Waldemar Maksymiec

Signaling responses in plants to heavy metal stress

Heavy metal toxicity is one of the major abiotic stresses leading to hazardous health effects in animals and plants. Because of their high reactivity they can directly influence growth, senescence and energy synthesis processes. In this review a new indirect mechanism of heavy metal action is proposed. This mechanism is connected with the generation of reactive oxygen species (especially H2O2) and jasmonate and ethylene signaling pathways and shows that toxicity symptoms observed in plants may result from direct heavy metal influence as well as the activity of some signaling molecules induced by the stress action.

In vivo response of photosynthetic apparatus of Phaseolus vulgaris L. to nickel toxicity

Summary We investigated the effect of in vivo Ni 2+ toxicity on the photosynthetic system of primary bean leaves ( Phaseolus vulgaris L. cv. Slowianka). The leaf area, chlorophyll and total carotenoid content showed a distinct decrease strictly related to the metal concentration in the nutrient medium and to the period of treatment. An analysis of the parameters of chlorophyll a fluorescence fast phase of induction kinetics revealed the lack of Ni 2+ effect of the primary photochemical processes in photosystem II. The measurements of photosynthetic electron transport under heavy metal influence, both in vivo and in vitro , and comparison with the results obtained from the analysis of the slow phase of the Kautsky curve allow us to postulate an indirect effect of nickel on photosystems related to the disturbances caused by the metal in the Calvin cycle reactions and down-regulation or even feedback inhibition of electron transport by the excessive amounts of ATP and NADPH accumulated due to non-efficient dark reactions.

Chlorophyll fluorescence in primary leaves of excess Cu-treated runner bean plants depends on their growth stages and the duration of Cu-action

Summary The functioning of the photosynthetic apparatus of runner bean plants (Phaseolus coccineus L., cv. Piekny Jaś) treated with excess Cu (20mgL−1 in the form of CuSO4 · 5H2O) at 5 successive growth stages of their primary leaves was studied. Chlorophyll fluorescence measurements were made on intact leaves or thylakoid membranes isolated from them after different times of Cu action from 3 to 12 days. It was shown that the plants exposed to copper at the intensive and intermediate growth stages were characterized by a lack of changes in the primary PS II photochemical processes. However, a simultaneous a potential quantum efficiency decrease was observed, pointing to intensified energy dissipation processes, as well as to a decrease of the vitality index values Rfd(except plants treated at the beginning of growth). The highest sensitivity of the photosynthetic apparatus to excess Cu occurred in the plants treated at advanced leaf growth stages. The presented data suggest that the observed fast decrease of photosynthetic efficency resulted from Cu-induced PSII complex destabilization connected with an inhibition of its donor side and the primary photochemical processes. At the same time as the inhibition of the electron flow through PSII a certain increase of the reduced QA pool occurred, pointing to a considerable weakening of reoxidation processes of the PSII acceptor side, possibly due to an inhibition of the Calvin cycle by Cu ions.

Different susceptibility of runner bean plants to excess copper as a function of the growth stages of primary leaves

Summary Runner bean plants ( Phaseolus coccineus L., cv. Piekny Jaś) grown hydroponically in Knop solution were treated with excess Cu (20mgL −1 in the form of CuSO 4 · 5H 2 O) at three different growth stages of primary leaves, and after 3–12 days of Cu action, respectively, their growth parameters and plastid pigment content were investigated. Moreover, photosynthetic O 2 evolution of chloroplasts isolated from plants treated with excess Cu at the different growth stages 10 days after the treatment was compared. The plants treated with Cu at the intensive leaf growth stage were characterized by fast and strong growth (measured as leaf area), a fresh weight decrease and a simultaneously increased chlorophyll content in comparison with the control, indicating some inhibition of elongation processes. Relatively smaller changes in growth parameters that were correlated with some modifications of the specific leaf area (calculated per fresh weight), leaf density and pigment composition were observed in the plants treated with Cu at an intermediate growth stage. However, at a longer exposure to Cu there occurred a tendency of Chl a/b and Chl/Car ratios to decrease. These changes, depending on the time of Cu action, were much more distinct in the plants treated with Cu at the stationary growth stage of primary leaves. A simultaneous decrease in leaf tissue density and enhanced specific leaf area, which contributes to an enlarged area for water loss, correlated with chlorophyll content reduction, indicated that a leaf senescence increase was presumably a consequence of water deficiency. In the plants treated with Cu at this stage of leaf growth, photosynthetic O 2 evolution decreased significantly.

