Agriculture Reference
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to drought imposed through 7.5 and 10 % PEG (MW 20,000), among other abiotic
and biotic stresses. Several lines of a transgenic European pear ( Pyrus communis L.
'Ballad') overexpressing the gene encoding for the apple Spd synthase (MdSPDS1)
were created by Agrobacterium -mediated transformation and tested for tolerance
to osmotic stress (300 mM mannitol, Wen et al. 2008 ). The transgenic line having
the highest Spd accumulation and expression level of MdSPDS1 (no. 32) showed
the strongest tolerance to this stress. On the tenth day after mannitol treatment, a
slight decrease in Put, and significant enhancements of Spd (33 %) and Spm titers,
and (Spd + Spm)/Put ratio were observed in the transgenic line, compared with the
wild type. Later, He et al. ( 2008 ) showed that the transgenic line contained superior
antioxidant enzyme activities, and less malondiahldehyde and H 2 O 2 than the wild
type, suggesting that transgenic plants were less stressed. In order to dissect the
roles of Put from the higher PAs Spd and Spm, Peremarti et al. (2009) generated
transgenic rice plants constitutively expressing an heterologous SAMDC gene from
D. stramonium so that the levels of higher PAs were increased without affecting Put
levels. Although such plants were not drought-tolerant, they returned to the normal
phenotype when stress was removed, whereas wild type plants could not recover.
Hazarika and Rajam ( 2011 ) generated transgenic tomato ( Lycopersicon esculentum
Mill.) plants with the human SAMDC gene, and evaluated the transgenic plants
for tolerance to drought, among other biotic and abiotic stresses. Transgenic plants
presented higher PAs levels and improved tolerance against drought, with respect
to untransformed control plants. In turn, transcription factors may influence PAs-
mediated adaptation to a variety of abiotic stresses (Chen et al. 2002 ). It was shown
that overexpression of CaPF1 ( a Capsicum annuum pathogen and freezing toler-
ance-related protein) in transgenic tissue of eastern white pine ( Pinus strobus L.),
prevented the decrease of PAs and resulted in a dramatic increase in tolerance to
drought, freezing, and salt stress (Tang et al. 2007 ). These authors suggested that
CaPF1 may influence, by a so far unknown mechanism on PA biosynthesis, enhanc-
ing stress tolerance in pine plants expressing the transgene.
Abscisic acid (ABA) is recognized as a major plant hormone during drought
stress, since it inhibits growth and stomatal opening. Upon water deficit, both ABA
biosynthesis in roots and its transport to the leaves are enhanced, leading to its ac-
cumulation in guard cells. In the stomata, ABA induces the release of water and
loss of turgor of guard cells, provoking the closure of the stomata pore (Anderson
et al. 1994 ; Allan et al. 1994 ). It is known that different Populus species and eco-
types may differ in their stomatal responsiveness to ABA (Chen et al. 2002 ; Yin
et al. 2004 ; Zhang 2009 et al.). Chen et al. ( 2002 ) reported that the drought-induced
decline of PAs concentrations in a sensitive Populus species was accompanied by
leaf shedding, whereas the tolerant species maintained higher PAs levels and did
not shed its leaves. The same authors showed that in water-stressed intact poplar,
xylem ABA reduces PAs contents, and hypothesized that this fact might intensify
the sensitivity of the leaf to ethylene, thus accelerating defoliation. Bae et al. ( 2008 )
reported that in cacao ( Theobroma cacao ), the induction by ABA (100 mM solution
applied to soil) of all five ESTs associated with PA biosynthesis (TcODC, TcADC,
TcSAMDC, TcSPMS and TcSPDS) was low, similarly to what has been observed in
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