Z. F. Rakhmankulova

Alternative Respiration Pathways and Secondary Metabolism in Plants with Different Adaptive Strategies under Mineral Deficiency

The costs of adaptation respiration, the contribution of different biochemical pathways of respiration and alternative oxidases to the adaptation to mineral deficiency were compared in plant species representing different types of adaptive strategies: ruderal (Ru) (Amaranthus retroflexus L. and Leonurus quinquelobatus Gilib.) and predominantly stress-tolerant type (St) (Dactylis glomerata L. and Medicago sativa L.). The adaptation component of the total dark respiration was calculated on the basis of the relative growth rate and the ratio of dark respiration to gross photosynthesis. Adaptation costs were greater in less tolerant species. The inhibitory analysis showed that the contribution of glycolytic pathway decreased and the proportion of oxidative pentose phosphate pathway and residual respiration increased in the intolerant Ru-species under the conditions of abiotic stress. Conversely, in the tolerant St-species, the contribution of glycolysis increased. In all species studied, the contribution of the cytochrome pathway of substrate oxidation was reduced and the contribution of the alternative cyanide-insensitive and residual respiration increased under conditions of mineral deficiency; this phenomenon was particularly pronounced in a less tolerant species L. quinquelobatus. In the species characterized by Ru-strategy, stress increased the content of phenolic compounds and shikimic acid. We suppose that the adaptation respiration costs are associated with alternative biochemical pathways and alternative cyanide-resistant oxidase. The role of these pathways in the plants with St-strategy consists in maintaining the active state of oxidative pathways, whereas in the plants with Ru-strategy, they serve to burn excess metabolites and enhance the synthesis of secondary metabolites, which perform protective functions.

Content of proline and flavonoids in the shoots of halophytes inhabiting the South Urals

Growth parameters, the content of proline and flavonoids. Na+ and K+ ion accumulation in the shoots of four species of halophytes from the family Chenopodiaceae inhabiting the territory of the South Urals at different levels of salinity were studied. Within a single taxonomic group, different strategies of salt tolerance related to the level of soil salinity and specific features of salt accumulation in the shoots were revealed. Differences were manifested in the contents of protective compounds (proline and flavonoids) in dependence on the Na+ and K+ ion accumulation in the plant shoots. To characterize the participation of these compounds in plant adaptation to salinity, the ratio proline/flavonoids (Pro/Fl) was introduced. The highest value of Pro/Fl (4.9) was observed in Artiplex tatarica L. growing at weak salinity; in this plant, salt accumulation in the shoots depended on the sodium content in soil. Species Bassia sedoides (Pall.) Aschers., Atriplex verrucifera M. Bieb, and Kochia prostrate (L.) Schrad., with salt accumulation in the shoots independent on the level of soil salinity, inhabited the regions with the higher salinity. In these species the ratio Pro/Fl was 0.2–1.4, i.e., flavonoids, which concentration was positively correlated with the K+ content, played a great and equal to proline role in protective mechanisms. In species B. sedoides and A. verrucifera, the high content of both flavonoids and K+ are indices correlating with high productivity and adaptation to the environment. Proline can be used as a marker of stress only in A. verrucifera, the most salt-tolerant among the studied halophytes. It is concluded that the Pro/Fl ratio in halophytes from the family Chenopodiaceae characterizes the strategies of adaptation depending on salt accumulation in plants.

Opposite roles of the Arabidopsis cytokinin receptors AHK2 and AHK3 in the expression of plastid genes and genes for the plastid transcriptional machinery during senescence

Key messageCytokinin membrane receptors of the Arabidopsis thaliana AHK2 and AHK3 play opposite roles in the expression of plastid genes and genes for the plastid transcriptional machinery during leaf senescenceAbstractLoss-of-function mutants of Arabidopsis thaliana were used to study the role of cytokinin receptors in the expression of chloroplast genes during leaf senescence. Accumulation of transcripts of several plastid-encoded genes is dependent on the АНК2/АНК3 receptor combination. АНК2 is particularly important at the final stage of plant development and, unlike АНК3, a positive regulator of leaf senescence. Cytokinin-dependent up-regulation of the nuclear encoded genes for chloroplast RNA polymerases RPOTp and RPOTmp suggests that the hormone controls plastid gene expression, at least in part, via the expression of nuclear genes for the plastid transcription machinery. This is further supported by cytokinin dependent regulation of genes for the nuclear encoded plastid σ-factors, SIG1-6, which code for components of the transcriptional apparatus in chloroplasts.

