Biology Reference
In-Depth Information
The phenylpropanoid pathway is stimulated by low temperatures; however,
variations are observed according to the genes/enzymes and organs/tissues
analysed. In maize seedlings transferred to low temperature (10
C) for 48 h,
PAL gene expression was constant during the first 12 h and then increased
during the next 12 h. 4CL gene expression was induced rapidly within 6 h
and remained unchanged for the following 36 h. When plants were returned
to ambient temperature (25
8
C) for 48 h, PAL and 4CL gene expression
returned to their initial expression levels (
Christie et al., 1994
). In the stem
of Arabidopsis plants exposed to light and subjected to low temperature,
PAL1 gene expression was observed in cortical cells in addition to the normal
expression pattern in protoxylem (
Leyva et al., 1995
). Cold acclimated
soybeans also showed a stimulation of PAL activity and accumulation of
hydroxycinnamic acid esters in root cells (
Janas et al., 2000
). In leaves of
winter oilseed rape plants (Brassica napus) subjected to cold acclimation and
freezing, PAL activity was enhanced in mesophyll cells accompanied by an
accumulation of hydroxycinnamic acids. The content of ferulic acid was
increased in mesophyll cell walls subjected to freezing following the period
of cold acclimation. It was hypothesized that these changes were associated
with an increase in cell wall rigidity to prevent mechanical stress caused by
water freezing. Inhibition of PAL activity abrogated the cold-promoted
accumulation of hydroxycinnamic acids (
Solecka and Kacperska, 2003
).
Analysis of lignin gene expression in winter barley exposed for 21 days to a
chilling temperature of 3
8
C and followed by 1 day at freezing temperature of
8
C showed that PAL, HCT, CCR1, CAD were upregulated in leaves after
1 day of acclimation and that high expression levels were maintained under
freezing. In contrast, however, most of the lignin genes were downregulated
in the crown after 1 day of acclimation (
Janska et al., 2011
). The authors
suggested that monolignols, rather than lignin polymers, were synthesized in
acclimated leaves since peroxidase genes involved in polymerization were
downregulated. This hypothesis could not be validated, as measurements of
lignin content were not performed in this study.
A study of oak (Quercus suber) exposed for 53 days to two temperature
treatments (10 and 28
3
8
C) showed that PAL and CHS (chalcone synthase,
gene encoding a flavonoid pathway enzyme) transcript levels were increased
at low temperature compared to high temperature. Conversely, CAD and
CS (chorismate synthase, gene encoding a shikimate pathway enzyme)
genes were more highly expressed under high temperature (
Chaves et al.,
2011
). The authors suggested that the coordinated expression of PAL and
CHS at 10
8
C could favour the synthesis of flavonoids, while high tempera-
ture would promote lignin synthesis. However, lignin levels were not eval-
uated in this study.
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