4 ACS and SAMDC Are Key Enzymes in Ethylene
and Polyamine Biosynthetic Pathways and MTA
Recycling
The biosynthesis of ethylene and polyamines are linked through the substrate
AdoMet, which can be converted into decarboxylated S-adenosyl-L-methionine
(dcAdoMet) and 1-aminocyclopropane-1-carboxylic acid (ACC), the ethylene precursor, through the action of S-adenosylmethionine decarboxylase (SAMDC) and
1-aminocyclopropane-1-carboxylate synthase (ACS), respectively (Fig. 3). In several physiological processes, ethylene and polyamines compete for their biosynthesis
because these two metabolic pathways use initially a common substrate: AdoMet.
Despite several studies that have revealed the competitive interaction between these
two pathways during the processes of fruit ripening (Tassoni et al. 2000; HarpazSaad et al. 2012), root elongation (Kumar et al. 1996; Locke et al. 2000; Tassoni
et al. 2000; Hummel et al. 2002), senescence (Pandey et al. 2000), and fruit or leaf
abscission (Saftner 1989; Gil-Amado and Gomez-Jimenez 2012; Harpaz-Saad et al.
2012), little is known about the cross regulations of these two pathways. In fact,
the partitioning of the AdoMet flux between these two pathways has never been
precisely estimated because of the difficulties in measuring the contents of the
different conjugated forms of ACC and polyamines that lead to underestimation of
the flux in one or the other of these pathways.
4.1 SAMDC Is the Rate-Limiting Step for Polyamine
Biosynthesis
Polyamine homeostasis is tightly regulated and achieved by the modulation of
polyamine biosynthesis, conjugation, transport, and catabolism (Tiburcio et al.
2014). In the polyamine pathway AdoMet is converted to decarboxylated
S-adenosyl-L-methionine (dcSAM) and MTA via SAMDC (Fig. 3). In Arabidopsis
SAMDC genes belong to a small family of four genes that are not functionally
redundant. Single and double mutants of SAMDC4 (bud2-1) and SAMDC1
(samdc1) increase root proliferation and growth by alteration of polyamine homeostasis (Kumar et al. 1996; Ge et al. 2006). Knockout of the SMDC4 gene affects
growth and development, whereas alteration of the SMDC1 gene displays a more
severe phenotype (Ge et al. 2006). The double mutant SMDC4-SMDC1 (bud2-1samdc1) exhibited embryo lethality, probably through spermidine deficiency (Imai
et al. 2004). SAMDC genes are regulated at the transcriptional level and induced by
many abiotic stresses such as drought, chilling, heat, hypoxia, ozone, UV, and heavy
metals (Alcázar et al. 2006). During tomato fruit ripening, it seems that no competition for AdoMet substrate occurs between SAMDC, ACS, and transmethylation
reactions, suggesting that these three pathways can operate simultaneously (Van de
Poel et al. 2012). This confirms studies where ethylene production was increased in
transgenic plants over-expressing SAMDC (Mehta et al. 2002).
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
265
and Polyamine Biosynthetic Pathways and MTA
Recycling
The biosynthesis of ethylene and polyamines are linked through the substrate
AdoMet, which can be converted into decarboxylated S-adenosyl-L-methionine
(dcAdoMet) and 1-aminocyclopropane-1-carboxylic acid (ACC), the ethylene precursor, through the action of S-adenosylmethionine decarboxylase (SAMDC) and
1-aminocyclopropane-1-carboxylate synthase (ACS), respectively (Fig. 3). In several physiological processes, ethylene and polyamines compete for their biosynthesis
because these two metabolic pathways use initially a common substrate: AdoMet.
Despite several studies that have revealed the competitive interaction between these
two pathways during the processes of fruit ripening (Tassoni et al. 2000; HarpazSaad et al. 2012), root elongation (Kumar et al. 1996; Locke et al. 2000; Tassoni
et al. 2000; Hummel et al. 2002), senescence (Pandey et al. 2000), and fruit or leaf
abscission (Saftner 1989; Gil-Amado and Gomez-Jimenez 2012; Harpaz-Saad et al.
2012), little is known about the cross regulations of these two pathways. In fact,
the partitioning of the AdoMet flux between these two pathways has never been
precisely estimated because of the difficulties in measuring the contents of the
different conjugated forms of ACC and polyamines that lead to underestimation of
the flux in one or the other of these pathways.
4.1 SAMDC Is the Rate-Limiting Step for Polyamine
Biosynthesis
Polyamine homeostasis is tightly regulated and achieved by the modulation of
polyamine biosynthesis, conjugation, transport, and catabolism (Tiburcio et al.
2014). In the polyamine pathway AdoMet is converted to decarboxylated
S-adenosyl-L-methionine (dcSAM) and MTA via SAMDC (Fig. 3). In Arabidopsis
SAMDC genes belong to a small family of four genes that are not functionally
redundant. Single and double mutants of SAMDC4 (bud2-1) and SAMDC1
(samdc1) increase root proliferation and growth by alteration of polyamine homeostasis (Kumar et al. 1996; Ge et al. 2006). Knockout of the SMDC4 gene affects
growth and development, whereas alteration of the SMDC1 gene displays a more
severe phenotype (Ge et al. 2006). The double mutant SMDC4-SMDC1 (bud2-1samdc1) exhibited embryo lethality, probably through spermidine deficiency (Imai
et al. 2004). SAMDC genes are regulated at the transcriptional level and induced by
many abiotic stresses such as drought, chilling, heat, hypoxia, ozone, UV, and heavy
metals (Alcázar et al. 2006). During tomato fruit ripening, it seems that no competition for AdoMet substrate occurs between SAMDC, ACS, and transmethylation
reactions, suggesting that these three pathways can operate simultaneously (Van de
Poel et al. 2012). This confirms studies where ethylene production was increased in
transgenic plants over-expressing SAMDC (Mehta et al. 2002).
From Aspartate to Ethylene: Central Role of N, C, and S Shuttles by. . .
265