M. S. Krasavina

Can salicylic acid affect the intercellular transport of the tobacco mosaic virus by changing plasmodesmal permeability

We studied the effects of salicylic acid (SA) on the plasmodesmal permeability as evaluated by the tobacco mosaic virus (TMV) spreading in tobacco Nicotiana glutinosaleaves, where TMV induces necrotic lesions. When leaves were treated with SA simultaneously with their viral inoculation, SA retarded the development of necrotic lesions and reduced their number. When inoculated leaves were kept on the SA solution at an elevated temperature (31°C) for a short period of time, the size of the necrotic lesions, which developed after leaf transfer to room temperature, was decreased. SA stimulated the formation of “rapid” callose involved in the control of the plasmodesmal permeability, which was assessed from fluorescence after tissue staining with Aniline Blue. On the basis of these data, we suggest that SA suppressed TMV spreading in the inoculated tobacco leaves by reducing the plasmodesmal permeability.

Ultrastructure of Chloroplasts in Phloem Companion Cells and Mesophyll Cells as Related to the Stimulation of Sink Activity by Cytokinins

Changes in the chloroplast ultrastructure and starch and lipid content in the mesophyll and phloem companion cells of the phloem were studied after induction of source and sink functions in leaf tissues. A detached sugar-beet leaf, one half of which was treated with water (source part) and the other half of which was treated with 10–4 M benzyladenine (BA) (acceptor part), was used as a model. After 65-h exposure to diffuse light, starch disappeared and lipid content increased in the source part of the leaf, with simultaneous disorganization of the chloroplast structure, which was most pronounced in the companion cells. Changeover from the source to sink function, induced by BA treatment, did not lead to marked destructive changes in the chloroplast structure of companion cells and resulted in the appearance of starch and in further increase in the level of lipids. Smaller amounts of starch also appeared in the mesophyll chloroplasts in the sink part of the leaf. We suppose that: (1) BA promotes the storage of assimilates, which are imported from the source part of the leaf to the companion cells, in the form of starch and lipids within chloroplasts; and this storage contributes to the maintenance of the sucrose concentration gradient in the conducting system between donor and sink parts of the leaf and, thus, activates metabolite inflow and (2) a barrier exists in the sink part of the leaf for assimilates destined to mesophyll cells, which restricts their export from the phloem.

Activity of sucrose synthase in trunk tissues of Karelian birch during cambial growth

Activity of sucrose synthase (SS) and the content of starch were studied in the xylem and phloem of two forms of 40-year-old trees of Betula pendula Roth: common silver birch (B. pendula var. pendula) and Karelian birch (B. pendula var. carelica). In order to reveal the initial signs of wood patterning, we used 8-year-old trees of Karelian birch that did not yet exhibit visible deviations from normal growth (trees with unpatterned wood) and those where formation of structural trunk anomalies already commenced (trees with patterned wood). Investigation was conducted during the period of cambial growth: in June growth was active and in July growth was suppressed by weather conditions. In June we found a high SS activity in the xylem of 40-year-old plants of common birch. Retardation of growth of common birch in July was accompanied by the decrease in SS activity in the xylem and by accumulation of starch, especially in the phloem. In 8-year-old trees of Karelian birch with unpatterned wood, the SS activity in the xylem was lower than in common birch trees, while the content of starch in the phloem was higher. In the xylem of trees with patterned wood, the highest activity of SS was approximately 2.5 times lower than in the trees with unpatterned wood. The lower SS activity in the xylem of patterned trunk trees was not associated with starch accumulation in the phloem. We propose that low SS activity during cambial growth reduces sink capacity of xylem tissues, thus leading to a considerable increase in the sucrose content in the phloem, which can alter the program of cell development in the cambial zone of Karelian birch.

Invertase activity in trunk tissues of Karelian birch

We investigated two forms of Betula pendula Roth: common silver birch (B. pendula var. pendula) and Karelian birch (B. pendula var. carelica) differing in wood texture. Activity of different forms of invertase in the tissues of birch trunk was determined for the first time. In the period of active operation of cambium, all three forms of invertase (vacuolar, cytoplasmic, and apoplastic) were present in the phloem and xylem of both forms of birch. Phloem and xylem of Karelian birch were notable for a high activity of apoplastic invertase. It was assumed that reprogramming of cell differentiation in cambial zone of Karelian birch causing a more pronounced parenchymatization of conducting tissues is related to an intense hydrolysis of sucrose in the apoplast. The obtained results corroborate the opinion formed in the literature about a possible participation of invertase in the synthesis of cell wall components.

The Role of Carbohydrates in Plant Resistance to Abiotic Stresses

The role of various osmotically active compound accumulations in plant resistance to low and high temperatures, drought, and salinity is considered. Attention is paid to the specific features of definite compound accumulation under different stresses, their involvement in the maintenance of the redox balance in the cells, and their function as signaling molecules. Despite an accumulation of sugars and other metabolites to maintain the osmotic and redox equilibriums, this process is not systematically correlated with plant resistance. Carbohydrate accumulation in the leaves may be determined by the inhibition of their efflux under the influence of stressors. In this case, a decrease in the assimilate influx to sinks results in the retardation of their growth and development, which should affect plant viability.

