Establishment of new crops for the production of natural rubber

Optimization and Scale-up of Inulin Extraction from Taraxacum kok-saghyz roots

The optimization and scale-up of inulin extraction from Taraxacum kok-saghyz Rodin was successfully performed. Evaluating solubility investigations, the extraction temperature was fixed at 85°C. The inulin stability regarding degradation or hydrolysis could be confirmed by extraction in the presence of model inulin. Confirming stability at the given conditions the isolation procedure was transferred from a 1 L- to a 1 m3-reactor. The Reynolds number was selected as the relevant dimensionless number that has to remain constant in both scales. The stirrer speed in the large scale was adjusted to 3.25 rpm regarding a 300 rpm stirrer speed in the 1 L-scale and relevant physical and process engineering parameters. Assumptions were confirmed by approximately homologous extraction kinetics in both scales. Since T. kok-saghyz is in the focus of research due to its rubber content side-product isolation from residual biomass it is of great economic interest. Inulin is one of these additional side-products that can be isolated in high quantity (~ 35% of dry mass) and with a high average degree of polymerization (15.5) in large scale with a purity of 77%.

Profiling Ethylene-Responsive Genes Expressed in the Latex of the Mature Virgin Rubber Trees Using cDNA Microarray

Ethylene is commonly used as a latex stimulant of Hevea brasiliensis by application of ethephon (chloro-2-ethylphosphonic acid); however, the molecular mechanism by which ethylene increases latex production is not clear. To better understand the effects of ethylene stimulation on the laticiferous cells of rubber trees, a latex expressed sequence tag (EST)-based complementary DNA microarray containing 2,973 unique genes (probes) was first developed and used to analyze the gene expression changes in the latex of the mature virgin rubber trees after ethephon treatment at three different time-points: 8, 24 and 48 h. Transcript levels of 163 genes were significantly altered with fold-change values ≥ 2 or ≤ –2 (q-value < 0.05) in ethephon-treated rubber trees compared with control trees. Of the 163 genes, 92 were up-regulated and 71 down-regulated. The microarray results were further confirmed using real-time quantitative reverse transcript-PCR for 20 selected genes. The 163 ethylene-responsive genes were involved in several biological processes including organic substance metabolism, cellular metabolism, primary metabolism, biosynthetic process, cellular response to stimulus and stress. The presented data suggest that the laticifer water circulation, production and scavenging of reactive oxygen species, sugar metabolism, and assembly and depolymerization of the latex actin cytoskeleton might play important roles in ethylene-induced increase of latex production. The results may provide useful insights into understanding the molecular mechanism underlying the effect of ethylene on latex metabolism of H. brasiliensis.

Population Genetics of the Rubber-Producing Russian Dandelion (Taraxacum kok-saghyz)

The Russian dandelion, Taraxacum kok-saghyz (TKS), is a perennial species native to Central Asia that produces high quality, natural rubber. Despite its potential to help maintain a stable worldwide rubber supply, little is known about genetic variation in this species. To facilitate future germplasm improvement efforts, we developed simple-sequence repeat (SSR) markers from available expressed-sequence tag (EST) data and used them to investigate patterns of population genetic diversity in this nascent crop species. We identified numerous SSRs (1,510 total) in 1,248 unigenes from a larger set of 6,960 unigenes (derived from 16,441 ESTs) and designed PCR primers targeting 767 of these loci. Screening of a subset of 192 of these primer pairs resulted in the identification of 48 pairs that appeared to produce single-locus polymorphisms. We then used the most reliable 17 of these primer pairs to genotype 176 individuals from 17 natural TKS populations. We observed an average of 4.8 alleles per locus with population-level expected heterozygosities ranging from 0.28 to 0.50. An average pairwise F_ST of 0.11 indicated moderate but statistically significant levels of genetic differentiation, though there was no clear geographic patterning to this differentiation. We also tested these 17 primer pairs in the widespread common dandelion, T. officinale, and a majority successfully produced apparently single-locus amplicons. This result demonstrates the potential utility of these markers for genetic analyses in other species in the genus.

Potential use of natural rubber to produce liquid fuels using hydrous pyrolysis – a review

Natural rubber is a tropical plantation crop that mainly consists of polyisoprene (cis-1,4-polyisoprene). It can be converted into fuels and other useful chemical commodities by depolymerization processes, with the hydrous pyrolysis being the most cost-effective.

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.

