Agriculture Reference
In-Depth Information
pounds. Arginine (Arg) and ornithine (Orn) are the precursors of plant PAs. Ornithine
decarboxylase (ODC; EC 4.1.1.17) converts Orn directly into Put. The other biosynthetic route
to Put, via arginine decarboxylase (ADC; EC 4.1.1.19), involves the production of the inter‐
mediate agmatine (Agm) followed by two successive steps catalysed by agmatine iminohy‐
drolase (AIH, EC 3.5.3.12) and
N
-carbamoylputrescine amidohydrolase (CPA, EC 3.5.1.53). In
animals and fungi Put is synthesized primarily through the activity of ODC while in plants
and bacteria the main pathway involves ADC. Aminopropyl groups, donated by decarboxy‐
lated S-adenosyl methionine (dcSAM), must be added to convert Put into Spd and Spm in a
reaction catalysed by spermidine synthase (SPDS; EC 2.5.1.16) and spermine synthase (SPMS;
EC 2.5.1.22), respectively (reviewed in [220]). Polyamines levels in plants increase under a
number of environmental stress conditions, including drought and salinity [221-223]. Several
biological roles were proposed for polyamines action in stress situations; PAs could act as
osmoprotectants, as scavengers of active oxygen species (AOS) or by stabilizing cellular
structures, such as thylakoid membranes [222, 224, 225]. The first reports of transgenic
approaches using genes responsible for PA biosynthesis were conducted in two species,
tobacco and rice [226-230]. Recently, new insights into the role and regulatory function of
polyamines in plant abiotic stress tolerance have been achieved, with several abiotic (salt,
drought, freezing, heat) stress tolerant transgenic plants overproducing polyamines being
described in the following reviews [220, 231-233].
Among abiotic stresses drought is the main abiotic factor as it affects 26% of arable area [229].
Plants respond to changes in water status by accumulating low molecular-weight osmolytes
including PAs. Polyamines may have a primary role of turgor maintenance but they may also
be involved in stabilizing proteins and cell structures. The polycationic nature of PAs at
physiological pH is believed to mediate their biological activity, since they are able to bind to
several negatively charged molecules, such as DNA, membrane phospholipids, pectic
polysaccharides and proteins [225].
In respect to the antioxidant activity of PAs, the research data is contradictory; on the one hand,
PAs have been suggested to protect cells against AOS and on the other hand, their catabolism
generates AOS [232]. PA catabolism produces H
2
O
2
, a signaling molecule that can act promot‐
ing activation of antioxidative defense response upon stress, but can also act as a peroxidation
agent. In a recent study, the effect of increased putrescine (Put) accumulation was found to
negatively impact the oxidative state of poplar cells in culture due to the enhanced turnover
of Put [233]. Gill and Tuteja [234] stated that, while increase Put accumulation may have a
protective role against AOS in plants, enhanced Put turnover can actually make them more
vulnerable to increased oxidative damage. The higher polyamines, Spd and Spm are believed
to be most efficient antioxidants and are considered scavengers of oxyradicals [235].
As plants with elevated putrescine contents are able to tolerate drought stress because Put has
a direct protective role in preventing the symptoms of dehydration, higher PAs (Spd and Spm)
appear to play an important in role in stress recovery [236]. Recently, transgenic rice plants
overexpressing
samdc
(S-Adenosyl methionine decaboxylase gene), with increased Spd and
Spm levels, were considered to be non drought tolerant, but showed a more robust recovery
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