Natural rubber biosynthesis—A living carbocationic polymerization?

The interaction networks of small rubber particle proteins in the latex of Taraxacum koksaghyz reveal diverse functions in stress responses and secondary metabolism

The Russian dandelion (Taraxacum koksaghyz) is a promising source of natural rubber (NR). The synthesis of NR takes place on the surface of organelles known as rubber particles, which are found in latex - the cytoplasm of specialized cells known as laticifers. As well as the enzymes directly responsible for NR synthesis, the rubber particles also contain small rubber particle proteins (SRPPs), the most abundant of which are SRPP3, 4 and 5. These three proteins support NR synthesis by maintaining rubber particle stability. We used homology-based searches to identify the whole TkSRPP gene family and qPCR to create their spatial expression profiles. Affinity enrichment-mass spectrometry was applied to identify TkSRPP3/4/5 protein interaction partners in T. koksaghyz latex and selected interaction partners were analyzed using qPCR, confocal laser scanning microscopy and heterologous expression in yeast. We identified 17 SRPP-like sequences in the T. koksaghyz genome, including three apparent pseudogenes, 10 paralogs arranged as an inverted repeat in a cluster with TkSRPP3/4/5, and one separate gene (TkSRPP6). Their sequence diversity and different expression profiles indicated distinct functions and the latex interactomes obtained for TkSRPP3/4/5 suggested that TkSRPP4 is a promiscuous hub protein that binds many partners from different compartments, whereas TkSRPP3 and 5 have more focused interactomes. Two interactors shared by TkSRPP3/4/5 (TkSRPP6 and TkUGT80B1) were chosen for independent validation and detailed characterization. TkUGT80B1 triterpenoid glycosylating activity provided first evidence for triterpenoid saponin synthesis in T. koksaghyz latex. Based on its identified interaction partners, TkSRPP4 appears to play a special role in the endoplasmic reticulum, interacting with lipidmodifying enzymes that may facilitate rubber particle formation. TkSRPP5 appears to be involved in GTPase-dependent signaling and TkSRPP3 may act as part of a kinase signaling cascade, with roles in stress tolerance. TkSRPP interaction with TkUGT80B1 draws a new connection between TkSRPPs and triterpenoid saponin synthesis in T. koksaghyz latex. Our data contribute to the functional differentiation between TkSRPP paralogs and demonstrate unexpected interactions that will help to further elucidate the network of proteins linking TkSRPPs, stress responses and NR biosynthesis within the cellular complexity of latex.

Proteomic and Targeted Lipidomic Analyses of Fluid and Rigid Rubber Particle Membrane Domains in Guayule

Rubber (cis-1,4-polyisoprene) is produced in cytosolic unilamellar vesicles called rubber particles (RPs), and the protein complex responsible for this synthesis, the rubber transferase (RTase), is embedded in, or tethered to, the membranes of these RPs. Solubilized enzyme activity is very difficult to achieve because the polymerization of highly hydrophilic substrates into hydrophobic polymers requires a polar/non-polar interface and a hydrophobic compartment. Using guayule (Parthenium argentatum) as a model rubber-producing species, we optimized methods to isolate RP unilamellear membranes and then a subset of membrane microdomains (detergent-resistant membranes) likely to contain protein complexes such as RTase. The phospholipid and sterol composition of these membranes and microdomains were analyzed using thin-layer chromatography (TLC) and liquid chromatography tandem mass spectroscopy (LC-MS/MS). Our data indicate that RP membranes consist predominantly of phosphatidic acid-containing membrane microdomains (DRMs or "lipid rafts"). Proteomic analyses of guayule RP membranes and membrane microdomains identified 80 putative membrane proteins covering 30 functional categories. From this population, we have tentatively identified several proteins in multiple functional domains associated with membrane microdomains which may be critical to RTase function. Definition of the mechanisms underlying rubber synthesis will provide targets for both metabolic engineering and breeding strategies designed to increase natural rubber production in latex-producing species.

Synthesis of Mechanically Robust Very High Molecular Weight Polyisoprene Particle Brushes by Atom Transfer Radical Polymerization

Linear polyisoprene (PI) and SiO2-g-PI particle brushes were synthesized by both conventional and activators regenerated by electron transfer (ARGET) atom transfer radical polymerization (ATRP). The morphology and solution state study on the particle brushes by transmission electron microscopy (TEM) and dynamic light scattering (DLS) confirmed the successful grafting of PI ligands on the silica surface. The presence of nanoparticle clusters suggests low grafting density (associated with the limited initiation efficiency of ARGET for PI). Nevertheless, particle brushes with very high molecular weights, Mn > 300,000, were prepared, which significantly improved the dispersion of silica nanoparticles and also contributed to excellent mechanical performance. The reinforcing effects of SiO2 nanofillers and very high molecular weight PI ligands were investigated by dynamic mechanical analysis (DMA) as well as computational simulation for the cured linear PI homopolymer/SiO2-g-PI particle brush bulk films.

