An ancestral role for 3-KETOACYL-COA SYNTHASE3 as a negative regulator of plant cuticular wax synthesis.

The plant cuticle, a structure primarily composed of wax and cutin, forms a continuous coating over most aerial plant surfaces. The cuticle plays important roles in plant tolerance to environmental stress, including stress imposed by drought. Some members of the 3-KETOACYL-COA SYNTHASE (KCS) family are known to act as metabolic enzymes involved in cuticular wax production. Here we report that Arabidopsis (Arabidopsis thaliana) KCS3, which was previously shown to lack canonical catalytic activity, instead functions as a negative regulator of wax metabolism by reducing the enzymatic activity of KCS6, a key KCS involved in wax production. We demonstrate that the role of KCS3 in regulating KCS6 activity involves physical interactions between specific subunits of the fatty acid elongation complex and is essential for maintaining wax homeostasis. We also show that the role of the KCS3-KCS6 module in regulating wax synthesis is highly conserved across diverse plant taxa from Arabidopsis to the moss Physcomitrium patens, pointing to a critical ancient and basal function of this module in finely regulating wax synthesis.

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.

Acetyl-CoA Carboxylase1 Influences ECERIFERUM2 Activity to Mediate the Synthesis of Very-Long-Chain Fatty Acid Past C28.

Cuticular wax is a protective layer on the aerial surfaces of land plants. In Arabidopsis (Arabidopsis thaliana), cuticular wax is mainly constituted of compounds derived from very-long-chain fatty acids (VLCFAs) with chain lengths longer than C28. CER2-LIKE (ECERIFERUM2-LIKE) proteins interact with CER6/KCS6 (ECERIFERUM6/ß-Ketoacyl-CoA Synthase6), the key enzyme of the fatty acid elongase complex, to modify its substrate specificity for VLCFA elongation past C28. However, the molecular regulatory mechanism of CER2-LIKE proteins remains unclear. Arabidopsis eceriferum19 (cer19) mutants display wax-deficient stems caused by loss of waxes longer than C28, indicating that CER19 may participate in the CER2-LIKE-mediated VLCFA elongation past C28. Using positional cloning and genetic complementation, we showed that CER19 encodes Acetyl-CoA Carboxylase1 (ACC1), which catalyzes the synthesis of malonyl-CoA, the essential substrate for the CER6/KCS6-mediated condensation reaction in VLCFA synthesis. We demonstrated that ACC1 physically interacts with CER2-LIKE proteins via split-ubiquitin yeast two-hybrid (SUY2H) and firefly luciferase complementation imaging (LCI) analysis. Additionally, heterologous expression in yeast and genetic analysis in Arabidopsis revealed that ACC1 affects CER2 activity to influence VLCFA elongation past C28. These findings imply that CER2-LIKE proteins might function as a link between ACC1 and CER6/KCS6 and subsequently enhance CER6/KCS6 binding to malonyl-CoA for further utilization in VLCFA elongation past C28. This information deepens our understanding of the complex mechanism of cuticular wax biosynthesis.

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.

Molecular mechanisms of cell death in bronchopulmonary dysplasia.

Bronchopulmonary dysplasia (BPD) in neonates is the most common pulmonary disease that causes neonatal mortality, has complex pathogenesis, and lacks effective treatment. It is associated with chronic obstructive pulmonary disease, pulmonary hypertension, and right ventricular hypertrophy. The occurrence and development of BPD involve various factors, of which premature birth is the most crucial reason for BPD. Under the premise of abnormal lung structure and functional product, newborns are susceptible to damage to oxides, free radicals, hypoxia, infections and so on. The most influential is oxidative stress, which induces cell death in different ways when the oxidative stress balance in the body is disrupted. Increasing evidence has shown that programmed cell death (PCD), including apoptosis, necrosis, autophagy, and ferroptosis, plays a significant role in the molecular and biological mechanisms of BPD and the further development of the disease. Understanding the mode of PCD and its signaling pathways can provide new therapeutic approaches and targets for the clinical treatment of BPD. This review elucidates the mechanism of BPD, focusing on the multiple types of PCD in BPD and their molecular mechanisms, which are mainly based on experimental results obtained in rodents.

