Rubber-producing Plant Research Articles

Genome-Wide Identification and Expression Analysis of the REF Genes in 17 Species

Natural rubber production currently relies heavily on a single species, Hevea brasiliensis, underscoring the urgent need to identify alternative sources to alleviate the strain on natural rubber production. The rubber elongation factor (REF) and small rubber particle protein (SRPP), both members of the REF/SRPP gene family, are crucial for natural rubber biosynthesis. However, research on the REF gene has predominantly focused on H. brasiliensis and Taraxacum kok-saghyz. We conducted a comprehensive genome-wide identification and characterization of the REF gene, identifying 87 REF protein sequences across 17 plants species. We observed a significant increase in the copy numbers and expression of REF genes in rubber-producing plants. Notably, in H. brasiliensis, T. kok-saghyz, Eucommia ulmoides, Lactuca sativa, and other rubber-yielding species, the number of REF genes has markedly increased. Furthermore, some REF genes in H. brasiliensis form a distinct clade in phylogenetic analyses and exhibit differences in conserved motif arrangements and tertiary protein structures compared to other REF genes. These findings suggest that REF genes in rubber-producing plants may have undergone independent evolution, leading to changes in copy number and structure. These alterations could contribute to the production of natural rubber in these species. The results of this study provide a scientific basis for further research into the mechanisms of rubber production in plants and for identifying potential rubber-producing species.

Structure characteristics, differentiation, and development of laticifer in rubber-producing plants

Structure characteristics, differentiation, and development of laticifer in rubber-producing plants

Rapid Determination of Taraxacum kok-saghyz Rubber Content Using a Pyrolyzer Hyphenated with a Miniaturized Mass Spectrometer

Taraxacum kok-saghyz (TKS), a rubber-producing plant with excellent potential, emerges as a viable substitute for rubber tree (Hevea brasiliensis). While natural rubber is a desirable material, conventional techniques for assessing rubber content have faced challenges in meeting practical production requirements. To address this issue, we have developed a pyrolysis–mass spectrometry (PY-MS) instrument for the quantitative evaluation of natural rubber (NR) content in rubber-producing plants. The derived standard curve equation, established for the detection of TKS dry weight through external standard calibration, demonstrates a correlation coefficient (R2) surpassing 0.99. The method exhibits commendable recovery rates (93.27–107.83%), relative standard deviations (RSD ≤ 3.93%), and a swift analysis time of merely 10 min per sample, thereby enabling accurate and efficient quantification of NR dry weight. Additionally, the PY-MS system we designed can be modified for vehicular use, enabling on-site, in situ analysis, and it provides substantial support for TKS breeding and propagation efforts. This approach possesses significant potential for extensive utilization in the assessment of rubber content in rubber-producing plants other than TKS. The integration of pyrolysis–mass spectrometry for the identification of polymers with high molecular weight offers a valuable pathway for the examination of diverse 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.

Effect of short-term seed priming on germination of wild-growing plant Scorzonera tau-saghyz

This study examines the efficiency of vermicompost tea, an organic fertilizer, in improving the process of seed germination in rubber-producing plants of the species Scorzonera tau-saghyz. Seeds collected from three-year-old plants within the Karatau State Nature Reserve in Kazakhstan were treated with 5% and 10% vermicompost tea for 2 hours and 4 hours’ exposure. Germination progress was closely monitored over a four-week period, revealing a significant correlation between higher concentrations and exposure times with increased rates of seed germination. In particular, tau-saghyz’ seeds treated with a 10% solution of vermicompost for 4 hours showed 1.3 times higher percentage of germination compared to seed pre-treatment by 5% solution and 1.9 times greater than that of the control. These results provide valuable information on optimizing seed treatment protocols to improve the germination and growth of Scorzonera tau-saghyz seedlings, offering potential approaches for biodiversity conservation and sustainable agricultural practices.

Advances in Genome Sequencing and Natural Rubber Biosynthesis in Rubber-Producing Plants.

