Isoprene: from the cell to the environment
Author(s)
Fortunati, Alessio
Date Issued
February 10, 2009
Type
Doctoral Thesis
Abstract
One of the most relevant aspects reported in the last Intergovernmental Panel on Climate Change is represented by the projections of global warming as well as the consequent possible effect of rising temperature on ecosystems. The expected negative
impacts of global climate change makes it very important to generate crop and forest plants that will have enhanced resistance and tolerance to environmental stress conditions and, in particular, to the
combined stress caused by sunlight, and rising temperature and drought. Drought and high temperatures are often concurrent factors in nature, and the combined effect of these two environmental constraints should be determined. Moreover, current global change models predict that lower frequency of rainfall days and longer dry
intervals associated with atmospheric warming will affect large areas
of the globe. Thus, it is of paramount importance to study the acclimatory responses of plants to the interactive effects between water stress and rising temperature.
Drought and temperature represent the main rising variables of the global warming, which could affect not only the plant response to environmental stress conditions, but also the atmospheric chemistry.
This is particularly true if we apply it to the forest ecosystems.
Terrestrial vegetation, especially tropical rain forest, releases vast quantities of carbon as volatile organic compounds (VOC) into the atmosphere of which the most important is isoprene. The emissions
of these volatile isoprenoids are strictly dependant on environmental factors and they are highly reactive with correspondingly short lifetimes. Volatile isoprenoids can be removed by oxidation reactions; this reactions chain is mainly initiated by hydroxyl radicals (OH),primarily formed through the photo-dissociation of ozone. The
consequence of forest degradation, especially in the Amazon region,would be to reduce the VOC emissions, with a subsequent impact on ozone levels at the surface. As pointed out by Betts et al. (2008) “Isoprene emissions have a significant impact on the projected future surface ozone levels. Ignoring vegetation changes has meant that
future simulated ozone levels were greater by 5-10 ppbv, owing to larger isoprene emissions. This may have implications for air quality in the region, with potential implications for the health of humans,
animals, ecosystems and crops”. All together, these considerations
reveal the possible impact of volatiles isoprenoids, and in particular isoprene, on the ecosystems, especially on forest ecosystems.
These metabolites are involved in numerous biological processes, such as electron transport, photosynthesis, hormonal regulation, membrane fluidity, and plant defense responses, or communication
with other organisms. In planta, isoprene is considered an important molecule for ameliorating abiotic stresses. Isoprene may increase thermotolerance particularly when leaves are exposed to transiently
high temperatures. Despite the large convincing experimental evidences, the mechanism by which isoprene exerts its protective action is not completely understood. The impact of drought on
isoprene emission has also been studied, since drought is a major limitation for plant growth worldwide, and the intensity and frequency of drought conditions is going to increase with current and future
climate change. The available evidence suggests that isoprene biosynthesis is relatively unaffected by drought. Episodes of severe drought stress may lead to suppression of photosynthesis, while
isoprene emission is only slightly reduced.
While numerous isoprenoids are formed in and emitted by leaf chloroplasts, where they likely exert their protective action, plants
also emit volatile compounds from their flowers. The (E)-- caryophyllene synthase (TPS27) catalyzes the formation of (-)-(E)-- caryophyllene the major sesquiterpene emitted from Arabidopsis
floral tissues. It has been hypothesized that (E)--caryophyllene can
play a role in the attraction of floral visitors, leading to low levels of
cross-pollination and increasing reproductive fitness in natural
populations. However, given the low levels of volatile emission, it is quite possible that the (E)--caryophyllene biosynthesis have a function besides, or in addition to, pollinator attraction.Though recent
studies support that constitutive (e.g. isoprene, monoterpenes and sesquiterpenes) terpene volatiles released from vegetative tissues of different plant species exposed to abiotic stress could serve as mediators of thermotolerance or in protecting cells against oxidative stress, there is limited information available about the possible roles of sesquiterpenes in the physiological response of plants to environmental stress conditions.
Many questions about the biological functions of isoprenoids are still open. Even when considering the large amount of studies on the ecological and physiological impact of the isoprenoids release into
the atmosphere from vegetation, there is only partial and incomplete
information about function, biosynthesis, and regulation of these compounds in the plant cell. It is important to fill this “gap” especially when considering the important role played by these compounds in
the defense system against environmental stress. In particular isoprene seems to protect leaves from high temperature and
oxidative damages. Given the specific and active function of isoprene in the plant defense system, it is expected that isoprene biosynthesis be specifically and strongly regulated. The goal of this work is to
provide an answer to some of these points not yet clarified. Using different plant species and different experimental approaches we will try to elucidate the regulation of isoprene function in the nature; it will be shown that isoprene is able to enhance thermo-tolerance also in
plants which naturally do not emit isoprene (like Arabidopsis), and that this protective role is not only directly played, scavenging ROS excess or helping membrane stability, but has also an indirect component, priming the defense system against thermal and
oxidative stress. New biophysical evidences supporting the hypothesis that isoprene may also be able to directly interact with
chloroplastic membranes increasing their stability during the stress are also presented.
Similarly to isoprene, many questions regarding another class of isoprenoids, the sesquiterpenes, are still open. Genetically
engineered model plant species, such as Arabidopsis thaliana, overexpressing enzymes that regulate sesquiterpene biosynthesis in
plants, may constitute a suitable tool to study the physiological role of
sesquiterpenes in plants. Following this line, a characterization of the plants overexpressing the TPS27 gene will be shown, and how those plants respond to environmental stress, compared to wild-type
plants, will be assessed. We used this tool to demonstrate that (E)-- caryophyllene is able to enhance plant thermotolerance, and also that this protective role is triggered by similar mechanisms to those
suggested for isoprene. Finally, all together these results reveal a pattern that could be assimilated to that also obtained with the Arabidopsis plants transformed to overexpress the isoprene synthase gene. We can surmise that all volatile isoprenoids exert a similar protective action increasing plant resistance to high temperatures. Looking beck the results obtained with isoprene and (E)--caryophyllene, looking on the recent variety of works on these
and other isoprenoids, and also on the evolutionary history of isoprenoid biosynthesis, it is possible that plants emit volatile compounds, as a part of the secondary metabolism, combining them
to release a bland of molecules with specific functions, independently
from the plant species. Plants have been developed evolving their capacity to regulate more and more finely the biosynthesis of even more specific family of isoprenoids to better respond to specific
environmental. Therefore, filogenetically distant compounds, like isoprene (an hemiterpene) and (E)--caryophyllene (a
sesquiterpene), synthesized and emitted by the same plant produce similar effect, showing the same biological function of thermal protection. This ‘merging’ hypothesis brings beck the explanations,
not only the thermo-tolerance, for the isoprenoid emission to a
unique multiple biological function: protection of plants against the
environmental stress conditions, and enhancement of the plant fitness. The ‘merging’ hypothesis overlap isoprenoids coming from different plant species suggesting that the biosynthesis and utilization or not of a specific volatile compound in two different species should
not be attributable to the fact that this compound may be or not ‘waste of carbon’, but because that plants have evolved in a different way, adapting their emission pattern to different environments, and,
consequently, selecting different isoprenoids to respond to similar phenomenon.
Additional information
Dottorato di ricerca in Ecologia forestale
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