Genome investigation suggests MdSHN3, an APETALA2-domain transcription factor gene, to be a positive regulator of apple fruit cuticle formation and an inhibitor of russet development

Insertion of the β-ketoacyl-CoA synthase MdKCS2 promoter segment causes wax biosynthesis difference in apple peel

Cuticular wax is essential for fruit to maintain moisture. Although the wax content of peel surface in apple (Malus spp.) varies, the detailed molecular mechanism remains unclear. Here, we identified the β-ketoacyl-CoA synthase 2 (MdKCS2) differentially expressed between apple peel with low and high wax content by integrating bulked segregant analysis-sequencing and RNA-seq. We found that a 63-bp insertion in the MdKCS2 promoter was the primary reason for apple peel with low wax content. The 63-bp insertion reduced MdKCS2 promoter activity and enhanced the DNA binding with the suppressor MdDOF4.6, decreasing wax biosynthesis by reducing C24 very-long-chain fatty acid (VLCFA). ECERIFERUM 2 (MdCER2) was co-expressed with MdKCS2 and suppressed by MdMYB56, MdbHLH137 and MdDOF4.6, further decreasing C29 alkane content in apple peel with low C24 VLCFA content. Overall, MdKCS2 and MdCER2 are coordinately involved in the wax production of apple peel surface.

Update on the structure and regulated biosynthesis of the apoplastic polymers cutin and suberin

The plant lipid polymers cutin and suberin play a critical role in many aspects of plant growth, development, and physiology. The mechanisms of cutin and suberin biosynthesis are relatively well understood thanks to just over 2 decades of work with primarily Arabidopsis (Arabidopsis thaliana) mutants. Recent advances in our understanding of cutin and suberin structure have arisen through the application of novel chemistries targeted at quantitative comprehension of intermolecular linkages, isolating intact suberins and cutins, and the application of advanced analytical techniques. The advent of high-throughput transcription factor binding assays and next-generation sequencing has facilitated the discovery of numerous cutin and suberin-regulating transcription factors and their gene promoter targets. Herein we provide an overview of aspects of cutin and suberin structure, biosynthesis, and transcriptional regulation of their synthesis highlighting recent developments in our understanding of these facets of cutin and suberin biology. We further identify outstanding questions in these respective areas and provide perspectives on how to advance the field to address these questions.

Apple russeting-causes, physiology and control measures: A review

MAIN CONCLUSION

This review serves as a critical framework for guiding future research into the causes of russeting and the development of effective control strategies to enhance fruit quality. Russeting is a condition characterized by the formation of brown, corky patches on fruit skin which significantly impairs both the quality and market value of apples. This phenomenon arises from a complex interplay of various biotic and abiotic factors. Among the abiotic factors, environmental conditions, such as light, temperature, and relative humidity, as well as nutrient imbalances and the application of agrochemicals are important, whereas biotic factors include the influence of yeasts, fungi, viruses, and bacteria. The susceptibility of apple cultivars to russeting varies with yellow-fleshed varieties generally exhibiting higher incidences compared to red-fleshed ones. While russeting is partly determined by varietal and genetic factors, it can be mitigated through the implementation of effective cultural practices, nutrient management, plant growth regulators, biological agents, and pesticides. Understanding these dynamics provides valuable insights for developing future research strategies aimed at improving fruit quality and production.

Revisiting plant cuticle biophysics

The plant cuticle is located at the interface of the plant with the environment, thus acting as a protective barrier against biotic and abiotic external stress factors, and regulating water loss. Additionally, it modulates mechanical stresses derived from internal tissues and also from the environment. Recent advances in the understanding of the hydric, mechanical, thermal, and, to a lower extent, optical and electric properties of the cuticle, as well as their phenomenological connections and relationships are reviewed. An equilibrium based on the interaction among the different biophysical properties is essential to ensure plant growth and development. The notable variability reported in cuticle geometry, surface topography, and microchemistry affects the analysis of some biophysical properties of the cuticle. This review aimed to provide an updated view of the plant cuticle, understood as a modification of the cell wall, in order to establish the state-of-the-art biophysics of the plant cuticle, and to serve as an inspiration for future research in the field.

NtWIN1 regulates the biosynthesis of scopoletin and chlorogenic acid by targeting NtF6'H1 and NtCCoAMT genes in Nicotiana tabacum

Scopoletin and chlorogenic acid (CGA) are important polyphenols that regulate plant growth, development, and stress resistance. The ERF transcription factor WAX INDUCER1 (WIN1) promotes the biosynthesis of cutin, suberine, and wax. However, its full roles in regulating the accumulation of plant secondary metabolites still remain to be further clarified. In this study, NtWIN1 gene encoding a SHINE-type AP2/ERF transcription factor of the Va subgroup was identified from N. tabacum. NtWIN1 showed high expression levels in tobacco stems, sepals, and pistils. Overexpression (OE) and knock-out of NtWIN1 showed that it promoted the accumulation of total polyphenols and altered their composition. Compare to that of WT plants, the CGA contents significantly increased by 25%-50% in the leaves, flowers, and capsules of OE lines, while the scopoletin contents in the OE plants significantly decreased by 30%-67%. In contrast, the CGA contents in ntwin1 lines reduced by 23%-26%, and the scopoletin contents in ntwin1 increased by 38%-75% compare to that of WT plants. Chromatin immunoprecipitation and Dual-Luc transcription activation assays showed that NtWIN1 could bind to the promoters of NtF6'H1 and NtCCoAMT, thereby modulating their expression. The scopoletin content in ntwin1/ntf6'h1 double mutant was significantly lower than that in ntwin1 and WT plants, but showed no significant differences with that in ntf6'h1 mutant, further indicating that the inhibition of NtWIN1 on scopoletin accumulation depends on the activity of NtF6'H1. Our study illustrates the new roles of NtWIN1, and provides a possible target for regulating the synthesis of polyphenols in tobacco.

Ripening and rot: How ripening processes influence disease susceptibility in fleshy fruits

Fleshy fruits become more susceptible to pathogen infection when they ripen; for example, changes in cell wall properties related to softening make it easier for pathogens to infect fruits. The need for high-quality fruit has driven extensive research on improving pathogen resistance in important fruit crops such as tomato (Solanum lycopersicum). In this review, we summarize current progress in understanding how changes in fruit properties during ripening affect infection by pathogens. These changes affect physical barriers that limit pathogen entry, such as the fruit epidermis and its cuticle, along with other defenses that limit pathogen growth, such as preformed and induced defense compounds. The plant immune system also protects ripening fruit by recognizing pathogens and initiating defense responses involving reactive oxygen species production, mitogen-activated protein kinase signaling cascades, and jasmonic acid, salicylic acid, ethylene, and abscisic acid signaling. These phytohormones regulate an intricate web of transcription factors (TFs) that activate resistance mechanisms, including the expression of pathogenesis-related genes. In tomato, ripening regulators, such as RIPENING INHIBITOR and NON_RIPENING, not only regulate ripening but also influence fruit defenses against pathogens. Moreover, members of the ETHYLENE RESPONSE FACTOR (ERF) family play pivotal and distinct roles in ripening and defense, with different members being regulated by different phytohormones. We also discuss the interaction of ripening-related and defense-related TFs with the Mediator transcription complex. As the ripening processes in climacteric and non-climacteric fruits share many similarities, these processes have broad applications across fruiting crops. Further research on the individual contributions of ERFs and other TFs will inform efforts to diminish disease susceptibility in ripe fruit, satisfy the growing demand for high-quality fruit and decrease food waste and related economic losses.

