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from drought compared to wild type [236]. The
de novo
synthesis of Spd and Spm in transgenic
plants under drought stress, at the expenses of Put, was responsible for the stress tolerance
observed in these plants.
The covalent linkage of PAs to proteins appeared to be of extreme importance in plant light-
induced stabilization of the photosynthetic complexes and Rubisco therefore exerting a
positive effect on photosynthesis and photo-protection. Also in the cytosol, they are involved,
mediated by transglutaminase (TGase) activity, in the modification of cytoskeletal proteins
and in the cell wall construction/organization [237]. In a recent study, the characterization at
the proteomic level of the TGase interaction with thylakoid proteins, demonstrated its
association with photosystem II (PSII) protein complexes using maize thylakoid protein
extracts [238]. Binding of Put to thylakoid membranes has been proposed to be a photoadap‐
tation response under controlled stress conditions. Campos and collaborators [238] results
reinforce the importance of the TGase in photo-protection by polyamine conjugation to light-
harvesting complex II (LHCII) proteins.
Recently, PAs were proposed to be components of signaling pathways and fulfill the role of
second messengers [220, 231]. Studies with ABA-deficient and ABA-insensitive Arabidopsis
mutants with differential abiotic stress adaptations [239] support the conclusion that the up-
regulation of PA biosynthetic genes and Put accumulation under water stress are mainly ABA-
dependent responses. To reinforce the fact that PAs biosynthesis may be regulated by ABA,
several stress-responsive elements, like drought responsive (DRE), low temperature-respon‐
sive (LTR) and ABA-responsive elements (ABRE and/or ABRE-related motifs) are present in
the promoters of the polyamine biosynthetic genes [239]. Liu
et al.
[240] also found that inward
potassium channels were targets for PA regulation of stomatal movements. Since ABA
signaling pathway in stomata regulation involves many different components including
signaling molecules like AOS, IP
3
, Ca
2+
and nitric oxide (NO), evidences point to an interplay
between ABA, polyamines, H
2
O
2
and NO in stomata regulation [220].
In our experiments, we transformed the model legume
Medicago truncatula
cv. Jemalong with
the arginine decarboxylase gene (
adc
) from
Avena sativa
to overexpress the heterologous ADC
enzyme aiming to increase the levels of polyamines in transgenic plants [241, 242]. Several
transgenic lines overexpressing This oat adc construct were obtained. The oat
adc
cDNA under
the control of a CaMV 35S constitutive promoter was previously transferred into rice plants
[228] and those authors found increased Put levels in regenerated plants and observed
minimized chlorophyll loss during drought stress. However, constitutive over-expression of
this gene severely affected developmental patterns of those plants. Afterwards, the same group
used the monocot maize's ubiquitin-1 (Ubi-1) promoter to overexpress the Datura
adc
gene
and found that transgenic plants, with increased Put levels, were tolerant to drought stress
[230]. The Ubi-1 promoter is known to contain a number of stress-responsive elements that
enhance transgene expression under drought stress [230] and hence function as a stress-
inducible promoter. Roy and Wu [229] also found that the expression of the
adc
transgene
under the control of an ABA-inducible promoter led to stress-induced upregulation of ADC
activity and polyamine accumulation in transgenic rice plants. Second-generation transgenic
rice plants showed an increase in biomass under salinity-stress conditions.
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