Construction of Supramolecular Polymers and Covalent Polymers via the Same Monomers

Nature owns the ability to construct structurally different polymers from the same monomers. While polymers can be classified as covalent polymers (CPs) and supramolecular polymers (SPs), it is still difficult to synthesize CPs and SPs using same monomers like nature do. Herein, M1 with two diazo salts on both the ends was designed. Additionally, hydroquinone was chosen to be M2 for the existence of two hydroxyl groups. When mixing at room temperature, M1 and M2 self-assembled to SPs via N…H hydrogen bonds. In another way, upon the exposure to ultraviolet irradiation when blending M1 with M2, CPs were fabricated in the presence of covalent bonds. The excellent thermal stability of CPs was determined by TGA and DSC, while the great corrosion resistance of covalent polymers was detected by acid or alkali immersion. In this way, constructing two kinds of polymers using the same monomers was successfully achieved. This shows tremendous potential in fields of polymer science, supramolecular chemistry, which would boom the development of polymers.

Natural rubber - Increasing diversity of an irreplaceable renewable resource

This paper discusses the importance of introducing domestic natural rubber production and presents the rediscovery of a rubber-producing species, Scorzonera tau-saghyz or "mountain gum", originally discovered in 1929 on the Karatau mountains in Kazakhstan. This plant could potentially also be cultivated in the U.S. In this exploratory work, roots (2-5 years old) were harvested on June 16, 2021 from wild strands in the Karatau mountains, Kumantas ridge, and Saraba, Kazakhstan, and processed at the Ohio State University. The rubber extraction method was based on an indigenous method in Kazakhstan to make natural chewing gum. Water extraction followed by purification yielded 16.2 wt% rubber from the dry roots, in comparison with 4-8 wt% from most rubber dandelion (Taraxacum kok-saghyz) plants, also a potential domestic rubber producing plant. High-resolution size exclusion chromatography was used to analyze rubber samples. The molecular weights and gel and oligomer contents were very similar to the rubber from Hevea brasiliensis, the current commercial source of natural rubber. More detailed investigations of this very interesting rubber-producing plant are in progress.

PNP-Ligated Rare-Earth Metal Catalysts for Efficient Polymerization of Isoprene

The tridentate PNP ligand-supported rare-earth metal complexes, i.e., bis[o-diphenylphosphinophenyl]amido-Re-bis[o-dimethylaminobenzyl], [(Ph2P-o-C6H4)2N]Re[(CH2-o-Me2N(C6H4))2]: (Re = Y, 1; Nd, 2; Gd, 3) were applied to isoprene polymerization. When activated with borate activator ([PhMe2NH][B(C6F5)4] (NH-BARF), catalysts 1 and 3 exhibited excellent catalytic efficiency in aromatic media, produced very-high to ultrahigh molecular weight (Mw over 130 × 104 g/moL) polyisoprene rubber (PIR), and the obtained PIR contained over 98% cis-1,4 head-to-tail repeating unites. In most cases, the borate-activated polymerization reaction proceeded in a quasi-living pattern (PDI = 1.2–1.5) under controlled monomer conversion; whereas, activated with the commercially available modified methylaluminoxane (MMAO3A) in aliphatic hydrocarbon media, complexes 1, 2 and 3 all showed high catalytic efficiency, produced high molecular weight PIR with narrow molecular weight distribution (PDI ≤ 2.0) and high cis-1,4 head-to tail repeating unites in the range of 91–95%. Thus, the catalyst systems that consisted of 1, 2 and 3/MMAO3A, are closely relevant to the current industrial polybutadiene rubber (PBR) and PIR production processes.

Design Strategy for Vulcanization Accelerator of Diphenylguanidine/Cyclodextrin Inclusion Complex for Natural Rubber Latex Foam with Enhancing Performance

Vulcanization is an essential process to obtain high-performance rubber products. Diphenylguanidine (DPG) is often used as the secondary accelerator in the vulcanization process of natural rubber (NR) latex. However, DPG would make NR latex emulsion exhibit gelation, resulting in the negative vulcanization efficiency. In addition, exposure to DPG might lead to some physiological diseases during the production process of DPG doped NR latex. Hydroxypropyl-β-cyclodextrin (HP-β-CD) with the hydrophobic interior and hydrophilic exterior has the advantages of good water solubility, high bioavailability, reliable stability, and low toxicity. In this study, the inclusion complex of diphenylguanidine-hydroxypropyl-β-cyclodextrin (DPG-HP-β-CD) is prepared by ball milling with a host-guest molar ratio of 1 : 1, which has also been applied to the foaming process of NR latex. The mechanical properties of DPG-HP-β-CD inclusion complex/natural rubber latex foam (DPG-HP-β-CD/NRLF) have been significantly improved, including the tensile strength, elongation at break, hardness, compression set, resilience, and antiaging performance. Further, the usage of DPG has been reduced, leading to the reduction of toxicity and environmental hazards.

