4.4 Are ACC and Polyamines Potential Ligands or
Modulators of Plant Glutamate Receptors?
Because of their role as neurotransmitters or modulators in animal cells, it is not
excluded that ACC, polyamines, GSH, and conjugated forms of ACC may also act
on plant glutamate receptors. Assuming ACC acts as a ligand or modulator on
the plant glutamate receptors (GLR), this would explain why levels of ACC are so
controlled in plant cells (Le Deunff and Lecourt 2016).
The Arabidopsis genome encodes for 20 GLR genes with coding sequences close
to the ionotropic glutamate receptors (iGluR) first identified and characterized in
mammals (Price et al. 2012; Weiland et al. 2016). Analyses of DNA sequences of
GLR have grouped these proteins into three clades (Davenport 2002; Chiu et al.
2002). Expression studies have shown that GLR genes were mainly expressed in
roots but also in leaves and reproductive organs (Chiu et al. 2002). The structure of
GLRs is organized into four different units comprised of an amino terminal domain
(ATD), a ligand binding domain (LBD), a trans-membrane domain (TMD) formed
by three complete trans-membrane domains (M1, M3, and M4) with one re-entrant
loop (M2) that forms the ion channel, and a C-terminal tail. Plant GLRs probably
form tetrameric channels selective for Na
+
, K
+ , and Ca
2+ as mammalian iGLuR
counterparts (Tapken and Hollmann 2008; Price et al. 2012). Heterologous expression in transfected human embryonic kidney and Xenopus oocytes associated with
patch-clamp studies have shown that AtGLR3.2, AtGLR3.4, and AtGLR1.4 can be
gated by a broad spectrum of amino acids such as Asn, Ser, Gly, Met, Trp, Phe, Leu,
Tyr, and Thr. Among these amino acids, Met was the most effective in AtGLR1.4
receptor whereas Arg was the most effective antagonist of Met effect (Vincill et al.
2012, 2013; Tapken et al. 2013). Moreover, the Cys and GSH tripeptides were the
most effective agonists for activating AtGLR3.3 to suppress growth of the tomato
bacterial pathogen Pseudomonas syringae in Arabidopsis leaves, suggesting that
the methylthio-group common for both ligands could specifically interact with the
receptor during the innate immunity response (Li et al. 2013).
If plants GLR are compared from an analogical perspective with their mammal’s
counterparts, very surprising results can be envisaged. Indeed, in mammals, the
ethylene precursor ACC is a synthetic molecule called ACPC and frequently used as
a partial agonist of iGluR on glycine binding sites (Nahum-Levy et al. 1999; Inanobe
et al. 2005). Similarly, polyamines such as spermine and spermidine stabilize the
formation of the LDB dimer in a tetrameric receptor and facilitate the attachment
of ATD lobes by “gluing” them together (Mony et al. 2009). Because polyamines
and ACC-ACP are directly produced downstream from AdoMet biosynthesis, further investigations are required because ACC and polyamines are involved in many
plant responses to biotic and abiotic stresses (Le Deunff and Lecourt 2016). GLR are
located in the plasma membrane but also in membranes of plastids and mitochondria
(Weiland et al. 2016). This suggests that they can react to a broad spectrum of amino
270
E. Le Deunff
Modulators of Plant Glutamate Receptors?
Because of their role as neurotransmitters or modulators in animal cells, it is not
excluded that ACC, polyamines, GSH, and conjugated forms of ACC may also act
on plant glutamate receptors. Assuming ACC acts as a ligand or modulator on
the plant glutamate receptors (GLR), this would explain why levels of ACC are so
controlled in plant cells (Le Deunff and Lecourt 2016).
The Arabidopsis genome encodes for 20 GLR genes with coding sequences close
to the ionotropic glutamate receptors (iGluR) first identified and characterized in
mammals (Price et al. 2012; Weiland et al. 2016). Analyses of DNA sequences of
GLR have grouped these proteins into three clades (Davenport 2002; Chiu et al.
2002). Expression studies have shown that GLR genes were mainly expressed in
roots but also in leaves and reproductive organs (Chiu et al. 2002). The structure of
GLRs is organized into four different units comprised of an amino terminal domain
(ATD), a ligand binding domain (LBD), a trans-membrane domain (TMD) formed
by three complete trans-membrane domains (M1, M3, and M4) with one re-entrant
loop (M2) that forms the ion channel, and a C-terminal tail. Plant GLRs probably
form tetrameric channels selective for Na
+
, K
+ , and Ca
2+ as mammalian iGLuR
counterparts (Tapken and Hollmann 2008; Price et al. 2012). Heterologous expression in transfected human embryonic kidney and Xenopus oocytes associated with
patch-clamp studies have shown that AtGLR3.2, AtGLR3.4, and AtGLR1.4 can be
gated by a broad spectrum of amino acids such as Asn, Ser, Gly, Met, Trp, Phe, Leu,
Tyr, and Thr. Among these amino acids, Met was the most effective in AtGLR1.4
receptor whereas Arg was the most effective antagonist of Met effect (Vincill et al.
2012, 2013; Tapken et al. 2013). Moreover, the Cys and GSH tripeptides were the
most effective agonists for activating AtGLR3.3 to suppress growth of the tomato
bacterial pathogen Pseudomonas syringae in Arabidopsis leaves, suggesting that
the methylthio-group common for both ligands could specifically interact with the
receptor during the innate immunity response (Li et al. 2013).
If plants GLR are compared from an analogical perspective with their mammal’s
counterparts, very surprising results can be envisaged. Indeed, in mammals, the
ethylene precursor ACC is a synthetic molecule called ACPC and frequently used as
a partial agonist of iGluR on glycine binding sites (Nahum-Levy et al. 1999; Inanobe
et al. 2005). Similarly, polyamines such as spermine and spermidine stabilize the
formation of the LDB dimer in a tetrameric receptor and facilitate the attachment
of ATD lobes by “gluing” them together (Mony et al. 2009). Because polyamines
and ACC-ACP are directly produced downstream from AdoMet biosynthesis, further investigations are required because ACC and polyamines are involved in many
plant responses to biotic and abiotic stresses (Le Deunff and Lecourt 2016). GLR are
located in the plasma membrane but also in membranes of plastids and mitochondria
(Weiland et al. 2016). This suggests that they can react to a broad spectrum of amino
270
E. Le Deunff