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

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

A reference-grade genome of the xerophyte Ammopiptanthus mongolicus sheds light on its evolution history in legumes and drought-tolerance mechanisms

Plants that grow in extreme environments represent unique sources of stress-resistance genes and mechanisms. Ammopiptanthus mongolicus (Leguminosae) is a xerophytic evergreen broadleaf shrub native to semi-arid and desert regions; however, its drought-tolerance mechanisms remain poorly understood. Here, we report the assembly of a reference-grade genome for A. mongolicus, describe its evolutionary history within the legume family, and examine its drought-tolerance mechanisms. The assembled genome is 843.07 Mb in length, with 98.7% of the sequences successfully anchored to the nine chromosomes of A. mongolicus. The genome is predicted to contain 47 611 protein-coding genes, and 70.71% of the genome is composed of repetitive sequences; these are dominated by transposable elements, particularly long-terminal-repeat retrotransposons. Evolutionary analyses revealed two whole-genome duplication (WGD) events at 130 and 58 million years ago (mya) that are shared by the genus Ammopiptanthus and other legumes, but no species-specific WGDs were found within this genus. Ancestral genome reconstruction revealed that the A. mongolicus genome has undergone fewer rearrangements than other genomes in the legume family, confirming its status as a "relict plant". Transcriptomic analyses demonstrated that genes involved in cuticular wax biosynthesis and transport are highly expressed, both under normal conditions and in response to polyethylene glycol-induced dehydration. Significant induction of genes related to ethylene biosynthesis and signaling was also observed in leaves under dehydration stress, suggesting that enhanced ethylene response and formation of thick waxy cuticles are two major mechanisms of drought tolerance in A. mongolicus. Ectopic expression of AmERF2, an ethylene response factor unique to A. mongolicus, can markedly increase the drought tolerance of transgenic Arabidopsis thaliana plants, demonstrating the potential for application of A. mongolicus genes in crop improvement.

NACs strike again: NOR-like1 is responsible for cuticle development in tomato fruit

This article comments on:

Liu G-S, Huang H, Grierson D, Gao Y, Ji X, Peng Z-Z, Li H-L, Niu X-L, Jia W, He J-L, Xiang L-T, Gao H-Y, Qu G-Q, Zhu H-L, Zhu B-Z, Luo Y-B, Fu D-Q. 2024. NAC transcription factor SlNOR-like1 plays a dual regulatory role in tomato fruit cuticle formation. Journal of Experimental Botany 75, 1903–1918.

Chemical and Transcriptomic Analyses of Leaf Cuticular Wax Metabolism in Ammopiptanthus mongolicus under Osmotic Stress

Plant cuticular wax forms a hydrophobic structure in the cuticle layer covering epidermis as the first barrier between plants and environments. Ammopiptanthus mongolicus, a leguminous desert shrub, exhibits high tolerances to multiple abiotic stress. The physiological, chemical, and transcriptomic analyses of epidermal permeability, cuticular wax metabolism and related gene expression profiles under osmotic stress in A. mongolicus leaves were performed. Physiological analyses revealed decreased leaf epidermal permeability under osmotic stress. Chemical analyses revealed saturated straight-chain alkanes as major components of leaf cuticular wax, and under osmotic stress, the contents of total wax and multiple alkane components significantly increased. Transcriptome analyses revealed the up-regulation of genes involved in biosynthesis of very-long-chain fatty acids and alkanes and wax transportation under osmotic stress. Weighted gene co-expression network analysis identified 17 modules and 6 hub genes related to wax accumulation, including 5 enzyme genes coding KCS, KCR, WAX2, FAR, and LACS, and an ABCG transporter gene. Our findings indicated that the leaf epidermal permeability of A. mongolicus decreased under osmotic stress to inhibit water loss via regulating the expression of wax-related enzyme and transporter genes, further promoting cuticular wax accumulation. This study provided new evidence for understanding the roles of cuticle lipids in abiotic stress tolerance of desert plants.

Influence of Cuticular Waxes from Triticale on Rumen Fermentation: A Metabolomic and Microbiome Profiling Study

Cuticular wax, a critical defense layer for plants, remains a relatively unexplored factor in rumen fermentation. We investigated the impact of cuticular wax on rumen fermentation using triticale as a model. In total, six wax classes were identified, including fatty acids, aldehydes, alkane, primary alcohol, alkyresorcinol, and β-diketone, with low-bloom lines predominated by 46.05% of primary alcohols and high-bloom lines by 35.64% of β-diketone. Low-wax addition (2.5 g/kg DM) increased the gas production by 19.25% (P < 0.05) and total volatile fatty acids by 6.34% (P > 0.05), and enriched key carbohydrate-fermenting rumen microbes like Saccharofermentans, Ruminococcus, and Prevotellaceae, when compared to non-wax groups. Metabolites linked to nucleotide metabolism, purine metabolism, and protein/fat digestion in the rumen showed a positive correlation with low-wax, benefiting rumen microbes. This study highlights the intricate interplay among cuticular wax, rumen microbiota, fermentation, and metabolomics in forage digestion, providing insights into livestock nutrition and forage utilization.

Trade-offs between the accumulation of cuticular wax and jasmonic acid-mediated herbivory resistance in maize

Plants have evolved complex physical and chemical defense systems that allow them to withstand herbivory infestation. Composed of a complex mixture of very-long-chain fatty acids (VLCFAs) and their derivatives, cuticular wax constitutes the first physical line of defense against herbivores. Here, we report the function of Glossy 8 (ZmGL8), which encodes a 3-ketoacyl reductase belonging to the fatty acid elongase complex, in orchestrating wax production and jasmonic acid (JA)-mediated defenses against herbivores in maize (Zea mays). The mutation of GL8 enhanced chemical defenses by activating the JA-dependent pathway. We observed a trade-off between wax accumulation and JA levels across maize glossy mutants and 24 globally collected maize inbred lines. In addition, we demonstrated that mutants defective in cuticular wax biosynthesis in Arabidopsis thaliana and maize exhibit enhanced chemical defenses. Comprehensive transcriptomic and lipidomic analyses indicated that the gl8 mutant confers chemical resistance to herbivores by remodeling VLCFA-related lipid metabolism and subsequent JA biosynthesis and signaling. These results suggest that VLCFA-related lipid metabolism has a critical role in regulating the trade-offs between cuticular wax and JA-mediated chemical defenses.

Changes in wax composition but not amount enhance cuticular transpiration

This study focuses on the role of the qualitative leaf wax composition in modulating the cuticular water loss using a Populus × canescens cer6 mutant line, which accumulates C34-C46 wax ester dimers and is reduced in wax monomers >C24. The two literature-based hypotheses to be tested were the importance of the amount of wax esters and the weighted mean carbon chain length in restricting cuticular water loss. The main results were acquired by chemical analysis of cuticular wax and gravimetric cuticular transpiration measurements. Besides additional physiological measurements, the leaf surface properties were characterised by scanning electron microscopy and spectrophotometric light reflectance quantification. Mutation of the CER6 gene resulted in striking changes in qualitative wax composition but not quantitative wax amount. Based on the strong accumulation of dimeric wax esters, the relative proportion of esters increased to >90%, and the weighted mean carbon chain length increased by >6 carbon atoms. These qualitative alterations were found to increase the cuticular transpiration of leaves by twofold. Our results do not support the hypotheses that enhanced amounts of wax esters or increased weighted mean carbon chain lengths of waxes lead to reduced cuticular transpiration.