Silencing the lettuce homologs of small rubber particle protein does not influence natural rubber biosynthesis in lettuce (Lactuca sativa)

Natural rubber, cis-1,4-polyisoprene, is an important raw material in chemical industries, but its biosynthetic mechanism remains elusive. Natural rubber is known to be synthesized in rubber particles suspended in laticifer cells in the Brazilian rubber tree (Hevea brasiliensis). In the rubber tree, rubber elongation factor (REF) and its homolog, small rubber particle protein (SRPP), were found to be the most abundant proteins in rubber particles, and they have been implicated in natural rubber biosynthesis. As lettuce (Lactuca sativa) can synthesize natural rubber, we utilized this annual, transformable plant to examine in planta roles of the lettuce REF/SRPP homologs by RNA interference. Among eight lettuce REF/SRPP homologs identified, transcripts of two genes (LsSRPP4 and LsSRPP8) accounted for more than 90% of total transcripts of REF/SRPP homologs in lettuce latex. LsSRPP4 displays a typical primary protein sequence as other REF/SRPP, while LsSRPP8 is twice as long as LsSRPP4. These two major LsSRPP transcripts were individually and simultaneously silenced by RNA interference, and relative abundance, polymer molecular weight, and polydispersity of natural rubber were analyzed from the LsSRPP4- and LsSRPP8-silenced transgenic lettuce. Despite previous data suggesting the implications of REF/SRPP in natural rubber biosynthesis, qualitative and quantitative alterations of natural rubber could not be observed in transgenic lettuce lines. It is concluded that lettuce REF/SRPP homologs are not critically important proteins in natural rubber biosynthesis in lettuce.

Identification of a Taraxacum brevicorniculatum rubber elongation factor protein that is localized on rubber particles and promotes rubber biosynthesis

Two protein families required for rubber biosynthesis in Taraxacum brevicorniculatum have recently been characterized, namely the cis-prenyltransferases (TbCPTs) and the small rubber particle proteins (TbSRPPs). The latter were shown to be the most abundant proteins on rubber particles, where rubber biosynthesis takes place. Here we identified a protein designated T. brevicorniculatum rubber elongation factor (TbREF) by using mass spectrometry to analyze rubber particle proteins. TbREF is homologous to the TbSRPPs but has a molecular mass that is atypical for the family. The promoter was shown to be active in laticifers, and the protein itself was localized on the rubber particle surface. In TbREF-silenced plants generated by RNA interference, the rubber content was significantly reduced, correlating with lower TbCPT protein levels and less TbCPT activity in the latex. However, the molecular mass of the rubber was not affected by TbREF silencing. The colloidal stability of rubber particles isolated from TbREF-silenced plants was also unchanged. This was not surprising because TbREF depletion did not affect the abundance of TbSRPPs, which are required for rubber particle stability. Our findings suggest that TbREF is an important component of the rubber biosynthesis machinery in T. brevicorniculatum, and may play a role in rubber particle biogenesis and influence rubber production.

Comparative transcriptome analysis of latex from rubber tree clone CATAS8-79 and PR107 reveals new cues for the regulation of latex regeneration and duration of latex flow

BACKGROUND

Rubber tree (Hevea brasiliensis Muell. Arg.) is the primarily commercial source of natural rubber in the world. Latex regeneration and duration of latex flow after tapping are the two factors that determine rubber yield of rubber tree, and exhibit a huge variation between rubber tree clones CATAS8-79 and PR107.

RESULTS

To dissect the molecular mechanism for the regulation of latex regeneration and duration of latex flow, we sequenced and comparatively analyzed latex of rubber tree clone CATAS8-79 and PR107 at transriptome level. More than 26 million clean reads were generated in each pool and 51,829 all-unigenes were totally assembled. A total of 6,726 unigenes with differential expression patterns were detected between CATAS8-79 and PR107. Functional analysis showed that genes related to mass of categories were differentially enriched between the two clones. Expression pattern of genes which were involved in latex regeneration and duration of latex flow upon successive tapping was analyzed by quantitative PCR. Several genes related to rubber biosynthesis, cellulose and lignin biosynthesis and rubber particle aggregation were differentially expressed between CATAS8-79 and PR107.

CONCLUSIONS

This is the first report about probing latex regeneration and duration of latex flow by comparative transcriptome analysis. Among all the suggested factors, it is more important that the level of endogenous jasmonates, carbohydrate metabolism, hydroxymethylglutaryl-CoA reductase (HMGR) and Hevea rubber transferase (HRT) in mevalonate (MVA) parthway for latex regeneration while the level of endogenous ethylene (ETH), lignin content of laticifer cell wall, antioxidants and glucanases for the duration of latex flow. These data will provide new cues for understanding the molecular mechanism for the regulation of latex regeneration and duration of latex flow in rubber tree.

Plant terpenes: defense responses, phylogenetic analysis, regulation and clinical applications

The terpenoids constitute the largest class of natural products and many interesting products are extensively applied in the industrial sector as flavors, fragrances, spices and are also used in perfumery and cosmetics. Many terpenoids have biological activities and also used for medical purposes. In higher plants, the conventional acetate-mevalonic acid pathway operates mainly in the cytosol and mitochondria and synthesizes sterols, sesquiterpenes and ubiquinones mainly. In the plastid, the non-mevalonic acid pathway takes place and synthesizes hemi-, mono-, sesqui-, and diterpenes along with carotenoids and phytol tail of chlorophyll. In this review paper, recent developments in the biosynthesis of terpenoids, indepth description of terpene synthases and their phylogenetic analysis, regulation of terpene biosynthesis as well as updates of terpenes which have entered in the clinical studies are reviewed thoroughly.

