Growth, rubber, and resin evaluation of two-year-old transgenic guayule

Guayule (Parthenium argentatum Gray) is a desert shrub that is a source of hypoallergenic, high-quality latex and rubber. Improvements in rubber content and yield have been made through conventional selection techniques. Further improvements are being attempted by transforming guayule with one of three genes encoding various allylic diphosphate synthases in the rubber biosynthesis pathway. The objective of this study was to evaluate the effect of these transgenes on growth, rubber and resin production, in field-grown guayule. Tissue culture-generated transgenic plants of the lines AZ 101, AZ-2 and N6-5 were planted in two field plots in 2001 and 2002. In both plots, plant height and width were measured monthly. Branches from each plant were sampled every four months starting at one year of growth. Resin and rubber were quantified by gravimetric analysis after being sequentially extracted with acetone (resin) and cyclohexane (rubber). The 2001 plot was harvested at the end of two years of growth. Transformation had no significant effect on growth of AZ-2 and N6-5 in the two years of the 2001 planting and the first year of the 2002 planting. In the 2001 planting, transformation appeared to have a drastic effect on the height and width of transformed AZ 101 compared with its empty vector control; however, the field in this study was not randomized and lacked non-transformed controls. In the 2002 planting, which was randomized and contained both positive and negative controls, the AZ 101 transformants were significantly larger than the empty vector AZ 101 control, but were not significantly different from the non-transformed controls. In the 2001 planting, resin concentration increased throughout the year up to January 2003, but decreased by the time of harvest in March 2003. Rubber concentration, on the other hand, was high in May 2002, but decreased throughout the summer, before steadily increasing during the winter months.