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also leads to significant increases in the levels of hsps90 dur-
ing thermal stress in
Brassica napus
(Dhaubhadel et al., 2002).
BR (24-epibrassinolide) treatments increase the gene expres-
sion of arginine decarboxylase (ADC) and improve IAA and
ABA levels under heavy metal chromium stress in
Raphanus
sativus
(Choudhary et al., 2012). ADC has an important role in
response to cold, oxidative stress, salt stress and seed develop-
ment. T-DNA knockout mutants of
OsGSK1
, a rice orthologue
of the BR-negative regulator BIN2, showed greater tolerance
to cold, heat, salt and drought stresses as compared with non-
transgenic plants (Koh et al., 2007).
Cytokinins
Cytokinins are plant hormones which are implicated in nearly
all aspects of plant growth and development, including apical
dominance, leaf senescence, nutrient signalling, shoot differ-
entiation, cell division and light responses (Kiba and Mizuno,
2003). Cytokinins homeostasis and signalling are changed and
regulated under various biotic and abiotic stresses. For exam-
ple,
Pseudomonas syringae
pv.
tomato
DC3000 (
Pst
), a bacte-
rial pathogen has a direct effect on cytokinins. The endogenous
cytokinins perceived by AHK2 and AHK3 receptors promote
SA signalling through ARR2 activation and association with
the promoters of pathogenesis-related (PR) genes, which lead to
enhanced plant immunity (Choi et al., 2010). Another instance
from a recent study has shown that cytokinin level and transport
are reduced by drought and/or salinity in various plant species
(Argueso et al., 2009; Perilli et al., 2010). Recent characterisation
of
Arabidopsis CKX
overexpression and the
ipt1
,
3
,
5
,
7
mutant
plants with reduced endogenous cytokinin levels revealed a strong
stress-tolerant phenotype that was associated with increased cell
membrane integrity and ABA hypersensitivity (Nishiyama et al.,
2011). The cold stress induces
ARR5
,
6
,
7
,
15
expression, and the
arr5
,
arr6
and
arr7
mutants exhibit enhanced freezing toler-
ance and ABA sensitivity similar to
ahk2
,
3
and
ahk3
,
4
mutants
(Jeon et al., 2010). In other recent studies, genome-wide microar-
ray analyses show that more than 10% transcriptional changes in
Arabidopsis
genes in the cytokinin-deficient
ipt1, 3,5,7
mutant
were compared to the wild type under both normal and salt stress
conditions (Nishiyama et al., 2012).
ethylene
Ethylene, as a gaseous plant hormone, is involved in many
aspects of the plant life cycle, including seed germination,
fruit ripening, organ abscission, pathogen response, senes-
cence, root hair development, root nodulation, abscission and
so on (Johnson and Ecker, 1998). The production of ethylene
