Agriculture Reference
In-Depth Information
ress has been made in the study of physiological mechanism(s) underlying iso-
prenoid synthesis under abiotic stress conditions, especially high temperatures and
oxidative stress conditions (Fineschi and Loreto
2012
).
Isoprenoids protect plants against different abiotic stresses through improving
the ability of plants to deal with cellular oxidative modifications, possibly through
reaction of isoprenoids with the oxidizing species, or alteration of ROS signaling, or
via membrane stabilization. It is postulated that dissolution of VOCs in membranes
coupled to interactions with membrane proteins can lead to changes in transmem-
brane potential and modulation of ion fluxes thereby inducing gene activity and a
subsequent cellular response to stress (Vickers et al.
2009a
). Plants have developed
an efficient antioxidant mechanisms for ROS detoxification (Ahmad et al.
2008
;
Gill and Tuteja
2010
; Ahmad and Umar
2011
). Isoprenes can boost plant's defense
system not only by keeping the membrane integrity intact and making it less sensi-
tive to denaturation, but also due to the fact that they have the capacity to quench
ROS produced under oxidative stress. Vickers et al. (
2009a
) discussed the possible
functions of isoprenes as natural antioxidant machinery in plants.
Plants are endowed with protective mechanisms to cope with a variety of abiotic
stresses. When the stress impact goes beyond a certain threshold, plants normally
experience stress, resulting in reduced growth and development. Most common and
ensuing response, thus, is the production of reactive oxygen spices (ROS). The
antioxidant effect of the isoprenoid compounds is mediated by their capacity to
swiftly combine with different ROS such as singlet oxygen, superoxide, hydro-
gen peroxide, hydroxyl radical that are released under stress regime (Holopainen
2004
; Fineschi and Loreto
2012
). Isoprenes are also known to alleviate visible dam-
age (necrosis) of leaves exposed to ozone through a mechanism involving release
of nitric oxide that interacts with increasing levels of ROS especially hydrogen
peroxide. The occurrence of conjugated double bonds (delocalized π-electrons) in
the isoprene molecule may mediate electron and energy transfers, conferring ROS-
scavenging ability (Vickers et al.
2009a
). Considering chloroplast as the site of iso-
prene biosynthesis (Logan et al.
2000
), the ROS scavenging ability of isoprene
molecule makes it important in plant defense against oxidative stress. Isoprenoids
including terpenoids have also been shown to confer a protective effect on photo-
synthetic process under heat and oxidative stress (Sharkey and Yeh
2001
). Isoprenes
have also been implicated to protect the photosynthetic system from thermal stress.
The mechanism underlying such protective nature is attributed to the stabilization
of membrane lipid bilayer by enhancing the hydrophobic (lipid-lipid, lipid-pro-
tein and/or protein-protein) interactions (Sharkey et al.
2008
). Based on modeling
studies with membranes, Siwko et al. (
2007
) demonstrated that isoprenes are able
to partition into the phospholipid membrane enhancing membrane order without
major alteration in the dynamic properties of the membrane.
Much less evidence has been accumulated so far on the role of volatile monoter-
penes in alleviating oxidative stress. In plants that don't emit monoterpenes, it has
been proved that photosynthesis becomes less sensitive to ozone that are externally
supplied with volatile monoterpenes (Loreto and Fares
2007
). In contrast, when
monoterpene synthesis is blocked, ROS rapidly accumulate. The highly volatile
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