Polyamines Contribution to the Improvement of Crop Plants Tolerance to Abiotic Stress - Crop Improvement Under Adverse Conditions - page 121

et al. 2007 ; Rodríguez et al. 2009 ) and wheat (Reggiani et al. 1994 ; El-Shintinawy
2000 ). Thus, under salinity, the pool of Put would be directed to Spd and finally, to
Spm synthesis (Groppa and Benavides 2008 ). In rice, (Krishnamurthy and Bhagwat
1989 ; Roy et al. 2005 ; Roychoudhury et al. 2008 ) wheat (El-bassiouny and Bekheta
2005 ) and barley (Liu et al. 2006 ), the buildup of the Spm level has been described
as an indicator of salt tolerance whereas Put accumulation has been associated with
salt sensitivity. Roy et al. ( 2005 ) clearly demonstrated that deficiencies of salt-sen-
sitive rice cultivars, due to high Na + accumulation or salinity stress-induced K + loss,
could be overcome by exogenously supplied Spd, necessary to Spm synthesis. In
general, plants respond to abiotic stress by increasing ADC activity (Bouchereau
et al. 1999 ). Roy and Wu ( 2001 ) reported that under salinity, rice plants transformed
with a gene encoding an oat ADC increased the PAs level and plant biomass as a
consequence of a higher ADC activity. Chattopadhyay et al. ( 1997 ) reported that
ADC transcripts and activity increased in rice cultivars as early as one hour after
the stress treatment was imposed, followed by a sharp decrease after prolonged salt
treatment, in the case of salt-sensitive cultivar. Roy and Wu ( 2002 ) transformed rice
plants with a Tritordeum SAMDC and observed a three-to-four-fold rise in Spd and
Spm levels in transformed plants under NaCl-derived stress. Li and Chen ( 2000 )
reported that the expression of the SAMDC1 gene in rice seedlings was dramatically
induced by salinity. The transcript levels of SAMDC1 in two rice varieties differing
in salt tolerance were found to be higher in the salt-tolerant than in the salt-sensitive
variety. However, authors reported that ADC and SAMDC transcript levels were
barely affected by NaCl treatment, although SPDS 2 in maize (Rodríguez-Kessler
et al. 2006 ) and rice (Imai et al. 2004 ), and SPDS 1 in maize (Jiménez-Bremont
et al. 2007 ) were upregulated by this treatment.
Although the mechanisms that govern PAs metabolism-mediated salt resistance
remain unclear, some reports have shed light in the last few years. Mansour and
Al-Mutawa ( 1999 ) reported that Spd or Spm but not Put alleviates the cellular al-
terations in wheat roots under saline stress, possibly by plasma membrane protec-
tion. Accordingly, Spm and Spd significantly prevented the leakage of electrolytes
and amino acids from roots and shoots of rice subjected to salinity (Chattopadhyay
et al. 2002 ). Saline stress-induced elevation of PAs levels may represent an adaptive
mechanism in which the uptake of Na + and leakage of K + from mesophyll cells are
reduced (Pang et al. 2007 ). Pre-treatment with PAs prevented salt-induced K + leak-
age in mature root zone of hydroponically grown maize, apparently by effect on cell
membrane transporters in a highly-specific way (Pandolfi et al. 2010 ). Shabala et al.
( 2007 ) showed that PAs treatment substantially reduced the NaCl-induced K + efflux
from the pea leaf mesophyll, most likely by blocking the non-selective cation chan-
nels. Zhao and Qin ( 2004 ) reported that exogenous PAs application could maintain
tonoplast integrity and function in barley seedlings under saline conditions. Lego-
cka and Kluk ( 2005 ) reported higher levels of PAs bound to microsomal membranes
in Lupinus luteus seedlings in salinity and proposed that PAs most likely stabilized
microsomal membrane surfaces, protecting them against NaCl stress damage.
Many authors suggested that PAs act as antioxidants under salinity and other
environmentally-adverse conditions, though their precise role as antioxidants is still