HbMYB44, a Rubber Tree MYB Transcription Factor With Versatile Functions in Modulating Multiple Phytohormone Signaling and Abiotic Stress Responses

The vital roles of R2R3-MYB transcription factors (TFs) in regulating stress response and phytohormone signaling have been thoroughly studied in numerous plant species, but the functions of these TFs in rubber tree are poorly understood. Rubber tree is the most important source of natural rubber but often suffers from various abiotic and biotic stresses that cause severe yield losses each year. In this study, we reported a novel MYB44 gene in rubber tree (named HbMYB44) and revealed its biological function. HbMYB44 was highly similar to AtMYB44 and clustered into subgroup 22. Transient expression indicated that HbMYB44 is a nuclear localized protein and displays transactivation activity at the C-terminus. HbMYB44 was ubiquitously expressed in rubber tree, and its expression was strongly induced by multiple phytohormones, drought stress, wounding, and H₂O₂ treatments. Furthermore, overexpression of HbMYB44 in Arabidopsis (OE) demonstrated that OE plants significantly enhanced stress tolerance, i.e., salt stress, osmotic stress, and drought stress. Additionally, HbMYB44 promoted recovery from root growth inhibition of OE plants caused by exogenous phytohormones (including abscisic acid, methyl jasmonic acid, gibberellic acid 3, and salicylic acid), but the opposite effect was present in response to ethephon. Interestingly, HbMYB44 increased the expression of its homologous genes and interacting protein-encoding genes in OE plants. Overall, HbMYB44 plays versatile functions in modulating multiple phytohormone signaling pathways and stress tolerance.

Comparative analysis of latex transcriptomes reveals the potential mechanisms underlying rubber molecular weight variations between the Hevea brasiliensis clones RRIM600 and Reyan7-33-97

BACKGROUND

The processabilities and mechanical properties of natural rubber depend greatly on its molecular weight (MW) and molecular weight distribution (MWD). However, the mechanisms underlying the regulation of molecular weight during rubber biosynthesis remain unclear.

RESULTS

In the present study, we determined the MW and particle size of latex from 1-year-old virgin trees and 30-year-old regularly tapped trees of the Hevea clones Reyan7-33-97 and RRIM600. The results showed that both the MW and the particle size of latex varied between these two clones and increased with tree age. Latex from RRIM600 trees had a smaller average particle size than that from Reyan7-33-97 trees of the same age. In 1-year-old trees, the Reyan7-33-97 latex displayed a slightly higher MW than that of RRIM600, whereas in 30-year-old trees, the RRIM600 latex had a significantly higher MW than the Reyan7-33-97 latex. Comparative analysis of the transcriptome profiles indicated that the average rubber particle size is negatively correlated with the expression levels of rubber particle associated proteins, and that the high-MW traits of latex are closely correlated with the enhanced expression of isopentenyl pyrophosphate (IPP) monomer-generating pathway genes and downstream allylic diphosphate (APP) initiator-consuming non-rubber pathways. By bioinformatics analysis, we further identified a group of transcription factors that potentially regulate the biosynthesis of IPP.

CONCLUSIONS

Altogether, our results revealed the potential regulatory mechanisms involving gene expression variations in IPP-generating pathways and the non-rubber isoprenoid pathways, which affect the ratios and contents of IPP and APP initiators, resulting in significant rubber MW variations among same-aged trees of the Hevea clones Reyan7-33-97 and RRIM600. Our findings provide a better understanding of rubber biosynthesis and lay the foundation for genetic improvement of rubber quality in H. brasiliensis.

Comparison of Morphological Characteristics and Determination of Different Patterns for Rubber Particles in Dandelion and Different Rubber Grass Varieties

Russian dandelion Taraxacum kok-saghyz (TKS) is one promising alternative crop for natural rubber production. However, it is easily confused with other dandelions. In this study, we performed a systematical comparison of the morphological characteristics for different TKS varieties and common dandelion Taraxacum officinale (TO). Our results demonstrated that several obvious differences in morphology can be found between TKS and TO. TO leaf is a pinnate shape, its margin is heavily jagged and its base is cuneate, but TKS leaf is more cuneate and its leaf margin is nearly smooth and round. There are obvious differences for the outer bracts of TO and TKS flower buds. TKS bracts are oblanceolate, apex obtuse, margin smooth and sinuate, and its outer layer of flower buds and faceplate involucre sepal is buckled inward to form a certain angle. TKS is self-incompatible, and its seeds are spindle-shaped achene and show upright plumpness. A large amount of laticifer cells and rubber particles can be detected from many TKS tissues, and dry roots of TKS contain high contents of natural rubber. Laticifer cells and rubber particles can only be examined in the vein, stem, and roots of TKS. Our statical results also revealed that the numbers of laticifer cells and rubber particles have a positive relationship with the rubber content in TKS roots. These morphological features can help us to easily distinguish TKS from common dandelion and approximately estimate the rubber content in the roots of different TKS varieties for TKS breeding in future.

