Arabidopsis cuticular waxes: advances in synthesis, export and regulation

Tea polyphenols coating improves physiological properties, microstructure and chemical composition of cuticle to suppress quality deterioration of passion fruit during cold storage

The plant cuticle plays a crucial role in modulating postharvest quality and extending shelf life of horticultural crops. Passion fruit often suffers from quality degradation primarily due to peel wrinkling after harvest. Tea polyphenols (TPs) hold potential for enhancing postharvest preservation. However, the specific effects of TPs coating on preservation of passion fruit, as well as the underlying mechanisms involving cuticle regulation, have not been thoroughly investigated. This study demonstrated that treating 'Qinmi no.9' passion fruit with TPs at a concentration of 0.1 g L-1 significantly mitigates weight loss, maintains firmness, and reduces cell membrane permeability during storage at 10 °C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that TPs treatment notably enhances cuticle thickness and structural integrity. Furthermore, gas chromatography-mass spectrometry (GC-MS) and metabolomics analyses indicated that TPs treatment obviously promotes the accumulation of palmitic acid, stearic acid, and their derivatives-primarily 12-Octadecenoic acid and 10(E)-Octadecenoic acid-as well as increases the levels of 11-Octadecenoic acid, primary alcohols such as 1-Eicosanol, and long-chain alkanes (including C31 and C32 alkanes) in the fruit peel cuticle. These biochemical changes contribute to the quality maintenance of passion fruit during cold storage. The findings suggest that TPs treatment is a promising biological strategy for extending shelf life and mitigating quality degradation by regulating cuticle metabolism in postharvest passion fruit.

Metabolic and gene-expression analyses reveal developmental dynamics of cutin deposition in pomegranate fruit grown under different environmental conditions

The chemical and transcriptional changes in the cuticle of pomegranate (Punica granatum L.) fruit grown under different environmental conditions were studied. We collected fruit from three orchards located in different regions in Israel, each with a distinct microclimate. Fruit were collected at six phenological stages, and cutin monomers in the fruit cuticle were profiled by gas chromatography-mass spectrometry (GC-MS), along with qPCR transcript-expression analyses of selected cutin-related genes. While fruit phenotypes were comparable along development in all three orchards, principal component analyses of cutin monomer profiles suggested clear separation between cuticle samples of young green fruit to those of maturing fruit. Moreover, total cutin contents in green fruit were lower in the orchard characterized by a hot and dry climate compared to orchards with moderate temperatures. The variances detected in total cutin contents between orchards corresponded well with the expression patterns of BODYGUARD, a key biosynthetic gene operating in the cutin biosynthetic pathway. Based on our extraction protocols, we found that the cutin polyester that builds the pomegranate fruit cuticle accumulates some levels of gallic acid-the precursor of punicalagin, a well-known potent antioxidant metabolite in pomegranate fruit. The gallic acid was also one of the predominant metabolites contributing to the variability between developmental stages and orchards, and its accumulation levels were opposite to the expression patterns of the UGT73AL1 gene which glycosylates gallic acid to synthesize punicalagin. To the best of our knowledge, this is the first detailed composition of the cutin polyester that forms the pomegranate fruit cuticle.

Compositional variances in petal cuticular wax of eight rose species and their impacts on vase life under water-loss stress

Cuticular wax is the first barrier between plants and the environment. Here, the densities of cuticular wax crystals on the petals of eight rose cultivars were determined to be sparse; the crystals were mostly granular and only a few rod-shaped crystals were observed in 'Sweet'. The total contents and chemical compositions of waxes were significantly different among the rose varieties. The waxes were mainly composed of n-alkanes, iso-alananes and alkenes. Under water-loss stress, 'Diana' and 'Carola' cultivars, having high petal wax contents, had low water permeability levels, long vase lives, high relative water contents and low relative conductivity levels. However, the low wax contents of the 'Jubilance' and 'Candy Avalanche' cultivars resulted in high water permeability levels and short vase lives. Pearson correlation analyses showed the total wax content in petal epidermis was positively correlated with vase life. The data provide novel insights into the compositional variances in the cuticular waxes of rose petals and their impacts on cut rose vase lives.

BnUC1 Is a Key Regulator of Epidermal Wax Biosynthesis and Lipid Transport in Brassica napus

The bHLH (basic helix-loop-helix) transcription factor AtCFLAP2 regulates epidermal wax accumulation, but the underlying molecular mechanism remains unknown. We obtained BnUC1mut (BnaA05g18250D homologous to AtCFLAP2) from a Brassica napus mutant with up-curling leaves (Bnuc1) and epidermal wax deficiency via map-based cloning. BnUC1mut contains a point mutation (N200S) in the conserved dimerization domain. Overexpressing BnUC1mut in ZS11 (Zhongshuang11) significantly decreased the leaf epidermal wax content, resulting in up-curled and glossy leaves. In contrast, knocking out BnUC1mut in ZS11-NIL (Zhongshuang11-near-isogenic line) restored the normal leaf phenotype (i.e., flat) and significantly increased the leaf epidermal wax content. The point mutation weakens the ability of BnUC1mut to bind to the promoters of VLCFA (very-long-chain fatty acids) synthesis-related genes, including KCS (β-ketoacyl coenzyme synthase) and LACS (long-chain acyl CoA synthetase), as well as lipid transport-related genes, including LTP (non-specific lipid transfer protein). The resulting sharp decrease in the transcription of genes affecting VLCFA biosynthesis and lipid transport disrupts the normal accumulation of leaf epidermal wax. Thus, BnUC1 influences epidermal wax formation by regulating the expression of LTP and genes associated with VLCFA biosynthesis. Our findings provide a foundation for future investigations on the mechanism mediating plant epidermal wax accumulation.

SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 13 (SLP13) together with SPL9 redundantly regulates wax biosynthesis under drought stress

Wax biosynthesis is closely controlled by many regulators under different environmental conditions. We have previously shown that the module miR156-SQUAMOSA PROMOTER BINDING PROTEIN-LIKE9 (SPL9)-DEWAX is involved in the diurnal regulation of wax production; however, it was not determined whether other SPLs are also involved in wax synthesis. Here, we report that SPL13 also regulates drought-induced wax production, by directly and indirectly affecting the expression of the two wax biosynthesis genes ECERIFERUM1 (CER1) and CER4, respectively. In addition, we show that SPL13 together with SPL9 redundantly regulates wax accumulation under both normal and drought stress conditions, and that simultaneous mutation of both genes additively increases cuticle permeability and decreases drought tolerance. However, in contrast to SPL9, SPL13 does not seem to participate in the DEWAX-mediated diurnal regulation of wax production.

Cutinsomes of Malus Mill. (Rosaceae) leaf and pericarp: genesis, localization, and transport

New data were obtained on specific bionanostructures, cutinsomes, which are involved in the formation of cuticles on the surface of leaf blades and pericarp of Malus domestica Borkh (Malus Mill., Rosaceae)introduced to the mountains at the altitudes of 1200 and 1700 m above sea level. Cutinsomes, which are electron-dense structures of spherical shape, have been identified by transmission electron microscopy. It was demonstrated that plastids can be involved in the synthesis of their constituent nanocomponents. The greatest number of nanoparticles was observed in the granal thylakoid lumen of the chloroplasts in palisade mesophyll cells and pericarp hypodermal cells. The transmembrane transport of cutinsomes into the cell wall cuticle proper by exocytosis has been visualized for the first time. The plasma membrane is directly involved in the excretion of nanostructures from the cell. Nanoparticles of cutinsomes in the form of necklace-like formations line up in a chain near cell walls, merge into larger conglomerates and are loaded into plasmalemma invaginations, and then, in membrane packing, they move into the cuticle, which covers both outer and inner cell walls of external tissues. The original materials obtained by us supplement the ideas about the non-enzymatic synthesis of cuticle components available in the literature and expand the cell compartment geography involved in this process.

