Specific features of source-sink relations in alloplasmic hybrid of winter wheat with alien cytoplasm of goatgrass with emphasis on resistance to low temperature stress

We studied the influence of alien cytoplasm of spring goatgrass Aegilops ovata L. on some physiological parameters in winter wheat (Triticum aestivum L.), Mironovskaya 808, under normal conditions and in the case of modified source-sink relations. Measurements of relative rates of plant dry matter growth and its distribution among organs, CO₂ exchange (photosynthesis upon light saturation and dark respiration), content of sugars (sucrose + glucose + fructose) and their ratio in leaves, frost hardiness, and indices of membrane stability and damage of leaves by frost have shown that, on average, alloplasmic hybrid differed from the initial cultivar by almost all parameters. Reduced frost hardiness, increased index of leaf damage by frost, lowered leaf content of sugars, and reduced sucrose/(glucose + fructose) ratio in the alloplasmic hybrid were combined with higher roots/leaves ratio, relative rate of dry matter growth, and photosynthesis and respiration rates. The alloplasmic hybrid was more tolerant to decreased source strength in source-sink relations as compared to the initial cultivar.     

The Decreased Cold-Resistance of Chilling-Sensitive Plants Is Related to Suppressed CO2 Assimilation in Leaves and Sugar Accumulation in Roots

Tomato (Lycopersicon esculentum L., cv. Sibirskii skorospelyi) and cucumber (Cucumis sativus L., cv. Konkurent) plants were grown in a soil culture in a greenhouse at an average daily temperature of 20°C and ambient illumination until the development of five and eight true leaves, respectively. During the subsequent three days, some plants were kept in a climatic chamber at 6°C in the light, whereas other plants remained in a greenhouse (control). The cold-resistance of cucumber leaves and roots, as assayed from the electrolyte leakage, was reduced after cold exposure stronger than cold-resistance of tomato organs. The ratio photosynthesis/dark respiration was lower in cucumber than in tomato leaves at all measurement temperatures. The concentrations of sugars (sucrose + glucose + fructose) increased in chilled tomato roots but decreased in cucumber roots. Cold exposure changed the activities of various invertase forms (soluble and insoluble acidic and alkaline invertases). The total invertase activity and the ratio of mono- to disaccharides increased. The lower cucumber cold-resistance is related to the higher sensitivity of its photosynthetic apparatus to chilling and, as a consequence, insufficient root supply with sugars.     

Involvement of cell-wall invertase in low-temperature hardening of tobacco plants

Specific features of low-temperature hardening (6 days at 8°C) of cold-sensitive tobacco plants (Nicotiana tabacum, cv. Samsun) related to changes in the cell-wall invertase activity were studied. During cold hardening, oppositely directed changes in this enzyme activity occurred in tobacco leaves and roots. In the leaves, cell-wall invertase was activated (approximately by 30%), the content of sugars increased (approximately by 25%), and the content of sucrose, the main transport form of sugars, in the apoplast reduced by three times; all these changes indicate that assimilate outflow from leaves to roots was inhibited. In contrast, in the root system, enzyme activity was decreased almost twice and the content of sugars in them was essentially unchanged. It is suggested that a strategy of low-temperature adaptation of cold-sensitive tobacco plants aimed at creating the high cold tolerance of aboveground parts, even at the expense of the root system, which, under conditions of native vegetation, is not practically exposed to damaging low temperatures.     

Effect of sugars on the development of oxidative stress induced by hypothermia in potato plants expressing yeast invertase gene

The influence of sugars on the development of oxidative stress induced by hypothermia was investigated in the leaves of two genotypes of potato (Solanum tuberosum L.) grown in vitro on the Murashige and Skoog medium supplemented with 2% sucrose. We used wild-type plants of potato, cv. Désirée, and potato plants expressing a yeast invertase gene under the control of the B33 class I patatin promoter and carrying a sequence of proteinase inhibitor II leader peptide for the apoplastic enzyme localization. At temperature of 22°C optimal for growth, expression of the yeast invertase gene in the leaves of transformed plants brought about a modification in the carbohydrate metabolism manifested in the activation of acid forms of invertase and accumulation of intracellular sugars (predominantly of sucrose because of its resynthesis). The exposure of plants to light under prolonged hypothermia (5°C, 6 days) activated all the forms of invertase (predominantly of acid invertase) and induced accumulation of sugars. In the leaves of potato expressing the yeast invertase gene, these processes were more intense. Under chilling, superoxide dismutase activity and the rate of lipid peroxidation in the leaves of investigated potato genotypes depended on the level of accumulated intracellular sugars. It was concluded that sugars play an important role as stabilizers of cellular membranes and scavengers of reactive oxygen species decelerating the processes of free radical oxidation of biomolecules upon the development of oxidative stress induced by hypothermia.     

Reorganization of chloroplast ultrastructure associated with low-temperature hardening of Arabidopsis plants

When following low-temperature acclimation (5 days at 2°C) of cold-resistant plants of Arabidopsis (Arabidopsis thaliana Heynh. (L.), ecotype Columbia) in relation to the changes in chloroplast ultrastructure, we registered the high efficiency of hardening and the ability of hardened plants to lower a threshold of frost damage by about 3°C. During hardening, the area of grana in the chloroplasts more than doubled, with considerably increased numbers of thylakoids per granum and thylakoids per chloroplast. The rate of apparent photosynthesis decreased to lesser extent than the rate of dark respiration, as a result the content of soluble sugars increased fourfold, ensuring an adaptive reorganization of metabolism, which enabled the hardened plants to survive even at below-zero temperatures (up to ?7°C). The authors conclude that a considerable increase in the number of thylakoids in the chloroplasts helps maintain photosynthesis at low above-zero temperatures and is a prerequisite for the accumulation of soluble sugars in Arabidopsis leaves.     

