Geoscience Reference
In-Depth Information
physiological processes, including plant immune response and
growth regulators. SA has an intensive role in signal-mediating
local and systemic plant defence gene expression and regula-
tion responses against pathogens (An and Mou, 2011; Rivas-
San Vicente and Plasencia, 2011). SA signalling is mediated
by both NPR1-dependent mechanisms and NPR1-independent
mechanisms. SA signalling pathway mediates through the
ankyrin repeat containing a BTB/POZ domain protein NPR1
(non-expresser of PR1), which was identified in mutant screens
of
Arabidopsis
(Cao et al., 1994). NPR1 is retained in the
cytoplasm as an oligomer in the absence of SA or pathogen
challenge. Upon SA or pathogen induction, NPR1 monomer is
released to enter the nucleus where it activates and regulates
defence-related gene expression (Mou et al., 2003). SA appli-
cation and pathogen challenge experiments augment NPR1
gene expression. Overexpression of
Arabidopsis
NPR1 or its
homologues confer broad resistance against diverse patho-
gens in multiple plant species (Cao et al., 1998; Chern et al.,
2001; Lin et al., 2004; Malnoy et al., 2007; Parkhi et al., 2010).
MKP1 and PTP1 regulate plant growth homeostasis, acting
as repressors of the stress-induced MAPK pathway involving
MPK3 and MPK6, which leads to SA biosynthesis and expres-
sion of PR genes (Bartels et al., 2009). The
mkp1
and
mkp1
ptp1 mutants have growth defects, increased levels of endog-
enous SA and constitutive defence responses including
PR
gene
expression and resistance to the bacterial pathogen
P. syrin-
gae
. SA also mediates the defence-signalling pathway through
NPR1-independent mechanism. For instance,
Arabidopsis
pro-
tein MYB30 positively regulates the pathogen-induced hyper-
sensitive response in an SA-dependent, NPR1-independent
manner (Raffaele et al., 2006). Another example for NPR1-
independent, SA-dependent gene expression regulation is the
constitutive defence mutants,
cpr5, cpr6
and
hrl1
(Clarke et al.,
2000; Devadas et al., 2002).
There are several reviews, which focus on the role of SA and
gene expression during plant defence mechanism in adverse envi-
ronmental condition. However, SA has also been identified as an
essential regulatory signal-mediating plant response to abiotic
stresses such as drought, chilling, heavy metal tolerance, heat
and osmotic stress (Borsani et al., 2001; Kang and Saltveit, 2002;
Chini et al., 2004; Freeman et al., 2005; Larkindale et al., 2005).
SA also acts as a negative regulator of seed germination, pre-
sumably due to an SA-induced oxidative stress (Xie et al., 2007).
However, some recent reports suggested that SA recovers the
seed germination under abiotic stress conditions. For example,