Plant and algal lipidomes: Analysis, composition, and their societal significance

Plants and algae play a crucial role in the earth's ecosystems. Through photosynthesis they convert light energy into chemical energy, capture CO2 and produce oxygen and energy-rich organic compounds. Photosynthetic organisms are primary producers and synthesize the essential omega 3 and omega 6 fatty acids. They have also unique and highly diverse complex lipids, such as glycolipids, phospholipids, triglycerides, sphingolipids and phytosterols, with nutritional and health benefits. Plant and algal lipids are useful in food, feed, nutraceutical, cosmeceutical and pharmaceutical industries but also for green chemistry and bioenergy. The analysis of plant and algal lipidomes represents a significant challenge due to the intricate and diverse nature of their composition, as well as their plasticity under changing environmental conditions. Optimization of analytical tools is crucial for an in-depth exploration of the lipidome of plants and algae. This review highlights how lipidomics analytical tools can be used to establish a complete mapping of plant and algal lipidomes. Acquiring this knowledge will pave the way for the use of plants and algae as sources of tailored lipids for both industrial and environmental applications. This aligns with the main challenges for society, upholding the natural resources of our planet and respecting their limits.

Compositional analysis of tuberose floral cuticle sheds light on scent volatile emission and cuticle deposition

The plant cuticle acts as a vital shield, safeguarding against environmental and biological stresses. Tuberose flowers, specifically the 'Calcutta Single' and 'Calcutta Double' cultivars of Agave amica (syn. Polianthes tuberosa), are highly prized in India for their aromatic and aesthetic appeal. This research examines the tuberose floral cuticle using GC-MS to identify key chemical components and their characteristics. The analysis uncovered six major chemical classes viz. fatty acids, fatty alcohols, esters, alkanes, alkenes, and aldehydes as the primary constituents of the cuticular wax. Fatty acids and fatty alcohols are dominant in both cultivars, with 'Calcutta Single' displaying a higher ester content than 'Calcutta Double'. This study represents the inaugural in-depth exploration of cuticular wax in tuberose flowers, presenting a comparative investigation into these prominent cultivars.

Genome-Wide Identification and Characterization of Maize Long-Chain Acyl-CoA Synthetases and Their Expression Profiles in Different Tissues and in Response to Multiple Abiotic Stresses

Long-chain acyl-CoA synthetases (LACSs) are essential enzymes that activate free fatty acids to fatty acyl-CoA thioesters, playing key roles in fatty acid (FA) catabolism, lipid synthesis and storage, epidermal wax synthesis, and stress tolerance. Despite their importance, comprehensive information about LACS genes in maize, a primary food crop, remains scarce. In the present work, eleven maize LACS genes were identified and mapped across five chromosomes. Three pairs of segmentally duplicated genes were detected in the maize LACS gene family, which underwent significant purifying selection (Ka/Ks < 1). Subsequently, phylogenetic analysis indicated that ZmLACS genes were divided into four subclasses, as supported by highly conserved motifs and gene structures. On the basis of the PlantCARE database, analysis of the ZmLACS promoter regions revealed various cis-regulatory elements related to tissue-specific expression, hormonal regulation, and abiotic stress response. RT-qPCR analysis showed that ZmLACS genes exhibit tissue-specific expression patterns and respond to diverse abiotic stresses including drought and salt, as well as phytohormone abscisic acid. Furthermore, using the STRING database, several proteins involved in fatty acid and complex lipid synthesis were identified to be the potential interaction partners of ZmLACS proteins, which was also confirmed by the yeast two-hybrid (Y2H) assay, enhancing our understanding of wax biosynthesis and regulatory mechanisms in response to abiotic stresses in maize. These findings provide a comprehensive understanding of ZmLACS genes and offer a theoretical foundation for future research on the biological functions of LACS genes in maize environmental adaptability.

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.

Characteristics, Relationships, and Anatomical Basis of Leaf Hydraulic Traits and Economic Traits in Temperate Desert Shrub Species

Shrubs are a key component of desert ecosystems, playing a crucial role in controlling desertification and promoting revegetation, yet their growth is often impeded by drought. Leaf hydraulic traits and economic traits are both involved in the process of water exchange for carbon dioxide. Exploring the characteristics, relationships, and anatomical basis of these two suites of traits is crucial to understanding the mechanism of desert shrubs adapting to the desert arid environment. However, the relationship between these two sets of traits currently remains ambiguous. This study explored the leaf hydraulic, economic, and anatomical traits of 19 desert shrub species. The key findings include the following: Relatively larger LT values and smaller SLA values were observed in desert shrubs, aligning with the "slow strategy" in the leaf economics spectrum. The relatively high P50leaf, low HSMleaf, negative TLPleaf, and positive HSMtlp values indicated that severe embolism occurs in the leaves during the dry season, while most species were able to maintain normal leaf expansion. This implies a "tolerance" leaf hydraulic strategy in response to arid stress. No significant relationship was observed between P50leaf and Kmax, indicating the absence of a trade-off between hydraulic efficiency and embolism resistance. Certain coupling relationships were observed between leaf hydraulic traits and economic traits, both of which were closely tied to anatomical structures. Out of all of the leaf traits, LT was the central trait of the leaf traits network. The positive correlation between C content and WPleaf and HSMleaf, as well as the positive correlation between N content and HSMtlp, suggested that the cost of leaf construction was synergistic with hydraulic safety. The negative correlation between SLA, P content, GCL, and SAI suggested a functional synergistic relationship between water use efficiency and gas exchange rate. In summary, this research revealed that the coupling relationship between leaf hydraulic traits and economic traits was one of the important physiological and ecological mechanisms of desert shrubs for adapting to desert habitats.

Evidence for within-species transition between drought response strategies in Nicotiana benthamiana

Nicotiana benthamiana is predominantly distributed in arid habitats across northern Australia. However, none of six geographically isolated accessions shows obvious xerophytic morphological features. To investigate how these tender-looking plants withstand drought, we examined their responses to water deprivation, assessed phenotypic, physiological, and cellular responses, and analysed cuticular wax composition and wax biosynthesis gene expression profiles. Results showed that the Central Australia (CA) accession, globally known as a research tool, has evolved a drought escape strategy with early vigour, short life cycle, and weak, water loss-limiting responses. By contrast, a northern Queensland (NQ) accession responded to drought by slowing growth, inhibiting flowering, increasing leaf cuticle thickness, and altering cuticular wax composition. Under water stress, NQ increased the heat stability and water impermeability of its cuticle by extending the carbon backbone of cuticular long-chain alkanes from c. 25 to 33. This correlated with rapid upregulation of at least five wax biosynthesis genes. In CA, the alkane chain lengths (c. 25) and gene expression profiles remained largely unaltered. This study highlights complex genetic and environmental control over cuticle composition and provides evidence for divergence into at least two fundamentally different drought response strategies within the N. benthamiana species in < 1 million years.

