Dealing with Environmental Stresses: Role of Polyamines in Stress Responses - Crop Improvement: New Approaches and Modern Techniques - page 464

revealed that diamine control the levels of ABA in response to cold by modulating
ABA biosynthesis (Cuevas et al. 2008 , 2009 ). In another study, PA levels specially
put levels increased in chickpea subjected to chilling temperature (Nayyar 2005 ).
Accumulation of put occurs as a rapid reaction to low temperature as seen in poplar
seedlings grown at 4°C (Renaut et al. 2005 ). Supplementation of growth medium
with spd prior to the cold treatment resulted higher cold tolerance in cucumber (He
et al. 2002 ). In cold-tolerant cultivars of cucumber, markedly increased level of spd
was observed during chilling as opposed to the cold sensitive cultivars (Shen et al.
2000 ). OsSPDS2 , a novel SPDS gene from rice was involved in chilling responses
in rice roots (Imai et al. 2004 ). Further, Arabidopsis plants overexpressing Cucur-
bita ficifolia SPDS1 showed increased tolerance to chilling and freezing tolerance
(Kasukabe et al. 2004 ). Microarray analysis revealed that increased transcription
of several stress responsive genes in the transgenic plants under chilling stress sug-
gesting an important role for spd as a signaling compound or as a regulator of stress
signaling pathways, leading to developed stress tolerance mechanisms (Kasukabe
et al. 2004 ).
Altered regulation of PA biosynthesis is observed as one of the mechanisms in-
volved in high-temperature tolerance. Overexpression of SAMDC (from Saccharo-
myces cerevisiae ) in tomato caused 1.7-2.4 fold higher levels of spd and spm pro-
duction under high temperature stress, enhanced antioxidant enzyme activity and
the protection of membrane lipid peroxidation (Cheng et al. 2009 ). The levels of
free and conjugated PAs and ADC were higher in calli of heat-tolerant rice cultivars
than in heat-sensitive cultivars under non-stressed conditions. Heat stress caused
greater accumulation of free and conjugated polyamines in calli of the heat-tolerant
cultivar (Roy and Ghosh 1996 ). PA catabolism was also associated with heat stress
as shown by increased PAO activity in heat-tolerant rice cultivars (Roy and Ghosh
1996 ). In another study, tobacco plants over-producing proline showed a transient
increase in the levels of free and conjugated Put and in the levels of free spd, nor-
spermidine (N-Spd) and spm after a 2-h lag phase (Cvikrová et al. 2012 ). The find-
ings indicate that proline and PA biosynthetic pathways act together in dealing with
heat stress conditions.
All these experimental results indicate that transgenic approach, which increases
PA biosynthesis could be a good strategy to improve the high and cold temperature
tolerance.
OxidativeStress
Oxidative stress is induced by various abiotic factors such as hyperoxia, light,
drought, high salinity, cold, metal ions, pollutants, xenobiotics and toxins, biotic
factors like pathogen infection and developmental transitions such as seed matura-
tion and aging of plant organs. Oxidative stress produces ROS in plant cells (Grene
2002 ). Under physiological steady state, ROS are scavenged by antioxidative de-
fence components that are often confined to particular cell compartments. In some