100 Years of Breeding Guayule - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 358

While these previous attempts to increase rubber yields have met with little or
limited success, the studies have shown that guayule can be successfully
transformed. It might be more effective in the short term to use biotechnology to
insert genes for other potentially useful traits such as herbicide tolerance, insect
resistance, and leaf senescence. Herbicide tolerance would be helpful during the
establishment phase of guayule since growth is slow during this period and weed
competition is high. Once established guayule has good tolerance to insect pests,
but during early stages of establishment and regrowth following harvest, the new
tissue is very susceptible to insect pests. These two traits will be even more
important to have if guayule production changes from plant establishment by
transplanting to a direct seeding system.
Current studies are underway at USDA-ARS to assemble a draft genome
sequence of the heterozygous, diploid guayule accession, PI 478663. Preliminary
results have indicated an estimated haploid nuclear genome size of ~1,100 Mb
[ 18 ]. Completion of this study should result in the development of markers that can
be used to enhance guayule germplasm.
Summary
Guayule has a long history of use as a source of natural rubber. Although similar in
quality to natural rubber from Hevea brasiliensis , guayule rubber was not compet-
itive economically until the occurrence of latex allergy in the general population.
Continued pressure on worldwide Hevea rubber supplies has contributed to
renewed interest in the use of guayule rubber in tire applications.
The available germplasm upon first glance appears to be rather narrow, but
because of the facultative nature of apomictic reproduction in guayule, genetic
variability is continually being released. Three collection trips, one in the early
1900s and two in the early 1940s, account for most of the germplasm used in current
breeding programs and what is available in the National Plant Germplasm System
today. A more recent collection trip in Texas found guayule was no longer growing
at many of the older sites. Collection of guayule germplasm from its natural habitat
in Mexico and the United States needs to be a top priority before it is all lost. In
conjunction with collection, research needs to be done on properly storing seed for
the long term. Much of the previously collected seed is no longer available because
it has lost viability.
The major differences in breeding guayule and other new crops compared to
traditional crops are that (1) the plant breeder starts with a different and frequently
unique and exotic germplasm base from which to develop a crop; (2) the breeder is
often totally unfamiliar with the species, the germplasm, and the potential end
products; (3) the traits to be improved frequently have not been identified by
researchers, industry, and/or growers; and (4) there is often a paucity of previous
research, including the appropriate technology for evaluating, selecting, and breed-
ing of the products and coproducts sought. New crop breeders must be flexible in