OASTL dimers requiring four molecules of PLP for functioning (Droux et al. 1998).
This complex, first found in bacteria, plays a major role in plants for modulation of
cysteine biosynthesis via bisulfide and OAS. Some authors consider that this complex could be a cellular sulfur sensor (Yi et al. 2010; Gerelova et al. 2017).
Serine is also indirectly involved in the last step of methionine biosynthesis through
the use of a polyglutamate form of tetrahydrofolate (5-methyl-THF: 5-CH 3 H 4 PteGlu n
with n > 3) as a methyl-group donor (Eichel et al. 1995). Thus, homocysteine (Hcy)
methylation to form methionine is catalyzed by methionine synthase (MS). The methyl
group given by 5-methyl-THF is provided by the β-carbon of serine after two consecutive reactions catalyzed in the plastids by serine hydroxymethyltransferase (THFT)
and 5,10-methylenetetrahydrofolate reductase (MTHFR). THFT enzyme uses serine as
a methylene group donor to convert tetrahydrofolate (H 4 PteGlu n ) into methylene-THF
(5,10-CH 2 H 4 PteGlu n ) and glycine (Fig. 3). Then, methylene-THF is converted to its
reduced form 5-methyl-THF (5-CH 3 H 4 PteGlu n ) by MTHFR. The one-carbon group of
5-methyl-THF is used by methionine synthase (MS) to convert Hcy to methionine
(Shah and Cossins 1970; Gorelova et al. 2017). Polyglutamate forms of THF are
synthetized by folylpolyglutamate synthase (FPGS), an enzyme localized in mitochondria, cytosol, and plastids that catalyzes the attachment of glutamate tail to a THF
molecule (Ravanel et al. 2001). In Arabidopsis, mutation of the AtDFB gene encoding
FPGS enzyme showed a disruption in primary root growth by an alteration of the
quiescent center of root meristem, suggesting that THF metabolism plays a major role
in root cell proliferation (Srivastava et al. 2011; Reyes-Hernández et al. 2014). Therefore, serine is doubly required for methionine biosynthesis via the cysteine biosynthesis
pathway and production of the polyglutamate form of 5-methyl-THF.
2.3 AdoMet Synthase Is a Fundamental Control Point
of the Methionine Pathway
The last step of the methionine pathway is catalyzed by methionine adenosyltransferase (SAM or MAT), a cytosolic enzyme that catalyzes conversion of methionine and ATP into AdoMet. In the Arabidopsis genome, SAM is encoded by three
genes (SAM1, SAM2, and SAM3) and their expression has been shown to be highly
regulated at transcriptional (Peleman et al. 1989a, b; Boerjan et al. 1994; Chen et al.
2016) and posttranscriptional levels (Mao et al. 2015; Jin et al. 2017) in response to
hormones, biotic and abiotic stress.
Recently, it has been shown in Arabidopsis that FERONIA (FER), a plasma
membrane receptor-like kinase, may negatively regulate SAM (Mao et al. 2015).
FERONIA receptor belongs to the CrRLK (Catharanthus roseus Receptor Like
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
261
This complex, first found in bacteria, plays a major role in plants for modulation of
cysteine biosynthesis via bisulfide and OAS. Some authors consider that this complex could be a cellular sulfur sensor (Yi et al. 2010; Gerelova et al. 2017).
Serine is also indirectly involved in the last step of methionine biosynthesis through
the use of a polyglutamate form of tetrahydrofolate (5-methyl-THF: 5-CH 3 H 4 PteGlu n
with n > 3) as a methyl-group donor (Eichel et al. 1995). Thus, homocysteine (Hcy)
methylation to form methionine is catalyzed by methionine synthase (MS). The methyl
group given by 5-methyl-THF is provided by the β-carbon of serine after two consecutive reactions catalyzed in the plastids by serine hydroxymethyltransferase (THFT)
and 5,10-methylenetetrahydrofolate reductase (MTHFR). THFT enzyme uses serine as
a methylene group donor to convert tetrahydrofolate (H 4 PteGlu n ) into methylene-THF
(5,10-CH 2 H 4 PteGlu n ) and glycine (Fig. 3). Then, methylene-THF is converted to its
reduced form 5-methyl-THF (5-CH 3 H 4 PteGlu n ) by MTHFR. The one-carbon group of
5-methyl-THF is used by methionine synthase (MS) to convert Hcy to methionine
(Shah and Cossins 1970; Gorelova et al. 2017). Polyglutamate forms of THF are
synthetized by folylpolyglutamate synthase (FPGS), an enzyme localized in mitochondria, cytosol, and plastids that catalyzes the attachment of glutamate tail to a THF
molecule (Ravanel et al. 2001). In Arabidopsis, mutation of the AtDFB gene encoding
FPGS enzyme showed a disruption in primary root growth by an alteration of the
quiescent center of root meristem, suggesting that THF metabolism plays a major role
in root cell proliferation (Srivastava et al. 2011; Reyes-Hernández et al. 2014). Therefore, serine is doubly required for methionine biosynthesis via the cysteine biosynthesis
pathway and production of the polyglutamate form of 5-methyl-THF.
2.3 AdoMet Synthase Is a Fundamental Control Point
of the Methionine Pathway
The last step of the methionine pathway is catalyzed by methionine adenosyltransferase (SAM or MAT), a cytosolic enzyme that catalyzes conversion of methionine and ATP into AdoMet. In the Arabidopsis genome, SAM is encoded by three
genes (SAM1, SAM2, and SAM3) and their expression has been shown to be highly
regulated at transcriptional (Peleman et al. 1989a, b; Boerjan et al. 1994; Chen et al.
2016) and posttranscriptional levels (Mao et al. 2015; Jin et al. 2017) in response to
hormones, biotic and abiotic stress.
Recently, it has been shown in Arabidopsis that FERONIA (FER), a plasma
membrane receptor-like kinase, may negatively regulate SAM (Mao et al. 2015).
FERONIA receptor belongs to the CrRLK (Catharanthus roseus Receptor Like
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
261