agonist on plant glutamate receptors (Le Deunff and Lecourt 2016). Finally, ethylene
acts on specific receptors and some components of its signaling pathway such as the
transcription factors ethylene insensitive 3 (EIN3) and its homologue EIN3-LIKE
1 (EIL1) play a major role in the regulation of auxin biosynthetic genes (He et al.
2011) and responses to glucose through the hexokinase1 (HXK1) C sensor
(Yanagisawa et al. 2003; Yoo et al. 2008).
So far, emphasis has been placed on the regulation of biosynthesis and signaling
cascades of both ethylene and auxins and their interactions on root and shoots
growth (Muday et al. 2012; Hu et al. 2017). However, a major challenge for the
next few years is to address the precise molecular mechanisms underlying the
complex signaling network that governs regulation of the metabolic interconnections
between mineral nutrition, amino acid biosynthesis, and the production of plant
growth regulators. In this aim, an alternative approach consists of stating that the
biosynthesis of hormones such as ethylene and IAA is highly regulated through the
aminotransferases network in primary metabolism (Le Deunff et al. 2016). Indeed,
aminotransferases are the key enzymes that ensure N, S, and C shuttlings but are also
essential in the biosynthesis of ethylene and auxin precursors Met, Trp, and Phe, and
in the production of major cofactors such as vitamins B 9 (folates) and B 6 .
This review focuses on the metabolic interconnections between ethylene biosynthesis and nitrogen metabolism and, to a lesser extent, sulfur metabolism. Because
the focus is mainly on the network of PLP-dependent aminotransferases during
ethylene and methionine biosynthesis, ethylene signaling is less discussed.
2 A Highly Regulated Pathway for the Biosynthesis
of Methionine and S-Adenosylmethionine (AdoMet):
The Ethylene Precursors
In plants, AA biosynthetic pathways can be organized depending on the origin of
their carbon skeleton coming from glycolysis, the citric acid cycle, and the oxidative
pentose phosphate pathway (Fig. 1; Coruzzi and Last 2000; Stitt et al. 2002).
The carbon skeleton of methionine is mainly synthesized via a branch of aspartate
pathway, but its sulfur atom is derived from cysteine and its methyl group from
the β-carbon of serine. Other amino acids such as Thr, Ile, and Lys also originate
from this aspartate-derived metabolic pathway (Azevedo et al. 2006; Jander and
Joshi 2009). Due to the importance in human diets of these four amino acids (Met,
Thr, Ile, and Lys), the biochemical regulatory mechanisms involved in the biosynthesis of aspartate-derived amino acids have been intensively studied. Thus, allosteric regulations of the branch-point enzymes by the pathway products have been
determined by genetic and biochemical approaches and are summarized in Fig. 2.
More details and explanations can be found in excellent reviews (Coruzzi and Last
2000; Azevedo et al. 2006; Jander and Joshi 2009; Galili 2011). Here we only focus
on methionine biosynthesis that competes with threonine biosynthesis because these
256
E. Le Deunff
acts on specific receptors and some components of its signaling pathway such as the
transcription factors ethylene insensitive 3 (EIN3) and its homologue EIN3-LIKE
1 (EIL1) play a major role in the regulation of auxin biosynthetic genes (He et al.
2011) and responses to glucose through the hexokinase1 (HXK1) C sensor
(Yanagisawa et al. 2003; Yoo et al. 2008).
So far, emphasis has been placed on the regulation of biosynthesis and signaling
cascades of both ethylene and auxins and their interactions on root and shoots
growth (Muday et al. 2012; Hu et al. 2017). However, a major challenge for the
next few years is to address the precise molecular mechanisms underlying the
complex signaling network that governs regulation of the metabolic interconnections
between mineral nutrition, amino acid biosynthesis, and the production of plant
growth regulators. In this aim, an alternative approach consists of stating that the
biosynthesis of hormones such as ethylene and IAA is highly regulated through the
aminotransferases network in primary metabolism (Le Deunff et al. 2016). Indeed,
aminotransferases are the key enzymes that ensure N, S, and C shuttlings but are also
essential in the biosynthesis of ethylene and auxin precursors Met, Trp, and Phe, and
in the production of major cofactors such as vitamins B 9 (folates) and B 6 .
This review focuses on the metabolic interconnections between ethylene biosynthesis and nitrogen metabolism and, to a lesser extent, sulfur metabolism. Because
the focus is mainly on the network of PLP-dependent aminotransferases during
ethylene and methionine biosynthesis, ethylene signaling is less discussed.
2 A Highly Regulated Pathway for the Biosynthesis
of Methionine and S-Adenosylmethionine (AdoMet):
The Ethylene Precursors
In plants, AA biosynthetic pathways can be organized depending on the origin of
their carbon skeleton coming from glycolysis, the citric acid cycle, and the oxidative
pentose phosphate pathway (Fig. 1; Coruzzi and Last 2000; Stitt et al. 2002).
The carbon skeleton of methionine is mainly synthesized via a branch of aspartate
pathway, but its sulfur atom is derived from cysteine and its methyl group from
the β-carbon of serine. Other amino acids such as Thr, Ile, and Lys also originate
from this aspartate-derived metabolic pathway (Azevedo et al. 2006; Jander and
Joshi 2009). Due to the importance in human diets of these four amino acids (Met,
Thr, Ile, and Lys), the biochemical regulatory mechanisms involved in the biosynthesis of aspartate-derived amino acids have been intensively studied. Thus, allosteric regulations of the branch-point enzymes by the pathway products have been
determined by genetic and biochemical approaches and are summarized in Fig. 2.
More details and explanations can be found in excellent reviews (Coruzzi and Last
2000; Azevedo et al. 2006; Jander and Joshi 2009; Galili 2011). Here we only focus
on methionine biosynthesis that competes with threonine biosynthesis because these
256
E. Le Deunff