Phenotypic Diversity in Leaf Cuticular Waxes in Brassica carinata Accessions

Brassica carinata has received considerable attention as a renewable biofuel crop for semi-arid zones due to its high oil content and polyunsaturated fatty acids contents. It is important to develop new drought-resistant cultivars of B. carinata production to expand its areas into more arid regions. The accumulation of leaf cuticular wax on plant surfaces is one mechanism that reduces non-stomatal water loss, thus increasing drought resistance in plants. To explore phenotypic variations in cuticular wax in B. carinata, leaf waxes were extracted and quantified from a diversity panel consisting of 315 accessions. The results indicate that the accessions have a wide range of total leaf wax content (289-1356 µg dm-2), wax classes, and their components. The C29 and C31 homologues of alkanes, C29 ketone homologue, C29 secondary alcohol, and C30 aldehyde were the most abundant leaf waxes extracted from B. carinata accessions. The high heritability values of these waxes point to the positive selection for high wax content during early generations of future B. carinata breeding programs. Positive correlation coefficients, combined with the effects of these waxes on leaf wax content accumulation, suggest that modifying specific wax content could increase the total wax content and enhance cuticle composition. The identified leaf wax content and compositions in B. carinata will lead to the future discovery of wax biosynthetic pathways, the dissection of its genetic regulatory networks, the identification of candidate genes controlling production of these waxes, and thus, develop and release new B. carinata drought-tolerant cultivars.

Fine Mapping and Identification of a Candidate Gene for the Glossy Green Trait in Cabbage (Brassica oleracea var. capitata)

In higher plants, cuticular wax deposited on the surface of epidermal cells plays an important role in protecting the plant from biotic and abiotic stresses; however, the molecular mechanism of cuticular wax production is not completely understood. In this study, we identified a glossy green mutant (98-1030gl) from the glaucous cabbage inbred line 98-1030. Scanning electron microscopy indicated that the amount of leaf cuticular wax significantly decreased in 98-1030gl. Genetic analysis showed that the glossy green trait was controlled by a single recessive gene. Bulked segregant analysis coupled with whole genome sequencing revealed that the candidate gene for the glossy green trait was located at 13,860,000-25,070,000 bp (11.21 Mb) on Chromosome 5. Based on the resequencing data of two parents and the F2 population, insertion-deletion markers were developed and used to reduce the candidate mapping region. The candidate gene (Bol026949) was then mapped in a 50.97 kb interval. Bol026949 belongs to the Agenet/Tudor domain protein family, whose members are predicted to be involved in chromatin remodeling and RNA transcription. Sequence analysis showed that a single nucleotide polymorphism mutation (C → G) in the second exon of Bol026949 could result in the premature termination of its protein translation in 98-1030gl. Phylogenetic analysis showed that Bol026949 is relatively conserved in cruciferous plants. Transcriptome profiling indicated that Bol026949 might participate in cuticular wax production by regulating the transcript levels of genes involved in the post-translational cellular process and phytohormone signaling. Our findings provide an important clue for dissecting the regulatory mechanisms of cuticular wax production in cruciferous crops.

Genetic variation in ZmWAX2 confers maize resistance to Fusarium verticillioides

Fusarium verticillioides (F. verticillioides) is a widely distributed phytopathogen that incites multiple destructive diseases in maize, posing a grave threat to corn yields and quality worldwide. However, there are few reports of resistance genes to F. verticillioides. Here, we reveal that a combination of two single nucleotide polymorphisms (SNPs) corresponding to ZmWAX2 gene associates with quantitative resistance variations to F. verticillioides in maize through a genome-wide association study. A lack of ZmWAX2 compromises maize resistance to F. verticillioides-caused seed rot, seedling blight and stalk rot by reducing cuticular wax deposition, while the transgenic plants overexpressing ZmWAX2 show significantly increased immunity to F. verticillioides. A natural occurrence of two 7-bp deletions within the promoter increases ZmWAX2 transcription, thus enhancing maize resistance to F. verticillioides. Upon Fusarium stalk rot, ZmWAX2 greatly promotes the yield and grain quality of maize. Our studies demonstrate that ZmWAX2 confers multiple disease resistances caused by F. verticillioides and can serve as an important gene target for the development of F. verticillioides-resistant maize varieties.

A glossy mutant in onion (Allium cepa L.) shows decreased expression of wax biosynthesis genes

Cuticular wax is a characteristic feature of land plants that provides protection against both biotic and abiotic stresses. In this study, a glossy mutant lacking an epicuticular wax layer was identified in the γ-irradiated M2 mutant population of the onion cultivar Bhima Super. The inheritance of the mutant's glossy phenotype was determined to be recessive and single locus. Scanning electron microscopy analysis showed poor accumulation of wax crystals in the glossy mutant, concentrated near the stomata. The plant height, number of leaves per plant, and stomatal parameters of the mutant were similar to the wild-type. RNA-seq was used to comprehend the expression variations of waxy cuticle-related genes in the glossy mutant and its wild-type waxy cultivars. Differential gene expression analysis of the RNA-seq data revealed that the genes involved in wax biosynthesis, such as AcCER1, AcCER26, AcMAH1, and AcWSD1, were downregulated by 2.72, 1.74, 2.59 and 2.12-fold, respectively, in the glossy mutant respectively. The expression patterns of these four unigenes were validated using semi-quantitative RT-PCR. The glossy mutant displayed a substantial 3.5-fold reduction in cuticular wax load compared to the wild-type due to the significant downregulation of these wax biosynthesis genes. These findings represent early advancements in understanding the molecular mechanisms of wax biosynthesis in onions. Furthermore, they provide a foundation for utilizing the glossy mutant trait in breeding programmes to enhance stress and pest resilience.

In Planta Study Localizes an Effector Candidate from Austropuccinia psidii Strain MF-1 to the Nucleus and Demonstrates In Vitro Cuticular Wax-Dependent Differential Expression

Austropuccinia psidii is a biotrophic fungus that causes myrtle rust. First described in Brazil, it has since spread to become a globally important pathogen that infects more than 480 myrtaceous species. One of the most important commercial crops affected by A. psidii is eucalypt, a widely grown forestry tree. The A. psidii-Eucalyptus spp. interaction is poorly understood, but pathogenesis is likely driven by pathogen-secreted effector molecules. Here, we identified and characterized a total of 255 virulence effector candidates using a genome assembly of A. psidii strain MF-1, which was recovered from Eucalyptus grandis in Brazil. We show that the expression of seven effector candidate genes is modulated by cell wax from leaves sourced from resistant and susceptible hosts. Two effector candidates with different subcellular localization predictions, and with specific gene expression profiles, were transiently expressed with GFP-fusions in Nicotiana benthamiana leaves. Interestingly, we observed the accumulation of an effector candidate, Ap28303, which was upregulated under cell wax from rust susceptible E. grandis and described as a peptidase inhibitor I9 domain-containing protein in the nucleus. This was in accordance with in silico analyses. Few studies have characterized nuclear effectors. Our findings open new perspectives on the study of A. psidii-Eucalyptus interactions by providing a potential entry point to understand how the pathogen manipulates its hosts in modulating physiology, structure, or function with effector proteins.

