Kinase) family and is involved in the elongation of root hairs and leaf cells (Guo
et al. 2009; Deslauriers and Larsen 2010; Duan et al. 2010). FER receptor ligand is
the RALF peptide (Rapid Alkalinization Factor) that is able to bind to the FER
receptor and trigger phosphorylation of many cytosolic proteins such as H
+
-ATPase
AHA2 involved in cell elongation (Haruta et al. 2014). Surprisingly, fer mutants
display a dwarf phenotype and a significant increase in ethylene production and
AdoMet contents. The dwarf phenotype of fer mutants was mimicked in transgenic
plants over-expressing SAM whereas sam1sam2 double mutant showed a wild-type
phenotype suggesting that FER receptor might be involved in inhibition of SAM
activity thereby reducing levels of AdoMet and ethylene. Further work is needed to
determine mechanisms by which FER receptor inhibits SAM1 and SAM2 activities
in responses to RALF peptide hormone and ethylene crosstalk.
SAM isoforms are also regulated at the posttranslational level by CPK28, a
calcium-dependent protein kinase, that interacts with phosphorylated forms of
SAM1, SAM2, and SAM3 to induce their degradation through the ubiquitin/26S
proteasome pathway (Jin et al. 2017). At the biochemical level, cpk8 mutants
display an increase in AdoMet contents and SAM protein levels, whereas at the
phenotypic level they present short hypocotyls and an increase in lignification
caused by ethylene overproduction. Inhibiting ACC synthetase (ACS) activity
with AVG treatment could restore the wild-type phenotype. Taken together
these results demonstrate that the last step of the methionine biosynthetic pathway
is highly regulated at the translational and posttranscriptional levels because AdoMet
is a regulatory node molecule involved in plant development through transmethylation reactions, crosstalk between ethylene and polyamine biosynthesis, and
stress signaling responses.
3 Because of Its Vital Cellular Functions AdoMet Requires
Two Salvage Pathways
In Lemna paucicostata, production of AdoMet consumes 80% of Met produced by
the methionine pathway whereas 20% of Met is involved for protein synthesis
(Giovanelli et al. 1985). AdoMet is one of the major cofactors used in nature with
ATP and ensures vital biochemical functions in plant and animal cells (Lu 2000).
Indeed, AdoMet is the methyl group donor during transmethylation reactions of
lignins, chlorophylls, proteins, phospholipids, and nucleic acids biosynthesis, the
precursor for biosynthesis of ethylene, polyamine, nicotianamine (siderophore), and
biotin, and it is also involved in many transsulfuration reactions (Lu 2000). Since
AdoMet is the end product of the methionine pathway and one of the major cofactors
used in nature, the lack of recycling of the AdoMet methylthio-group and adenosine
moiety reduces biosynthesis of ethylene and polyamines by a restriction in sulfur
availability (Baur and Yang 1972; Bürstenbinder et al. 2007). From a literature
survey, it appears that the cellular AdoMet homeostasis is controlled at several
262
E. Le Deunff
et al. 2009; Deslauriers and Larsen 2010; Duan et al. 2010). FER receptor ligand is
the RALF peptide (Rapid Alkalinization Factor) that is able to bind to the FER
receptor and trigger phosphorylation of many cytosolic proteins such as H
+
-ATPase
AHA2 involved in cell elongation (Haruta et al. 2014). Surprisingly, fer mutants
display a dwarf phenotype and a significant increase in ethylene production and
AdoMet contents. The dwarf phenotype of fer mutants was mimicked in transgenic
plants over-expressing SAM whereas sam1sam2 double mutant showed a wild-type
phenotype suggesting that FER receptor might be involved in inhibition of SAM
activity thereby reducing levels of AdoMet and ethylene. Further work is needed to
determine mechanisms by which FER receptor inhibits SAM1 and SAM2 activities
in responses to RALF peptide hormone and ethylene crosstalk.
SAM isoforms are also regulated at the posttranslational level by CPK28, a
calcium-dependent protein kinase, that interacts with phosphorylated forms of
SAM1, SAM2, and SAM3 to induce their degradation through the ubiquitin/26S
proteasome pathway (Jin et al. 2017). At the biochemical level, cpk8 mutants
display an increase in AdoMet contents and SAM protein levels, whereas at the
phenotypic level they present short hypocotyls and an increase in lignification
caused by ethylene overproduction. Inhibiting ACC synthetase (ACS) activity
with AVG treatment could restore the wild-type phenotype. Taken together
these results demonstrate that the last step of the methionine biosynthetic pathway
is highly regulated at the translational and posttranscriptional levels because AdoMet
is a regulatory node molecule involved in plant development through transmethylation reactions, crosstalk between ethylene and polyamine biosynthesis, and
stress signaling responses.
3 Because of Its Vital Cellular Functions AdoMet Requires
Two Salvage Pathways
In Lemna paucicostata, production of AdoMet consumes 80% of Met produced by
the methionine pathway whereas 20% of Met is involved for protein synthesis
(Giovanelli et al. 1985). AdoMet is one of the major cofactors used in nature with
ATP and ensures vital biochemical functions in plant and animal cells (Lu 2000).
Indeed, AdoMet is the methyl group donor during transmethylation reactions of
lignins, chlorophylls, proteins, phospholipids, and nucleic acids biosynthesis, the
precursor for biosynthesis of ethylene, polyamine, nicotianamine (siderophore), and
biotin, and it is also involved in many transsulfuration reactions (Lu 2000). Since
AdoMet is the end product of the methionine pathway and one of the major cofactors
used in nature, the lack of recycling of the AdoMet methylthio-group and adenosine
moiety reduces biosynthesis of ethylene and polyamines by a restriction in sulfur
availability (Baur and Yang 1972; Bürstenbinder et al. 2007). From a literature
survey, it appears that the cellular AdoMet homeostasis is controlled at several
262
E. Le Deunff