Polyamines and Regulation of Ripening and Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 329

firmness and delayed color development in GA-vacuum infiltrated or heat-treated (45 ◦ C)
lemons were associated with increases in free Put and Spd during storage at 15 ◦ C (Valero
et al., 1998). GA-treated lemons also showed a decrease in ABA (abscisic acid) suggesting a
possible interaction between PA and ABA in delaying senescence. Thus, there appears to be
a cross talk between PAs and various hormones in modulating these physiological responses
(Srivastava and Handa, 2005). Evidence for direct roles of PAs has begun to emerge from
transgenic model plants such as tomato, tobacco, Arabidopsis , and rice. In such instances,
overexpression of PA biosynthetic genes was shown to impart tolerance of the transgenic
plant to various abiotic stresses such as salt, drought, cold, dehydration, and K + deficiency
(Capell et al., 1998, 2004; Roy and Wu, 2001; Kumria and Rajam, 2002; Perez-Amador et al.,
2002; Urano et al., 2003; Waie and Rajam, 2003; Armengaud et al., 2004; Hummel et al.,
2004; Alcazar et al., 2006; Wi et al., 2006).
PAs may ameliorate oxidative stress in plants as they are known to scavenge free radicals
such as superoxide (O 2 ) and hydroxyl radicals (OH − ) formed enzymatically or chemically
(Drolet et al., 1986). Hydrogen peroxide generated due to PA catabolism is required for
lignification and cross-linking of extensins in response to stress and wounding (Cona et al.,
2006). A PA-interacting protein showed 60% identity with catalase suggesting an interac-
tion of PAs with the ROS pathway (Votyakova et al., 1999). Pretreatment with Spd prevented
increase in hydrogen peroxide (H 2 O 2 ) and NADPH-dependent superoxide in microsomes
and protected a chilling-sensitive cucumber from chilling injury (Shen et al., 2000). Ara-
bidopsis plants overexpressing Cucurbita ficifoli Spd synthase gene showed tolerance to
paraquat, an oxidative stress inducer (Kasukabe et al., 2004). Also, PAs protected sunflower
leaf discs against oxidative stress induced by metals (Groppa et al., 2001).
Exogenous application of Spd to oat plants subjected to osmotic stress showed stabiliza-
tion of native structure of thylakoid proteins D1 and D2, cytochrome b 559 , and Rubisco, in-
dicating protection of the photosynthetic machinery. This effect could be mediated through
diaminopropane formed during PA catabolism (Besford et al., 1993). PAs regulated the
voltage-dependent inward K + channel, KAT1 in the plasma membrane of the guard cells,
causing a decrease in stomatal aperture, which suggests a role for PAs in stress response
through stomatal regulation ( Liu et al., 2000). In addition, synthesis of uncommon PAs
and interaction with various hormones may also play a role in stress adaptation. However,
various signal transduction mechanisms involved in plant responses to abiotic stresses are
still not fully understood. Hence, effects of PAs in stress responses should be interpreted
with caution, and further information regarding the mechanisms involved is required.

15.10 Polyamines and biotic stress response

An increase of cell wall-bound PAs concomitant with a decrease in free PAs has been
reported in diseased organs of Vitis vinifera infected with Eutypa lata (grapevine dieback).
Levels of conjugated PAs decreased in diseased and increased in healthy organs in response
to eutypiosis, which suggested a role for PAs in responses to E. lata infection (Rifai et al.,
2004). Treatment of apples with three PA biosynthesis inhibitors, α -DFMO, DFMA, and

α -methylornithine (MeOrn), individually or in combination with CaCl 2 , reduced growth of

Botrytis cinerea and Penicillium expansum and reduced soft rot symptoms associated with
these pathogens (Saftner et al., 1997).