Roles of the MYB94/FUSED LEAVES1 (ZmFDL1) and GLOSSY2 (ZmGL2) genes in cuticle biosynthesis and potential impacts on Fusarium verticillioides growth on maize silks

Maize silks, the stigmatic portions of the female flowers, have an important role in reproductive development. Silks also provide entry points for pathogens into host tissues since fungal hyphae move along the surface of the silks to reach the site of infection, i.e., the developing kernel. The outer extracellular surface of the silk is covered by a protective hydrophobic cuticle, comprised of a complex array of long-chain hydrocarbons and small amounts of very long chain fatty acids and fatty alcohols. This work illustrates that two previously characterized cuticle-related genes separately exert roles on maize silk cuticle deposition and function. ZmMYB94/FUSED LEAVES 1 (ZmFDL1) MYB transcription factor is a key regulator of cuticle deposition in maize seedlings. The ZmGLOSSY2 (ZmGL2) gene, a putative member of the BAHD superfamily of acyltransferases with close sequence similarity to the Arabidopsis AtCER2 gene, is involved in the elongation of the fatty acid chains that serve as precursors of the waxes on young leaves. In silks, lack of ZmFDL1 action generates a decrease in the accumulation of a wide number of compounds, including alkanes and alkenes of 20 carbons or greater and affects the expression of cuticle-related genes. These results suggest that ZmFDL1 retains a regulatory role in silks, which might be exerted across the entire wax biosynthesis pathway. Separately, a comparison between gl2-ref and wild-type silks reveals differences in the abundance of specific cuticular wax constituents, particularly those of longer unsaturated carbon chain lengths. The inferred role of ZmGL2 is to control the chain lengths of unsaturated hydrocarbons. The treatment of maize silks with Fusarium verticillioides conidia suspension results in altered transcript levels of ZmFDL1 and ZmGL2 genes. In addition, an increase in fungal growth was observed on gl2-ref mutant silks 72 hours after Fusarium infection. These findings suggest that the silk cuticle plays an active role in the response to F. verticillioides infection.

Cuticular Wax Triterpenes Maintain Storage Quality of Blueberries by Reducing Water Loss

Cuticular wax contributes to maintaining postharvest storage quality against fruit water loss and softening. Triterpenoids, such as oleanolic acid (OA) and ursolic acid (UA), are the main components in blueberry cuticular wax, but their role in water migration during the storage of blueberries remains to be determined. Here, we examined the relationship between the content of OA and UA and the storage quality of blueberry fruit (25 °C). The results revealed that the UA content during eight-day postharvest storage ranged from 58 to 77 μg cm-2, which was negatively related to weight loss. Additionally, we investigated the effect of exogenous OA and UA on water migration in the blueberry fruit during storage at room temperature; the weight loss was significantly lower (by 22%) with UA treatment than in the control fruit. Our findings indicate that OA and UA effectively affect water migration in blueberry fruit during postharvest storage, which could contribute to improving postharvest preservation techniques.

Mechanistic Insight into the Internalization, Distribution, and Autophagy Process of Manganese Nanoparticles in Capsicum annuum L.: Evidence from Orthogonal Microscopic Analysis

Orthogonal techniques were used to track manganese nanoparticles (MnNPs) in Capsicum annuum L. leaf tissue and cell compartments and subsequently to explain the mechanism of uptake, translocation, and cellular interaction. C. annuum L was cultivated and foliarly exposed to MnNPs (100 mg/L, 50 mL/per leaf) before analysis by using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) as well as dark-field hyperspectral and two-photon microscopy. We visualized the internalization of MnNP aggregates from the leaf surface and observed particle accumulation in the leaf cuticle and epidermis as well as spongy mesophyll and guard cells. These techniques enabled a description of how MnNPs cross different plant tissues as well as selectively accumulate and translocate in specific cells. We also imaged abundant fluorescent vesicles and vacuoles containing MnNPs, indicating likely induction of autophagy processes in C. annuum L., which is the bio-response upon storing or transforming the particles. These findings highlight the importance of utilizing orthogonal techniques to characterize nanoscale material fate and distribution with complex biological matrices and demonstrate that such an approach offers a significant mechanistic understanding that can inform both risk assessment and efforts aimed at applying nanotechnology to agriculture.

Bacterium-enabled transient gene activation by artificial transcription factors for resolving gene regulation in maize

Understanding gene regulatory networks is essential to elucidate developmental processes and environmental responses. Here, we studied regulation of a maize (Zea mays) transcription factor gene using designer Transcription Activator-Like effectors (dTALes), which are synthetic type III TALes of the bacterial genus Xanthomonas and serve as inducers of disease susceptibility gene transcription in host cells. The maize pathogen Xanthomonas vasicola pv. vasculorum was used to introduce two independent dTALes into maize cells to induced expression of the gene glossy3 (gl3), which encodes a MYB transcription factor involved in biosynthesis of cuticular wax. RNA-seq analysis of leaf samples identified, in addition to gl3, 146 genes altered in expression by the two dTALes. Nine of the ten genes known to be involved in cuticular wax biosynthesis were up-regulated by at least one of the two dTALes. A gene previously unknown to be associated with gl3, Zm00001d017418, which encodes aldehyde dehydrogenase, was also expressed in a dTALe-dependent manner. A chemically induced mutant and a CRISPR-Cas9 mutant of Zm00001d017418 both exhibited glossy leaf phenotypes, indicating that Zm00001d017418 is involved in biosynthesis of cuticular waxes. Bacterial protein delivery of dTALes proved to be a straightforward and practical approach for the analysis and discovery of pathway-specific genes in maize.

Uptake, Translocation, and Terminal Residue of Chlorantraniliprole and Difenoconazole in Rice: Effect of the Mixed-Application with Adjuvant

Plant oil adjuvants are widely used to improve the utilization rate of pesticides. In this study, the uptake, translocation, and terminal residue of chlorantraniliprole and difenoconazole spraying with plant oil adjuvant in rice (Oryza sativa L.) were evaluated. After being mixed with the tank-mixed plant oil adjuvant, the cuticular wax of rice leaf was destroyed, which decreased the hydrophobicity of the rice leaf and facilitated the wetting, spreading, and penetration of pesticides onto the rice leaf. Additionally, the adjuvant promoted the translocation of difenoconazole from leaves to stems, but had little effect on the translocation of difenoconazole from leaves to roots, while inhibiting chlorantraniliprole translocation. Although adjuvant increased the initial deposition of chlorantraniliprole and difenoconazole on rice, the terminal residue was not significantly affected. The findings can promote the safe use of chlorantraniliprole and difenoconazole in rice production, especially when used with plant oil adjuvants. In the future, studies on more rice cultivars will be necessary to determine the generality of the conclusions.

Massive increases in C31 alkane on Zygophyllum xanthoxylum leaves contribute to its excellent abiotic stress tolerance

BACKGROUND AND AIMS

Desert plants possess excellent water-conservation capacities to survive in extreme environments. Cuticular wax plays a pivotal role in reducing water loss through plant aerial surfaces. However, the role of cuticular wax in water retention by desert plants is poorly understood.

METHODS

We investigated leaf epidermal morphology and wax composition of five desert shrubs from north-west China and characterized the wax morphology and composition for the typical xerophyte Zygophyllum xanthoxylum under salt, drought and heat treatments. Moreover, we examined leaf water loss and chlorophyll leaching of Z. xanthoxylum and analysed their relationships with wax composition under the above treatments.

KEY RESULTS

The leaf epidermis of Z. xanthoxylum was densely covered by cuticular wax, whereas the other four desert shrubs had trichomes or cuticular folds in addition to cuticular wax. The total amount of cuticular wax on leaves of Z. xanthoxylum and Ammopiptanthus mongolicus was significantly higher than that of the other three shrubs. Strikingly, C31 alkane, the most abundant component, composed >71 % of total alkanes in Z. xanthoxylum, which was higher than for the other four shrubs studied here. Salt, drought and heat treatments resulted in significant increases in the amount of cuticular wax. Of these treatments, the combined drought plus 45 °C treatment led to the largest increase (107 %) in the total amount of cuticular wax, attributable primarily to an increase of 122 % in C31 alkane. Moreover, the proportion of C31 alkane within total alkanes remained >75 % in all the above treatments. Notably, the water loss and chlorophyll leaching were reduced, which was negatively correlated with C31 alkane content.

