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

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.

Cuticular wax metabolism responses to atmospheric water stress on the exocarp surface of litchi fruit after harvest

Litchi fruit is susceptible to pericarp browning, which is largely due to the oxidation of phenols in pericarp. However, the response of cuticular waxes to water loss of litchi after harvest is less mentioned. In this study, litchi fruits were stored under ambient, dry, water-sufficient, and packing conditions, while rapid pericarp browning and water loss from the pericarp were observed under the water-deficient conditions. The coverage of cuticular waxes on the fruit surface increased following the development of pericarp browning, during which quantities of very-long-chain (VLC) fatty acids, primary alcohols, and n-alkanes changed significantly. Genes involved in the metabolism of such compounds were upregulated, including LcLACS2, LcKCS1, LcKCR1, LcHACD, and LcECR for elongation of fatty acids, LcCER1 and LcWAX2 for n-alkanes, and LcCER4 for primary alcohols. These findings reveal that cuticular wax metabolism may take part in the response of litchi to water-deficient and pericarp browning during storage.

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.

Cuticles and postharvest life of tomato fruit: A rigid cover for aerial epidermis or a multifaceted guard of freshness?

Fruit cuticle is a specialized cell wall hydrophobic architecture covering the aerial surfaces of fruit, which forms the interface between the fruit and its environment. As a specialized seed-bearing organ, fruit utilize cuticles as physical barriers, water permeation regulator and resistance to pathogens, thus appealing extensive research interests for its potential values in developing postharvest freshness-keeping strategies. Here, we provide an overview for the composition and functions of fruit cuticles, mainly focusing on its functions in mechanical support, water permeability barrier and protection over pathogens, further introduce key mechanisms implicated in fruit cuticle biosynthesis. Moreover, currently available state-of-art techniques for examining compositional diversity and architecture of fruit are also compared.

Transcriptomics and gas chromatography-mass spectrometry metabolomics reveal the mechanism of heat shock combined with 1-methylcyclopropene to regulate the cuticle wax of jujube fruit during storage

Cuticle wax is closely related to fruit quality during storage. In this study, changes in epidermal wax morphology, composition, and genes regulation induced by heat shock (HT), 1-methylcyclopropene (1-MCP) or their combination (HT + 1-MCP) were investigated in jujube fruit during cold storage. HT, 1-MCP, or HT + 1-MCP caused a smoother wax layer and fewer micro-cracks compared to the control (CK) during cold storage. It was confirmed that acids and terpenoids were the main wax components by gas chromatography-mass spectrometry. HT + 1-MCP and 1-MCP treatments could significantly increase (p < 0.05) the wax content at 45 d of cold storage. The transcriptomics results indicated that HT + 1-MCP treatment up-regulated FATB, FATB, FAB2, FAD2 and CYP716A, and maintained the wax content of jujube fruit during cold storage. These results could provide new perspective for regulating the cuticle characteristics to extend the shelf life of jujube fruit.

Cranberry fruit epicuticular wax benefits and identification of a wax-associated molecular marker

BACKGROUND

As the global climate changes, periods of abiotic stress throughout the North American cranberry growing regions will become more common. One consequence of high temperature extremes and drought conditions is sunscald. Scalding damages the developing berry and reduces yields through fruit tissue damage and/or secondary pathogen infection. Irrigation runs to cool the fruit is the primary approach to controlling sunscald. However, it is water intensive and can increase fungal-incited fruit rot. Epicuticular wax functions as a barrier to various environmental stresses in other fruit crops and may be a promising feature to mitigate sunscald in cranberry. In this study we assessed the function of epicuticular wax in cranberries to attenuate stresses associated with sunscald by subjecting high and low epicuticular wax cranberries to controlled desiccation and light/heat exposure. A cranberry population that segregates for epicuticular wax was phenotyped for epicuticular fruit wax levels and genotyped using GBS. Quantitative trait loci (QTL) analyses of these data identified a locus associated with epicuticular wax phenotype. A SNP marker was developed in the QTL region to be used for marker assisted selection.

RESULTS

Cranberries with high epicuticular wax lost less mass percent and maintained a lower surface temperature following heat/light and desiccation experiments as compared to fruit with low wax. QTL analysis identified a marker on chromosome 1 at position 38,782,094 bp associated with the epicuticular wax phenotype. Genotyping assays revealed that cranberry selections homozygous for a selected SNP have consistently high epicuticular wax scores. A candidate gene (GL1-9), associated with epicuticular wax synthesis, was also identified near this QTL region.