Chilling Tolerance of Potato Plants Transformed with a Yeast-Derived Invertase Gene under the Control of the B33 Patatin Promoter

Tolerance to chilling was compared under in vitro conditions in potato plants (Solanum tuberosum L., cv. Désirée) transformed with a yeast-derived invertase gene under the control of the B33 class 1 tuber-specific promoter (the B33-inv plants) and potato plants transformed only with a reporter gene (the control plants). The expression of the inserted yeast invertase gene was proved by following the acid and alkaline invertase activities and sugar contents in the leaves under the optimum temperature (22°C). The total activities of acid and alkaline invertases in the B33-inv plants exceeded those in the control plants by the factors of 2–3 and 1.3, respectively. In the B33-inv plants, the activity of acid invertase twice exceeded that of the alkaline invertase, whereas the difference equaled 12% in the control plants. The contents of sucrose and glucose increased in the B33-inv plants by 21 and 13%, respectively, as compared to the control. Chilling at +3 and –1°C for 1, 3, and 6 h did not affect the rate of lipid peroxidation, as measured by the content of malonic dialdehyde (MDA) in the leaves of the genotypes under study. Only the longer exposures (24 h at +3 and –1°C and 7 days at +5°C) produced a significant decline in the MDA content in the B33-invplants, as compared to the control. Following short freezing (20 min at –9°C), the content of MDA increased by 50% in the leaves of the control plants, while in the B33-inv plants, cold-treated and control plants did not differ in the MDA content. The authors presume that the potato plants transformed with the yeast invertase gene acquire a higher tolerance to low temperatures as compared to the control plants, apparently due to the changes in sugar ratio produced by the foreign invertase.     

The involvement of apoplastic invertase in the formation of resistance of cold-tolerant plants to hypothermia

Involvement of apoplastic invertase in the formation of resistance of cold-tolerant plants to hypothermia was established for Solanum tuberosum L. cv. Désirée and a transformant of the same cultivar expressing the yeast suc2 gene encoding apoplastic invertase. The dependence of the formation of increased constitutive and stress-induced cold-tolerance on the activity of apoplastic invertase and, consequently, on the level of intracellular sugars in the roots and leaves was demonstrated.     

Morphophysiological monitoring of winter wheat in spring in the context of global climate warming

Data on morphophysiological monitoring of winter wheat (Triticum aestivum L.) cultivar Mironovskaya 808 grown in Hoagland and Arnon solution in a greenhouse and transferred to natural conditions in March–April 2004 with the mean daily temperature of 0.6 ± 0.7°C within the exposure period of 42 days are presented. Water content, dry weight of plants and their organs, frost hardiness of plants, degree of tissue damage by frost, CO₂ metabolism (photosynthesis and respiration), concentrations of sugars in tissues and proportions between different sugar forms, and activities of soluble and insoluble acid and alkaline phosphatases were monitored. Monitoring was carried out for three experimental variants simulating different microclimatic conditions in spring: after snow melting (experiment I), under ice crust (experiment II), and under snow cover (experiment III). Plants in experiments III and II demonstrated a higher water content in tissues, lower frost hardiness, higher rates of biomass loss, lower concentration of sugars and lower di-to monosaccharide ratio in tissues, and higher total invertase activity, particularly, cell wall-associated acid invertase activity. The dark respiration rates at 0°C did not significantly differ between experimental variants. The photosynthetic capacity at this measurement temperature was maintained in all experimental variants being most pronounced in experiment II with the most intense photoinhibition under natural conditions. Comparison of experiments III and II with experiment I is used to discuss the negative effect of changes in certain microclimatic indices associated with global warming and leading to plant exhaustion and death from frost in spring.     

The changes in invertase activity and the content of sugars in the course of adaptation of potato plants to hypothermia

The pattern of changes in the activity of various forms of invertase (acid soluble, alkaline, and acid insoluble) and the content of sugars (glucose, fructose, and sucrose) in the course of plant adaptation to prolonged (6 days) hypothermia (5°C) was investigated in the leaves of potato plants (Solanum tuberosum L., cv. Desiree) produced in vitro. We used the wild-type plants as a control and transformed plants with carbohydrate metabolism modified by inserting the yeast gene for invertase (apoplastic enzyme). In the course of adaptation to hypothermia, the activity of acid invertase was shown to rise and the content of sucrose and glucose to increase in the leaves of both genotypes. The greatest activity of acid invertases by the third day of cold acclimation corresponded to the peak level of sugars; in transformed plants, these characteristics exceeded those in the control plants. The transformed plants were more cold resistant than the control plants as suggested by the lack of disturbance of ion permeability of their membranes. It was concluded that owing to accumulation of low-molecular carbohydrates in the course of cold acclimation caused by activation of acid invertase cold resistant plants better adapt to temperature drop.     

Apoplastic sugars and cell-wall invertase are involved in formation of the tolerance of cold-resistant potato plants to hypothermia

We studied the involvement of apoplastic sugars (glucose, fructose, and sucrose) and the cell-wall invertase (CWI) in the formation of the tolerance of cold-resistant potato plants (Solanum tuberosum L., cv Désirée) to hypothermia. The activity of CW1 and the content in the cell and the apoplast substrate (sucrose) and the reaction products of this enzyme (glucose and fructose) have a significant influence on the formation of the tolerance of cold-resistant potato plants to hypothermia.