7 Are Aminotransferases Potential Targets for the Improvement of Nitrogen Use Efficiency
(NUE)? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
7.1 Over-expression of the Tryptophan Aminotransferase Genes Improves Nitrogen Use
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
7.2 Over-expression of Alanine Aminotransferase Genes Improves Nitrogen Use
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
7.3 Could the Under-expression of ACC Synthase Genes Improve Nitrogen Use
Efficiency? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280
8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282
Abstract Ethylene biosynthesis originates from three amino acids: aspartate, cysteine, and methionine. In the aspartate-derived amino acid pathway, the ethylene
pathway requires no less than seven aminotransferases that connect the metabolisms
of nitrogen (N), sulfur (S), and carbon (C). Aminotransferases are fundamental
enzymes in plants involved in N, S, and C shuttling through their implication
in amino acid biosynthesis and catabolism. The role of these enzymes in the biosynthesis of hormones such as ethylene and auxins (IAA and PAA) is frequently
overlooked. The functioning of aminotransferases is dependent on an essential
cofactor: pyridoxal-5
0 -phosphate (PLP). This phosphorylated form of vitamin B 6
is synthesized from glutamine, the first product of N assimilation produced by the
GS/GOGAT cycle after reduction of nitrate and the glyceraldehyde 3-phosphate
(G3P) and ribose 5-phosphate (5RP) provided by the glycolytic and pentose phosphate pathways, respectively. Here we review the recent progress in characterization
of the aspartate-derived metabolic pathway with a particular focus on methionine
biosynthesis and its salvage pathway (Yang cycle) related to ethylene and polyamine
biosynthesis. Emphasis is placed on the key role of aminotransferases in regulating
these pathways and their relation with aromatic amino acid biosynthesis and catabolism. Indeed, the promiscuity of certain aminotransferases extends their catalytic
function and gives them a key role in the metabolism of ethylene, IAA, and aromatic
amino acids. In this respect, recent studies have identified specific aminotransferases
as being the main targets involved in the root morphogenetic program in response
to environmental cues, nutrient availability, and energy status. Thus, genetically
engineered plants for some aminotransferases, such as ACC synthase and tryptophan
aminotransferase, demonstrate a great potential to produce crop species with
enhanced exploratory root growth and a better nitrogen use efficiency. How the
network of aminotransferases is involved in nitrogen-sensing systems such as plant
glutamate receptors, TOR, and GCN2 kinases is now becoming a fundamental issue.
The use of specific and nonspecific inhibitors of the catalytic activity of certain
aminotransferases should help future pharmacological and genetic approaches to
unravel their role in N, S, and C sensory systems.
254
E. Le Deunff
(NUE)? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
7.1 Over-expression of the Tryptophan Aminotransferase Genes Improves Nitrogen Use
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
7.2 Over-expression of Alanine Aminotransferase Genes Improves Nitrogen Use
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
7.3 Could the Under-expression of ACC Synthase Genes Improve Nitrogen Use
Efficiency? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280
8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282
Abstract Ethylene biosynthesis originates from three amino acids: aspartate, cysteine, and methionine. In the aspartate-derived amino acid pathway, the ethylene
pathway requires no less than seven aminotransferases that connect the metabolisms
of nitrogen (N), sulfur (S), and carbon (C). Aminotransferases are fundamental
enzymes in plants involved in N, S, and C shuttling through their implication
in amino acid biosynthesis and catabolism. The role of these enzymes in the biosynthesis of hormones such as ethylene and auxins (IAA and PAA) is frequently
overlooked. The functioning of aminotransferases is dependent on an essential
cofactor: pyridoxal-5
0 -phosphate (PLP). This phosphorylated form of vitamin B 6
is synthesized from glutamine, the first product of N assimilation produced by the
GS/GOGAT cycle after reduction of nitrate and the glyceraldehyde 3-phosphate
(G3P) and ribose 5-phosphate (5RP) provided by the glycolytic and pentose phosphate pathways, respectively. Here we review the recent progress in characterization
of the aspartate-derived metabolic pathway with a particular focus on methionine
biosynthesis and its salvage pathway (Yang cycle) related to ethylene and polyamine
biosynthesis. Emphasis is placed on the key role of aminotransferases in regulating
these pathways and their relation with aromatic amino acid biosynthesis and catabolism. Indeed, the promiscuity of certain aminotransferases extends their catalytic
function and gives them a key role in the metabolism of ethylene, IAA, and aromatic
amino acids. In this respect, recent studies have identified specific aminotransferases
as being the main targets involved in the root morphogenetic program in response
to environmental cues, nutrient availability, and energy status. Thus, genetically
engineered plants for some aminotransferases, such as ACC synthase and tryptophan
aminotransferase, demonstrate a great potential to produce crop species with
enhanced exploratory root growth and a better nitrogen use efficiency. How the
network of aminotransferases is involved in nitrogen-sensing systems such as plant
glutamate receptors, TOR, and GCN2 kinases is now becoming a fundamental issue.
The use of specific and nonspecific inhibitors of the catalytic activity of certain
aminotransferases should help future pharmacological and genetic approaches to
unravel their role in N, S, and C sensory systems.
254
E. Le Deunff