CONCLUSION

Zygophyllum xanthoxylum could serve as a model desert plant for study of the function of cuticular wax in water retention because of its relatively uncomplicated leaf surface and because it accumulates C31 alkane massively to reduce cuticular permeability and resist abiotic stressors.

The Relationships between Waxes and Storage Quality Indexes of Fruits of Three Plum Cultivars

In the present study, the cuticular wax morphology, composition and the relationship with storage quality in three plum cultivars of Prunus salicina 'Kongxin' (KXL), Prunus salicina 'Fengtang' (FTL) and Prunus salicina 'Cuihong' (CHL) were investigated during storage at room temperature of 25 ± 1 °C. The results illustrated that the highest cuticular wax concentration was discovered in KXL, followed by FTL and the lowest in CHL. The fruit wax composition of the three plum cultivars was similar and principally composed of alkanes, alcohols, fatty acids, ketones, aldehydes, esters, triterpenes and olefins. Alcohols, alkanes and triterpenes were the dominant fruit wax compounds of the three plum cultivars. After storage for 20 d at room temperature, the variation of cuticular wax crystal structure and composition showed significant cultivar-associated differences. The total wax content decreased for FTL and CHL and increased for KXL, and the wax crystal degraded and melted together over time. The higher contents of the main components in the three plum cultivars were nonacosane, 1-triacontanol, 1-heneicosanol, nonacosan-10-one, octacosanal, ursolic aldehyde and oleic acid. Alcohols, triterpenes, fatty acids and aldehydes were most dramatically correlated with the softening of fruit and storage quality, and alkanes, esters and olefins were most significantly correlated with the water loss. Nonacosane and ursolic aldehyde can enhance the water retention of fruit. Overall, this study will provide a theoretical reference for the further precise development of edible plum fruit wax.

How Does Stomatal Density and Residual Transpiration Contribute to Osmotic Stress Tolerance?

Osmotic stress that is induced by salinity and drought affects plant growth and development, resulting in significant losses to global crop production. Consequently, there is a strong need to develop stress-tolerant crops with a higher water use efficiency through breeding programs. Water use efficiency could be improved by decreasing stomatal transpiration without causing a reduction in CO₂ uptake under osmotic stress conditions. The genetic manipulation of stomatal density could be one of the most promising strategies for breeders to achieve this goal. On the other hand, a substantial amount of water loss occurs across the cuticle without any contribution to carbon gain when the stomata are closed and under osmotic stress. The minimization of cuticular (otherwise known as residual) transpiration also determines the fitness and survival capacity of the plant under the conditions of a water deficit. The deposition of cuticular wax on the leaf epidermis acts as a limiting barrier for residual transpiration. However, the causal relationship between the frequency of stomatal density and plant osmotic stress tolerance and the link between residual transpiration and cuticular wax is not always straightforward, with controversial reports available in the literature. In this review, we focus on these controversies and explore the potential physiological and molecular aspects of controlling stomatal and residual transpiration water loss for improving water use efficiency under osmotic stress conditions via a comparative analysis of the performance of domesticated crops and their wild relatives.

The Plant Fatty Acyl Reductases

Fatty acyl reductase (FAR) is a crucial enzyme that catalyzes the NADPH-dependent reduction of fatty acyl-CoA or acyl-ACP substrates to primary fatty alcohols, which in turn acts as intermediate metabolites or metabolic end products to participate in the formation of plant extracellular lipid protective barriers (e.g., cuticular wax, sporopollenin, suberin, and taproot wax). FARs are widely present across plant evolution processes and play conserved roles during lipid synthesis. In this review, we provide a comprehensive view of FAR family enzymes, including phylogenetic analysis, conserved structural domains, substrate specificity, subcellular localization, tissue-specific expression patterns, their varied functions in lipid biosynthesis, and the regulation mechanism of FAR activity. Finally, we pose several questions to be addressed, such as the roles of FARs in tryphine, the interactions between transcription factors (TFs) and FARs in various environments, and the identification of post-transcriptional, translational, and post-translational regulators.

Comparative Analysis of Physiological, Hormonal and Transcriptomic Responses Reveal Mechanisms of Saline-Alkali Tolerance in Autotetraploid Rice (Oryza sativa L.)

Saline-alkali soil has posed challenges to the growth of agricultural crops, while polyploidy often show greater adaptability in diverse and extreme environments including saline-alkali stress, but its defense mechanisms in rice remain elusive. Herein, we explored the mechanisms of enhanced saline-alkali tolerance of autotetraploid rice 93-11T relative to diploid rice 93-11D, based on physiological, hormonal and transcriptomic profilings. Physiologically, the enhanced saline-alkali tolerance in 93-11T was manifested in higher soluble sugar accumulation and stronger superoxide dismutase (SOD) and peroxidase (POD) activities in leaves during 24 h after saline-alkali shock. Furthermore, various hormone levels in leaves of 93-11T altered greatly, such as the negative correlation between salicylic acid (SA) and the other four hormones changed to positive correlation due to polyploidy. Global transcriptome profiling revealed that the upregulated differentially expressed genes (DEGs) in leaves and roots of 93-11T were more abundant than that in 93-11D, and there were more DEGs in roots than in leaves under saline-alkali stress. Genes related to phytohormone signal transduction of auxin (AUX) and SA in roots, lignin biosynthesis in leaves or roots, and wax biosynthesis in leaves were obviously upregulated in 93-11T compared with 93-11D under saline-alkali condition. Collectively, 93-11T subjected to saline-alkali stress possibly possesses higher osmotic regulation ability due to cuticular wax synthesis, stronger negative regulation of reactive oxygen species (ROS) production by increasing the SA levels and maintaining relative lower levels of IAA, and higher antioxidant capacity by increasing activities of SOD and POD, as well as lignin biosynthesis. Our research provides new insights for exploring the mechanisms of saline-alkali tolerance in polyploid rice and discovering new gene targets for rice genetic improvement.

Melatonin Treatment Affects Wax Composition and Maintains Storage Quality in 'Kongxin' Plum (Prunus salicina L. cv) during Postharvest

Cuticular wax is an essential barrier against biological and abiotic stress and is also an important factor affecting fruit storage quality. This paper investigated the effect of melatonin treatment on cuticular wax and the storage quality of plum fruit at low temperature storage of 4 ± 1 °C. 'Kongxin' plum was treated with 150 μmol·L^-1 melatonin, dried overnight at room temperature 25 ± 1 °C, and then stored at 4 ± 1 °C for 40 d. The microstructure of the fruit epidermis was examined after 0, 20, and 40 d of storage, and the wax composition and fruit storage quality were measured at 10 d intervals. The results demonstrated that melatonin promoted the disintegration and thickening of rod-shaped waxy crystals of 'Kongxin' plum fruit and inhibited the combination of disintegrated wax and inner wax. Melatonin maintained fruit firmness and decreased the correlation between fruit firmness and other storage quality parameters. The correlation between firmness and wax composition was enhanced. Melatonin promoted long-chain alkanes that were positively correlated with firmness and water retention and strengthened the correlation between the length of the alkane chain and storage quality parameters but reduced the difference between alkane isomers and storage quality parameters.

