1 Introduction
Although N fertilization is one of the major components in the success of the green
revolution and plant growth and productivity, the basis of the profound remodeling
induced by nitrogen in nitrogen (N), sulfur (S), and carbon (C) allocation between
shoots and roots remains always elusive (Good et al. 2004; Hirel et al. 2007;
Masclaux-Daubresse et al. 2010). Thus, it is well known that N deficiency induces
proliferation of roots at the expense of shoots whereas excess of N induced the
opposite situation (Scheible et al. 1997). However, the basis of these physiological
and molecular switches is still not understood (Scheible et al. 1997; Le Ny et al.
2013). For example, conventional approaches to study root functional and structural
responses based on nitrate availability under homogeneous and heterogeneous
N supply conditions are hampered by the fact that there is a very strong coordination
and integration of absorption and assimilation of N with C fixation at primary
metabolism level (Wang et al. 2003; Scheible et al. 2004; Bi et al. 2007; Nero
et al. 2009; Bussell et al. 2013). NO 3
À uptake and its utilization by plants to match
the N demand are sensitively regulated by internal N-sensing mechanisms that
control functional and structural responses via a myriad of signaling molecules.
Although many candidate systems for N detection have been proposed such as TOR
(Target Of Rapamycin) and GCN2 (General Control Non-derepressible 2) kinases as
well as GLR (Glutamate Like Receptor) receptors, most approaches for testing these
N sensory systems ignore the involvement of hormones such as ethylene and
IAA (Lam et al. 2006; Castilho et al. 2014; Xiong and Sheen 2014). However,
compared to nitrate, ethylene and IAA play a major role in root growth and
development because they act at very low concentrations (nM and μM) and at
very short-term durations (min to hours) on root and shoot development (Robinson
2005; Le Deunff et al. 2016). Moreover, their interactions are mostly involved in the
primary root and lateral root (LR) development as well as root hair growth
(Stepanova et al. 2007; Muday et al. 2012; Hu et al. 2017). Interestingly, plant
hormones such as ethylene, indole-3-acetic acid (IAA), and phenylacetic acid (PAA)
are derived from L-methionine (Met), L-tryptophan (Trp), and L-phenylalanine
(Phe) amino acids, respectively (Giovanelli et al. 1985, Stepanova et al. 2008;
Sugawara et al. 2015). This confers to these plant hormones a specific place in N,
S, and C primary metabolism in relation with growth during the plant life cycle.
Among these hormones, ethylene occupies a special place since it is a volatile
molecule involved in alarm, stress, and senescence (Abeles et al. 1992), but is also
a powerful natural anesthetic (Baluska et al. 2016). Moreover, the first precursor of
ethylene, methionine, is an essential amino acid at the intersection of the metabolism
of N, S, and C. Its second precursor, S-adenosyl-L-methionine (AdoMet), is a
fundamental cofactor involved in plant transmethylation reactions that is used
by folates (THF) as a relay molecule to extend their capacities for methylation
(Cossins 2000; Lu 2000; Gerelova et al. 2017). Finally, the direct precursor of
ethylene, ACC, is a non-proteinogenic amino acid whose production is exquisitely
regulated (Wang et al. 2002; Stepanova et al. 2007) and has a potential role as an
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
255
Although N fertilization is one of the major components in the success of the green
revolution and plant growth and productivity, the basis of the profound remodeling
induced by nitrogen in nitrogen (N), sulfur (S), and carbon (C) allocation between
shoots and roots remains always elusive (Good et al. 2004; Hirel et al. 2007;
Masclaux-Daubresse et al. 2010). Thus, it is well known that N deficiency induces
proliferation of roots at the expense of shoots whereas excess of N induced the
opposite situation (Scheible et al. 1997). However, the basis of these physiological
and molecular switches is still not understood (Scheible et al. 1997; Le Ny et al.
2013). For example, conventional approaches to study root functional and structural
responses based on nitrate availability under homogeneous and heterogeneous
N supply conditions are hampered by the fact that there is a very strong coordination
and integration of absorption and assimilation of N with C fixation at primary
metabolism level (Wang et al. 2003; Scheible et al. 2004; Bi et al. 2007; Nero
et al. 2009; Bussell et al. 2013). NO 3
À uptake and its utilization by plants to match
the N demand are sensitively regulated by internal N-sensing mechanisms that
control functional and structural responses via a myriad of signaling molecules.
Although many candidate systems for N detection have been proposed such as TOR
(Target Of Rapamycin) and GCN2 (General Control Non-derepressible 2) kinases as
well as GLR (Glutamate Like Receptor) receptors, most approaches for testing these
N sensory systems ignore the involvement of hormones such as ethylene and
IAA (Lam et al. 2006; Castilho et al. 2014; Xiong and Sheen 2014). However,
compared to nitrate, ethylene and IAA play a major role in root growth and
development because they act at very low concentrations (nM and μM) and at
very short-term durations (min to hours) on root and shoot development (Robinson
2005; Le Deunff et al. 2016). Moreover, their interactions are mostly involved in the
primary root and lateral root (LR) development as well as root hair growth
(Stepanova et al. 2007; Muday et al. 2012; Hu et al. 2017). Interestingly, plant
hormones such as ethylene, indole-3-acetic acid (IAA), and phenylacetic acid (PAA)
are derived from L-methionine (Met), L-tryptophan (Trp), and L-phenylalanine
(Phe) amino acids, respectively (Giovanelli et al. 1985, Stepanova et al. 2008;
Sugawara et al. 2015). This confers to these plant hormones a specific place in N,
S, and C primary metabolism in relation with growth during the plant life cycle.
Among these hormones, ethylene occupies a special place since it is a volatile
molecule involved in alarm, stress, and senescence (Abeles et al. 1992), but is also
a powerful natural anesthetic (Baluska et al. 2016). Moreover, the first precursor of
ethylene, methionine, is an essential amino acid at the intersection of the metabolism
of N, S, and C. Its second precursor, S-adenosyl-L-methionine (AdoMet), is a
fundamental cofactor involved in plant transmethylation reactions that is used
by folates (THF) as a relay molecule to extend their capacities for methylation
(Cossins 2000; Lu 2000; Gerelova et al. 2017). Finally, the direct precursor of
ethylene, ACC, is a non-proteinogenic amino acid whose production is exquisitely
regulated (Wang et al. 2002; Stepanova et al. 2007) and has a potential role as an
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
255