New insights into natural rubber biosynthesis from rubber-deficient lettuce mutants expressing goldenrod and guayule cis-prenyltransferase

Lactuca sativa (lettuce) was used as a model organism to study natural rubber (NR) biosynthesis in planta. Four NR deficient lettuce lines were produced using CRISPR-Cas9 gene-editing to knock-out the function of cis-prenyltransferase 3 (LsCPT3). The phenotype of mutants was assessed and transcripts in the latex of wild-type and lscpt3 mutant lettuce lines were measured using Next-Generation Sequencing. With no obvious physiological abnormalities, one of the lscpt3 mutant lines were used as a genetic background for complementation experiments using CPT genes from the related species which produce high and low molecular weight NR, Parthenium argentatum (guayule) and Solidago canadensis (golden rod), respectively. Surprisingly, when expressed in lettuce laticifers the guayule and golden rod CPTs are capable of producing NR with a molecular weight 1.8- and 14-fold greater than in the native plants.