Natural rubber: A vital commodity
Natural rubber is an extremely important, agricultural resource vital to all modern economies. It is curiously underappreciated because many people assume that all rubber is synthetic . However, the total rubber market is about 45 % natural rubber because synthetic rubber, which is made from fossil reserves, cannot replace natural rubber in many applications . For example, the higher the performance of a tire, the greater the percentage of natural rubber in the tire. Commercial airline tires are 100 % natural rubber: of course, there are many other components in tires, like the reinforcing filler Carbon Black, vulcanization chemicals, steel, fabric and more. About 50,000 different products across all sectors are made with natural rubber, including gloves, bushings, isolators, seals, tubing, adhesives, balloons and condoms (Indian Rubber Board, 2013) [2-4]. The United States and other countries with temperate climates must import all the natural rubber they need. In 2018, the United States imported $1.5 billion worth of natural rubber and $13 billion worth of tires . The natural rubber, 13.96 million metric tons of it, was collected by poorly paid workers tapping trees by hand  – about 42 billion liters of latex dribbled into little cups.
Unlike all other agricultural products, natural rubber has no backup, and biodiversification is an important goal for the industry . Rubber trees are restricted to tropical regions and are produced as clonal scions grafted onto seedling root stocks . Differences among clones are small, and genetic uniformity makes crop failure a serious risk to the global rubber supply . Very little rubber is produced in South America today because of the endemic fatal rubber tree disease, South American Leaf Blight (SALB) - a fungal disease caused by Microcyclus ulei . If this disease establishes itself in southeast Asia, the industry could collapse, and rubber supplies could be lost very quickly . The region already struggles to control other serious pathogens like white root rot and yellow stripe.
Demand for natural rubber and latex is constantly increasing (5.2 % /year), labor costs are increasing, deforestation moratoria restricts expansion of new rubber tree plantings and climate change erodes suitable growing acreage. This means that even if rubber tree plantations persist indefinitely, rubber trees will not be able to fully address the doubling of natural rubber production expected to build up during the next 20 years. Meeting this demand, while minimizing environmental and ecological impacts, is going to need biodiversification of the natural rubber supply, and this means farming of other rubber crops in temperate regions [1, 10-12].
Out of about 2,500 other rubber-producing species, two species, namely guayule (Parthenium argentatum) and rubber dandelion (Taraxacum kok-saghyz), are being developed as temperate zone crops [ 11, 12]. Guayule is a native of the Chihuahuan desert of North America , and rubber dandelion is native to Kazakhstan and Uzbekistan . Thus, rubber dandelion is suited for regions with serious winters (northern United States, Canada northern Europe) while guayule is suited to semi-arid regions with cool, not frigid winters, as occur in Mediterranean regions, the southwestern United States, Mexico, South Africa, and Australia.
These two alternative rubber-producing species must be domesticated before they can be grown as field crops and suitable, cost-effective, sustainable rubber extraction processes are required to extract the rubber from the roots of the dandelion and the bark parenchyma cells of guayule. Guayule farming is practically established, but rubber dandelion cultivation is still far from being a commercial farm system because the plants are slow to establish and grow and are easily outcompeted by weeds . To become truly domesticated crops, on mechanized farms, both species will benefit from increased rubber accumulation, vigor and herbicide tolerance. Also, rubber dandelion can be cultivated in high density indoor hydroponic or aeroponic farms that can support high roots biomass .
A shift to sustainable rubber production
Fortunately, recent progress in genome-supported conventional and molecular breeding and genome-informed gene editing approaches supports rapid progress in plant domestication. These approaches can improve herbicide tolerance or resistance, and route photosynthetic carbon to generate more vigorous plants containing higher rubber quantities. Understanding how rubber biosynthesis is regulated at the gene and enzyme/metabolic levels can focus additional gene improvement approaches to increase rubber yield and provide molecular markers and genetic maps which can be used to significantly accelerate conventional breeding. Considerable progress is finally being made in the biology of rubber biosynthesis, especially since the genomes of both alternative rubber crops and the rubber tree were published and became available to scientists [17-19]. It is now clear that the rubber polymerization is mediated by a rubber particle membrane-bound protein complex, although its tertiary structure has not yet been elucidated . Also, the necessary enabling tissue culture and gene insertion and editing technologies have been developed in recent years. Both alternative species are amenable to Agrobacterium tumafaciens and gene gun (ballistic) transformation methods, CRISPR/Cas9 gene-editing [21-23], and rubber dandelion can be rapidly modified or edited using a novel Agrobacterium rhizogenes stable root transformation method .
Rubber production, as it currently stands, is unsustainable. Monocultures of rubber trees have caused irremediable ecological harm to once rich ecosystems in southeast Asian countries. Synthetic rubber production generates large quantities of greenhouse gases and depletes fossil hydrocarbon reserves. As alternative rubber crops begin to be farmed, we have an unprecedented opportunity to establish and expand a new industry in a responsible, sustainable and ecologically-sound manner. Risks and impacts can be assessed and addressed proactively, instead of the usual reactive practices of the modern world to clean up a problem after it has occurred. For example, as discussed above, new traits will be progressively introduced into guayule and rubber dandelion. The potential ecological risk imposed by unintended transfer of a trait to a related species is the same whatever the origin of that trait, i.e. whether a trait is enhanced through natural selection, mutagenesis followed by selection, targeted editing or foreign gene introduction. The most obvious issue in the context of rubber crops is the perceived risk of trait transfer from rubber dandelion to common weedy dandelion (Taraxacum officinale). Proactive research on this question has proved that common dandelions in North America are apomictic triploids which cannot accept pollen from other plants and produce seed genetically identical to the mother [15, 25]. Proactive research can be done to select the best growing and processing sites, enhance ecosystem services and safeguard watersheds and soils.
In conclusion, dandelion and guayule rubber crops will secure rubber supplies, calm global natural rubber price volatility largely caused by futures traders and provide new crop choices to farmers and indoor growers while reducing our dependence on rubber imports. In the long term, temperate countries could fully supply their own rubber needs and then export excess rubber to other countries.
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