lignin biosynthesis (de la Torre et al. 2014a, b). Unfortunately, AdoMet and Met
levels have not been measured so that it is difficult to evaluate the impact of this
enzyme on the last step of the methionine pathway. Likewise, it is likely that the
cytosolic aspartate aminotransferases ASP2 and ASP3 could also catalyze transamination of KMTB to methionine when the last step of Yang cycle occurs in the
cytosol (Miesak and Coruzzi 2002). In summary, the functioning of AAT/PAT,
SAV1/ISS1, and PPY-AT aminotransferases could be a major metabolic crossroad
interconnecting the metabolism of methionine and aromatic amino acids (Trp, Phe,
and Tyr) with biosynthesis of ethylene, IAA, PAA, and phenylpropanoids.
5.4 Is Tryptophan Aminotransferase Also Involved
in Histidine Catabolism?
Although histidine catabolism is well established in animal cells, this pathway has
not yet been established in plant cells (Hildebrandt et al. 2015). A surprising result
has been recently obtained with the human fungal pathogen Candida glabrata and
Saccharomyces cerevisiae that initialize His degradation via the aromatic aminotransferase ARO8. In yeast, ARO8 is known to be an AroAT that in vitro converts
Phe, Tyr, and Trp as amino acid donor to phenylpyruvate, α-ketoglutarate, and
pyruvate as amino acid acceptors (Iraqui et al. 1998; Urrestarazu et al. 1998).
However, in both fungi ARO8 is ten-fold up-regulated by exogenous His and is
also able to transfer the donor amino group of His to the α-ketoglutarate acceptor to
produce imidazol-5-yl-pyruvate and glutamate (Brunke et al. 2014). Furthermore,
heterologous expression of SAV1/ISS1 or TAA1 Arabidopsis genes can rescue the
yeast mutant Aro8Aro9 auxotroph for Phe and Tyr (Pieck et al. 2015). In B. napus
seedlings, in vivo inhibition of TAA and ACS activities by root treatment with
10 μM AVG (Soeno et al. 2010) induced a 2.5-fold and 8-fold increase of His levels
in the roots and shoots, respectively (Fig. 6). However, treatment with 1 mM
glutamate was able to restore control levels of amino acids in roots and shoots
with the exception of His (Le Deunff et al. 2018). In yeast, His starvation imposed by
3-aminotriazole induced the bZIP transcription factor GCN4 that initiates transcription of 539 genes such as genes encoding amino acids transporters and biosynthetic
enzymes as well as genes involved in PLP biosynthesis (Niederberger et al. 1981;
Natarajan et al. 2001; Hinnebusch 2005). In Arabidopsis, His homeostasis is crucial
for root development since the hap1 mutant of plastidial histidinol phosphate
aminotransferases (HPA), the eighth enzyme of His biosynthesis, displays a defect
in root meristem maintenance and reduction in root development (Mo et al. 2006).
Taken together, these results suggest that TAA could be involved in His catabolism
in plants and play a major role in the regulation of amino acid biosynthesis.
Therefore, TAA/SAV1/ISS1 interconversion system could play a major role as a
metabolic crossroad in the amino acid imbalance and changes in C/N ratio acting on
N sensory systems such as the GCN4, GCN2, and TOR kinases signaling involved
in plant nutrition.
274
E. Le Deunff
levels have not been measured so that it is difficult to evaluate the impact of this
enzyme on the last step of the methionine pathway. Likewise, it is likely that the
cytosolic aspartate aminotransferases ASP2 and ASP3 could also catalyze transamination of KMTB to methionine when the last step of Yang cycle occurs in the
cytosol (Miesak and Coruzzi 2002). In summary, the functioning of AAT/PAT,
SAV1/ISS1, and PPY-AT aminotransferases could be a major metabolic crossroad
interconnecting the metabolism of methionine and aromatic amino acids (Trp, Phe,
and Tyr) with biosynthesis of ethylene, IAA, PAA, and phenylpropanoids.
5.4 Is Tryptophan Aminotransferase Also Involved
in Histidine Catabolism?
Although histidine catabolism is well established in animal cells, this pathway has
not yet been established in plant cells (Hildebrandt et al. 2015). A surprising result
has been recently obtained with the human fungal pathogen Candida glabrata and
Saccharomyces cerevisiae that initialize His degradation via the aromatic aminotransferase ARO8. In yeast, ARO8 is known to be an AroAT that in vitro converts
Phe, Tyr, and Trp as amino acid donor to phenylpyruvate, α-ketoglutarate, and
pyruvate as amino acid acceptors (Iraqui et al. 1998; Urrestarazu et al. 1998).
However, in both fungi ARO8 is ten-fold up-regulated by exogenous His and is
also able to transfer the donor amino group of His to the α-ketoglutarate acceptor to
produce imidazol-5-yl-pyruvate and glutamate (Brunke et al. 2014). Furthermore,
heterologous expression of SAV1/ISS1 or TAA1 Arabidopsis genes can rescue the
yeast mutant Aro8Aro9 auxotroph for Phe and Tyr (Pieck et al. 2015). In B. napus
seedlings, in vivo inhibition of TAA and ACS activities by root treatment with
10 μM AVG (Soeno et al. 2010) induced a 2.5-fold and 8-fold increase of His levels
in the roots and shoots, respectively (Fig. 6). However, treatment with 1 mM
glutamate was able to restore control levels of amino acids in roots and shoots
with the exception of His (Le Deunff et al. 2018). In yeast, His starvation imposed by
3-aminotriazole induced the bZIP transcription factor GCN4 that initiates transcription of 539 genes such as genes encoding amino acids transporters and biosynthetic
enzymes as well as genes involved in PLP biosynthesis (Niederberger et al. 1981;
Natarajan et al. 2001; Hinnebusch 2005). In Arabidopsis, His homeostasis is crucial
for root development since the hap1 mutant of plastidial histidinol phosphate
aminotransferases (HPA), the eighth enzyme of His biosynthesis, displays a defect
in root meristem maintenance and reduction in root development (Mo et al. 2006).
Taken together, these results suggest that TAA could be involved in His catabolism
in plants and play a major role in the regulation of amino acid biosynthesis.
Therefore, TAA/SAV1/ISS1 interconversion system could play a major role as a
metabolic crossroad in the amino acid imbalance and changes in C/N ratio acting on
N sensory systems such as the GCN4, GCN2, and TOR kinases signaling involved
in plant nutrition.
274
E. Le Deunff