Enzymatic and non-enzymatic defense mechanisms against ultraviolet-B radiation in two Anabaena species

Enzymatic and non-enzymatic defense strategies against ultraviolet-B radiation (UV-B, 280–315 nm) were studied in Anabaena doliolum and Anabaena strain L31, two of the most common strains of Indian cyanobacteria. Upon UV-B irradiation, both strains showed a 2–5-fold increase in antioxidative enzymes, such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and peroxidase (POD), as compared to non-irradiated control cultures. These enzymes scavenge damaging reactive oxygen species (ROS), generated by UV-B radiation inside the cells. In addition, these organisms also synthesize mycosporine-like amino acids (MAAs) which are able to carry out UV-screening and/at the same time as UV-quenching. The identification and characterization of three types of MAAs from both Anabaena species were performed using absorption spectroscopy, high performance liquid chromatography (HPLC), electro spray ionization-mass spectrometry (ESI-MS), Fourier Transform Infrared (FTIR) spectroscopy and Nuclear Magnetic Resonance (NMR) spectroscopy. Shinorine was found to be the most common MAA in both Anabaena species while porphyra-334 and mycosporine-glycine were present only in A. doliolum. The results of the present investigation clearly demonstrate that both enzymatic and non-enzymatic defense mechanisms are being employed by A. doliolum and Anabaena strain L31 to counteract the damaging effects of UV-B radiation.     

The role of proline-associated pentose phosphate pathway in cool-season turfgrasses after UV-B exposure

Physiological adjustments of cool-season turfgrasses were investigated to determine the role of proline-associated pentose phosphate pathway for phenolic biosynthesis and stimulation of antioxidant response system following UV-B exposure. Creeping bentgrass, Kentucky bluegrass, tall fescue, and perennial ryegrass plugs were subjected to artificial UV-B exposure (biologically effective UVB_BE radiation 8 kJ m⁻² d⁻¹.) for one week with 10-h photoperiod. Significant accumulation of phenolics and stimulation of antioxidant enzyme activity was observed in all investigated cool-season turfgrasses after UV-B exposure and this induction corroborated with higher glucose-6-phosphate dehydrogenase activity and high accumulation of proline. Guaiacol peroxidase activity also increased in all investigated cool-season turfgrass species after UV-B exposure. In this study, the shift of carbon flux from glycolysis to pentose phosphate pathway following UV-B exposure and as a result of that, the higher accumulation of phenolics and stimulation of antioxidant response system provides an insight to understand a probable defense mechanism of cool-season turfgrasses against UV stress.     

Ultraviolet-B-induced DNA damage and photorepair in the cyanobacterium Anabaena variabilis PCC 7937

The impact of simulated solar radiation on DNA and the mitigation of DNA-damaging effects by photoreactivation was studied in a cyanobacterium Anabaena variabilis PCC 7937. Cultures were irradiated under 295, 320 and 395 nm cut-off filters as well as seven other filters such as WG 280, WG 295, WG 305, WG 320, WG 335, WG 345 and GG 400. Growth of the test organism was found to be affected mostly under UV-B radiation as compared to PAR and PAR + UV-A radiations. Amplification of 16s rDNA and RAPD profile was significantly affected following exposure of genomic DNA to UV-B radiation. The formation of T<>T CPDs was recorded only in the cultures irradiated with UV-B radiation (i.e., under 295 nm as well as under WG 280, WG 295 and WG 305 nm cut-off filters), but maximum yield was found under 280 nm cut-off filter. Furthermore, the considerable induction of thymine dimers was observed with increasing UV-irradiation times. Fluorometric analysis of DNA unwinding (FADU) assay for UV-induced DNA strand breaks exhibited the maximum loss in the percentage of dsDNA under UV-B radiation followed by UV-A and PAR in comparison to the light control samples. We observed that T<>T CPD repair is light-dependent, since these lesions were more efficiently removed upon exposure to visible light than in the darkness. Blue radiation was found to be the most effective in photoreactivation than any other wavebands of light. Furthermore, the rate of photoreactivation was measured under varying temperatures (10, 20 and 30 °C); the repair rate was found to be the maximum at 20 °C under white fluorescent light. Our results indicate that photoreactivation play an important role in survival of the organism under natural conditions in spite of being exposed to the UV-B component present in the solar drops.     

