) was analyzed in cuticle samples isolated from "Celeste" and "Somerset" cherries (Prunus avium L.) after cold storage at 0 °C. Total cuticle amounts per surface unit (μg cm(-2)) increased along with cold storage. The triterpene ursolic acid, the alkane nonacosane, linoleic acid, and β-sitosterol were the most abundant components of cuticular waxes, whereas cutin composition was dominated by C18-type monomers. In spite of being comprised of similar chemical families, cultivar-related differences were found regarding the abundance and the evolution of some compound families during cold storage. To the best of our knowledge, this is the first report on changes in cuticle composition of sweet cherry during postharvest storage.

Comparative analysis of surface wax in mature fruits between Satsuma mandarin (Citrus unshiu) and 'Newhall' navel orange (Citrus sinensis) from the perspective of crystal morphology, chemical composition and key gene expression

Surface wax of mature Satsuma mandarin (Citrus unshiu) and 'Newhall' navel orange (Citrus sinensis) was analysed by crystal morphology, chemical composition, and gene expression levels. The epicuticular and total waxes of both citrus cultivars were mostly composed of aldehydes, alkanes, fatty acids and primary alcohols. The epicuticular wax accounted for 80% of the total wax in the Newhall fruits and was higher than that in the Satsuma fruits. Scanning electron microscopy showed that larger and more wax platelets were deposited on the surface of Newhall fruits than on the Satsuma fruits. Moreover, the expression levels of genes involved in the wax formation were consistent with the biochemical and crystal morphological analyses. These diversities of fruit wax between the two cultivars may contribute to the differences of fruit postharvest storage properties, which can provide important information for the production of synthetic wax for citrus fruits.