Identification and Characterization of Glycoproteins and Their Responsive Patterns upon Ethylene Stimulation in the Rubber Latex

Natural rubber is an important industrial material, which is obtained from the only commercially cultivated rubber tree, Hevea brasiliensis. In rubber latex production, ethylene has been extensively used as a stimulant. Recent research showed that post-translational modifications (PTMs) of latex proteins, such as phosphorylation, glycosylation and ubiquitination, are crucial in natural rubber biosynthesis. In this study, comparative proteomics was performed to identify the glycosylated proteins in rubber latex treated with ethylene for different days. Combined with Pro-Q Glycoprotein gel staining and mass spectrometry techniques, we provided the first visual profiling of glycoproteomics of rubber latex and finally identified 144 glycosylated protein species, including 65 differentially accumulated proteins (DAPs) after treating with ethylene for three and/or five days. Gene Ontology (GO) functional annotation showed that these ethylene-responsive glycoproteins are mainly involved in cell parts, membrane components and metabolism. Pathway analysis demonstrated that these glycosylated rubber latex proteins are mainly involved in carbohydrate metabolism, energy metabolism, degradation function and cellular processes in rubber latex metabolism. Protein-protein interaction analysis revealed that these DAPs are mainly centered on acetyl-CoA acetyltransferase and hydroxymethylglutaryl-CoA synthase (HMGS) in the mevalonate pathway for natural rubber biosynthesis. In our glycoproteomics, three protein isoforms of HMGS2 were identified from rubber latex, and only one HMGS2 isoform was sharply increased in rubber latex by ethylene treatment for five days. Furthermore, the HbHMGS2 gene was over-expressed in a model rubber-producing grass Taraxacum Kok-saghyz and rubber content in the roots of transgenic rubber grass was significantly increased over that in the wild type plant, indicating HMGS2 is the key component for natural rubber production.

Natural rubber biosynthesis in plants, the rubber transferase complex, and metabolic engineering progress and prospects

Natural rubber (NR) is a nonfungible and valuable biopolymer, used to manufacture ~50 000 rubber products, including tires and medical gloves. Current production of NR is derived entirely from the para rubber tree (Hevea brasiliensis). The increasing demand for NR, coupled with limitations and vulnerability of H. brasiliensis production systems, has induced increasing interest among scientists and companies in potential alternative NR crops. Genetic/metabolic pathway engineering approaches, to generate NR-enriched genotypes of alternative NR plants, are of great importance. However, although our knowledge of rubber biochemistry has significantly advanced, our current understanding of NR biosynthesis, the biosynthetic machinery and the molecular mechanisms involved remains incomplete. Two spatially separated metabolic pathways provide precursors for NR biosynthesis in plants and their genes and enzymes/complexes are quite well understood. In contrast, understanding of the proteins and genes involved in the final step(s)-the synthesis of the high molecular weight rubber polymer itself-is only now beginning to emerge. In this review, we provide a critical evaluation of recent research developments in NR biosynthesis, in vitro reconstitution, and the genetic and metabolic pathway engineering advances intended to improve NR content in plants, including H. brasiliensis, two other prospective alternative rubber crops, namely the rubber dandelion and guayule, and model species, such as lettuce. We describe a new model of the rubber transferase complex, which integrates these developments. In addition, we highlight the current challenges in NR biosynthesis research and future perspectives on metabolic pathway engineering of NR to speed alternative rubber crop commercial development.

Proteomic Landscape Has Revealed Small Rubber Particles Are Crucial Rubber Biosynthetic Machines for Ethylene-Stimulation in Natural Rubber Production

Rubber particles are a specific organelle for natural rubber biosynthesis (NRB) and storage. Ethylene can significantly improve rubber latex production by increasing the generation of small rubber particles (SRPs), regulating protein accumulation, and activating many enzyme activities. We conducted a quantitative proteomics study of different SRPs upon ethylene stimulation by differential in-gel electrophoresis (DIGE) and using isobaric tags for relative and absolute quantification (iTRAQ) methods. In DIGE, 79 differentially accumulated proteins (DAPs) were determined as ethylene responsive proteins. Our results show that the abundance of many NRB-related proteins has been sharply induced upon ethylene stimulation. Among them, 23 proteins were identified as rubber elongation factor (REF) and small rubber particle protein (SRPP) family members, including 16 REF and 7 SRPP isoforms. Then, 138 unique phosphorylated peptides, containing 129 phosphorylated amino acids from the 64 REF/SRPP family members, were identified, and most serine and threonine were phosphorylated. Furthermore, we identified 226 DAPs from more than 2000 SRP proteins by iTRAQ. Integrative analysis revealed that almost all NRB-related proteins can be detected in SRPs, and many proteins are positively responsive to ethylene stimulation. These results indicate that ethylene may stimulate latex production by regulating the accumulation of some key proteins. The phosphorylation modification of REF and SRPP isoforms might be crucial for NRB, and SRP may act as a complex natural rubber biosynthetic machine.

