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 124

extreme external conditions may really contribute to an increase in plant resistance
and productivity. These goals are achieved not only by means of selection of stress
tolerant cultivars, but also through the purposeful manipulation of adaptation with
the help of natural plant growth regulators. To those, in particular, can salicylic
acid be attributed.

3.1 Cadmium Stress

Due to the progressive contamination of soils with salts of toxic heavy metal
(HM), causing not only a decrease in yield, but also the deterioration of soil quality
(Pál et al. 2006 ; DalCorso et al. 2008 ). The investigation of the molecular
mechanisms of plant resistance to HM becomes a serious goal. Cadmium can be
attributed to the most toxic of HM, because it is not a necessary element for the
normal functioning of plants and does not perform any physiological function in
plants (DalCorso et al. 2008 ).
The reactions of plants in response to cadmium, mainly, are nonspecific and are
similar to those of the effects of other HM. The most characteristic of them could
be the accumulation of ABA, stomatal closure, the inhibition of uptake and
transport of water, the inhibition of chlorophyll synthesis and photosynthesis, the
imbalance of pro- and antioxidant components and disturbance of integrity of the
cell membrane structure, which in general is reflected in a delay in plant growth
and development and reduction of its productivity (Hsu and Kao 2005 ; Pál et al.
2006 ; DalCorso et al. 2008 ; Yadav 2010 ).
In recent years, special attention is paid to the study of molecular mechanisms of
plant resistance to cadmium and other HM as well as of the ways of their regulation.
Thus, transcriptomic and proteomic analysis allowed to identify a wide range of
proteins involved in the responses of plants to cadmium. Among them an important
place is occupied by the proteins associated with antioxidant protection and
detoxification of ROS (Fusco et al. 2005 ; Ahsan et al. 2009 ; Wang et al. 2011 ).
In addition, there has also been identified proteins involved in phytohormone sig-
naling including salicylic acid (Ahsan et al. 2009 ), which indicates the involvement
of this hormone in the regulation of plant resistance to HM. Since exposure to
cadmium causes a disturbance of water relations, it is not surprising that under these
conditions there has been detected the activation of the transcription of ABA
sensitive genes (Fusco et al. 2005 ), playing, as is known, a key role in the regulation
of plant defense responses during dehydration. Those, in particular, include genes
coding for proteins dehydrins and lectin wheat germ agglutinin, whose synthesis
and accumulation increases sharply under disturbance of the water regime,

including the effects of HM (Hara 2010 ; Bezrukova et al. 2011 ).
To date, a lot of information accumulated, indicating the effectiveness of
application of salicylic acid for reduction of the toxic effects of cadmium on
different crops (Metwally et al. 2003 ; Janda et al. 2007 ; Meng et al. 2009 ; Hayat
et al. 2010 ). Thus, it is shown that pretreatment with SA helps to prevent