Péter Nyitrai

Characterization of the stimulating effect of low-dose stressors in maize and bean seedlings

The effect of some more or less harmful compounds like Cd, Pb, Ni, Ti salts and DCMU at low concentrations on the development of chloroplasts in maize and bean seedlings was investigated. Chlorophyll content, chlorophyll a/b ratio, photosynthetic activity (14CO2 fixation), chlorophyll-protein composition of thylakoid membranes, fluorescence spectra of chloroplasts, fluorescence induction parameters of leaves and electron microscopic structure of maize and bean chloroplasts as well as growth parameters were studied. Stimulation of chlorophyll synthesis and photosynthetic activity was observed at different intervals during all of the treatments, while chlorophyll a/b ratios and fluorescence properties of leaves or chloroplasts did not change considerably except in DCMU treated plants. Heavy metal treatments increased the amount of photosystem I and light-harvesting complex II, while decreased amount of photosystem I and higher amount of light-harvesting complex II was found in DCMU treated thylakoids. Electron microscopy showed only sligth differences in the morphology of chloroplast lamellar system (mostly in DCMU treated plants), while the status of the plasmalemma and tonoplast seemed to be altered as a result of certain metal treatments. Results showed the expression of a cytokinin-like effect on the development of chloroplasts. It is assumed, that these low-dose stressors generate non-specific alarm reactions in plants, which may involve changes of the hormonal balance.

Investigation into the mechanism of stimulation by low-concentration stressors in barley seedlings

Beneficial effects of low-concentration chemical stressors have been investigated previously in different model systems. The symptoms of stimulation are known from earlier studies, but information about the mechanism is at an initial stage. In the present work, the mechanism of stimulation of low-concentration Cd (5 x 10(-8)M) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU, 10(-7)M) was investigated in barley seedlings. In treated plants, the amount of cytokinins increased in roots and, after being transported to the leaves, they caused stimulation there. To identify the signal transduction pathway(s) involved in the primary stimulation of cytokinin synthesis (and/or activation) in roots, specific phosphatidylinositol-4,5-bisphosphate-inositol-1,4,5-triphosphate/diacylglycerol (PIP(2)-IP(3)/DAG) and mitogen activated protein kinase (MAPK) signaling pathway inhibitors were added to the nutrient solution, and all proved to be effective, eliminating the stimulation by the stressors. Measurements of superoxide dismutase (SOD, EC 1.15.1.1) activity and the amount of malonyldialdehyde (MDA) showed that the increased amount of Cd did not cause oxidative stress in the roots, and no oxidative stress was found in the leaves, where Cd did not even accumulate. DCMU slightly increased the activity of SOD after 1 week in roots, but did not cause lipid peroxidation. In leaves, there was no oxidative stress upon treatment with DCMU. Thus, oxidative stress cannot be responsible for the stimulation with low-concentration stressors, as they changed the activity of SOD differently, while being equally stimulative for the plants.

On the mechanism of rejuvenation of ageing detached bean leaves by low-concentration stressors.

The effect of low concentrations of some stress-inducing compounds of different toxicity and chemical nature, such as Cd and Pb salts or DCMU, was investigated on the senescence of chloroplasts in detached primary leaves of bean (Phaseolus vulgaris L.). After 1 week of senescence followed by root development from the petiole, these agents stimulated chlorophyll accumulation and photosynthetic activity ((14)CO(2) fixation) as compared to the control, thus inducing rejuvenation. Low-concentration stressors increased the level of active cytokinins in roots and leaves during the treatment, as monitored by the Amaranthus betacyanin bioassay and high-pressure liquid chromatography. The lithium ion, an inhibitor of the PIP(2)-IP(3)/DAG signal transduction pathway, abolished the stimulating effect of stressors, both in roots (retarding cytokinin synthesis) and consequently also in leaves (reducing cytokinin-dependent chlorophyll accumulation). This suggests the involvement of the PIP(2)-IP(3)/DAG signal transduction pathway in generation of these consecutive organ-specific responses.

Artificial Tripartite Symbiosis Involving a Green Alga (Chlamydomonas), a Bacterium (Azotobacter) and a Fungus (Alternaria): Morphological and Physiological Characterization

A long-living artificial tripartite symbiosis involving a green alga (Chlamydomonas), a bacterium (Azotobacter) and a fungus (Alternaria) was established on carbon- and nitrogen-free medium. The basis of the interdependence is the complementation of photosynthetic CO2 assimilation and atmospheric nitrogen fixation. Green color of the colonies indicated that the algal cells had enough nitrogen to synthesize chlorophylls. The chlorophyll content was nearly 40 % of the control cells. The relatively high rate of photosynthetic oxygen evolution proved that nitrogen was effectively used for building up a well functioning photosynthetic apparatus. This was supported by the analysis of photosystems and ultrastructural investigations. In comparison with degreened algae cultured on nitrogen-free medium, the chloroplasts in the symbiont algal cells contained a well-developed, stacked thylakoid membrane system without extreme starch or lipid accumulation. The occurrence of the fungus in the association greatly increased the chlorophyll content. Far fewer types of amino acids were excreted by the tripartite cultures than by pure cultures. Cystathionine, which is a common intermediate in the sulfur-containing amino acid metabolism, was produced in high quantities by the tripartite symbiosis. This can mostly be attributed to the activity of the fungus.