Beyond skin-deep: targeting the plant surface for crop improvement

The above-ground plant surface is a well-adapted tissue layer that acts as an interface between the plant and its surrounding environment. As such, its primary role is to protect against desiccation and maintain the gaseous exchange required for photosynthesis. Further, this surface layer provides a barrier against pathogens and herbivory, while attracting pollinators and agents of seed dispersal. In the context of agriculture, the plant surface is strongly linked to post-harvest crop quality and yield. The epidermal layer contains several unique cell types adapted for these functions, while the non-lignified above-ground plant organs are covered by a hydrophobic cuticular membrane. This review aims to provide an overview of the latest understanding of the molecular mechanisms underlying crop cuticle and epidermal cell formation, with focus placed on genetic elements contributing towards quality, yield, drought tolerance, herbivory defence, pathogen resistance, pollinator attraction, and sterility, while highlighting the inter-relatedness of plant surface development and traits. Potential crop improvement strategies utilizing this knowledge are outlined in the context of the recent development of new breeding techniques.

Time course of changes in the transcriptome during russet induction in apple fruit

BACKGROUND

Russeting is a major problem in many fruit crops. Russeting is caused by environmental factors such as wounding or moisture exposure of the fruit surface. Despite extensive research, the molecular sequence that triggers russet initiation remains unclear. Here, we present high-resolution transcriptomic data by controlled russet induction at very early stages of fruit development. During Phase I, a patch of the fruit surface is exposed to surface moisture. For Phase II, moisture exposure is terminated, and the formerly exposed surface remains dry. We targeted differentially expressed transcripts as soon as 24 h after russet induction.

RESULTS

During moisture exposure (Phase I) of 'Pinova' apple, transcripts associated with the cell cycle, cell wall, and cuticle synthesis (SHN3) decrease, while those related to abiotic stress increase. NAC35 and MYB17 were the earliest induced genes during Phase I. They are therefore linked to the initial processes of cuticle microcracking. After moisture removal (Phase II), the expression of genes related to meristematic activity increased (WOX4 within 24 h, MYB84 within 48 h). Genes related to lignin synthesis (MYB52) and suberin synthesis (MYB93, WRKY56) were upregulated within 3 d after moisture removal. WOX4 and AP2B3 are the earliest differentially expressed genes induced in Phase II. They are therefore linked to early events in periderm formation. The expression profiles were consistent between two different seasons and mirrored differences in russet susceptibility in a comparison of cultivars. Furthermore, expression profiles during Phase II of moisture induction were largely identical to those following wounding.

CONCLUSIONS

The combination of a unique controlled russet induction technique with high-resolution transcriptomic data allowed for the very first time to analyse the formation of cuticular microcracks and periderm in apple fruit immediately after the onset of triggering factors. This data provides valuable insights into the spatial-temporal dynamics of russeting, including the synthesis of cuticles, dedifferentiation of cells, and impregnation of cell walls with suberin and lignin.

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.

Microscopic and metabolic investigations disclose the factors that lead to skin cracking in chili-type pepper fruit varieties

The hydrophobic cuticle encasing the fruit skin surface plays critical roles during fruit development and post-harvest. Skin failure often results in the fruit surface cracking and forming a wound-periderm tissue made of suberin and lignin. The factors that make the fruit skin susceptible to cracking have yet to be fully understood. Herein, we investigated two varieties of chili peppers (Capsicum annuum L.), Numex Garnet, whose fruit has intact skin, and Vezena Slatka, whose fruit has cracked skin. Microscopical observations, gas chromatography-mass spectrometry, biochemical and gene expression assays revealed that Vezena Slatka fruit form a thicker cuticle with greater levels of cutin monomers and hydroxycinnamic acids, and highly express key cutin-related genes. The skin of these fruit also had a lower epidermal cell density due to cells with very large perimeters, and highly express genes involved in epidermal cell differentiation. We demonstrate that skin cracking in the Vezena Slatka fruit is accompanied by a spatial accumulation of lignin-like polyphenolic compounds, without the formation of a typical wound-periderm tissues made of suberized cells. Lastly, we establish that skin cracking in chili-type pepper significantly affects fruit quality during post-harvest storage in a temperature-dependent manner. In conclusion, our data highlight cuticle thickness and epidermal cell density as two critical factors determining fruit skin susceptibility to cracking in chili-type pepper fruit.

AaWIN1, an AP2/ERF protein, positively regulates glandular secretory trichome initiation in Artemisia annua

Exploring the genetic network of glandular trichomes and manipulating genes relevant to secondary metabolite biosynthesis are of great importance and value. Artemisinin, a key antimalarial drug ingredient, is synthesized and stored in glandular secretory trichomes (GSTs) in Artemisia annua. WIN/SHN proteins, a clade of AP2/ERF family, are known as regulators for cuticle biosynthesis. However, their function in glandular trichome development is less unknown. In this study, we identified a WIN/SHN gene from A. annua and named it as AaWIN1. AaWIN1 was predominantly expressed in buds, flowers and trichomes, and encoded a nuclear-localized protein. Overexpressing AaWIN1 in A. annua significantly increased the density of GST as well as the artemisinin content. Furthermore, AaGSW2 was reported to play an important role in promoting GST initiation, and the expression of AaGSW2 was induced in AaWIN1-overexpression lines. AaMIXTA1, a MYB protein positively regulating trichome initiation and cuticle biosynthesis, was confirmed to interact with AaWIN1. In addition, the ectopic expression of AaWIN1 resulted in slender and curled leaves, fewer trichomes, and rising expressions of cuticle biosynthesis genes in Arabidopsis thaliana. Taken together, based on phenotype observations, content measurements and gene expression detections, AaWIN1 was considered as a positive regulator for GST initiation in A. annua.

