Drought is a major environmental factor limiting crop productivity. Considering a significant area of crop production under water-limited rained conditions, there is a great need to develop production systems to sustain yield potentials under drought stress. Silicon has recently been recognized as an important element in plant nutrition. In this study, it was shown that supplying soybean with soluble silicon in the soil could improve vegetative growth and drought tolerance under water limiting conditions. In order to understand the molecular mechanism how silicon alleviates drought stress, the effects of silicon application on protein expression and antioxidant enzymes were examined. Soybean plants were grown in sand-containing pots supplied with 4 millimolar solutions of sodium silicate. To cancel the effect of sodium, the same amount of sodium chloride was used along with control plants. Soluble proteins were isolated from the leaves and roots of silicon-treated and control plants subjected to water deficit stress. Two-dimensional gel electrophoresis and mass spectrometry approaches were used to identify differentially expressed leaf and root proteins in response to silicon application under water deficit stress. Proteins that showed differential expression in response to silicon application included metabolic enzymes and proteins involved in the proteasome-dependent degradation pathway. These results indicate that silicon application could affect enzymes important for carbohydrate metabolism and stabilize aldehyde dehydrogenases and malic enzyme under water deficit stress, which may be attributable to drought tolerance.

Stomata are small pores that are located on the surface of epidermal leaves, and they can regulate the uptake of CO2 and prevent water lose by opening and closing the pores. Stomata of plants can be regulated by external condition such as CO2, biotic and abiotic stresses and internal factors. CAM (crassulacean acid metabolism) plants adapt to hot and dry environments by closing stomata during the day and opening stomata during the cool night. However, it is still unclear how CAM plants open their stomata during the night and close them during the day. In this study, a number of factors were evaluated for their potential roles in promoting stomatal opening in the model CAM plant Kalanchoe laxiflora. Citrate is an important organic acid and it accumulates during the night in CAM plants. It is shown in this study that citrate promoted stomatal opening in detached leaf epidermis of Kalanchoe laxiflora. Further, the cytokinin zeatin is also shown to stimulate stomatal opening in detached leave of Kalanchoe laxiflora. Melatonin is an important regulator of circadian rhythms in mammals and has been implicated in regulation of plant abiotic stress responses. Melatonin was detected in the leaves of Kalanchoe laxiflora. It promoted stomatal opening in detached epidermis of Kalanchoe laxiflora. Together, these results suggest that stomata of Kalanchoe laxiflora respond to citrate and malate which are the main organic acids accumulate during nighttime and also to some signaling molecules (zeatin, melatonin, and serotonin) by opening stomata during dark period.

Biotic stress in the form of viral infection, as well as abiotic salt stress, cause leaves injuries, stomata closure and decreased rate of photosynthesis. These factors lead to the limitation of plant growth and to reduced amount of coenzyme NADPH. However NADPH is an important coenzyme for many metabolic pathways such as synthesis of fatty acids, amino acids and secondary metabolites involved in stress responses. NADPH is also a coenzyme for key enzymes of antioxidant system and for many regulatory enzymes. NADP-dependent enzymes are alternative source of NADPH in plants under stress conditions. In this work, activities of four NADP-dependent enzymes: Glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49), NADP-isocitrate dehydrogenase (NADP-ICDH, EC 1.1.1.42), NADP-malic enzyme (decarboxylating) (NADP-ME, EC 1.1.1.40) and Shikimate dehydrogenase (SDH, EC 1.1.1.25) were studied. Activities of all these enzymes but SDH increased in leaves of tobacco plants (Nicotiana tabacum L.) infected by PVYNTN , The most sensitive enzymes to viral infection were NADP-ICDH and NADP-ME, whose activity was increased in comparison with control plants 3-fold and 2,4-fold, respectively. Changes in activity of studied enzymes were also determined in plants exposed to viral infection in combination with heat-shock...