Whole-transcriptome survey of the putative ATP-binding cassette (ABC) transporter family genes in the latex-producing laticifers of Hevea brasiliensis

The ATP-binding cassette (ABC) proteins or transporters constitute a large protein family in plants and are involved in many different cellular functions and processes, including solute transportation, channel regulation and molecular switches, etc. Through transcriptome sequencing, a transcriptome-wide survey and expression analysis of the ABC protein genes were carried out using the laticiferous latex from Hevea brasiliensis (rubber tree). A total of 46 putative ABC family proteins were identified in the H. brasiliensis latex. These consisted of 12 ‘full-size’, 21 ‘half-size’ and 13 other putative ABC proteins, and all of them showed strong conservation with their Arabidopsis thaliana counterparts. This study indicated that all eight plant ABC protein paralog subfamilies were identified in the H. brasiliensis latex, of which ABCB, ABCG and ABCI were the most abundant. Real-time quantitative reverse transcription-polymerase chain reaction assays demonstrated that gene expression of several latex ABC proteins was regulated by ethylene, jasmonic acid or bark tapping (a wound stress) stimulation, and that HbABCB15, HbABCB19, HbABCD1 and HbABCG21 responded most significantly of all to the abiotic stresses. The identification and expression analysis of the latex ABC family proteins could facilitate further investigation into their physiological involvement in latex metabolism and rubber biosynthesis by H. brasiliensis.

A lettuce (Lactuca sativa) homolog of human Nogo-B receptor interacts with cis-prenyltransferase and is necessary for natural rubber biosynthesis

Natural rubber (cis-1,4-polyisoprene) is an indispensable biopolymer used to manufacture diverse consumer products. Although a major source of natural rubber is the rubber tree (Hevea brasiliensis), lettuce (Lactuca sativa) is also known to synthesize natural rubber. Here, we report that an unusual cis-prenyltransferase-like 2 (CPTL2) that lacks the conserved motifs of conventional cis-prenyltransferase is required for natural rubber biosynthesis in lettuce. CPTL2, identified from the lettuce rubber particle proteome, displays homology to a human NogoB receptor and is predominantly expressed in latex. Multiple transgenic lettuces expressing CPTL2-RNAi constructs showed that a decrease of CPTL2 transcripts (3-15% CPTL2 expression relative to controls) coincided with the reduction of natural rubber as low as 5%. We also identified a conventional cis-prenyltransferase 3 (CPT3), exclusively expressed in latex. In subcellular localization studies using fluorescent proteins, cytosolic CPT3 was relocalized to endoplasmic reticulum by co-occurrence of CPTL2 in tobacco and yeast at the log phase. Furthermore, yeast two-hybrid data showed that CPTL2 and CPT3 interact. Yeast microsomes containing CPTL2/CPT3 showed enhanced synthesis of short cis-polyisoprenes, but natural rubber could not be synthesized in vitro. Intriguingly, a homologous pair CPTL1/CPT1, which displays ubiquitous expressions in lettuce, showed a potent dolichol biosynthetic activity in vitro. Taken together, our data suggest that CPTL2 is a scaffolding protein that tethers CPT3 on endoplasmic reticulum and is necessary for natural rubber biosynthesis in planta, but yeast-expressed CPTL2 and CPT3 alone could not synthesize high molecular weight natural rubber in vitro.

Involvement of ethylene-responsive microRNAs and their targets in increased latex yield in the rubber tree in response to ethylene treatment

The rubber tree is an economically important plant that produces natural rubber for various industrial uses. The application of ethylene contributes to increased latex production in rubber trees; however, the molecular biology behind the effects of ethylene on latex yield remains to be elucidated. Recently, the intersection between microRNA (miRNA) regulation and phytohormone responses has been revealed. Insight into the regulation of miRNAs and their target genes should help to determine the functional importance of miRNAs as well as the role of miRNAs in signaling under ethylene stimulation in the rubber tree. In this study, hbr-miR159 and hbr-miR166 were down-regulated in bark under ethylene treatment. The ethylene also down-regulated ATHB15-like (Class III Homeodomain Leucine Zipper, HD-ZIP III) which have been extensively implicated in the regulation of primary and secondary vascular tissue pattern formation. The strong negative-regulation of ARF6/ARF8 caused by hbr-miR167 involved in an attenuation of vascular development and may gradually lead to bark dryness syndrome in the long term ethylene treatment. The negative correlation of hbr-miR172 and its target REF3 in the inner soft bark under ethylene treatment results in dramatic increases in latex yield in the ethylene-sensitive clone of the rubber tree. The overall results suggested that the differential expression of HD-ZIP III, miR167/ARF6, ARF8, and miR172/REF3 and related genes may play possible roles in the response to ethylene treatment, resulting in longer latex flow and increased latex yield.

