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

Freezing Stress
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with physiological data reflecting photosynthetic acclimation to mild stress and photosynthetic
failure due to severe stress. Greater changes in gene transcript levels under
mild stress was associated with physiological acclimation. The number of clones showing
either positive or negative changes in transcript levels increased from 5.8 % to 8.6 %
between mild cycles 1 and 2 suggesting the behavior of these genes was correlated
with acclimation. The number of genes showing negative changes indicates the importance
of down regulation as well as up regulation in acclimation to drought stress. No
comparable changes occurred when plants were grown under severe water deficit.
Group 2 LEAs, flavanoid enzymes and genes associated with mitochondrial electron
transport were upregulated during photosynthetic acclimation under mild drought stress.
Carbon metabolism enzymes, that provide carbon compounds to other metabolic processes,
pyruvate kinase and pyruvate dehydrogenase, increased during mild stress.
During severe stress, reduced transcript levels of genes associated with the reductive
pentose phosphate pathway was observed, suggesting decreased photosynthesis which
was correlated with decreased photosynthetic rate. Regulation of genes associated
with polyamines involved in senescence and dormancy were observed.
A number of microarray analysis projects have been published that isolated
and characterized genes involved in cold acclimation. Three studies were on
Arabidopsis, which has limited hardiness (Seki et al., 2001; Fowler and Thomashow
2002; Kreps et al., 2002) and one on sugarcane (Saccharum sp), a chilling sensitive
plant which has no potential to cold acclimate (Nogueira et al., 2003). Fowler and
Thomashow (2002) treated Arabidopsis plants for 7 days at 4 °C and profiled the expression
patterns of approximately 8000 cDNAs using micro-array analysis and found
306 genes were cold responsive with 218 genes up regulated and 88 genes downregulated.
Of the genes that were up-regulated, 64 genes were expressed during the
cold treatment and 156 genes were transiently expressed. Over 50 genes expressed
during the cold treatment had not been previously identified as cold responsive. Seki
et al. (2001) studied the expression patterns of approximately 1300 full length cDNAs
isolated from Arabidopsis under control, drought and cold temperature treatments
against cDNA isolated from transgenic and wild type Arabidopsis and demonstrated
19 genes induced by cold, of which 10 genes were not previously identified as cold
responsive. Nogueira et al. (2003) identified numerous cold inducible ESTs, from sugarcane
showing a chilling sensitive plant has the ability to respond to acclimating
temperatures.
The majority of work regarding cold acclimation has been conducted in cereals
and Arabidopsis. Arabidopsis is ideal to study because it is a small plant with a
short life cycle and a small genome that has been recently sequenced (The Arabidopsis
Genome Initiative 2000). Much knowledge has been gained from Arabidopsis and as a
result comparisons have been made with B. napus, to determine if differences in gene
regulation exist (Weretilnyk et al., 1993; Girke et al., 2000). B. napus is an economically
important crop grown for its edible oil. In Canada, 6,669,000 tonnes of canola was
produced in 2003 (Statistics Canada, 2003) worth approximately 2.3 billion dollars to the