ACC Is Conjugated Under Different Forms as Many Signal Molecules The cellular
levels of ACC can be decreased by conversion under different conjugated forms.
Thus, ACC can be converted in tomato fruits by N-malonyltransferase (AMT)
into 1-malonyl-ACC (MACC, Martin et al. 1995) and in Arabidopsis vegetative
tissues by JA-aminosynthetase (JAR1, Jasmonic acid resistance 1) into jasmonylACC (JA-ACC; Staswick and Tiryaki 2004). ACC can also be conjugated to
the reduced glutathione (GSH). GSH is a very abundant cellular tripeptide (γ-GluCys-Gly) that serves as an essential antioxidant by scavenging reactive oxygen
species during plant stresses (Noctor et al. 2011). γ-glutamyltranspeptidase (GGT)
converted GSH and ACC into 1-(γ-glutamylamino)-ACC (GACC) in plant tissues
(Amrhein et al. 1981).
ACC Can Be Stored in the Vacuole or Transported Long-Distance Another strategy
developed by the plant for buffering an excess of ACC synthesis and ethylene
production is ACC storage into the vacuole or long-distance translocation through
the xylem (Bradford and Yang 1980; O’Neill 1997; Finlayson et al. 1999) and the
phloem (Voesenek et al. 1990). Although ACC compartmentalization and storage
into the vacuole have been studied for a long time, the characterization of genes
encoding transporters of influx and efflux is not yet achieved (Bouzayen et al. 1991;
Tophof et al. 1989; Saftner and Martin 1993). However, it has been recently
demonstrated that ACC transport across the plasma membrane is provided by the
lysine histidine transporter1 (LHT1). Indeed, neutral amino acids such as alanine and
glycine have a competitive effect on the ACC-induced triple response. Likewise,
are2 mutants (ACC-resistant 2) defective in LHT1 exhibit a dose-dependent
response to ACC (Shin et al. 2015). Therefore, long-distance circulation and shortdistance accumulation of ACC are dependent of amino acids transporters that have
not been yet completely characterized.
ACC Is Catabolized by a PLP-Dependent Deaminase A recent study has shown
that there exists in the Arabidopsis genome an ACC deaminase gene (AtACD1)
encoding a protein homolog to bacterial and fungal ACD and previously identified
as a D-cysteine desulfhydrase (D-CDes) (Riemenschneider et al. 2005). ACD1
is a PLP-dependent enzyme that belongs to the β family of aminotransferases
(Christen and Mehta 2001) and converts ACC into α-ketobutyrate and ammonia,
two important molecules involved in the GS/GOGAT cycle during N assimilation
(McDonnell et al. 2009). The transgenic lines over-expressing and under-expressing
the AtACD1 gene showed a significant modulation in the triple response of Arabidopsis
etiolated seedlings. Likewise, antisense lines exhibited 70% reduction in ACD activity
and a significant increase in ethylene production (McDonnell et al. 2009). These results
strongly suggest that ACD is responsible for maintenance and control of ACC pools
in vivo but the underlying regulatory mechanisms at the transcriptional and posttranscriptional levels of this gene remain to be discovered.
ACC Treatment Reduces Aspartate Levels by an Unknown Mechanism Seedling
treatment by exogenous ACC induces a rapid decrease in primary and lateral
root growth within minutes after application (Le et al. 2001; Swarup et al. 2007;
268
E. Le Deunff
levels of ACC can be decreased by conversion under different conjugated forms.
Thus, ACC can be converted in tomato fruits by N-malonyltransferase (AMT)
into 1-malonyl-ACC (MACC, Martin et al. 1995) and in Arabidopsis vegetative
tissues by JA-aminosynthetase (JAR1, Jasmonic acid resistance 1) into jasmonylACC (JA-ACC; Staswick and Tiryaki 2004). ACC can also be conjugated to
the reduced glutathione (GSH). GSH is a very abundant cellular tripeptide (γ-GluCys-Gly) that serves as an essential antioxidant by scavenging reactive oxygen
species during plant stresses (Noctor et al. 2011). γ-glutamyltranspeptidase (GGT)
converted GSH and ACC into 1-(γ-glutamylamino)-ACC (GACC) in plant tissues
(Amrhein et al. 1981).
ACC Can Be Stored in the Vacuole or Transported Long-Distance Another strategy
developed by the plant for buffering an excess of ACC synthesis and ethylene
production is ACC storage into the vacuole or long-distance translocation through
the xylem (Bradford and Yang 1980; O’Neill 1997; Finlayson et al. 1999) and the
phloem (Voesenek et al. 1990). Although ACC compartmentalization and storage
into the vacuole have been studied for a long time, the characterization of genes
encoding transporters of influx and efflux is not yet achieved (Bouzayen et al. 1991;
Tophof et al. 1989; Saftner and Martin 1993). However, it has been recently
demonstrated that ACC transport across the plasma membrane is provided by the
lysine histidine transporter1 (LHT1). Indeed, neutral amino acids such as alanine and
glycine have a competitive effect on the ACC-induced triple response. Likewise,
are2 mutants (ACC-resistant 2) defective in LHT1 exhibit a dose-dependent
response to ACC (Shin et al. 2015). Therefore, long-distance circulation and shortdistance accumulation of ACC are dependent of amino acids transporters that have
not been yet completely characterized.
ACC Is Catabolized by a PLP-Dependent Deaminase A recent study has shown
that there exists in the Arabidopsis genome an ACC deaminase gene (AtACD1)
encoding a protein homolog to bacterial and fungal ACD and previously identified
as a D-cysteine desulfhydrase (D-CDes) (Riemenschneider et al. 2005). ACD1
is a PLP-dependent enzyme that belongs to the β family of aminotransferases
(Christen and Mehta 2001) and converts ACC into α-ketobutyrate and ammonia,
two important molecules involved in the GS/GOGAT cycle during N assimilation
(McDonnell et al. 2009). The transgenic lines over-expressing and under-expressing
the AtACD1 gene showed a significant modulation in the triple response of Arabidopsis
etiolated seedlings. Likewise, antisense lines exhibited 70% reduction in ACD activity
and a significant increase in ethylene production (McDonnell et al. 2009). These results
strongly suggest that ACD is responsible for maintenance and control of ACC pools
in vivo but the underlying regulatory mechanisms at the transcriptional and posttranscriptional levels of this gene remain to be discovered.
ACC Treatment Reduces Aspartate Levels by an Unknown Mechanism Seedling
treatment by exogenous ACC induces a rapid decrease in primary and lateral
root growth within minutes after application (Le et al. 2001; Swarup et al. 2007;
268
E. Le Deunff