Isoprene is a hydrocarbon volatile compound emitted in high quantities
by many woody plant species, with significant impact on atmospheric
chemistry.
The Australian Blue Mountains and the Blue Ridge Mountains in the
Eastern United States are so called because of the spectral properties of
the huge amounts of isoprenes emitted from the trees growing there.

Although a positive correlation has been observed between leaf
temperature and isoprene emission in plants, the physiological role of
isoprene emission, which is clearly quite costly to the plant, is still
under vigorous debate.

One of the most popular hypotheses suggests that isoprene protects the
metabolic processes in the leaf, in particular photosynthesis (the process
by which plants use light energy to fix CO2 and produce their own "food"),
against thermal stress.

To test this hypothesis, scientists Katja Behnke and Jörg-Peter
Schnitzler from the Institute for Meteorology and Climate Research of the
Research Centre Karlsruhe and Garmisch-Partenkirchen in Germany, together
with colleagues from the Universities of Braunschweig and Göttingen, also in
Germany, and British Columbia, in Canada, recently applied genetic
engineering techniques to obtain transgenic Grey poplar (Populus x
canescens) trees with decreased isoprene emission, and examined their
tolerance to heat.

Behnke et al. engineered such poplar trees by suppressing the
expression of the gene encoding isoprene synthase (ISPS), the enzyme
producing isoprene, by RNA interference (RNAi). They then subjected these
trees to transient heat phases of 38-42°C, each followed by phases of
recovery at 30°C, and measured the performance of photosynthesis.

In these experiments, Behnke et al. observed that photosynthesis in
trees that no longer emitted isoprenes was much less efficient under such
repeated "heat shocks" (a situation that is similar to what happens in
nature, where temperatures around the leaves often oscillate, with short
heat spikes). Thus, their results clearly indicate that isoprenes have an
important role in protecting the leaves from the harmful effects of high
ambient temperature.

How does isoprene confer heat tolerance? Does isoprene act as an
antioxidant due to its chemical reactivity? And more generally: Is this
effect of significance under natural conditions for poplar and other
isoprene-emitting species? The researchers aim to analyse the biophysical
and biochemical mechanisms of heat effects on photosynthesis and
chloroplasts, and future long-term field trials will test whether the
isoprene effect represents a positive adaptive trait for isoprene-producing
species.


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