Biology Reference
In-Depth Information
UV-B radiation induced an increase in lignin content (56%) in comparison
with plants exposed to ambient UV-B (
Rozema et al., 1997
). This lignin
accumulation limits the microbial degradability in leaf litter. Similarly,
cucumber (Cucumis sativus) seedlings exposed to continuous UV-B radiation
for 15 days showed an accumulation of phenolic compounds in epidermal
cells and sharp-headed trichomes of cotyledons. In addition, lignin was
deposited in cotyledon sharp-headed trichomes over a short period from
day 9 to day 15 (
Yamasaki et al., 2007
). It was suggested that sharp-headed
trichomes may play an important role in sensing and transmitting signals
during defence mechanism induction.
Detailed information on the effects of UV-B radiation on phenylpropa-
noid metabolism has come from studies of different cell culture systems.
UV-B radiation induced the expression of the gene encoding DAHPS
(3-deoxyarabinoheptulosonate-7-phosphate synthase), an enzyme of the shi-
kimate pathway providing substrates for the phenylpropanoid pathway in
parsley cultures. DAHPS was induced immediately after applying UV-B radi-
ation followed by PAL gene induction 2 h later (
Logemann et al., 2000
).
Similarly, transcript levels of the gene encoding chorismate mutase, the enzyme
providing phenylalanine and tyrosine, were increased inmaize plants irradiated
with UV-B (
Casati and Walbot, 2003
). UV-B radiation applied to suspension-
cultured cells of carrot (Daucus carota) immediately induced the expression of
the DcMYB1 gene followed by the expression of DcPAL1. DcMYB1 encodes
an MYB transcription factor and is similar to the AtMYB15 gene that is
induced by wounding and controls the expression of genes in the shikimate
pathway (
Maeda et al., 2005
). In this study, the stimulation of the shikimate
pathway was necessary to provide sufficient precursors for the production of
cell wall and phenolic UV-B-absorbing compounds.
In another study, increased production of UV-B-absorbing compounds
(sinapate esters) was shown to coincide with enhanced expression of the C4H
gene.
Jin et al. (2000)
characterized an Arabidopsis mutant for the R2R3
MYB gene AtMYB4. The mutant is tolerant to UV-B radiation, accumulates
high transcript levels of C4H and contains high levels of sinapate ester.
Analyses indicated that UV-B radiation repressed AtMYB4 expression in
Arabidopsis wild-type plants associated with C4H gene induction and phe-
nolic accumulation. In this case, the AtMYB4 acts as a repressor of the C4H
gene (
Jin et al., 2000
).
Another Arabidopsis lignin mutant is similarly compromised in its UV-B
tolerance. The fah-1 mutant is unable to synthesize the F5H enzyme resulting
in the absence of sinapic acid and its ester sinapoyl malate (
Chapple et al.,
1992
). Although the mutant is still able to produce flavonoids, it is more
sensitive to UV-B radiation than WT plants and UV-B exposure leads to an
