Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

Freezing Stress
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Whereas, in the field, symptoms of photoinhibition may only be apparent in the field
prior to freeze occurring under full sunlight or when day temperatures approach 0 o C.
At this time, the plants have reached their full cold hardiness potential and have also
acclimated to high light and low temperature conditions (Strand and Öquist 1985).
However, if plants undergo photoinhibition during the initial stages of hardening, the
full hardiness potential will not be achieved because adequate photosynthates are not
produced to drive the metabolism necessary for cold acclimation to occur. Thus, controlled
environment chambers may produce an unrealistic picture of the process of cold
acclimation. Cold acclimation is a multigene trait and it is very difficult to ascertain
which genes are causal and which are induced by unrealistic, artificial conditions. The
experimenter may argue since changes in nature are so slow and subtle it is difficult or
impossible to measure changes in gene expression associated with cold acclimation,
but the genes and proteins isolated from plants treated in this manner may represent a
shock versus a stress response. This may or may not be true, but if many genes are
upregulated that are only associated with a shock and not acclimation, how will the
causal genes be discovered?
Artificial cold acclimating conditions can be a major limitation in identifying
genes or proteins causally associated with freezing tolerance. Sudden changes in
temperature can put the plant in a state of cold shock and the genes upregulated under
these conditions may be more closely related to drought and chilling injury. For example,
hardy temperate plants such as winter cereals of canola when transferred from a
20 o C to 4 o C display symptoms of wilting for the first 24 to 48 hours of transfer. Such
dramatic changes in temperature rarely occur in nature and are not characteristic of the
natural environmental signals. During natural cold acclimation which requires days to
weeks, membranes become less saturated (Yoshida 1984), metabolism adjusts for low
temperature growth (Levitt 1980) and photoinhibition is compensated for (Strand and
Öquist 1985). In addition, pots in controlled environment chambers cool rapidly in
contrast to the large mass of the earth’s surface. Therefore, the roots undergo a shock
that alters their hydraulic conductivity.
Therefore, some of the changes that occur when plants are exposed to rapid
changes in temperature (20 o C to 4 o C within minutes) stress may not be related to cold
acclimation. For example, Hammond et al. (2003) in a study on phosphate starvation
reported the expression of 60 genes was transiently up-regulated four hours after withdrawing
P. Several of these genes are involved in cell rescue and defense. These
authors concluded that many of these genes are ubiquitous “shock” response genes
up-regulated that have been shown to be upregulated by various pathogens and environmental
perturbations (Desikan et al., 2001, Kreps et al., 2002). The following is a list
of shock associated proteins; chitinases, peroxidases, and PR-1 like protein (Mackerness
et al., 2001); cytochrome P450, a C2H2-type zinc finger protein and a blue copperbinding
protein (Desikan et al., 2001) and proteins associated with reactive oxygen
species (Gonzalez-Meler et al., 2001). Following the initial shock the genes up-regulated
may be more closely related to the stress of interest. This lull in differential gene