Characterization of Cuticular Wax in Tea Plant and Its Modification in Response to Low Temperature

Cuticular wax ubiquitously covers the outer layer of plants and protects them against various abiotic and biotic stresses. Nevertheless, the characteristics of cuticular wax and its role in cold resistance in tea plants remain unclear. In our study, cuticular wax from different tissues, cultivars, and leaves during different spatio-temporal growth stages were characterized and compared in tea plants. The composition, distribution pattern, and structural profile of cuticular wax showed considerable tissue specificity, particularly in petals and seeds. During the spatial development of tea leaves, total wax content increased from the first to fifth leaf in June, while a decreasing pattern was observed in September. Additionally, the total wax content and number of wax compounds were enhanced, and the wax composition significantly varied with leaf growth from June to September. Ten cultivars showed considerable differences in total wax content and composition, such as the predominance of saturated fatty acids and primary alcohols in SYH and HJY cultivars, respectively. Correlation analysis suggested that n-hexadecanoic acid is positively related to cold resistance in tea plants. Further transcriptome analysis from cold-sensitive AJBC, cold-tolerant CYQ, and EC 12 cultivars indicated that the inducible expression of wax-related genes was associated with the cold tolerance of different cultivars in response to cold stress. Our results revealed the characterization of cuticular wax in tea plants and provided new insights into its modification in cold tolerance.

PtoMYB142, a poplar R2R3-MYB transcription factor, contributes to drought tolerance by regulating wax biosynthesis

Drought is one of the main environmental factors that limit plant development and growth. Accordingly, plants have evolved strategies to prevent water loss under drought stress, such as stomatal closure, maintenance of root water uptake, enhancement of stem water transport, and synthesis and deposition of cuticular wax. However, the molecular evidence of cuticular wax biosynthesis regulation in response to drought is limited in woody plants. Here, we identified an MYB transcription factor, Populus tomentosa Carr. MYB transcription factor (PtoMYB142), in response to drought stress from P. tomentosa. Over-expression of PtoMYB142 (PtoMYB142-OE) resulted in increased wax accumulation in poplar leaves, and significantly enhanced drought resistance. We found that the expression of wax biosynthesis genes CER4 and 3-ketoacyl CoA synthase (KCS) were markedly induced under drought stress, and significantly up-regulated in PtoMYB142-OE lines. Biochemical analysis confirmed that PtoMYB142 could directly bind to the promoter of CER4 and KCS6, and regulate their expression in P. tomentosa. Taken together, this study reveals that PtoMYB142 regulates cuticular wax biosynthesis to adapt to water-deficient conditions.

Genome-wide analysis of the WSD family in sunflower and functional identification of HaWSD9 involvement in wax ester biosynthesis and osmotic stress

The wax esters are important cuticular wax composition that cover the outer surface of plant organs and play a critical role in protection and energy metabolism. Wax ester synthesis in plant is catalyzed by a bifunctional wax ester synthase/acyl-CoA: diacylglycerol acyltransferase (WSD). Sunflower (Helianthus annuus L.) is an important oil crop in the world; however, little is known about WSD in sunflower. In this study, we identified and performed a functional analysis of twelve HaWSD genes from sunflower genome. Tissue-specific expression revealed that 12 HaWSD genes were differentially expressed in various organs and tissues of sunflower, except seeds. HaWSD genes were highly induced by salinity, drought, cold, and abscisic acid (ABA) in sunflower. To ascertain their function, HaWSD9, with highly expressed levels in stems and leaves, was cloned and expressed in a yeast mutant defective in triacylglycerol (TAG) biosynthesis. HaWSD9 complemented the phenotype by producing wax ester but not TAG in vivo, indicating that it functions as a wax ester synthase. Subcellular localization analysis indicated that HaWSD9 was located in the endoplasmic reticulum (ER). Heterologous introduction of HaWSD9 into Arabidopsis wsd1 mutant exhibited increased epicuticular wax crystals and cuticular wax contents on the stems. As compared with the wsd1 mutant, HaWSD9 overexpressing transgenic Arabidopsis showed less cuticle permeability, chlorophyll leaching and water loss rate. Further analysis showed that the HaWSD9 transgenics enhanced tolerance to ABA, mannitol, drought and salinity, and maintained higher leaf relative water content (RWC) than the wsd1 mutant under drought stress, suggesting that HaWSD9 play an important physiological role in stress response as well as wax synthase. These results contribute to understanding the function of HaWSD genes in wax ester synthesis and stress tolerance in sunflower.

Temperature has a major effect on the cuticular wax composition of bilberry (Vaccinium myrtillus L.) fruit

Cuticle is the first layer protecting plants against external biotic and abiotic factors and is responsive to climatic factors as well as determined by genetic adaptations. In this study, the chemical composition of bilberry fruit cuticular wax was investigated through a latitudinal gradient from Latvia (56°N 24°E) through Finland (65°N 25°E) to northern Norway (69°N 18°E) in two seasons 2018 and 2019. Changes in the major cuticular wax compounds, including triterpenoids, fatty acids, alkanes, aldehydes, ketones, and primary alcohols, were detected by GC-MS analysis. Generally, a decreasing trend in the proportion of triterpenoids from southern to northern latitudes, accompanied with an increase in proportion of fatty acids, aldehydes, and alkanes, in bilberry fruit cuticular wax was observed. A correlation analysis between climatic factors with proportion of wax compounds indicated that temperature was the main factor affecting the cuticular wax composition in bilberries. A controlled phytotron experiment with southern and northern bilberry ecotypes confirmed the major effect of temperature on bilberry fruit cuticular wax load and composition. Elevated temperature increased wax load most in berries of northern ecotypes. The level of triterpenoids was higher, while levels of fatty acids and alkanes were lower, in wax of bilberry fruits ripened at 18°C compared to 12°C in both northern and southern ecotypes. Based on our results, it can be postulated that the predicted increase in temperature due to climate change leads to alterations in fruit cuticular wax load and composition. In northern ecotypes, the alterations were especially evident.

The histidine phosphotransfer AHP4 plays a negative role in Arabidopsis plant response to drought

Cytokinin plays an important role in plant stress responses via a multistep signaling pathway, involving the histidine phosphotransfer proteins (HPs). In Arabidopsis thaliana, the AHP2, AHP3 and AHP5 proteins are known to affect drought responses; however, the role of AHP4 in drought adaptation remains undetermined. In the present study, using a loss-of-function approach we showed that AHP4 possesses an important role in the response of Arabidopsis to drought. This is evidenced by the higher survival rates of ahp4 than wild-type (WT) plants under drought conditions, which is accompanied by the downregulated AHP4 expression in WT during periods of dehydration. Comparative transcriptome analysis of ahp4 and WT plants revealed AHP4-mediated expression of several dehydration- and/or abscisic acid-responsive genes involved in modulation of various physiological and biochemical processes important for plant drought acclimation. In comparison with WT, ahp4 plants showed increased wax crystal accumulation in stems, thicker cuticles in leaves, greater sensitivity to exogenous abscisic acid at germination, narrow stomatal apertures, heightened leaf temperatures during dehydration, and longer root length under osmotic stress. In addition, ahp4 plants showed greater photosynthetic efficiency, lower levels of reactive oxygen species, reduced electrolyte leakage and lipid peroxidation, and increased anthocyanin contents under drought, when compared with WT. These differences displayed in ahp4 plants are likely due to upregulation of genes that encode enzymes involved in reactive oxygen species scavenging and non-enzymatic antioxidant metabolism. Overall, our findings suggest that AHP4 plays a crucial role in plant drought adaptation.

Effects of cuticular wax on the postharvest physiology in fragrant pear at different storages

BACKGROUND

Epidermal wax is an important factor affecting the storage quality of fruits and vegetables. Previous studies have shown that the epidermal wax of fruits undergoes significant changes during storage, but there are few studies on the effects of different storage methods on the changes in waxes and the relationship with storage quality. To investigate the effect of cuticular wax on the postharvest physiology in fragrant pear, equal numbers of fragrant pear fruits were stored in room temperature storage (control), cold storage and controlled atmosphere (CA) storage environs, respectively.

