Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 196

level and the activity of phenylalanine ammonia-lyase (an enzyme also involved in
SA biosynthesis) rapidly increased during the first hour, and then declined (Wang
et al. 2005 ). The determination of the SA and ABA contents during heat accli-
mation in the presence of SA and/or ABA biosynthesis inhibitors revealed an
interesting relationship between the endogenous SA and ABA levels. An SA
biosynthesis-related benzoic acid-2-hydroxylase enzyme inhibitor was unable to
influence the maximum peak of free SA content, but the peak of SA-2-O-b- D -
glucose (SAG) was missing, while treatment with an ABA biosynthesis inhibitor
resulted in the disappearance of the free SA peak during heat acclimation. The
ABA peak induced by heat treatment disappeared in the presence of ABA bio-
synthesis inhibitor, rather than in that of the SA synthesis inhibitor (Liu et al.
2006 ). It was concluded that the inhibition of ABA biosynthesis did not lead to as
great a loss of thermotolerance as that of SA biosynthesis. It was also found that
the importance of conjugated SA in the development of thermotolerance was
analogous to that of free SA or heat acclimation.
However, the use of Arabidopsis genotypes with modified SA signalling
showed that SA-dependent signalling plays a role in the maintenance of basal
thermotolerance: 0.5 or 1 mM SA pre-treatment promoted basal thermotolerance
in 3-week-old Arabidopsis plants (Clarke et al. 2004 ).The level of endogenous SA
correlated with basal thermotolerance. It was also shown that SA is not essential
for acquired thermotolerance: all the genotypes could be heat acclimated irre-
spective of their endogenous SA content. It may be that SA is dispensable because
heat acclimation is always initiated by some other key factor(s), or because SA is
only one of multiple alternative acclimation signals. If SA was a potential mediator
of a heat-induced acclimation response, the SA levels should be heat-inducible.
A moderate, transient increase in glucosylated SA during heat treatment was also
detected in wild-type Arabidopsis plants. Given the significant basal levels of SA
in Arabidopsis and mustard, the extent to which such changes in the SA metab-
olism provide additional thermotolerance is uncertain, but they suggest that despite
the metabolic stress caused by heat treatment, the plants actively maintained the
biosynthesis of this hormone. In plants, SA is subject to glucosylation, making it
more suitable for translocation (Seo et al. 1995 ) or vacuolar localization (Dean
et al. 2003 ). Heat-induced increases in SA were not apparent in the other Ara-
bidopsis genotypes, although this might be because of their altered SA metabolism
or signalling (Clarke et al. 2004 ).
2.5 Cold Stress
2.5.1 Low Temperature and Cold Tolerance
Each plant has a unique temperature requirement, which is optimum for proper
growth and development. For crops, low temperature is one of the most important
factors restricting cultivation in a given area. Plants are said to be cold-sensitive