Furthermore, it was found in Arabidopsis that under nitrogen limitation, aminotransferase TAR2 controls LR proliferation, suggesting that TAR2 is required for the
reprogramming of root architecture in response to low nitrogen availability (Ma et al.
2014). Thus, transgenic plants over-expressing the TAR2 gene showed a significant
increase in the number of LRs under low and high nitrogen conditions. In order to
improve NUE in crop species, over-expression of the TAR2 gene has been recently
engineered in wheat (Triticum aestivum). After a selection among the 12 TAR2
alleles present in the wheat genome, the best candidate TaTAR2.1 gene expressed in
different organs was over-expressed by using a constitutive promoter (Shao et al.
2017). The engineered plants grown under controlled-environment and field conditions showed an increase in LR-growth under low N supply conditions at the plantlet
stage. At maturity and whatever the N supply levels applied, plants also displayed a
significant increase in the biomass, plant height, spike number, and grain yield (Shao
et al. 2017). Taken together, these results indicate that TAR2 genes show a potential
for engineering crop plants for improving gain of yield under nitrogen-limiting
conditions.
7.2 Over-expression of Alanine Aminotransferase Genes
Improves Nitrogen Use Efficiency
Alanine aminotransferase (AlaAT) is a PLP-dependent aminotransferase that catalyzes the amino group transfer of alanine to α-ketoglutarate to form glutamate and
pyruvate. This aminotransferase was used to engineer nitrogen use efficiency in
maize, rice, and canola (Good et al. 2007; Shrawat et al. 2008). Over-expression of
AlaAT from barley (Hordeum vulgare, HvAlaAT) under the control of root-specific
and constitutive promoters OsAnt1 and btg26 in rice and canola, respectively,
induces a significant increase in NUE and root biomass in these crop species
(Good et al. 2007; Shrawat et al. 2008). Especially, under N-limiting conditions,
canola and rice plants exhibit increased biomass and yields. In field trials with 40%
less applied N, canola maintains yields. The physiological reasons for this improvement remain elusive. It is assumed that the decrease in some amino acids in shoots
such as Gln and Glu could alleviate the repression of these AA on nitrate uptake
transporters and increase N uptake (Good et al. 2007). However, it is not excluded
that the overproduction of pyruvate can play a major role in both gluconeogenesis
and auxins biosynthesis (IAA and PAA). Indeed, auxins production needs pyruvate
for TAA/TAR activities since pyruvate is one of the 2-oxoacid acceptors used for
their functioning (Le Deunff et al. 2016). Similarly, phosphoenolpyruvate
carboxykinase PEPCK activity (catalysing the reversible conversion of oxaloacetate
to phosphoenolpyryvate, see Fig. 1) is required for sink tissues metabolically active
such as root, stem, and leaves (Malone et al. 2007). Unfortunately, no study of
transcriptional regulation of AlaAT or PEPCK promoters has been done as with
the TAA1 promoter in Arabidopsis. Therefore, it is difficult to know which
endogenous and exogenous signals are involved in the regulation of AlaAT and
PEPCK expression.
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
279
reprogramming of root architecture in response to low nitrogen availability (Ma et al.
2014). Thus, transgenic plants over-expressing the TAR2 gene showed a significant
increase in the number of LRs under low and high nitrogen conditions. In order to
improve NUE in crop species, over-expression of the TAR2 gene has been recently
engineered in wheat (Triticum aestivum). After a selection among the 12 TAR2
alleles present in the wheat genome, the best candidate TaTAR2.1 gene expressed in
different organs was over-expressed by using a constitutive promoter (Shao et al.
2017). The engineered plants grown under controlled-environment and field conditions showed an increase in LR-growth under low N supply conditions at the plantlet
stage. At maturity and whatever the N supply levels applied, plants also displayed a
significant increase in the biomass, plant height, spike number, and grain yield (Shao
et al. 2017). Taken together, these results indicate that TAR2 genes show a potential
for engineering crop plants for improving gain of yield under nitrogen-limiting
conditions.
7.2 Over-expression of Alanine Aminotransferase Genes
Improves Nitrogen Use Efficiency
Alanine aminotransferase (AlaAT) is a PLP-dependent aminotransferase that catalyzes the amino group transfer of alanine to α-ketoglutarate to form glutamate and
pyruvate. This aminotransferase was used to engineer nitrogen use efficiency in
maize, rice, and canola (Good et al. 2007; Shrawat et al. 2008). Over-expression of
AlaAT from barley (Hordeum vulgare, HvAlaAT) under the control of root-specific
and constitutive promoters OsAnt1 and btg26 in rice and canola, respectively,
induces a significant increase in NUE and root biomass in these crop species
(Good et al. 2007; Shrawat et al. 2008). Especially, under N-limiting conditions,
canola and rice plants exhibit increased biomass and yields. In field trials with 40%
less applied N, canola maintains yields. The physiological reasons for this improvement remain elusive. It is assumed that the decrease in some amino acids in shoots
such as Gln and Glu could alleviate the repression of these AA on nitrate uptake
transporters and increase N uptake (Good et al. 2007). However, it is not excluded
that the overproduction of pyruvate can play a major role in both gluconeogenesis
and auxins biosynthesis (IAA and PAA). Indeed, auxins production needs pyruvate
for TAA/TAR activities since pyruvate is one of the 2-oxoacid acceptors used for
their functioning (Le Deunff et al. 2016). Similarly, phosphoenolpyruvate
carboxykinase PEPCK activity (catalysing the reversible conversion of oxaloacetate
to phosphoenolpyryvate, see Fig. 1) is required for sink tissues metabolically active
such as root, stem, and leaves (Malone et al. 2007). Unfortunately, no study of
transcriptional regulation of AlaAT or PEPCK promoters has been done as with
the TAA1 promoter in Arabidopsis. Therefore, it is difficult to know which
endogenous and exogenous signals are involved in the regulation of AlaAT and
PEPCK expression.
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
279