The Arabidopsis cytochrome P450 enzyme CYP96A4 is involved in the wound-induced biosynthesis of cuticular wax and cutin monomers

The plant cuticle is composed of cuticular wax and cutin polymers and plays an essential role in plant tolerance to diverse abiotic and biotic stresses. Several stresses, including water deficit and salinity, regulate the synthesis of cuticular wax and cutin monomers. However, the effect of wounding on wax and cutin monomer production and the associated molecular mechanisms remain unclear. In this study, we determined that the accumulation of wax and cutin monomers in Arabidopsis leaves is positively regulated by wounding primarily through the jasmonic acid (JA) signaling pathway. Moreover, we observed that a wound- and JA-responsive gene (CYP96A4) encoding an ER-localized cytochrome P450 enzyme was highly expressed in leaves. Further analyses indicated that wound-induced wax and cutin monomer production was severely inhibited in the cyp96a4 mutant. Furthermore, CYP96A4 interacted with CER1 and CER3, the core enzymes in the alkane-forming pathway associated with wax biosynthesis, and modulated CER3 activity to influence aldehyde production in wax synthesis. In addition, transcripts of MYC2 and JAZ1, key genes in JA signaling pathway, were significantly reduced in cyp96a4 mutant. Collectively, these findings demonstrate that CYP96A4 functions as a cofactor of the alkane synthesis complex or participates in JA signaling pathway that contributes to cuticular wax biosynthesis and cutin monomer formation in response to wounding.

AtMYB41 acts as a dual-function transcription factor that regulates the formation of lipids in an organ- and development-dependent manner

The plant cuticle controls non-stomatal water loss and can serve as a barrier against biotic agents, whereas the heteropolymer suberin and its associated waxes are deposited constitutively at specific cell wall locations. While several transcription factors controlling cuticle formation have been identified, those involved in the transcriptional regulation of suberin biosynthesis remain poorly characterized. The major goal of this study was to further analyse the function of the R2R3-Myeloblastosis (MYB) transcription factor AtMYB41 in formation of the cuticle, suberin, and suberin-associated waxes throughout plant development. For functional analysis, the organ-specific expression pattern of AtMYB41 was analysed and Atmyb41ge alleles were generated using the CRISPR/Cas9 system. These were investigated for root growth and water permeability upon stress. In addition, the fatty acid, wax, cutin, and suberin monomer composition of different organs was evaluated by gas chromatography. The characterization of Atmyb41ge mutants revealed that AtMYB41 negatively regulates the production of cuticular lipids and fatty acid biosynthesis in leaves and seeds, respectively. Remarkably, biochemical analyses indicate that AtMYB41 also positively regulates the formation of cuticular waxes in stems of Arabidopsis thaliana. Overall, these results suggest that the AtMYB41 acts as a negative regulator of cuticle and fatty acid biosynthesis in leaves and seeds, respectively, but also as a positive regulator of wax production in A. thaliana stems.

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.

Broad Chain-Length Specificity of the Alkane-Forming Enzymes NoCER1A and NoCER3A/B in Nymphaea odorata

Many terrestrial plants produce large quantities of alkanes for use in epicuticular wax and the pollen coat. However, their carbon chains must be long to be useful as fuel or as a petrochemical feedstock. Here, we focus on Nymphaea odorata, which produces relatively short alkanes in its anthers. We identified orthologs of the Arabidopsis alkane biosynthesis genes AtCER1 and AtCER3 in N. odorata and designated them NoCER1A, NoCER3A and NoCER3B. Expression analysis of NoCER1A and NoCER3A/B in Arabidopsis cer mutants revealed that the N. odorata enzymes cooperated with the Arabidopsis enzymes and that the NoCER1A produced shorter alkanes than AtCER1, regardless of which CER3 protein it interacted with. These results indicate that AtCER1 frequently uses a C30 substrate, whereas NoCER1A, NoCER3A/B and AtCER3 react with a broad range of substrate chain lengths. The incorporation of shorter alkanes disturbed the formation of wax crystals required for water-repellent activity in stems, suggesting that chain-length specificity is important for surface cleaning. Moreover, cultured tobacco cells expressing NoCER1A and NoCER3A/B effectively produced C19-C23 alkanes, indicating that the introduction of the two enzymes is sufficient to produce alkanes. Taken together, our findings suggest that these N. odorata enzymes may be useful for the biological production of alkanes of specific lengths. 3D modeling revealed that CER1s and CER3s share a similar structure that consists of N- and C-terminal domains, in which their predicted active sites are respectively located. We predicted the complex structure of both enzymes and found a cavity that connects their active sites.

A long non-coding RNA functions as a competitive endogenous RNA to modulate TaNAC018 by acting as a decoy for tae-miR6206

Increasing evidence indicates a strong correlation between the deposition of cuticular waxes and drought tolerance. However, the precise regulatory mechanism remains elusive. Here, we conducted a comprehensive transcriptome analysis of two wheat (Triticum aestivum) near-isogenic lines, the glaucous line G-JM38 rich in cuticular waxes and the non-glaucous line NG-JM31. We identified 85,143 protein-coding mRNAs, 4,485 lncRNAs, and 1,130 miRNAs. Using the lncRNA-miRNA-mRNA network and endogenous target mimic (eTM) prediction, we discovered that lncRNA35557 acted as an eTM for the miRNA tae-miR6206, effectively preventing tae-miR6206 from cleaving the NAC transcription factor gene TaNAC018. This lncRNA-miRNA interaction led to higher transcript abundance for TaNAC018 and enhanced drought-stress tolerance. Additionally, treatment with mannitol and abscisic acid (ABA) each influenced the levels of tae-miR6206, lncRNA35557, and TaNAC018 transcript. The ectopic expression of TaNAC018 in Arabidopsis also improved tolerance toward mannitol and ABA treatment, whereas knocking down TaNAC018 transcript levels via virus-induced gene silencing in wheat rendered seedlings more sensitive to mannitol stress. Our results indicate that lncRNA35557 functions as a competing endogenous RNA to modulate TaNAC018 expression by acting as a decoy target for tae-miR6206 in glaucous wheat, suggesting that non-coding RNA has important roles in the regulatory mechanisms responsible for wheat stress tolerance.

Divergent evolution of the alcohol-forming pathway of wax biosynthesis among bryophytes

The plant cuticle is a hydrophobic barrier, which seals the epidermal surface of most aboveground organs. While the cuticle biosynthesis of angiosperms has been intensively studied, knowledge about its existence and composition in nonvascular plants is scarce. Here, we identified and characterized homologs of Arabidopsis thaliana fatty acyl-CoA reductase (FAR) ECERIFERUM 4 (AtCER4) and bifunctional wax ester synthase/acyl-CoA:diacylglycerol acyltransferase 1 (AtWSD1) in the liverwort Marchantia polymorpha (MpFAR2 and MpWSD1) and the moss Physcomitrium patens (PpFAR2A, PpFAR2B, and PpWSD1). Although bryophyte harbor similar compound classes as described for angiosperm cuticles, their biosynthesis may not be fully conserved between the bryophytes M. polymorpha and P. patens or between these bryophytes and angiosperms. While PpFAR2A and PpFAR2B contribute to the production of primary alcohols in P. patens, loss of MpFAR2 function does not affect the wax profile of M. polymorpha. By contrast, MpWSD1 acts as the major wax ester-producing enzyme in M. polymorpha, whereas mutations of PpWSD1 do not affect the wax ester levels of P. patens. Our results suggest that the biosynthetic enzymes involved in primary alcohol and wax ester formation in land plants have either evolved multiple times independently or undergone pronounced radiation followed by the formation of lineage-specific toolkits.