Fine-tuning the activities of ß-KETOACYL-COA SYNTHASE 3 (KCS3) and KCS12 in Arabidopsis is essential for maintaining cuticle integrity.

The plant cuticle, consisting of wax and cutin, is involved in adaptations to various environments. ß-Ketoacyl-CoA synthases (KCSs) usually serve as a component of the fatty acid elongation complex that participates in the production of very long-chain fatty acids and provides precursors for the synthesis of various lipids, including wax; however, we recently reported that KCS3 and KCS12 negatively regulate wax biosynthesis. In this current study, we observed that unlike KCS3-overexpressing (OE) lines, KCS12-OE lines had fused floral organs because of abnormal cuticle biosynthesis. This prompted us to compare the functions of KCS3 and KCS12 during cuticle formation. Mutation of KCS3 caused greater effects on wax production, whereas mutation of KCS12 exerted more severe effects on cutin synthesis. The double-mutant kcs3 kcs12 had significantly increased wax and cutin contents compared to either single-mutant, suggesting that KCS12 and KCS3 have additive effects on cuticle biosynthesis. Cuticle permeability was greater for the double-mutant than for the single mutants, which ultimately led to increased susceptibility to drought stress and floral-organ fusion. Taken together, our results demonstrate the regulatory roles of KCS3 and KCS12 during cuticle biosynthesis, and show that maintaining KCS3 and KCS12 expression at certain levels is essential for the formation of a functional cuticle layer.

Multifunctional cellulosic materials prepared by a reactive DES based zero-waste system.

Energy consumption and post-treatment of chemical reagent residues are important issues that hinder the sustainable production of the natural building blocks of cellulose nanofibrils (CNFs). In this study, we realize a low-energy, zero-waste process for CNF production by designing a novel reactive deep eutectic solvent (DES), the residue of which can be directly used as a plant growth regulator. After pretreatment with the DES, cellulose fibers self-delaminate into thin layers referred to as pseudo-CNFs, as their strength, toughness and transmittance are comparable to those of CNFs. Pseudo-CNFs break into smaller particles during recycling and thus display unique mechanical upcycling. After facile fibrillation, the obtained CNFs can independently form freestanding sub-micrometer films that show a strong, full coloration, which is demonstrated for the first time. Our concept can enable a green process, and the developed cellulosic materials may find various applications as structural materials and optical coatings.

Arabidopsis ACYL-ACTIVATING ENZYME 9 (AAE9) encoding an isobutyl-CoA synthetase is a key factor connecting branched-chain amino acid catabolism with iso-branched wax biosynthesis.

Iso-branched wax compounds are well known in plants, but their biosynthetic pathways are still mostly unknown. It has been speculated that branched waxes are derived from branched-chain amino acid (BCAA) catabolism, but the evidence for this is very limited. Gas chromatography-flame ionisation detection (GC-FID) analysis revealed that mutations in two subunits of the branched-chain ketoacid dehydrogenase (BCKDH) complex, a key enzyme complex in the degradation of BCAAs, significantly decreased the amounts of branched wax compounds, indicating that BCAA degradation may be integral to the synthesis of iso-branched wax. Substrate feeding studies further revealed that the metabolic precursor of iso-branched wax compounds is isobutyric acid (iBA), which is derived from valine degradation in Arabidopsis. We also isolated a novel mutant and found that its branched wax deficient phenotype could not be rescued by iBA. Map-based cloning together with complementation analysis revealed that mutation in ACYL-ACTIVATING ENZYME 9 (AAE9) is responsible for this phenotype. Genetic and enzyme activity analysis demonstrated that AAE9 is located downstream of the BCAA degradation pathway, and that it activates iBA to isobutyryl-CoA for use on branched wax synthesis. Taken together, our study demonstrates that AAE9 is a key factor connecting BCAA catabolism with branched wax biosynthesis.

