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      Species name: Guayule (Parthenium argenatum A. Gray)
      Overview
    Guayule, botany, taxonomy and distribution.
    Guayule (Parthenium argenatum A. Gray) is a small low, branching shrub endemic to the desert regions of Northern Mexico and southern Texas whose economic value lies in its high-quality rubber. Rubber is an isoprenoid (cis-1,4polyisoprene) compound found in over 2,000 plants species. In most species, rubber accumulates in discrete subcellular organelles called rubber particles which are confined to latex vessels, best exemplified from the commercial source of rubber, Hevea brasiliensis. However, rubber biogenesis in guayule is unique in that its rubber producing parenchyma and epithelial cells remain intact for the life of the plant (Artschwager, 1943, 1945). Extraction requires processing the entire plant.

    Guayule genetics.
    Guayule populations exhibit a natural polyploidy series ranging from diploid (2n=2x=36) to pentaploid (2n=5x = 90), with tetraploid (4x) being the predominant ploidy level (Gore et al., 2011). In general, diploids reproduce sexually and are self-incompatible (Gore et al., 2011). Self-incompatibility has been established in 40 genera in Asteraceae (Charlesworth, 1985) including Parthenium. Guayule exhibits homomorphic sporophytic self-incompatability (Gerstel, 1950). This type of self-incompatiblity is rare in dicotyledonous angiosperms, occurring in only five other families (Hiscock and KŁes, 1999). Thus, guayule offers an interesting opportunity to study self-incompatibility within the composites.

    Guayule economic use.
    In the early part of the 20th century, guayule provided about 50% of the rubber used in the U.S., which imported the product from Mexico (National Academy of Sciences, 1977). Rubber production in Mexico from guayule declined rapidly due to unsustainable harvesting of wild plants and the Mexican revolution. The US was soon dependent on rubber from Hevea. During WWII, 90% of the U.S. rubber supply was cut off and subsequently the Emergency Rubber Project was initiated with over 1,000 scientists, 9,000 workers and 13,000 ha of research fields working to produce over one billion guayule seedlings and 1.4 million kg of rubber resin. Guayule rubber plants in Salinas and Bakersfield CA produced 15 tons of rubber daily. Guayule production and research quickly ended after WWII being replaced by increased rubber production and synthetic elastomers, which had both become much more economically feasible. In addition to its novel rubber biosynthesis and rare self-incompatibility mechanism, there remains some interest in developing the shrub as an alternative natural rubber source and the species shows high potential as a source of high grade hypoallergenic natural rubber (van Beilen and Poirier, 2007).

    Guayule cultural production.
    Guayule is propagated through seed but germination is typically low and variable. Osmopriming has been used to increase the germination percentage and uniformity (Jorge et al., 2006). Guayule is once again being developed as a new industrial crop suitable for cultivation in arid and semiarid regions (Jorge et al., 2007) and efforts are being made to improve seed quality and germination (Jorge et al., 2007).

      Resources
      Images of Parthenium argenatum
      
      Figure 1: Parthenium argenatum A. Gray accession 11591. (photo by D.W. Still)
      
      Figure 2: Guayule accession 11591 flower. (photo by D.W. Still)
      
      Figure 3: Guayule accession 11591 leaves and stems. (photo by D.W. Still)
      CGP Activities
    The CGP produced Illumina ESTs from leaves, stem, flower and roots from accession 11591. This accession is an open-pollinated, triploid, phenotypically uniform cultivar from which several single-plant selections have been made (Gore et al., 2011). The Elastomics Project conducted by the USDA Crop Improvement and Utilization Research Unit (Albany, CA) sequenced accession AZ2 from cold- acclimated bark. The CGP EST sequences are being assembled.
      Publications
    Artschwager, E. 1943. Contribution to the morphology and anatomy of guayule (Parthenium argentatum). USDA Technical Bulletin 842.

    Artschwager, E. 1945. Growth studies on guayule. USDA Technical Bulletin 885.

    Charlesworth, D. 1985. Distribution of dioecy and self-incompatibility in angiosperms. In P. J. Greenwoog, P. H. Harvey, and M. Slatkin, [eds.], Evolution: essays in honor of John Maynard Smith, 237?268. Cambridge University Press, Cambridge, UK.

    Gerstel, D.U. 1950. Self-incompatibility studies in guayule. Genetics 35: 482?506.

    Gore M.A., Coyle, G. Friebe, B., Coffelt, T.A., and M.E. Salvucci. 2011. Complex ploidy level variation in guayule breeding programs. Crop Science 51:210-216.

    Hiscock, S.J., and U. K?es. 1999. Cellular and molecular mechanisms of sexual incompatibility in plants and fungi. International Review of Cytology 193: 165?195.

    Jorge, M.H.A., Veatch-Blohm, M.E., and D.T. Ray. 2007. Quality of guayule seed separated by physical attributes. Industrial Crops and Products 25: 55-62.

    Jorge, M.H.A., Veatch-Blohm, M.E., Ray, D.T., and M.A. Foster. 2006. Guayule seed germination under different conditioning treatments. Industrial Crops Production 24:60?65.

    National Academy of Sciences. 1977. Guayule: An alternative source of natural rubber. Report of an Ad Hoc Panel of the Board on Agriculture and Renewable Resources. Library of Congress Catalog Number 76-62525.

    Van Beilen J.B., and Y. Poirier. 2007. Establishment of new crops for the production of natural rubber. Trends in Biotechnology 25: 522-529.

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