The characteristics and potential applications of structural lipid droplet proteins in plants

Plant cytosolic lipid droplets are storage organelles that accumulate hydrophobic molecules. They are found in many tissues and their general structure includes an outer lipid monolayer with integral and associated proteins surrounding a hydrophobic core. Two distinct types can be distinguished, which we define here as oleosin-based lipid droplets (OLDs) and non-oleosin-based lipid droplets (NOLDs). OLDs are the best characterized lipid droplets in plants. They are primarily restricted to seeds and other germinative tissues, their surface is covered with oleosin-family proteins to maintain stability, they store triacylglycerols (TAGs) and they are used as a source of energy (and possibly signaling molecules) during the germination of seeds and pollen. Less is known about NOLDs. They are more abundant than OLDs and are distributed in many tissues, they accumulate not only TAGs but also other hydrophobic molecules such as natural rubber, and the structural proteins that stabilize them are unrelated to oleosins. In many species these proteins are members of the rubber elongation factor superfamily. NOLDs are not typically used for energy storage but instead accumulate hydrophobic compounds required for environmental interactions such as pathogen defense. There are many potential applications of NOLDs including the engineering of lipid production in plants and the generation of artificial oil bodies.

Erasing the past: a new identity for the Damoclean pathogen causing South American leaf blight of rubber

BACKGROUND

South American leaf blight (SALB) of rubber has been the main constraint to production in its neotropical centre of origin since commercial plantations were first established. The fungal causal agent was identified and described more than a century ago but its precise placement within the Ascomycota still remains uncertain. Indeed, such is the ambiguity surrounding the pathogen that each of the spore morphs would, according to their present classification, be placed in different ascomycete families: the Microcyclus sexual morph in the Planistromellaceae and the two purported asexual morphs--Fusicladium and Aposphaeria--in the Venturiaceae and Lophiostomataceae, respectively. Given the historical importance of the fungus and the ever-menacing threat that it poses to rubber production in the Palaeotropics--and, thus to the rubber industry and to the global economy--its phylogeny, as well as its biology, should be resolved as a matter of urgency.

METHODS AND RESULTS

Here, six genomic regions (LSU rRNA, mtSSU, MCM7, EF-1α, Act and ITS) were used for reconstructing the molecular phylogeny of the SALB fungus based on material collected throughout Brazil. The analyses support the classification of the fungus in the family Mycosphaerellaceae s. str. (Capnodiales, Dothideomycetes) and place it firmly within the clade Pseudocercospora s. str., now accepted as one of the distinct genera within Mycosphaerellaceae. The new combination Pseudocercospora ulei is proposed and the life cycle of the fungus is confirmed, based on both experimental and phylogenetic evidence, with the Aposphaeria morph shown to have a spermatial rather than an infective-dispersal function.

CONCLUSIONS

Because the phylogeny of the SALB fungus has now been clarified, new insights of its epidemiology and genomics can be gained following comparison with closely-related, better-researched crop pathogens.

ATP citrate lyase activity is post-translationally regulated by sink strength and impacts the wax, cutin and rubber biosynthetic pathways

Cytosolic acetyl-CoA is involved in the synthesis of a variety of compounds, including waxes, sterols and rubber, and is generated by the ATP citrate lyase (ACL). Plants over-expressing ACL were generated in an effort to understand the contribution of ACL activity to the carbon flux of acetyl-CoA to metabolic pathways occurring in the cytosol. Transgenic Arabidopsis plants synthesizing the polyester polyhydroxybutyrate (PHB) from cytosolic acetyl-CoA have reduced growth and wax content, consistent with a reduction in the availability of cytosolic acetyl-CoA to endogenous pathways. Increasing the ACL activity via the over-expression of the ACLA and ACLB subunits reversed the phenotypes associated with PHB synthesis while maintaining polymer synthesis. PHB production by itself was associated with an increase in ACL activity that occurred in the absence of changes in steady-state mRNA or protein level, indicating a post-translational regulation of ACL activity in response to sink strength. Over-expression of ACL in Arabidopsis was associated with a 30% increase in wax on stems, while over-expression of a chimeric homomeric ACL in the laticifer of roots of dandelion led to a four- and two-fold increase in rubber and triterpene content, respectively. Synthesis of PHB and over-expression of ACL also changed the amount of the cutin monomer octadecadien-1,18-dioic acid, revealing an unsuspected link between cytosolic acetyl-CoA and cutin biosynthesis. Together, these results reveal the complexity of ACL regulation and its central role in influencing the carbon flux to metabolic pathways using cytosolic acetyl-CoA, including wax and polyisoprenoids.

Arabidopsis AtPAP1 transcription factor induces anthocyanin production in transgenic Taraxacum brevicorniculatum

This study developed a new purple coloured Taraxacum brevicorniculatum plant through genetic transformation using the Arabidopsis AtPAP1 gene, which overproduced anthocyanins in its vegetative tissues. Rubber-producing Taraxacum plants synthesise high-quality natural rubber (NR) in their roots and so are a promising alternative global source of this raw material. A major factor in its commercialization is the need for multipurpose exploitation of the whole plant. To add value to the aerial tissues, red/purple plants of the rubber-producing Taraxacum brevicorniculatum species were developed through heterologous expression of the production of anthocyanin pigment 1 (AtPAP1) transcription factor from Arabidopsis thaliana. The vegetative tissue of the transgenic plants showed an average of a 48-fold increase in total anthocyanin content over control levels, but with the exception of pigmentation, the transgenic plants were phenotypically comparable to controls and displayed similar growth vigor. Southern blot analysis confirmed that the AtPAP1 gene had been integrated into the genome of the high anthocyanin Taraxacum plants. The AtPAP1 expression levels were estimated by quantitative real-time PCR and were highly correlated with the levels of total anthocyanins in five independent transgenic lines. High levels of three cyanidin glycosides found in the purple plants were characterized by high performance liquid chromatography-mass spectrum analysis. The presence of NR was verified by NMR and infrared spectroscopy, and confirmed that NR biosynthesis had not been affected in the transgenic Taraxacum lines. In addition, other major phenylpropanoid products such as chlorogenic acid and quercetin glycosides were also enhanced in the transgenic Taraxacum. The red/purple transgenic Taraxacum lines described in this study would increase the future application of the species as a rubber-producing crop due to its additional health benefits.