MdDEWAX decreases plant drought resistance by regulating wax biosynthesis

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

Dynamic changes to the plant cuticle include the production of volatile cuticular wax-derived compounds

The cuticle is a hydrophobic structure that seals plant aerial surfaces from the surrounding environment. To better understand how cuticular wax composition changes over development, we conducted an untargeted screen of leaf surface lipids from black cottonwood (Populus trichocarpa). We observed major shifts to the lipid profile across development, from a phenolic and terpene-dominated profile in young leaves to an aliphatic wax-dominated profile in mature leaves. Contrary to the general pattern, levels of aliphatic cis-9-alkenes decreased in older leaves following their accumulation. A thorough examination revealed that the decrease in cis-9-alkenes was accompanied by a concomitant increase in aldehydes, one of them being the volatile compound nonanal. By applying exogenous alkenes to P. trichocarpa leaves, we show that unsaturated waxes in the cuticle undergo spontaneous oxidative cleavage to generate aldehydes and that this process occurs similarly in other alkene-accumulating systems such as balsam poplar (Populus balsamifera) leaves and corn (Zea mays) silk. Moreover, we show that the production of cuticular wax-derived compounds can be extended to other wax components. In bread wheat (Triticum aestivum), 9-hydroxy-14,16-hentriacontanedione likely decomposes to generate 2-heptadecanone and 7-octyloxepan-2-one (a caprolactone). These findings highlight an unusual route to the production of plant volatiles that are structurally encoded within cuticular wax precursors. These processes could play a role in modulating ecological interactions and open the possibility for engineering bioactive volatile compounds into plant waxes.

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.

Epicuticular wax accumulation and regulation of wax pathway gene expression during bioenergy Sorghum stem development

Bioenergy sorghum is a drought-tolerant high-biomass C4 grass targeted for production on annual cropland marginal for food crops due primarily to abiotic constraints. To better understand the overall contribution of stem wax to bioenergy sorghum's resilience, the current study characterized sorghum stem cuticular wax loads, composition, morphometrics, wax pathway gene expression and regulation using vegetative phase Wray, R07020, and TX08001 genotypes. Wax loads on sorghum stems (~103-215 µg/cm2) were much higher than Arabidopsis stem and leaf wax loads. Wax on developing sorghum stem internodes was enriched in C28/30 primary alcohols (~65%) while stem wax on fully developed stems was enriched in C28/30 aldehydes (~80%). Scanning Electron Microscopy showed minimal wax on internodes prior to the onset of elongation and that wax tubules first appear associated with cork-silica cell complexes when internode cell elongation is complete. Sorghum homologs of genes involved in wax biosynthesis/transport were differentially expressed in the stem epidermis. Expression of many wax pathway genes (i.e., SbKCS6, SbCER3-1, SbWSD1, SbABCG12, SbABCG11) is low in immature apical internodes then increases at the onset of stem wax accumulation. SbCER4 is expressed relatively early in stem development consistent with accumulation of C28/30 primary alcohols on developing apical internodes. High expression of two SbCER3 homologs in fully elongated internodes is consistent with a role in production of C28/30 aldehydes. Gene regulatory network analysis aided the identification of sorghum homologs of transcription factors that regulate wax biosynthesis (i.e., SbSHN1, SbWRI1/3, SbMYB94/96/30/60, MYS1) and other transcription factors that could regulate and specify expression of the wax pathway in epidermal cells during cuticle development.

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.

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.

OsFAR1 is involved in primary fatty alcohol biosynthesis and promotes drought tolerance in rice

MAIN CONCLUSION

OsFAR1 encodes a fatty acyl-CoA reductase involved in biosynthesis of primary alcohols and plays an important role in drought stress response in rice. Cuticular waxes cover the outermost surface of terrestrial plants and contribute to inhibiting nonstomatal water loss and improving plant drought resistance. Primary alcohols are the most abundant components in the leaf cuticular waxes of rice (Oryza sativa), but the biosynthesis and regulation of primary alcohol remain largely unknown in rice. Here, we identified and characterized an OsFAR1 gene belonging to the fatty acyl-CoA reductases (FARs) via a homology-based approach in rice. OsFAR1 was activated by abiotic stresses and abscisic acid, resulting in increased production of primary alcohol in rice. Heterologous expression of OsFAR1 enhanced the amounts of C22:0 and C24:0 primary alcohols in yeast (Saccharomyces cerevisiae) and C24:0 to C32:0 primary alcohols in Arabidopsis. Similarly, OsFAR1 overexpression significantly increased the content of C24:0 to C30:0 primary alcohols on rice leaves. Finally, OsFAR1 overexpression lines exhibited reduced cuticle permeability and enhanced drought tolerance in rice and Arabidopsis. Taken together, our results demonstrate that OsFAR1 is involved in rice primary alcohol biosynthesis and plays an important role in responding to drought and other environmental stresses.

Genome-Wide Investigation and Functional Analysis Reveal That CsKCS3 and CsKCS18 Are Required for Tea Cuticle Wax Formation

Cuticular wax is a complex mixture of very long-chain fatty acids (VLCFAs) and their derivatives that constitute a natural barrier against biotic and abiotic stresses on the aerial surface of terrestrial plants. In tea plants, leaf cuticular wax also contributes to the unique flavor and quality of tea products. However, the mechanism of wax formation in tea cuticles is still unclear. The cuticular wax content of 108 germplasms (Niaowang species) was investigated in this study. The transcriptome analysis of germplasms with high, medium, and low cuticular wax content revealed that the expression levels of CsKCS3 and CsKCS18 were strongly associated with the high content of cuticular wax in leaves. Hence, silencing CsKCS3 and CsKCS18 using virus-induced gene silencing (VIGS) inhibited the synthesis of cuticular wax and caffeine in tea leaves, indicating that expression of these genes is necessary for the synthesis of cuticular wax in tea leaves. The findings contribute to a better understanding of the molecular mechanism of cuticular wax formation in tea leaves. The study also revealed new candidate target genes for further improving tea quality and flavor and cultivating high-stress-resistant tea germplasms.

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.

Metabolome and transcriptome profiling in different bagging pear fruit reveals that PbKCS10 affects the occurrence of superficial scald via regulating the wax formation

Superficial scald is a physiological disorder of fruit, which is easy to occur during long-term cold storage after harvest. Different preharvest bagging treatments (no bagging, polyethylene bagging and non-woven fabric bagging) were used to explore the occurrence mechanism of superficial scald. UHPLC-MS analysis, GC-MS analysis and RNA-seq revealed the influence of the wax of 'Chili' on the occurrence of superficial scald. The wax content and wax components (Lupeol, lup-20(29)-en-3-one, heptacosane, 9-octadecenoic acid, eicosanoic acid, cis-11-eicosenoic acid) were significantly higher in the fruit bagged with non-woven fabric (NWF, with low incidence of superficial scald) than that in fruit bagged with polyethylene (PE, high incidence of superficial scald). Transcriptomics and qRT-PCR data identified a wax synthesis gene, PbKCS10, which exhibited high expression levels in fruit with low of superficial scald. The results of gene function showed that PbKCS10 reduced the occurrence of superficial scald by increasing the wax formation.

