Oleoresin sesquiterpene volatiles biosynthesis (Oryza sativa)

• General background*

Oleoresin (also simply termed "resin" or "pitch") is a viscous secretion common in coniferous trees, which is involved in the chemical and physical defense of conifers against predators (such as beetles and fungal pathogens). The accumulated resin is released upon tissue injury and/or produced at the site of infestation (for review, see [Trapp01]). The predator is killed, encased in resin and expelled from the bore point of entry (a process called "pitching out"). This process has the dual function of killing and expelling the predator, but also to form a physical barrier around the wound through evaporation of the oleoresin turpentine which allows the resin acids to seal the wound [Croteau85, Gijzen93]. Oleoresin is composed in roughly equal parts of volatile turpentine (a mixture of monoterpenes and sesquiterpenes) and rosin (also known as diterpene resin acids). The exact composition of turpentine and rosin varies from one conifer species to the next, as well as depending on the resin-producing tissue analyzed [Trapp01]. The main constituents of these fractions have been regrouped in the following pathways: superpathway of oleoresin turpentine biosynthesis and superpathway of diterpene resin acids biosynthesis.

Concerning this pathway:

Compared to monoterpenes, fewer sesquiterpene synthases from conifers have been identified and studied to date. Generally, the longer chain of (2E,6E)-farnesyl diphosphate compared to that of geranyl diphosphate, and the additional double bond, allows for a greater diversity of products being formed by those enzymes. Two coniferous enzymes γ-humulene synthase and δ-selinene synthase from Grand Fir (Abies grandis) produces each in excess of 30 different sesquiterpenes olefins. In contrast, (E)-α-bisabolene synthase catalyzes only the synthesis of (E)-α-bisabolene [Bohlmann98]. Importantly, this compound has been shown to be converted by methyl-jasmonate-induced suspension cell cultures of Grand Fir into todomatuic acid, a sesquiterpene derivative structurally similar to the insect juvenile hormone III [Bohlmann98], and can disrupt insect reproduction and/or larval development [Trapp01].