Guayule plant-breeding (Parthenium argentatum A. Gray) provides a fascinating story intertwining geopolitical and economic events that can influence strategic decisions to domesticate and breed wild species, to an equal, and maybe even larger extent, than the straightforward breeding and industrial science potential. I am not a specialist in Guayule, but have been fortunate to learn from experts. Much of what I am writing about is based on published literature and knowledge gained from visiting the Bridgestone R&D center in Arizona as part of NRGene’s ongoing collaboration with Bridgestone.
Let’s start by talking about the natural rubber industry and work our way to Guayule.
Rubber is an organic polymer possessing both viscosity and elasticity features and is used in numerous products such as tires, gloves, sportswear and others. It can be made from petroleum products or extracted from different plant species. Natural rubber (i.e. made from plants)) has better elasticity and durability properties as compared to synthetic rubber derivatives, thus making it essential in the tire industry and especially for high-pressure tires needed for aircraft and agricultural machinery. Small vehicle tires can be made using lower quality synthetic rubber. According to Statista, the worldwide production of rubber in 2019 amounted to 15 and 13 million metric tons for synthetic and natural rubber, respectively.
There are over 2,000 plant species that produce rubber yet only two have been primarily domesticated and commercially used. One is the rubber tree (Hevea brasiliensis), native to the Amazon basin and almost the sole source of rubber, and the other is Guayule, a perennial small shrub found in the southwestern US and northern Mexican deserts. Both Guayule and rubber tree have been harvested from the wild since the turn of the 20th century. But as wild growing plants were approaching extinction, what remained could not meet the increasing global demand and thus efforts were directed towards agricultural production. Modern breeding of both plants began based on mass selection and vegetative propagation using grafting in havea and apomixis in Guayule.
While Havea domestication, breeding, industrialized processing and rubber production have evolved steadily over the last century, Guayule breeding was intermittent, surging and waning four times during the same period. Rubber production from Guayule started in Mexico but was halted during the Mexican revolution in 1912. It then regained momentum by the agronomic and breeding studies carried out by the Intercontinental Rubber Company in San Diego, owned by the Rockefeller family. This ended due to the Great Depression of the 1930s and was briefly, but aggressively, revived during WW2 through US congress legislation. The “Emergency Rubber Project” was aimed to overcome the Japanese army cutting off the rubber supply from southeast Asia. After extensive, but short-lived research the project stopped in 1945 at the end of the war. However, the 1970’s price surge in global oil led in 1978 to the enactment of the US Congress “Native Latex Commercialization and Economic Development Act”, which supported Guayule research in a slow and steady pace until the early 1990’s. Then the Havea-based latex allergy offered Guayule research a fourth push as Guayule was discovered to be hypoallergenic. This century length roller-coaster led to the reality that although much crucial knowledge and knowhow on Guayule already existed for a while, the fact is that it never reached large industrial-level use, even though in principle, it could have been just as successful as the Havea rubber tree.
Guayule has several advantages over Havea. It is a low input crop with the potential to reduce the environmental impact and contribute to sustainable development. In addition to its rubber being hypoallergenic, harvesting can be fully mechanized as opposed to the labor-intensive rubber collecting process for Havea. But Guayule does have several disadvantages that have presented major barriers to its commercialization. Being only partly domesticated, it cannot tolerate the low winter temperatures in much of Europe and the US. Although current varieties can be grown immediately, the species is relatively undeveloped, thus there is much effort needed to improve rubber yield and quality, and to reduce water requirements and other agronomic issues.
Many of these obstacles can be overcame through molecular breeding practices, common today in high volume row crops such as maize, wheat, cotton and others. These crops have a cultivation history of dozens of years, with constant improvements to their yield, disease resistance and other important traits. The introduction of genomics into the breeding processes, coupled with computational analytical capabilities, significantly sped up the development of new varieties. The ability to analyze all the genetic content, to map specific traits and to predict computationally the desired outcome, enabled researchers to domesticate wild species and skip years of development. This is precisely what we are hoping to contribute to with Guayule.
The fact that most natural rubber production is principally from one source in Southeast Asia creates risks for industries around the world relying on natural rubber. Climate changes, pests and diseases, and regional geopolitics may individually or collectively lead to an unstable or interrupted supply. Therefore, investing in research and development to secure an alternative source for natural rubber, such as Guayule, has become a pressing strategic goal.