Polyamines and Regulation of Ripening and Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 328

15.8 Polyamines and senescence

PAs have been linked with cell death-related processes in many organisms and are considered
as juvenility factors (Thomas and Thomas, 2001; Serafini-Fracassini et al., 2002). In many
plant systems, leaf and fruit aging and senescence is correlated with a decrease in PA levels.
Exogenous application of PAs often delays or prevents progression of senescence (Kaur-
Sawhney et al., 1982; Sood and Nagar, 2003). Tomato fruits treated with UV (3.7

10 3 J/m 2 )
showed delayed ripening and senescence, which was attributed in part to the maintenance
of a high level of Put (Maharaj et al., 1999). Treatment of hypodermal-mesocarp tissues of
“Honey Dew” melon fruits with Spd or Spm reduced membrane peroxidation indicated by
lower production of malondialdehyde, decreased lipoxygenase and phospholipase-D activ-
ities, and decreased perturbation of plasma membrane as indicated by higher H + -ATPase
activity (Lester, 2000). These data support a role for membrane lipid and PA interaction
during senescence. A PA conjugate, N -4-hexanoyl-Spd, accumulates in senescing petals
and ovaries of pea. Conjugation of Spd with hexanoic acid can reduce the positive charge
and affect interactions with anionic groups present on membrane phospholipids and nu-
cleic acids, leading to membrane destabilization and senescence (Seiler, 1987; Pereza-
mador et al., 1996). PAs, mainly Spm, retarded the senescence of leaf discs of two diverse
species of rose, whereas PA synthesis inhibitors such as difluoromethylarginine (DFMA)
and methylglyoxal-bis-guanylhydrazone (MGBG) promoted senescence. This retardation
of senescence by PAs may be due to the inhibition of enzymes such as peroxidase and
cellulase (Sood and Nagar, 2003). Spd-inhibited fruitlet abscission in grapevine increased
soluble sugar content but reduced the levels of amino acids. These studies indicate a direct or
indirect link of PAs with regulation of abscission via sugars and/or amino acids (Aziz, 2003).
Cut carnation flowers, treated with 10 mM Spd, exhibited a delay in senescence. The
treated petals accumulated free and PCA-soluble Spd, which correlated with stabilization
and reduced degradation of DNA (Tassoni et al., 2006a). Spermine was also shown to delay
senescence of cut carnation flowers through reduction of ethylene production (Lee et al.,
1997) as previously shown for other systems (Apelbaum et al., 1981). In the ovary of
senescent Hibiscus flowers, the levels of PA-conjugates bound to small molecules decreased
(Seo et al., 2007). Senescence in nodules is mainly associated with loss of ability to fix
nitrogen, due to decreased nitrogenase activity, and with lower leghaemoglobin (LB) levels.
Analysis of developing root nodules in five genotypes of Vigna showed a peak in nitrogenase
activity, LB content, and levels of free Put during flowering. This strong correlation between
nitrogenase activity, LB, and free Put suggests a role for Put-mediated sequestration of N 2
through nitrogenase in nodules (Lahiri et al., 2004).

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15.9 Polyamines and abiotic stress response

PAs have been implicated in alleviating stress-induced injury to various fruit crops.
Pomegranate fruit treated with Put and Spd under pressure infiltration or immersion ex-
hibited reduced chilling injury and delayed deterioration of fruit quality (Mirdehghan et al.,
2007a). Treatment of fruit with GA (gibberellin) and Put increased fruit firmness and reduced
susceptibility to mechanical stress in peaches with a concomitant reduction in respiration
and ethylene production (Martınez-Romero et al., 2000). These effects were attributed to
an increase in Spd levels rather than an indirect effect of respiration or ethylene. Increased