M. V. Bezrukova

CHANGES IN THE HORMONAL STATUS OF WHEAT SEEDLINGS INDUCED BY SALICYLIC ACID AND SALINITY

The treatment of wheat plants with 0.05 mM salicylic acid (SA) increased the level of cell division within the apical meristem of seedling roots causing an increase in plant growth and an elevated wheat productivity. It was found that SA treatment caused accumulation of both abscisic acid (ABA) and indoleacetic acid (IAA) in the wheat seedlings but did not influence cytokinin content. SA treatment reduced the damaging action of salinity on seedling growth and accelerated reparation of the growth processes. SA-treatment diminished changes in phytohormones levels in wheat seedlings under salinity. It prevented any decrease in IAA and cytokinin contents and thus reduced stress-induced inhibition of plant growth. A high ABA level was also maintained in SA-treated wheat seedlings providing the development of antistress reactions, for example, maintenance of proline accumulation. Thus SAs protective action includes the development of antistress programs and acceleration of growth processes recovery after the removal of stress factors.

WGA reduces the level of oxidative stress in wheat seedlings under salinity

Protective effect of exogenous wheat germ agglutinin (WGA) on wheat seedling (Triticum aestivum L.) during salinity stress was studied. In particular, we examined the state of pro- and antioxidant systems as well as the level of peroxide oxidation of lipids and electrolyte leakage under control conditions and when stressed with NaCl. Generation of superoxide anions and activity of both superoxide dismutase (SOD) and peroxidase increased during saline stress. Accumulation of O2·− resulted in peroxide oxidation of lipids and electrolyte leakage in response to stress. The injurious effect of salinity on root growth of seedlings was manifested by a decreased mitotic index (MI) in apical root meristem. This study show that WGA pretreatment decreased salt-induced superoxide anion generation, SOD and peroxidase activities, levels of lipid peroxidation and electrolytes leakage as well as correlating with a reduction in the inhibition of root apical meristem mitotic activity in salt-treated plants. This suggests that exogenous WGA reduced the detrimental effects of salinity-induced oxidative stress in wheat seedlings. Thus WGA effects on a balance of reactive oxygen species (ROS) and activities of antioxidant enzymes may provide an important contribution to a range of the defense reactions induced by this lectin in wheat plants.

Stimulation of wheat germ agglutinin gene expression in root seedlings by 24-epibrassinolide

The effects of 24-epibrassinolide (EB) on the growth of four-day-old seedlings of Triticum aestivum L. roots, the accumulation of wheat germ agglutinin (WGA) in them, and its gene expression were studied. EB stimulated growth with two optimum concentrations, 0.4 nM and 0.4 μM. Both concentrations enhanced WGA accumulation in roots, which resulted from the stimulation of its gene expression. The latter was shown by dot-blot hybridization of transcripts with a DNA probe corresponding to WGA. Since EB did not affect the level of endogenous ABA, its direct effect on WGA-gene transcription in wheat roots is suggested.

Wheat germ agglutinin regulates cell division in wheat seedling roots

The mitogenic activity of wheat germ agglutinin (WGA) has been studied in roots of 4-day-old wheat seedlings. WGA had a more pronounced stimulating effect on cell division than the known mitogens concanavalin A and phytohemagglutinin whereas gliadin had no effect. Treatment of wheat seedling roots with exogenous WGA led to the accumulation of indoleacetic acid and cytokinins, hormones that play an important role in the activation of plant cell growth. The data on the combined effect of 24-epibrassinolide and WGA on cell division and accumulation of phytohormones in seedling roots support a possible link between the endogenous WGA level and hormonal regulation of cell division in the root meristem of wheat plants.

