7 Are Aminotransferases Potential Targets
for the Improvement of Nitrogen Use Efficiency (NUE)?
An alternative approach to address the metabolic interconnections between mineral
nutrition and the production of plant growth regulators consists of proposing that the
biosynthesis of hormones such as ethylene and IAA is highly regulated through the
aminotransferase network in primary metabolism (Le Deunff et al. 2016; Le Deunff
et al. 2018 submitted). Indeed, aminotransferases are the key enzymes that ensure N,
S, and C shuttling through production of amino and organic acids but also biosynthesis of ethylene and auxins via Met, Trp, and Phe. Therefore, using mutants or
nonspecific inhibitors of aminotransferases involved in ethylene and auxins biosynthesis can modulate the aminotransferase network and impact nitrogen use efficiency
(NUE). It is likely that such approaches may disrupt N sensory systems such as TOR
and GNC2 kinase signaling, the plant GCN4 transcription factor homolog, or GLR
receptors. We present here some recent results in favor of this assumption.
7.1 Over-expression of the Tryptophan Aminotransferase
Genes Improves Nitrogen Use Efficiency
In Arabidopsis the members of the Trp aminotransferases namely TAA1 (also
known as WEI8, SAV3 and TIR2) and their related proteins TAR1 and TAR2
play critical role in plant development in response to stresses (Stepanova et al.
2008; Ma et al. 2014). Thus, the Arabidopsis double mutant wei8tar2 displays a
dwarf and bushy plant phenotype with reduced vasculature and abnormal flowers
with complete sterility, whereas the triple mutant wei8tar2tar1 is rootless, seedlinglethal, and impaired in embryo patterning (Stepanova et al. 2008). These mutants are
insensitive to ethylene and ACC treatments because they lack the triple response.
A recent study has shown that TAA1 is highly transcriptionally regulated by ARR
transcription factors (Arapidopsis response regulator) that can bind after dimerization at two different sites. One site is present in the TAA1 promoter region whereas
the other is found in the second intron of the gene sequence (Yan et al. 2017).
Thus, transcriptional responses are modulated by a subset of ARRs implicated in
binding on site 1 or 2. The ARRs biosynthesis is regulated by cytokinin, ethylene,
light, and developmental signals. Especially, ethylene response is mediated by the
EIN3 transcription factor, which is able to bind to the ARR12 protein and modulate
TAA gene expression (Yan et al. 2017). These results explain previous data indicating that TAA1 transcription is activated by EIN3 (He et al. 2011). The major role
of TAA1 aminotransferase in plant development was also demonstrated in maize
(Zea mays) and rice (Oryza sativa) where the loss of function of two co-ortholog
genes ZmVT2 (Vanishing Tassel2) and OsFIB (Fish Bone) caused dramatic effects
on vegetative and reproductive development associated with a reduction in IAA
levels (Phillips et al. 2011; Yoshikawa et al. 2014).
278
E. Le Deunff
for the Improvement of Nitrogen Use Efficiency (NUE)?
An alternative approach to address the metabolic interconnections between mineral
nutrition and the production of plant growth regulators consists of proposing that the
biosynthesis of hormones such as ethylene and IAA is highly regulated through the
aminotransferase network in primary metabolism (Le Deunff et al. 2016; Le Deunff
et al. 2018 submitted). Indeed, aminotransferases are the key enzymes that ensure N,
S, and C shuttling through production of amino and organic acids but also biosynthesis of ethylene and auxins via Met, Trp, and Phe. Therefore, using mutants or
nonspecific inhibitors of aminotransferases involved in ethylene and auxins biosynthesis can modulate the aminotransferase network and impact nitrogen use efficiency
(NUE). It is likely that such approaches may disrupt N sensory systems such as TOR
and GNC2 kinase signaling, the plant GCN4 transcription factor homolog, or GLR
receptors. We present here some recent results in favor of this assumption.
7.1 Over-expression of the Tryptophan Aminotransferase
Genes Improves Nitrogen Use Efficiency
In Arabidopsis the members of the Trp aminotransferases namely TAA1 (also
known as WEI8, SAV3 and TIR2) and their related proteins TAR1 and TAR2
play critical role in plant development in response to stresses (Stepanova et al.
2008; Ma et al. 2014). Thus, the Arabidopsis double mutant wei8tar2 displays a
dwarf and bushy plant phenotype with reduced vasculature and abnormal flowers
with complete sterility, whereas the triple mutant wei8tar2tar1 is rootless, seedlinglethal, and impaired in embryo patterning (Stepanova et al. 2008). These mutants are
insensitive to ethylene and ACC treatments because they lack the triple response.
A recent study has shown that TAA1 is highly transcriptionally regulated by ARR
transcription factors (Arapidopsis response regulator) that can bind after dimerization at two different sites. One site is present in the TAA1 promoter region whereas
the other is found in the second intron of the gene sequence (Yan et al. 2017).
Thus, transcriptional responses are modulated by a subset of ARRs implicated in
binding on site 1 or 2. The ARRs biosynthesis is regulated by cytokinin, ethylene,
light, and developmental signals. Especially, ethylene response is mediated by the
EIN3 transcription factor, which is able to bind to the ARR12 protein and modulate
TAA gene expression (Yan et al. 2017). These results explain previous data indicating that TAA1 transcription is activated by EIN3 (He et al. 2011). The major role
of TAA1 aminotransferase in plant development was also demonstrated in maize
(Zea mays) and rice (Oryza sativa) where the loss of function of two co-ortholog
genes ZmVT2 (Vanishing Tassel2) and OsFIB (Fish Bone) caused dramatic effects
on vegetative and reproductive development associated with a reduction in IAA
levels (Phillips et al. 2011; Yoshikawa et al. 2014).
278
E. Le Deunff