Biosynthesis of Organic Compounds Emitted by Plants

Abstract

Trees produce a wide spectrum of volatile organic compounds (VOCs) including isoprene and monoterpenes, as well as oxygenated compounds like aldehydes, alcohols and carboxylic acids. In recent years, much progress has been made regarding the elucidation of metabolic pathways leading to the production of these compounds. This is particularly true for the biosynthesis of the isoprenoid precursors, isopentenyl diphosphate (IDP) and dimethyl allyl diphosphate (DMADP). In addition to the classical mevalonate pathway which leads to the biosynthesis of these compounds, recent studies indicate the presence of a non-mevalonate pathway originating from pyruvate and glyceraldehyde-3-phosphate (GAP), also leading to isoprenoid precursors. This new 1-deoxy-D-xylulose-5-phosphate (DOXP) pathway is probably responsible for the formation of all plastid-derived isoprenoid compounds in plants, including carotenoids, plastochinones, the prenyl side chains of chlorophyll, as well as monoterpenes and diterpenes. Because all plastidic isoprenoids studied so far are formed via this new pathway, it is assumed that isoprene synthesized in the chloroplasts is also produced via this metabolic route. Among the 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 such as seed maturation, cell expansion, cell wall degradation, leaf abscission and senescence of plant tissues. The production of C-2 compounds, however, seems mainly to be associated with changing environmental conditions, particularly during stress. Acetaldehyde, for example, is produced in the leaves of trees if the roots are exposed to anaerobic conditions which in nature may be caused by flooding. As a consequence of anaerobiosis, roots produce ethanol through alcoholic fermentation. Ethanol is loaded into the xylem, transported to the leaves and oxidized there under aerobic conditions, thereby releasing acetaldehyde and acetic acid.

Key words

Acetaldehyde - acetic acid - ethanol - formaldehyde - formic acid - isoprene - monoterpenes