CONCLUSIONS

Our results suggest that high cranberry epicuticular wax load may help reduce the effects of heat/light and water stress: two primary contributors to sunscald. Further, the molecular marker identified in this study can be used in marker assisted selection to screen cranberry seedlings for the potential to have high fruit epicuticular wax. This work serves to advance the genetic improvement of cranberry crops in the face of global climate change.

The Effects of Epicuticular Wax on Anthracnose Resistance of Sorghum bicolor

Cuticular waxes are mixtures of hydrophobic compounds covering land plant surfaces and play key roles in plant resistance to abiotic and biotic stresses. However, it is still not clear whether the epicuticular wax could protect the plants from infection by anthracnose, one of the most important plant diseases worldwide, which seriously infects sorghum and causes great yield loss. In this study, Sorghum bicolor L., an important C4 crop with high wax coverage, was selected to analyze the relationship between epicuticular wax (EW) and anthracnose resistance. In vitro analysis indicated that the sorghum leaf wax significantly inhibited the anthracnose mycelium growth of anthracnose on potato dextrose agar (PDA) medium, with the plaque diameter smaller than that grown on medium without wax. Then, the EWs were removed from the intact leaf with gum acacia, followed by the inoculation of Colletotrichum sublineola. The results indicated that the disease lesion was remarkably aggravated on leaves without EW, which showed decreased net photosynthetic rate and increased intercellular CO₂ concentrations and malonaldehyde content three days after inoculation. Transcriptome analysis further indicated that 1546 and 2843 differentially expressed genes (DEGs) were regulated by C. sublineola infection in plants with and without EW, respectively. Among the DEG encoded proteins and enriched pathways regulated by anthracnose infection, the cascade of the mitogen-activated protein kinases (MAPK) signaling pathway, ABC transporters, sulfur metabolism, benzoxazinoid biosynthesis, and photosynthesis were mainly regulated in plants without EW. Overall, the EW increases plant resistance to C. sublineola by affecting physiological and transcriptome responses through sorghum epicuticular wax, improving our understanding of its roles in defending plants from fungi and ultimately benefiting sorghum resistance breeding.

Comparative Untargeted Metabolic Profiling of Different Parts of Citrus sinensis Fruits via Liquid Chromatography-Mass Spectrometry Coupled with Multivariate Data Analyses to Unravel Authenticity

Differences between seven authentic samples of Citrus sinensis var. Valencia peel (albedo and flavedo) and juices from Spain and Uruguay, in addition to a concentrate obtained from Brazil, were investigated by untargeted metabolic profiling. Sixty-six metabolites were detected by nano-liquid chromatography coupled to a high-resolution electrospray-ionization quadrupole time-of-flight mass spectrometer (nLC-ESI-qTOF-MS) belonging to phenolic acids, coumarins, flavonoid glycosides, limonoids, terpenes, and fatty acids. Eleven metabolites were detected for the first time in Citrus sinensis and identified as citroside A, sinapic acid pentoside, apigenin-C-hexosyl-O-pentoside, chrysoeriol-C-hexoside, di-hexosyl-diosmetin, perilloside A, gingerol, ionone epoxide hydroxy-sphingenine, xanthomicrol, and coumaryl alcohol-O-hexoside. Some flavonoids were completely absent from the juice, while present most prominently in the Citrus peel, conveying more industrial and economic prospects to the latter. Multivariate data analyses clarified that the differences among orange parts overweighed the geographical source. PCA analysis of ESI-(-)-mode data revealed for hydroxylinoleic acid abundance in flavedo peel from Uruguay the most distant cluster from all others. The PCA analysis of ESI-(+)-mode data provided a clear segregation of the different Citrus sinensis parts primarily due to the large diversity of flavonoids and coumarins among the studied samples.

The role of cuticle in fruit shelf-life

Ensuring the availability of high-quality fresh fruits requires the development of strategies to maintain prolonged shelf-life. The plant cuticle is a modification of the outer epidermal cell wall and, as such, acts as a barrier with the environment. Understanding how the cuticle naturally changes during postharvest is crucial to address the potential effect of different storage conditions on the cuticle biophysical properties. The impact of different cuticle traits in fruit water loss, its relevance in several fruit-skin disorders, and its participation in postharvest decay caused by pathogens are discussed. Future challenges to study in vivo the physicochemical properties of the cuticle are also addressed.