Characterization of MdMYB68, a suberin master regulator in russeted apples

INTRODUCTION

Apple russeting is mainly due to the accumulation of suberin in the cell wall in response to defects and damages in the cuticle layer. Over the last decades, massive efforts have been done to better understand the complex interplay between pathways involved in the suberization process in model plants. However, the regulation mechanisms which orchestrate this complex process are still under investigation. Our previous studies highlighted a number of transcription factor candidates from the Myeloblastosis (MYB) transcription factor family which might regulate suberization in russeted or suberized apple fruit skin. Among these, we identified MdMYB68, which was co-expressed with number of well-known key suberin biosynthesis genes.

METHOD

To validate the MdMYB68 function, we conducted an heterologous transient expression in Nicotiana benthamiana combined with whole gene expression profiling analysis (RNA-Seq), quantification of lipids and cell wall monosaccharides, and microscopy.

RESULTS

MdMYB68 overexpression is able to trigger the expression of the whole suberin biosynthesis pathway. The lipid content analysis confirmed that MdMYB68 regulates the deposition of suberin in cell walls. Furthermore, we also investigated the alteration of the non-lipid cell wall components and showed that MdMYB68 triggers a massive modification of hemicelluloses and pectins. These results were finally supported by the microscopy.

DISCUSSION

Once again, we demonstrated that the heterologous transient expression in N. benthamiana coupled with RNA-seq is a powerful and efficient tool to investigate the function of suberin related transcription factors. Here, we suggest MdMYB68 as a new regulator of the aliphatic and aromatic suberin deposition in apple fruit, and further describe, for the first time, rearrangements occurring in the carbohydrate cell wall matrix, preparing this suberin deposition.

Fine mapping and candidate gene analysis of CaFCD1 affecting cuticle biosynthesis in Capsicum annuum L

CaFCD1 gene regulates pepper cuticle biosynthesis. Pepper (Capsicum annuum L.) is an economically important vegetable crop that easily loses water after harvesting, which seriously affects the quality of its product. The cuticle is the lipid water-retaining layer on the outside of the fruit epidermis, which regulates the biological properties and reduces the rate of water-loss. However, the key genes involved in pepper fruit cuticle development are poorly understood. In this study, a pepper fruit cuticle development mutant fcd1 (fruit cuticle deficiency 1) was obtained by ethyl methanesulfonate mutagenesis. The mutant has great defects in fruit cuticle development, and the fruit water-loss rate of fcd1is significantly higher than that of the wild-type '8214' line. Genetic analysis suggested that the phenotype of the mutant fcd1 cuticle development defect was controlled by a recessive candidate gene CaFCD1 (Capsicum annuum fruit cuticle deficiency 1) on chromosome 12, which is mainly transcribed during fruit development. In fcd1, a base substitution within the CaFCD1 domain resulted in the premature termination of transcription, which affected cutin and wax biosynthesis in pepper fruit, as revealed by the GC-MS and RNA-seq analysis. Furthermore, the yeast one-hybrid and dual-luciferase reporter assays verified that the cutin synthesis protein CaCD2 was directly bound to the promoter of CaFCD1, suggesting that CaFCD1 may be a hub node in the cutin and wax biosynthetic regulatory network in pepper. This study provides a reference for candidate genes of cuticle synthesis and lays a foundation for breeding excellent pepper varieties.

Molecular mechanisms involved in fruit cracking: A review

Several fleshy fruits are highly affected by cracking, a severe physiological disorder that compromises their quality and causes high economical losses to the producers. Cracking can occur due to physiological, genetic or environmental factors and may happen during fruit growth, development and ripening. Moreover, in fleshy fruits, exocarp plays an important role, acting as a mechanical protective barrier, defending against biotic or abiotic factors. Thus, when biochemical properties of the cuticle + epidermis + hypodermis are affected, cracks appear in the fruit skin. The identification of genes involved in development such as cell wall modifications, biosynthesis and transport of cuticular waxes, cuticular membrane deposition and associated transcription factors provides new insights to better understand how fruit cracking is affected by genetic factors. Amongst the major environmental stresses causing cracking are excessive water during fruit development, leading to imbalances in cations such as Ca. This review focus on expression of key genes in these pathways, in their influence in affected fruits and the potential for molecular breeding programs, aiming to develop cultivars more resistant to cracking under adverse environmental conditions.

A banana transcriptional repressor MaAP2a participates in fruit starch degradation during postharvest ripening

Fruit postharvest ripening is a crucial course for many fruits with significant conversion of biosubstance, which forms an intricate regulatory network. Ethylene facilitates the ripening process in banana with a remarkable change of fruit starch, but the mechanism adjusting the expression of starch degradation-related enzyme genes is incompletely discovered. Here, we describe a banana APETALA2 transcription factor (MaAP2a) identified as a transcriptional repressor with its powerful transcriptional inhibitory activity. The transcriptional level of MaAP2a gradually decreased with the transition of banana fruit ripening, suggesting a passive role of MaAP2a in banana fruit ripening. Moreover, MaAP2a is a classic nucleoprotein and encompasses transcriptional repressor domain (EAR, LxLxLx). More specifically, protein-DNA interaction assays found that MaAP2a repressed the expression of 15 starch degradation-related genes comprising MaGWD1, MaPWD1, MaSEX4, MaLSF1, MaBAM1-MaBAM3, MaAMY2B/2C/3A/3C, MaMEX1/2, and MapGlcT2-1/2-2 via binding to the GCC-box or AT-rich motif of their promoters. Overall, these results reveal an original MaAP2a-mediated negative regulatory network involved in banana postharvest starch breakdown, which advances our cognition on banana fruit ripening and offers additional reference values for banana varietal improvement.

Transcriptome and metabolome analyses reveal phenotype formation differences between russet and non-russet apples

The apple is an economically important fruit, and fruit russeting is not conducive to its appearance. Although studies have examined fruit russeting, its mechanism remains unclear. Two apple strains of the F₁ hybrid population derived from 'Fuji' and 'Golden Delicious' were used in this study. We found that the skin of russet apples was rough and fissured, while that of non-russet apples was smooth and waxy. Chemical staining, LC- and GC-MS showed that both lignin and suberin were increased in russet apple skin. Meanwhile, genes involved in lignin and suberin synthetic pathways were upregulated in russet apple skin. Additionally, we found many differentially expressed genes (DEGs¹) involved in hormone biosynthesis and signaling and stress responses in the two apple strains. We found that WRKY13 may influence russeting by regulating lignin synthesis. Our study identified several candidate metabolites and genes, which will provide a good foundation for further research.