Bioengineering of plant (tri)terpenoids: from metabolic engineering of plants to synthetic biology in vivo and in vitro

Terpenoids constitute a large and diverse class of natural products that serve many functions in nature. Most of the tens of thousands of the discovered terpenoids are synthesized by plants, where they function as primary metabolites involved in growth and development, or as secondary metabolites that optimize the interaction between the plant and its environment. Several plant terpenoids are economically important molecules that serve many applications as pharmaceuticals, pesticides, etc. Major challenges for the commercialization of plant-derived terpenoids include their low production levels in planta and the continuous demand of industry for novel molecules with new or superior biological activities. Here, we highlight several synthetic biology methods to enhance and diversify the production of plant terpenoids, with a foresight towards triterpenoid engineering, the least engineered class of bioactive terpenoids. Increased or cheaper production of valuable triterpenoids may be obtained by 'classic' metabolic engineering of plants or by heterologous production of the compounds in other plants or microbes. Novel triterpenoid structures can be generated through combinatorial biosynthesis or directed enzyme evolution approaches. In its ultimate form, synthetic biology may lead to the production of large amounts of plant triterpenoids in in vitro systems or custom-designed artificial biological systems.

Comparative analysis of latex transcriptome reveals putative molecular mechanisms underlying super productivity of Hevea brasiliensis

Increasing demand for natural rubber prompts studies into the mechanisms governing the productivity of rubber tree (Heveabrasiliensis). It is very interesting to notice that a rubber tree of clone PR107 in Yunnan, China is reported to yield more than 20 times higher than the average rubber tree. This super-high-yielding (SHY) rubber tree (designated as SY107), produced 4.12 kg of latex (cytoplasm of rubber producing laticifers, containing about 30% of rubber) per tapping, more than 7-fold higher than that of the control. This rubber tree is therefore a good material to study how the rubber production is regulated at a molecular aspect. A comprehensive cDNA-AFLP transcript profiling was performed on the latex of SY107 and its average counterparts by using the 384 selective primer pairs for two restriction enzyme combinations (ApoI/MseI and TaqI/MseI). A total of 746 differentially expressed (DE) transcript-derived fragments (TDFs) were identified, of which the expression patterns of 453 TDFs were further confirmed by RT-PCR. These RT-PCR confirmed TDFs represented 352 non-redundant genes, of which 215 had known or partially known functions and were grouped into 10 functional categories. The top three largest categories were transcription and protein synthesis (representing 24.7% of the total genes), defense and stress (15.3%), and primary and secondary metabolism (14.0%). Detailed analysis of the DE-genes suggests notable characteristics of SHY phenotype in improved sucrose loading capability, rubber biosynthesis-preferred sugar utilization, enhanced general metabolism and timely stress alleviation. However, the SHY phenotype has little correlation with rubber-biosynthesis pathway genes.

Histochemical study of trans-polyisoprene accumulation by spectral confocal laser scanning microscopy and a specific dye showing fluorescence solvatochromism in the rubber-producing plant, Eucommia ulmoides Oliver

A microscopic technique combining spectral confocal laser scanning microscopy with a lipophilic fluorescent dye, Nile red, which can emit trans-polyisoprene specific fluorescence, was developed, and unmixed images of synthesized trans-polyisoprene in situ in Eucommia ulmoides were successfully obtained. The images showed that trans-polyisoprene was initially synthesized as granules in non-articulated laticifers that changed shape to fibers during laticifer maturation. Non-articulated laticifers are developed from single laticiferous cells, which are differentiated from surrounding parenchyma cells in the cambium. Therefore, these observations suggested that trans-polyisoprene biosynthesis first started in laticifer cells as granules and then the granules accumulated and fused in the inner space of the laticifers over time. Finally, laticifers were filled with the synthesized trans-polyisoprene, which formed a fibrous structure fitting the laticifers shape. Both trans- and cis-polyisoprene are among the most important polymers naturally produced by plants, and this microscopic technique combined with histological study should provide useful information in the fields of plant histology, bioindustry and phytochemistry.

