Plants as factories for technical materials.

Until the latter part of the 19th century, humans were largely dependent upon contemporaneous biological sources for the production of all organic materials. Plants and animals provided the only sources of fibers, coatings, lubricants, solvents, dyes, waxes, fillers, and insulation, fragrances, detergents, sizing, leather, wood, paper, rubber, and many other types of materials. As recently as 1930, 30% of industrial organic chemicals were derived from plants (13). The discovery of extensive petroleum reserves and advances in chemistry and petroleum engineering resulted in a major shift to reliance on fossil sources of organic feedstocks and the development of materials such as inexpensive plastics, with properties that could not be duplicated by abundantly available natural materials. Nevertheless, many important materials are still derived from plants and animals. Wood, cork, paper, and leather remain ubiquitous. Cotton, ramie, hemp, flax, sisal, wool, and silk are also important sources of fiber for many applications. Rubber from natural latex is still the only material that can be used to produce tires that will reliably withstand the forces associated with airplane landings. Linseed oil is still used to make paint, although linoleum is no longer produced. Thus for many applications, biological sources can still be used to produce materials on the scale necessary to meet the needs of populous industrialized nations. The traditional strategy for using plants has been to modify, by breeding and selection, a species that produces something useful so that it is suited to our needs. Many of the important food species such as maize or the many varieties of Brassica oleracea no longer bear much resemblance to wild progenitors because of strong selection for useful traits. Attempts have been made to improve jojoba for production of wax esters and guayule for latex production and species such as crambe, meadowfoam, Euphorbia lagascae, Lesquerella fendleri, and Cuphea sp. for technically useful oils (5). However, these initiatives have met with limited success because of an inability to develop lines with acceptable production, quality, and agronomic properties. Except for relatively small-scale production of jojoba and guayule, there does not appear to have been a newly created field or plantation crop for production of technical materials during the last century. Several species that produced useful materials such as kapok, which was used for applications such as waterproof fiber filling in life vests, have declined because of the labor costs associated with harvesting the materials. Interest in the possibility of using genetically engineered plants as factories seems to have several motivations (19); in the short term, it would be desirable to diversify crop production by producing high value technical materials in crop plants and create potentially large new markets for excess agricultural production. In the longer term, the concept addresses a widely held social goal of developing more sustainable and environmentally benign methods of meeting our needs for materials that are currently produced by chemical synthesis from declining petroleum or coal feedstocks. In addition, it is possible to envision the production in plants of novel biologically inspired materials, with properties not easily simulated through chemical synthesis. Lingering concern about the consequences of the 1973 oil embargo appear to have been the motivation for the first article that clearly described the concept of using genetic engineering to produce an industrial product in plants. Even before the first paper describing the production of a transgenic plant had appeared (24), Melvin Calvin (1) explicitly outlined the basic steps by which Euphorbia lathyrus could be engineered to produce industrial quantities of sesquiterpenes. This paper and related work from Calvin and colleagues established the concept of engineering plants as factories, but because of economic considerations, the work on E. lathyrus was discontinued. In retrospect it was unrealistic to expect that the first applications of genetic engineering would be directed toward creation of a new crop for a nonexistent market. Calvin’s ideas remain interesting, but until the price of petroleum increases substantially there will not be sufficient economic incentive to attempt the engineering of E. lathyrus. Perhaps the major lesson from Calvin’s work was the necessity of minimizing the threshold for the introduction of a new crop. In practice this has focused attention on using genetic engineering to make incremental changes in plants that are already grown on a large scale. 1 This work was supported in part by the U.S. Department of Energy (DOE–FG02– 00ER20133). * Corresponding author; e-mail [email protected]; fax 650 –325– 6857.