Cytokinin oxidase is involved in the regulation of cytokinin content by 24-epibrassinolide in wheat seedlings

Fast and stable 2-fold accumulation of cytokinins (CKs) was detected initially in roots and then in shoots of 4-day-old wheat (Triticum aestivum L.) seedlings in the course of their treatment with 0.4μM 24-epibrassinolide (EBR). Elevated cytokinin level has been maintained only in the presence of EBR, while the hormone removal has led to return of cytokinin concentration to the control level initially in the roots and then in the shoots. EBR-induced accumulation of cytokinins was accompanied by inhibition of both cytokinin oxidase (CKX) (cytokinin oxidase/dehydrogenase, EC 1.5.99.12) activity and expression of the gene coding for this enzyme, and on the contrary the decline in CKs level resulted in increase in these characteristics up to the control level in roots and then in shoots. Sharp accumulation of cytokinin O-glucosides has been discovered in response to EBR-treatment suggesting fast EBR-induced activation of production of cytokinins, which excessive amounts were transferred into the storage forms. The obtained data provide evidence for the involvement of EBR in regulation of cytokinin level in wheat seedlings.

The role of endogenous ABA in cold-induced expression of the TADHN dehydrin gene in wheat seedlings

The effect of fluridone, an efficient inhibitor of ABA synthesis, on expression of the TADHN gene encoding dehydrin was studied in four-day-old wheat (Triticum aestivum L.) seedlings exposed to low temperature (6°C). It was found that complete inhibition of a rapid cold-induced transient ABA formation reduced but did not eliminate cold-induced enhancement of this dehydrin gene transcription. These results argue for an important role of endogenous ABA in the induction of the TADHN gene expression in wheat seedlings under low temperature. Thus, under hypothermia, TADHN gene expression is controlled by both low temperature and endogenous ABA.

Effect of temperature shock on the dynamics of abscisic acid and wheat germ agglutinin accumulation in wheat cell culture

Abscisic acid (ABA) and lectin content was immunoassayed in wheat cell cultures affected by temperature stress. The elevated temperature (40°C) resulted in a 7-fold increase in the level of ABA and a 10-fold increase in that of lectin. The increase in the lectin content in cells was preceded by ABA accumulation. It is suggested that this ABA increase induces the synthesis of lectin, which in addition to stress proteins, play an important role in controlling mechanisms of plant adaptation to unfavourable environments.

Induction of wheat germ agglutinin synthesis by abscisic and gibberellic acids in roots of wheat seedlings

The dynamics of ABA and wheat germ agglutinin (WGA) content and WGA-geneexpression have been studied in roots of 4-d-old wheat seedlings treated with 4 ABA and 0.3 GA. Both hormones caused rapidand significant effects on WGA accumulation, though GA might control thisprocess independently of ABA, since the content of endogenous ABA did not changeafter GA-treatment. The results of dot-blot analysis of transcripts of RNA usingthe 32P labelled WGA-gene as a probe showed that a two-fold increasein WGA content resulted from ABA- and GA-induced activation of WGA-geneexpression.

Lectin involvement in the development of wheat tolerance to cadmium toxicity

In the roots of bread wheat (Triticum aestivum L.) seedlings, the effects of pretreatment with 28 nM wheat germ agglutinin (WGA) and successive action of 1 mM cadmium acetate on growth, phytohormone balance, lignin deposition, and also cadmium accumulation and distribution were studied. Priority data on cadmium-induced ABA-mediated reversible accumulation of WGA in the roots, which was accompanied by its excretion in the medium of seedling incubation, were obtained. Pretreatment with WGA exerted a clear protective effect on seedling growth in the presence of cadmium, which was based on a decrease in the amplitude of stress-induced shifts in the balance between IAA and ABA and preventing the reduction in the cytokinin level. Acceleration of lignification of the cell walls in the basal parts of roots of seedlings pretreated with WGA and subjected to stress is shown, and this limits cadmium entry into the plant.

The involvement of wheat and common bean lectins in the control of cell division in the root apical meristems of various plant species

The effects of wheat germ agglutinin (WGA) and phytohemagglutinin (PHA) at the concentration of 1 mg/l on the rate of cell division in the root apical meristem of wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), rice (Oryza sativa L.), and common bean (Phaseolus vulgaris L.) seedlings were compared. WGA enhanced cell division in the roots of barley and rice approximately similarly as in wheat roots but did not affect division of meristematic cells in the roots of common bean seedlings. In contrast PGA enhanced mitotic activity in the root apical meristem of common bean seedlings but did not affect division in the wheat and barley roots. Seedling treatment with lectins shifted the hormonal balance in them toward accumulation of growth activators (IAA and cytokinins). The relationship between lectin and hormonal systems in the control of cell division is discussed.