Photosynthetic and biochemical characterization of pigments and UV-absorbing compounds in <Emphasis Type="Italic">Phormidium tenue</Emphasis> due to UV-B radiation

We report the effect of UV-B radiation (0.8 ± 0.1 mW cm⁻²) and UV-B radiation supplemented with low-intensity PAR (∼80 μmol photons m⁻² s⁻¹) on the photosynthesis, photosynthetic pigments, phosphoglycolipids, oxidative damage, enzymatic antioxidants, and UV-absorbing compounds in Phormidium tenue, a marine cyanobacterium. UV-B radiation resulted in a decline in photosynthesis and photosynthetic pigments leading to lower biomass. P. tenue synthesized UV-absorbing compounds like mycosporine-like amino acids (MAAs) and scytonemin in response to UV-B radiation. Quantity of MAAs and scytonemin was higher when UV-B was supplemented with low-level PAR. UV-B treatment also resulted in quantitative changes in phosphoglycolipids of the membrane. The UV-B treatment resulted in a slight increase in the level of peroxidation of cell membrane and very little increase in the activity of superoxide dismutase (SOD). Results indicate that UV-B affected photosynthesis and that the main protective system was the synthesis of MAAs and scytonemin-like compounds rather than antioxidant enzymes such as SOD.     

Photooxidation and antioxidant responses in the earthworm Amynthas gracilis exposed to environmental levels of ultraviolet B radiation

Ultraviolet (UV) radiation leads to photooxidation in various organisms. Our previous study demonstrated that ultraviolet B (UV-B) radiation is lethal for particular species of earthworms, but the mechanisms responsible for the lethality are unclear. In our current study, we investigated that ultraviolet light causes photooxidative damage and reduces antioxidant responses in the earthworm Amynthas gracilis. Intact earthworms and skin/muscle tissue extracts were exposed to UV-B radiation for in vivo and in vitro studies. Both in vitro and in vivo results showed that the products of photooxidative damage, MDA and H₂O₂, increased after UV-B exposure. Glutathione peroxidase (GPx) and catalase were inhibited immediately after exposure to high doses (3000 J/m²) of UV-B radiation in vivo. Catalase activity was increased following a low UV-B dose (500 J/m²) in vivo, but decreased in response to all dosage levels in vitro. These data indicate that a relationship exists between UV-B induced damage and photooxidation and also that catalase and GPx act as important antioxidants to prevent photooxidation. According to these data, A. gracilis exhibits high sensitivity to environmental levels of UV-B. Therefore, A. gracilis represents a sensitive and cost-effective model organism for investigations of UV-radiation damage and environmental UV stress.     

Daily variations of the antioxidant defense system of the lithodid crab Lithodes santolla

Several physiological processes can induce daily variations in aerobic metabolism. Lithodes santolla is a decapod crustacean of special concern since it is a sub-Antarctic species of commercial interest. The aim of this work was to study in L. santolla the daily variations in levels of enzymatic and non-enzymatic antioxidants, lipid peroxidation and protein oxidation, and haemolymphatic pH. Males of L. santolla of commercial size were randomly dissected every 4 h during a period of 24 h. Enzymatic activities of superoxide dismutase (SOD), catalase (CAT), glutathione-S-transferase and glutathione peroxidase were determined in samples of gills, muscle, hepatopancreas and haemolymph. Ascorbic acid, total glutathione, lipid peroxidation and protein oxidation were also determined in all tissues. Gills showed the highest enzymatic activity and hepatopancreas the highest concentration of non-enzymatic antioxidants. Maximum antioxidant activity was during the dark phase in gills and during the photophase in the haemolymph. Muscle showed significant daily variations, with peaks during the photophase and scotophase. Overall, an antioxidant protective mechanism is present in all tissues, where SOD and CAT represent the first line of defense. The defense mechanism in L. santolla seems to be more active during the dark phase, with slight differences among the analyzed tissues, indicating a higher metabolic rate.     