Proteomic Landscape of the Mature Roots in a Rubber-Producing Grass Taraxacum Kok-saghyz

The rubber grass Taraxacum kok-saghyz (TKS) contains large amounts of natural rubber (cis-1,4-polyisoprene) in its enlarged roots and it is an alternative crop source of natural rubber. Natural rubber biosynthesis (NRB) and storage in the mature roots of TKS is a cascade process involving many genes, proteins and their cofactors. The TKS genome has just been annotated and many NRB-related genes have been determined. However, there is limited knowledge about the protein regulation mechanism for NRB in TKS roots. We identified 371 protein species from the mature roots of TKS by combining two-dimensional gel electrophoresis (2-DE) and mass spectrometry (MS). Meanwhile, a large-scale shotgun analysis of proteins in TKS roots at the enlargement stage was performed, and 3545 individual proteins were determined. Subsequently, all identified proteins from 2-DE gel and shotgun MS in TKS roots were subject to gene ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses and most proteins were involved in carbon metabolic process with catalytic activity in membrane-bounded organelles, followed by proteins with binding ability, transportation and phenylpropanoid biosynthesis activities. Fifty-eight NRB-related proteins, including eight small rubber particle protein (SRPP) and two rubber elongation factor(REF) members, were identified from the TKS roots, and these proteins were involved in both mevalonate acid (MVA) and methylerythritol phosphate (MEP) pathways. To our best knowledge, it is the first high-resolution draft proteome map of the mature TKS roots. Our proteomics of TKS roots revealed both MVA and MEP pathways are important for NRB, and SRPP might be more important than REF for NRB in TKS roots. These findings would not only deepen our understanding of the TKS root proteome, but also provide new evidence on the roles of these NRB-related proteins in the mature TKS roots.

Power Law Distribution Concerning Absolute Free Energies of Linear Sulfur Chains, Polythiazyls, Polyisoprenes, Linear trans-Polyenes, and Polyynes

After quantum calculations of absolute free energies of polyatomic sulfur derivatives or hydrocarbon oligomer compounds, we have studied the variations of G _n as a function of the number of repeating units n (sulfur atoms or hydrocarbon units, n = 2 to about 60), and observed that these values can be correlated by (a) a nearly perfect linear relationship, G _n = An + B (A and B, constants), with a high accuracy which enables an extrapolation for higher values of n, and (b) a power law:, δ G = G _n/ n - G_{( n-1)} /( n - 1) = C/ n ^d ( C and d, constants). From comparison of variations of the C-C bond lengths, we show that the conjugation of trans-polyenes (up to C₆₀H₆₂) is more important than for polyynes (up to C₆₀H₂).

Biosynthesis of Natural Rubber: Current State and Perspectives

Natural rubber is a kind of indispensable biopolymers with great use and strategic importance in human society. However, its production relies almost exclusively on rubber-producing plants Hevea brasiliensis, which have high requirements for growth conditions, and the mechanism of natural rubber biosynthesis remains largely unknown. In the past two decades, details of the rubber chain polymerization and proteins involved in natural rubber biosynthesis have been investigated intensively. Meanwhile, omics and other advanced biotechnologies bring new insight into rubber production and development of new rubber-producing plants. This review summarizes the achievements of the past two decades in understanding the biosynthesis of natural rubber, especially the massive information obtained from the omics analyses. Possibilities of natural rubber biosynthesis in vitro or in genetically engineered microorganisms are also discussed.

Subcellular proteome profiles of different latex fractions revealed washed solutions from rubber particles contain crucial enzymes for natural rubber biosynthesis

Rubber particle (RP) is a specific organelle for natural rubber biosynthesis (NRB) and storage in rubber tree Hevea brasiliensis. NRB is processed by RP membrane-localized proteins, which were traditionally purified by repeated washing. However, we noticed many proteins in the discarded washing solutions (WS) from RP. Here, we compared the proteome profiles of WS, C-serum (CS) and RP by 2-DE, and identified 233 abundant proteins from WS by mass spectrometry. Many spots on 2-DE gels were identified as different protein species. We further performed shotgun analysis of CS, WS and RP and identified 1837, 1799 and 1020 unique proteins, respectively. Together with 2-DE, we finally identified 1825 proteins from WS, 246 were WS-specific. These WS-specific proteins were annotated in Gene Ontology, indicating most abundant pathways are organic substance metabolic process, protein degradation, primary metabolic process, and energy metabolism. Protein-protein interaction analysis revealed these WS-specific proteins are mainly involved in ribosomal metabolism, proteasome system, vacuolar protein sorting and endocytosis. Label free and Western blotting revealed many WS-specific proteins and protein complexes are crucial for NRB initiation. These findings not only deepen our understanding of WS proteome, but also provide new evidences on the roles of RP membrane proteins in NRB.

