I. Biosynthesis of volatile plant secondary metabolites
and its interconnection with primary metabolism
A relatively large group of plant natural products consists of volatile
organic compounds (VOCs), lipophilic liquids with low molecular
weight and high vapor pressure at ambient temperatures. Physical
properties of these compounds allow them to freely cross cellular
membranes and be released into the surrounding environment
(Pichersky et al., 2006). Over the years > 1700 VOCs have been
identified from 90 different plant families belonging to both
angio- and gymnosperms (Knudsen et al., 2006). Biosynthesis of
VOCs depends on the availability of carbon, nitrogen and sulfur as
well as energy provided by primary metabolism. Therefore, the
availability of these building blocks has a major impact on the
concentration of any secondary metabolite, including VOCs,
demonstrating the high degree of connectivity between primary
and secondary metabolism. Biosynthesis of the wide array of
different VOCs branches off from only a few primary metabolic
pathways (Fig. 1). Based on their biosynthetic origin, all VOCs
are divided into several classes, including terpenoids,