Solving the regulation puzzle of periderm development using advances in fruit skin

Periderm protects enlarged organs of most dicots and gymnosperms as a barrier to water loss and disease invasion during their secondary growth. Its development undergoes a complex process with genetically controlled and environmental stress-induced characters. Different development of periderm makes the full and partial russet of fruit skin, which diverges in inheritance with qualitative and quantitative characters, respectively, in pear pome. In addition to its specific genetics, fruit periderm has similar development and structure as that of stem and other organs, making it an appropriate material for periderm research. Recently, progress in histochemical as well as transcriptome and proteome analyses, and quantitative trait locus (QTL) mapping have revealed the regulatory molecular mechanism in the periderm based on the identification of switch genes. In this review, we concentrate on the periderm development, propose the conservation of periderm regulation between fruit and other plant organs based on their morphological and molecular characteristics, and summarize a regulatory network with the elicitors and repressors for the tissue development. Spontaneous programmed-cell death (PCD) or environmental stress produces the original signal that triggers the development of periderm. Spatio-temporal specific PCD produced by PyPPCD1 gene and its homologs can play a key role in the coordinated regulation of cell death related tissue development.

Functional copy number variation of CsSHINE1 is associated with fruit skin netting intensity in cucumber, Cucumis sativus

Fruit skin netting in cucumber (Cucumis sativus) is associated with important fruit quality attributes. Two simply inherited genes H (Heavy netting) and Rs (Russet skin) control skin netting, but their molecular basis is unknown. Here, we reported map-based cloning and functional characterization of the candidate gene for the Rs locus that encodes CsSHINE1 (CsSHN1), an AP2 domain containing ethylene-responsive transcription factor protein. Comparative phenotypic analysis in near-isogenic lines revealed that fruit with netted skin had different epidermal structures from that with smooth skin including thicker cuticles, smaller, palisade-shaped epidermal and sub-epidermal cells with heavily suberized and lignified cell walls, higher peroxidase activities, which suggests multiple functions of CsSHN1 in regulating fruit skin netting and epidermal cell patterning. Among three representative cucumber inbred lines, three haplotypes at three polymorphic sites were identified inside CsSHN1: a functional copy in Gy14 (wild type) with light fruit skin netting, a copy number variant with two tandemly arrayed functional copies in WI7120 with heavy skin netting, and a loss-of-function copy in 9930 with smooth skin. The expression level of CsSHN1 in fruit exocarp of three lines was positively correlated with the skin netting intensity. Comparative analysis between cucumber and melon revealed conserved and divergent genetic mechanisms underlying fruit skin netting/reticulation that may reflect the different selection histories in the two crops. A discussion was made on genetic basis of fruit skin netting in the context of natural and artificial selections of fruit quality-related epidermal features during cucumber breeding.

A Collection of Melon (Cucumis melo) Fruit Cultivars with Varied Skin Appearances Provide Insight to the Contribution of Suberin in Periderm Formation and Reticulation

At times of fruit skin failure, reticulation made of a wound-periderm is formed below the cracked skin in order to seal the damaged tissue. Preceding investigations shed light on the mechanisms underlying the formation of fruit skin reticulation, demonstrating that the walls of periderm cells are heavily suberized and lignified. However, the relative contribution of the suberin pathway to these processes, as well as the association between suberin contents in the periderm tissue and reticulation degree, are largely unknown. To strengthen our understanding on these important physiological and agricultural aspects, we comparatively profiled skin tissues of a collection of smooth- and reticulated-skin melon (Cucumis melo) cultivars for suberin monomer composition via gas chromatography-mass spectrometry (GC-MS). This metabolite profiling approach accompanied by statistical tools highlighted the fundamental chemical differences between the skin of smooth fruit made of a typical cuticle, to the skin of reticulated fruit made of large amounts of archetypal suberin building blocks including hydroxycinnamic acids, very long chain fatty acids, fatty alcohols, α-hydroxyacids, ω-hydroxyacids, and α,ω-diacids. Next, using image analysis we generated 'reticulation maps' and calculated the relative densities of reticulation. We then performed correlation assays in order to monitor suberin monomers that specifically correlate well with reticulation degree. Nonetheless, total suberin contents and most suberin building blocks did not show high correlations with reticulation degree, further suggesting that additional factors are likely to influence and regulate these processes. Altogether, the data provided vital information regarding the relative contribution of the suberin pathway to periderm formation and skin reticulation.

Two R2R3-MYB genes cooperatively control trichome development and cuticular wax biosynthesis in Prunus persica

The fruit surface has an enormous impact on the external appearance and postharvest shelf-life of fruit. Here, we report two functionally redundant genes, PpMYB25 and PpMYB26, involved in regulation of fruit skin texture in peach. PpMYB25 can activate transcription of PpMYB26 and they both induce trichome development and cuticular wax accumulation, resulting in peach fruit with a fuzzy and dull appearance. By contrast, nonfunctional mutation of PpMYB25 caused by an insertional retrotransposon in the last exon in nectarine fails to activate transcription of PpMYB26, resulting in nectarine fruit with a smooth and shiny appearance due to loss of trichome initiation and decreased cuticular wax accumulation. Secondary cell wall biosynthesis in peach fruit pubescence is controlled by a transcriptional regulatory network, including the master regulator PpNAC43 and its downstream MYB transcription factors such as PpMYB42, PpMYB46 and PpMYB83. Our results show that PpMYB25 and PpMYB26 coordinately regulate fruit pubescence and cuticular wax accumulation and their simultaneous perturbation results in the origin of nectarine, which is botanically classified as a subspecies of peach.

Advances in Understanding the Causes, Molecular Mechanism, and Perspectives of Russeting on Tree Fruit

The external quality of fruit is one of its most important qualities; good external quality attracts consumers easily and increases the value of fruit. Fruit russeting is one of the factors that influences the external quality of fruit and has been studied in most horticultural plants. However, the molecular mechanism of russeting has never been discussed so far. In this review, we summarize the research progress on fruit russeting, including causes, microscopic histomorphology, composition, genetics, and regulation and made a series of elaboration on the current research on fruit russeting. This study aims to provide insights into the mechanisms underlying fruit russeting. It also puts forward ideas for research on fruit russeting, which may provide a reference for future research.

Identification of Novel Candidate Genes Involved in Apple Cuticle Integrity and Russeting-Associated Triterpene Synthesis Using Metabolomic, Proteomic, and Transcriptomic Data

Apple russeting develops on the fruit surface when skin integrity has been lost. It induces a modification of fruit wax composition, including its triterpene profile. In the present work, we studied two closely related apple varieties, 'Reinette grise du Canada' and 'Reinette blanche du Canada', which display russeted and non-russeted skin phenotypes, respectively, during fruit development. To better understand the molecular events associated with russeting and the differential triterpene composition, metabolomics data were generated using liquid chromatography coupled to high-resolution mass spectrometry (LC-HRMS) and combined with proteomic and transcriptomic data. Our results indicated lower expression of genes linked to cuticle biosynthesis (cutin and wax) in russet apple throughout fruit development, along with an alteration of the specialized metabolism pathways, including triterpene and phenylpropanoid. We identified a lipid transfer protein (LTP3) as a novel player in cuticle formation, possibly involved in the transport of both cutin and wax components in apple skin. Metabolomic data highlighted for the first time a large diversity of triterpene-hydroxycinnamates in russeted tissues, accumulation of which was highly correlated with suberin-related genes, including some enzymes belonging to the BAHD (HXXXD-motif) acyltransferase family. Overall, this study increases our understanding about the crosstalk between triterpene and suberin pathways.