SIGNIFICANCE

Natural rubber is stored in rubber particle from the rubber tree. Rubber particles were traditionally purified by repeated washing, but many proteins were identified from the washing solutions (WS). We obtained the first visualization proteome profiles with 1825 proteins from WS, including 246 WS-specific ones. These WS proteins contain almost all enzymes for polyisoprene initiation and may play important roles in rubber biosynthesis.

Quantitation of isoprenoids for natural rubber biosynthesis in natural rubber latex by liquid chromatography with tandem mass spectrometry

The natural rubber molecule is one of the end products of isoprenoids metabolism in the plant. Dimethylallyl diphosphate (DMAPP) and farnesyl pyrophosphate (FPP) are two typical isoprenoids which control the rate of biosynthesis and the molecular weight of natural rubber. A rapid, nonradioactive method for quantitation of DMAPP and FPP in natural rubber latex by liquid chromatography tandem with mass spectrometry (LC-MS/MS) was reported. DMAPP and FPP were determined in the multiple reaction monitoring mode(MRM)followed by separation with a silica-based C18 column. The external standard quantitative method was established, and the results showed limits of quantitation (LOQs) were 28 ng/ml and 33 ng/ml for DMAPP and FPP, respectively. The concentrations were detected 70-96 ng/ml and 242-375 ng/ml for these two isoprenoids in natural rubber latex. Recoveries of the method were in the range 81-93%. Daytime comparison experiments found that FPP had better stability than DMAPP.

The 14-3-3 protein HbGF14a interacts with a RING zinc finger protein to regulate expression of the rubber transferase gene in Hevea brasiliensis

Hevea brasiliensis is a key commercial source of natural rubber (cis 1,4-polyisoprene). In H. brasiliensis, rubber transferase is responsible for cis-1,4-polymerization of isoprene units from isopentenyl diphosphate and thus affects the yield of rubber. Little is known about the regulatory mechanisms of the rubber transferase gene at a molecular level. In this study we show that the 5'UTR intron of the promoter of the rubber transferase gene (HRT2) suppresses the expression of HRT2. A H. brasiliensis RING zinc finger protein (designated as HbRZFP1) was able to interact specifically with the HRT2 promoter to down-regulate its transcription in vivo. A 14-3-3 protein (named as HbGF14a) was identified as interacting with HbRZFP1, both in yeast and in planta. Transient co-expression of HbGF14a and HbRZFP1-encoding cDNAs resulted in HbRZFP1-mediated HRT2 transcription inhibition being relieved. HbGF14a repressed the protein-DNA binding of HbRZFP1 with the HRT2 promoter in yeast. We propose a regulatory mechanism by which the binding of HbGF14a to HbRZFP1 interferes with the interaction of HbRZFP1 with the HRT2 promoter, thereby repressing the protein-DNA binding between them. This study provides new insights into the role of HbGF14a in mediating expression of the rubber transferase gene in Hevea brasiliensis.

Nucleophile-initiated anionic polymerization of zwitterionic monomers derived from vinylpyridines in aqueous media under ambient aerobic conditions

Anionic polymerization of vinylpyridine based zwitterionic monomers using nucleophile initiators under natural conditions and DFT calculations for such polymerization are reported here.

New insight into the polymerization mechanism of 1,3-dienes cationic polymerization. IV. Mechanism of unsaturation loss in the polymerization of isoprene

The mechanism of obtaining a polymer with reduced unsaturation is proposed, which includes the formation of branched structures.

Comparative Transcriptome Analysis of Latex Reveals Molecular Mechanisms Underlying Increased Rubber Yield in Hevea brasiliensis Self-Rooting Juvenile Clones

Rubber tree (Hevea brasiliensis) self-rooting juvenile clones (JCs) are promising planting materials for rubber production. In a comparative trial between self-rooting JCs and donor clones (DCs), self-rooting JCs exhibited better performance in rubber yield. To study the molecular mechanism associated with higher rubber yield in self-rooting JCs, we sequenced and comparatively analyzed the latex of rubber tree self-rooting JCs and DCs at the transcriptome level. Total raw reads of 34,632,012 and 35,913,020 bp were obtained from the library of self-rooting JCs and DCs, respectively, by using Illumina HiSeq 2000 sequencing technology. De novo assemblies yielded 54689 unigenes from the library of self-rooting JCs and DCs. Among 54689 genes, 1716 genes were identified as differentially expressed between self-rooting JCs and DCs via comparative transcript profiling. Functional analysis showed that the genes related to the mass of categories were differentially enriched between the two clones. Several genes involved in carbohydrate metabolism, hormone metabolism and reactive oxygen species scavenging were up-regulated in self-rooting JCs, suggesting that the self-rooting JCs provide sufficient molecular basis for the increased rubber yielding, especially in the aspects of improved latex metabolisms and latex flow. Some genes encoding epigenetic modification enzymes were also differentially expressed between self-rooting JCs and DCs. Epigenetic modifications may lead to gene differential expression between self-rooting JCs and DCs. These data will provide new cues to understand the molecular mechanism underlying the improved rubber yield of H. brasiliensis self-rooting clones.

