Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

322 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
hydroxyl group on the ribose moiety of MTR by MTR kinase yielding 5-methylthioribose-
1-phosphate (MTR-P). MTR-P undergoes subsequent enzymatic isomerization, dehydration,
and oxidative decarboxylation to 2-keto-4-methylthiobutyrate, which is the immediate
precursor of methionine.
15.3 Polyamine catabolism
Amine oxidases such as diamine oxidase (DAO) and PA oxidase (PAO) are primarily responsible
for PA catabolism (Fig. 15.1). DAO oxidizes Put to pyrroline, hydrogen peroxide,
and ammonia. Pyrroline can be further catalyzed to form γ -aminobutyric acid (GABA)
that is subsequently converted to succinate and incorporated into the Krebs cycle, thus
recycling carbon and nitrogen from Put. PAO catalyzes production of pyrroline and 1,5-
diabicyclononane, from Spd and Spm, respectively, and generates diaminopropane (DAP)
and hydrogen peroxide. DAP subsequently produces β-alanine (Bouchereau et al., 1999;
Tavladoraki et al., 2006). Association of amine oxidases with primary and secondary cell
wall tissues during specific developmental processes has been reported (Rea et al., 2004).
Hydrogen peroxide released through these catabolic reactions has been suggested to control
processes such as lignification, suberization, and cell wall stiffening. In vertebrates and
yeast, acetylated forms of Spm and Spd can be converted back to Put by the activity of
Spd/Spm acetyl transferase (SSAT). Activity of SSAT has not yet been demonstrated in
plant tissues. However, acetyl PAs are present in sugar-beet seedlings (Christ et al., 1989),
Helianthus tuberosus chloroplasts (Del-Duca et al., 1995) and various organs of Arabidopsis,
suggesting that PA interconversion may also occur in plants (Tassoni et al., 2000). Also,
yeast and animal Spm oxidases can oxidize Spm back to Spd, suggesting SSAT-independent
back conversion of PAs (Cona et al., 2006). An Arabidopsis PAO was shown to catalyze production
of Spd and nor-Spd from Spm and nor-Spm, respectively (Tavladoraki et al., 2006).
15.3.1 Conjugated and bound polyamines
PAs can exist as free, bound, and conjugated forms in most plant systems. PAs are conjugated
to hydroxycinnamic acids by an amide linkage using esters of CoA catalyzed by
a class of enzymes called transferases. They occur both as basic and neutral forms. In the
basic form, one amine group of PAs is associated with phenolic cinnamic acid, and in
the neutral form each terminal amine group of an aliphatic amine is linked with cinnamic
acid (Martin-Tanguy, 1997). Spd, homo-Spd, and Spm conjugate with fatty acids as well
as cinnamoyl compounds (Martin-Tanguy, 2001). Conjugation of PAs, through binding of
hydroxycinnamic acid, is thought to have important roles in long-distance translocation of
PAs and floral induction (Martin-Tanguy, 1985, 1997). Owing to their cationic nature, PAs
are bound with several proteins such as transglutaminases (discussed later in the chapter)
and other macromolecules in tobacco, oats, and petunia (Apelbaum et al., 1988; Mizrahi et
al., 1989).
15.4 Uncommon polyamines
Several uncommon PAs such as homo-Spd, aminopropyl-cadavarine, thermo-Spm, nor-Spd,
nor-Spm, caldopentamine, homocaldopentamine, caldohexamine, and homocaldohexamine