Biosynthesis of organic compounds emitted by plants

Trees produce a wide spectrum of volatile organic compounds (VOCs) includin g isoprene and monoterpenes, as well as oxygenated compounds like aldehydes , alcohols and carboxylic acids. In recent years, much progress has been ma de regarding the elucidation of metabolic pathways leading to the productio n of these compounds. This is particularly true for the biosynthesis of the isoprenoid precursors, isopentenyl diphosphate (IDP) and dimethyl allyl di phosphate (DMADP). In addition to the classical mevalonate pathway which le ads to the biosynthesis of these compounds, recent studies indicate the pre sence of a non-mevalonate pathway originating from pyruvate and glyceraldeh yde-3-phosphate (GAP), also leading to isoprenoid precursors. This new 1-de oxy-D-xylulose-5-phosphate (DOXP) pathway is probably responsible for the f ormation of all plastid-derived isoprenoid compounds in plants, including c arotenoids, plastochinones, the prenyl side chains of chlorophyll, as well as monoterpenes and diterpenes. Because all plastidic isoprenoids studied s o far are formed via this new pathway, it is assumed that isoprene synthesi zed in the chloroplasts is also produced via this metabolic route. Among th e oxygenated hydrocarbons which are emitted by the leaves of trees, C-1 and C-2 aldehydes, alcohols and carboxylic acids are of great importance. C-1 compounds are synthesized during many growth and developmental processes su ch as seed maturation, cell expansion, cell wall degradation, leaf abscissi on and senescence of plant tissues. The production of C-2 compounds, howeve r, seems mainly to be associated with changing environmental conditions, pa rticularly during stress. Acetaldehyde, for example, is produced in the lea ves of trees if the roots are exposed to anaerobic conditions which in natu re may be caused by flooding. As a consequence of anaerobiosis, roots produ ce ethanol through alcoholic fermentation. Ethanol is loaded into the xylem , transported to the leaves and oxidized there under aerobic conditions, th ereby releasing acetaldehyde and acetic acid.