Interference of nickel with the photosynthetic apparatus of Zea mays.

Photosynthetic capacity of plants in heavy metals-polluted environment depends on growth stage of plants. Previously, Ni effect on the process of photosynthesis was studied in fully developed plants. The objective of our studies was the response of the developing photosynthetic apparatus to Ni. Changes in chlorophyll a fluorescence induction kinetics: t(1/2), F(v), F(m), F(0), F(v)/F(m), the content of chlorophyll a, b and carotenoids (x+c), as well as the ratios of chlorophyll (a+b) to the total carotenoids [(a+b)/(x+c)] and chlorophyll a/b (Chl a/b) were followed in leaves of 3 day old seedlings of Zea mays exposed to 0, 100 and 200 microNi for 8 and 13 days. The values of t(1/2), F(v), F(m) markedly declined in seedlings treated with 200 microM Ni for both periods, while the F(v)/F(m) ratio decreased in those exposed to 200 microM Ni for 13 days. The content of (x+c) declined gradually with time of plant exposure to the metal. The (a+b)/(x+c) ratio was reduced under Ni treatment, particularly after 8 days. The Chl a/b ratio increased in seedlings treated with the metal for 8 days, but decreased after 13-day-exposure, indicating different response of Chl b to Ni depending on the exposure time. During growth under Ni stress maize invested primarily in Cars and the reaction centre pigment-Chl a. The protective mechanisms in the Ni stressed plant are discussed.

Effect of BSO-supplemented heavy metals on antioxidant enzymes in Arabidopsis thaliana.

Activities of the ascorbate-glutathione cycle enzymes were investigated in leaves of Arabidopsis thaliana plants grown for 7 and 14 days in modified Hoagland nutrient solution containing Cd and Cu alone or supplemented with buthionine sulfoximine (BSO), an inhibitor of glutathione synthesis. In A. thaliana treated with Cd+BSO, the modifying BSO effect involved dehydroascorbate reductase (DHAR) activity after 7 days of treatment and ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR) and glutathione reductase (GR) activities after 14 days. DHAR activity increased, while activities of APX, MDHAR and GR decreased in comparison to that found with Cd alone. BSO supplied in combination with Cu modified the metal effect on APX activity after 7 days of exposure and on MDHAR activity after 14 days. Cu+BSO enhanced APX activity, but decreased MDHAR activity as compared to that with Cu alone. Similarities and varieties in the modifying BSO effect, depending on the metal, have been discussed. The modifying BSO effect was more pronounced in the plants exposed to Cd than Cu, and was opposite after 7 and 14 days. However, differences between the effects of the individual metals on the enzymes were greater after 7 days of plant exposure.

Role of polyisoprenoids in tobacco resistance against biotic stresses.

Infection with avirulent pathogens, tobacco mosaic virus (TMV) or Pseudomonas syringae pv. tabaci induced accumulation of polyisoprenoid alcohols, solanesol and a family of polyprenols [from polyprenol composed of 14 isoprene units (Pren-14) to -18, with Pren-16 dominating] in the leaves of resistant tobacco plants Nicotiana tabacum cv. Samsun NN. Upon TMV infection, solanesol content was increased seven- and eight-fold in the inoculated and upper leaves, respectively, while polyprenol content was increased 2.5- and 2-fold in the inoculated and upper leaves, respectively, on the seventh day post-infection. Accumulation of polyisoprenoid alcohols was also stimulated by exogenously applied hydrogen peroxide but not by exogenous salicylic acid (SA). On the contrary, neither inoculation of the leaves of susceptible tobacco plants nor wounding of tobacco leaves caused an increase in polyisoprenoid content. Taken together, these results indicate that polyisoprenoid alcohols might be involved in plant resistance against pathogens. A putative role of accumulated polyisoprenoids in plant response to pathogen attack is discussed. Similarly, the content of plastoquinone (PQ) was increased two-fold in TMV-inoculated and upper leaves of resistant plants. Accumulation of PQ was also stimulated by hydrogen peroxide, bacteria (P. syringae) and SA. The role of PQ in antioxidant defense in cellular membranous compartments is discussed in the context of the enzymatic antioxidant machinery activated in tobacco leaves subjected to viral infection. Elevated activity of several antioxidant enzymes (ascorbate peroxidase, guaiacol peroxidase, glutathione reductase and superoxide dismutase, especially the CuZn superoxide dismutase isoform) and high, but transient elevation of catalase was found in inoculated leaves of resistant tobacco plants but not in susceptible plants.

The redox state and activity of superoxide dismutase classes in Arabidopsis thaliana under cadmium or copper stress.