The Effects of Storage Temperature, Light Illumination, and Low-Temperature Plasma on Fruit Rot and Change in Quality of Postharvest Gannan Navel Oranges

Gannan navel orange (Citrus sinensis Osbeck cv. Newhall) is an economically important fruit, but postharvest loss occurs easily during storage. In this study, the effects of different temperatures, light illuminations, and low-temperature plasma treatments on the water loss and quality of the Gannan navel orange were investigated. The fruit began to rot after 90 d of storage at 5 °C and 20-45 d at 26 °C. Navel oranges stored at 26 °C had 7.2-fold and 3.1-fold higher rates of water loss at the early and late storage stages, respectively, as compared with those stored at 5 °C. Storage at 5 °C decreased the contents of total soluble solids at the early storage stage and the contents of titratable acids at the late storage stage, whereas storage at 26 °C decreased the contents of total soluble solids at the late storage stage and the contents of titratable acids at the early storage stage, respectively. Application of low-temperature plasma produced by air ionization for 6 min, or continuous blue or red light illumination significantly inhibited water loss within 7 and 21 d of storage at 22 °C, respectively, but exhibited no significant effect on fruit quality. Furthermore, the low-temperature plasma treatment protected against fruit rot. Thus, treatment with low-temperature plasma followed by storage at a low temperature under continuous red or blue light illumination was of potential value as a green technology for preserving Gannan navel orange during storage.

Comprehensive Genome-Wide Identification and Expression Profiling of Eceriferum (CER) Gene Family in Passion Fruit (Passiflora edulis) Under Fusarium kyushuense and Drought Stress Conditions

Plant surfaces are covered with cuticle wax and are the first barrier between a plant and environmental stresses. Eceriferum (CER) is an important gene family involved in wax biosynthesis and stress resistance. In this study, for the first time, 34 CER genes were identified in the passion fruit (Passiflora edulis) genome, and PeCER proteins varied in physicochemical properties. A phylogenetic tree was constructed and divided into seven clades to identify the evolutionary relationship with other plant species. Gene structure analyses revealed that conserved motifs ranged from 1 to 24, and that exons ranged from 1 to 29. The cis-element analysis provides insight into possible roles of PeCER genes in plant growth, development and stress responses. The syntenic analysis revealed that segmental (six gene pairs) and tandem (six gene pairs) gene duplication played an important role in the expansion of PeCER genes and underwent a strong purifying selection. In addition, 12 putative ped-miRNAs were identified to be targeting 16 PeCER genes, and PeCER6 was the most targeted by four miRNAs including ped-miR157a-5p, ped-miR164b-5p, ped-miR319b, and ped-miR319l. Potential transcription factors (TFs) such as ERF, AP2, MYB, and bZIP were predicted and visualized in a TF regulatory network interacting with PeCER genes. GO and KEGG annotation analysis revealed that PeCER genes were highly related to fatty acid, cutin, and wax biosynthesis, plant-pathogen interactions, and stress response pathways. The hypothesis that most PeCER proteins were predicted to localize to the plasma membrane was validated by transient expression assays of PeCER32 protein in onion epidermal cells. qRT-PCR expression results showed that most of the PeCER genes including PeCER1, PeCER11, PeCER15, PeCER17, and PeCER32 were upregulated under drought and Fusarium kyushuense stress conditions compared to controls. These findings provide a foundation for further studies on functions of PeCER genes to further facilitate the genetic modification of passion fruit wax biosynthesis and stress resistance.

Transcriptome and Physiological Analyses of a Navel Orange Mutant with Improved Drought Tolerance and Water Use Efficiency Caused by Increases of Cuticular Wax Accumulation and ROS Scavenging Capacity

Drought is one of the main abiotic stresses limiting the quality and yield of citrus. Cuticular waxes play an important role in regulating plant drought tolerance and water use efficiency (WUE). However, the contribution of cuticular waxes to drought tolerance, WUE and the underlying molecular mechanism is still largely unknown in citrus. 'Longhuihong' (MT) is a bud mutant of 'Newhall' navel orange with curly and bright leaves. In this study, significant increases in the amounts of total waxes and aliphatic wax compounds, including n-alkanes, n-primary alcohols and n-aldehydes, were overserved in MT leaves, which led to the decrease in cuticular permeability and finally resulted in the improvements in drought tolerance and WUE. Compared to WT leaves, MT leaves possessed much lower contents of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), significantly higher levels of proline and soluble sugar, and enhanced superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) activities under drought stress, which might reduce reactive oxygen species (ROS) damage, improve osmotic regulation and cell membrane stability, and finally, enhance MT tolerance to drought stress. Transcriptome sequencing results showed that seven structural genes were involved in wax biosynthesis and export, MAPK cascade, and ROS scavenging, and seven genes encoding transcription factors might play an important role in promoting cuticular wax accumulation, improving drought tolerance and WUE in MT plants. Our results not only confirmed the important role of cuticular waxes in regulating citrus drought resistance and WUE but also provided various candidate genes for improving citrus drought tolerance and WUE.

