Terry A. Coffelt

Transcriptome and gene expression analysis in cold-acclimated guayule (Parthenium argentatum) rubber-producing tissue

Natural rubber biosynthesis in guayule (Parthenium argentatum Gray) is associated with moderately cold night temperatures. To begin to dissect the molecular events triggered by cold temperatures that govern rubber synthesis induction in guayule, the transcriptome of bark tissue, where rubber is produced, was investigated. A total of 11,748 quality expressed sequence tags (ESTs) were obtained. The vast majority of ESTs encoded proteins that are similar to stress-related proteins, whereas those encoding rubber biosynthesis-related proteins comprised just over one percent of the ESTs. Sequence information derived from the ESTs was used to design primers for quantitative analysis of the expression of genes that encode selected enzymes and proteins with potential impact on rubber biosynthesis in field-grown guayule plants, including 3-hydroxy-3-methylglutaryl-CoA synthase, 3-hydroxy-3-methylglutaryl-CoA reductase, farnesyl pyrophosphate synthase, squalene synthase, small rubber particle protein, allene oxide synthase, and cis-prenyl transferase. Gene expression was studied for field-grown plants during the normal course of seasonal variation in temperature (monthly average maximum 41.7 °C to minimum 0 °C, from November 2005 through March 2007) and rubber transferase enzymatic activity was also evaluated. Levels of gene expression did not correlate with air temperatures nor with rubber transferase activity. Interestingly, a sudden increase in night temperature 10 days before harvest took place in advance of the highest CPT gene expression level.

Heritability of height, width, resin, rubber, and latex in guayule (Parthenium argentatum)

Abstract Plant breeding in guayule, a rubber and latex producing plant, has not been as effective as predicted or desired. A surprisingly large amount of variability has been reported in this apomictic crop for traits such as plant height, width, resin, rubber, and latex contents. This study was designed to calculate the proportion of the total measured variability due to the environment and the proportion due to genetic influences within and between three-released germplasm lines. Plant heights were measured at 1, 2 and 3-years-of-age; plant width, and latex contents at 2 and 3-years-of-age; and resin and rubber content at 2-years-of-age. Broad-sense-heritabilities were estimated for each trait and year by dividing the genotypic by the total variance. To estimate the genetic component of the measured variance for each trait, the environment effects (variance from clonally propagated plants) were subtracted from the total variance (variance from open-pollinated (OP) seed propagated plants). In general, the variances of the means for the measured traits were lower in the clonally propagated plants compared with the apomictic OP seed propagated plants. The heritability estimates calculated for each trait differed from year to year. For instance, heritability for plant height was estimated in line AZ-2 to be 0.84 at 1 year of age; 0.47 at 2 years of age; and 0.0 at 3 years of age. These values imply that a large portion of the observed variation in this line is attributed to genetic effects in the first 2 years of growth. As the plant grows over several seasons, the environment effects compound, masking the genetic effects, making effective selection choices more difficult. Heritability for latex content for the same line was estimated to be 0.97 for the second year and 0.55 for the third year. Selections for the measured traits in this study appear to be most effective during the first and second years of growth, with effectiveness diminishing during the third year. Most selections previous to this study were performed between 3 and 5 years of growth, thus suggesting one reason for the lack of significant progress in most breeding programs.

Biobased composition boards made from cotton gin and guayule wastes: select physical and mechanical properties

Using biomass materials such as cotton gin byproducts (CGB) and guayule wastes in value-added products can help the economics of these crops, and additionally, aid in alleviating waste management and environmental problems. A study was conducted to assess important physical and mechanical properties of composition boards made from select CGB and guayule waste. Boards were made from five different ratios of cotton gin and guayule wastes: 100 : 0, 75 : 25, 50 : 50, 25 : 75, and 0 : 100. Overall, the biobased CGB and guayule waste boards showed great potential. However, refinement is needed to further enhance the performance of these biomasses for composite board applications.