Natural rubber (cis-1,4-polyisoprene, NR) is an important raw material utilized widely in the manufacturing of medical, agricultural, and industrial products. Rubber tree (Hevea brasiliensis) and several alternative rubber-producing plants (Taraxacum kok-saghyz, Lactuca sativa, and Parthenium argentatum) have the capability to produce high-quality NR. With the progress of genome sequencing, similar rubber biosynthesis pathways have been discovered among different rubber-producing plant species. NR is synthesized and stored in rubber particles, which are specialized organelles comprising a hydrophobic NR core surrounded by a lipid monolayer and membrane-bound proteins. The rubber transferase complex is considered to be the pivotal enzyme involved in catalyzing NR biosynthesis. However, the exact compositions of the RT complex in rubber-producing plants remain elusive and poorly understood. Here, we review the progress of genome sequencing, natural rubber biosynthesis, and the components of the RT complex in rubber-producing plants. We emphasize that identifying the detailed components of the RT complex holds great significance for exploring the mechanism of NR biosynthesis and accelerating molecular breeding in rubber-producing plants.

Extractable latex yield from Taraxacum kok-saghyz roots is enhanced by increasing rubber particle buoyancy

Taraxacum kok-saghyz (TK) is a leading alternative rubber-producing plant that produces natural rubber (NR) latex in laticifers within its roots. Unlike the tropical rubber tree, Hevea brasiliensis, TK can be grown in temperate regions, such as the northern United States, as the basis of a domestic rubber production industry. To support such an industry, efficient, scalable rubber extraction techniques are needed. In this study, aqueous extraction was used to recover TK natural rubber latex (TNRL) from four harvests of greenhouse-produced TK roots with a new technique that improves rubber particle buoyancy in root homogenate by chelation. The new method includes harvest, cleaning, and homogenization of freshly harvested roots, followed by centrifugation, vacuum collection of latex, and purification. Treatment of homogenate with ethylenediaminetetraacetic acid (EDTA), a divalent cation chelator, allowed more than twice as much latex to be extracted from fresh homogenate. EDTA-treated rubber particles contained lower concentrations of divalent cations than untreated ones in fresh homogenate and homogenate stored for two weeks. Long term (three months) storage of root homogenate similarly yielded more latex than fresh homogenate and had low divalent cation rubber particle concentrations, indicating that heavy cations had slowly leached from the particles into the aqueous medium during storage. Rubber particle size was larger in this treatment suggestive of rubber particle agglomeration but this was prevented and reversed by EDTA. Removal of divalent cations via chelation increased particle packing density of particles in concentrated latex and appeared to increase the gel content of dried latex samples. Molecular characterization of dried samples of purified TNRL established that the rubber was of the high molecular weight needed for product manufacturing.

Novel Mechanisms Underlying Rubber Accumulation and Programmed Cell Death in Laticiferous Canals of Decaisnea insignis Fruits: Cytological and Transcriptomic Analyses.

Natural rubber is one of the most important industrial raw materials, and its biosynthesis is still a fascinating process that is still largely unknown. In this research, we studied Decaisnea insignis, a unique rubber-producing plant that is different from other rubber-producing species due to the presence of lactiferous canals in its pericarp. The present study aims to provide novel insights into the mechanisms underlying rubber accumulation and PCD by subjecting the Decaisnea insignis laticiferous canals to light microscopy, TUNEL assay, and DAPI staining, as well as viability analysis, cellular ultrastructure analysis, and molecular analysis using light microscopy, scanning electron microscopy, immunofluorescence labeling, transmission electron microscopy, and transcriptome sequencing. At the cellular level, the origin of small rubber particles in the laticiferous canals had no morphological correlation with other organelles, and these particles were freely produced in the cytosol. The volume of the rubber particles increased at the sunken and expanding stage, which were identified as having the characteristics of programmed cell death (PCD); meanwhile, plenty of the rubber precursors or rubber particles were engulfed by the vacuoles, indicating a vacuole-mediated autophagy process. The accumulation of rubber particles occurred after the degeneration of protoplasts, suggesting a close association between rubber biosynthesis and PCD. The molecular analysis revealed the expression patterns of key genes involved in rubber biosynthesis. The upstream genes DiIPP, DiFPP, and DiGGPPS showed a decreasing trend during fruit ripening, while DiHRT, which is responsible for rubber particle extension, exhibited the highest expression level during the rubber particle formation. Moreover, the transcription factors related to PCD, DiLSD1, and DiLOL2 showed a negative correlation with the expression pattern of DiHRT, thus exhibiting strict rules of sequential expression during rubber biosynthesis. Additionally, the expression trends of DiXCP1 and DiCEP1, which act as proteases during PCD, were positively correlated with DiGGPPS expression. In conclusion, the findings suggest that the autophagic PCD may play a crucial role in rubber accumulation in D. insignis. Further research is still needed to fully understand the complex regulatory network underlying rubber biosynthesis in plants.