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.

Ethylene-driven changes in epicuticular wax metabolism in citrus fruit

Epicuticular waxes are important natural compounds that influence cuticle properties and can protect fruit from factors that harm its external quality. We demonstrated that, at a dose that reduces postharvest citrus fruit quality loss (4 d 2 µL L^-1), ethylene redirected epicuticular wax metabolism towards the synthesis of primary alcohols, mostly behenyl alcohol, by favouring the acyl-reduction pathway. This treatment also reduced the synthesis of terpenoids by redirecting the mevalonate pathway towards farnesol accumulation to the detriment of the accumulation of most triterpenoids, but not of their precursor squalene. Moreover, the 4 d ethylene treatment sharply increased the synthesis of docosane and lignoceric acid and lowered that of cerotic acid. Longer ethylene exposure (8 d) reversed some of these effects by lowering the contents of most alcohols, lignoceric acid and squalene, while increasing that of its derivative sitosterol. The 8 d ethylene treatment also increased farnesol and docosane contents.

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.

A Guide to Elucidate the Hidden Multicomponent Layered Structure of Plant Cuticles by Raman Imaging

The cuticle covers almost all plant organs as the outermost layer and serves as a transpiration barrier, sunscreen, and first line of defense against pathogens. Waxes, fatty acids, and aromatic components build chemically and structurally diverse layers with different functionality. So far, electron microscopy has elucidated structure, while isolation, extraction, and analysis procedures have revealed chemistry. With this method paper, we close the missing link by demonstrating how Raman microscopy gives detailed information about chemistry and structure of the native cuticle on the microscale. We introduce an optimized experimental workflow, covering the whole process of sample preparation, Raman imaging experiment, data analysis, and interpretation and show the versatility of the approach on cuticles of a spruce needle, a tomato peel, and an Arabidopsis stem. We include laser polarization experiments to deduce the orientation of molecules and multivariate data analysis to separate cuticle layers and verify their molecular composition. Based on the three investigated cuticles, we discuss the chemical and structural diversity and validate our findings by comparing models based on our spectroscopic data with the current view of the cuticle. We amend the model by adding the distribution of cinnamic acids and flavonoids within the cuticle layers and their transition to the epidermal layer. Raman imaging proves as a non-destructive and fast approach to assess the chemical and structural variability in space and time. It might become a valuable tool to tackle knowledge gaps in plant cuticle research.

Soluble cuticular wax composition and antimicrobial activity of the fruits of Chaenomeles species and an interspecific hybrid

Plants of the genus Chaenomeles Lindl. (Rosaceae) naturally grow in Southeast Asia and represent the richest resource of biologically active compounds with beneficial properties for humans. Plants of C. japonica (Thunb.) Lindl. and C. speciosa (Sweet) Nakai species, and interspecific hybrid C. × superba (Frahm) Rehder (C. japonica × C. speciosa, Superba group) have been successfully introduced in the steppe zone of Ukraine and bear fruits. In this study, we evaluated chemical composition of fruit cuticular waxes and antimicrobial activity of fruit extracts. The soluble waxes were characterized using gas chromatography-mass spectrometry (GC-MS), and 26–36 compounds, representing 91.7–96.6% of the total soluble cuticular waxes, were identified. Waxes of Chaenomeles fruits belonged to six classes, namely fatty acids, alcohols, aldehydes, esters, ethers and alkanes. Aldehydes 7-hexadecenal and heptacosanal, and alkanes hexatriacontane and tetrapentacontane were the main constituents in the soluble cuticular waxes of C. speciosa and C. × superba fruits, accounting for more than half of the total contents. However, alkane tetrapentacontane, alcohol 8,10-hexadecadien-1-ol and heptacosanal prevailed in C. japonica fruit waxes. Isopropanolic fruit extracts exhibited dose-dependent antimicrobial activity against four Gram-negative bacteria, five Gram-positive bacteria and one fungal strain in the disc diffusion assay. In general, extracts from the Chaenomeles fruits demonstrated higher activity against Gram+ bacteria than Gram- strains. The strongest inhibiting activity was shown against Staphylococcus epidermidis (by the fruit extracts of C. × superba and C. speciosa), Micrococcus lysodeikticus and Candida albicans (both by C. × superba fruit extract). Results of the study confirmed accumulation of the bioactive compounds in the fruit waxes of different Chaenomeles species and antimicrobial ability of Chaenomeles fruits as well. These findings revealed the bioactive compounds in fruit cuticular waxes and suggested health-promoting properties of introduced Chaenomeles species.