RESULTS

Gas chromatography-mass spectrometry analysis revealed that the prevailing compositions of cuticular wax of fragrant pear were alkanes, alkenes, alcohols, aldehydes, esters and fatty acids. Compared with the control, cold storage and CA storage significantly inhibited changes in postharvest physiology, total wax contents and wax compositions of fragrant pear, and the effects of CA storage were more pronounced than cold storage. Under different storage methods, total wax contents and wax compositions show different correlations with various physiological indicators.

CONCLUSION

The results obtained in the present study indicate that cold storage and CA storage altered the fragrant pear cuticular wax contents and constituents, thus changing the postharvest physiology quality. The changes in the metabolism of wax components caused by the changes in storage environment mainly affect the changes in the hardness of fragrant pears. The present study provides a theoretical basis for the preservation and storage of fruits. © 2022 Society of Chemical Industry.

Regulatory mechanisms underlying cuticular wax biosynthesis

Plants are sessile organisms that have developed hydrophobic cuticles that cover their aerial epidermal cells to protect them from terrestrial stresses. The cuticle layer is mainly composed of cutin, a polyester of hydroxy and epoxy fatty acids, and cuticular wax, a mixture of very-long-chain fatty acids (>20 carbon atoms) and their derivatives, aldehydes, alkanes, ketones, alcohols, and wax esters. During the last 30 years, forward and reverse genetic, transcriptomic, and biochemical approaches have enabled the identification of key enzymes, transporters, and regulators involved in the biosynthesis of cutin and cuticular waxes. In particular, cuticular wax biosynthesis is significantly influenced in an organ-specific manner or by environmental conditions, and is controlled using a variety of regulators. Recent studies on the regulatory mechanisms underlying cuticular wax biosynthesis have enabled us to understand how plants finely control carbon metabolic pathways to balance between optimal growth and development and defense against abiotic and biotic stresses. In this review, we summarize the regulatory mechanisms underlying cuticular wax biosynthesis at the transcriptional, post-transcriptional, post-translational, and epigenetic levels.

Increased cuticular wax deposition does not change residual foliar transpiration

The effect of contrasting environmental growth conditions (in vitro tissue culture, ex vitro acclimatisation, climate chamber, greenhouse and outdoor) on leaf development, cuticular wax composition, and foliar transpiration of detached leaves of the Populus × canescens clone 84 K were investigated. Our results show that total amounts of cuticular wax increased more than 10-fold when cultivated in different growth conditions, whereas qualitative wax composition did not change. With exception of plants directly taken from tissue culture showing rapid dehydration, rates of water loss (residual foliar transpiration) of intact but detached leaves were constant and independent from growth conditions and thus independent from increasing wax amounts. Since cuticular transpiration measured with isolated astomatous P. × canescens cuticles was identical to residual foliar transpiration rates of detached leaves, our results confirm that cuticular transpiration of P. × canescens leaves can be predicted with high accuracy from residual transpiration of detached leaves after stomatal closure. Our results convincingly show that more than 10-fold increased wax amounts in P. × canescens cuticles do not lead to decreased rates of residual (cuticular) transpiration.

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.

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

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

KLU/CYP78A5, a Cytochrome P450 Monooxygenase Identified via Fox Hunting, Contributes to Cuticle Biosynthesis and Improves Various Abiotic Stress Tolerances

Acquired osmotolerance after salt stress is widespread among Arabidopsis thaliana (Arabidopsis) accessions. Most salt-tolerant accessions exhibit acquired osmotolerance, whereas Col-0 does not. To identify genes that can confer acquired osmotolerance to Col-0 plants, we performed full-length cDNA overexpression (FOX) hunting using full-length cDNAs of halophyte Eutrema salsugineum, a close relative of Arabidopsis. We identified EsCYP78A5 as a gene that can confer acquired osmotolerance to Col-0 wild-type (WT) plants. EsCYP78A5 encodes a cytochrome P450 monooxygenase and the Arabidopsis ortholog is known as KLU. We also demonstrated that transgenic Col-0 plants overexpressing AtKLU (AtKLUox) exhibited acquired osmotolerance. Interestingly, KLU overexpression improved not only acquired osmotolerance but also osmo-shock, salt-shock, oxidative, and heat-stress tolerances. Under normal conditions, the AtKLUox plants showed growth retardation with shiny green leaves. The AtKLUox plants also accumulated higher anthocyanin levels and developed denser cuticular wax than WT plants. Compared to WT plants, the AtKLUox plants accumulated significantly higher levels of cutin monomers and very-long-chain fatty acids, which play an important role in the development of cuticular wax and membrane lipids. Endoplasmic reticulum (ER) stress induced by osmotic or heat stress was reduced in AtKLUox plants compared to WT plants. These findings suggest that KLU is involved in the cuticle biosynthesis, accumulation of cuticular wax, and reduction of ER stress induced by abiotic stresses, leading to the observed abiotic stress tolerances.

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.

ECERIFERUM 10 Encoding an Enoyl-CoA Reductase Plays a Crucial Role in Osmotolerance and Cuticular Wax Loading in Arabidopsis

Acquired osmotolerance induced after salt stress is widespread across Arabidopsis thaliana (Arabidopsis) accessions (e.g., Bu-5). However, it remains unclear how this osmotolerance is established. Here, we isolated a mutant showing an acquired osmotolerance-defective phenotype (aod2) from an ion-beam-mutagenized M2 population of Bu-5. aod2 was impaired not only in acquired osmotolerance but also in osmo-shock, salt-shock, and long-term heat tolerances compared with Bu-5, and it displayed abnormal morphology, including small, wrinkled leaves, and zigzag-shaped stems. Genetic analyses of aod2 revealed that a 439-kbp region of chromosome 4 was translocated to chromosome 3 at the causal locus for the osmosensitive phenotype. The causal gene of the aod2 phenotype was identical to ECERIFERUM 10 (CER10), which encodes an enoyl-coenzyme A reductase that is involved in the elongation reactions of very-long-chain fatty acids (VLCFAs) for subsequent derivatization into cuticular waxes, storage lipids, and sphingolipids. The major components of the cuticular wax were accumulated in response to osmotic stress in both Bu-5 WT and aod2. However, less fatty acids, primary alcohols, and aldehydes with chain length ≥ C30 were accumulated in aod2. In addition, aod2 exhibited a dramatic reduction in the number of epicuticular wax crystals on its stems. Endoplasmic reticulum stress mediated by bZIP60 was increased in aod2 under osmotic stress. The only cer10 showed the most pronounced loss of epidermal cuticular wax and most osmosensitive phenotype among four Col-0-background cuticular wax-related mutants. Together, the present findings suggest that CER10/AOD2 plays a crucial role in Arabidopsis osmotolerance through VLCFA metabolism involved in cuticular wax formation and endocytic membrane trafficking.

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.