UVB-induced DNA damage, generation of reactive oxygen species, and inflammation are effectively attenuated by the flavonoid luteolin in vitro and in vivo

Ultraviolet (UV) radiation induces DNA damage, oxidative stress, and inflammatory processes in human keratinocytes resulting in skin inflammation, photoaging, and photocarcinogenesis. The flavonoid luteolin is one of the most potent antioxidative plant polyphenols. We investigated the UV protective and antioxidant properties of luteolin in human keratinocytes in vitro, ex vivo, and in vivo. Spectrophotometric measurements revealed extinction maxima of luteolin in the UVB and UVA range. UV transmission below 370 nm was < 10%. In human skin, luteolin effectively reduced the formation of UVB-induced cyclobutane pyrimidine dimers. The free radical scavenging activity of luteolin was assessed in various cell-free and cell-based assays. In the cell-free DPPH assay the half-maximal effective concentration (EC₅₀) of luteolin (12 μg/ml) was comparable to those of Trolox (25 μg/ml) and N-acetylcysteine (32 μg/ml). In contrast, in the H₂DCFDA assay performed with UVB-irradiated keratinocytes, luteolin (EC₅₀ 3 μg/ml) was much more effective compared to Trolox (EC₅₀ 12 μg/ml) and N-acetylcysteine (EC₅₀ 847 μg/ml). Luteolin also inhibited both UVB-induced skin erythema and the upregulation of cyclooxygenase-2 and prostaglandin E₂ production in human skin via interference with the MAPK pathway. These data suggest that luteolin may protect human skin from UVB-induced damage by a combination of UV-absorbing, DNA-protective, antioxidant, and anti-inflammatory properties.     

Metabolism of terpenes in the response of grape (Vitis vinifera L.) leaf tissues to UV-B radiation

This study investigated the terpene profiles as determined by GC–EIMS analysis of in vitro cultured plants of Vitis vinifera exposed to a “field-like” dose of UV-B (4.75 kJ m⁻² d⁻¹) administered at two different fluence rates (low, 16 h at 8.25 μW cm⁻², and high 4 h at 33 μW cm⁻²). Low UV-B treatment increased levels of the membrane-related triterpenes sitosterol, stigmasterol and lupeol, more notable in young leaves, suggesting elicitation of a mechanism for grapevine acclimation. By contrast, accumulation of compounds with antioxidant properties, diterpenes α and γ tocopherol and phytol, the sesquiterpene E-nerolidol and the monoterpenes carene, α-pinene and terpinolene had maximum accumulation under high UV-B, which was accentuated in mature leaves. Also the levels of the sesquiterpenic stress-related hormone abscisic acid (ABA) increased under high UV-B, although 24 h post irradiation ABA concentrations decreased. Such increments of antioxidant terpenes along with ABA suggest elicitation of mechanism of defense. The adaptative responses induced by relatively low UV-B irradiations as suggested by synthesis of terpenes related with membrane stability correlated with augments in terpene synthase activity.     

Effect of seed pretreatment by magnetic field on the sensitivity of cucumber (Cucumis sativus) seedlings to ultraviolet-B radiation

The effects of seed pretreatment by magnetic field (MF) on the impacts of ultraviolet-B (UV-B) radiation were tested using cucumber (Cucumis sativus) seedlings in phytotron. Soaked cucumber seeds were placed in MF of various strengths (0, 0.2 and 0.45 T). After germination the seeds were sowed in homogeneous garden soil and grown, then cucumber seedlings were exposed to 0 (as control) and 3.5 kJ m⁻² UV-B irradiation, respectively. Some effects of UV-B radiation and MF-pretreatment as well as their combination were investigated. MF-pretreatment increased seed germination rate, seedling growth and development, although also increased lipid oxidation and ascorbic acid contents. On the other hand, our results provided evidence that seed MF-pretreatment increased the sensitivity of cucumber seedlings to UV-B radiation. The seedling growth and development were significantly decreased by the combination of UV-B irradiation and MF-pretreatment. This combination also increased oxidative pressure and decreased actual quantum yield of PS II. Leaf UV-B absorbing compound was increased by MF-pretreatment or UV-B irradiation, whereas their combination significantly decreased it. These results suggested that the harmful effects of combination were partially due to the inhibition of secondary metabolism.     

Partial protection of photosynthetic apparatus from UV-B-induced damage by UV-A radiation

Alteration in the photosynthetic apparatus of clusterbean (Cyamopsis tetraganoloba) cotyledons owing to UV-B irradiation in the absence or presence of UV-A radiation (UV-A + UV-B) during steady phase of its growth has been studied. UV-B radiation induces a decline in the photosynthetic pigments content and O₂ evolution along with a modification in the absorption spectra of chloroplasts. UV-A + UV-B irradiation moderately reverses these changes. The partial restoration of F_V/F_M value and other fluorescence transient parameters in UV-A + UV-B treated sample compared to that of UV-B treated one suggest that UV-A helps in developing a protective pathway against UV-B-induced impairment. UV-B-mediated alteration in S state transition of Mn cluster associated with oxygen evolving complex, as appeared from TL glow curves, is retrieved by UV-A radiation and Car is considered to negotiate against UV-B-induced damage of photosynthetic apparatus.     

