Citrus fruit flavor and aroma biosynthesis: isolation, functional characterization, and developmental regulation of Cstps1, a key gene in the production of the sesquiterpene aroma compound valencene

Abstract

SummaryCitrus fruits possess unique aromas rarely found in other fruit species. While fruit flavor is composed of complex combinations of soluble and volatile compounds, several low‐abundance sesquiterpenes, such as valencene, nootkatone, alpha‐sinensal, and beta‐sinensal, stand out in citrus as important flavor and aroma compounds. The profile of terpenoid volatiles in various citrus species and their importance as aroma compounds have been studied in detail, but much is still lacking in our understanding of the physiological, biochemical, and genetic regulation of their production. Here, we report on the isolation, functional expression, and developmental regulation of Cstps1, a sesquiterpene synthase‐encoding gene, involved in citrus aroma formation. The recombinant enzyme encoded by Cstps1 was shown to convert farnesyl diphosphate to a single sesquiterpene product identified as valencene by gas chromatography–mass spectrometry (GC–MS). Phylogenetic analysis of plant terpene synthase genes localized Cstps1 to the group of angiosperm sesquiterpene synthases. Within this group, Cstps1 belongs to a subgroup of citrus sesquiterpene synthases. Cstps1 was found to be developmentally regulated: transcript was found to accumulate only towards fruit maturation, corresponding well with the timing of valencene accumulation in fruit. Although citrus fruits are non‐climacteric, valencene accumulation and Cstps1 expression were found to be responsive to ethylene, providing further evidence for the role of ethylene in the final stages of citrus fruit ripening. Isolation of the gene encoding valencene synthase provides a tool for an in‐depth study of the regulation of aroma compound biosynthesis in citrus and for metabolic engineering for fruit flavor characteristics.

Terpene production in the peel of sweet orange fruits

“…The limonene synthase gene showed expression in all analyzed tissues, fruit and flower with the highest levels. This probably reflects the role of terpenes as an important group of secondary metabolites involved in fruit and flower aroma (Sharon-Asa et al, 2003). In agreement with in silico data, expression of the limonene synthase gene was higher in fruit tissue.…”

Peroxisomal Localization of Arabidopsis Isopentenyl Diphosphate Isomerases Suggests That Part of the Plant Isoprenoid Mevalonic Acid Pathway Is Compartmentalized to Peroxisomes

“…(2) Constitutive overexpression in tobacco of a nonregulated form of HMGR (lacking the N-terminal membrane anchor) resulted in a 3-to 10-fold increase in total sterol levels but no increase in sesquiterpene levels (Chappell et al, 1995). (3) Constitutive overexpression of citrus valencene synthase (Sharon-Asa et al, 2003) in transgenic tobacco did not result in synthesis of the sesquiterpene valencene. However, treatment of these transgenic plants with elicitor resulted in valencene synthesis (S. Tal-Meshulam, A. Frydman, E. Bar, E. Lewinsohn, and Y. Eyal, unpublished data).…”

Citrus plastid-related gene profiling based on expressed sequence tag analyses

“…The former is associated with phytochrome-mediated light perception, and it has been already correlated with acclimatization to low temperatures in Poncirus (Zhang et al, 2005). Terpene synthase catalyzes a key step on sesquiterpene metabolism, and it has been described in Citrus due to its importance to typical citric flavor (Sharon-Asa et al, 2003).…”