Molecular Cloning and Functional Analysis of GmLACS2-3 Reveals Its Involvement in Cutin and Suberin Biosynthesis along with Abiotic Stress Tolerance

Cutin and wax are the main precursors of the cuticle that covers the aerial parts of plants and provide protection against biotic and abiotic stresses. Long-chain acyl-CoA synthetases (LACSs) play diversified roles in the synthesis of cutin, wax, and triacylglycerol (TAG). Most of the information concerned with LACS functions is obtained from model plants, whereas the roles of LACS genes in Glycine max are less known. Here, we have identified 19 LACS genes in Glycine max, an important crop plant, and further focused our attention on 4 LACS2 genes (named as GmLACS2-1, 2, 3, 4, respectively). These GmLACS2 genes display different expression patterns in various organs and also show different responses to abiotic stresses, implying that these genes might play diversified functions during plant growth and against stresses. To further identify the role of GmLACS2-3, greatly induced by abiotic stresses, we transformed a construct containing its full length of coding sequence into Arabidopsis. The expression of GmLACS2-3 in an Arabidopsis atlacs2 mutant greatly suppressed its phenotype, suggesting it plays conserved roles with that of AtLACS2. The overexpression of GmLACS2-3 in wild-type plants significantly increased the amounts of cutin and suberin but had little effect on wax amounts, indicating the specific role of GmLACS2-3 in the synthesis of cutin and suberin. In addition, these GmLACS2-3 overexpressing plants showed enhanced drought tolerance. Taken together, our study deepens our understanding of the functions of LACS genes in different plants and also provides a clue for cultivating crops with strong drought resistance.

Chemical Composition, Crystal Morphology, and Key Gene Expression of the Cuticular Waxes of Goji (Lycium barbarum L.) Berries

The cuticular wax of fruit is closely related to quality, storability, and pathogen susceptibility after harvest. However, little is known about the cuticular wax of goji berry (Lycium barbarum L.) cultivars. In the present study, the chemical composition, crystal structures, and expression levels of associated genes of the cuticular wax of six goji cultivars were investigated. We detected 70 epicuticular wax compounds in six goji cultivars. Among them, fatty acids, alkanes, and primary alcohols were the major components of the cuticular wax of goji berries, which were related to the formation of irregular lamellar crystal structures. The terpenoid compounds in the cuticular wax of goji berries were highly resistant to Alternaria rot. Moreover, the CER1, CER6, LACS1, MAH1, LTP4, ABC11, MYB96, and WIN1 genes in goji berries might be closely related to wax synthesis. These results provide valuable information for breeding and screening goji cultivars suitable for postharvest storage.

Environment-Driven Adaptations of Leaf Cuticular Waxes Are Inheritable for Medicago ruthenica

Cuticular waxes covering the plant surface play pivotal roles in helping plants adapt to changing environments. However, it is still not clear whether the responses of plant cuticular waxes to their growing environments are inheritable. We collected seeds of Medicago ruthenica (a perennial legume) populations from 30 growing sites in northern China and examined the variations of leaf cuticular waxes in a common garden experiment. Four wax genes, MrFAR3-1, MrFAR3-2, MrCER1, and MrKCS1, involved in biosynthesis of predominant wax classes (primary alcohol and alkane) and wax precursors, were isolated to test the contributions of genetic variations of the coding sequences (CDS) and the promoter sequences and epigenetic modifications. The plasticity responses of the cuticular waxes were further validated by two stress-modeling experiments (drought and enhancing ultraviolet B). Great variations in total wax coverage and abundance of wax classes or wax compounds were observed among M. ruthenica populations in a common garden experiment. Stress-modeling experiments further validated that M. ruthenica would alter leaf wax depositions under changed growing conditions. The transcriptional levels of the wax genes were positively or negatively correlated with amounts of cuticular waxes. However, the analysis of promoter methylation showed that the methylation level of the promoter region was not associated with their expressions. Although both promoter sequences and CDS showed a number of polymorphic sites, the promoters were not naturally selected and insignificant difference could be observed in the numbers and types of acting elements of the four wax genes among populations. In contrast, the CDS of the wax genes were naturally selected, with a number of missense mutations resulting in alterations of the amino acid as well as their isoelectric points and polarities, which could impact on enzyme function/activity. We conclude that long-term adaptation under certain environments would induce genetic mutation of wax biosynthesis genes, resulting in inheritable alterations of cuticular wax depositions.

A co-opted steroid synthesis gene, maintained in sorghum but not maize, is associated with a divergence in leaf wax chemistry

Virtually all above-ground plant surfaces, such as leaf and stem exteriors, are covered in a cuticle: a wax-infused polyester. This waxy biocomposite is the largest interface between Earth’s biosphere and atmosphere. Its chemical composition is not only highly tuned to mediate nonstomatal water loss, but it also self-assembles to produce superhydrophobic surfaces, protects against UV radiation, and contains bioactive compounds that help resist microbial attack. Developing fundamental knowledge of waxy biocomposites, particularly those on crop species, is a prerequisite for an understanding of their structure–function relationships. Here, we uncover a likely genetic basis for the presence and absence, respectively, of triterpenoids in the leaf waxes of sorghum and maize—compounds previously associated with creating heat-tolerant cuticular water barriers.

Virtually all land plants are coated in a cuticle, a waxy polyester that prevents nonstomatal water loss and is important for heat and drought tolerance. Here, we describe a likely genetic basis for a divergence in cuticular wax chemistry between Sorghum bicolor, a drought tolerant crop widely cultivated in hot climates, and its close relative Zea mays (maize). Combining chemical analyses, heterologous expression, and comparative genomics, we reveal that: 1) sorghum and maize leaf waxes are similar at the juvenile stage but, after the juvenile-to-adult transition, sorghum leaf waxes are rich in triterpenoids that are absent from maize; 2) biosynthesis of the majority of sorghum leaf triterpenoids is mediated by a gene that maize and sorghum both inherited from a common ancestor but that is only functionally maintained in sorghum; and 3) sorghum leaf triterpenoids accumulate in a spatial pattern that was previously shown to strengthen the cuticle and decrease water loss at high temperatures. These findings uncover the possibility for resurrection of a cuticular triterpenoid-synthesizing gene in maize that could create a more heat-tolerant water barrier on the plant’s leaf surfaces. They also provide a fundamental understanding of sorghum leaf waxes that will inform efforts to divert surface carbon to intracellular storage for bioenergy and bioproduct innovations.

Dissecting the Roles of Cuticular Wax in Plant Resistance to Shoot Dehydration and Low-Temperature Stress in Arabidopsis

Cuticular waxes are a mixture of hydrophobic very-long-chain fatty acids and their derivatives accumulated in the plant cuticle. Most studies define the role of cuticular wax largely based on reducing nonstomatal water loss. The present study investigated the role of cuticular wax in reducing both low-temperature and dehydration stress in plants using Arabidopsis thaliana mutants and transgenic genotypes altered in the formation of cuticular wax. cer3-6, a known Arabidopsis wax-deficient mutant (with distinct reduction in aldehydes, n-alkanes, secondary n-alcohols, and ketones compared to wild type (WT)), was most sensitive to water loss, while dewax, a known wax overproducer (greater alkanes and ketones compared to WT), was more resistant to dehydration compared to WT. Furthermore, cold-acclimated cer3-6 froze at warmer temperatures, while cold-acclimated dewax displayed freezing exotherms at colder temperatures compared to WT. Gas Chromatography-Mass Spectroscopy (GC-MS) analysis identified a characteristic decrease in the accumulation of certain waxes (e.g., alkanes, alcohols) in Arabidopsis cuticles under cold acclimation, which was additionally reduced in cer3-6. Conversely, the dewax mutant showed a greater ability to accumulate waxes under cold acclimation. Fourier Transform Infrared Spectroscopy (FTIR) also supported observations in cuticular wax deposition under cold acclimation. Our data indicate cuticular alkane waxes along with alcohols and fatty acids can facilitate avoidance of both ice formation and leaf water loss under dehydration stress and are promising genetic targets of interest.

