Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

POLYAMINES AND REGULATION OF RIPENING AND SENESCENCE 323
occur in microorganisms and have been speculated to offer protection in mediating growth
responses under extreme environmental conditions (Kuehn et al., 1990). Occurrence of
such uncommon PAs has also been reported in an osmotic stress-tolerant strain of alfalfa
where Spd synthase give rise to thermospermine in addition to Spd and Spm (Bagga et al.,
1997). PAs also lead to the formation of several alkaloid compounds of pharmacological
interest in many plants. Compounds such as tropane alkaloids and nicotine are derived
from Put through the activity of Put N-methyl transferase (Hibi et al., 1994). Additionally,
alkaloids can also be derived through conjugation products of other PAs (Smith et al., 1983).
Piperidine and quinolizidine alkaloids are derived from cadaverine (Wink and Hartman,
1982).
15.5 Polyamine transport and localization
Long-distance transport of PAs has been reported in plants, but the presence of a PA
transporter has not yet been demonstrated (Martin-Tanguy, 1985). However, radiolabeling
of zucchini hypocotyls with [ 14 C]-Spd showed binding to 66-kDa and 44-kDa
plasma membrane proteins suggesting the presence of putative Spd-binding polypeptides
in plants (Tassoni et al., 1998). However, multiple PA transporter genes have been
reported in Escherichia coli and yeast (Kashiwagi et al., 1990, 1997; Tomitori et al.,
2001).
The ODC pathway for PA synthesis was predominantly localized to the cytoplasm in the
algae, Chlamydomonas reinhardtii (Voigt et al., 2000). In higher plants, both nuclear and
cytoplasmic localization of this enzyme has been reported (Slocum and Flores, 1991). In
tobacco, ADC is mainly localized to the chloroplast of photosynthetic tissues such as leaves
and stems and to the nucleus in nonphotosynthetic tissues like roots and flowers, suggesting
its role in photosynthesis and cellular signaling processes such as protein phosphorylation.
ADC does not possess transmembrane domains and hence, localization to thylakoid
membranes may be due to a predicted putative amphiphilic α-helix-forming region with
high hydrophobicity (Borrell et al., 1995). Spd synthase has been localized to the chloroplast
(Cohen et al., 1981) and cytoplasm (Sindhu and Cohen, 1984). Additionally, the PA
catabolic enzyme PAO was localized to the cell wall (Kaur-Sawhney et al., 1981; Slocum
and Furey, 1991).
15.6 Polyamines, regulators of postharvest processes
PAs have been implicated in a multitude of growth and developmental processes such as
embryogenesis, root formation, floral initiation and development, pollen tube growth, fruit
development including ripening, senescence, biotic, and abiotic stress responses (Slocum
and Flores, 1991; Cohen, 1998; Cassol and Mattoo, 2003; Kaur-Sawhney et al., 2003). Fruit
ripening and senescence are two processes that are closely associated with the shelf life of
fresh produce and produce destined for processing. In vitro and pharmacological experiments
with PAs have led to the suggestion that PAs are important plant growth regulators
with the ability to inhibit ripening and senescence in a number of plant tissues (Galston and
Kaur-Sawhney, 1995; Cassol and Mattoo, 2003). PAs may also play a significant role by
regulating biotic and abiotic stress responses that limit quality and postharvest shelf life of
produce.