Organization of Thylakoid Membranes in Low-Light Grown Maize Seedlings. Effect of Lincomycin Treatment

Summary Etiolated maize seedlings were greened under extreme low-light intensity (25 lux). Pigment content, chlorophyll-protein composition, polypeptide pattern, photochemical activity and ultrastructure of mesophyll chloroplasts were studied. It was found that mainly chlorophyll a protein complexes were formed. The amount of light-harvesting chlorophyll-protein complexes and grana stacks were strongly reduced. The light intensity dependence of photochemical activities of these plastids referred to the presence of photosystems I and II of smaller size. After lincomycin treatment (inhibiting the synthesis of reaction centers and inner antennae) stabilization of light-harvesting chlorophyll a/b complexes of photosystems I and II were allowed. The role of the light regime and chlorophyll accumulation in the assembly and stabilization of light-harvesting chlorophyll-protein complexes is discussed.

Structural and functional changes in the photosynthetic apparatus of Chlamydomonas reinhardtii during nitrogen deprivation and replenishment

Nitrogen is an essential factor for normal plant and algal development. As a component of nucleic acids, proteins, and chlorophyll (Chl) molecules, it has a crucial role in the organization of a functioning photosynthetic apparatus. Our aim was to study the effects of nitrogen starvation in cultures of the unicellular green alga, Chlamydomonas reinhardtii, maintained on nitrogen-free, and then on nitrogen-containing medium. During the three-week-long degreening process, considerable changes were observed in the Chl content, the ratio of Chl-protein complexes, and photosynthetic activity of the cultures as well as in the ultrastructure of single chloroplasts. The regreening process was much faster then the degradation; total greening of the cells occurred within four days. The rate of regeneration depended on the nitrogen content. At least 50% of the normal nitrogen content of Tris-Acetate-Phosphate (TAP) medium was required in the medium for the complete regreening of the cells and regeneration of chloroplasts.

Relative Accumulation of LHCP II in Mesophyll Plastids of Intermittently Illuminated Maize Seedlings under Lincomycin Treatment

Summary It was found that chlorophyll b can be synthesized, apoLHCP II and LHCP II can be accumulated in mesophyll plastids of maize seedlings greened under intermittent light (IML), when treated with lincomycin. As opposed to untreated IML plastids the relative amount and spectral characteristics of LHCP II and the LHCP/CPa ratio of these thylakoids were similar to those of untreated mature ones. The development of numerous bithylakoids and small grana in treated IML plastids and the saturation of their DCPIP reduction rate at low light intensities refer to a usual structural arrangement and normal functioning of LHCP II. Possible reasons of LHCP II accumulation in these treated plastids are discussed.

Herbicide-Affected Plant Metabolism Reduces Virus Propagation

Abstract It has been previously shown that certain herbicides or plant extracts inhibited the viral infection. The goal of this study was to investigate the effect of Obuda pepper virus (ObPV) infection and herbicide or plant extract treatments on the photosynthetic processes of the host plants to get informations about the interactions of these factors. In Capsicum annuum- ObPV host-virus relations the virus infection slightly increased the activity of photosystem II (PSII), as it was supposed from fluorescence induction parameters. Chlorophyll content of leaves was also elevated probably due to virus-induced growth inhibition. The herbicide Stomp (active ingredient: pendimethalin) incorporated into the soil one week before planting (preplant treatment) together with virus infection even strengthened these effects in agreement with previous observations that this herbicide always did not prevent virus infection or reduce virus concentration in hosts. In ObPV-infected Nicotiana tabacum the structural changes showed similar tendency like in ObPV-infected C. annuum, but PSII efficiency did not significantly differ from that of the control. However, non-photochemical quenching (NPQ) increased because of the strongly decreasing CO2 fixation activity. Though simultaneous application of a water extract of Cirsium arvense shoot caused a little stronger inhibition of CO2 fixation, little loss in production was obtained due to significant reduction in virus concentration. In Solanum nigrum-ObPV relation the slightly increasing tendency of the values of actual PSII quantum efficiency could be related to the probably elevated ratio of reaction centre components (increased chlorophyll a/b ratio) in the thylakoids. Application of the herbicide Fusilade S (active ingredient: fluazifop-P-butyl) at 4-6 leaf stage as a postemergence treatment practically prevented systemic virus infection and the virus-induced changes of photosynthesis are probably due to inhibiting the virus infection/replication process.

How does a little stress stimulate a plant

In contrast to the damaging effect of high-concentration chemical stressors, the same agents in very low (submicromolar) concentrations have a positive effect on the treated plants, which is non-specific (independent of the chemical nature of the agent). The direct responses depend on the treated organ. When leaves are treated, the effects include an increase in chlorophyll content, CO2 fixation, and delaying senescence of chloroplasts. When roots are treated, the direct effect is an increased cytokinin synthesis. This hormone, after being transported to the shoot, exerts secondary effects, which are similar to the primary ones in leaves. The signalization routes involved in the primary effects proved to be the phosphoinositide and MAPK pathways in any stimulated organ. In this mini-review we summarize our current knowledge about the effects of low-concentration stressors and their mechanism of action with the help of the four used model systems: detached non-rooting and rooting leaves, hydroponically treated and sprayed seedlings.

Accumulation of LHCI in Picea and Maize Seedlings Greened Under Different Conditions

Chlorophyll a/b containing light-harvesting complexes (LHCs) are encoded in the nucleus and built in the thylakoids post-translationally (1). Factors affecting their synthesis, accumulation in thylakoids and assembly into larger complexes have not been completely understood yet. To investigate some aspects of this regulation, dark and light grown Picea abies seedlings, capable of chlorophyll synthesis in the dark (2), were used and etiolated maize seedlings were greened in the presence of lincomycin (LM) — a specific inhibitor of chloroplastic translation (3), or gabaculine (GAB) — an inhibitor of δ-ALA synthesis (4), or kinetin. Different light regimes including intermittent (IML) and continuous light (CL) of moderate (Co- control) and extremely low light (LL) intensity were used.