Variation in Petal and Leaf Wax Deposition Affects Cuticular Transpiration in Cut Lily Flowers

The vase life of cut flowers is largely affected by post-harvest water loss. Cuticular wax is the primary barrier to uncontrolled water loss for aerial plant organs. Studies on leaf cuticular transpiration have been widely conducted; however, little is known about cuticular transpiration in flowers. Here, the cuticular transpiration rate and wax composition of three lily cultivars were determined. The minimum water conductance of tepal cuticles was higher at the green bud than open flower stage. Lily cuticular transpiration exhibited cultivar- and organ-specific differences, where transpiration from the tepals was higher than leaves and was higher in the 'Huang Tianba' than 'Tiber' cultivar. The overall wax coverage of the tepals was higher compared to that of the leaves. Very-long-chain aliphatics were the main wax constituents and were dominated by n-alkanes with carbon (C) chain lengths of C₂₇ and C₂₉, and C₂₉ and C₃₁ in the tepal and leaf waxes, respectively. Primary alcohols and fatty acids as well as small amounts of alkyl esters, ketones, and branched or unsaturated n-alkanes were also detected in both tepal and leaf waxes, depending on the cultivar and organ. In addition, the chain-length distributions were similar between compound classes within cultivars, whereas the predominant C-chain lengths were substantially different between organs. This suggests that the less effective transpiration barrier provided by the tepal waxes may result from the shorter C-chain aliphatics in the tepal cuticle, compared to those in the leaf cuticle. These findings provide further insights to support the exploration of potential techniques for extending the shelf life of cut flowers based on cuticular transpiration barrier properties.

Variations in Triterpenoid Deposition in Cuticular Waxes during Development and Maturation of Selected Fruits of Rosaceae Family

The process of fruit ripening involves many chemical changes occurring not only in the mesocarp but also in the epicarp, including changes in the triterpenoid content of fruit cuticular waxes that can modify the susceptibility to pathogens and mechanical properties of the fruit surface. The aim of the study was the determination of the ripening-related changes in the triterpenoid content of fruit cuticular waxes of three plant species from the Rosaceae family, including rugosa rose (Rosa rugosa), black chokeberry (Aronia melanocarpa var. “Galicjanka”) and apple (Malus domestica var. “Antonovka”). The triterpenoid and steroid content in chloroform-soluble cuticular waxes was determined by a GC-MS/FID method at four different phenological stages. The profile of identified compounds was rather similar in selected fruit samples with triterpenoids with ursane-, oleanane- and lupane-type carbon skeletons, prevalence of ursolic acid and the composition of steroids. Increasing accumulation of triterpenoids and steroids, as well as the progressive enrichment of the composition of these compounds in cuticular wax during fruit development, was observed. The changes in triterpenoid content resulted from modifications of metabolic pathways, particularly hydroxylation and esterification, that can alter interactions with complementary functional groups of aliphatic constituents and lead to important changes in fruit surface quality.

EgMIXTA1, a MYB-Type Transcription Factor, Promotes Cuticular Wax Formation in Eustoma grandiflorum Leaves

In the aerial plant organs, cuticular wax forms a hydrophobic layer that can protect cells from dehydration, repel pathogen attacks, and prevent organ fusion during development. The MIXTA gene encodes an MYB-like transcription factor, which is associated with epicuticular wax biosynthesis to increase the wax load on the surface of leaves. In this study, the AmMIXTA-homologous gene EgMIXTA1 was functionally characterized in the Eustoma grandiflorum. EgMIXTA1 was ubiquitously, but highly, expressed in leaves and buds. We identified the Eustoma MIXTA homolog and developed the plants for overexpression. EgMIXTA1-overexpressing plants had more wax crystal deposition on the leaf surface compared to wild-type and considerably more overall cuticular wax. In the leaves of the overexpression line, the cuticular transpiration occurred more slowly than in those of non-transgenic plants. Analysis of gene expression indicated that several genes, such as EgCER3, EgCER6, EgCER10, EgKCS1, EgKCR1, and EgCYP77A6, which are known to be involved in wax biosynthesis, were induced by EgMIXTA1-overexpression lines. Expression of another gene, WAX INDUCER1/SHINE1, encoding a transcription factor that stimulates the production of cutin, was also significantly higher in the overexpressors than in wild-type. However, the expression of a lipid-related gene, EgABCG12, did not change relative to the wild-type. These results suggest that EgMIXTA1 is involved in the biosynthesis of cuticular waxes.

An apple long-chain acyl-CoA synthetase 2 gene enhances plant resistance to abiotic stress by regulating the accumulation of cuticular wax

Apple cuticular wax can protect plants from environmental stress, determine fruit luster and improve postharvest fruit storage quality. In recent years, dry weather, soil salinization and adverse environmental conditions have led to declines in apple fruit quality. However, few studies have reported the molecular mechanisms of apple cuticular wax biosynthesis. In this study, we identified a long-chain acyl-CoA synthetase MdLACS2 gene from apple. The MdLACS2 protein contained an AMP-binding domain and demonstrated long-chain acyl-CoA synthetase activity. MdLACS2 transgenic Arabidopsis exhibited reductions in epidermal permeability and water loss; change in the expression of genes related to cuticular wax biosynthesis, transport and transcriptional regulation; and differences in the composition and ultrastructure of cuticular wax. Moreover, the accumulation of cuticular wax enhanced the resistance of MdLACS2 transgenic plants to drought and salt stress. The main protein functional interaction networks of LACS2 were predicted, revealing a preliminary molecular regulation pathway for MdLACS2-mediated wax biosynthesis in apple. Our study provides candidate genes for breeding apple varieties and rootstocks with better fruit quality and higher stress resistance.

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.

Wax biosynthesis in response to danger: its regulation upon abiotic and biotic stress

The plant cuticle is the first physical barrier between land plants and their terrestrial environment. It consists of the polyester scaffold cutin embedded and sealed with organic, solvent-extractable cuticular waxes. Cuticular wax ultrastructure and chemical composition differ with plant species, developmental stage and physiological state. Despite this complexity, cuticular wax consistently serves a critical role in restricting nonstomatal water loss. It also protects the plant against other environmental stresses, including desiccation, UV radiation, microorganisms and insects. Within the broader context of plant responses to abiotic and biotic stresses, our knowledge of the explicit roles of wax crystalline structures and chemical compounds is lacking. In this review, we summarize our current knowledge of wax biosynthesis and regulation in relation to abiotic and biotic stresses and stress responses.

WOOLLY, interacting with MYB transcription factor MYB31, regulates cuticular wax biosynthesis by modulating CER6 expression in tomato

Cuticular waxes play a crucial role not only in plant defense against biotic and abiotic stresses, but also in the quality and storability of fruits, such as the tomato (Solanum lycopersicum). Although the biosynthetic pathways of waxes have been extensively characterized, the regulatory mechanisms underlying wax biosynthesis in tomato remain largely unclear. Here, we show that Woolly (Wo), a multicellular trichome regulator, is involved in modulating wax biosynthesis in tomato. Wo enhances the expression of the wax biosynthetic genes SlCER6, SlKCR1, and SlPAS2, and the wax transporter gene SlLTP, and thereby promotes wax accumulation. Furthermore, Wo directly binds to the L1-box in the promoter of SlCER6, an essential element of the very-long-chain fatty acid elongase complex. Intriguingly, overexpression (OE) or knock-down of SlMYB31, an MYB transcription factor that physically interacts with Wo in vivo and in vitro, produces marked changes in wax composition, and whereas Wo knock-down inhibits wax accumulation in SlMYB31-OE lines, SlMYB31 knock-down inhibits wax accumulation in Wo-OE lines, implying that these two genes function in the same pathway. Lastly, SlCER6 expression is induced by abscisic acid in a manner that is partially dependent on Wo. These results demonstrate that Wo and SlMYB31 cooperatively control tomato cuticular wax biosynthesis by regulating the expression of SlCER6.