Genome-Wide Analysis of SPL Gene Families Illuminate the Evolution Patterns in Three Rubber-Producing Plants

Transcription factors SQUAMOSA Promoter-binding Protein-like (SPL) play a crucial role in regulating plant response to stress, root development, and flower production. However, analysis of SPL gene families in the three rubber-producing plants Taraxacum kok-saghyz, Hevea brasiliensis, and Eucommia ulmoides, renowned for their natural rubber production, has not yet been conducted. In this study, we utilized reference genomes to perform genome-wide analysis, and obtained new insights on the evolution of SPL gene families in these three rubber-producing plants. Our results revealed the following: (1) T. kok-saghyz, H. brasiliensis, and E. ulmoides harbored 25, 16, and 13 SPL genes, respectively, containing conserved structural domains of SBP. (2) A phylogenetic analysis categorized 90 SPL proteins from 25 TkSPLs, 16 HbSPLs, 13 EuSPLs, 17 AtSPLs, and 19 OsSPLs into eight groups. (3) Analysis of cis-acting elements demonstrated that the promoters of EuSPLs contained a significant number of light response elements, hormone regulatory elements, and stress response elements. (4) Transcriptome data analysis revealed that the EuSPL8 gene had strong expression in bark, as well as TkSPL4 and TkSPL8 exhibit high expression levels specifically in roots and latex. This study provides valuable insights into the biological functions of the SPL gene family in the three rubber plants and might serve as a reference for identifying efficient genes.

Genomic insight into domestication of rubber tree

Understanding the genetic basis of rubber tree (Hevea brasiliensis) domestication is crucial for further improving natural rubber production to meet its increasing demand worldwide. Here we provide a high-quality H. brasiliensis genome assembly (1.58 Gb, contig N50 of 11.21 megabases), present a map of genome variations by resequencing 335 accessions and reveal domestication-related molecular signals and a major domestication trait, the higher number of laticifer rings. We further show that HbPSK5, encoding the small-peptide hormone phytosulfokine (PSK), is a key domestication gene and closely correlated with the major domestication trait. The transcriptional activation of HbPSK5 by myelocytomatosis (MYC) members links PSK signaling to jasmonates in regulating the laticifer differentiation in rubber tree. Heterologous overexpression of HbPSK5 in Russian dandelion (Taraxacum kok-saghyz) can increase rubber content by promoting laticifer formation. Our results provide an insight into target genes for improving rubber tree and accelerating the domestication of other rubber-producing plants.

Back to the 'roots' of research: a hypothesis-driven approach provides predictive understanding of plant-herbivore-microbe interactions.

This article is a Commentary on Böttner et al. (2023), 239: 1475–1489.

Quantitative Detection of Natural Rubber Content in Eucommia ulmoides by Portable Pyrolysis-Membrane Inlet Mass Spectrometry.