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.

Fatty alcohol oxidase 3 (FAO3) and FAO4b connect the alcohol- and alkane-forming pathways in Arabidopsis stem wax biosynthesis.

The alcohol- and alkane-forming pathways in cuticular wax biosynthesis are well characterized in Arabidopsis. However, potential interactions between the two pathways remain unclear. Here, we reveal that mutation of CER4, the key gene in the alcohol-forming pathway, also led to a deficiency in the alkane-forming pathway in distal stems. To trace the connection between the two pathways, we characterized two homologs of fatty alcohol oxidase (FAO), FAO3 and FAO4b, which were highly expressed in distal stems and localized to the endoplasmic reticulum. The amounts of waxes from the alkane-forming pathway were significantly decreased in stems of fao4b and much lower in fao3 fao4b plants, indicative of an overlapping function for the two proteins in wax synthesis. Additionally, overexpression of FAO3 and FAO4b in Arabidopsis resulted in a dramatic reduction of primary alcohols and significant increases of aldehydes and related waxes. Moreover, expressing FAO3 or FAO4b led to significantly decreased amounts of C18-C26 alcohols in yeast co-expressing CER4 and FAR1. Collectively, these findings demonstrate that FAO3 and FAO4b are functionally redundant in suppressing accumulation of primary alcohols and contributing to aldehyde production, which provides a missing and long-sought-after link between these two pathways in wax biosynthesis.

ArabidopsisKCS5 and KCS6 Play Redundant Roles in Wax Synthesis.

3-ketoacyl-CoA synthases (KCSs), as components of a fatty acid elongase (FAE) complex, play key roles in determining the chain length of very-long-chain fatty acids (VLCFAs). KCS6, taking a predominate role during the elongation from C26 to C28, is well known to play an important role in wax synthesis. KCS5 is one paralog of KCS6 and its role in wax synthesis remains unknown. Wax phenotype analysis showed that in kcs5 mutants, the total amounts of wax components derived from carbon 32 (C32) and C34 were apparently decreased in leaves, and those of C26 to C32 derivatives were obviously decreased in flowers. Heterologous yeast expression analysis showed that KCS5 alone displayed specificity towards C24 to C28 acids, and its coordination with CER2 and CER26 catalyzed the elongation of acids exceeding C28, especially displaying higher activity towards C28 acids than KCS6. BiLC experiments identified that KCS5 physically interacts with CER2 and CER26. Wax phenotype analysis of different organs in kcs5 and kcs6 single or double mutants showed that KCS6 mutation causes greater effects on the wax synthesis than KCS5 mutation in the tested organs, and simultaneous repression of both protein activities caused additive effects, suggesting that during the wax biosynthesis process, KCS5 and KCS6 play redundant roles, among which KCS6 plays a major role. In addition, simultaneous mutations of two genes nearly block drought-induced wax production, indicating that the reactions catalyzed by KCS5 and KCS6 play a critical role in the wax biosynthesis in response to drought.

Overexpression of ß-Ketoacyl CoA Synthase 2B.1 from Chenopodium quinoa Promotes Suberin Monomers' Production and Salt Tolerance in Arabidopsis thaliana.

