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 134

to the protection of plants from the damaging effect of osmotic and oxidative stress
induced by these adverse environmental factors. Important role in protecting
cellular structures from osmotic and oxidative stress is performed by dehydrins,
belonging to the group of 2 LEA proteins, which are just as WGA responsive to
abscisic acid (RAB) proteins, having also chelating properties. We found that the
presence of fluridone in the medium for incubation of the seedling prevents SA-
induced reversible enhancement of transcription TADHN dehydrin gene. It was
shown that pretreatment with fluridone prevents the SA-induced rapid production
of H 2 O 2 , activation of peroxidase and PAL, involved in the formation of lignin,
which is reflected in the inhibition of SA-induced deposition of lignin in the cell
walls of the central cylinder of the basal part of roots. Moreover, treatment of these
plants with ABA restores completely the intensity of the deposition of lignin in the
cell walls of roots.
Pretreatment with SA substantially reduces the damaging effects of cadmium
stress and sodium chloride salinity on growth processes of wheat seedlings. This
seems to be primarily due to the fact that the seedlings pretreated with SA and
subjected to stress factors, are characterized by lower amplitude of imbalance of
ABA, IAA and cytokinins, which could be due to the triggering in the course of
pretreatment with SA (prior to stress) of the various protective mechanisms
forming the basis of its pre-adaptive effect on plants, in particular, the activation of
the systems osmoprotection and antioxidant defense, including the accumulation
of WGA, and TADHN transcripts. In this regard, it is not surprising that under
saline conditions or when exposed to toxic cadmium ions stress-induced activation
of these important components of protection in the SA-pretreated seedlings occurs
at notably lower level, which in general is reflected in a decrease in the levels of
MDA and electrolyte leakage from plant tissues, as well as in maintaining the
growth of these plants, at least at the level of control. It should be noted that the
seedlings pre-treated with SA, in contrast to untreated with SA, are characterized
by an additional acceleration of deposition of lignin in the cell walls of roots,
contributing to the strengthening of their barrier properties, and inhibition of the
entry of toxic ions into the internal tissues of roots, which is clearly demonstrated
in particular for cadmium by histochemical methods using dithizone reagent.
The important role of maintaining high concentration of ABA in the devel-
opment of the resistance of SA-pretreated wheat seedlings to cadmium and salinity
was manifested in a significant inhibition of all studied defense responses in
seedlings pretreated with SA in combination with fluridone. Despite the fact that
fluridon completely prevents maintenance of elevated ABA content in plants
pretreated with SA, attention is drawn to the fact of existence of alternative ABA-
independent pathways of regulation of stress-induced activation of protective
mechanisms, although, as can be seen in terms of growth, MDA level and release
of electrolytes from the tissues, this was not sufficient for effective development of
SA-induced wheat plant resistance.
Thus, the summary of the data presented in this chapter, obtained by using an
inhibitor of ABA synthesis, fluridone, clearly demonstrates the key role of
endogenous ABA in the implementation of pre-adaptation and protective action of