In-depth proteome analysis of the rubber particle of Hevea brasiliensis (para rubber tree)

The rubber particle is a special organelle in which natural rubber is synthesised and stored in the laticifers of Hevea brasiliensis. To better understand the biological functions of rubber particles and to identify the candidate rubber biosynthesis-related proteins, a comprehensive proteome analysis was performed on H. brasiliensis rubber particles using shotgun tandem mass spectrometry profiling approaches-resulting in a thorough report on the rubber particle proteins. A total of 186 rubber particle proteins were identified, with a range in relative molecular mass of 3.9-194.2 kDa and in isoelectric point values of 4.0-11.2. The rubber particle proteins were analysed for gene ontology and could be categorised into eight major groups according to their functions: including rubber biosynthesis, stress- or defence-related responses, protein processing and folding, signal transduction and cellular transport. In addition to well-known rubber biosynthesis-related proteins such as rubber elongation factor (REF), small rubber particle protein (SRPP) and cis-prenyl transferase (CPT), many proteins were firstly identified to be on the rubber particles, including cyclophilin, phospholipase D, cytochrome P450, small GTP-binding protein, clathrin, eukaryotic translation initiation factor, annexin, ABC transporter, translationally controlled tumour protein, ubiquitin-conjugating enzymes, and several homologues of REF, SRPP and CPT. A procedure of multiple reaction monitoring was established for further protein validation. This comprehensive proteome data of rubber particles would facilitate investigation into molecular mechanisms of biogenesis, self-homeostasis and rubber biosynthesis of the rubber particle, and might serve as valuable biomarkers in molecular breeding studies of H. brasiliensis and other alternative rubber-producing species.

Metabolic engineering of plant monoterpenes, sesquiterpenes and diterpenes--current status and future opportunities

Terpenoids (a.k.a. isoprenoids) represent the most diverse class of natural products found in plants, with tens of thousands of reported structures. Plant-derived terpenoids have a multitude of pharmaceutical and industrial applications, but the natural resources for their extraction are often limited and, in many cases, synthetic routes are not commercially viable. Some of the most valuable terpenoids are not accumulated in model plants or crops, and genetic resources for breeding of terpenoid natural product traits are thus poorly developed. At present, metabolic engineering, either in the native producer or a heterologous host, is the only realistic alternative to improve yield and accessibility. In this review article, we will evaluate the state of the art of modulating the biosynthetic pathways for the production of mono-, sesqui- and diterpenes in plants.

Construction and analysis of EST libraries of the trans-polyisoprene producing plant, Eucommia ulmoides Oliver

Eucommia ulmoides Oliver is one of a few woody plants capable of producing abundant quantities of trans-polyisoprene rubber in their leaves, barks, and seed coats. One cDNA library each was constructed from its outer stem tissue and inner stem tissue. They comprised a total of 27,752 expressed sequence tags (ESTs) representing 10,520 unigenes made up of 4,302 contigs and 6,218 singletons. Homologues of genes coding for rubber particle membrane proteins that participate in the synthesis of high-molecular poly-isoprene in latex were isolated, as well as those encoding known major latex proteins (MLPs). MLPs extensively shared ESTs, indicating their abundant expression during trans-polyisoprene rubber biosynthesis. The six mevalonate pathway genes which are implicated in the synthesis of isopentenyl diphosphate (IPP), a starting material of poly-isoprene biosynthesis, were isolated, and their role in IPP biosynthesis was confirmed by functional complementation of suitable yeast mutants. Genes encoding five full-length trans-isoprenyl diphosphate synthases were also isolated, and two among those synthesized farnesyl diphosphate from IPP and dimethylallyl diphosphate, an assumed intermediate of rubber biosynthesis. This study should provide a valuable resource for further studies of rubber synthesis in E. ulmoides.

Proteome analysis of the large and the small rubber particles of Hevea brasiliensis using 2D-DIGE

The rubber particle is a specialized organelle in which natural rubber is synthesised and stored in the laticifers of Hevea brasiliensis (para rubber tree). It has been demonstrated that the small rubber particles (SRPs) has higher rubber biosynthesis ratio than the large rubber particles (LRPs), but the underlying molecular mechanism still remains unknown. In this study, LRPs and SRPs were firstly separated from the fresh latex using differential centrifugation, and two-dimensional difference in-gel electrophoresis (2D-DIGE) combined with MALDI-TOF/TOF was then applied to investigate the proteomic alterations associated with the changed rubber biosynthesis capacity between LRPs and SRPs. A total of 53 spots corresponding to 22 gene products, were significantly altered with the |ratio|≥2.0 and T value ≤0.05, among which 15 proteins were up-regulated and 7 were down-regulated in the SRPs compared with the LRPs. The 15 up-regulated proteins in the SRPs included small rubber particle protein (SRPP), 3-hydroxy-3-methylglutaryl-CoA synthase (HMGCS), phospholipase D alpha (PLD α), ethylene response factor 2, eukaryotic translation initiation factor 5A isoform IV (eIF 5A-4), 70-kDa heat shock cognate protein (HSC 70), several unknown proteins, etc., whereas the 7 up-regulated proteins in the LRPs were rubber elongation factor (REF, 19.6kDa), ASR-like protein 1, REF-like stress-related protein 1, a putative phosphoglyceride transfer family protein, β-1,3-glucanase, a putative retroelement, and a hypothetical protein. Since several proteins related to rubber biosynthesis were differentially expressed between LRPs and SRPs, the comparative proteome data may provide useful insights into understanding the mechanism involved in rubber biosynthesis and latex coagulation in H. brasiliensis.

