100 Years of Breeding Guayule - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 356

relationships between latex concentration and yield and rubber concentration and
yield will need to be done before meaningful breeding programs can be started.
In a recent study, Foster et al. [ 38 ] evaluated eight guayule lines for cold
tolerance and rubber, resin, and biomass yields over a three-year period (2006-
2009). Two lines, 11591 and N6-5, had the least cold damage and hold promise for
rubber production on the Texas High Plains. Certain production criteria make the
Texas High Plains an ideal guayule production site: the long-term annual rainfall
averages 460 mm, irrigation water salinity is less than 1 E.C. and is pumped from
only 909 m, and center pivots are available for establishing guayule by direct
seeding. This study shows that sufficient variability exists within the current
germplasm pool to select for cold tolerance to expand the potential production
area of guayule.
Guayule still contains many wild characteristics such as indeterminate
flowering, seed shattering, natural seed dormancy, and both sexual and asexual
reproduction occurring in the same plant, which are desirable to change to facilitate
commercialization [ 5 , 39 ]. To overcome these undesirable characteristics, long-
term well-supported breeding efforts will be needed. Hopefully the renewed interest
in guayule as a commercial crop will lead to these efforts.
Biotechnology
Limited studies utilizing biotechnology, chemical, molecular, or other new methods
of improvement have been conducted in guayule. Most have involved trying to
understand the rubber synthesis pathway or to modify the rubber biosynthesis
pathway through genetic engineering in guayule. The first chemical studies with
isozymes [ 40 ] involved the development of isozyme markers to identify genetic
differences among diploids, but they are not in use in the current breeding programs
since the emphasis is currently on developing polyploid lines.
Rubber synthesis in guayule is temperature dependent, with highest accumula-
tion in the cold winter months [ 41 , 42 ]. It is important to dissect the effect of cold
temperature on this biosynthetic pathway to understand the regulation of rubber
biosynthesis in guayule is critical to realize the goal of improving guayule as a
domestic rubber crop, by means of breeding or genetic engineering, to achieve high
yields of natural rubber.
Ponciano et al. [ 43 ] utilized an expressed sequence tag (EST) collection to
analyze of the transcriptome of cold-acclimated guayule to search for genes
involved in rubber biosynthesis, including the elusive member(s) of the rubber
transferase enzyme complex. They found that cold-acclimated, rubber-producing
guayule tissue is enriched with allene oxide synthase (AOS) transcript, but not
much by other transcripts encoding proteins believed to be associated with rubber
particles (e.g., cis-prenyltransferase (CPT) and small rubber particle protein
(SRPP)) or the isoprenoid pathway enzymes that make the precursors for rubber
biosynthesis (MEV enzymes and farnesyl pyrophosphate synthase (FPPS)).