Photosynthetically active and UV radiation act in an antagonistic way in regulating buoyancy of Arthrospira (Spirulina) platensis (cyanobacterium)

Buoyancy provided by gas vesicles has been suggested to play an important role in regulating vertical distribution and nutrient acquisition in cyanobacteria. However, little is known about how changes in UV radiation (UVR, 280–400 nm) would affect the buoyancy. We have shown here that the floatation activity of the economically important cyanobacterium Arthrospira platensis (D-0083) decreased with increased photosynthetic rates associated with increased photosynthetically active radiation (PAR), but it decreased less in the presence of UVR, which resulted in inhibitory effects. When the cells were grown under isoenergetic levels of solar PAR or UVR alone, they migrated downward under PAR but maintained buoyant under UVR. The buoyancy regulation of A. platensis depended on the exposed levels of PAR as well as UVR, which affected photosynthesis and growth in an antagonistic way. The buoyancy of A. platensis in water columns is likely to be dependant on diurnal photosynthetic performance regulated by solar radiation, and can hardly be considered as an active strategy to gain more energy during sunrise/sunset or to escape from harmful irradiation during the noon period.     

Changes in oxidative stress defense system in wheat (Triticum aestivum L.) and mung bean (Vigna radiata L.) cultivars grown with and without mineral nutrients and irradiated by supplemental ultraviolet-B

Field study was conducted to evaluate the inter- and intra-specific variations in sensitivity of two cultivars each of wheat (Triticum aestivum L. cv. HD 2329 and HUW 234) and mung bean (Vigna radiata L. cv. Malviya Jyoti and Malviya Janpriya) to supplemental levels of UV-B irradiation (sUV-B, 280–315 nm) with and without recommended levels of mineral nutrients. Results showed decrease in photosynthetic pigments and biomass of all the four cultivars due to sUV-B radiation. Antioxidative defense system was activated in all the cultivars after irradiation with sUV-B. SOD, peroxidase and total thiol contents increased, while catalase activity and ascorbic acid contents decreased under sUV-B irradiation. On the basis of biomass, UV-B sensitivity can be arranged in decreasing order as: Malviya Janpriya < Malviya Jyoti < HD 2329 < HUW 234. Application of mineral nutrients (N, P and K) showed significant positive response in all cultivars by ameliorating the negative impact of sUV-B.     

Dynamic response of UV-absorbing compounds, quantum yield and the xanthophyll cycle to diel changes in UV-B and photosynthetic radiations in an aquatic liverwort

We studied the diel responses of the liverwort Jungermannia exsertifolia subsp. cordifolia to radiation changes under laboratory conditions. The samples were exposed to three radiation regimes: P (only PAR), PA (PAR + UV-A), and PAB (PAR + UV-A + UV-B). The day was divided in four periods: darkness, a first low-PAR period, the high-PAR plus UV period, and a second low-PAR period. After 15 days of culture, we measured photosynthetic pigments, chlorophyll fluorescence and UV-absorbing compounds in the four periods of the day on two consecutive days. With respect to UV-absorbing compounds, we analyzed their global amount (as the bulk UV absorbance of methanolic extracts) and the concentration of seven hydroxycinnamic acid derivatives, both in the soluble (mainly vacuolar) and insoluble (cell wall-bound) fractions of the plant extracts. PAB samples increased the bulk UV absorbance of the soluble and insoluble fractions, and the concentrations of p-coumaroylmalic acid in the soluble fraction and p-coumaric acid in the cell wall. Most of these variables showed significant diel changes and responded within a few hours to radiation changes (more strongly to UV-B), increasing at the end of the period of high-PAR plus UV. F_v/F_m, Φ_PSII, NPQ and the components of the xanthophyll cycle showed significant and quick diel changes in response to high PAR, UV-A and UV-B radiation, indicating dynamic photoinhibition and protection of PSII from excess radiation through the xanthophyll cycle. Thus, the liverwort showed a dynamic protection and acclimation capacity to the irradiance level and spectral characteristics of the radiation received.     

