F. M. Shakirova

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.

Cytokinin Stimulates Chloroplast Transcription in Detached Barley Leaves

Chloroplasts are among the main targets of cytokinin action in the plant cell. We report here on the activation of transcription by cytokinin as detected by run-on assays with chloroplasts isolated from apical parts of first leaves detached from 9-d-old barley (Hordeum vulgare) seedlings and incubated for 3 h on a 2.2 × 10−5 m solution of benzyladenine (BA). Northern-blot analysis also detected a BA-induced increase in the accumulation of chloroplast mRNAs. A prerequisite for BA activation of chloroplast transcription was preincubation of leaves for 24 h on water in the light, resulting in a decreased chloroplast transcription and a drastic accumulation of abscisic acid. Cytokinin enhanced the transcription of several chloroplast genes above the initial level measured before BA treatment, and in the case of rrn16 and petD even before preincubation. Cytokinin effects on basal (youngest), middle, and apical (oldest) segments of primary leaves detached from plants of different ages revealed an age dependence of chloroplast gene response to BA. BA-induced stimulation of transcription of rrn16, rrn23, rps4, rps16, rbcL, atpB, and ndhC required light during the period of preincubation and was further enhanced by light during the incubation on BA, whereas activation of transcription of trnEY, rps14, rpl16, matK, petD, and petLG depended on light during both periods. Our data reveal positive and differential effects of cytokinin on the transcription of chloroplast genes that were dependent on light and on the age (developmental stage) of cells and leaves.

Effect of salicylic acid on the activity of antioxidant enzymes in wheat under conditions of salination

The effect of pretreatment with 0.05 mM salicylic acid (SA) on the activity of superoxide dismutase (SOD) and peroxidase in the roots of four-day-old seedlings of wheat (Triticum aestivum L.) was studied under conditions of salination. The level of the stress-induced accumulation of active oxygen species and, therefore, activities of SOD and peroxidase in seedlings pretreated with SA were significantly lower than in untreated seedlings, which indicates that these enzymes contribute to the protective effect of SA on plants under conditions of salination.

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.

Changes in the Phytohormone Levels in Wheat Calli as Affected by Salicylic Acid and Infection with Tilletia caries, a Bunt Pathogenic Agent

The role of salicylic acid (SA) in growth regulation and the change in the levels of phytohormones (IAA, ABA, and cytokinins) were studied in the wheat calli co-cultured with bunt pathogen Tilletia caries. Calli infection with T. caries resulted in the hypertrophied callus growth and simultaneous increase in phytohormone level. The addition of SA to the nutrient media decreased the callus growth induced by the pathogen, whereas the level of investigated phytohormones was not affected. In the SA-treated infected calli, the formation of necrotic lesions was observed in the zones of contact of the fungal mycelium with callus cells that limited pathogen growth. The authors suggest that the stabilization of the hormonal balance of plant cells at pathogenesis is one of the possible mechanisms of the SA protective action in vitro and in vivo. Hence, co-culturing wheat calli and T. caries fungus appeared to be a convenient model for assessing SA protective action.

NOR Activity in Wheat Species with Different Ploidy Levels Treated with Phytohormones

The effects of the phytohormones 6-benzylaminopurine (BAP) and 24-epibrassinolide (EB) on the sizes of nucleoli in the interphase nuclei of root meristem were studied using the silver-staining procedure in wheat species with different ploidy levels (a polyploid series). In addition, the effects of the phytohormones on the cell mitotic activity in the roots of 5-day-old seedlings were studied. The higher the wheat species ploidy level, the higher its sensitivities to BAP and EB were. In diploid wheat, the maximum increase in the nucleolar organizing region (NOR) activity was observed after treatment with considerably higher phytohormone concentrations compared to tetra- and hexaploid wheat species. The phytohormone treatment increased both the sizes and the number of nucleoli in meristematic cells of seedling roots in all wheat species studied. It was assumed that the differences between the responses of wheat species with three different ploidy levels to different concentrations of phytohormones were related to their effects on the methylation/demethylation of cytosine residues in the rDNA promoter region.