Very-long-chain fatty acids (VLCFAs) are precursors for the synthesis of various lipids, such as triacylglycerols, sphingolipids, cuticular waxes, and suberin monomers, which play important roles in plant growth and stress responses. However, the underlying molecular mechanism regulating VLCFAs' biosynthesis in quinoa (Chenopodium quinoa Willd.) remains unclear. In this study, we identified and functionally characterized putative 3-ketoacyl-CoA synthases (KCSs) from quinoa. Among these KCS genes, CqKCS2B.1 showed high transcript levels in the root tissues and these were rapidly induced by salt stress. CqKCS2B.1 was localized to the endoplasmic reticulum. Overexpression of CqKCS2B.1 in Arabidopsis resulted in significantly longer primary roots and more lateral roots. Ectopic expression of CqKCS2B.1 in Arabidopsis promoted the accumulation of suberin monomers. The occurrence of VLCFAs with C22-C24 chain lengths in the overexpression lines suggested that CqKCS2B.1 plays an important role in the elongation of VLCFAs from C20 to C24. The transgenic lines of overexpressed CqKCS2B.1 showed increased salt tolerance, as indicated by an increased germination rate and improved plant growth and survival under salt stress. These findings highlight the significant role of CqKCS2B.1 in VLCFAs' production, thereby regulating suberin biosynthesis and responses to salt stress. CqKCS2B.1 could be utilized as a candidate gene locus to breed superior, stress-tolerant quinoa cultivars.

CER16 Inhibits Post-Transcriptional Gene Silencing of CER3 to Regulate Alkane Biosynthesis.

The aerial surfaces of land plants have a protective layer of cuticular wax. Alkanes are common components of these waxes, and their abundance is affected by a range of stresses. The CER16 protein has been implicated in alkane biosynthesis in the cuticular wax of Arabidopsis (Arabidopsis thaliana). Here, we identified two new mutant alleles of CER16 in Arabidopsis resulting in production of less wax with dramatically fewer alkanes than the wild type. Map-based cloning with genetic analysis revealed that the cer16 phenotype was caused by complete loss of AT5G44150, encoding a protein with no known domains or motifs. Comparative transcriptomic analysis revealed that transcripts of CER3, previously shown to play a principal role in alkane production, were markedly reduced in the cer16 mutants. To define the relationship between CER3 and CER16, we transformed the full CER3 gene into a cer16 mutant. Transgenic CER3 expression was silenced, and levels of small interfering RNAs targeting CER3 were significantly increased. Mutating two major components of the RNA-silencing machinery in a cer16 genetic background restored CER3 transcript levels to wild-type levels, with the stems restored to wild-type glaucousness. We suggest that CER16 deficiency induces post-transcriptional gene silencing of both endogenous and exogenous expression of CER3.

GDSL lipase occluded stomatal pore 1 is required for wax biosynthesis and stomatal cuticular ledge formation.

The plant leaf surface is coated with a waterproof cuticle layer. Cuticle facing the stomatal pore surface needs to be sculpted to form outer cuticular ledge (OCL) after stomatal maturation for efficient gas exchange. Here, we characterized the roles of Arabidopsis GDSL lipase, Occlusion of Stomatal Pore 1 (OSP1), in wax biosynthesis and stomatal OCL formation. OSP1 mutation results in significant reduction in leaf wax synthesis and occlusion of stomata, leading to increased epidermal permeability, decreased transpiration rate, and enhanced drought tolerance. We demonstrated that OSP1 activity is critical for its role in wax biosynthesis and stomatal function. In vitro enzymatic assays demonstrated that OSP1 possesses thioesterase activity, particularly on C22:0 and C26:0 acyl-CoAs. Genetic interaction analyses with CER1 (ECERIFERUM 1), CER3 (ECERIFERUM 3) and MAH1 (Mid-chain Alkane Hydroxylase 1) in wax biosynthesis and stomatal OCL formation showed that OSP1 may act upstream of CER3 in wax biosynthesis, and implicate that wax composition percentage changes and keeping ketones in a lower level play roles, at least partially, in forming stomatal ledges. Our findings provided insights into the molecular mechanism mediating wax biosynthesis and highlighted the link between wax biosynthesis and the process of stomatal OCL formation.

The R2R3-MYB transcription factor AtMYB49 modulates salt tolerance in Arabidopsis by modulating the cuticle formation and antioxidant defence.