The redox state of glutathione and ascorbate as well as the activity of superoxide dismutase classes were determined in leaves of Arabidopsis thaliana grown for seven days in the nutrient solution containing 0, 5 and 50 microM Cd or Cu excess. A decrease in GSH/GSSG ratio was found in plants under Cd and Cu stress. In the plants exposed to Cu stress the activity of all SOD classes increased. However, in the plants treated with Cd the activity of FeSOD and MnSOD was elevated, but CuZnSOD activity was diminished in comparison with control. In these plants the activity of SOD classes was dependent on both the GSH/GSSG and AA/DHA ratios, while in those exposed to Cu excess - on the GSH/GSSG ratio. Differences were shown in the changes both in redox state and activity of SOD classes caused by the metals differing in physiochemical properties. Moreover, relationships between changes in SOD class activities and ROS levels were discussed.

Growth parameters and photosynthetic pigments in leaf segments of Zea mays exposed to cadmium, as related to protection mechanisms.

The influence of cadmium on growth parameters and photosynthetic pigment content was studied in maize leaf segments differing in tissue maturity. Experiments were carried out with maize seedlings Zea mays L. cv. Hidosil treated with 25, 50, 100, 150 and 200 microM Cd for 14 days under low light conditions. Tissue age-dependent decrease of fresh mass, dry mass and area of the leaf segments was correlated with Cd concentration in the growth medium. Cd induced changes in chlorophyll and carotenoid contents, and the specific areas and densities of the leaf segments were dependent on age and metal concentration. Results are discussed with respect to the protection mechanisms in the leaf segments.

Age-dependent response of maize leaf segments to cadmium treatment: effect on chlorophyll fluorescence and phytochelatin accumulation.

The relationship between the age of leaf tissue and response of the photosynthetic apparatus and phytochelatin accumulation to Cd treatment was studied. Studies were carried out with seedlings of Zea mays L. cv. Hidosil grown in the presence of 100-200 mumol/L Cd for 14 days under low light conditions. The third leaf was divided into 3 segments of equal length differing in the stage of tissue maturity and used for measurements of chlorophyll content, chlorophyll fluorescence, glutathione and phytochelatin content and Cd accumulation. A close relationship between the age of leaf tissue and response of the photosynthetic apparatus to Cd was shown. Cadmium (200 mumol/L) reduced photochemical processes more in older than younger leaf segments as seen in the Chl fluorescence parameters Fv/F0, and t1/2, while the chlorophyll fluorescence decrease ratio (Rfd) was inhibited more strongly in younger ones. Fv/Fm was slightly affected. Cd-induced enhancement of GSH content was correlated with higher phytochelatin accumulation to a greater extent in younger than in older leaf segments. Phytochelatin level corresponded to changes of photochemical processes in older leaves. The peptide thiol:Cd molar ratio for the phytochelatins varied depending on Cd concentration and age of leaf segments. The protective role of phytochelatins for the photosynthetic apparatus is discussed.

The activity of antioxidant enzymes in Arabidopsis thaliana exposed to colchicine and H2O2.

Studies on the possible interference of colchicine and H2O2 with the activity of some antioxidant enzymes were carried out on Arabidopsis thaliana v. Columbia grown in Murashige and Skooge nutrient medium. Measurements of superoxide dismutase (SOD), guaiacol peroxidase (POX), ascorbate peroxidase (APX) and catalase (CAT) activities were conducted spectrophotometrically. In the presence of colchicine, SOD activity increased, while CAT, APX and POX activities decreased. Inhibitory H2O2 effects on the activity of the enzymes were found. Colchicine pre-treatment resulted in an increase in CAT activity and a further increase in SOD activity in plants treated with H2O2.

Copper-induced oxidative stress and antioxidant defence in Arabidopsis thaliana.

Content of reactive oxygen species (ROS): O2*-, H2O2 and OH* as well as activities of antioxidant enzymes: superoxide dismutase (SOD), guaiacol peroxidase (POX) and catalase (CAT) were studied in leaves of Arabidopsis thaliana ecotype Columbia, treated with Cu excess (0, 5, 25, 30, 50, 75, 100, 150 and 300 microM). After 7 days of Cu action ROS content and the activity of SOD and POX increased, while CAT activity decreased in comparison with control. Activities of SOD, POX and CAT were correlated both with Cu concentration (0-75 microM) in the growth medium and with OH* content in leaves. Close correlation was also found between OH* content and Cu concentration. Oxidative stress in A. thaliana under Cu treatment expressed in elevated content of O2*-, H2O2 and OH* in leaves. To overcome it very active the dismutase- and peroxidase-related (and not catalase-related, as in other plants) ROS scavenging system operated in A. thaliana. Visual symptoms of phytotoxicity: chlorosis, necrosis and violet colouring of leaves as well as a reduction of shoot biomass occurred in plants.