WAX INDUCER 1 Regulates β-Diketone Biosynthesis by Mediating Expression of the Cer-cqu Gene Cluster in Barley

Plant surface properties are crucial determinants of resilience to abiotic and biotic stresses. The outer layer of the plant cuticle consists of chemically diverse epicuticular waxes. The WAX INDUCER1/SHINE subfamily of APETALA2/ETHYLENE RESPONSIVE FACTORS regulates cuticle properties in plants. In this study, four barley genes homologous to the Arabidopsis thaliana AtWIN1 gene were mutated using RNA-guided Cas9 endonuclease. Mutations in one of them, the HvWIN1 gene, caused a recessive glossy sheath phenotype associated with β-diketone deficiency. A complementation test for win1 knockout (KO) and cer-x mutants showed that Cer-X and WIN1 are allelic variants of the same genomic locus. A comparison of the transcriptome from leaf sheaths of win1 KO and wild-type plants revealed a specific and strong downregulation of a large gene cluster residing at the previously known Cer-cqu locus. Our findings allowed us to postulate that the WIN1 transcription factor in barley is a master mediator of the β-diketone biosynthesis pathway acting through developmental stage- and organ-specific transactivation of the Cer-cqu gene cluster.

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

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

Roles of very long-chain fatty acids in compound leaf patterning in Medicago truncatula

Plant cuticles are composed of hydrophobic cuticular waxes and cutin. Very long-chain fatty acids (VLCFAs) are components of epidermal waxes and the plasma membrane and are involved in organ morphogenesis. By screening a barrelclover (Medicago truncatula) mutant population tagged by the transposable element of tobacco (Nicotiana tabacum) cell type1 (Tnt1), we identified two types of mutants with unopened flower phenotypes, named unopened flower1 (uof1) and uof2. Both UOF1 and UOF2 encode enzymes that are involved in the biosynthesis of VLCFAs and cuticular wax. Comparative analysis of the mutants indicated that the mutation in UOF1, but not UOF2, leads to the increased number of leaflets in M. truncatula. UOF1 was specifically expressed in the outermost cell layer (L1) of the shoot apical meristem (SAM) and leaf primordia. The uof1 mutants displayed defects in VLCFA-mediated plasma membrane integrity, resulting in the disordered localization of the PIN-FORMED1 (PIN1) ortholog SMOOTH LEAF MARGIN1 (SLM1) in M. truncatula. Our work demonstrates that the UOF1-mediated biosynthesis of VLCFAs in L1 is critical for compound leaf patterning, which is associated with the polarization of the auxin efflux carrier in M. truncatula.

Diversified chemical profiles of cuticular wax on alpine meadow plants of the Qinghai-Tibet Plateau

The alpine meadow plants showed great intra- and inter-genera variations of chemical profiles of cuticular waxes. Developing an understanding of wax structure-function relationships that will help us tackle global climate change requires a detailed understanding of plant wax chemistry. The goal in this study was to provide a catalog of wax structures, abundances, and compositions on alpine meadow plants. Here, leaf waxes from 33 plant species belonging to 11 families were sampled from alpine meadows of the east side of the Qinghai-Tibet Plateau. Across these species, total wax coverage varied from 2.30 μg cm^-2 to 40.70 μg cm^-2, showing variation both within as well as between genera and suggesting that wax variation is subject to both environmental and genetic effects. Across all wax samples, more than 140 wax compounds belonging to 13 wax compound classes were identified, including both ubiquitous wax compounds and lineage-specific compounds. Among the ubiquitous compounds (primary alcohols, alkyl esters, aldehydes, alkanes, and fatty acids), chain length profiles across a wide range of species point to key differences in the chain length specificity of alcohol and alkane formation machinery. The lineage-specific wax compound classes (diols, secondary alcohols, lactones, iso-alkanes, alkyl resorcinols, phenylethyl esters, cinnamate esters, alkyl benzoates, and triterpenoids) nearly all consisted of isomers with varying chain lengths or functional group positions, making the diversity of specialized wax compounds immense. The comparison of species relationships between chemical data and genetic data highlighted the importance of inferring phylogenetic relationships from data sets that contain a large number of variables that do not respond to environmental stimuli.

Morphological and antioxidant responses of Nopalea cochenillifera cv. Maya (edible Opuntia sp. "Kasugai Saboten") to chilling acclimatization

To clarify the wintering ability of the cactus Nopalea cochenillifera cv. Maya (edible Opuntia sp., common name "Kasugai Saboten"), we investigated the effects of temperature and antioxidant capacity on chilling acclimatization. We analyzed the anatomy of cladode chlorenchyma tissue of plants exposed to light under chilling. We found that chilling acclimatization can be achieved by exposure to approximately 15 °C for 2 weeks and suggest that it is affected by whether or not antioxidant capacity can recover. The overwintering cacti had the thinnest cuticle but firm cuticular wax, which is important in the acquisition of low temperature tolerance under strong light. In cacti with severe chilling injury, round swollen nuclei with clumping chloroplasts were localized in the upper part (axial side) of the cell, as though pushed up by large vacuoles in the lower part. In overwintering cacti, chloroplasts were arranged on the lateral side of the cell as in control plants, but they formed pockets: invaginations with a thin layer of chloroplast stroma that surrounded mitochondria and peroxisomes. Specific cellular structural changes depended on the degree of chilling stress and provide useful insights linking chloroplast behavior and structural changes to the environmental stress response.

Tackling functional redundancy of Arabidopsis fatty acid elongase complexes

Very-long-chain fatty acids (VLCFA) are precursors for various lipids playing important physiological and structural roles in plants. Throughout plant tissues, VLCFA are present in multiple lipid classes essential for membrane homeostasis, and also stored in triacylglycerols. VLCFA and their derivatives are also highly abundant in lipid barriers, such as cuticular waxes in aerial epidermal cells and suberin monomers in roots. VLCFA are produced by the fatty acid elongase (FAE), which is an integral endoplasmic reticulum membrane multi-enzymatic complex consisting of four core enzymes. The 3-ketoacyl-CoA synthase (KCS) catalyzes the first reaction of the elongation and determines the chain-length substrate specificity of each elongation cycle, whereas the other three enzymes have broad substrate specificities and are shared by all FAE complexes. Consistent with the co-existence of multiple FAE complexes, performing sequential and/or parallel reactions to produce the broad chain-length-range of VLCFA found in plants, twenty-one KCS genes have been identified in the genome of Arabidopsis thaliana. Using CRISPR-Cas9 technology, we established an expression platform to reconstitute the different Arabidopsis FAE complexes in yeast. The VLCFA produced in these yeast strains were analyzed in detail to characterize the substrate specificity of all KCS candidates. Additionally, Arabidopsis candidate proteins were transiently expressed in Nicotiana benthamiana leaves to explore their activity and localization in planta. This work sheds light on the genetic and biochemical redundancy of fatty acid elongation in plants.

