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plants showed augmented Spd levels whereas they did not present any difference in
organ phenotype, compared to the wild type. Transgenic plants displayed reduced
sensitivity to chilling (0 °C) injury with respect to wild type plants. Also, overex-
pressing SAMDC in transgenic plants showed augmented PAs levels, having all
transgenic lines higher free Put contents than the wild-type. Hazarika and Rajam
( 2011 ) generated transgenic tomato plants that constitutively expressed the human
SAMDC gene. These plants showed higher levels of PAs and cold tolerance, com-
pared to untransformed plants. Using A. thaliana T-DNA mutants for adc1 and adc2
genes, Cuevas et al. ( 2008 ) demonstrated improved sensitivity of these mutants
during acclimation response to chilling and freezing. After cold treatment (4 °C
for three weeks), mutant plants showed diminished Put accumulation, and reduced
expression of 9-cis-epoxycarotenoid dioxygenase (NCED3), a key gene involved in
ABA biosynthesis. These authors suggested that Put-controlled ABA levels through
modulation of ABA biosynthesis.
In conclusion, low temperatures provoke intense changes in PAs levels. The
most common response related with PAs metabolism is the buildup of Put content
(Guye et al 1986 ; Nadeau et al. 1987 ; Lee et al. 1995 , 1997 ; Rácz et al. 1996 ; Szalai
et al. 1997 , 2009 ; Kim et al. 2002 ; Németh et al. 2002 ; Oufir et al. 2008 ), but the
magnitude of this response is related to stress severity, as it was shown in C. sativus
(Shen et al. 2000 ; Zhang et al. 2009 ). Furthermore, changes in PAs levels depend on
the genetic background of plants exposed to cold treatment (Guye et al. 1986 ; Pil-
lai and Akiyama 2004 ; Oufir et al. 2008 ; Szalai et al. 2009 ). On the other hand, the
relationship between ABA and PAs is not clear: While Lee et al. ( 1997 ) observed
that ABA levels controlled Put titers in rice under cold stress, Cuevas et al. ( 2008 )
showed the opposite in Arabidopsis and Kim et al. ( 2002 ) reported that ABA and
Put act independently from each other in leaves of tomato. It is noteworthy that both
rice and tomato are species adapted to warm climate, with minimum temperatures
of growth higher than those used in these experiments, whereas A. thaliana is a
species adapted to cold environment. On this basis, it is possible that the relation
between ABA and PAs is dependent either on the stress severity or the interaction
between genotype and environment.
6   Conclusion and Future Perspectives
Despite the significant information denoting that PAs are implicated in diverse
plant physiological processes, there is still no agreement on whether PAs operate
as second messengers or they may be regarded as phytohormones. Crop protection
against drought, salt and cold stresses provided by polyamines would presumably
rely on their ability to act as antioxidants, stabilizer of nucleic acids, cytosolic pH
and biomembranes, and also on their effect as compatible osmolytes. However, the
precise mechanism of action of PAs during the plant tolerance response to stress
is complex and not fully understood. Neither is the metabolic regulation of the en-
zymes that synthesize these molecules.
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