HbMADS4, a MADS-box Transcription Factor from Hevea brasiliensis, Negatively Regulates HbSRPP

In plants MADS-box transcription factors (TFs) play important roles in growth and development. However, no plant MADS-box gene has been identified to have a function related to secondary metabolites regulation. Here, a MADS-box TF gene, designated as HbMADS4, was isolated from Hevea brasiliensis by the yeast one-hybrid experiment to screen the latex cDNA library using the promoter of the gene encoding H. brasiliensis small rubber particle protein (HbSRPP) as bait. HbMADS4 was 984-bp containing 633-bp open reading frame encoding a deduced protein of 230 amino acid residues with a typical conserved MADS-box motif at the N terminus. HbMADS4 was preferentially expressed in the latex, but little expression was detected in the leaves, flowers, and roots. Its expression was inducible by methyl jasmonate and ethylene. Furthermore, transient over-expression and over-expression of HbMADS4 in transgenic tobacco plants significantly suppressed the activity of the HbSRP promoter. Altogether, it is proposed that HbMADS4 is a negative regulator of HbSRPP which participates in the biosynthesis of natural rubber.

Archaeal Inorganic Pyrophosphatase Displays Robust Activity under High-Salt Conditions and in Organic Solvents

Soluble inorganic pyrophosphatases (PPAs) that hydrolyze inorganic pyrophosphate (PPi) to orthophosphate (Pi) are commonly used to accelerate and detect biosynthetic reactions that generate PPi as a by-product. Current PPAs are inactivated by high salt concentrations and organic solvents, which limits the extent of their use. Here we report a class A type PPA of the haloarchaeon Haloferax volcanii (HvPPA) that is thermostable and displays robust PPi-hydrolyzing activity under conditions of 25% (vol/vol) organic solvent and salt concentrations from 25 mM to 3 M. HvPPA was purified to homogeneity as a homohexamer by a rapid two-step method and was found to display non-Michaelis-Menten kinetics with a Vmax of 465 U · mg(-1) for PPi hydrolysis (optimal at 42°C and pH 8.5) and Hill coefficients that indicated cooperative binding to PPi and Mg(2+). Similarly to other class A type PPAs, HvPPA was inhibited by sodium fluoride; however, hierarchical clustering and three-dimensional (3D) homology modeling revealed HvPPA to be distinct in structure from characterized PPAs. In particular, HvPPA was highly negative in surface charge, which explained its extreme resistance to organic solvents. To demonstrate that HvPPA could drive thermodynamically unfavorable reactions to completion under conditions of reduced water activity, a novel coupled assay was developed; HvPPA hydrolyzed the PPi by-product generated in 2 M NaCl by UbaA (a "salt-loving" noncanonical E1 enzyme that adenylates ubiquitin-like proteins in the presence of ATP). Overall, we demonstrate HvPPA to be useful for hydrolyzing PPi under conditions of reduced water activity that are a hurdle to current PPA-based technologies.

Comprehensive Proteomics Analysis of Laticifer Latex Reveals New Insights into Ethylene Stimulation of Natural Rubber Production

Ethylene is a stimulant to increase natural rubber latex. After ethylene application, both fresh yield and dry matter of latex are substantially improved. Moreover, we found that ethylene improves the generation of small rubber particles. However, most genes involved in rubber biosynthesis are inhibited by exogenous ethylene. Therefore, we conducted a proteomics analysis of ethylene-stimulated rubber latex, and identified 287 abundant proteins as well as 143 ethylene responsive latex proteins (ERLPs) with mass spectrometry from the 2-DE and DIGE gels, respectively. In addition, more than 1,600 proteins, including 404 ERLPs, were identified by iTRAQ. Functional classification of ERLPs revealed that enzymes involved in post-translational modification, carbohydrate metabolism, hydrolase activity, and kinase activity were overrepresented. Some enzymes for rubber particle aggregation were inhibited to prolong latex flow, and thus finally improved latex production. Phosphoproteomics analysis identified 59 differential phosphoproteins; notably, specific isoforms of rubber elongation factor and small rubber particle protein that were phosphorylated mainly at serine residues. This post-translational modification and isoform-specific phosphorylation might be important for ethylene-stimulated latex production. These results not only deepen our understanding of the rubber latex proteome but also provide new insights into the use of ethylene to stimulate rubber latex production.