Rubber elongation factor (REF), a major allergen component in Hevea brasiliensis latex has amyloid properties

REF (Hevb1) and SRPP (Hevb3) are two major components of Hevea brasiliensis latex, well known for their allergenic properties. They are obviously taking part in the biosynthesis of natural rubber, but their exact function is still unclear. They could be involved in defense/stress mechanisms after tapping or directly acting on the isoprenoid biosynthetic pathway. The structure of these two proteins is still not described. In this work, it was discovered that REF has amyloid properties, contrary to SRPP. We investigated their structure by CD, TEM, ATR-FTIR and WAXS and neatly showed the presence of β-sheet organized aggregates for REF, whereas SRPP mainly fold as a helical protein. Both proteins are highly hydrophobic but differ in their interaction with lipid monolayers used to mimic the monomembrane surrounding the rubber particles. Ellipsometry experiments showed that REF seems to penetrate deeply into the monolayer and SRPP only binds to the lipid surface. These results could therefore clarify the role of these two paralogous proteins in latex production, either in the coagulation of natural rubber or in stress-related responses. To our knowledge, this is the first report of an amyloid formed from a plant protein. This suggests also the presence of functional amyloid in the plant kingdom.

Transcriptome and gene expression analysis in cold-acclimated guayule (Parthenium argentatum) rubber-producing tissue

Natural rubber biosynthesis in guayule (Parthenium argentatum Gray) is associated with moderately cold night temperatures. To begin to dissect the molecular events triggered by cold temperatures that govern rubber synthesis induction in guayule, the transcriptome of bark tissue, where rubber is produced, was investigated. A total of 11,748 quality expressed sequence tags (ESTs) were obtained. The vast majority of ESTs encoded proteins that are similar to stress-related proteins, whereas those encoding rubber biosynthesis-related proteins comprised just over one percent of the ESTs. Sequence information derived from the ESTs was used to design primers for quantitative analysis of the expression of genes that encode selected enzymes and proteins with potential impact on rubber biosynthesis in field-grown guayule plants, including 3-hydroxy-3-methylglutaryl-CoA synthase, 3-hydroxy-3-methylglutaryl-CoA reductase, farnesyl pyrophosphate synthase, squalene synthase, small rubber particle protein, allene oxide synthase, and cis-prenyl transferase. Gene expression was studied for field-grown plants during the normal course of seasonal variation in temperature (monthly average maximum 41.7 °C to minimum 0 °C, from November 2005 through March 2007) and rubber transferase enzymatic activity was also evaluated. Levels of gene expression did not correlate with air temperatures nor with rubber transferase activity. Interestingly, a sudden increase in night temperature 10 days before harvest took place in advance of the highest CPT gene expression level.

Photosynthesis and assimilate partitioning between carbohydrates and isoprenoid products in vegetatively active and dormant guayule: physiological and environmental constraints on rubber accumulation in a semiarid shrub

The stems and roots of the semiarid shrub guayule, Parthenium argentatum, contain a significant amount of natural rubber. Rubber accumulates in guayule when plants are vegetatively and reproductively dormant, complicating the relationship between growth/reproduction and product synthesis. To evaluate the factors regulating the partitioning of carbon to rubber, carbon assimilation and partitioning were measured in guayule plants that were grown under simulated summer- and winter-like conditions and under winter-like conditions with CO(2) enrichment. These conditions were used to induce vegetatively active and dormant states and to increase the source strength of vegetatively dormant plants, respectively. Rates of CO(2) assimilation, measured under growth temperatures and CO(2) , were similar for plants grown under summer- and winter-like conditions, but were higher with elevated CO(2) . After 5 months, plants grown under summer-like conditions had the greatest aboveground biomass, but the lowest levels of non-structural carbohydrates and rubber. In contrast, the amount of resin in the stems was similar under all growth conditions. Emission of biogenic volatile compounds was more than three-fold higher in plants grown under summer- compared with winter-like conditions. Taken together, the results show that guayule plants maintain a high rate of photosynthesis and accumulate non-structural carbohydrates and rubber in the vegetatively dormant state, but emit volatile compounds at a lower rate when compared with more vegetatively active plants. Enrichment with CO(2) in the vegetatively dormant state increased carbohydrate content but not the amount of rubber, suggesting that partitioning of assimilate to rubber is limited by sink strength in guayule.

Characterization of rubber particles and rubber chain elongation in Taraxacum koksaghyz

Background

Natural rubber is a biopolymer with exceptional qualities that cannot be completely replaced using synthetic alternatives. Although several key enzymes in the rubber biosynthetic pathway have been isolated, mainly from plants such as Hevea brasiliensis, Ficus spec. and the desert shrub Parthenium argentatum, there have been no in planta functional studies, e.g. by RNA interference, due to the absence of efficient and reproducible protocols for genetic engineering. In contrast, the Russian dandelion Taraxacum koksaghyz, which has long been considered as a potential alternative source of low-cost natural rubber, has a rapid life cycle and can be genetically transformed using a simple and reliable procedure. However, there is very little molecular data available for either the rubber polymer itself or its biosynthesis in T. koksaghyz.