Effects of UV radiation on five marine microphytobenthic Wadden sea diatoms isolated from the Solthörn tidal flat (Lower Saxony,southern North Sea) – Part I: growth and antioxidative defence strategies

The unconsolidated sediment of intertidal mudflats constitutes a highly unstable environment, due to continuously changing water levels and currents as well as temporary exposure to the air. Therefore, diatoms inhabiting marine intertidal areas are subjected to strongly changing surface light and UV intensities due to exposure at low tide. Five marine intertidal diatoms (Achnanthes exigua, Cocconeis peltoides, Diploneis littoralis, Navicula digitoradiata and Amphora exigua) were isolated from the Solthörn tidal flat (Lower Saxony, southern North Sea). Semi-continuous cultures were used to determine the effect of UV radiation (photosynthetically active radiation only [PAR], PAR+UV-B, PAR+UV-A, PAR+UV-B+UV-A) during short- and long-term exposure (6 h or 30 days). Growth rates, chlorophyll a (chl a), antioxidant capacities, accumulation of phenolic compounds (e.g. flavonoids) and DMSP, and activities of antioxidant enzymes (superoxide dismutase, ascorbate peroxidase, monodehydroascorbate reductase and glutathione reductase) were assessed. UV-A had only minor effects on cells, while growth rate, chl a content and protein content were significantly reduced after long-term UV-B exposure. Achnanthes exigua extracts showed the highest antioxidant capacity. The highest activity of SOD, APX and MDHAR was found under long-term combined UV exposure (PAR+UV-B+UV-A). Overall, the antioxidative defence of the five isolates was stimulated during exposure to UV radiation, as may be found during emersion. Emersion induces oxidative stress and, as a result, growth of the five diatom taxa was inhibited to suit changing environmental conditions. All five taxa tested in the present study showed species-specific acclimatization potentials, providing possible explanations for variability in population, species composition and ecosystem structures in the face of climatic variations.     

The biliprotein C-phycocyanin modulates the early radiation response: A pilot study

This study analyzed the effects of biliprotein C-phycocyanin (C-PC) on the enzymatic antioxidant defence system in lymphocytes of nuclear power-plant workers and non-exposed controls. Changes in the protein levels of manganese super oxide dismutase (MnSOD), catalase and glutathione-S-transferase (GST) were used as markers for early biological effects of a single in vitro exposure of cells to: (i) 2 Gy gamma rays; (ii) 5 μM C-PC; and (iii) a combination of C-PC plus irradiation with 2 Gy. The results showed that C-PC selectively stimulated the lymphocyte antioxidant defence system of occupationally exposed subjects. The activation of the antioxidant protective mechanisms as part of the early radiation response was probably related to the chronic low-dose occupational exposure. The modulating capacity of C-PC at the molecular level may be of interest for the protection of occupationally exposed persons.     

Induction of heme oxygenase-1 by chamomile protects murine macrophages against oxidative stress

AimsProtection of cells from oxidative insult may be possible through direct scavenging of reactive oxygen species, or through stimulation of intracellular antioxidant defense mechanisms by induction of antioxidant gene expression. In this study we investigated the cytoprotective effect of chamomile and elucidated the underlying mechanisms.Main methodsThe cytoprotective effect of chamomile was examined on H₂O₂-induced cellular stress in RAW 264.7 murine macrophages.Key findingsRAW 264.7 murine macrophages treated with chamomile were protected from cell death caused by H₂O₂. Treatment with 50 μM H₂O₂ for 6 h caused significant increase in cellular stress accompanied by cell death in RAW 264.7 macrophages. Pretreatment with chamomile at 10–20 μg/mL for 16 h followed by H₂O₂ treatment protected the macrophages against cell death. Chamomile exposure significantly increased the expression of antioxidant enzymes viz. heme oxygenase-1 (HO-1), peroxiredoxin-1 (Prx-1), and thioredoxin-1 (Trx-1) in a dose-dependent manner, compared with their respective controls. Chamomile increased nuclear translocation of Nrf2 with increased phosphorylated Nrf2 levels, and binding to the antioxidant response element in the nucleus.SignificanceThese molecular findings for the first time provide insights into the mechanisms underlying the induction of phase 2 enzymes through the Keap1-Nrf2 signaling pathway by chamomile, and provide evidence that chamomile possesses antioxidant and cytoprotective properties.     