Jasmonic acid and heavy metals in Arabidopsis plants - a similar physiological response to both stressors?

Summary Methyl ester of jasmonic acid (JAMe) in in vivo conditions inhibited growth and pigment accumulation and also decreased the efficiency of some photosynthetic processes in Arabidopsis plants. A similar effect of excess Cu and Cd ions, depending on their concentration, was observed. Salicylhydroxamate and propyl gallate, inhibitors of jasmonic acid synthesis, prevented the inhibitory effect of heavy metals on chlorophyll accumulation and photosynthetic activity, but not that on leaf growth (at a higher Cd content) and on energy dissipative processes in the light-harvesting antennae. JAMe at 10−6 mol/L and 10−5 mol/L showed a protective effect against Cu and Cd ions, not observed at 10−4 mol/L. These results indicate that JAMe strongly influences Cu and Cd toxicity in Arabidopsis plants. This effect is partially metal-specific and depends on jasmonic acid concentration and its internal or external origin.

Conformational rearrangements in light-harvesting complex II accompanying light-induced chlorophyll a fluorescence quenching

Light-induced chlorophyll a (Chl a) fluorescence quenching was studied in light-harvesting complex of photosystem II (LHCII). Fluorescence intensity decreased by ca. 20% in the course of 20 min illumination (412 nm, 36 micromol m(-2) s(-1)) and was totally reversible within 30 min dark adaptation. The pronounced quenching was observed only in LHCII in an aggregated form and exclusively in the presence of molecular oxygen. Structural rearrangement of LHCII correlated to the quenching was monitored by measuring changes in UV-Visible light absorption spectra, and by measuring Fourier-transform infrared spectroscopy (FTIR) in the Amide I region of the protein (1600-1700 cm(-1)). The light-induced structural rearrangement of LHCII was interpreted as a partial disaggregation of the complex based on the decrease in the light scattering signal and the characteristic features observed in the FTIR spectra: the relative increase in the intensity of the band at 1653 cm(-1), corresponding to a protein in the alpha-helical structure at the expense of the band centered at 1621 cm(-1), characteristic of aggregated forms. The fact that the light-driven isomerization of the all-trans violaxanthin to the 13-cis form was not observed under the non-oxygenic conditions coincided with the lack of large-scale conformational reorganization of LHCII. The kinetics of this large-scale structural effect does not correspond to the light-induced fluorescence quenching, in contrast to the kinetics of structural changes in LHCII observable at low oxygen concentrations. Photo-conversion of 5% of the pool of all-trans violaxanthin to 9-cis isomer was observed under such conditions. Possible involvement of the violaxanthin isomerization in the process of structural rearrangements and excitation quenching in LHCII is discussed.

Changes in acyl lipid and fatty acid composition in thylakoids of copper non-tolerant spinach exposed to excess copper

Summary The effect of toxic concentrations of Cu on acyl lipid and fatty acid composition in thylakoid membranes of non-tolerant spinach plants exposed to excess Cu was investigated. In Cu-treated plants a decrease in acyl lipid content and changes in its molar distribution were found. Thylakoid lipid degradation was accompanied by changes in fatty acid composition, especially that of 18:3, 16:1 and 16: 0. The obtained results are discussed in terms of the indirect effect of long term action of excess Cu on the thylakoid membrane structure and decrease of its photochemical activities.

