Polyamines and Regulation of Ripening and Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 330

15.11 Molecular basis of polyamine effects
15.11.1 Cellular effects of polyamines
PAs affect many cellular processes including cell proliferation, cell division and differen-
tiation, apoptosis, homeostasis, gene expression, protein, and DNA synthesis (Tabor and
Tabor, 1984; Slocum and Flores, 1991; Cohen, 1998; Igarashi and Kashiwagi, 2000). How-
ever, in spite of these varied effects of PAs, little is understood about the mode of action at
the cellular level. All these physiological roles suggested for PAs in fruit growth, ripening,
shelf life and stress tolerance may be partly due to their cationic nature that facilitates PAs
binding to proteins and membranes. PAs also bind to nucleic acids and may thereby regulate
gene expression and modulate homeostasis as reflected by changes in global gene expres-
sion (Srivastava et al., 2007) and modulation of metabolite (Mattoo et al., 2006) and protein
profiles (Mattoo et al., 2007). Understanding the cellular roles of PAs in mediating these
physiological parameters is important to comprehend the nature of PA action. However,
studies of cellular roles of PAs in plants are limited, and current knowledge of their mode
of action is derived largely from non-plant systems.
15.11.2 Interaction of polyamines with DNA, RNA, and proteins
Spd and Spm can bind to DNA and modulate their stability and conformation, and influence
chromatin remodeling (Raspaud et al., 1999; Childs et al., 2003; Keniry, 2003). Triplex
DNA exhibited highest aggregation in the presence of PAs, but higher concentration of PAs
resolubilized these aggregates with single-stranded DNA-PA complex being the easiest to
solubilize (Childs et al., 2003). Molecular dynamics simulations and UV absorption studies
indicate that PAs preferred stabilization of A-DNA over B-DNA conformation and also
induced B-Z transition (Bryson and Greenall, 2000). Uranyl photoprobing was used to show
that PAs preferentially bind to bent adenine tracks of double-stranded DNA (Lindemose
et al., 2005). PAs have been implicated in increasing core nucleosome stability and also
in facilitating in vitro condensation of nucleosomal fibers (Makarov et al., 1987; Morgan,
1987). Nuclear aggregates of PAs have been suggested to play a crucial role in protecting
genomic DNA and its conformation (Agostino et al., 2005). About 90% of Spd and 50% of
the Put exist as PA-RNA complexes in E.coli (Igarashi and Kashiwagi, 2006). PAs control
phosphorylation of nucleolar proteins in pea (Datta et al., 1987). Covalent binding of PAs
possibly through the activity of transglutaminases (TGases) may aid in storage and transport
of these compounds. A role for PAs in buffering cellular pH has also been proposed (Galston
and Kaur-Sawhney, 1995).
15.11.3 Polyamine interactions with membranes
PAs may influence fruit ripening, senescence, improvement of fruit shelf life including firm-
ness through their effects on stabilization of membranes and prevention of protein, nucleic
acid, and chlorophyll degradation, processes that are associated with senescence (Ben-
Arie et al., 1982; Galston and Kaur-Sawhney, 1987; Evans and Malmberg, 1989; Brune
et al., 1991; Pandey et al., 2000). Anti-senescence role of PAs in membrane stabiliza-
tion is attributed to their ability to bind and interact with negatively charged phospholipid