Dealing with Environmental Stresses: Role of Polyamines in Stress Responses - Crop Improvement: New Approaches and Modern Techniques - page 459

involving molecular synthesis, enzyme induction and membrane transport. Over
the years, attempts were made to enhance the salt tolerance of several salt sensitive
crop species using classical plant breeding and molecular biological approaches
(Taiz and Zeiger 2006 ).
Significance of PAs, CuAO/DAO and PAO in salt stress tolerance is described in
several plant species (reviewed in Alcázar et al. 2006b , 2010a ; Cona et al. 2006 ; Liu
et al. 2007 ). Accumulation of increased amounts of PAs in rice, tobacco and Arabi-
dopsis is reported to enhance the tolerance to high salinity conditions. In salt-tolerant
cultivars of rice, accumulated amount of spd and spm was higher than in the salt-
sensitive rice cultivars while the latter showed higher amounts of put accumulation
(Basu and Ghosh 1991 ). The quantification of PA levels in salt-sensitive rice culti-
vars showed that salt-sensitivity is related with the differences in PA accumulation
in the shoot system under high salinity, specifically put to a higher level and spd and
spm to lower levels (Krishnamurthy and Bhagwat 1989 ). In salt-tolerant cultivars of
rice, root plasma membranes were rich in spd and spm while in salt-sensitive culti-
vars root plasma membranes were rich in put (Roy et al. 2005 ). In barley seedlings,
salt injuries caused by high concentration of NaCl could be partially attenuate by
exogenous application of put and spd (0.5 mM) (Zhao and Qin 2004 ). In roots and
leaves of Lupinus luteus growing on high salinity conditions, accumulated increased
amounts of PAs bound to microsomal membranes implying less injuries caused by
salt stress (Legocka and Kluk 2005 ). There are more reports on adverse salinity-me-
diated increase in PA amounts in a number of plant species. For example, an increase
in free, acid-soluble bound and total spm was observed in leaf tissues of sunflower
exposed to increasing concentrations of NaCl (Mutlu and Bozcuk 2005 ), In spinach,
lettuce, melon, pepper, broccoli, tomato and wheat high salt concentrations resulted
in substantial accumulation of spd and spm (El-Shintinawy 2000 ; Zapata et al. 2004 ).
Experimental evidences indicate that salt stress induced PA-mediated responses
mainly rely on the activation of arginine decarboxylase ( ADC2 ) and amine oxi-
dases. In Arabidopsis , strikingly increased expression level of ADC2 and spermine
synthase ( SPMS ) was observed under high salinity (Soyka and Heyer 1999 ). Fur-
ther, mutants defect in PA biosynthesis displayed increased sensitivity to salt stress
(Soyka and Heyer 1999 ; Yamaguchi et al. 2006 ). External supplementation of spm
to spm-deficient mutants suppressed the salt sensitivity of these mutants (Yamagu-
chi et al. 2006 ). Arabidopsis mutants spe1-1 and spe2-1 with reduced ADC activ-
ity showed no accumulation of PAs in response to salt treatment, demonstrating
the importance of ADC activity in salt tolerance (Kasinathan and Wingler 2004 ).
Moreover, rice over-expressing oat ADC showed increased plant biomass under
salinity indicating higher PA production by enhanced ADC activity (Roy and Wu
2001 ). In another study, salt stress resulted in an induction of AtADC2 transcripts
in Arabidopsis correlating with the accumulation of free put (Urano et al. 2004 ).
Rice varieties exposed to high salinity showed an increase in transcript levels of
S -adenosylmethionine decarboxylase ( SAMDC1 ) and in the salt-tolerant variety,
transcription of SAMDC1 was higher than in the salt-sensitive variety (Li and Chen
2000 ). These experimental evidences indicate an obvious protective function of spd
and spm in salt stress tolerance. Presence of a pool of put may be a prerequisite