Effect of NaCl and isoosmotic polyethylene glycol stress on gas exchange in shoots of the C4 xerohalophyte Haloxylon aphyllum (Chenopodiaceae)

The effects of NaCl (200 mM) and osmotic stress generated by polyethylene glycol (PEG) on PSII maximal quantum efficiency, photosynthetic CO2/H2O gas exchange at two CO2 concentrations, content of chlorophyll, proline, and malondialdehyde were investigated in shoots of C4 xerohalophyte Haloxylon aphyllum (Chenopodiaceae). The PEG treatment induced a low water osmotic potential (−0.4 MPa) and inhibited photosynthesis (by a factor of 2) and transpiration (by a factor of 4). The NaCl treatment, at equal osmoticity conditions, reduced transpiration (by a factor of 2) and stimulated photosynthesis (by a factor of 2.5). Only the PEG-treated plants showed osmotic stress effects, which were demonstrated by an increase in proline and malondialdehyde contents in the shoot tissue. The data indicated that the halophilic character of this species was essential for maintaining the plant water status and photosynthesis under osmoticity induced by NaCl treatment. Herewith, the presence of C4-type photosynthesis appeared to be just an auxiliary mechanism, because this xerohalophyte did not reveal the efficiency in water use typical for C4 plants under osmotic stress, in the absence of a saline substrate.

Respiratory supercomplexes of plant mitochondria: Structure and possible functions

Over the last few years a vast amount of information has accumulated on the structural organization of mitochondrial electron-transport chain. The respiratory complexes were shown to be organized into sophisticated dynamic structures, so-called supercomplexes and megacomplexes that ensure sustained and effective operation of the respiratory electron-transport chain. This review considers structural and functional features of plant supercomplexes, as well as their possible role in energy-related adaptation of plants to stresses. It is proposed that supercomplexes provide the first-line defense mechanism that protects the electron-transport chain against reactive oxygen species in stressed plants. Subsequent protective responses involve the alternative respiratory pathways, uncoupling mechanisms, and other antioxidant systems.

Changes in photosynthesis caused by adaptation of maize seedlings to short-term drought

Detached leaf is in the state of increasing water deficit; it is a good experimental model for looking into the hardening effect of adaptation of eight-day-old maize (Zea mays L.) seedlings to short-term drought (five days without watering). The light stage of photosynthesis and photosynthetic CO2/H2O exchange in detached leaves were studied. Specific surface density of leaf tissue (SSDL), the content of chlorophylls a and b, proline, MDA as well as photosynthetic parameters: quantum yield of photosystem II fluorescence, assimilation of CO2, and transpiration at room temperature and light saturation (density of PAR quantum flux of 2000 μmol/(m2 s)) at normal and half atmospheric CO2 concentration were determined. The leaves of seedlings exposed to short-term drought differed from control material by a greater SSDL and higher content of proline. The hardening effect of the stress agent on the dark stage of photosynthesis was detected; it was expressed in the maintenance of the higher photosynthetic CO2 assimilation against control material due to the elevation of stomatal conductance for CO2 diffusing into the leaf. Judging from the lack of differences in the MDA content, short-term drought did not injure photosynthetic membranes. In detached leaves of experimental maize seedlings, photosynthesis was maintained on a higher level than in control material.

New method for quantitative determination of water potential of mesophyll cells’ apoplast in substomatal cavity of the leaf

A new method for quantitative determination of water potential of mesophyll cells’ apoplast in substomatal cavity of the leaves of herbaceous (maize, millet, wheat, amaranth, and seepweed) and woody (larch, pine, and birch) plants by means of modern instruments designed to assess photosynthetic(СО2/Н2О) gas exchange is described. The method consists in determination in the light of such a level of humidity above the leaf surface, which would nullify transpiration without a noticeable suppression of photosynthetic assimilation of СО2; this makes it possible to calculate the value of water potential at the interface between aqueous and gaseous phases of mesophyll cells’ apoplast in the substomatal cavity.