Potassium in the Apoplast of the Root Sink Region

Using carboxyfluorescein, a fluorochrome transported along the phloem, we demonstrated that symplasmic phloem unloading in the watermelon root occurred in the basal zone of the meristem adjusting to the elongation zone. In the similar zones of maize and pumpkin roots, a high level of potassium was detected by X-ray microanalysis in the cell walls and intercellular spaces. Potassium concentration in these compartments comprised two-thirds of that in the cytoplasm. Such proportion between potassium concentrations in the cytoplasm and apoplast was characteristic of both the cortex and stele. Since potassium is a dominant osmotically active component in root tissues, such a proportion between its intracellular and apoplastic concentrations provides for a low turgor pressure in the cells of the sink region, in the phloem in particular. This might increase a turgor pressure gradient along the translocation route between source and sink tissues, which is a driving force for phloem assimilate transport.

Resistance of Plants to Cu Stress: Transgenesis

Abstract Copper (Cu) is reported to play a dual role in plants. At low concentrations, it functions as a necessary element participating in carbon assimilation, transport of organic substances, growth, and metabolism of cell walls. Cu needs to sustain the plant based on its role in enzymatic catalysis, in redox reactions, and in the electron transport in the respiratory and photosynthetic chains. In high concentrations, it has a toxic effect. This is discussed, as are mechanisms of maintenance of intracellular Cu-homeostasis, including regulation of Cu translocation, its complexation with organic substances in plant and in soil, and its sequestration in cell organelles (vacuole) and organs (roots). A necessity to study transport processes (translocation of Cu and other ions essential for plants as well as assimilates) is discussed. Cu complexation process with natural and artificially synthesized chelators, the interaction of plants with microorganisms must be applied in the development of practical methods for phytoremediation. Because multifactorial defense reactions of plants in stress conditions and necessity of integrated regulation of protection against various stresses, it seems preferable to create transgenic plants expressing transcription factors regulating expression of several groups of genes. This chapter describes transgenic plants expressing Osmyb-gene which have increased resistance to such diverse factors as cold, drought, and Cu.

Effect of ion channel blockers and H+-ATPase inhibitors on generation of local electrical response in a cucumber leaf

Electric potential difference was measured with extracellular electrodes between the leaf surface of 2-week-old cucumber (Cucumis sativus L.) plants and soil solution. When the leaf region with a diameter of 5 mm was gradually cooled during a 105-s period to 8–9°C, the temperature drop induced a local (confined to the cooled area) nonpropagating pulse-wise electric activity. The cessation of cooling was followed by gradual (within 12–15 min) restoration of the initial potential difference. Two peaks of electric potential with amplitudes of 100–120 mV usually appeared upon cooling. The first depolarizing stage of the pulse activity was sensitive to inhibition of voltage-gated and mechanosensitive calcium channels of plasmalemma by lanthanum and gadolinium chlorides and to verapamil treatment. Furthermore, the inhibition of this stage by ruthenium red implies the release of calcium ions from intracellular stores. The initial slow depolarization was followed by a fast depolarizing shift, which was sensitive to La3+ and the anion channel inhibitor 4-acetamido-4′-isothiocyano-stilbene-2,2′-dilsulfonic acid. At the next stage repolarization developed, which was sensitive to potassium channel blockers, tetraethylammonium and quinine sulfate. The influence of ion channels blockers indicates that generation of local bioelectric response is based on fluxes of the same ion species that are involved in the action potential. The depolarization stage was due to the transient Ca2+ influx into the cytosol from the apoplast and intracellular stores, together with the anion efflux from the cell; the repolarization stage involved potassium ions. Both stages of electric pulse generation were retarded by the H+-ATPase inhibitors, sodium orthovanadate and dicyclohexylcarbodiimide, which implies the involvement of the proton pump in the origin of electric pulses examined.

Salicylic acid can regulate phloem unloading in the root tip.

In three-day-old maize (Zea mays L.) seedlings, we removed the endosperm, coleoptile with leaflets, and adventitious roots. Primary roots were exposed to 0–10−3 M salicylic acid (SA) for 1–5 h; scutellum, to 10−2 M 2-desoxy-D-glucose (2dG). 2dG-sucrose synthesized from 2dG was transported from scutella to the roots along the phloem. Its accumulation in 5-mm-long root tips was the measure of phloem unloading. At the concentrations higher than 10−4 M, SA suppressed unloading. Simultaneously, the uptake of 14C-5,5-dimethyloxazolidinedione (DMO) by root segments was inhibited, indicating cytoplasm acidification. 10−3 M SA also inhibited root respiration and growth. The lower SA concentrations (10−5 and 10−6 M) activated unloading under conditions of weak sucrose phloem transport to the root. They did not affect DMO uptake, respiration, and growth. 10−4 M SA stimulated unloading during 1- or 2-h exposure but did not affect it at longer treatments. A dependence of SA action on its concentration and exposure duration implies its involvement in the control of phloem unloading in the root tip.