Results

We established a method for the purification of rubber particles - the active sites of rubber biosynthesis - from T. koksaghyz latex. Photon correlation spectroscopy and transmission electron microscopy revealed an average particle size of 320 nm, and ¹³C nuclear magnetic resonance (NMR) spectroscopy confirmed that isolated rubber particles contain poly(cis-1,4-isoprene) with a purity >95%. Size exclusion chromatography indicated that the weight average molecular mass (w) of T. koksaghyz natural rubber is 4,000-5,000 kDa. Rubber particles showed rubber transferase activity of 0.2 pmol min^-1 mg^-1. Ex vivo rubber biosynthesis experiments resulted in a skewed unimodal distribution of [1-¹⁴C]isopentenyl pyrophosphate (IPP) incorporation at a w of 2,500 kDa. Characterization of recently isolated cis-prenyltransferases (CPTs) from T. koksaghyz revealed that these enzymes are associated with rubber particles and are able to produce long-chain polyprenols in yeast.

Conclusions

T. koksaghyz rubber particles are similar to those described for H. brasiliensis. They contain very pure, high molecular mass poly(cis-1,4-isoprene) and the chain elongation process can be studied ex vivo. Because of their localization on rubber particles and their activity in yeast, we propose that the recently described T. koksaghyz CPTs are the major rubber chain elongating enzymes in this species. T. koksaghyz is amenable to genetic analysis and modification, and therefore could be used as a model species for the investigation and comparison of rubber biosynthesis.

The second green revolution? Production of plant-based biodegradable plastics

Biodegradable plastics are those that can be completely degraded in landfills, composters or sewage treatment plants by the action of naturally occurring micro-organisms. Truly biodegradable plastics leave no toxic, visible or distinguishable residues following degradation. Their biodegradability contrasts sharply with most petroleum-based plastics, which are essentially indestructible in a biological context. Because of the ubiquitous use of petroleum-based plastics, their persistence in the environment and their fossil-fuel derivation, alternatives to these traditional plastics are being explored. Issues surrounding waste management of traditional and biodegradable polymers are discussed in the context of reducing environmental pressures and carbon footprints. The main thrust of the present review addresses the development of plant-based biodegradable polymers. Plants naturally produce numerous polymers, including rubber, starch, cellulose and storage proteins, all of which have been exploited for biodegradable plastic production. Bacterial bioreactors fed with renewable resources from plants – so-called ‘white biotechnology’ – have also been successful in producing biodegradable polymers. In addition to these methods of exploiting plant materials for biodegradable polymer production, the present review also addresses the advances in synthesizing novel polymers within transgenic plants, especially those in the polyhydroxyalkanoate class. Although there is a stigma associated with transgenic plants, especially food crops, plant-based biodegradable polymers, produced as value-added co-products, or, from marginal land (non-food), crops such as switchgrass (Panicum virgatum L.), have the potential to become viable alternatives to petroleum-based plastics and an environmentally benign and carbon-neutral source of polymers.

Production of renewable polymers from crop plants

Plants produce a range of biopolymers for purposes such as maintenance of structural integrity, carbon storage, and defense against pathogens and desiccation. Several of these natural polymers are used by humans as food and materials, and increasingly as an energy carrier. In this review, we focus on plant biopolymers that are used as materials in bulk applications, such as plastics and elastomers, in the context of depleting resources and climate change, and consider technical and scientific bottlenecks in the production of novel or improved materials in transgenic or alternative crop plants. The biopolymers discussed are natural rubber and several polymers that are not naturally produced in plants, such as polyhydroxyalkanoates, fibrous proteins and poly-amino acids. In addition, monomers or precursors for the chemical synthesis of biopolymers, such as 4-hydroxybenzoate, itaconic acid, fructose and sorbitol, are discussed briefly.

Guayule and Russian dandelion as alternative sources of natural rubber

Natural rubber, obtained almost exclusively from the Para rubber tree (Hevea brasiliensis), is a unique biopolymer of strategic importance that, in many of its most significant applications, cannot be replaced by synthetic rubber alternatives. Several pressing motives lead to the search for alternative sources of natural rubber. These include increased evidence of allergenic reactions to Hevea rubber, the danger that the fungal pathogen Microcyclus ulei, causative agent of South American Leaf Blight (SALB), might spread to Southeast Asia, which would severely disrupt rubber production, potential shortages of supply due to increasing demand and changes in land use, and a general trend towards the replacement of petroleum-derived chemicals with renewables. Two plant species have received considerable attention as potential alternative sources of natural rubber: the Mexican shrub Guayule (Parthenium argentatum Gray) and the Russian dandelion (Taraxacum koksaghyz). This review will summarize the current production methods and applications of natural rubber (dry rubber and latex), the threats to the production of natural rubber from the rubber tree, and describe the current knowledge of the production of natural rubber from guayule and Russian dandelion.