Flowering and seed yield of lesquerella as affected by nitrogen fertilization and seeding rate

The lesquerolic acid in lesquerella seed can be used in industrial applications such as greases, cosmetics, polishes, inks, and coatings. Successful commercialization of lesquerella will depend on the development of improved cultural practices. Lesquerella flowers are bright yellow and are prominently displayed. As a result, many cultural practices could be tied to flowering in a qualitative way. A new method of estimating flowers such as those of lesquerella using digital images has been developed that is rapid, not labor intensive, and can be automated. The purpose of this study was to determine the effects of nitrogen fertilizer and plant density on flowering of lesquerella and to develop relationships between seed yield and flowering. The lesquerella crop was planted on 15 October 1997, at the Maricopa Agricultural Center, approximately 40 km south of Phoenix, Arizona on a variable Mohall sandy loam (fine-loamy, mixed hyperthermic, Typic Haplargid). The experimental design was a complete factorial of three fertilizer rates and four seeding rates. Ammonium sulfate at rates of 0, 60 and 120 kg of N ha−1 was applied at flowering on 18 March 1998. Digital images of the plots were taken periodically from 19 March 1998 to 4 June 1998 using a color digital camera. Images were acquired between 1030 and 1300 h MST. In this experiment, the crop did not respond to seeding rate. Flowers present at initial bloom could be used to estimate stand establishment. The early flowers did not contribute much to final yield, but flowers present in the first 3 weeks of May were a good predictor of yield. Flowering increased with N additions and noticeable peaks in flowering occurred after irrigations. The new method was verified as a viable method for estimating flower number. The method of flower estimation should also be useful for plant breeders for selection of earlier maturing lines, which would increase the potential for use of lesquerella in rotational systems.

A Century of Guayule: Comprehensive Genetic Characterization of the Guayule (Parthenium argentatum A. Gray) USDA Germplasm Collection

The fragility of a single-source, geographically concentrated supply of natural rubber, a critical material of the modern economy, has brought guayule (Parthenium argentatum A. Gray) to the forefront as an alternative source of natural rubber. The improvement of guayule for commercial-scale production has been limited by the lack of genomic tools and well-characterized genetic resources required for genomics-assisted breeding. To address this issue, we developed nearly 50,000 single nucleotide polymorphism (SNP) genetic markers and genotyped 69 accessions of guayule and its sister taxa mariola (Parthenium incanum Kunth), representing the entire available NALPGRU germplasm collection. We identified multiple interspecific hybrid accessions previously considered guayule, including six guayule-mariola hybrids and non-mariola interspecific hybrid accessions AZ-2 and AZ-3, two commonly used high-yielding cultivars. We dissected genetic diversity within the collection to identify a highly diverse subset of guayule accessions, and showed that wild guayule stands in Big Bend National Park, Texas, USA have the potential to provide hitherto untapped guayule genetic diversity. Together, these results provide the most thorough genetic characterization of guayule germplasm to date and lay the foundation for rapid genetic improvement of commercial guayule germplasm. Key Results Six guayule accessions are guayule-mariola hybrids Guayule collections from Big Bend National Park contain novel guayule genotypes not present in collections from Mexico Commonly cultivated accessions AZ2 and AZ3 contain introgressions from other Parthenium species The triploid accessions 11591, 11646, N576, N565, N565II, and RICHARDSON are generally indistinguishable from each other with respect to genetic background and likely represent the 4265-I source genotype (Johnson, 1950) Open pollinated and purposefully outcrossed tetraploid selections derived from 4265-I incorporate further genetic diversity and form distinct genotypes

100 Years of Breeding Guayule

Guayule has been known for 100 years as a potential source of natural rubber. Breeding efforts have been sporadic limiting progress in guayule breeding compared to other crops. Even though the genetic base appears to be rather narrow, it has not hindered guayule breeding programs. The most extensively employed breeding approach has been single-plant selections. The primary objective for guayule breeding programs has been increased rubber yield. 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. 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 and insect resistance. For future progress in guayule breeding to be made, much work remains to be done.