The results can be compared with th

The results can be compared with the gene expression profile
reported in former work in which the grape whole transcriptome
response to UV-B was analyzed in an analogous experiment
(Pontin et al., 2010). The experimental design used allowed to
compare two situations; one in which a field-like UV-B dose
(4.75 kJ m2 d1) was administrated at a low irradiance, other
where the same dose was given in a rather high irradiance but in
a narrower period, and a control where UV-B was excluded. This
experimental approach allowed comparison and contrast of two
purported responses, one in which the UV-B signal would induce
adaptation of the plants to this radiation, and the other in which
the plant may respond to UV-B-induced damage. In plant tissues,
UV-B exposure causes an increase of the excitation energy, which
may lead to reduction of oxygen and therefore to generation of
ROS. These compounds are mainly responsible for UV-B-mediated
damage, but can also act as second messengers in different signaling
pathways (Hideg et al., 2002). In fact, ROS are considered signaling
molecules that initiate defense responses and their levels
increase after UV-B pulses (Allan and Fluhr, 1997). In this report,
it is shown that UV-B induces in grape leaf tissues a modulating effect
on isoprenoid metabolism and this was differentially affected
by fluence rate of UV-B.
The synthesis of the terpenes is initiated from isopentenyl and
dimethylallyl diphosphate precursors. Two independent pathways
contribute in higher plants to the formation of building block (C5)
of isoprenoids. In general, the cytosolic mevalonate pathway
(MVA) provides the precursors for sesquiterpenes, and sterols,
whereas the plastidial methylerythritol pathway (MEP) furnishes
the monoterpenes, diterpenes, tetraterpenes, prenyl moieties of
chlorophyll, plastoquinone, and tocopherol (Hampel et al., 2005;
McGarvey and Croteau, 1995; Tholl, 2006). The C5 units are condensed
by prenyltransferases to synthesize geranyl diphosphate
(C10), farnesyl diphosphate (C15) and geranylgeranyl diphosphate
(C20). These prenyl diphosphates in turn undergo a wide range of
cyclizations and other transformations to produce the various
structural types of mono-, sesqui- and diterpenes. These steps
are catalyzed by terpene synthases (TPS), a very large family of
enzymes with multiple representatives in all plant species studied
so far (Degenhardt et al., 2009; Facchini and Chappell, 1992; Steele
et al., 1998; Wise et al., 1998), including grapevine (Lücker et al.,
2004; Martin et al., 2010). Grapevine leaf tissues exposed to
UV-B radiation showed increased TPS activity, and the response
was different depending on the irradiance level and the leaves
ontogeny. In this sense, the amount of substrate 3H-FPP (12) transformed
via the mevalonate pathway (MVA; Lichtenthaler et al.,
1997) was higher under low UV-B, and mainly in young leaves exposed
to both UV-B treatments. Such increase in TPS activity was
associated with augments in the membrane content of sitosterol
(1), stigmasterol (2) and lupeol (3), that is, enhancement of the sterol-
structural defense. This is in agreement with Sabater-Jara et al.
(2010), who observed that accumulation of sterols was induced as
defense response in elicited cell cultures of Capsicum annuum.