TAA/TARs enzymes (Soeno et al. 2010). Due to the structural analogies of this
family of inhibitors (Table 1) and the promiscuity of aminotransferases belonging to
subgroup I, caution is now required when using these inhibitors to validate molecular studies with mutational approaches or transgenic plants on ethylene and IAA
biosynthesis and signaling (Le Deunff and Lecourt 2016; Le Deunff et al. 2016).
Moreover, cellular compartmentalization of their target enzymes in the peroxisomes,
plastids, mitochondria, and cytosol probably modulate their effect in term of specificity, permeability, and stability during in vivo treatments. However the broad
spectrum of action of these compounds remains a powerful tool (Le Deunff and
Lecourt 2016). Indeed, they create an imbalance in the amino acid levels which
makes it possible to test potential candidates for N sensory systems such as the TOR
and GCN2 signaling pathways or to discover new targets involved in N detection
and C/N ratio changes in primary metabolism (Le Deunff et al. 2018, submitted).
6.1 In Search for Highly Specific Inhibitors of ACS and TAA
Aminotransferases
Because ethylene and auxin biosynthetic enzymes such as ACS and TAA are
encoded by redundant genes in the Arabidopsis genome, the mutational approaches
on single or multiple genes by reverse and forward genetics are often associated with
plant lethality, poor growth, and sterility (Tsuchisaka et al. 2009; Stepanova et al.
2008). Chemical genetic or genomic approaches can overcome these limitations by
selecting for small bioactive molecules able to inhibit specifically protein activity in
any tissue and at any time during plant development and avoiding side effects
(Zheng and Chan 2002; Blackwell and Zhao 2003; Robert et al. 2009). Recently,
these chemical strategies have been used in Arabidopsis to find out new specific
inhibitors of ACC synthase (Lin et al. 2010) and TAA enzymes (Soeno et al. 2010;
Nakamura et al. 2016). Thus, more specific inhibitors of the ACS enzyme formed
from a quinazolinone backbone (called compounds 9393, 9370, and 7303 compounds) have been discovered from their ability to suppress the constitutive triple
response of ethylene overproducer mutant eto1-4 (Lin et al. 2010). Similarly,
41 compounds derived from the 2-(aminooxy)-3-(naphthalen-2-yl)propanoic acid
(KOK1169/AONP) backbone were able, in vivo and in vitro, to reduce TAA
aminotransferase more specifically than L-α-aminooxy-phenylpropionic acid
(AOPP) (Nakamura et al. 2016). Today, these compounds form a new class of
specific TAA inhibitors designated as “pyruvamine.” Therefore, it is reasonable to
assume that the use of these specific and less specific inhibitors in differential
transcriptomic studies should lead to the discovery of metabolic hubs in N and C
metabolism or N sensory systems involved in the regulation of the root morphogenetic program.
276
E. Le Deunff
family of inhibitors (Table 1) and the promiscuity of aminotransferases belonging to
subgroup I, caution is now required when using these inhibitors to validate molecular studies with mutational approaches or transgenic plants on ethylene and IAA
biosynthesis and signaling (Le Deunff and Lecourt 2016; Le Deunff et al. 2016).
Moreover, cellular compartmentalization of their target enzymes in the peroxisomes,
plastids, mitochondria, and cytosol probably modulate their effect in term of specificity, permeability, and stability during in vivo treatments. However the broad
spectrum of action of these compounds remains a powerful tool (Le Deunff and
Lecourt 2016). Indeed, they create an imbalance in the amino acid levels which
makes it possible to test potential candidates for N sensory systems such as the TOR
and GCN2 signaling pathways or to discover new targets involved in N detection
and C/N ratio changes in primary metabolism (Le Deunff et al. 2018, submitted).
6.1 In Search for Highly Specific Inhibitors of ACS and TAA
Aminotransferases
Because ethylene and auxin biosynthetic enzymes such as ACS and TAA are
encoded by redundant genes in the Arabidopsis genome, the mutational approaches
on single or multiple genes by reverse and forward genetics are often associated with
plant lethality, poor growth, and sterility (Tsuchisaka et al. 2009; Stepanova et al.
2008). Chemical genetic or genomic approaches can overcome these limitations by
selecting for small bioactive molecules able to inhibit specifically protein activity in
any tissue and at any time during plant development and avoiding side effects
(Zheng and Chan 2002; Blackwell and Zhao 2003; Robert et al. 2009). Recently,
these chemical strategies have been used in Arabidopsis to find out new specific
inhibitors of ACC synthase (Lin et al. 2010) and TAA enzymes (Soeno et al. 2010;
Nakamura et al. 2016). Thus, more specific inhibitors of the ACS enzyme formed
from a quinazolinone backbone (called compounds 9393, 9370, and 7303 compounds) have been discovered from their ability to suppress the constitutive triple
response of ethylene overproducer mutant eto1-4 (Lin et al. 2010). Similarly,
41 compounds derived from the 2-(aminooxy)-3-(naphthalen-2-yl)propanoic acid
(KOK1169/AONP) backbone were able, in vivo and in vitro, to reduce TAA
aminotransferase more specifically than L-α-aminooxy-phenylpropionic acid
(AOPP) (Nakamura et al. 2016). Today, these compounds form a new class of
specific TAA inhibitors designated as “pyruvamine.” Therefore, it is reasonable to
assume that the use of these specific and less specific inhibitors in differential
transcriptomic studies should lead to the discovery of metabolic hubs in N and C
metabolism or N sensory systems involved in the regulation of the root morphogenetic program.
276
E. Le Deunff