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Background  Latex from the dandelion species  Taraxacum brevicorniculatum  contains many high-value isoprenoid end products, e.g. triterpenes and polyisoprenes such as natural rubber. The isopentenyl pyrophosphate units required as precursors for these isoprenoids are provided by the mevalonate (MVA) pathway. The key enzyme in this pathway is 3-hydroxy-methyl-glutaryl-CoA reductase (HMGR) and its activity has been thoroughly characterized in many plant species including dandelion. However, two enzymes acting upstream of HMGR have not been characterized in dandelion latex: ATP citrate lyase (ACL), which provides the acetyl-CoA utilized in the MVA pathway, and acetoacetyl-CoA thiolase (AACT), which catalyzes the first step in the pathway to produce acetoacetyl-CoA. Here we isolated  ACL  and  AACT  genes from  T. brevicorniculatum  latex and characterized their expression profiles. We also overexpressed the well-characterized  HMGR ,  ACL  and  AACT  genes from  Arabidopsis thaliana  in  T. brevicorniculatum  to determine their impact on isoprenoid end products in the latex.    Results  The spatial and temporal expression profiles of  T. brevicorniculatum ACL  and  AACT  revealed their pivotal role in the synthesis of precursors necessary for isoprenoid biosynthesis in latex. The overexpression of  A. thaliana ACL  and  AACT  and  HMGR  in  T. brevicorniculatum  latex resulted in the accumulation of all three enzymes, increased the corresponding enzymatic activities and ultimately increased sterol levels by ~5-fold and pentacyclic triterpene and  cis -1,4-isoprene levels by ~2-fold. Remarkably high levels of the triterpene precursor squalene were also detected in the triple-transgenic lines (up to 32 mg/g root dry weight) leading to the formation of numerous lipid droplets which were observed in root cross-sections.    Conclusions  We could show the effective expression of up to three transgenes in  T. brevicorniculatum  latex which led to increased enzymatic activity and resulted in high level squalene accumulation in the dandelion roots up to an industrially relevant amount. Our data provide insight into the regulation of the MVA pathway in dandelion latex and can be used as a basis for metabolic engineering to enhance the production of isoprenoid end products in this specialized tissue.    Electronic supplementary material  The online version of this article (doi:10.1186/s12870-017-1036-0) contains supplementary material, which is available to authorized users.

Isoprenoid biosynthesis in dandelion latex is enhanced by the overexpression of three key enzymes involved in the mevalonate pathway