Many plants form and emit in the atmosphere foliar volatile organic compounds belonging to the isoprenoid family. It is now known that the isoprenoids emitted by the vegetation are directly formed from photosynthetic carbon through a specific, and entirely chloroplastic, metabolic pathway. But it is still unclear why plants invest on average 2 to 10% of the photosynthetic Carbon for forming these compounds apparently useless in the metabolism and therefore re-emitted in the atmosphere.
At the IBAF (Institute of Agro-environmental and Forest Biology) we have carried out a seies of original experiments aiming at clarifying the role of isoprenoids emitted by plants. Our results clearly indicated that isoprenoids have a fundamental role in the protection of leaves against oxidative stresses that denature membranes and may severely damage plants. Leaves emitting isoprene (the basic compound of all isoprenoids) or fumigated with exogenous isoprene resist to acute or chronic exposure to high ozone levels by far better than leaves on which isoprene formation is inhibited or naturally absent. Isoprene emitting leaves exposed to ozone do not alter photosynthesis and chlorophyll fluorescence, do not increase the formation of oxygenated products (e.g. hydrogen peroxide) or of products of cellular membrane degradation, show a well-preserved anatomy both at mesophyll and ultrastructural level, and do not show visible damages even day after the ozone exposure. However, isoprene emitting leaves uptake more ozone than those on which isoprene emission is absent or inhibited. This indicates that gaseous isoprene reacts with ozone or ozone toxic by-products (peroxynitrites) within the leaf mesophyll. This reaction quenches the concentration of ozone reaching the sites where oxidation products are toxic (predominantly cellular membranes) and eventually reduces damages. Apparently the anti-oxidant action of isoprene is very efficient and effectively cooperate or replace other defense mechanisms against oxidative stresses, many of which are also activated by the synthesis of more complex isoprenoids (e.g. carotenoids).
Isoprenoid synthesis and emission is a character that has been often lost in the most recent plant species. Our results suggest that this trait was essential to resist violent oxidative stresses and could become again useful once these stresses occur more frequently as it is nowadays the case because of the anthropic and industrial pollution and of the consequent enrichment of the tropospheric ozone layer. The isoprenoids are molecules relatively easy to manipulate because the synthesis is controlled by several but well described environmental and genetic factors. If the role of isoprenoids in nature is so important as we demonstrated in our laboratory experiments, then it will be relatively easy to transfer this important trait in plants of agricultural and forest interest to increment their productivity or their survival rate in harsh environments characterized by heavy oxidative pressure, and more efficiently phytoremediate gaseous pollutants.
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