Dandelion rubber – bad weeds grow tall
Natural rubber is a unique high molecular weight biopolymer derived from plants. It was discovered by the indigenous peoples of Mesoamerica 3,500 years ago. Chemically speaking, natural rubber consists of cis-1,4-polyisoprene with a molecular weight of about 10^6 g mol^-1 and also contains a small percentage of substances (<1%) which contribute to the special properties of this polymer. It is characterized by elasticity, tensile strength and heat resistance, and cannot be replaced by synthetic alternatives in most applications. Rubber-based materials have to withstand extremely high shear forces and temperature fluctuations such as in aircraft and truck tires. There are also about 40,000 other items we use in everyday life, including about 400 products in medicine, that rely on the use of natural rubber (Figure 1).
This biopolymer is obtained from the Hevea brasiliensis (rubber tree), which is grown on large plantations mainly in Southeast Asia (80% of the world production is from Malaysia, Indonesia and Thailand). The production area extends through the tropics, though it can no longer be cultivated in plantations in South America due to fungal infections with Microcyclus ulei which causes South America Leaf Blight (SALB). Natural rubber is deposited as rubber particles which are enclosed in a monolayer of species-specific fatty acids and associated proteins formed in the cytoplasm of specific cell types. The size of the rubber particles varies from species to species and in H. brasiliensis has been described as between 0.2 to 1.0 microns.
The last step in rubber biosynthesis is the enzymatic stereospecific, sequential addition of the C5 body isopentenyl pyrophosphate (IPP) to an allylic pyrophosphate via head-to-tail condensation. The polymerization of the IPP units takes place at the border between the polar mono-molecular phospholipid membrane and the surrounding polar cytosol. The hydrophobic polyisoprene chains accumulate by diffusion inside the rubber particles. The elongation of the isoprene chains and thus the molecular weight of each rubber molecule is determined by several factors, such as, for example, the concentration of the initial substrates. So far chemists have failed to replicate this process in detail and there are no available synthetic rubbers with such extremely high molecular masses, stereo specificity and rheological properties.
The worldwide demand for both synthetic and natural rubber is rapidly growing. This is a result of the progress of industrialization in countries like China and India (Figure 1). The long-term rise in oil prices will also lead to a rise in synthetic rubber prices, which will increase the use of natural rubber in the industry. This growing global demand for natural rubber is accompanied by the threat of SALB to large plantations. Similarly, the expansion of oil palm plantations for biofuel production, in Malaysia from 100,000 hectares to 2,000,000 hectares in the last decade, has led to a lack of H. brasiliensis.
Overall, oil palm and para rubber tree plantations cover 70% of the cultivated area of Malaysia, causing socio-economic problems in the country. Similar developments are being observed in Thailand and Indonesia. Additionally, a cartel of natural rubber producers has turned this raw material into a strategic material, so that large latex and natural rubber processing companies have been forced to have their own plantations in the natural rubber producing countries or to relocate industrial facilities to these countries. This has contributed to significant losses in European and other non-rubber producing economic regions.
The increasing demand and simultaneous restriction of production of and trade in natural rubber from H. brasiliensis make the search for alternative sources for this essential commodity indispensable. In addition to increasing demand, allergic reactions to natural rubber from H. brasiliensis are becoming an increasingly severe problem. In laboratories and medical professions, around 17% of all staff is already allergic to at least one of the present proteins in natural rubber.
As early as the 1940s, the shortage of natural rubber production and trade restrictions during the Second World War forced the governments in Russia, Germany and the United States to identify plants that produce natural rubber of similar quality to H. brasiliensis. Today about 2,500 species of plants that synthesize rubber have been described, but only a few of them are able to produce rubber of high molecular weight (>10^5 g mol^-1).The only plant that produces natural rubber in temperate latitudes, and thus of interest to Northern Europe and Northern America as an attractive source of this raw material, is the dandelion species Taraxacum koksaghyz, which was originally derived from Kazakhstan (Figure 2).
This plant produced natural rubber in the milk ducts of its taproot that accounts for around 15-40% of dry matter. The individual rubber particles of the T. koksaghyz have an average size of 0.3-0.4 microns and the rubber molecules have a molecular weight of 2x10^6 g mol^-1 with low dispersity (<1.8) (Schmidt et al., 2010a). One great advantage of the dandelion is its modest needs. It grows on soils that are unsuitable for the production of crops, and it could be easily cultivated in temperate regions. This could create a new lucrative crop for farmers, especially in view of the valuable constituents in addition to natural rubber produced in dandelions. By the end of World War II, 200 tons rubber and 42,000 litres of bioethanol were obtained from T. koksaghyz roots.
Under greenhouse conditions the milky sap (latex) from which the natural rubber can then be isolated can be harvested after just 4 months. The amount of natural rubber in the latex increases during the growth period and reaches a maximum after 6-8 months. During the winter months, the dandelion sloughs off the rubber from the root to the outside and the synthesis is begins again the following spring. In initial field trials T. koksaghyz was grown as annual crop from March to October and small quantities of dandelion rubber were produced. It was noted that the natural rubber and latex derived causes no known allergies and could therefore be extremely interesting for use in medical products. Further investigation as to its elasticity before and after vulcanization or potential applicability in areas of dip coating will now be carried out. It must therefore now be produced in larger quantities, making efficient methods for extracting the natural rubber and latex indispensable.
A limiting factor in rubber production from T. koksaghyz is the coagulation of the latex during the harvesting process, when colloidal stability is destroyed and the ingredients clump together. H. brasiliensis latex can be collected by wounding the bark. Collection from dandelions, however, requires a destructive procedure. First the root is harvested, as with beets or carrots, and then the latex is extracted by soaking and rolling the macerated plant tissue. The ingredients of the latex coagulate very quickly and the rubber part adheres to the plant tissue. In the BioSysPro project funded by the German Federal Ministry of Education and Research, a polyphenol oxidase (PPO) was identified as the primary latex coagulation factor (Wahler et al, 2009). This PPO is the main protein in the latex. The specific inhibition of PPO in T. koksaghyz led to a marked decrease in the coagulation of the latex which in turn led to a four- to fivefold increased yield. This procedure had no influence on the subsequent separation of natural rubber from the latex. T. koksaghyz latex PPO deficient plants represent an enormous development of T. koksaghyz as an alternative source for natural rubber, and should now be generated for cultivation using modern breeding techniques.
A research project, “EU PEARLS” (EU-based Production and Exploitation of Alternative Rubber and Latex Sources) brought together cooperating partners from university, research centres and companies from Europe to establish alternative rubber crops. In this project, the rubber biosynthesis in T. koksaghyz and Parthenium argentatum – another potential alternative rubber crop – will be explored. An exchange with researchers in the U.S. who are also working on plants as alternative sources of natural rubber will ensure rapid progress. Overall, the various projects aim at establishing sustainable alternative sources of natural rubber and recyclable cis-1,4-polyisoprene in Europe are likely to prevent future production shortfalls.
Schmidt, T., Lenders, M., Hillebrand, A., van Deenen, N., Munt, O., Reichelt, R., Eisenreich, W., Fischer, R., Prüfer, D. & Schulze Gronover, C. Characterization of rubber particles and rubber chain elongation in Taraxacum koksaghyz. BMC Biochemistry 2010, 11:11
Wahler, D., Schulze Gronover, C., Richter, C., Foucu, F., Twyman, R.M., Moerschbacher, B.M., Fischer, R., Muth, J. & Prüfer, D. Polyphenoloxidase silencing affects latex coagulation in Taraxacum spp. Plant Physiology 2009, 151: 334-346.