Sesquiterpene aroma biosynthesis in melon (Cucumis melo) rinds

As part of a combined chemical, biochemical and molecular study on the factors that affect melon unique aromas, the differential accumulation of volatile sesquiterpenes in melon varieties was studied. Sesquiterpenes were present mainly in the rinds of climacteric varieties, and generally absent in non-climacteric varieties. In the green-fleshed climacteric 'Noy Yizre'el' variety the main sesquiterpenes accumulated were delta-cadinene, gamma-cadinene and alpha-copaene, while alpha-farnesene is the main sesquiterpene in the orange-fleshed climacteric 'Duice' variety. The rinds of mature fruits displayed high levels of sesquiterpene synthase activity but the activity in each variety generated different sesquiterpenes in accordance to the compositions found in rinds. Two new members of the Tps gene family termed CmTpsNY and CmTpsDul were identified from the EST melon database. The gene product Of CmTpsNY produced mainly delta-copaene, alpha-copaene, beta-caryophyllene, gemacrene D, alpha-muurolene, gamma-cadinene, delta-cadinene, and alpha-cadinene from farnesyldiphosphate, when overexpressed in E. coli while CmTpsDul produced only alpha-farnesene. CmTpsNY was mostly expressed in 'Noy Yizre'el' rind while CmTpsDul expression was specific to 'Dulce' rind. Our results indicate that different sesquiterpene synthases encoded by different members of the Tps gene family are active in melon varieties and this specificity is associated with the accumulation of sesquiterpenes.