Geoscience Reference
In-Depth Information
is tightly regulated by internal signals during development and
in response to environmental stimuli from biotic (e.g. fungal
and bacterial disease) and abiotic stresses, such as wounding,
hypoxia, ozone, chilling or freezing. Signalling responses to
ethylene in
Arabidopsis
are regulated and transferred by five
receptors called ethylene receptor1 (ETR1), ETR2, ethylene
response sensor1 (ERS1), ERS2 and ethylene insensitive4
(EIN4) (Chang et al., 1993; Hua et al., 1998). The receptors acti-
vate Raf-like protein kinase, CTR1, which negatively regulates
downstream ethylene-signalling events (Kieber et al., 1993).
During either biotic or abiotic stress condition, plants produce
increased levels of ethylene, called stress ethylene, which is able
to initiate various ethylene-regulated responses (Abeles et al.,
1992). Plants deficient in ethylene signalling may show either
increased susceptibility or increased resistance during biotic
stress. For example, in soybean, mutants with reduced ethyl-
ene sensitivity produce less-severe chlorotic symptoms when
challenged with the virulent strains
P. syringae
pv.
glycinea
and
Phytophthora sojae
, whereas virulent strains of the fungi
Septoria glycines
and
Rhizoctonia solani
cause more severe
symptoms (Hoffman et al., 1999).
Arabidopsis
plants with
defects in ethylene perception (
ein2
) or JA signalling (coi1) fail
to induce a subset of
PR
gene expression, including the plant
defensin gene
PDF1.2
, a basic chitinase (
PR-3
) and an acidic
have-in-like protein (
PR-4
), resulting in enhanced susceptibil-
ity to certain pathogens (Penninckx et al., 1998). Peng et al.
(2005) investigated the responses of
Arabidopsis ACS
genes
to hypoxia stress and they found that
ACS2, ACS6, ACS7
and
ACS9
were specifically induced during hypoxia. As an environ-
mental signal integrator, ethylene transmits salt stress signalling
and enhances plant survival in a DELLA protein-dependent
manner (Achard et al., 2006).
Arabidopsis
NAC-type tran-
scription factor gene AtNAC2 incorporates the environmental
and endogenous stimuli into the process of plant lateral root
development. It has been shown that in the
ein2-1
and
ein3-1
mutant, the salt induction of AtNAC2 was affected (He et al.,
2005). Ethylene signal pathways are also essential for defence
against ultraviolet B (UV-B) damage. In
etr1-1
(insensitive to
ethylene) mutant plants, the UV-B-induced up-regulation of
PR-1
and
PDF1·2
transcript levels was considerably reduced
compared with wild-type plants, indicating a role of ethylene in
the up-regulation of these genes in response to UV-B exposure
(Mackerness, 1999). Another protein in the ethylene-signalling
pathway, ethylene response factor protein (JERF3) activates the
gene expression through transcription, resulting in decreased
