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Vol. 24 No. 4 - October 2018

Strategies to reduce arsenic uptake by rice

By: Nandita Singh

Arsenic is a known carcinogen and it has been reported that it is a cancer promoter rather than an initiator. It has epidermological as well as serious health effects like cancer, hyperkeratosis on human beings. Arsenic in human beings enters through contaminated food and water. In food arsenic is found in those areas where there is contamination in soil or irrigation water, the vegetables and crops grown there are source in human beings. Arsenic contamination has become a major problem in many parts of the world. Rice is the staple food for the people of arsenic contaminated areas of South and South-East Asia. Rice grains have higher arsenic levels than other cereals like wheat and barley. In India, West Bengal is most affected where rice is the major crop grown. The arsenic contaminated water has been extensively used for paddy irrigation which has resulted in high deposition of arsenic in topsoil and uptake in rice grain, increasing threat to the sustainable agriculture in this region. The arsenic concentration in rice varies with the rice varieties. The selection of suitable rice genotype is essential to limit As concentration in rice. There is a need to breed rice cultivars with high tolerance to As in the soil and which prevent its partitioning into grain. But the problem arises due to continuous addition of arsenic in soil through the contaminated water.

Paddy rice is characterized by iron plaque on root surfaces. Ferrous ions form from the reduction of ferric ions under the reducing conditions such as paddy fields. Paddy rice roots can release oxygen and oxidants into the rhizosphere, thereby oxidize the ferrous ions transported to roots from paddy soils into ferric irons with the precipitation of iron oxides or hydroxides. Under field conditions, iron plaque has also served as a restraint on the uptake of metal(loids) such as As by plants. This is probably due to its adsorption or co-precipitation processes. At the same time, the sink-like characteristic of iron plaque lead to the concentrated metal(loid)s in the rhizosphere. In some cases, iron plaque may release these metal(loid)s and subsequently enhance uptake. For example, iron plaque can diminish the inhibition effect of phosphate on paddy rice's arsenate uptake. The experiments with Fe/Mn plaque formation on root and As uptake in different genotypes, showed that root oxidation significantly influences As mobility in rhizoshphere. Genotype with higher radial oxygen loss (ROL) induces more Fe plaque formation and sequesters more As in iron plaque and rhizoshphere soil, leading to the reduction of As accumulation in rice plants. The Fe/ Mn plaque formation on the root surface play a crucial role on the genotypic variations in As uptake. The differences in iron plaque formation among rice genotypes have been noted. Specifically, the interactions between genotype, environment, and environment x genotype affect the uptake and accumulation of As. Furthermore, Hu and his group have reported that both As and phosphate concentrations in iron plaque had a strong positive correlation with the amounts of Fe in iron plaque (DCB-extractable) for three rice cultivars. The degree of radio oxygen loss (ROL) has been used to evaluate root aeration and has been found to have a strong correlation with As tolerance and accumulation in rice.

Development of efficient, cost effective and environment friendly remediation method is needed for As removal from contaminated soil and water. Phytoremediation, the use of plants to clean up pollutants, is steadily gaining acceptance. Pteris vittata (Chinese brake fern) is a well-known As hyperaccumulator. The plant possesses an exceptional ability to take up, translocate and tolerate As. When grown in As-contaminated soils, P. vittata accumulates As in fronds often more than 10 times the concentration in the soil. In glasshouse pot experiments, P. vittata removed between 0.1 and 26% of the soil As, depending on soil As concentration and bioavailability, and other soil factors. In a small scale field trial, Ma and her group from University of Florida reported that the mean concentration of soil As was decreased from 190 to 140 mg kg_1 after three harvests of P. vittata. However, many plants have been reported as As hyper-peraccumulators. Xie and his group have listed ferns as naturally evolved As accumulators, and the majority of them are members of the Pteris genus. Beside these, efforts have led to the identification of other potential As-accumulator aquatic and terrestrial plants (Singh et al. 2010; Ozturk et al. 2010; Tripathi et al. 2012). However, there is a need to screen more plants to find the best suited option for a particular area. The use of indigenous plants with high tolerance and accumulation capacity for As could be a very convenient approach to As phytoremediation. From phytoremediation perspective, plants should display, (i) high uptake rate; (ii) tolerance to high concentration of As; (iii) high translocation to shoot system; (iv) efficient system to tolerate high As level in plant parts.

