Geoscience Reference
In-Depth Information
search for metabolic and genetic factors involved in cold toler-
ance. One of the immediate responses of cold-tolerant plants to
low temperature is an increase in the proportion of unsaturated
fatty acid bound to lipids associated with the plasma membrane,
mitochondrial membranes as well as thylakoid membranes.
Various biophysical measurements indicate that this ensures
the membrane can remain in a more fluid and less gel-like state
at lower temperature which enhances membrane stability and
function at these low temperatures. A change in the membrane
from the fluid state to a more solid state is marked by an abrupt
change in the membrane activity. The temperature at which this
transition occurs is known as the transition temperature. This
means that at temperatures above the transition temperature,
the membrane remains fluid but becomes more solid or gel like
at temperatures below the transition temperature. This allows
higher activity of membrane process at lower temperature.
Cold acclimation of herbaceous plants induces change in gene
expression. During acclimation, there are changes in m-RNA
transcription and increases in protein synthesised. A major class
of cold-induced genes encode homologs of late embryogenesis
active proteins that are synthesised late in embryogenesis and
during dehydration stress. These polypeptides fall into a number
of families based on amino acid sequence similarities. However,
these proteins encoded by cold-regulated genes share common
physical properties.
The promoter regions contain cold-regulated genes that are
activated in response to low temperature and dehydration stress.
Analyses of these promoter regions identify a DNA regulatory
element called dehydration responsive element (DRE). The
DRE has a conserved core C-repeat sequence of CCGAC that
imparts responsiveness to low temperature and dehydration.
Specific proteins that bind to the DRE are called C-repeat bind-
ing factors (CBFs). Thus, CBFs are transcriptional activators
that are involved in regulating the expression of cold-regulated
genes. It is concluded that cold acclimation is regulated by a
family of CBF transcriptional factors.
Q
10
is the
temperature
dependent for
plant respiration
Enzymes and enzyme reactions are sensitive to temperature.
Enzyme reactions typically are considered to have a Q
10
of
about 2, which means that the rate of the reaction doubles for
each 10°C rise in temperature. The rate of reaction increases
with temperature until an optimum is reached, beyond which
the rate usually declines sharply; the decline in enzyme activ-
ity is normally caused by thermal denaturation as a result of
protein unfolding. It is usually assumed that Q
10
is independent