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

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

The Plant Metabolic Changes and the Physiological and Signaling Functions in the Responses to Abiotic Stress

Global climate change has altered, and will further alter, rainfall patterns and temperatures likely causing more frequent drought and heat waves, which will consequently exacerbate abiotic stresses of plants and significantly decrease the yield and quality of crops. On the one hand, the global demand for food is ever-increasing owing to the rapid increase of the human population. On the other hand, metabolic responses are one of the most important mechanisms by which plants adapt to and survive to abiotic stresses. Here we therefore summarize recent progresses including the plant primary and secondary metabolic responses to abiotic stresses and their function in plant resistance acting as antioxidants, osmoregulatory, and signaling factors, which enrich our knowledge concerning commonalities of plant metabolic responses to abiotic stresses, including their involvement in signaling processes. Finally, we discuss potential methods of metabolic fortification of crops in order to improve their abiotic stress tolerance.

Genome-Wide Identification of the CER Gene Family and Significant Features in Climate Adaptation of Castanea mollissima

The plant cuticle is the outermost layer of the aerial organs and an important barrier against biotic and abiotic stresses. The climate varies greatly between the north and south of China, with large differences in temperature and humidity, but Chinese chestnut is found in both regions. This study investigated the relationship between the wax layer of chestnut leaves and environmental adaptation. Firstly, semi-thin sections were used to verify that there is a significant difference in the thickness of the epicuticular wax layer between wild chestnut leaves in northwest and southeast China. Secondly, a whole-genome selective sweep was used to resequence wild chestnut samples from two typical regional populations, and significant genetic divergence was identified between the two populations in the CmCER1-1, CmCER1-5 and CmCER3 genes. Thirty-four CER genes were identified in the whole chestnut genome, and a series of predictive analyses were performed on the identified CmCER genes. The expression patterns of CmCER genes were classified into three trends-upregulation, upregulation followed by downregulation and continuous downregulation-when chestnut seedlings were treated with drought stress. Analysis of cultivars from two resource beds in Beijing and Liyang showed that the wax layer of the northern variety was thicker than that of the southern variety. For the Y-2 (Castanea mollissima genome sequencing material) cultivar, there were significant differences in the expression of CmCER1-1, CmCER1-5 and CmCER3 between the southern variety and the northern one-year-grafted variety. Therefore, this study suggests that the CER family genes play a role in environmental adaptations in chestnut, laying the foundation for further exploration of CmCER genes. It also demonstrates the importance of studying the adaptation of Chinese chestnut wax biosynthesis to the southern and northern environments.

Two Arabidopsis MYB-SHAQKYF transcription repressors regulate leaf wax biosynthesis via transcriptional suppression on DEWAX

Plant cuticular wax accumulation limits nonstomatal transpiration and is regulated by external environmental stresses. DEWAX (DECREASE WAX BIOSYNTHESIS) plays a vital role in diurnal wax biosynthesis. However, how DEWAX expression is controlled and the molecular mechanism of wax biosynthesis regulated by the diurnal cycle remains largely unknown. Here, we identified two Arabidopsis MYB-SHAQKYF transcription factors, MYS1 and MYS2, as new regulators in wax biosynthesis and drought tolerance. Mutations of both MYS1 and MYS2 caused significantly reduced leaf wax, whereas overexpression of MYS1 or MYS2 increased leaf wax biosynthesis and enhanced drought tolerance. Our results demonstrated that MYS1 and MYS2 act as transcription repressors and directly suppress DEWAX expression via ethylene response factor-associated amphiphilic repression motifs. Genetic interaction analysis with DEWAX, SPL9 (SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 9), and CER1 (ECERIFERUM 1) in wax biosynthesis and under drought stresses demonstrated that MYS1 and MYS2 act upstream of the DEWAX-SPL9 module, thus regulating CER1 expression. Expression analysis suggested that the diurnal expression pattern of DEWAX is partly regulated by MYS1 and MYS2. Our findings demonstrate the roles of two unidentified transcription repressors, MYS1 and MYS2, in wax biosynthesis and provide insights into the mechanism of diurnal cycle-regulated wax biosynthesis.

Transcriptome Profiling of the Resistance Response of Musa acuminata subsp. burmannicoides, var. Calcutta 4 to Pseudocercospora musae

Banana (Musa spp.), which is one of the world’s most popular and most traded fruits, is highly susceptible to pests and diseases. Pseudocercospora musae, responsible for Sigatoka leaf spot disease, is a principal fungal pathogen of Musa spp., resulting in serious economic damage to cultivars in the Cavendish subgroup. The aim of this study was to characterize genetic components of the early immune response to P. musae in Musa acuminata subsp. burmannicoides, var. Calcutta 4, a resistant wild diploid. Leaf RNA samples were extracted from Calcutta 4 three days after inoculation with fungal conidiospores, with paired-end sequencing conducted in inoculated and non-inoculated controls using lllumina HiSeq 4000 technology. Following mapping to the reference M. acuminata ssp. malaccensis var. Pahang genome, differentially expressed genes (DEGs) were identified and expression representation analyzed on the basis of gene ontology enrichment, Kyoto Encyclopedia of Genes and Genomes orthology and MapMan pathway analysis. Sequence data mapped to 29,757 gene transcript models in the reference Musa genome. A total of 1073 DEGs were identified in pathogen-inoculated cDNA libraries, in comparison to non-inoculated controls, with 32% overexpressed. GO enrichment analysis revealed common assignment to terms that included chitin binding, chitinase activity, pattern binding, oxidoreductase activity and transcription factor (TF) activity. Allocation to KEGG pathways revealed DEGs associated with environmental information processing, signaling, biosynthesis of secondary metabolites, and metabolism of terpenoids and polyketides. With 144 up-regulated DEGs potentially involved in biotic stress response pathways, including genes involved in cell wall reinforcement, PTI responses, TF regulation, phytohormone signaling and secondary metabolism, data demonstrated diverse early-stage defense responses to P. musae. With increased understanding of the defense responses occurring during the incompatible interaction in resistant Calcutta 4, these data are appropriate for the development of effective disease management approaches based on genetic improvement through introgression of candidate genes in superior cultivars.

The AP2 Transcription Factor BrSHINE3 Regulates Wax Accumulation in Nonheading Chinese Cabbage

Wax is an acellular structural substance attached to the surface of plant tissues. It forms a protective barrier on the epidermis of plants and plays an important role in resisting abiotic and biotic stresses. In this paper, nonheading Chinese cabbage varieties with and without wax powder were observed using scanning electron microscopy, and the surface of waxy plants was covered with a layer of densely arranged waxy crystals, thus differentiating them from the surface of waxless plants. A genetic analysis showed that wax powder formation in nonheading Chinese cabbage was controlled by a pair of dominant genes. A preliminary bulked segregant analysis sequencing (BSA-seq) assay showed that one gene was located at the end of chromosome A09. Within this interval, we identified BraA09000626, encoding an AP2 transcription factor homologous to Arabidopsis AtSHINE3, and we named it BrSHINE3. By comparing the CDS of the gene in the two parental plants, a 35 bp deletion in the BrSHINE3 gene of waxless plants resulted in a frameshift mutation. Tissue analysis showed that BrSHINE3 was expressed at significantly higher levels in waxy plant rosette stage petioles and bolting stage stems than in the tissues of waxless plants. We speculate that this deletion in BrSHINE3 bases in the waxless material may inhibit wax synthesis. The overexpression of BrSHINE3 in Arabidopsis induced the accumulation of wax on the stem surface, indicating that BrSHINE3 is a key gene that regulates the formation of wax powder in nonheading Chinese cabbage. The analysis of the subcellular localization showed that BrSHINE3 is mainly located in the nucleus and chloroplast of tobacco leaves, suggesting that the gene may function as a transcription factor. Subsequent transcriptome analysis of the homology of BrSHINE3 downstream genes in nonheading Chinese cabbage showed that these genes were downregulated in waxless materials. These findings provide a basis for a better understanding of the nonheading Chinese cabbage epidermal wax synthesis pathway and provide important information for the molecular-assisted breeding of nonheading Chinese cabbage.