Eucommia ulmoides gum (EUG) is a natural polymer predominantly consisting of trans-1,4-polyisoprene. Due to its excellent crystallization efficiency and rubber-plastic duality, EUG finds applications in various fields, including medical equipment, national defense, and civil industry. Here, we devised a portable pyrolysis-membrane inlet mass spectrometry (PY-MIMS) approach to rapidly, accurately, and quantitatively identify rubber content in Eucommia ulmoides (EU). EUG is first introduced into the pyrolyzer and pyrolyzed into tiny molecules, which are then dissolved and diffusively transported via the polydimethylsiloxane (PDMS) membrane before entering the quadrupole mass spectrometer for quantitative analysis. The results indicate that the limit of detection (LOD) for EUG is 1.36 μg/mg, and the recovery rate ranges from 95.04% to 104.96%. Compared to the result of pyrolysis-gas chromatography (PY-GC), the average relative error is 1.153%, and the detection time was reduced to less than 5 min, demonstrating that the procedure was reliable, accurate, and efficient. The method has the potential to be employed to precisely identify the rubber content of natural rubber-producing plants such as Eucommia ulmoides, Taraxacum kok-saghyz (TKS), Guayule, and Thorn lettuce.

Beyond natural rubber: Taraxacum kok-saghyz and Taraxacum brevicorniculatum as sources of bioactive compounds

Taraxacum kok-saghyz (TKS) and T. brevicorniculatum (TB) are broadly investigated as natural rubber-producing plants and possible alternative supply sources for this relevant raw material. To fully exploit Taraxacum as a profitable crop, all the potential co-products should be investigated, through in-depth analysis of metabolites present in different organs of the plant. In the present study, natural rubber (NR), inulin, and resin content was measured by accelerated solvent extraction from the roots of TB and TKS, highlighting a 5-fold more content of NR in TKS compared to TB. Moreover, the chemical composition of both acetone and methanolic extracts from the roots and leaves of TKS and TB has been characterized by ultra-high-pressure liquid chromatography/tandem mass spectrometry (UHPLC-HRMS) technique and the content of target compounds between TKS and TB was also compared. The analysis resulted in the detection of 55 metabolites, whose identification was discussed based on chemical classes and the extraction method. Thus, sesquiterpenoids, fatty acids and their derivatives, phenolic compounds, mainly caftaric and chicoric acid, were identified. Hence, both the leaves and roots of TB, and especially of TKS, are rich in a wide variety of high-value-added compounds exploitable along with NR and inulin to increase the commercial value of these two dandelion species.

Understanding the biosynthesis and regulatory mechanisms of bioactive compounds in Taraxacum species (dandelions), a model system for natural rubber, food, and medicinal plant biology

Plants that belong to the genus Taraxacum are commonly referred to as dandelions; these are some of the most evolved plants in the Asteraceae, with more than 2000 species distributed worldwide. Dandelions not only include medicinal plants but also rubber-producing plants. Recent advances in biotechnology and the growing demand for dandelions have increased research attention toward dandelions. Owing to the availability of the whole genome sequence of Taraxacum kok-saghyz, and Taraxacum mongolicum (accession no. GWHBCHF00000000 and GWHAAAA00000000), and large transcriptome and metabolome databases for T. kok-saghyz, Taraxacum officinale, T. mongolicum, and Taraxacum antungense (accession no. PRJNA378120, PRJEB48186, PRJNA897666, and PRJNA578773), significant progress has been made in understanding the biosynthesis and regulatory mechanisms of phenolic acids, flavonoids, terpenoids, rubber, and other bioactive compounds. This review focuses on Taraxacum spp.; specifically, We describe the following: (1) the recent results of genetic transformation systems, (2) biosynthetic pathways of bioactive compounds and natural rubber, (3) cross-talk in the biosynthesis of different secondary metabolites, (4) the relevance of the environment and the reproductive mode of dandelions in secondary metabolite biosynthesis, and (5) future research directives for dandelions. This review also lays the foundation for further research on other medicinal and edible plants.

Genome-Wide Identification and Spatial Expression Analysis of Histone Modification Gene Families in the Rubber Dandelion Taraxacum kok-saghyz.

