S. V. Veselova

Role of cytokinins in the regulation of stomatal conductance of wheat seedlings under conditions of rapidly changing local temperature

The influence of an air temperature increase by 4°C and nutrient solution cooling down to 5 ± 1°C on stomatal conductance and hormone level of seven-day-old wheat (Triticum durum L., cv. Bezenchukskaya 139) seedlings was studied. An elevated air temperature resulted in a rapid rise of stomatal conductance preceded by the increase in the level of cytokinins in leaves. Cooling of the nutrient solution induced gradual stomatal closure along with a decreasing cytokinin level in leaves. Hormone concentration in the xylem sap of wheat seedlings was determined, and the rate of hormone transport from the roots to shoots was calculated. The role of cytokinins in the regulation of stomatal conductance under conditions of local thermal treatments is discussed.

Stomata Response to Changes in Temperature and Humidity in Wheat Cultivars Grown under Contrasting Climatic Conditions

Stomatal response to changes in temperature and humidity was studied in wheat (Triticum aestivum L.) cv. Iren’ cultivated under conditions of high water supply and cv. Kazakhstanskaya 10, which is relatively drought tolerant. Experiments were performed under both laboratory and field conditions. It was demonstrated that stomata of cv. Kazakhstanskaya 10 plants closed rapidly with reducing humidity (the response of the first type), whereas, in cv. Iren’, this response was less expressed and, under conditions of a high water content in soil, stomatal conductance could increase in response to reduced humidity (the response of the second type). At an increased stomatal conductance and transpiration, water content in cv. Iren’ plants was maintained due to the increase in hydraulic conductance and water inflow from the roots. A possible role of the first-type response (rapid stomata closure) for growth maintenance under drought and of the second-type response (a parallel increase in the stomatal and hydraulic conductance) for providing of rapid growth and high productivity under sufficient water supply is discussed. A possibility to use the type of stomata behavior for cultivar assessment is considered.

The Role of Hormones in Fast Growth Responses of Wheat Plants to Osmotic and Cold Shocks

The effects of nutrient-solution cooling and PEG addition to the nutrient solution on the phytohormone content, the rate of leaf growth, leaf extensibility under the influence of external mechanical action, osmotic potential, and transpiration were studied in seven-day-old wheat plants. Leaf growth rapidly ceased, and the transpiration rate was reduced in both treatments. Growth cessation induced by PEG was transient, and growth resumption was preceded by an increase in the leaf extensibility. The functional role of auxin accumulation in plant shoots in the control of extensibility as well as the relationship between the ABA accumulation and a decrease in the cytokinin content, on the one hand, and reduced transpiration, on the other hand, under stress conditions are discussed.

Plant growth-promoting bacteria in regulation of plant resistance to stress factors

The review considers the mechanisms underlying the ability of plant growth-promoting bacteria (PGPB) to enhance plant tolerance to biotic and abiotic stresses. The improved tolerance is determined by a number of microbiological factors such as: supply of mineral nutrients; production of hormones and signaling metabolites; syntheses of antibiotics, biosurfactants, siderophores, chitinases, glucanases, proteases and lipases and other defence proteins; and induction of plant systemic resistance to pathogens and herbivores. Possible mechanisms of interactions in the plant–PGPB–pathogen (pest) system are considered. The PGPB may exert their role either directly, through elimination of harmful organisms by antimicrobial substances and hydrolytic enzymes, or indirectly, through activation of host defense systems and the induction of systemic resistance. New approaches to the design of broad-spectrum biopesticides on the basis of PGPB are considered.

The effect of rapid temperature increase on the growth rate of wheat leaves

The growth rate of the first leaf of eight-day-old wheat plants was measured using a DLT-2 highly sensitive linear displacement transducer. Leaf extensibility was evaluated from the growth rate under the increase in the pulling force by 2 g. An increase in the air temperature resulted in the doubling of the transpiration rate and immediate slowing of the leaf growth followed by the leaf shrinkage. However, growth was later resumed almost completely. Heat treatment did not induce any changes in the leaf extensibility, indicating that cell-wall mechanical properties were not changed. Growth retardation was supposed to result from a decrease in the water content in the leaf tissues because the balance between water influx from roots and its loss through transpiration was shifted toward the water loss. An initial drop in the relative water content (RWC) indicates such a misbalance. Subsequent growth resumption coincided with a decreased water deficiency. Since the rate of transpiration was not reduced, RWC and growth rate restoring evidently occurred due to the activated water uptake by roots, which can be explained by the increased hydraulic permeability detected in our experiments.

Roles of ethylene and cytokinins in development of defense responses in Triticum aestivum plants infected with Septoria nodorum

Effects of ethephon (2-chloroethylphosphonic acid, ET), which is a producer of ethylene, and 1-methylcyclopropene (1-MCP), which inhibits ethylene binding with the corresponding receptors, on defense responses caused by the causal agent of leaf blotch (Septoria nodorum Berk.) in leaves of soft spring wheat (Triticum aestivum L.) of cultivars contrast in the resistance to the pathogen were studied. After treatment with 1-MCP, an induction of wheat resistance to the disease, more prominent in the susceptible cv. Kazakhstanskaya 10 than in the resistant cv. Omskaya 35, was found. The rise in the resistance was accompanied by rise in zeatin content in leaves, enhanced generation of hydrogen peroxide (most likely, due to the decreased catalase activity and increased peroxidase activity), and accumulation of transcripts of marker genes of the salicylate signaling pathway (PR-1 and PR-2). On the contrary, in ET-treated plants, all the studied defense responses were inhibited, and the pathogen developed more intensively. The effect of ethylene on zeatin distribution in infected wheat leaves of the susceptible cv. Kazakhstanskaya 10 was also found. In the 1-MCP-treated wheat leaves, cytokinins were localized in mesophyll cells and cell walls. In the ET-treated leaves, cell walls were free of zeatin, and the hormone concentrated in developing hyphae of the pathogen. The results allow for the hypothesis that wheat plant resistance is controlled by antagonistic interaction of signaling pathways of salicylic acid and ethylene with participation of cytokinins.