The metabolic and proteomic repertoires of periderm tissue in skin of the reticulated Sikkim cucumber fruit

Suberized and/or lignified (i.e. lignosuberized) periderm tissue appears often on surface of fleshy fruit skin by mechanical damage caused following environmental cues or developmental programs. The mechanisms underlying lignosuberization remain largely unknown to date. Here, we combined an assortment of microscopical techniques with an integrative multi-omics approach comprising proteomics, metabolomics and lipidomics to identify novel molecular components involved in fruit skin lignosuberization. We chose to investigate the corky Sikkim cucumber (Cucumis sativus var. sikkimensis) fruit. During development, the skin of this unique species undergoes massive cracking and is coated with a thick corky layer, making it an excellent model system for revealing fundamental cellular machineries involved in fruit skin lignosuberization. The large-scale data generated provides a significant source for the field of skin periderm tissue formation in fleshy fruit and suberin metabolism.

Changes in Morphology, Metabolism and Composition of Cuticular Wax in Zucchini Fruit During Postharvest Cold Storage

Cuticle composition is an important economic trait in agriculture, as it is the first protective barrier of the plant against environmental conditions. The main goal of this work was to study the role of the cuticular wax in maintaining the postharvest quality of zucchini fruit, by comparing two commercial varieties with contrasting behavior against low temperatures; the cold-tolerant variety 'Natura', and the cold-sensitive 'Sinatra', as well as 'Sinatra' fruit with induced-chilling tolerance through a preconditioning treatment (15°C for 48 h). The freshly-harvested 'Natura' fruit had a well-detectable cuticle with a significant lower permeability and a subset of 15 up-regulated cuticle-related genes. SEM showed that zucchini epicuticular waxes mainly consisted of round-shaped crystals and clusters of them, and areas with more dense crystal deposition were found in fruit of 'Natura' and of preconditioned 'Sinatra'. The cuticular wax load per surface was higher in 'Natura' than in 'Sinatra' fruit at harvest and after 14 days at 4°C. In addition, total cuticular wax load only increased in 'Natura' and preconditioned 'Sinatra' fruit with cold storage. With respect to the chemical composition of the waxes, the most abundant components were alkanes, in both 'Natura' and 'Sinatra', with similar values at harvest. The total alkane content only increased in 'Natura' fruit and in the preconditioned 'Sinatra' fruit after cold storage, whereas the amount of total acids decreased, with the lowest values observed in the fruit that showed less chilling injury (CI) and weight loss. Two esters were detected, and their content also decreased with the storage in both varieties, with a greater reduction observed in the cold-tolerant variety in response to low temperature. Gene expression analysis showed significant differences between varieties, especially in CpCER1-like and CpCER3-like genes, involved in alkane production, as well as in the transcription factors CpWIN1-like and CpFUL1-like, associated with cuticle development and epidermal wax accumulation in other species. These results suggest an important role of the alkane biosynthetic pathway and cuticle morphology in maintaining the postharvest quality of zucchini fruit during the storage at low temperatures.

Dissecting the molecular mechanism of russeting in sand pear (Pyrus pyrifolia Nakai) by metabolomics, transcriptomics, and proteomics

Brown coloration and a rough appearance as russet and semi-russet (partial russet) are features unique to the popular Asian sand pear (Pyrus pyrifolia Nakai). The degree of russeting is different between different genotypes. Russeting is sensitive to water fluctuations, where excessive rainwater can trigger/stimulate its development. However, the molecular mechanism of russeting is currently unclear. Here, we employed multi-omics, i.e., metabolomics, transcriptomics, and proteomics, and analyzed the effect of different sand pear genotypes and artificial rainfall on russeting of pear fruits. This led to the identification of 79, 64, and 29 differentially produced/expressed metabolites, transcripts, and proteins that are involved in the biosynthesis of suberin, phenylpropane, cutin, and waxes. Further analysis of these differentially expressed genes and their encoded proteins revealed that four of them exhibited high expression at both transcript and protein levels. Transient expression of one such gene, PbHHT1 (accession number 103966555), which encodes ω-hydroxypalmitate-O-feruloyl transferase, in young green non-russet fruits triggered premature suberization in the russeting pear genotypes. This coincided with increased production of 16-feruloyloxypalmitic acid, a conjugated compound between phenols and esters during the polymerization for suberin formation. Collectively, our data from the combined three omics demonstrate that russeting in sand pear is a complex process involving the biosynthesis and transport of suberin and many other secondary metabolites.

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.

Peridermal fruit skin formation in Actinidia sp. (kiwifruit) is associated with genetic loci controlling russeting and cuticle formation

BACKGROUND

The skin (exocarp) of fleshy fruit is hugely diverse across species. Most fruit types have a live epidermal skin covered by a layer of cuticle made up of cutin while a few create an outermost layer of dead cells (peridermal layer).

RESULTS

In this study we undertook crosses between epidermal and peridermal skinned kiwifruit, and showed that epidermal skin is a semi-dominant trait. Furthermore, backcrossing these epidermal skinned hybrids to a peridermal skinned fruit created a diverse range of phenotypes ranging from epidermal skinned fruit, through fruit with varying degrees of patches of periderm (russeting), to fruit with a complete periderm. Quantitative trait locus (QTL) analysis of this population suggested that periderm formation was associated with four loci. These QTLs were aligned either to ones associated with russet formation on chromosome 19 and 24, or cuticle integrity and coverage located on chromosomes 3, 11 and 24.

CONCLUSION

From the segregation of skin type and QTL analysis, it appears that skin development in kiwifruit is controlled by two competing factors, cuticle strength and propensity to russet. A strong cuticle will inhibit russeting while a strong propensity to russet can create a continuous dead skinned periderm.

An integrated metabolic and transcriptomic analysis reveals the mechanism through which fruit bagging alleviates exocarp semi-russeting in pear fruit

Fruit semi-russeting is an undesirable quality trait that occurs in fruit production. It is reported that preharvest fruit bagging could effectively alleviate fruit exocarp semi-russeting, but the physiological and molecular mechanisms remain unclear. In the present study, we performed an in-depth investigation into pear fruit semi-russeting from morphologic, metabolic and transcriptomic perspectives by comparing control (semi-russeted) and bagged (non-russeted) 'Cuiguan' pear fruits. The results showed that significant changes in cutin and suberin resulted in pear fruit semi-russeting. Compared with the skin of bagged fruits, the skin of the control fruits presented reduced cutin contents accompanied by an accumulation of suberin, which resulted in fruit semi-russeting; α, ω-dicarboxylic acids accounted for the largest proportion of typical suberin monomers. Moreover, combined transcriptomic and metabolic analysis revealed a series of genes involved in cutin and suberin biosynthesis, transport and polymerization differentially expressed between the two groups. Furthermore, the expression levels of genes involved in the stress response and in hormone biosynthesis and signaling were significantly altered in fruits with contrasting phenotypes. Finally, a number of transcription factors, including those of the MYB, NAC, bHLH and bZIP families, were differentially expressed. Taken together, the results suggest that the multilayered mechanism through which bagging alleviates pear fruit semi-russeting is complex, and the large number of candidate genes identified provides a good foundation for future functional studies.

