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 Research: Engineering New Domestic Sources of Natural Rubber

Natural rubber is considered a vital raw material by developed countries and is valued for its high performance characteristics. Synthetic rubber, derived from petroleum, is not as elastic or resilient and does not have the heat transfer properties of natural rubber. Although synthetic rubber is often blended with natural rubber, various products, such as airplane tires, cannot be made without the natural form. Also, synthetic rubber is a non-renewable resource whereas natural rubber should be available indefinitely from renewable plant sources. The only commercial source of natural rubber, at the moment, is the Brazilian rubber tree [Hevea brasiliensis (A. Juss.) Mill. Arg.]. The rubber is harvested by tapping into the pipe-like network of latex-containing laticifers that run beneath the bark, a labor-intensive procedure. The expense of tapping and the tree's tropical growth requirements make H. brasiliensis unsuitable for cultivation within the United States. However, because natural rubber is the second most costly raw material imported into the United States after petroleum, there is strong commercial incentive to develop a domestic rubber crop. Moreover, as plantation-grown H. brasiliensis is derived from clonal material grafted onto seedling root stocks all plants of a commercial line are genetically identical to each other. Thus, H. brasiliensis is vulnerable to crop failure should a particularly virulent disease arise. An alternative rubber crop capable of rapid scale up, using fast-growing annual plants or fermentation in a bioreactor, could furnish a protective buffer in the event of an import shortfall. Even if the crop was not profitable initially, the commercial competitiveness of domestic rubber should steadily improve if natural rubber prices increase as anticipated (Greek 1992).

We are attempting to develop a domestic source of natural rubber using a biotechnological approach. To this end, we intend to clarify the biochemistry of rubber formation and identify and isolate the enzymes and genes responsible for the cis-1,4-polymerization of isoprene unique to rubber producing plants. Once accomplished, it should be possible to isolate, then insert and express the appropriate genes into annual plants and/or microorganisms. These systems would then be optimized to produce large amounts of high quality rubber.

A considerable body of information exists on the biological mechanism of rubber biosynthesis and on the adjacent portions of the isoprenoid pathway. This includes the isolation and cloning of genes for enzymes involved in the production of allylic pyrophosphate initiators for new rubber molecules (Anderson et al. 1989a,b). However, before transformation experiments on potential domestic rubber-producing species can be realistically begun, a definitive isolation of the rubber transferase enzyme responsible for rubber molecule elongation, and then its gene, is required.

Parthenium argentatum Gray (guayule) is a promising candidate for a domestic commercial source as it produces high quality rubber in its bark. Unlike, H. brasiliensis, which has the complex laticiferous anatomy to support its rubber production (d'Auzac et al. 1989), P. argentatum simply produces rubber in generalized parenchyma cells in its bark tissue (Backhaus 1985). Furthermore, P. argentatum is native to the warm arid regions of the southwestern United States and is being cultivated there in various preliminary trials. Disadvantages exist in obtaining the rubber from this perennial species as the destructive harvest of mature plants is required. Woody shrubs, at least three years old, must be ground up before the rubber can be extracted. Also, high yields only result when the crop is irrigated and fertilized, and P. argentatum cannot tolerate the severe winters of the northern United States. These characteristics of the crop make it unsuitable for an emergency supply as it could not be rapidly scaled up. Nonetheless, this species is a good model system for studying rubber biosynthesis, and provides a source of genes that may be useful in increasing its own rubber yield and/or for transformation of other species.

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