Salt stress activates defence responses in plants, including changes in leaf surface structure. Here, we showed that the transcriptional activation of cutin deposition and antioxidant defence by the R2R3-type MYB transcription factor AtMYB49 contributed to salt tolerance in Arabidopsis thaliana. Characterization of loss-of-function myb49 mutants, and chimeric AtMYB49-SRDX-overexpressing SRDX49 transcriptional repressor and AtMYB49-overexpressing (OX49) overexpressor plants demonstrated a positive role of AtMYB49 in salt tolerance. Transcriptome analysis revealed that many genes belonging to the category "cutin, suberin and wax biosyntheses" were markedly up-regulated and down-regulated in OX49 and SRDX49 plants, respectively, under normal and/or salt stress conditions. Some of these differentially expressed genes, including MYB41, ASFT, FACT and CYP86B1, were also shown to be the direct targets of AtMYB49 and activated by AtMYB49. Biochemical analysis indicated that AtMYB49 modulated cutin deposition in the leaves. Importantly, cuticular transpiration, chlorophyll leaching and toluidine blue-staining assays revealed a link between increased AtMYB49-mediated cutin deposition in leaves and enhanced salt tolerance. Additionally, increased AtMYB49 expression elevated Ca2+ level in leaves and improved antioxidant capacity by up-regulating genes encoding peroxidases and late embryogenesis abundant proteins. These results suggest that genetic manipulation of AtMYB49 may provide a novel way to improve salt tolerance in plants.

Deciphering the Novel Role of AtMIN7 in Cuticle Formation and Defense against the Bacterial Pathogen Infection.

The cuticle is the outermost layer of plant aerial tissue that interacts with the environment and protects plants against water loss and various biotic and abiotic stresses. ADP ribosylation factor guanine nucleotide exchange factor proteins (ARF-GEFs) are key components of the vesicle trafficking system. Our study discovers that AtMIN7, an Arabidopsis ARF-GEF, is critical for cuticle formation and related leaf surface defense against the bacterial pathogen Pseudomonas syringae pathovar tomato (Pto). Our transmission electron microscopy and scanning electron microscopy studies indicate that the atmin7 mutant leaves have a thinner cuticular layer, defective stomata structure, and impaired cuticle ledge of stomata compared to the leaves of wild type plants. GC-MS analysis further revealed that the amount of cutin monomers was significantly reduced in atmin7 mutant plants. Furthermore, the exogenous application of either of three plant hormones-salicylic acid, jasmonic acid, or abscisic acid-enhanced the cuticle formation in atmin7 mutant leaves and the related defense responses to the bacterial Pto infection. Thus, transport of cutin-related components by AtMIN7 may contribute to its impact on cuticle formation and related defense function.

Evolutionarily conserved function of the sacred lotus (Nelumbo nucifera Gaertn.) CER2-LIKE family in very-long-chain fatty acid elongation.

MAIN CONCLUSION: Identification of NnCER2 and NnCER2-LIKE from Nelumbo nucifera, which are required for the very-long-chain fatty acid elongation, provides new evidence that CER2 proteins are evolutionarily conserved across the eudicots. CER2-LIKE family proteins have been described as core components of the fatty acid elongase complex in Arabidopsis, maize, and rice, having specific function in synthesis of the C30 to C34 fatty acyl-CoA precursors of cuticular waxes. Little is known about the functional conservation in this gene family across species. In this study, two CER2-LIKE family proteins, NnCER2 and NnCER2-LIKE, were characterized from sacred lotus (Nelumbo nucifera), which is an ancient basal eudicot. The transcriptional expression of NnCER2 and NnCER2-LIKE was found in floating leaf blades, emergent petioles and vertical leaves, petals, and anthers. The NnCER2 and NnCER2-LIKE proteins were localized to the endoplasmic reticulum and nucleus. Overexpressing NnCER2 and NnCER2-LIKE in Arabidopsis led to alteration of cuticle wax structure in inflorescence stems, and this was associated with elevated 30, 32, and 34 carbon length wax compounds, and their derivatives. The different substrate specificities of NnCER2 and NnCER2-LIKE were explored using co-expression with AtCER6 in yeast cells. These findings provide clear evidence that the function of CER2 family proteins in producing VLCFAs is highly conserved across the eudicots.