Plant Transcription Factors Involved in Drought and Associated Stresses

Transcription factors (TFs) play a significant role in signal transduction networks spanning the perception of a stress signal and the expression of corresponding stress-responsive genes. TFs are multi-functional proteins that may simultaneously control numerous pathways during stresses in plants-this makes them powerful tools for the manipulation of regulatory and stress-responsive pathways. In recent years, the structure-function relationships of numerous plant TFs involved in drought and associated stresses have been defined, which prompted devising practical strategies for engineering plants with enhanced stress tolerance. Vast data have emerged on purposely basic leucine zipper (bZIP), WRKY, homeodomain-leucine zipper (HD-Zip), myeloblastoma (MYB), drought-response elements binding proteins/C-repeat binding factor (DREB/CBF), shine (SHN), and wax production-like (WXPL) TFs that reflect the understanding of their 3D structure and how the structure relates to function. Consequently, this information is useful in the tailored design of variant TFs that enhances our understanding of their functional states, such as oligomerization, post-translational modification patterns, protein-protein interactions, and their abilities to recognize downstream target DNA sequences. Here, we report on the progress of TFs based on their interaction pathway participation in stress-responsive networks, and pinpoint strategies and applications for crops and the impact of these strategies for improving plant stress tolerance.

Differential regulation of triterpene biosynthesis induced by an early failure in cuticle formation in apple

Waxy apple cuticles predominantly accumulate ursane-type triterpenes, but the profile shifts with the induction of skin russeting towards lupane-type triterpenes. We previously characterised several key enzymes in the ursane-type and lupane-type triterpene pathways, but this switch in triterpene metabolism associated with loss of cuticle integrity is not fully understood. To analyse the relationship between triterpene biosynthesis and russeting, we used microscopy, RNA-sequencing and metabolite profiling during apple fruit development. We compared the skin of three genetically-close clones of 'Golden Delicious' (with waxy, partially russeted and fully russeted skin). We identified a unique molecular profile for the russet clone, including low transcript abundance of multiple cuticle-specific metabolic pathways in the early stages of fruit development. Using correlation analyses between gene transcription and metabolite concentration we found MYB transcription factors strongly associated with lupane-type triterpene biosynthesis. We showed how their transcription changed with the onset of cuticle cracking followed by russeting and that one factor, MYB66, was able to bind the promoter of the oxidosqualene cyclase OSC5, to drive the production of lupeol derivatives. These results provide insights into the breakdown of cuticle integrity leading to russet and how this drives MYB-regulated changes to triterpene biosynthesis.

Russeting in Apple is Initiated after Exposure to Moisture Ends: Molecular and Biochemical Evidence

Exposure of the fruit surface to moisture during early development is causal in russeting of apple (Malus × domestica Borkh.). Moisture exposure results in formation of microcracks and decreased cuticle thickness. Periderm differentiation begins in the hypodermis, but only after discontinuation of moisture exposure. Expressions of selected genes involved in cutin, wax and suberin synthesis were quantified, as were the wax, cutin and suberin compositions. Experiments were conducted in two phases. In Phase I (31 days after full bloom) the fruit surface was exposed to moisture for 6 or 12 d. Phase II was after moisture exposure had been discontinued. Unexposed areas on the same fruit served as unexposed controls. During Phase I, cutin and wax synthesis genes were down-regulated only in the moisture-exposed patches. During Phase II, suberin synthesis genes were up-regulated only in the moisture-exposed patches. The expressions of cutin and wax genes in the moisture-exposed patches increased slightly during Phase II, but the levels of expression were much lower than in the control patches. Amounts and compositions of cutin, wax and suberin were consistent with the gene expressions. Thus, moisture-induced russet is a two-step process: moisture exposure reduces cutin and wax synthesis, moisture removal triggers suberin synthesis.

A review of current knowledge about the formation of native peridermal exocarp in fruit

The outer skin layer in any plant is essential in offering a protective barrier against water loss and pathogen attack. Within fleshy fruit, the skin supports internal cell layers and can provide the initial cues in attracting seed-dispersing animals. The skin of a fruit, termed the exocarp, is a key element of consumer preference and a target for many breeding programs. Across fruiting species there is a huge diversity of exocarp types and these range from a simple single living cell layer (epidermis) often covered with a waxy layer, to complex multicellular suberised and dead cell layers (periderm), with various intermediate russet forms in between. Each exocarp can be interspersed with other structures such as hairs or spines. The epidermis has been well characterised and remains pluripotent with the help of the cells immediately under the epidermis. The periderm, in contrast, is the result of secondary meristematic activity, which replaces the epidermal layers, and is not well characterised in fruits. In this review we explore the structure, composition and mechanisms that control the development of a periderm type fruit exocarp. We draw upon literature from non-fleshy fruit species that form periderm tissue, from which a considerable amount of research has been undertaken.

Lignin and Quercetin Synthesis Underlies Berry Russeting in 'Sunshine Muscat' Grape

In order to further explore the mechanism of 'sunshine muscat' grape russet formation, transcriptomic and metabolomic analyses were performed on 'sunshine muscat' grape peels with and without russet. A total of 1491 differentially expressed genes (DEGs) were discovered based on these analyses. The phenylpropane synthesis pathway was the key metabolic pathway identified, and 28 DEGs related to phenylpropane synthesis pathway were screened, of which 16 were related to lignin synthesis. In addition, 60 differential metabolites were screened. There were 29 phenolic substances among the differential metabolites, which were all up-regulated and 10 were quercetin-related glycosides. Our results indicate that phenols likely play a dominant role in the formation of 'sunshine muscat' grape russet, and the synthesis of lignin and quercetin may be the key factors underlying russet formation.

