Biology Reference
In-Depth Information
acceleration in the degradation of photosystem II D1, D2 proteins (
Booij-
James et al., 2000
).
UV-B radiation can also affect lignin gene expression in organs not directly
exposed to this stress. In one recent study, it was shown that PAL, CCR,
CAD, genes were upregulated in light-shielded organs of maize plants where
only the canopy leaves were exposed to radiation. The authors suggest that
this might be part of a general acclimation response to UV-B radiation
(
Casati et al., 2011
).
The phenylpropanoid and lignin pathways seem to be necessary for both
acclimation and defence responses to UV-B light in both short- or long-term
exposure situations. Carbon partitioning into and within these pathways also
appears to play a central role in privileging one or the other response.
H. LIGHT
Light is one of the most important environmental factors for plant develop-
ment and the energy source for photosynthesis. The photosynthetically
active radiation (400-700 nm) regulates a number of different physiological
processes and metabolic pathways.
Phenylpropanoid and lignin biosynthetic pathways are regulated by the
photoperiod. In Arabidopsis, the genes involved in monolignol biosynthesis
(PAL1, C4H1, CCR1 and CAD6) display a diurnal cycle of expression
with two peaks—one of them just before dawn and the other one between
0.5 and 4 h after the beginning of the photoperiod (
Rogers et al., 2005
). Some
of these genes and others of the monolignol pathway (C4H1, COMT,
CCoAOMT1, CCR1 and CAD6) show a circadian oscillation in their tran-
script abundance. Similar results were obtained from developing xylem of
Eucalyptus trees (
Solomon et al., 2010
). One peak of expression levels
occurred in mid-afternoon and may be associated with producing phenolic
compounds for oxidative stress protection, and the other one occurred just
before dawn possibly for the reinforcement of new cell walls.
The phenylpropanoid pathway is also controlled by light quality, as well as
by light quantity. Roots of Arabidopsis plants lacking phytochrome B (PHYB)
contained less monolignol glucosides than wild-type plants (
Hemm et al.,
2004
). Such an observation could suggest that the photoreceptor for red
light, PHYB, is important for phenylpropanoid pathway activity. The photo-
receptor for blue-light cryptochrome (CRY2), acting in concert with PHYB, is
also required for the accumulation of phenylpropanoids. Members of the PAL
gene family are differentially regulated according to the duration of light
treatment or wavelength and flux rate. The PAL3 gene was significantly
upregulated in roots of Arabidopsis plants exposed for 1 h to red light but