The Acyl Desaturase CER17 Is Involved in Producing Wax Unsaturated Primary Alcohols and Cutin Monomers.

We report n-6 monounsaturated primary alcohols (C26:1, C28:1, and C30:1 homologs) in the cuticular waxes of Arabidopsis (Arabidopsis thaliana) inflorescence stem, a class of wax not previously reported in Arabidopsis. The Arabidopsis cer17 mutant was completely deficient in these monounsaturated alcohols, and CER17 was found to encode a predicted ACYL-COENZYME A DESATURASE LIKE4 (ADS4). Studies of the Arabidopsis cer4 mutant and yeast variously expressing CER4 (a predicted fatty acyl-CoA reductase) with CER17/ADS4, demonstrated CER4's principal role in synthesis of these monounsaturated alcohols. Besides unsaturated alcohol deficiency, cer17 mutants exhibited a thickened and irregular cuticle ultrastructure and increased amounts of cutin monomers. Although unsaturated alcohols were absent throughout the cer17 stem, the mutation's effects on cutin monomers and cuticle ultrastructure were much more severe in distal than basal stems, consistent with observations that the CER17/ADS4 transcript was much more abundant in distal than basal stems. Furthermore, distal but not basal stems of a double mutant deficient for both CER17/ADS4 and LONG-CHAIN ACYL-COA SYNTHETASE1 produced even more cutin monomers and a thicker and more disorganized cuticle ultrastructure and higher cuticle permeability than observed for wild type or either mutant parent, indicating a dramatic genetic interaction on conversion of very long chain acyl-CoA precursors. These results provide evidence that CER17/ADS4 performs n-6 desaturation of very long chain acyl-CoAs in both distal and basal stems and has a major function associated with governing cutin monomer amounts primarily in the distal segments of the inflorescence stem.

Self-healing polymer materials constructed by macrocycle-based host-guest interactions.

Self-healing polymers, which can spontaneously recover themselves after being ruptured, result in enhanced lifetimes for materials and open up a fascinating direction in material science. Macrocycle-based host-guest interactions, one of the most crucial non-covalent interactions, play a key role in self-healing material fabrication. This review aims to highlight the very recent and important progress made in the area of self-healing polymer materials by focusing on cyclodextrins (CDs), crown ethers, cucurbit[n]urils (CBs), calix[n]arenes and pillar[n]arenes with special guest groups and tailored structures. In addition, we also propose future research directions and hope that this review can in a way reflect the current situation and future trends in this developing area.

Enhancing Two-Electron Reaction Contribution in MnO2 Cathode Material by Structural Engineering for Stable Cycling in Aqueous Zn Batteries.

MnO2 is a promising cathode for aqueous Zn batteries. However, the cycling stability is seriously hindered by active material dissolution, and the pre-addition of Mn2+ salts in electrolytes is widely required. Herein, we propose a structural engineering strategy for MnO2 to enhance the capacity contribution from the reversible two-electron transfer reaction of MnO2/Mn2+ and realize stable cycling in Mn2+-free electrolytes. By compositing with MoO3, MnO2 exhibits weakened Mn-O bonds, more oxygen vacancies, spontaneous generation of structural water, and thus a lowered energy barrier for Mn release during discharge. Meanwhile, the composite material presents stronger electrostatic attractions for dissolved Mn2+, which ensures highly reversible re-deposition during charge. As a result, the mass ratios between materials undergoing reversible two-electron and one-electron transfer reactions increase from 0.85 in MnO2 to 1.68 in the MnO2/MoO3 composite material. In the ZnSO4 electrolyte, the MnO2/MoO3 cathode achieves 92.6% capacity retention after 300 cycles at 0.1 A g-1 (>1900 h), superior to 62.7% for MnO2. MnO2/MoO3 also retains 80.1% capacity after 16 000 cycles at 1 A g-1 (>3200 h). This work presents an effective path to realize stable cycling of MnO2 in Zn batteries.