Advances in Biosynthesis, Regulation, and Function of Apple Cuticular Wax

A layer of cuticular wax is deposited on the surface of terrestrial plants, which reduces the damage caused by environmental stress and maintains growth in a relatively stable internal environment. Apple cuticular wax is an important part of the fruit epidermis that plays an essential role in apple development, storage, and adaptation to environmental stress. The formation of cuticular wax has been described at the transcriptional, post-transcriptional, and translational levels in Arabidopsis, whereas less research has been performed on apple cuticular wax. Here, we provide a brief overview of how apple cuticular wax is formed, as well as its structure, composition, and function. An association among the environment, genes, and apple cuticular wax deposition was revealed. Cuticular wax prevents fruit rust from occurring on apple. Taken together, a detailed understanding of apple cuticular wax is discussed. The results will act as a reference for extending the storage period and increasing the commodity value of apple.

The expanding roles of APETALA2/Ethylene Responsive Factors and their potential applications in crop improvement

Understanding the molecular basis of the gene-regulatory networks underlying agronomic traits or plant responses to abiotic/biotic stresses is very important for crop improvement. In this context, transcription factors, which either singularly or in conjugation directly control the expression of many target genes, are suitable candidates for improving agronomic traits via genetic engineering. In this regard, members of one of the largest class of plant-specific APETALA2/Ethylene Response Factor (AP2/ERF) superfamily, which is implicated in various aspects of development and plant stress adaptation responses, are considered high-value targets for crop improvement. Besides their long-known regulatory roles in mediating plant responses to abiotic stresses such as drought and submergence, the novel roles of AP2/ERFs during fruit ripening or secondary metabolites production have also recently emerged. The astounding functional plasticity of AP2/ERF members is considered to be achieved by their interplay with other regulatory networks and signalling pathways. In this review, we have integrated the recently accumulated evidence from functional genomics studies and described their newly emerged functions in plants. The key structural features of AP2/ERF proteins and the modes of their action are briefly summarized. The importance of AP2/ERFs in plant development and stress responses and a summary of the event of their successful applications in crop improvement programs are also provided. Altogether, we envisage that the synthesized information presented in this review will be useful to design effective strategies for improving agronomic traits in crop plants.

An Integrated Transcriptome and Proteome Analysis Reveals New Insights into Russeting of Bagging and Non-Bagging "Golden Delicious" Apple

Apple skin russeting naturally occurs in many varieties, particularly in “Golden Delicious” and its pedigree, and is regarded as a non-invasive physiological disorder partly caused by excessive deposition of lignin. However, the understanding of its molecular mechanism is still limited. In this study, we used iTRAQ (isobaric tags for relative and absolute quantitation) and RNA-seq to detect the changes in the expression levels of genes and proteins in three developmental stages of russeting formation, in russeted (non-bagging) and non-russeted (bagging) skin of “Golden Delicious” apple. 2856 differentially expressed genes and 942 differentially expressed proteins in the comparison groups were detected at the transcript level and protein level, respectively. A correlation analysis of the transcriptomics and proteomics data revealed that four genes (MD03G1059200, MD08G1009200, MD17G1092400, and MD17G1225100) involved in lignin biosynthesis are significant changed during apple russeting formation. Additionally, 92 transcription factors, including 4 LIM transcription factors, may be involved in apple russeting formation. Among them, one LIM transcription factor (MD15G1068200) was capable of binding to the PAL-box like (CCACTTGAGTAC) element, which indicated it was potentially involved in lignin biosynthesis. This study will provide further views on the molecular mechanisms controlling apple russeting formation.

Multifaceted analyses disclose the role of fruit size and skin-russeting in the accumulation pattern of phenolic compounds in apple

Fruits are nowadays considered important suppliers of anti-oxidant molecules. Apples are particularly rich in phenolic compounds, non-nutritional phytochemicals that play active roles in controlling severe chronic diseases. In this work, 19 phenolic compounds were investigated in both skin and pulp tissues of seven apple accessions across the Malus genus collected at two stages: during fruit development and at harvest. The primary difference in phenolic concentration between wild and domesticated accessions, especially in the pulp, could be explained by the larger growth rate of the domesticated varieties. The proposed dilution effect was also confirmed through the observation of the increased content of procyanidin B2+B4 and phloridzin in russet-skinned apples, known to have higher concentrations of these compounds. The metabolite screening was also accompanied by the expression analysis of 16 polyphenolic genes showing, for nine elements, a higher expression at harvest than during fruit development. Finally, a polyphenolic comparison with red-fleshed apples was also carried out, underlying a larger amount of procyanidins and quercetin-3rhamnoside in the white-fleshed accessions. The results presented and discussed in this work suggest that specific white-fleshed apples, especially with russeted-skin, may play an important role in ameliorating the nutraceutical potential of apple fruit.

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.

A Multilevel Study of Melon Fruit Reticulation Provides Insight into Skin Ligno-Suberization Hallmarks

The skin of fleshy fruit is typically covered by a thick cuticle. Some fruit species develop different forms of layers directly above their skin. Reticulation, for example, is a specialized suberin-based coating that ornaments some commercially important melon (Cucumis melo) fruit and is an important quality trait. Despite its importance, the structural, molecular, and biochemical features associated with reticulation are not fully understood. Here, we performed a multilevel investigation of structural attributes, chemical composition, and gene expression profiles on a set of reticulated and smooth skin melons. High-resolution microscopy, surface profiling, and histochemical staining assays show that reticulation comprises cells with heavily suberized walls accumulating large amounts of typical suberin monomers, as well as lignified cells localized underneath the specialized suberized cell layer. Reticulated skin was characterized by induced expression of biosynthetic genes acting in the core phenylpropanoid, suberin, lignin, and lignan pathways. Transcripts of genes associated with lipid polymer assembly, cell wall organization, and loosening were highly enriched in reticulated skin tissue. These signatures were exclusive to reticulated structures and absent in both the smooth surfaces observed in between reticulated regions and in the skin of smooth fruit. Our data provide important insights into the molecular and metabolic bases of reticulation and its tight association with skin ligno-suberization during melon fruit development. Moreover, these insights are likely to contribute to melon breeding programs aimed at improving postharvest qualities associated with fleshy fruit surface layers.