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.

ECERIFERUM1-6A is required for the synthesis of cuticular wax alkanes and promotes drought tolerance in wheat

Cuticular waxes cover the aerial surfaces of land plants and protect them from various environmental stresses. Alkanes are major wax components and contribute to plant drought tolerance, but the biosynthesis and regulation of alkanes remain largely unknown in wheat (Triticum aestivum L.). Here, we identified and functionally characterized a key alkane biosynthesis gene ECERIFERUM1-6A (TaCER1-6A) from wheat. The clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9)-mediated knockout mutation in TaCER1-6A greatly reduced the contents of C27, C29, C31, and C33 alkanes in wheat leaves, while TaCER1-6A overexpression significantly increased the contents of these alkanes in wheat leaves, suggesting that TaCER1-6A is specifically involved in the biosynthesis of C27, C29, C31, and C33 alkanes on wheat leaf surfaces. TaCER1-6A knockout lines exhibited increased cuticle permeability and reduced drought tolerance, whereas TaCER1-6A overexpression lines displayed reduced cuticle permeability and enhanced drought tolerance. TaCER1-6A was highly expressed in flag leaf blades and seedling leaf blades and could respond to abiotic stresses and abscisic acid. TaCER1-6A was located in the endoplasmic reticulum, which is the subcellular compartment responsible for wax biosynthesis. A total of three haplotypes (HapI/II/III) of TaCER1-6A were identified in 43 wheat accessions, and HapI was the dominant haplotype (95%) in these wheat varieties. Additionally, we identified two R2R3-MYB transcription factors TaMYB96-2D and TaMYB96-5D that bound directly to the conserved motif CAACCA in promoters of the cuticular wax biosynthesis genes TaCER1-6A, TaCER1-1A, and fatty acyl-CoA reductase4. Collectively, these results suggest that TaCER1-6A is required for C27, C29, C31, and C33 alkanes biosynthesis and improves drought tolerance in wheat.

An apple long-chain acyl-CoA synthase, MdLACS1, enhances biotic and abiotic stress resistance in plants

Epidermal waxes are part of the outermost hydrophobic structures of apples and play a significant role in enhancing apple resistance and improving fruit quality. The biosynthetic precursors of epidermal waxes are very long-chain fatty acids (VLCFAs), which are made into different wax components through various wax synthesis pathways. In Arabidopsis thaliana, the AtLACS1 protein can activate the alkane synthesis pathway to produce very long-chain acyl CoAs (VLC-acyl-CoAs), which provide substrates for wax synthesis, from VLCFAs. The apple protein MdLACS1, encoded by the MdLACS1 gene, belongs to the AMP-binding superfamily and has long-chain acyl coenzyme A synthase activity, but its function in apple remains unclear. Here, we identified MdLACS1 in apple (Malus × domestica) and analyzed its function. Our results suggest that MdLACS1 promotes wax synthesis and improves biotic and abiotic stress tolerance, which were directly or indirectly dependent on wax. Our study further refines the molecular mechanism of wax biosynthesis in apples and elucidates the physiological function of wax in resistance to external stresses. These findings provide candidate genes for the synergistic enhancement of apple fruit quality and stress tolerance.

Genome-wide identification and characterization of the KCS gene family in sorghum (Sorghum bicolor (L.) Moench)

The aboveground parts of plants are covered with cuticle, a hydrophobic layer composed of cutin polyester and cuticular wax that can protect plants from various environmental stresses. β-Ketoacyl-CoA synthase (KCS) is the key rate-limiting enzyme in plant wax synthesis. Although the properties of KCS family genes have been investigated in many plant species, the understanding of this gene family in sorghum is still limited. Here, a total of 25 SbKCS genes were identified in the sorghum genome, which were named from SbKCS1 to SbKCS25. Evolutionary analysis among different species divided the KCS family into five subfamilies and the SbKCSs were more closely related to maize, implying a closer evolutionary relationship between sorghum and maize. All SbKCS genes were located on chromosomes 1, 2, 3, 4, 5, 6, 9 and 10, respectively, while Chr 1 and Chr 10 contained more KCS genes than other chromosomes. The prediction results of subcellular localization showed that SbKCSs were mainly expressed in the plasma membrane and mitochondria. Gene structure analysis revealed that there was 0-1 intron in the sorghum KCS family and SbKCSs within the same subgroup were similar. Multiple cis-acting elements related to abiotic stress, light and hormone response were enriched in the promoters of SbKCS genes, which indicated the functional diversity among these genes. The three-dimensional structure analysis showed that a compact spherical space structure was formed by various secondary bonds to maintain the stability of SbKCS proteins, which was necessary for their biological activity. qRT-PCR results revealed that nine randomly selected SbKCS genes expressed differently under drought and salt treatments, among which SbKCS8 showed the greatest fold of expression difference at 12 h after drought and salt stresses, which suggested that the SbKCS genes played a potential role in abiotic stress responses. Taken together, these results provided an insight into investigating the functions of KCS family in sorghum and in response to abiotic stress.

Cloning, characterization and functional analysis of NtMYB306a gene reveals its role in wax alkane biosynthesis of tobacco trichomes and stress tolerance

Trichomes are specialized hair-like organs found on epidermal cells of many terrestrial plants, which protect plant from excessive transpiration and numerous abiotic and biotic stresses. However, the genetic basis and underlying mechanisms are largely unknown in Nicotiana tabacum (common tobacco), an established model system for genetic engineering and plant breeding. In present study, we identified, cloned and characterized an unknown function transcription factor NtMYB306a from tobacco cultivar K326 trichomes. Results obtained from sequence phylogenetic tree analysis showed that NtMYB306a-encoded protein belonged to S1 subgroup of the plants' R2R3-MYB transcription factors (TFs). Observation of the green fluorescent signals from NtMYB306a-GFP fusion protein construct exhibited that NtMYB306a was localized in nucleus. In yeast transactivation assays, the transformed yeast containing pGBKT7-NtMYB306a construct was able to grow on SD/-Trp-Ade+X-α-gal selection media, signifying that NtMYB306a exhibits transcriptional activation activity. Results from qRT-PCR, in-situ hybridization and GUS staining of transgenic tobacco plants revealed that NtMYB306a is primarily expressed in tobacco trichomes, especially tall glandular trichomes (TGTs) and short glandular trichomes (SGTs). RNA sequencing (RNA-seq) and qRT-PCR analysis of the NtMYB306a-overexpressing transgenic tobacco line revealed that NtMYB306a activated the expression of a set of key target genes which were associated with wax alkane biosynthesis. Gas Chromatography-Mass Spectrometry (GC-MS) exhibited that the total alkane contents and the contents of n-C28, n-C29, n-C31, and ai-C31 alkanes in leaf exudates of NtMYB306a-OE lines (OE-3, OE-13, and OE-20) were significantly greater when compared to WT. Besides, the promoter region of NtMYB306a contained numerous stress-responsive cis-acting elements, and their differential expression towards salicylic acid and cold stress treatments reflected their roles in signal transduction and cold-stress tolerance. Together, these results suggest that NtMYB306a is necessarily a positive regulator of alkane metabolism in tobacco trichomes that does not affect the number and morphology of tobacco trichomes, and that it can be used as a candidate gene for improving stress resistance and the quality of tobacco.