Taraxacum kok-saghyz (Tks), also known as the Russian dandelion, is a recognized alternative source of natural rubber quite comparable, for quality and use, to the one obtained from the so-called rubber tree, Hevea brasiliensis. In addition to that, Tks roots produce several other compounds, including inulin, whose use in pharmaceutical and dietary products is quite extensive. Histone-modifying genes (HMGs) catalyze a series of post-translational modifications that affect chromatin organization and conformation, which, in turn, regulate many downstream processes, including gene expression. In this study, we present the first analysis of HMGs in Tks. Altogether, we identified 154 putative Tks homologs: 60 HMTs, 34 HDMs, 42 HATs, and 18 HDACs. Interestingly, whilst most of the classes showed similar numbers in other plant species, including M. truncatula and A. thaliana, HATs and HMT-PRMTs were indeed more abundant in Tks. Composition and structure analysis of Tks HMG proteins showed, for some classes, the presence of novel domains, suggesting a divergence from the canonical HMG model. The analysis of publicly available transcriptome datasets, combined with spatial expression of different developmental tissues, allowed us to identify several HMGs with a putative role in metabolite biosynthesis. Overall, our work describes HMG genomic organization and sets the premises for the functional characterization of epigenetic modifications in rubber-producing plants.

Comparative full-length transcriptome analysis provides novel insights into the regulatory mechanism of natural rubber biosynthesis in Taraxacum kok-saghyz Rodin roots

Taraxacum kok-saghyz Rodin (TKS) is a promising alternative rubber-producing plant due to the high-quality natural rubber (NR) synthesized in its root. Nevertheless, the comprehensive molecular mechanisms of NR biosynthesis in TKS roots remains segmented and elusive. In the current study, to elucidate the regulatory mechanism of NR synthesis at the transcriptional level, high-quality full-length root transcriptomic data of the TKS were generated based on Oxford Nanopore Technology (ONT). The transcripts with alternative splicing events, polyadenylation sites, and simple sequence repeats (SSRs) were screened. Then, comparative transcriptome analysis was conducted between the lines containing high rubber (HR) and low rubber (LR) contents, and a total of 3134 differentially expressed genes (DEGs) were identified. Among the genes, 1157 DEGs were upregulated, and 1977 DEGs were downregulated in the HR lines vs. the LR lines. The genes involved in NR synthesis were upregulated in HR lines. In contrast, the key genes involved in the synthesis of squalene, phenylpropanoid, fatty acids and ketone, which require the same substrates (isopentenyl pyrophosphate or acetyl-CoA) as NR biosynthesis, were downregulated in HR lines. The results indicated that the expression levels of NR synthesis-related genes were proportional to the NR production, and competition between NR and some metabolites synthesis existed when common substrates were used. Hence, this work provides novel insights into the regulatory mechanism of NR biosynthesis in TKS. Furthermore, the genes or pathways related to NR synthesis identified in this study will be important targets for genetic modification and molecular breeding in further.

Molecular Genetic Analysis of Natural Rubber Biosynthesis

Natural rubber (cis-1,4-polyisoprene) is an irreplaceable starting material for thousands of products, many of which are absolutely essential, in particular for modern medicine. Rubber is found in the latex of an aqueous emulsion contained in the specialized structures of rubber-producing plants, laticifers. More than 2500 plant species are capable of synthesizing natural rubber. However, Hevea brasiliensis (Brazilian rubber tree, Hevea) is currently the only commercially important species producing raw material for rubber production. Several proteins play key roles in the biosynthesis of natural rubber. Cis-prenyltransferases, activators of cis-prenyltransferases, and small proteins associated with rubber particles are among them. These proteins are directly involved in the synthesis of the polyisoprene chain. In this review, the molecular genetic aspects of the functioning of these proteins in the biosynthesis of natural rubber are analyzed. In particular, the cloning, expression, and functional analysis of cis-prenyltransferase coding genes from Arabidopsis thaliana, Hevea brasiliensis, and Taraxacum kok-saghyz are described, and the role of small proteins associated with rubber particles in the biosynthesis of natural rubber are investigated. Special attention is paid to the analysis of the effect of the expression level of the cis-prenyltransferase coding genes and the cis-prenyltransferase activator coding gene on the production of natural rubber in plant cells.