Soluble cuticular wax composition and antimicrobial activity of the fruits of Chaenomeles species and an interspecific hybrid

Plants of the genus Chaenomeles Lindl. (Rosaceae) naturally grow in Southeast Asia and represent the richest resource of biologically active compounds with beneficial properties for humans. Plants of C. japonica (Thunb.) Lindl. and C. speciosa (Sweet) Nakai species, and interspecific hybrid C. × superba (Frahm) Rehder (C. japonica × C. speciosa, Superba group) have been successfully introduced in the steppe zone of Ukraine and bear fruits. In this study, we evaluated chemical composition of fruit cuticular waxes and antimicrobial activity of fruit extracts. The soluble waxes were characterized using gas chromatography-mass spectrometry (GC-MS), and 26–36 compounds, representing 91.7–96.6% of the total soluble cuticular waxes, were identified. Waxes of Chaenomeles fruits belonged to six classes, namely fatty acids, alcohols, aldehydes, esters, ethers and alkanes. Aldehydes 7-hexadecenal and heptacosanal, and alkanes hexatriacontane and tetrapentacontane were the main constituents in the soluble cuticular waxes of C. speciosa and C. × superba fruits, accounting for more than half of the total contents. However, alkane tetrapentacontane, alcohol 8,10-hexadecadien-1-ol and heptacosanal prevailed in C. japonica fruit waxes. Isopropanolic fruit extracts exhibited dose-dependent antimicrobial activity against four Gram-negative bacteria, five Gram-positive bacteria and one fungal strain in the disc diffusion assay. In general, extracts from the Chaenomeles fruits demonstrated higher activity against Gram+ bacteria than Gram- strains. The strongest inhibiting activity was shown against Staphylococcus epidermidis (by the fruit extracts of C. × superba and C. speciosa), Micrococcus lysodeikticus and Candida albicans (both by C. × superba fruit extract). Results of the study confirmed accumulation of the bioactive compounds in the fruit waxes of different Chaenomeles species and antimicrobial ability of Chaenomeles fruits as well. These findings revealed the bioactive compounds in fruit cuticular waxes and suggested health-promoting properties of introduced Chaenomeles species.

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.

Albedo- and Flavedo-Specific Transcriptome Profiling Related to Penicillium digitatum Infection in Citrus Fruit

Penicillium digitatum is the main postharvest pathogen of citrus fruit. Although the inner fruit peel part (albedo) is less resistant than the outer part (flavedo) to P. digitatum, the global mechanisms involved in their different susceptibility remain unknown. Here, we examine transcriptome differences between both tissues at fruit harvest and in their early responses to infection. At harvest, not only was secondary metabolism, involving phenylpropanoids, waxes, and terpenoids, generally induced in flavedo vs. albedo, but also energy metabolism, transcription factors (TFs), and biotic stress-related hormones and proteins too. Flavedo-specific induced responses to infection might be regulated in part by ERF1 TF, and are related to structural plant cell wall reinforcement. Other induced responses may be related to H₂O₂, the synthesis of phenylpropanoids, and the stress-related proteins required to maintain basal defense responses against virulent pathogens, whereas P. digitatum represses some hydrolase-encoding genes that play different functions and auxin-responsive genes in this peel tissue. In infected albedo, the repression of transport and signal transduction prevail, as does the induction of not only the processes related to the synthesis of flavonoids, indole glucosinolates, cutin, and oxylipins, but also the specific genes that elicit plant immunity against pathogens.

Update on Cuticular Wax Biosynthesis and Its Roles in Plant Disease Resistance

The aerial surface of higher plants is covered by a hydrophobic layer of cuticular waxes to protect plant tissues against enormous environmental challenges including the infection of various pathogens. As the first contact site between plants and pathogens, the layer of cuticular waxes could function as a plant physical barrier that limits the entry of pathogens, acts as a reservoir of signals to trigger plant defense responses, and even gives cues exploited by pathogens to initiate their infection processes. Past decades have seen unprecedented proceedings in understanding the molecular mechanisms underlying the biosynthesis of plant cuticular waxes and their functions regulating plant–pathogen interactions. In this review, we summarized the recent progress in the molecular biology of cuticular wax biosynthesis and highlighted its multiple roles in plant disease resistance against bacterial, fungal, and insect pathogens.