Endogenous ABA as a Hormonal Intermediate in the Salicylic Acid Induced Protection of Wheat Plants Against Toxic Ions - Salicylic Acid: Plant Growth and Development - page 119

accumulation of abscisic acid (ABA) and indoleacetic acid (IAA) on the back-
ground of the absence of changes in cytokinin level. This allowed us to suggest
that endogenous ABA may serve as a hormonal intermediate in the realization of
SA-induced pre-adaptation of plant to the forthcoming stress (Shakirova et al.
2003 ; Shakirova 2007 ).
Increased synthesis and accumulation of ABA having frequently a transitory
pattern may be characterized as the universal plant response to stressful impacts
leading to disturbance of water relations (Xiong et al. 2002 ). ABA is known to play a
key role in regulation of stomatal closure (Wilkinson and Davies 2010 ), resulting in
a decline in transpiration and reduction of water loss. Stomatal closure is one of early
plant responses to salinity caused by ABA-induced increase in Ca 2+ concentration in
cytoplasm, subsequent activation of ion channels in plasmalema and turgor losses by
guard cells also linked with ABA-induced enhancement of H 2 O 2 production serving
as ABA signal intermediate in stomatal closure (Kim et al. 2010 ). At the same time
ABA is involved in up-regulation of antioxidant enzyme genes and enhancement of
corresponding enzyme activity (Xiong 2007 ) providing a protection against oxi-
dative stress caused by conditions unfavourable for plant growth.
ABA is of pivotal importance for the induction of biosynthesis and accumu-
lation of prolin, which functions as osmoprotectant participating in the stabiliza-
tion of biopolymers and cell membranes and protection against injurious action of
reactive oxygen species (ROS) (Yu et al. 2008 ; Szabados and Savoure 2009 ), as
well as for production of many other ABA-induced components of plant protection
(Rock et al. 2010 ). Among ABA-induced genes, important role belongs to those
for dehydrins. Their massive accumulation is observed in plant seed embryos
during their dehydration. However, sharp increase in expression of dehydrin genes
and accumulation of their protein products is registered in vegetative plant tissues
subjected to dehydration, dehydrins being the most abundant among stress proteins
induced under these conditions (Close 1996 ; Hara 2010 ).
The gene coding for wheat germ agglutinin (WGA) also belongs to ABA-
responsive genes (Skriver and Mundy 1990 ; Shakirova et al. 2001 ). WGA, being a
typical representative of cereal lectins, is a constitutive wheat protein, whose
presence in plant tissues increases significantly during ontogenesis. Thus, signifi-
cant reversible increase in WGA content was observed in wheat plants in response
to salinity, drought, osmotic stress and heat shock (Cammue et al. 1989 ; Shakirova
et al. 1993 , 1996 ; Singh et al. 2000 ; Shakirova and Bezrukova 2007 ). Data showing
a decline in stress-induced oxidative damage in seedlings pretreated with WGA and
an accelerated restoration of growth processes during the post-stress period in these
plants confirm that WGA is an active participant in ABA-induced wheat resistance
(Bezrukova et al. 2008 ). It is of interest that in the series of components of plant
protection controlled by exogenous and endogenous ABA there are also those
involved in the range of protective action of SA (Shakirova and Bezrukova 1997 ;
Shakirova 2001 , 2007 ; Shakirova et al. 2003 ; Fatkhutdinova et al. 2004 ; Rajjou
et al. 2006 ; Hayat et al. 2010 ; Nazar et al. 2011 ). These data indicate in favor of
possible implication of endogenous ABA as a hormonal intermediate in the regu-
lation of realization of pre-adaptive and protective action of SA on plants.