Dealing with Environmental Stresses: Role of Polyamines in Stress Responses - Crop Improvement: New Approaches and Modern Techniques - page 461

socio-economic impacts and even famine in some less developed countries. It is
being predicted that wide spread drought conditions may happen more regularly,
severely and last for extended periods in coming decades as a result of global warm-
ing and climate change. Strategies for efficient management of droughts include
strategic management of agriculture. Apart from efficient practices for water and
soil management and crop planning, several attempts have been paid to improve the
drought tolerant capacity of plants especially important crop plants by conventional
as well as by modern genetic engineering techniques.
Drought resistance mechanisms vary with climate and soil conditions and the
capacity of drought resistant vary greatly among plant species. Plants exhibit vari-
ous responses to water stress such as inhibition of leaf expansion which occur as a
result of decrease in turgor, limits in photosynthesis, leaf abscission, increased wax
deposition on the leaf surface, altered energy dissipation from leaves, root exten-
sion into deeper wetter soil and stomatal closure. Sensing of water deficiency and
signal transduction leads to the induction of genes responsible for acclimation and
adaptation to stress. Several of these genes encode enzymes associated with os-
motic adjustment (Taiz and Zeiger 2006 ). Considerable effort have been directed
towards identifying traits associated with drought resistance of crop plants and with
the availability of molecular techniques crops have been modified for improved
drought resistance.
Among the many components, PAs are identified as one such component hav-
ing regulatory function in water stress. In water stressed plants endogenous levels
of PAs are considerably increased confirming stress-specific roles of PAs (Galston
et al. 1997 ; Groppa and Benavides 2008 ). For example, in wheat seedlings exposed
to drought, PAs especially spd accumulated to a higher level in comparison to the
control plants (Kubis and Krzywanski 1989 ), chickpea plants responded to onset
of drought by increasing the endogenous spd levels and total PA content in the
roots (Nayyar and Chander 2004 ; Nayyar et al. 2005 ), and in water stressed V. faba
leaves, free spd levels increased considerably (Liu et al. 2000 ). The Arabidopsis
double mutant acl5/spms that produce lower spm is hypersensitive to drought stress
(Kusano et al. 2007b ). Manipulation of PA biosynthesis pathway caused altered
drought resistant capacities in many plant species. In Arabidopsis , overexpression
of ADC2 increased put levels and drought tolerance was enhanced by stimulation
of stomatal closure (Alcázar et al. 2010b ). Rice plants overexpressing ADC gene of
oat showed improved drought tolerance by increasing put levels and reducing chlo-
rophyll loss (Capell et al. 1998 , 2004 ). Up-regulation of ADC gene was observed
in the osmotically-stressed oat leaves, indicating increased PA synthesis leading to
stress tolerance (Galston et al. 1997 ). Overexpression of S -adenosyl methionine
decarboxylase in Arabidopsis lead to increased spm levels and enhanced the expres-
sion of a key ABA biosynthesis gene NCED3 (Alcázar et al. 2006a ). During drought
conditions, accumulation of put was impaired in the ABA-deficient and ABA-in-
sensitive mutants (Alcázar et al. 2006a ). During water stress, ABA is known to
modulate PA metabolism by up-regulating the expression of ADC2 , spermidine syn-
thase ( SPDS1 ) and spermine synthase ( SPMS ) genes (Alcázar et al. 2006a ). A study
by Lie et al. ( 2000 ) revealed that physiological function of elevated PA levels during