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a rate-limiting step for synthesis of Spd and Spm (Mehta et al., 2002). Cadaverine, a less
widely distributed polyamine, is produced as a catabolic derivative of lysine through the
activity of lysine decarboxylase (Bouchereau et al., 1999).
15.2.2 Polyamine and ethylene biosynthesis share a common precursor
Ethylene has tremendous impact on fruit ripening and has been termed as “ripening hor-
mone” (Mattoo and White, 1991; Abeles et al., 1992; Fluhr and Mattoo, 1996). PA and
ethylene biosynthetic pathways share a common intermediate, SAM. It has been suggested
that competition for this substrate may tightly regulate internal levels of PAs and ethylene
(Kushad and Dumbroff, 1991; Mattoo and White, 1991; Escribano and Merodio, 1994;
Fluhr and Mattoo, 1996; Mehta et al., 1999; Pandey et al., 2000). Ethylene is a plant hor-
mone that initiates fruit ripening in climacteric fruits and promotes senescence in leaves
(see Mattoo and Suttle, 1991; Abeles et al., 1992; Mattoo and Handa, 2004). Ethylene has
multiple influences on plant growth and development, in addition to enhancing ripening
and senescence, thereby generating significant interest in understanding the interactions
between ethylene and PAs in affecting various biochemical and physiological processes
in fruits and vegetables. Ethylene is synthesized from SAM by sequential action of two
enzymes: 1-aminocyclopropane-1-carboxylate (ACC) synthase and ACC oxidase. In vitro
studies suggest that PAs inhibit ethylene biosynthesis in a variety of fruit and vegetative
tissues, while ethylene suppresses the accumulation of PAs (Apelbaum et al., 1981; Li et al.,
1992; Cassol and Mattoo, 2003). Inhibition of ethylene biosynthesis by PAs results in chan-
neling of SAM into PA biosynthesis (Ben-Arie et al., 1982; Even-Chen et al., 1982; Roberts
et al., 1984). These studies led to a hypothesis that a cross-talk exists between these two
apparently antagonistic biosynthetic pathways (Mehta et al., 1997). However, increased
ethylene production in tomato (Saftner and Baldi, 1990), cherimoya (Escribano and Mero-
dio, 1994), and melon (Martinez-Madrid et al., 2002) is not always accompanied by a
decline in Put. This suggests alternative interactions between ethylene and PA pathways in
these systems. Watercore-affected apple fruit produced more ethylene and contained higher
levels of Put, Spd, ACC, and 1-malonylamino-cyclopropane-l-carboxylic acid, a conjugate
of ACC (Wang and Faust, 1992). Interestingly, accumulation of higher PAs, Spd, and Spm in
transgenic tomato fruits expressing yeast SAM decarboxylase also resulted in several-fold
higher ethylene production in these fruits as compared to the controls (Mehta et al., 2002).
These results together suggest that PAs and ethylene pathways can be simultaneously active
in fruits. Hence, more work is needed to show that production of SAM is actually a rate-
limiting step in regulating levels of PA and ethylene in physiological processes (Matilla,
1996; Walden et al., 1997; Matilla, 2000; Mattoo et al., 2003).
15.2.3 Polyamine biosynthesis and methionine cycle
As mentioned above, methionine (Met) plays a central role in the production of PA and
ethylene pathway. Methionine cycle facilitates recycling of Met from methylthioadeno-
sine (MTA), a byproduct of PA and ethylene biosynthesis (Fig. 15.1) allowing for con-
tinued flux of Met into PA and ethylene (Wang et al., 1982; Yang and Hoffman, 1984;
Miyazaki and Yang, 1987; Sauter et al., 2004; Katharina et al., 2007). MTA is depurinated
by MTA nucleosidase to 5-methylthioribose (MTR) followed by phosphorylation of the C-1
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