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Magazanik
2009
; Vatsa et al.
2011
). The ionotropic glutamate receptor consists of a
ligand binding domain and a channel-forming domain (Price et al.
2012
). The iono-
tropic glutamate receptor-like channels possess a large N-terminal extracellular
domain, three transmembrane regions, a hydrophobic loop defi ning the pore region
and a cytosolic C-terminal domain (Tapken and Hollmann
2008
; Kwaaitaal et al.
2011
). Twenty glutamate receptor-like channels have been detected in
A
.
thaliana
genome (Chiu et al.
1999
,
2002
; Wheeler and Brownlee
2008
). Glutamate receptor-
like genes have been shown to form Ca
2+
channels (Michard et al.
2011
). Ca
2+
permeability of a glutamate receptor channel in
Arabidopsis
has been demonstrated
(Vincill et al.
2012
). Exogenous glutamate application triggers a large transient
elevation in [Ca
2+
]
cyt
and a membrane depolarization in
Arabidopsis
(Dennison and
Spalding
2000
).
In
A. thaliana
overexpressing a full-length cDNA clone (
RsGluR
) encoding a
putative glutamate receptor from small radish, glutamate treatment triggered greater
Ca
2+
infl ux in the root cells of transgenic plants than in those of the wild type.
Jasmonate-responsive genes including defensins and JA-biosynthetic genes were
upregulated in the transgenic plants.
RsGluR
overexpression also inhibited growth
of the fungal pathogen
Botrytis cinerea
. The RsGluR is a glutamate -gated Ca
2+
channel located in the plasma membrane of higher plants and plays a direct or
indirect role in defense against pathogen infection by triggering JA biosynthesis
(Kang et al.
2006a
,
b
).
The glutamate receptors were found to be activated by the PAMP cryptogein and
they were involved in triggering Ca
2+
infl ux (Vatsa et al.
2011
). The downstream
event in the glutamate receptor-mediated signaling pathway included NO produc-
tion (Vatsa et al.
2011
). Kwaaitaal et al. (
2011
) showed that the initiation of innate
immune responses upon the PAMPs fl g22, elf18 and chitin recognition involves
apoplastic Ca
2+
infl ux via glutamate receptor-like channels in
A
.
thaliana
. The
downstream events in the glutamate receptor channels-mediated immune response
signaling pathway included mitogen-activated protein kinase cascade, activation
of calcium-dependent protein kinase (CDPK) and accumulation of defense gene
transcripts (Kwaaitaal et al.
2011
).
4.3.4
Annexins as Calcium Transporters
Elevation of cytoplasmic free calcium ([Ca
2+
]
cyt
) is a regulatory step in plant innate
immunity and it relies spatiotemporal control of Ca
2+
-permeable channel activity at
endomembranes and the plasma membrane (McAinsh and Pittman
2009
). The
plant genomes encode multiple potential Ca
2+
-permeable channel subunits that
could contribute to [Ca
2+
]
cyt
elevation (McAinsh and Pittman
2009
; Ward et al.
2009
). Plant annexins appear capable of mediating passive, channel-like Ca
2+
transport (Mortimer et al.
2008
; Laohavisit et al.
2009
,
2010
; Laohavisit and Davies
2011
). Annexins are membrane binding proteins that can form Ca
2+
-permeable
conductances
in vitro
(Laohavisit et al.
2012
). Pepper annexin mediates Ca
2+
infl ux
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