Agriculture Reference
In-Depth Information
of ROS levels. The addition of Spm to the substrate, led to drought-stressed
Pinus
strobus
seedlings to sustain higher photosynthesis and lower transpiration rates (Is-
lam et al.
2003
).
There is an amount of evidence supporting the influence of PAs on membrane-
associated enzymes activities (Srivastava and Rajbabu
1983
; Reggiani et al.
1992
).
Reggiani et al. (
1992
) reported that PM- H
+
-ATPase from rice coleoptiles is activat-
ed by PAs. Under osmotic stress, less membrane peroxidation, greater H
+
-ATPase
activity and reduced senescence were registered in honey brew (
Cucumis melo
L.) supplemented with exogenous Spd or Spm, compared with the corresponding
control without PAs (Lester
2000
). Treatment with PEG brought about signifi-
cantly higher increments of noncovalently conjugated-Spd and Spm contents, and
H
+
-ATPase activity in root plasma membranes of a drought-tolerant than those
found in a drought-sensitive wheat cultivar (Liu et al.
2004
). In addition, exog-
enously added Spd alleviated osmotic stress injury in drought-sensitive seedlings,
parallely enhancing the root PM-H
+
-ATPase activity hugely. Later, it was shown
that treatment with methylglyoxyl-bis (guanylhydrazone) (MGBG), an inhibitor of
SAMDC, aggravated PEG injury to drought-tolerant seedlings, with a concomitant
reduction of the root PM-H + -ATPase activity (Liu et al.
2005
). These results point-
ed to a possible involvement of these PAs in PM-H
+
-ATPase activity and water
stress tolerance of wheat seedlings.
The over-expression of genes-encoding enzymes that mediate in diverse path-
ways of PAs anabolism has become a promising approach for obtaining transgenic
plants with higher drought stress tolerance. The introduction of a human SAMDC
(EC 4.1.1.50) gene under the control of a constitutive promoter (CaMV35S) in to-
bacco (
Nicotiana tabacum
var.
xanthi
) led to increased conjugated Spd and Put
titers and improved drought tolerance (10 % (w/v) PEG, MW 20,000), as well as
tolerence to other abiotic and biotic stresses (Waie and Rajam
2003
). Sweet potato
(
Ipomoea batatas
, cv. Kokei 14) plants transformed with the
Cucurbita ficifolia
-
derived Spd synthase gene
FSPD1
, doubled their Spd content and produced higher
storage tissue biomass, compared with the wild type (Kasukabe et al. 2006). In
addition, transgenic plants were more tolerant to paraquat (a powerful oxidative
stress inducer) than the wild-type, suggesting that the observed improved toler-
ance may be in part due to enhanced oxidative stress tolerance. Previously, Capell
et al. (
2004
) generated transgenic rice plants expressing the
Datura stramonium
adc
gene and evaluated their response to drought stress. They observed that wild-
type plants responded to the onset of drought stress by increasing endogenous Put
levels, but not those of Spd and Spm (the agents that are believed to protect plants
under stress). In contrast, transgenic plants expressing
D. stramonium adc
showed
improved drought tolerance, in parallel with much higher levels of Put, what led to
increased Spd and Spm synthesis.
Prabhavathi and Rajam (
2007
) introduced in eggplants (
Solanum melongena
)
the gene encoding an ADC enzyme under the control of a constitutive promoter
of cauliflower mosaic virus CaMV35S. Transgenic seedlings of this crop showed
enhanced PAs level due to the augmented ADC activity, and also higher DAO activ-
ity. PAs-accumulating transgenic eggplants exhibited an augmented tolerance level
