methylation, probably because the methylthio-group of AdoMet is more reactive
(Fig. 3). Therefore, the biosynthesis of methionine strongly depends on serine
metabolism. In this respect, recent studies have demonstrated that the mutants of
genes involved in the plastidial phosphorylated pathway of serine biosynthesis
(PPSB) displayed arrested root development and exhibited a strong impairment of
carbon and nitrogen metabolisms (Muños-Berthomeu et al. 2009, 2010; CascalesMiñana et al. 2013). Furthermore, at the root morphological level, recent results in
Arabidopsis showed that folic acid is essential in root organogenesis via root cell
proliferation in meristems (Reyes-Hernández et al. 2014) and maturation of lateral
root primordia upstream of auxin and independently of TOR kinase signaling
(Ayala-Rodriguez et al. 2017; Li et al. 2017).
3.2 Ethylene and Polyamines Are By-Products Resulting
from the Recycling of the Methylthio-Group of MTA
Issued from AdoMet
5
0 -Methylthioadenosine (MTA), the by-product of the ACC synthase (ACS) and
spermidine synthase (SPDS) reactions, is salvaged for the regeneration of the
methylthio-group and adenine moiety through the methionine cycle, also known
as the Yang or MTA cycle or methionine salvage pathway (Miyazaki and Yang
1987; Sekowska et al. 2004; Pommerrenig et al. 2011; Sauter et al. 2013). In plants,
this cycle has been recently revised (Pommerrenig et al. 2011), and it is now
composed of six reaction steps (Fig. 3) that involve successively: (1) the conversion
of MTA into 5-methylthioribose (MTR) and adenine by MTA nucleosidase (MTN;
Adams and Yang 1977; Wang et al. 1982; Rzewuski et al. 2007), (2) the phosphorylation of MTR into 5-methylthioribose-1-phosphate (MTR-1P) by the MTR
kinase (MTK; Kushad et al. 1982; Sauter et al. 2004), (3) the isomerization of
MTR-1P to 5-methylthioribulose-1-phosphate (MTRu-1P) by MTR-P isomerase
(MTI; Pommerrenig et al. 2011), (4) then dehydratase-enolase-phosphatase (DEP)
ensures the conversion of MTRu-1P into 1,2-dihydro-3-keto-5-methylthiopentene
(DHKMP; Pommerrenig et al. 2011), (5) acireductone dioxygenase (ARD) in the
presence of dioxygen catalyzes the conversion of DHKMP into 2-keto-4-methylthio
butyrate (KMTB; Sauter et al. 2005; Bürstenbinder et al. 2007), and (6) finally
transamination of KMTB to methionine is catalyzed by unknown aminotransferases
(Kushad et al. 1983; Pommerrenig et al. 2011). In plants and many microorganisms,
different unknown plastidial or cytosolic aspartate aminotransferases (AAT) are
presumed to be involved in the last steps of the methionine regeneration pathway
(Berger et al. 2003; Sekowska et al. 2004; Pommerrenig et al. 2011).
264
E. Le Deunff
(Fig. 3). Therefore, the biosynthesis of methionine strongly depends on serine
metabolism. In this respect, recent studies have demonstrated that the mutants of
genes involved in the plastidial phosphorylated pathway of serine biosynthesis
(PPSB) displayed arrested root development and exhibited a strong impairment of
carbon and nitrogen metabolisms (Muños-Berthomeu et al. 2009, 2010; CascalesMiñana et al. 2013). Furthermore, at the root morphological level, recent results in
Arabidopsis showed that folic acid is essential in root organogenesis via root cell
proliferation in meristems (Reyes-Hernández et al. 2014) and maturation of lateral
root primordia upstream of auxin and independently of TOR kinase signaling
(Ayala-Rodriguez et al. 2017; Li et al. 2017).
3.2 Ethylene and Polyamines Are By-Products Resulting
from the Recycling of the Methylthio-Group of MTA
Issued from AdoMet
5
0 -Methylthioadenosine (MTA), the by-product of the ACC synthase (ACS) and
spermidine synthase (SPDS) reactions, is salvaged for the regeneration of the
methylthio-group and adenine moiety through the methionine cycle, also known
as the Yang or MTA cycle or methionine salvage pathway (Miyazaki and Yang
1987; Sekowska et al. 2004; Pommerrenig et al. 2011; Sauter et al. 2013). In plants,
this cycle has been recently revised (Pommerrenig et al. 2011), and it is now
composed of six reaction steps (Fig. 3) that involve successively: (1) the conversion
of MTA into 5-methylthioribose (MTR) and adenine by MTA nucleosidase (MTN;
Adams and Yang 1977; Wang et al. 1982; Rzewuski et al. 2007), (2) the phosphorylation of MTR into 5-methylthioribose-1-phosphate (MTR-1P) by the MTR
kinase (MTK; Kushad et al. 1982; Sauter et al. 2004), (3) the isomerization of
MTR-1P to 5-methylthioribulose-1-phosphate (MTRu-1P) by MTR-P isomerase
(MTI; Pommerrenig et al. 2011), (4) then dehydratase-enolase-phosphatase (DEP)
ensures the conversion of MTRu-1P into 1,2-dihydro-3-keto-5-methylthiopentene
(DHKMP; Pommerrenig et al. 2011), (5) acireductone dioxygenase (ARD) in the
presence of dioxygen catalyzes the conversion of DHKMP into 2-keto-4-methylthio
butyrate (KMTB; Sauter et al. 2005; Bürstenbinder et al. 2007), and (6) finally
transamination of KMTB to methionine is catalyzed by unknown aminotransferases
(Kushad et al. 1983; Pommerrenig et al. 2011). In plants and many microorganisms,
different unknown plastidial or cytosolic aspartate aminotransferases (AAT) are
presumed to be involved in the last steps of the methionine regeneration pathway
(Berger et al. 2003; Sekowska et al. 2004; Pommerrenig et al. 2011).
264
E. Le Deunff