Gas Chromatography-Mass Spectrometry Metabolite Analysis Combined with Transcriptomic and Proteomic Provide New Insights into Revealing Cuticle Formation during Pepper Development

The cuticle plays an important role for the quality of pepper fruit. However, the molecular mechanism of cuticle formation in pepper fruit remains unclear. Our results showed that the wax was continuously accumulated during pepper development, while the cutin monomer first increased and then decreased. Hexadecanoic acid and 10,16-hydroxyhexadecanoic acid were the main components of wax and cutin, respectively. Combined with transcriptome and proteome, the formation patterns of wax and cutin polyester network for pepper cuticle was proposed. The 18 pairs of consistent expression genes and proteins involved in cuticle formation were revealed. Meanwhile, 12 key genes were screened from fatty acid biosynthesis, biosynthesis of unsaturated fatty acids, fatty acid elongation, cutin, suberine, and wax biosynthesis, glycerolipid metabolism, and transport pathway. This study would provide important candidate genes and theoretical basis for the molecular mechanism of cuticle formation, which is essential for the breeding of peppers.

Inactivation of BoORP3a, an oxysterol-binding protein, causes a low wax phenotype in ornamental kale

Identifying genes associated with wax deposition may contribute to the genetic improvement of ornamental kale. Here, we characterized a candidate gene for wax contents, BoORP3a, encoding an oxysterol-binding protein. We sequenced the BoORP3a gene and coding sequence from the high-wax line S0835 and the low-wax line F0819, which revealed 12 single nucleotide polymorphisms between the two lines, of which six caused five amino acids substitutions. BoORP3a appeared to be relatively well conserved in Brassicaceae, as determined by a phylogenetic analysis, and localized to the endoplasmic reticulum and the nucleus. To confirm the role of BoORP3a in wax deposition, we generated three orp3a mutants in a high-wax kale background via CRISPR/Cas9-mediated genome editing. Importantly, all three mutants exhibited lower wax contents and glossy leaves. Overall, these data suggest that BoORP3a may participate in cuticular wax deposition in ornamental kale.

Cytochrome b 5: A versatile electron carrier and regulator for plant metabolism

Cytochrome b ₅ (CB5) is a small heme-binding protein, known as an electron donor delivering reducing power to the terminal enzymes involved in oxidative reactions. In plants, the CB5 protein family is substantially expanded both in its isoform numbers and cellular functions, compared to its yeast and mammalian counterparts. As an electron carrier, plant CB5 proteins function not only in fatty acid desaturation, hydroxylation and elongation, but also in the formation of specialized metabolites such as flavonoids, phenolic esters, and heteropolymer lignin. Furthermore, plant CB5s are found to interact with different non-catalytic proteins such as ethylene signaling regulator, cell death inhibitor, and sugar transporters, implicating their versatile regulatory roles in coordinating different metabolic and cellular processes, presumably in respect to the cellular redox status and/or carbon availability. Compared to the plentiful studies on biochemistry and cellular functions of mammalian CB5 proteins, the cellular and metabolic roles of plant CB5 proteins have received far less attention. This article summarizes the fragmentary information pertaining to the discovery of plant CB5 proteins, and discusses the conventional and peculiar functions that plant CB5s might play in different metabolic and cellular processes. Gaining comprehensive insight into the biological functions of CB5 proteins could offer effective biotechnological solutions to tailor plant chemodiversity and cellular responses to environment stimuli.

BrWAX3, Encoding a β-ketoacyl-CoA Synthase, Plays an Essential Role in Cuticular Wax Biosynthesis in Chinese Cabbage

In this study, we identified a novel glossy mutant from Chinese cabbage, named SD369, and all wax monomers longer than 26 carbons were significantly decreased. Inheritance analysis revealed that the glossy trait of SD369 was controlled by a single recessive locus, BrWAX3. We fine-mapped the BrWAX3 locus to an interval of 161.82 kb on chromosome A09. According to the annotated genome of Brassica rapa, Bra024749 (BrCER60.A09), encoding a β-ketoacyl-CoA synthase, was identified as the candidate gene. Expression analysis showed that BrCER60.A09 was significantly downregulated in all aerial organs of glossy plants. Subcellular localization indicated that the BrCER60.A09 protein functions in the endoplasmic reticulum. A 5567-bp insertion was identified in exon 1 of BrCER60.A09 in SD369, which lead to a premature stop codon, thus causing a loss of function of the BrCER60.A09 enzyme. Moreover, comparative transcriptome analysis revealed that the ‘cutin, suberine, and wax biosynthesis’ pathway was significantly enriched, and genes involved in this pathway were almost upregulated in glossy plants. Further, two functional markers, BrWAX3-InDel and BrWAX3-KASP1, were developed and validated. Overall, these results provide a new information for the cuticular wax biosynthesis and provide applicable markers for marker-assisted selection (MAS)-based breeding of Brassica rapa.

ZmEREB46, a maize ortholog of Arabidopsis WAX INDUCER1/SHINE1, is involved in the biosynthesis of leaf epicuticular very-long-chain waxes and drought tolerance

The aerial surfaces of plants are covered by a layer of cuticular wax that is composed of long-chain hydrocarbon compounds for protection against adverse environmental conditions. The current study identified a maize (Zea mays L.) APETALA2/ethylene-responsive element-binding protein (AP2/EREBP)-type transcription factor, ZmEREB46. Ectopic expression of ZmEREB46 in Arabidopsis increased the accumulation of epicuticular wax on the leaves and enhanced the drought tolerance of plants. The amounts of C24/C32 fatty acids, C32/C34 aldehydes, C32/C34 1-alcohols and C31 alkanes in zmereb46 (ZmEREB46 knockout mutant) leaves were reduced. The amount of leaf total epicuticular wax decreased approximately 50% in zmereb46. Compared to wild-type LH244 leaves, the cuticle permeability of zmereb46 leaves was increased, which resulted from decreased epicuticular wax load and a thinner cuticle layer. ZmEREB46 had transcriptional activation activity and directly bound to promoter regions of ZmCER2, ZmCER3.2 and ZmKCS1. The zmereb46 seedlings also exhibited reduced drought tolerance. These results, and the observations in ZmEREB46-overexpressing lines, suggest that ZmEREB46 is involved in cuticular metabolism by influencing the biosynthesis of very-long-chain waxes and participates in the cutin biosynthesis pathway. These results are helpful to further analyze the regulatory network of wax accumulation in maize.