Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 207

The interactions between ethylene, JA and SA induced by ozone, and the inhi-
bition of the ethylene or SA pathways by JA have also been investigated. Pre-
treatment with methyl JA hindered the accumulation of SA or H 2 O 2 , thus preventing
ozone-induced necrosis (Rao et al. 2000 ). It was also shown that ROS-mediated SA
biosynthesis and that the SA signalling pathway in an ozone-sensitive ecotype was
influenced by the JA pathway (Rao et al. 2000 ). The results of a cDNA microarray
assay indicated that ozone-induced defence gene expression was mainly regulated
by ethylene and JA, and that the SA pathway acted as a strong antagonist to gene
expression induced by ethylene and JA signalling (Tamaoki et al. 2003 ). During
ozone exposure, transgenic plants with reduced ozone-induced ethylene production
accumulated less SA than wild-type plants. Ozone increased the activity of phen-
ylalanine ammonia-lyase and the transcript levels of the chorismate mutase and
phenylalanine ammonialyase genes in wild-type tobacco, but their induction was
suppressed in transgenic plants. These results indicate that ethylene promotes SA
accumulation by regulating the expression of chorismate mutase and phenylalanine
ammonialyase genes in tobacco plants, exposed to ozone (Ogawa et al. 2005 ).
Contrary to these results, the NahG transgenic line and the npr1Arabidopsis mutant
failed to produce ethylene in response to ozone, indicating that the SA pathway is
required for ethylene signalling (Rao et al. 2002 ).
During the investigation of complex phytohormone responses during the cold
acclimation of two wheat cultivars differing in cold tolerance, three phases were
distinguished (Kosová et al. 2012 ). The alarm phase was characterized by an
increase in ABA and in the content of ACC, and by a decrease in CK, GA 3 and
IAA. The upregulation of the ABA content coincided with the rapid down regu-
lation of SA and JA. During the acclimation phase the ABA level was down
regulated, while the levels of CK, GA 3 and IAA exhibited a transient maximum.
The decrease in ABA during the acclimation phase coincided with the elevation of
ACC and SA. The plants become resistant after 21 days of cold treatment. The
ABA content was maintained at a higher level and the SA level exhibited a gradual
decrease in winter wheat leaves, but it was still significantly higher than that in
spring wheat. The JA content was also higher in the leaves of the tolerant geno-
type, the increase occurring after 7-21 days of cold treatment. As JA inhibits cell
division and growth in a similar manner to ABA, it is possible that JA may replace
ABA in suppressing growth during later stage of cold stress. The ACC levels did
not differ between the genotypes. Parallel to this, lower levels of the growth-
promoting hormones CK, GA 3 and IAA were observed in tolerant genotypes,
while relatively high CK levels were maintained in the sensitive genotypes with a
mild elevation in the IAA levels. These results indicated that the cross-talk
between the different plant hormones results in synergetic or antagonic interactions
that play a crucial role in the response of plants to cold stress, as in the case of
other abiotic stresses.
In another study on Pinus radiate a decrease in CK level was found to be the
primary signal of drought, while IAA and ABA accumulation represented a sec-
ondary signal. While the CK level further decreased, the ABA and IAA levels
doubled. At the end of the drought period, less tolerant genotypes exhibited an over