Different responses of two ecotypes of C3–C4 xero-halophyte Bassia sedoides to osmotic and ionic factors of salt stress

The effects of low and moderate salinity (100 and 200 mM NaCl, respectively) and iso-osmotic stress generated by polyethylene glycol PEG (1) (–0.3 MPa) and PEG (2) (–0.6 MPa) on maximum quantum yield of photosystem II (PSII), growth, photosynthesis, transpiration, dark respiration, water use efficiency (WUE), water content, chlorophyll, proline, Na+ and K+ concentrations were investigated in shoots of two ecotypes С3–С4 xero-halophyte Bassia sedoides (Pall.) Aschers. Plants were grown from seeds of two Southern Urals populations (Makan and Podolsk) differing in their bioproductivity. Aboveground biomass of the Makan plants was approximately 10-fold higher than that of the Podolsk ecotype. The plants of both ecotypes were sensitive to water deficit. They showed similar decrease in biomass, water content, net photosynthesis and transpiration intensity under both low and moderate osmotic stress (PEG). However, the content of сhlorophyll and free proline in shoots of the Podolsk plants increased under moderate osmotic stress (PEG(2)). Under salinity the differences between transpiration, Fv/Fm, WUE, water content, chlorophyll and proline concentrations in shoots of two ecotypes were no found. But, the Podolsk plants showed decrease in the growth parameters (1.5-fold), increase in the dark respiration intensity (2-fold) and the Na+/K+ ratio (1.2-fold) under moderate salinity (200 mM NaCl). Thus, the reduction of bioproductivity of the Podolsk ecotype under salinity was the result of ionic rather than osmotic factor of salinity. In the Podolsk plants the additional transpiration costs and consumption of assimilates (correspondingly) increased with the toxic sodium ion accumulation under salinity. This led to decrease in the growth parameters. Thus, two B. sedoides ecotypes have different adaptive strategies of tolerance to the ionic factor of salt stress at the level of the physiological processes associated with the dark CO2 gas exchange. Moreover, in less tolerant and productive Podolsk ecotype the increase in proline content in shoots characterized comparatively low adaptation to osmotic factor, and the increase in dark respiration and the Na+/K+ ratio pointed to relatively low resistance to ion factor of salinity as compared with the Makan ecotype.

Photorespiration: Its Role in the Productive Process and Evolution of С 4 Plants

The review surveys the modern concepts of photorespiration, its role in the productive process, redox-regulation, and integration of energy-transforming processes of the photosynthesizing cell and, also, in evolution of C4 plants. Special attention is given to methods to explore photorespiration. Principal strategies that are aimed at improvements in photosynthesis intensity and in plant growth by manipulations with photorespiration based on up-to-date technologies are considered.

Role of Photorespiration and Cyclic Electron Transport in C4 Photosynthesis Evolution in the C3–C4 Intermediate Species Sedobassia sedoides

Plants from two Sedobassia sedoides (Pall.) Aschers populations (Makan and Valitovo) (Chenopodiaceae) with C2 photosynthesis (precursor of C4 photosynthesis in phylogenesis) and photorespiratory CO2-concentrating mechanism were studied. Genetic polymorphism and isotope discrimination (δ13С) levels of the plants were determined under natural conditions, and their morpho-physiological parameters such as fresh and dry biomass of the above ground parts of plants, functioning of photosystem I (PSI) and photosystem II (PSII), intensity of net photosynthesis (A), transpiration (E), photorespiration and water use efficiency (WUE) of plants were calculated under control and salinine conditions (0 and 200 mM NaCl). Results of the population-genetic analysis showed that the Makan population is polymorphic (plastic) and the Valitovo population is monomorphic (narrowly specialized). There were no significant differences between the populations based on δ13С values or growth parameters, PSII, A, E and WUE under control conditions. Under saline conditions, dry biomass accumulation decreased in the Makan population by 15% and by more than 2- fold in the Valitovo population. Population differences were revealed in terms of photorespiration intensity and P700 oxidation kinetics under control and saline conditions. Under control conditions, Makan plants were characterized by a higher photorespiration intensity, which decreased by 2-fold under saline conditions to the photorespiration level of Valitovo plants. Cyclic electron transport activity was minimal in the control Makan plants, and it increased by almost 2-fold under saline conditions to the level of that in Valitovo plants under control and saline conditions. Under control conditions, photosynthesis in Makan plants can be specified as the proto-Kranz type (transitional type from C3 to C2) and that in Valitovo plants can be specified as the C2 type (C4 photosynthesis with photorespiratory CO2-concentrating mechanism), based on their photorespiration level and cyclic electron transport activity. Under saline conditions, Makan plants exhibited features of C2 photosynthesis. Intraspecific functional differences of photosynthesis were revealed in different populations of intermediate C3–C4 plant species S. sedoides which reflect the initial stages of formation of a photorespiratory CO2-concentrating mechanism during C4 photosynthesis evolution, accompanied by decrease in salt tolerance.