Lipid Composition of Latex and Rubber Particles in Hevea brasiliensis and Taraxacum kok-saghyz.

Natural rubber is usually synthesized in the rubber particles present in the latex of rubber-producing plants such as the Pará rubber tree (Hevea brasiliensis) and rubber dandelion (Taraxacum kok-saghyz). Since the detailed lipid compositions of fresh latex and rubber particles of the plants are poorly known, the present study reports detailed compound lipid composition, focusing on phospholipids and galactolipids in the latex and rubber particles of the plants. In the fresh latex and rubber particles of both plants, phospholipids were much more dominant (85–99%) compared to galactolipids. Among the nine classes of phospholipids, phosphatidylcholines (PCs) were most abundant, at ~80%, in both plants. Among PCs, PC (36:4) and PC (34:2) were most abundant in the rubber tree and rubber dandelion, respectively. Two classes of galactolipids, monogalactosyl diacylglycerol and digalactosyl diacylglycerol, were detected as 12% and 1%, respectively, of total compound lipids in rubber tree, whereas their percentages in the rubber dandelion were negligible (< 1%). Overall, the compound lipid composition differed only slightly between the fresh latex and the rubber particles of both rubber plants. These results provide fundamental data on the lipid composition of rubber particles in two rubber-producing plants, which can serve as a basis for artificial rubber particle production in the future.

Identification of histone methylation modifiers and their expression patterns during somatic embryogenesis in Hevea brasiliensis.

Histone methylation plays a crucial role in various biological processes, from heterochromatin formation to transcriptional regulation. Currently, no information is available regarding histone methylation modifiers in the important rubber-producing plant Hevea brasiliensis. Here, we identified 47 histone methyltransferase (HMT) genes and 25 histone demethylase (HDM) genes as possible members of the histone methylation modifiers in the rubber tree genome. According to the structural features of HMT and HDM, the HbHMTs were classified into two groups (HbPRMs and HbSDGs), the HbHDMs have two groups (HbLSDs and HbJMJs). Expression patterns were analyzed in five different tissues and at different phases of somatic embryogenesis. HbSDG10, 21, 25, 33, HbJMJ2, 18, 20 were with high expression at different phases of somatic embryogenesis. HbSDG10,14, 20, 21, 33 and HbPRMT4 were expressed highly in anther, HbSDG14, 20, 21, 22, 23, 33, 35 and HbPRMT1 HbJMJ7 and HbLSD1, 2, 3, 4 showed high expression levels in callus. HbSDG1, 7, 10, 13, 14, 18, 19, 21, 22, 23, 35, HbPRMT1, 8, HbJMJ5, 7, 11, 16, 20 and HbLSD2, 3, 4 were expressed highly in somatic embryo. HbSDG10, 21, 25, 33, HbLSD2, 3 were expressed highly in bud of regenerated plant. The analyses reveal that HbHMTs and HbHDMs exhibit different expression patterns at different phases during somatic embryogenesis, implying that some HbHMTs and HbHDMs play important roles during somatic embryogenesis. This study provide fundamental information for further studies on histone methylation in Hevea brasiliensis.

Trial and Error: The Introduction of Plants and Animals to German Micronesia 1885–1914

ABSTRACTIn the absence of real strategic interest in her Micronesian possessions, Imperial Germany set out to administer the Islands Territory of Micronesia with as little financial effort as possible. To maximize returns from the colonies, the German administrators assessed the agricultural potential of the islands and set about introducing plant varieties for trial and future commercial planting. This paper traces the history of the animal and plant introductions to Micronesia and compiles the available evidence. The administrators were caught in an economic mindset circumscribed by copra production as well as by the need to grow crops of strategic potential, such as rubber-producing plants. While initially reliant on the interest and initiative of the local administrators, agricultural experimentation became increasingly centralized. The absence of fully trained agricultural advisors, especially in the later years of the German colonial administration, meant that plants of great commercial potential, such as sugarcane in the Marianas, were never trialled.