The effect of cadmium-nickel interactions on superoxide production, cell viability and membrane potential (EM) in roots of two maize cultivars

Effects of CdCl2, NiCl2 or both on superoxide production, viability and membrane potential (EM) of root cells in meristematic (MZ) and differentiation (DZ) zones of two maize cultivars (cv. Premia and cv. Blitz) were studied. Plants were supplied with 10 and 100 μM concentrations of heavy metals (HM). The responses in the studied parameters to HM were concentration- and time-dependent, and were found only in the cells of MZ. The treatment of roots with Cd-stimulated massive superoxide production, although to different extent depending on the cultivar, root zone, and metal concentration. The stimulating effect of Ni on oxidative burst in Cd-treated maize roots was related to an increased Cd-induced superoxide production. The cell death appeared between 24 and 48 h and between 12 and 24 h of the 10 μM and 100 μM metal treatments, respectively. This was in accordance with Cd-induced ROS (superoxide) production and the EM decline in the corresponding time periods. Cell viability, EM changes and partially superoxide production indicate that the impact of the metals on the studied parameters declined in the order Cd+Ni > Cd > Ni and that cv. Blitz tends to respond more sensitively than cv. Premia.

A Lotus japonicus mutant defective in nitrate uptake is also affected in the nitrate response to nodulation

A chlorate-resistant mutant (Ljclo1) of the model legume Lotus japonicus was identified that showed normal levels of nitrate reductase enzyme activity but had decreased uptake of nitrate, as determined from nitrate depletion and electrophysiological measurements. The data suggest that the mutant could be affected specifically in the low-affinity but not in the high-affinity nitrate transport system, and also showed decreased uptake of chlorate. Back-crosses of the mutant plant to the wild type indicated that it is affected in a single Mendelian recessive trait. Thus, the mutation produced in Ljclo1 may be related to some of the low-affinity nitrate transporters or to a regulatory mechanism associated with nitrate/chlorate uptake. Both size and chlorophyll content in young leaves of the mutant plants were significantly reduced compared to the wild type. In addition, nodulation performance of the mutant plants was similar to the wild type in the absence of any exogenous nitrate. However, the nodule:root biomass ratio in mutant plants was considerably reduced in the presence of 1-2 mm nitrate. The levels of several transcripts for nitrate transport and assimilation genes were determined for the wild type and mutant plants and were slightly different. The results suggest interdependence between nitrate uptake, plant growth and nodulation in Ljclo1 mutant plants.

Glutathione peroxidase expression and activity in barley root tip after short-term treatment with cadmium, hydrogen peroxide and t-butyl hydroperoxide

The purpose of this study was to analyse the alterations of glutathione peroxidase (GPX) expression and activity during the recovery period after a short-term treatment of barley root tip with cadmium (Cd) and hydrogen peroxide (H(2)O(2)). The transcript level of GPX increased as early as 1 h and GPX activity 3 h after short-term treatment independently of Cd concentration. In 15 μM Cd-treated roots, its expression reached a peak within 2 h and sustained until 3 h, after which it gradually declined. After 6 h of short-term Cd treatment, the activity of GPX was the highest in the 15-μM Cd-treated roots. At higher Cd concentrations, the activity of GPX was lower than in 15 μM Cd-treated roots, but still higher than in control roots. A considerable increase in H(2)O(2) production was observed even after only 1 h of short-term exposure of roots to 30 and 60 μM Cd, while after 15 μM Cd exposure, its production increased 3 h after the treatment. Lipid peroxidation increased even 1 h after short-term treatment in a Cd concentration-dependent manner. A considerable decrease of GPX activity was observed after the exposure of roots to H(2)O(2) or t-butyl hydroperoxide in a concentration-dependent manner despite that its expression increased even 1 h after short-term treatment. Presumable, under high acute Cd stress, rapid accumulation of H(2)O(2) leads to the disturbance of basal metabolic processes affecting also GPX activity. In contrast, high GPX activity under moderate Cd stress maintains cell function despite the high rate of H(2)O(2) metabolism in root tip.

Early Zn2+-induced effects on membrane potential account for primary heavy metal susceptibility in tolerant and sensitive Arabidopsis species

BACKGROUND AND AIMS

Uptake of heavy metals by plant root cells depends on electro-physiological parameters of the plasma membrane. In this study, responses of the plasma membrane in root cells were analysed where early reactions to the metal ion-induced stress are localized. Three different Arabidopsis species with diverse strategies of their adaptation to heavy metals were compared: sensitive Arabidopsis thaliana and tolerant A. halleri and A. arenosa.