QTL and Transcriptomic Analyses Implicate Cuticle Transcription Factor SHINE as a Source of Natural Variation for Epidermal Traits in Cucumber Fruit

The fruit surface is a unique tissue with multiple roles influencing fruit development, post-harvest storage and quality, and consumer acceptability. Serving as the first line of protection against herbivores, pathogens, and abiotic stress, the surface can vary markedly among species, cultivars within species, and developmental stage. In this study we explore developmental changes and natural variation of cucumber (Cucumis sativus L.) fruit surface properties using two cucumber lines which vary greatly for these traits and for which draft genomes and a single nucleotide polymorphism (SNP) array are available: Chinese fresh market type, Chinese Long '9930' (CL9930), and pickling type, 'Gy14'. Thin-section samples were prepared from the mid-region of fruit harvested at 0, 4, 8, 12, 16, 20, 24 and 30 days post pollination (dpp), stained with Sudan IV and evaluated for cuticle thickness, depth of wax intercalation between epidermal cells, epidermal cell size and shape, and number and size of lipid droplets. 'Gy14' is characterized by columnar shaped epidermal cells, a 2-3 fold thicker cuticular layer than CL9930, increased cuticular intercalations between cells and a larger number and larger sized lipid droplets. In both lines maximal deposition of cuticle and increase in epidermal size coincided with exponential fruit growth and was largely completed by approximately 16 dpp. Phenotyping and quantitative trait locus mapping (QTL) of fruit sampled from an F₇:F₈ Gy14 × CL9930 recombinant inbred line (RIL) population identified QTL regions on chromosomes 1, 4 and 5. Strong QTL for epidermal cell height, cuticle thickness, intercalation depth, and diameter of lipid droplets co-localized on chromosome 1. SSR markers on chromosome 1 were used to screen for recombinants in an extended RIL population to refine the QTL region. Further fine mapping by KASP assay combined with gene expression profiling suggested a small number of candidate genes. Tissue specificity, developmental analysis of expression, allelic diversity and gene function implicate the regulatory factor CsSHINE1/WIN1 as a source of natural variation for cucumber fruit epidermal traits.

Identification of candidate genes mediating apple fruit-cracking resistance following the application of gibberellic acids 4 + 7 and the cytokinin 6-benzyladenine

Calyx-end cracking in 'Pink Lady' apple is treated by a solution of gibberellic acids 4 and 7 (GA₄₊₇) and the cytokinin 6-benzyladenine (BA). Although the GA₄₊₇ and BA mixture is applied early in apple fruit development, it mitigates cracking that becomes evident in the mature fruit, implying a long-term treatment effect. The reduced incidence of peel cracking is associated with increased epidermal cell density, which is maintained until fruit maturation. Presently, the expression of genes that have been previously reported to be associated with epidermal cell patterning and cuticle formation, or cracking resistance, was monitored in the peel during fruit development and following GA₄₊₇ and BA treatment. For most of the genes whose expression is naturally upregulated during fruit development, the early GA₄₊₇ and BA treatment maintained or further increased the high expression level in the mature peel. Where the expression of a gene was downregulated during development, no change was detected in the treated mature peel. Gene-networking analysis supported the interaction between gene clusters of cell-wall synthesis, cuticle formation and GA signaling. Overall, the data suggested that the GA₄₊₇ and BA treatment did not modify developmental cues, but promoted or enhanced the innate developmental program.

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.

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.

MdMyb93 is a regulator of suberin deposition in russeted apple fruit skins

A comparison of the transcriptomes of russeted vs nonrusseted apple skins previously highlighted a tight relationship between a gene encoding an MYB-type transcription factor, MdMYB93, and some key suberin biosynthetic genes. The present work assesses the role of this transcription factor in the suberization process. A phylogenetic analysis of MdMYB93 and Arabidopsis thaliana MYBs was performed and the function of MdMYB93 was further investigated using Agrobacterium-mediated transient overexpression in Nicotiana benthamiana leaves. An RNA-Seq analysis was performed to highlight the MdMYB93-regulated genes. Ultraperformance liquid chromatography-triple time-of-flight (UPLC-TripleTOF) and GC-MS were used to investigate alterations in phenylpropanoid, soluble-free lipid and lipid polyester contents. A massive accumulation of suberin and its biosynthetic precursors in MdMYB93 agroinfiltrated leaves was accompanied by a remobilization of phenylpropanoids and an increased amount of lignin precursors. Gene expression profiling displayed a concomitant alteration of lipid and phenylpropanoid metabolism, cell wall development, and extracellular transport, with a large number of induced transcripts predicted to be involved in suberin deposition. The present work supports a major role of MdMYB93 in the regulation of suberin deposition in russeted apple skins, from the synthesis of monomeric precursors, their transport, polymerization, and final deposition as suberin in primary cell wall.

MYB107 and MYB9 Homologs Regulate Suberin Deposition in Angiosperms

Suberin, a polymer composed of both aliphatic and aromatic domains, is deposited as a rough matrix upon plant surface damage and during normal growth in the root endodermis, bark, specialized organs (e.g., potato [Solanum tuberosum] tubers), and seed coats. To identify genes associated with the developmental control of suberin deposition, we investigated the chemical composition and transcriptomes of suberized tomato (Solanum lycopersicum) and russet apple (Malus x domestica) fruit surfaces. Consequently, a gene expression signature for suberin polymer assembly was revealed that is highly conserved in angiosperms. Seed permeability assays of knockout mutants corresponding to signature genes revealed regulatory proteins (i.e., AtMYB9 and AtMYB107) required for suberin assembly in the Arabidopsis thaliana seed coat. Seeds of myb107 and myb9 Arabidopsis mutants displayed a significant reduction in suberin monomers and altered levels of other seed coat-associated metabolites. They also exhibited increased permeability, and lower germination capacities under osmotic and salt stress. AtMYB9 and AtMYB107 appear to synchronize the transcriptional induction of aliphatic and aromatic monomer biosynthesis and transport and suberin polymerization in the seed outer integument layer. Collectively, our findings establish a regulatory system controlling developmentally deposited suberin, which likely differs from the one of stress-induced polymer assembly recognized to date.

Overexpression of the Transcription Factors GmSHN1 and GmSHN9 Differentially Regulates Wax and Cutin Biosynthesis, Alters Cuticle Properties, and Changes Leaf Phenotypes in Arabidopsis

SHINE (SHN/WIN) clade proteins, transcription factors of the plant-specific APETALA 2/ethylene-responsive element binding factor (AP2/ERF) family, have been proven to be involved in wax and cutin biosynthesis. Glycine max is an important economic crop, but its molecular mechanism of wax biosynthesis is rarely characterized. In this study, 10 homologs of Arabidopsis SHN genes were identified from soybean. These homologs were different in gene structures and organ expression patterns. Constitutive expression of each of the soybean SHN genes in Arabidopsis led to different leaf phenotypes, as well as different levels of glossiness on leaf surfaces. Overexpression of GmSHN1 and GmSHN9 in Arabidopsis exhibited 7.8-fold and 9.9-fold up-regulation of leaf cuticle wax productions, respectively. C31 and C29 alkanes contributed most to the increased wax contents. Total cutin contents of leaves were increased 11.4-fold in GmSHN1 overexpressors and 5.7-fold in GmSHN9 overexpressors, mainly through increasing C16:0 di-OH and dioic acids. GmSHN1 and GmSHN9 also altered leaf cuticle membrane ultrastructure and increased water loss rate in transgenic Arabidopsis plants. Transcript levels of many wax and cutin biosynthesis and leaf development related genes were altered in GmSHN1 and GmSHN9 overexpressors. Overall, these results suggest that GmSHN1 and GmSHN9 may differentially regulate the leaf development process as well as wax and cutin biosynthesis.