Arsenic induced oxidative stress and role of scavenging enzymes in phytoremediation by Pteris vittata and Eichhornia crassipes

Arsenic (As) pollution is on the rise globally and is a looming environmental threat. Arsenic is found chiefly as arsenate (AsV) and arsenite (AsIII) showing a broad range of solubility controlled by the ionic environment and pH. Arsenate behaves as a phosphate chemical analog, taking the help of phosphate transporters to enter the plant system, resulting in disparity of phosphate supply (Finnegan and Chen, 2012). Within the cell, arsenate disrupts the phosphate-controlled metabolism including ATP synthesis causing toxicity. Arsenite toxicity is mainly due to its affinity to react with thiol (-SH) groups of enzymes and proteins with cysteine residues, disturbing their structure and function (Hasanuzzaman et al ., 2015).

Phytoremediation, an important eco-friendly technology, uses different techniques, such as uptake, transport, translocation, and detoxification, to remediate such harmful metals and metalloids. Hyperaccumulator plants take up metals and metalloids from soils, translocate and store them in their above ground biomass (Reeves, 2006). Similarly, arsenic hyperaccumulator plants have also developed different approaches to accumulate and withstand high concentrations of As. Arsenic toxicity induces oxidative stress due to abundant production of reactive oxygen species (ROS). The hyperaccumulator plants by their strong antioxidative defenses could constitute an important arsenic detoxification strategy.

The objective of this work was to compare the arsenic phytoremediation abilities of two common plants Pteris vittata and Eichhornia crassipes both growing abundantly under natural tropical conditions of India. Arsenic pollution is a growing menace in major parts of West Bengal, India and Bangladesh. The two selected plant species, if found effective in arsenic removal could serve as cheap and easily available green sources for detoxification. An analyses of their reactive oxygen species, taking total peroxide and malondialdehyde contents as the parameters, could indicate the extent of arsenic induced oxidative stress, while the activities of the scavenging enzymes catalase, peroxidase and superoxide dismutase could indicate the comparative effectiveness of the two plants in arsenic detoxification.

MATERIALS AND METHODS

Pteris vittata and Eichhornia crassipes used for the experimental work were ensured to be of identical age. Pteris plants were germinated from spores and 4 month old plants were acclimatized in a hydroponic system to promote root growth. After acclimatization in 0.2 strength Hoagland nutrient solution for 2 weeks, the plants were transferred into 0.2-strength Hoagland nutrient solution containing different concentrations of arsenic as As (V), as sodium arsenate (Na 2 HAsO 4. 7H 2 O). The finally selected concentrations of arsenic are 0, 130-133 and 267-270 µM. The plants were harvested at three intervals - 1, 5 and 10 days post arsenic treatment. Eichhornia was collected from local ponds and thereby grown, acclimatized and allowed to reproduce under laboratory conditions. The daughter plants obtained were of the same age and were taken as test materials. Eichhornia was also subjected to the same treatment with arsenic in the form of sodium arsenate. The treated plants were collected at three intervals, i.e. 1, 5 and 10 days post arsenic treatment. Control sets were maintained throughout for comparison.

The test materials were subjected to biochemical and enzymatic analyses using standard methods and tests. Estimation of total peroxide was done by ferrithiocyanate method of Thurman et al. (1972) and Malondialdehyde (MDA) was estimated by the method of Heath and Packer (1968). Catalase enzyme activity was assayed by the method of Gasper and Lacoppe (1968), peroxidase enzyme activity was assayed spectrophotometrically according to Chance and Maehly (1955) and superoxide dismutase (SOD) was assayed by the method of Marshall and Worsfold (1978). All the experiments were carried out in triplicates and then subjected to statistical analyses using analysis of variance (ANOVA) table.

RESULTS

The results showed that total peroxide content (Fig.1) and MDA content (Fig.2) was significantly higher in all samples of Eichhornia as compared to Pteris throughout the experimental period. The results of the enzymatic analyses revealed an identical trend in all the three assayed enzymes viz. catalase (Fig.3), peroxidase (Fig.4) and SOD (Fig.5). The three scavenging enzymes exhibited higher activities in Pteris with increasing arsenic concentration while Eichhornia showed a reverse trend.

1 Day                          5 Day                            10 Day

Arsenic Concentration |1M

Figure. 1. Total Peroxide Content

P – Pteris , E - Eichhornia

S.E.= 0.74

C.D. =0.78 (5%)

1 Day                       5Day                     10 Day

Arsenic Concentration цМ

Figure 2. Malondialdehyde Content

P – Pteris , E - Eichhornia

S.E.= 0.73

C.D. =0.75 (5%)

1 Day                        5 Day                     10 Day

Arsenic Concentration цМ

Figure 3. Catalase enzyme activity P – Pteris , E - Eichhornia

S.E.= 0.26

C.D. =0.27 (5%)

1 Day                  5 Day                   10 Day

Arsenic Concentration цМ

Figure 4. Peroxidase enzyme activity

P – Pteris , E - Eichhornia

S.E.= 0.07

C.D. =0.07 (5%)

1 Day                         5 Day                     ю Day

Arsenic Concentration цМ

Figure 5. Superoxide dismutase (SOD) enzyme activity

P – Pteris , E - Eichhornia

S.E.= 0.07

C.D. =0.07 (5%)

DISCUSSION