METHODS

Plants of A. thaliana Col-0 ecotype and plants of A. arenosa and A. halleri originating from natural metallicolous populations were exposed to high concentrations of Zn(2+). Plants were tested for root growth rate, cellular tolerance, plant morphology and cell death in the root apex. In addition, the membrane potential (E(M)) of mature cortical root cells and changes in the pH of the liquid culture media were measured.

KEY RESULTS

Primary roots of A. halleri and A. arenosa plants grew significantly better at increased Zn(2+) concentrations than A. thaliana plants. Elevated Zn(2+) concentrations in the culture medium induced rapid changes in E(M). The reaction was species-specific and concentration-dependent. Arabidopsis halleri revealed the highest insensitivity of the plasma membrane and the highest survival rate under prolonged treatment with extra-high concentrations. Plants were able to effectively adjust the pH in the control, but much less at Zn(2+)-induced lower pH.

CONCLUSIONS

The results indicate a similar mode of early reaction to Zn(2+), but with different extent in tolerant and sensitive species of Arabidopsis. The sensitivity of A. thaliana and a high tolerance of A. halleri and A. arenosa were demonstrated. Plasma membrane depolarization was lowest in the hyperaccumulator A. halleri and highest in A. thaliana. This indicates that rapid membrane voltage changes are an excellent tool to monitor the effects of heavy metals.

Growth and functional responses of different cultivars of Lotus corniculatus to aluminum and low pH stress

Aluminum toxicity is an important stress factor in acid soils. Growth, respiration and permeability properties of root cells were studied in five cultivars of Lotus corniculatus subjected to aluminum (Al) or low pH stress. The cultivars showed significant differences in root elongation under stress conditions, which correlated with changes in membrane potential (E(M)) of root cortical cells. A pH drop from 5.5 to 4.0 resulted in significant membrane depolarization and root growth inhibition. The strongest inhibition was observed in cv. São Gabriel (33.6%) and least in cv. UFRGS (25.8%). Application of an extremely high Al concentration (2mM) stopped the root growth in cv. INIA Draco, while inhibition in cv. UFRGS reached only 75%. The E(M) values of cortical cells of Lotus roots varied between -115 and -144mV. Treatment with 250microM of AlCl(3) (pH 4) resulted in rapid membrane depolarization. The extent of the membrane depolarization ranged between 51mV (cv. UFGRS) and 16mV (cv. INIA Draco). The membrane depolarization was followed by a loss of K(+) from Al-treated roots (2mM Al) and resulted in a decrease of the diffusion potential (E(D)). The total amount of K(+) in Al-treated roots dropped from 31.4 to 16.8micromolg(-1) FW in sensitive cv. INIA Draco, or from 26.1 to 22.7micromolg(-1) FW in tolerant cv. UFGRS. The rate of root respiration under control conditions as well as under Al treatment was higher in cv. INIA Draco than in cv. UFRGS. Al-induced inhibition of root respiration was 21-34% of the control.

Aluminium toxicity in plants: internalization of aluminium into cells of the transition zone in Arabidopsis root apices related to changes in plasma membrane potential, endosomal behaviour, and nitric oxide production

The extent of aluminium internalization during the recovery from aluminium stress in living roots of Arabidopsis thaliana was studied by non-invasive in vivo microscopy in real time. Aluminium exposure caused rapid depolarization of the plasma membrane. The extent of depolarization depends on the developmental state of the root cells; it was much more extensive in cells of the distal than in the proximal portion of the transition zone. Also full recovery of the membrane potential after removal of external aluminium was slower in cells of the distal transition zone than of its proximal part. Using morin, a vital marker dye for aluminium, and FM4-64, a marker for endosomal/vacuolar membranes, an extensive aluminium internalization was recorded during the recovery phase into endosomal/vacuolar compartments in the most aluminium-sensitive cells. Interestingly, aluminium interfered with FM4-64 internalization and inhibited the formation of brefeldin A-induced compartments in these cells. By contrast, there was no detectable uptake of aluminium into cells of the proximal part of the transition zone and the whole elongation region. Moreover, cells of the distal portion of the transition zone emitted large amounts of nitric oxide (NO) and this was blocked by aluminium treatment. These data suggest that aluminium internalization is related to the most sensitive status of the distal portion of the transition zone towards aluminium. Aluminium in these root cells has impact on endosomes and NO production.

Effects of calcium ions on electrical responses of gastric gland cells to histamine and pentagastrin

The effect of Ca ions on electrical responses of gastric gland cells on histamine and pentagastrin was investigated using intracellular glass microelectrodes. It was established that in low-calcium solutions hyperpolarization induced by these secretagogues was diminished. In calcium-free solutions and in solutions with blockers of the calcium current hyperpolarization induced by histamine and pentagastrin was not observed. It was suggested that external calcium ions are necessary for hyperpolarization responses to histamine and pentagastrin actions on gastric gland cells to occur.