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post-transcriptional level while in the RNAi lines, there was no isoprene emission.
The researchers also exposed transgenic lines to high temperature with three tem-
porary heat stages (38-40 °C), followed by recovery at 30 °C. During heat stress,
the non-isoprene-emitting transgenic poplars exhibited low rates of net assimilation
and photosynthetic electron transport, compared to situation where there was no
stress. The poplars plants in which isoprene was repressed had an increased zeaxan-
thin in the absence of stress, suggesting increased non-photochemical quenching
or may indicate an increased necessity for antioxidants (Behnke et al.
2007
). This
study demonstrated that down-regulation of isoprene can influence thermotolerance
and induce increased energy dissipation by non-photochemical quenching path-
ways. Isoprene synthase transcription has been shown to increase as leaves undergo
maturity (Wiberley et al.
2005
) and is temperature- and light responsive (Sasaki
et al.
2005
; Cinege et al.
2008
). Variation in the accumulation of isoprene synthase
protein is also observed under different environmental conditions (Schnitzler et al.
2005
; Wiberley et al.
2009
; Calfapietra et al.
2007
).
Transgenic tobacco (
Nicotiana tabacum
L.) plants transformed with an isoprene
synthase gene (from poplar) showed isoprene emission at comparable amounts to
a natural situation. These transgenic plants when subjected to heat and combined
heat/light exhibited considerable tolerance to stress-induced oxidative stress (Vick-
ers et al.
2009b
). Further, Vickers et al. (
2011
) used transgenic tobacco lines harbor-
ing a poplar isoprene synthase gene and then examined control of isoprene emis-
sion. In mature transgenic tobacco leaves, it was observed that primary controls on
isoprene emission was thought to be via the substrate supply and changes in enzyme
kinetics rather than changes in isoprene synthase levels or post-translational regula-
tion of activity. The transgenic tobacco plants also had emission patterns remark-
ably similar to naturally emitting plants under a wide variety of conditions and the
emissions correlated with photosynthetic rates in developing and mature leaves, and
with the amount of isoprene synthase protein in mature leaves. Isoprene synthase
protein levels did not change under short-term increase in heat/light, despite an
increase in emissions under these conditions. In a study with a halophytes (
Kande-
lia candel
) and
Bruguiera gymnorrhiza
, mRNA expression of four oxidosqualene
cyclase (
OSC
) genes namely,
KcMS
multifunctional terpenoid synthase
and
Kc-
CAS cyloartenol synthase
(
K. candel
),
BgbAS ß-amyrin synthase
and
BgLUS lupeol
synthase
(
B. gymnorrhiza
) in relation to salt concentration was analyzed (Basyunia
et al.
2009
). The mRNA levels of
KcMS
in both roots and leaves of
K. candel
and
BgLUS
and
BgbAS
in the roots of
B. gymnorrhiza
increased with salt concentration.
This result suggested that the function of terpenoids in root is associated with the
salt stress.
Attempts have been made to over-accumulate isoprenoids in transgenic plants
to study their role in stress alleviation. Over-expression of
Hevea brasiliensis
3-hy-
droxy-3-methylglutaryl coenzyme A reductase (HMGR) in transgenic tobacco led
to an increase in sterol production (Schaller et al.
1995
). Neelakandan et al. (
2011
)
over-expressed Arabidopsis HMGR1 in soybean, resulting in greater seed sterol
content. The
Populus alba
isoprene synthase gene was introduced into
Arabidopsis
and has shown to confer elevated heat tolerance in the transgenic lines over wild
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