Furthermore this conclusion is also corroborated by the overaccumulation of
anthocyanin in ethylene signaling mutants etr1, ein2, ein3/eil1, and rdh3 (root hair
defective3) subjected to nitrogen deficiency, whereas exogenous application of ACC
almost completely suppresses anthocyanin accumulation in Col-0 WT plants and
ACC effect was attenuated in the ethylene signaling mutants (Wang et al. 2015).
Anthocyanin is accumulated under low nitrate levels because nitrate relieves the
repression of lateral organ boundary (LDB) transcription factors on anthocyanin
biosynthetic genes (Rubin et al. 2009). The LDB genes also repress many other
N-responsive genes such as nitrate reductase genes NIA2 and NIA2 and nitrate
transporters AtNRT1.1 (NPF6.3), AtNRT2.1, and AtNRT2.5. Regulation by ethylene of nitrate responsive genes such as nitrate transporters is also demonstrated
during N-induced nutritional stress. Indeed, rapid changes in nitrate supply such
as transfers from high to low concentration and vice versa induce a burst of ethylene
production in the roots associated with a differential expression of AtNRT2.1
and AtNRT1.1 (NPF6.3) nitrate transporters genes (Tian et al. 2009; Zheng et al.
2013b). In response to nitrate limitation, induction of AtNRT2.1 plays a positive role
on ethylene biosynthesis and signaling pathway but the ethylene signaling components such as EIN3/EIL1 induce in turn the repression of AtNRT2.2 (Zheng et al.
2013b). In response to nitrate excess, expression of AtNRT1.1 (NPF6.3)
and AtNRT2.1 genes is respectively up- and down-regulated (Tian et al. 2009).
However, these responses are abolished in etr1-3 and ein2-1 ethylene receptor
mutants, again demonstrating that components of ethylene signaling are probably
involved in nitrate nutritional responses.
8 Conclusion
Ethylene biosynthesis from the amino acid aspartate needs at least seven different
aminotransferases belonging to the subfamilies I and γ of the α family: AAT,
AAT/PAT, CgS, CbL, Vas1/ISS1, ACS and an unknown AAT aminotransferase
to complete definitively the Yang cycle. Five of these aminotransferases play a
fundamental role in N, S and C shuttling in plant cells (namely CgS, CbL, ACS,
AAT/PAT, VAS1/ISS1) and three of them (VAS1/ISS1, AAT/PAT, and an unknown
AAT) connect the aspartate/methionine metabolism to aromatic amino acid biosynthesis and catabolism in a unique and major network of aminotransferases in the
plastids.. Because of the promiscuous nature of the aminotransferases for their substrates, it is likely that different aminotransferases can be involved in the last step of the
Yang cycle within plastids or the cytosol. The functioning of these aminotransferases
depends directly on the availability of PLP that is synthesized from glutamine, the first
amino acid produced by the glutamine synthetase in the nitrate reduction/assimilation
pathway. Understanding how the genes encoding these aminotransferases and their
cofactor are transcriptionally and translationally regulated is a major challenge in
unraveling the interconnection between Trp, Met, and Phe amino acid biosynthesis,
hormone production, and nitrogen metabolism. Involvement of N, S, and C sensory
systems in the regulation of these aminotransferases such as hexokinase, glutamate like
receptors, TOR, and GCN2 kinases seems highly probable.
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
281
anthocyanin in ethylene signaling mutants etr1, ein2, ein3/eil1, and rdh3 (root hair
defective3) subjected to nitrogen deficiency, whereas exogenous application of ACC
almost completely suppresses anthocyanin accumulation in Col-0 WT plants and
ACC effect was attenuated in the ethylene signaling mutants (Wang et al. 2015).
Anthocyanin is accumulated under low nitrate levels because nitrate relieves the
repression of lateral organ boundary (LDB) transcription factors on anthocyanin
biosynthetic genes (Rubin et al. 2009). The LDB genes also repress many other
N-responsive genes such as nitrate reductase genes NIA2 and NIA2 and nitrate
transporters AtNRT1.1 (NPF6.3), AtNRT2.1, and AtNRT2.5. Regulation by ethylene of nitrate responsive genes such as nitrate transporters is also demonstrated
during N-induced nutritional stress. Indeed, rapid changes in nitrate supply such
as transfers from high to low concentration and vice versa induce a burst of ethylene
production in the roots associated with a differential expression of AtNRT2.1
and AtNRT1.1 (NPF6.3) nitrate transporters genes (Tian et al. 2009; Zheng et al.
2013b). In response to nitrate limitation, induction of AtNRT2.1 plays a positive role
on ethylene biosynthesis and signaling pathway but the ethylene signaling components such as EIN3/EIL1 induce in turn the repression of AtNRT2.2 (Zheng et al.
2013b). In response to nitrate excess, expression of AtNRT1.1 (NPF6.3)
and AtNRT2.1 genes is respectively up- and down-regulated (Tian et al. 2009).
However, these responses are abolished in etr1-3 and ein2-1 ethylene receptor
mutants, again demonstrating that components of ethylene signaling are probably
involved in nitrate nutritional responses.
8 Conclusion
Ethylene biosynthesis from the amino acid aspartate needs at least seven different
aminotransferases belonging to the subfamilies I and γ of the α family: AAT,
AAT/PAT, CgS, CbL, Vas1/ISS1, ACS and an unknown AAT aminotransferase
to complete definitively the Yang cycle. Five of these aminotransferases play a
fundamental role in N, S and C shuttling in plant cells (namely CgS, CbL, ACS,
AAT/PAT, VAS1/ISS1) and three of them (VAS1/ISS1, AAT/PAT, and an unknown
AAT) connect the aspartate/methionine metabolism to aromatic amino acid biosynthesis and catabolism in a unique and major network of aminotransferases in the
plastids.. Because of the promiscuous nature of the aminotransferases for their substrates, it is likely that different aminotransferases can be involved in the last step of the
Yang cycle within plastids or the cytosol. The functioning of these aminotransferases
depends directly on the availability of PLP that is synthesized from glutamine, the first
amino acid produced by the glutamine synthetase in the nitrate reduction/assimilation
pathway. Understanding how the genes encoding these aminotransferases and their
cofactor are transcriptionally and translationally regulated is a major challenge in
unraveling the interconnection between Trp, Met, and Phe amino acid biosynthesis,
hormone production, and nitrogen metabolism. Involvement of N, S, and C sensory
systems in the regulation of these aminotransferases such as hexokinase, glutamate like
receptors, TOR, and GCN2 kinases seems highly probable.
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
281