Another possibility is in situ arsenic phytostabilization, i.e., using metal tolerant plants for retaining the metal maximally in roots, thus reducing leaching and uptake in plant parts, further preventing transfer to food chain. The field utilization potential of most of these hyperaccumulator plants are very less due to their small biomass and slow growth rate. Rhizoremediation, involving both plants and the rhizospheric microbes, is an efficient bioremediation process for contaminant degradation and/or promoting plant growth in presence of plant growth promoting bacteria (PGPB). Different metal tolerance mechanisms have also been discovered in various microbes: exclusion, active removal, biosorption, precipitation or bioaccumulation, both in external and intracellular spaces. Some bacterial strains are also known to play an important role in the biochemical cycle of As, through its conversion to species with different solubility, mobility, bioavaiability and toxicity. The known mechanism of arsenic resistance in microorganisms requires the ars operon and is based on energy dependent efflux of both arsenate [As(V)] and arsenite [As(III)] from the cell [20]. In this operon the gene arsC is particularly interesting because its product, a cytoplasmic arsenate reductase, catalyzes reduction of less toxic As(V)to more toxic As(III), which may be transported out of the cell by arsAB, As chemiosmotic efflux system and by ATPase membrane system. Rhizospheric bacteria have found to enhance, reduce, or have no effect on the metal uptake. For instance Srivastava et al. (2013) identified a bacteria Staphylococcus arlettae strain NBRIEAG-6, was able to remove arsenic from liquid media and possesses arsC gene, gene responsible for arsenate reductase activity. The biochemical profiling of the isolated strain showed that it had the capacity of producing indole acetic acid (IAA), siderophores and 1-aminocyclopropane-1-carboxylic acid(ACC) deaminase. The microbial inoculation significantly (p < 0.05) increased biomass, protein, chlorophyll and carotenoids contents in test plant. Moreover, bacteria NBRIEAG-6 has the ability to help B. junceato accumulate As maximally in plant root, and can be accounted as a new bacteria for As phytostabilization. Plants from As contaminated soils are generally mycorrhizal, indicating that the symbionts can evolve As-tolerance. However, conflicting results are reported in the literature concerning the role of mycorrhizae (including AM) in the absorption and translocation of metals into the plant, some reports indicating exclusion, some others accumulation.

Arsenic-contaminated irrigation water could increase the As level in soil and its subsequently accumulation in rice grains; however, the arsenic risk assessment of rice based on the total content of As in the soil and irrigated groundwater can be misleading because arsenic accumulation in plants is largely influenced by a variety of factors, including soil physicochemical parameters; other elements such as iron, phosphorus, sulfur, and silicon concentrations; and environmental conditions that control As availability and uptake in the soil-rhizosphere-plant system. Environmental conditions can be managed by changing irrigation practices. For example, the flooding of the paddy soil mobilizes As in the soil solution and can increase As accumulation in rice. Therefore, changing agricultural practices to aerobic rice cultivation throughout the entire season may be a viable strategy to mitigate this problem.

The uptake of arsenite in rice usually occurs through the silicon transport pathway. Therefore, the application of silica fertilizer in soil is suggested to decrease the transfer of arsenic from the soil and irrigation water to rice. Furthermore, phosphate fertilization is suggested to lower arsenate uptake in plants because both compounds enter rice via the same transporters. However, there are arguments in certain cases because under flooding conditions, As is present as arsenite, which cannot compete with phosphate; furthermore, phosphate increases As mobility because it competes with arsenate for the adsorption site on Fe-oxides/hydroxides.

Researchers and practitioners are trying their level best to mitigate the problem of As contamination in rice. However, the solution strategies vary considerably with various factors, such as cultural practices, soil, water, and environmental/economic conditions, etc. The contemporary work on rice to explain arsenic uptake, transport, and metabolism processes at rhizosphere, may help to formulate better plans. Common agronomical practices like rain water harvesting for crop irrigation, use of natural components that help in arsenic methylation, and biotechnological approaches may explore how to reduce arsenic uptake by food crops.

 

Consultant Scientist, CSIR-National Botanical Research Institute, Lucknow, India, <nanditasingh8@yahoo.co.in>

 


This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.


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