Molecular Architecture of Plant Thylakoids under Physiological and Light Stress Conditions: A Study of Lipid–Light-Harvesting Complex II Model Membranes

The organization of plant thylakoid membranes under physiological and light stress conditions was analyzed in studies of model membranes formed with galactolipids and LHCII. The results show adaptation of an organization pattern of lipid-protein membranes to better fulfill two opposite physiological functions: harvesting of light quanta versus quenching of excess energy. In this study, we analyzed multibilayer lipid-protein membranes composed of the photosynthetic light-harvesting complex II (LHCII; isolated from spinach [Spinacia oleracea]) and the plant lipids monogalcatosyldiacylglycerol and digalactosyldiacylglycerol. Two types of pigment-protein complexes were analyzed: those isolated from dark-adapted leaves (LHCII) and those from leaves preilluminated with high-intensity light (LHCII-HL). The LHCII-HL complexes were found to be partially phosphorylated and contained zeaxanthin. The results of the x-ray diffraction, infrared imaging microscopy, confocal laser scanning microscopy, and transmission electron microscopy revealed that lipid-LHCII membranes assemble into planar multibilayers, in contrast with the lipid-LHCII-HL membranes, which form less ordered structures. In both systems, the protein formed supramolecular structures. In the case of LHCII-HL, these structures spanned the multibilayer membranes and were perpendicular to the membrane plane, whereas in LHCII, the structures were lamellar and within the plane of the membranes. Lamellar aggregates of LHCII-HL have been shown, by fluorescence lifetime imaging microscopy, to be particularly active in excitation energy quenching. Both types of structures were stabilized by intermolecular hydrogen bonds. We conclude that the formation of trans-layer, rivet-like structures of LHCII is an important determinant underlying the spontaneous formation and stabilization of the thylakoid grana structures, since the lamellar aggregates are well suited to dissipate excess energy upon overexcitation.

Heavy Metal Interactions with Plant Nutrients

According to the classical definition “Interactions between nutrients occur when the supply of one nutrient affects the absorption, distribution or function of another nutrient. Thus, depending on nutrient supply, interactions between nutrients can either induce deficiencies or toxicities and can modify growth response” (Robson and Pitman, 1983). There are many nonspecific as well as specific interactions between mineral nutrients of plants (Robson and Pitman, 1983; Marschner 1988). When contents of any mineral nutrients are near the deficiency range the importance of interactions between two mineral nutrients increases. Specific interactions, e.g. competition between nutrients at the cellular level or replacement of one nutrient by another, are also important in evaluating critical toxicity contents (Foy et al., 1978; Marschner, 1988).

The in vivo and in vitro influence of methyl jasmonate on oxidative processes in Arabidopsis thaliana leaves

In Arabidopsis thaliana leaves a strong increase of H2O2 content was induced by application of methyl jasmonate (JAMe) through the root system, but the induction only slightly depended on JAMe concentration. The activity of superoxide dismutase and ascorbic acid peroxidase increased at lower JAMe concentrations and decreased at higher ones. Catalase activity decreased proportionally to JAMe concentration (in comparison with control plants). The sum of ascorbic acid and dehydroascorbate content at 10−6 M JAMe was similar to the control, but at higher concentrations it increased, especially due to a higher ascorbate accumulation. Methyl jasmonate applied directly to the extract of leaves (in vitro experiment) also induced a strong increase in H2O2 level, even at a low concentration (10−8 M). Since lower JAMe concentrations induced weak superoxide dismutase and did not change catalase and peroxidase activity, it is suggested that in this case a high level of hydrogen peroxide was not the result of the activity of the mentioned enzymes. JAMe-induction of H2O2 increase at the highest JAMe concentration resulted from SOD activity. Our in vivo and in vitro experiments suggest that jasmonate can influence oxidative stress not only through gene expression but also by its direct effect on enzyme activity.