Farnesol is utilized for isoprenoid biosynthesis in plant cells via farnesyl pyrophosphate formed by successive monophosphorylation reactions

The ability of Nicotiana tabacum cell cultures to utilize farnesol (F- OH) for sterol and sesquiterpene biosynthesis was investigated. [3H]F-OH was readily incorporated into sterols by rapidly growing cell cultures. However, the incorporation rate into sterols was reduced by greater than 70% in elicitor-treated cell cultures whereas a substantial proportion of the radioactivity was redirected into capsidiol, an extracellular sesquiterpene phytoalexin. The incorporation of [³H]F-OH into sterols was inhibited by squalestatin 1, suggesting that [³H]F-OH was incorporated via farnesyl pyrophosphate (F-P-P). Consistent with this possibility, N. tabacum proteins were metabolically labeled with [³H]F-OH or [³H]geranylgeraniol ([³H]GG- OH). Kinase activities converting F-OH to farnesyl monophosphate (F-P) and, subsequently, F-P-P were demonstrated directly by in vitro enzymatic studies. [³H]F-P and [³H]F-P-P were synthesized when exogenous [³H]F-OH was incubated with microsomal fractions and CTP. The kinetics of formation suggested a precursor-product relationship between [³H]F-P and [³H]F-P-P. In agreement with this kinetic pattern of labeling, [³²P]F-P and [³²P]F- P-P were synthesized when microsomal fractions were incubated with F-OH and F-P, respectively, with [γ-³²P]CTP serving as the phosphoryl donor. Under similar conditions, the microsomal fractions catalyzed the enzymatic conversion of [³H]GG-OH to [³H]geranylgeranyl monophosphate and [³H]geranylgeranyl pyrophosphate ([³H]GG-P-P) in CTP-dependent reactions. A novel biosynthetic mechanism involving two successive monophosphorylation reactions was supported by the observation that [³H]CTP was formed when microsomes were incubated with [³H]CDP and either F-P-P or GG-P-P, but not F-P. These results document the presence of at least two CTP-mediated kinases that provide a mechanism for the utilization of F-OH and GG-OH for the biosynthesis of isoprenoid lipids and protein isoprenylation.