Toward a photosynthetic microbial platform for terpenoid engineering

Plant terpenoids are among the most diverse group of naturally-occurring organic compounds known, and several are used in contemporary consumer products. Terpene synthase enzymes catalyze complex rearrangements of carbon skeleton precursors to yield thousands of unique chemical structures that range in size from the simplest five carbon isoprene unit to the long polymers of rubber. Such chemical diversity has established plant terpenoids as valuable commodity chemicals with applications in the pharmaceutical, neutraceutical, cosmetic, and food industries. More recently, terpenoids have received attention as a renewable alternative to petroleum-derived fuels and as the building blocks of synthetic biopolymers. However, the current plant- and petrochemical-based supplies of commodity terpenoids have major limitations. Photosynthetic microorganisms provide an opportunity to generate terpenoids in a renewable manner, employing a single consolidated host organism that is able to use solar energy, H2O and CO2 as the primary inputs for terpenoid biosynthesis. Advances in synthetic biology have seen important breakthroughs in microbial terpenoid engineering, traditionally via fermentative pathways in yeast and Escherichia coli. This review draws on the knowledge obtained from heterotrophic microbial engineering to propose strategies for the development of microbial photosynthetic platforms for industrial terpenoid production. The importance of utilizing the wealth of genetic information provided by nature to unravel the regulatory mechanisms of terpenoid biosynthesis is highlighted.

Recent progress in the Lewis acid co-initiated cationic polymerization of isobutylene and 1,3-dienes

This article reviews recent approaches towards the synthesis of exo-olefin terminated polyisobutylenes and well-defined poly(1,3-diene)s based on their scientific and industrial relevance.

Carbocationic polymerization of isoprene using cumyl initiators: progress in understanding side reactions

The cationic polymerization of isoprene using cumyl chloride/B(C₆F₅)₃ and cumyl ether/TiCl₄ systems was investigated in dichloromethane or in dichloromethane/methylcyclohexane mixtures varying the polymerization conditions.

Physiological and molecular responses to variation of light intensity in rubber Tree (Hevea brasiliensis Muell. Arg.)

Light is one of most important factors to plants because it is necessary for photosynthesis. In this study, physiological and gene expression analyses under different light intensities were performed in the seedlings of rubber tree (Hevea brasiliensis) clone GT1. When light intensity increased from 20 to 1000 µmol m(-2) s(-1), there was no effect on the maximal quantum yield of photosystem II (PSII) photochemistry (Fv/Fm), indicating that high light intensity did not damage the structure and function of PSII reaction center. However, the effective photochemical quantum yield of PSII (Y(II)), photochemical quenching coefficient (qP), electron transfer rate (ETR), and coefficient of photochemical fluorescence quenching assuming interconnected PSII antennae (qL) were increased significantly as the light intensity increased, reached a maximum at 200 µmol m(-2) s(-1), but decreased from 400 µmol m(-2) s(-1). These results suggested that the PSII photochemistry showed an optimum performance at 200 µmol m(-2) s(-1) light intensity. The chlorophyll content was increased along with the increase of light intensity when it was no more than 400 µmol m(-2) s(-1). Since increasing light intensity caused significant increase in H2O2 content and decreases in the per unit activity of antioxidant enzymes SOD and POD, but the malondialdehyde (MDA) content was preserved at a low level even under high light intensity of 1000 µmol m(-2) s(-1), suggesting that high light irradiation did not induce membrane lipid peroxidation in rubber tree. Moreover, expressions of antioxidant-related genes were significantly up-regulated with the increase of light intensity. They reached the maximum expression at 400 µmol m(-2) s(-1), but decreased at 1000 µmol m(-2) s(-1). In conclusion, rubber tree could endure strong light irradiation via a specific mechanism. Adaptation to high light intensity is a complex process by regulating antioxidant enzymes activities, chloroplast formation, and related genes expressions in rubber tree.

Thermal degradation kinetics and mechanism of epoxidized natural rubber

Thermal resistance is one of the most dominative properties for polymer materials. Thermal degradation mechanisms of epoxidized natural rubber (ENR) and NR are studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The results show that, the introduction of epoxy groups into the NR molecular main chain leads to a remarkable change in the degradation mechanism. The thermal stability of ENR is worse than that of NR. For the first thermooxidative degradation stage, the thermal decomposition mechanism of ENR is similar to that of NR, which corresponds to a mechanism involving one-dimensional diffusion. For the second stage, the thermal decomposition mechanism of ENR is a three-dimensional diffusion, which is more complex than that of NR. Kinetic analysis showed that activation energy (Eα), activation entropy (ΔH) and activation Gibbs energy (ΔG) values are all positive, indicating that the thermooxidative degradation process of ENR is non-spontaneous.