Two Oxidosqualene Cyclases Responsible for Biosynthesis of Tomato Fruit Cuticular Triterpenoids

The first committed step in triterpenoid biosynthesis is the cyclization of epoxysqualene into various triterpene alcohol isomers, a reaction catalyzed by oxidosqualene cyclases (OSCs). The different OSCs have characteristic product specificities, which are mainly due to differences in the numbers of high-energy intermediates the enzymes can stabilize. The goal of this investigation was to clone and characterize OSCs from tomato (Solanum lycopersicum), a species known to accumulate δ-amyrin in its fruit cuticular wax, in order to gain insights into the enzymatic formation of this particular triterpenoid. We used a homology-based approach to isolate two tomato OSCs and tested their biochemical properties by heterologous expression in yeast as well as overexpression in tomato. One of the enzymes was found to be a product-specific β-amyrin synthase, while the other one was a multifunctional OSC synthesizing 48% δ-amyrin and six other products. The product spectra of both OSCs together account for both the range and the relative amounts of the triterpenoids found in the fruit cuticle. Both enzymes were expressed exclusively in the epidermis of the tomato fruit, indicating that their major function is to form the cuticular triterpenoids. The relative expression levels of both OSC genes, determined by quantitative reverse transcription-polymerase chain reaction, were consistent with product profiles in fruit and leaves of the tomato cultivar MicroTom. However, the transcript ratios were only partially consistent with the differences in amounts of product triterpenoids between the tomato cultivars MicroTom, M82, and Ailsa Craig; thus, transcriptional control of the two OSCs alone cannot explain the fruit triterpenoid profiles of the cultivars.