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Natural rubber biosynthesized via the isoprenoid pathway by domestic plant sources, such as guayule (Parthenium argentatum) may be more economically sustainable with improved yields, through breeding or targeted metabolic engineering of the isoprenoid pathway. The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) is considered a key regulatory enzyme of isoprenoid carbon flux in mammals, in microbial systems, and possibly in plants. The objective of our study was to modify isoprenoid production in guayule through overexpression of the HMGR gene. A transformation plasmid pND4-HMGR(tAN) was constructed with a modified binary vector and the HMGR gene from Aspergillus nidulans containing only the 465 amino acid catalytic domain driven by a constitutive promoter. Five independent transgenic lines were obtained via leaf disc Agrobacterium-mediated transformation. In the laboratory, the NR content of 2-month-old in vitro plantlets showed a 65% increase in rubber over the vector control for one line (HMGR6), and lower resin production for another (HMGR2). In field evaluations, the genetically modified HMGR6 line was differentiated from control lines in size, biomass, and plant morphology descriptors, but not in rubber or resin content. Remarkably, the survival rate of all HMGR-modified plants following pollarding harvest, was better than controls, with the highest survival rate for line HMGR6. In summary, we report the first genetic modification of guayule to overexpress the isoprenoid pathway enzyme HMGR. Survival during regrowth was significantly improved for HMGR overexpressing plants, suggesting enhanced carbon flux to important secondary isoprenoid metabolites, such as growth phytohormones. |
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