Differential induction of enzymes and antioxidants of the antioxidative defense system in Anabaena doliolum exposed to heat stress

Anabaena doliolum subjected to 43, 48, 53 and 58 °C temperature for 1, 2, 3 and 4 h, showed temperature and time-dependent increase in H₂O₂ production and MDA contents. All the measured enzymes of the antioxidative defense system (SOD, CAT, APX and GR) showed increase in their activities at 43 °C after 1 h of treatment, but at higher temperature their activity declined. The content of antioxidants (ASC, GSH, and α-TOC) increased significantly with rise in temperature as well as duration of treatment. This study clearly demonstrates that when enzymatic defense system becomes inactive, the antioxidants (GSH, and α-TOC) are induced to protect the cyanobacterium from heat stress. One of the major roles of these antioxidants appears to be the protection of PSII as reflected by an effect on O₂ evolution up to 53 °C.     

Differential response of leaf gas exchange to enhanced ultraviolet-B (UV-B) radiation in three species of herbaceous climbingplants

We measured diurnal changes in photosynthetic rate, transpiration rate, stomatal conductance and water use efficiency in three species of herbaceous climbing plants (Luffa cylindrica, Trichosanthes kirilowii and Dioscorea opposita) exposed to two intensities of UV-B radiation: 3.0 μw cm^?2 (R1) and 8.0 μw cm^?2 UV-B (R2) radiation under ambient growth conditions. Responses differed per species and per treatment. In Luffa all values increased compared to the Control in both treatments, except for stomatal conductance in R2. In Trichosanthes photosynthetic rates and water use efficiency increased, while the transpiration rates decreased under both treatments, and stomatal conductance was lower in R1. In Dioscorea photosynthetic rates and water use efficiency decreased under both treatments, while the transpiration rates and stomatal conductance increased. The results suggested that to some extent increased UV-B radiation was beneficial to the growth of L. cylindrica and T. kirilowii, but detrimental to D. opposita.     

Arsenic mitigates cadmium toxicity in rice seedlings

Two contrasting rice (Oryza sativa L.) cultivars, i.e. Wuyujing 3 (WYJ3, Cd-tolerant) and Shanyou 63 (SY63, Cd-sensitive), were grown on a red soil (Ultisol) to study both individual and combined phytotoxicity of arsenic (As) and cadmium (Cd) in terms of Cd and As availability, their uptake and accumulation, antioxidant defense activity and oxidative damage. The antioxidant defense system examined in this study included enzymatic and non-enzymatic molecular antioxidants such as superoxide dismutase (SOD), peroxidase (POD), glutathione (GSH) and ascorbic acid (AsA). Results showed that As or Cd treatment decreased root and shoot biomass in both cultivars compared with their corresponding control (no Cd or As treatment), although less severe inhibition of plant growth was observed in WYJ3 than in SY63. Moreover, rice growth was inhibited more severely by Cd treatment than by As treatment, which could be explained by the higher amount of available Cd (60%) (0.1 M HCl-extractable Cd) compared to the lower amount of available As (15%) (0.5 M NaH₂PO₄-extractable As) in their postharvest soils. However, shoot biomass in cultivar SY63, and root and shoot biomass in cultivar WYJ3 were significantly higher in the As plus Cd treatment than in the Cd treatment alone, showing that the combined toxicity of these two heavy metals was not additive and on the contrary, As mitigated Cd-induced growth inhibition. The As plus Cd treatment also significantly decreased As or Cd concentrations both in roots and in shoots of the two rice cultivars compared with the As or Cd treatment alone, respectively. On the other hand, treatment with As or Cd alone significantly decreased the SOD and POD activities, and GSH and AsA concentrations, while the activities of these enzymes and the concentrations of GSH and AsA were significantly higher in the As plus Cd treatment than in the Cd treatment alone, resulting in less severe oxidative damage as indicated by the lower concentration of MDA in the As plus Cd treatment (P < 0.05). However, no significant difference was observed in the antioxidant defense activity between the As plus Cd treatment and the As treatment alone. These results suggest that the combined toxicity of As and Cd in rice is lower than that of individual Cd or As, which might be attributed to the decreased uptake and accumulation of Cd and As, and the less oxidative stress caused by the interactive effects of As with Cd both in rhizosphere and in plants.