Heavy metal contamination of soil and water causing toxicity/stress has become one important constraint to crop productivity and quality. This situation has further worsened by the increasing population growth and inherent food demand. It have been reported in several studies that counterbalancing toxicity, due to heavy metal requires complex mechanisms at molecular, biochemical, physiological, cellular, tissue and whole plant level, which might manifest in terms of improved crop productivity. Recent advances in various disciplines of biological sciences such as metabolomics, transcriptomics, proteomics etc. have assisted in the characterization of metabolites, transcription factors, stress-inducible proteins involved in heavy metal tolerance, which in turn can be utilized for generating heavy metal tolerant crops. This review summarizes various tolerance strategies of plants under heavy metal toxicity, covering the role of metabolites (metabolomics), trace elements (ionomics), transcription factors (transcriptomics), various stress-inducible proteins (proteomics) as well as the role of plant hormones. We also provide a glance at strategies adopted by metal accumulating plants also known as “metallophytes”.

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Heavy Metal Tolerance in Plants: Role of Transcriptomics, Proteomics, Metabolomics, and Ionomics.

Chromium (Cr) is the second most common metal contaminant in ground water, soil, and sediments due to its wide industrial application, hence posing a serious environmental concern. Among various valence states, Cr(III) and Cr(VI) are the most stable forms. Cr(VI) is the most persistent in the soil and is highly toxic for biota. Since Cr is a non-essential element for plants, there is no uptake mechanism; Cr is taken up along essential elements such as sulfate through sulfate transporters. Cr accumulation in plants causes high toxicity in terms of reduction in growth and biomass accumulation, and Cr induces structural alterations. Cr interferes with photosynthetic and respiration processes, and water and minerals uptake mechanism. Various enzymatic activities related to starch and nitrogen metabolism are decreased by Cr toxicity either by direct interference with the enzymes or through the production of reactive oxygen species. Cr causes oxidative damage by destruction of membrane lipids and DNA damage. Cr may even cause the death of plant species. Few plant species are able to accumulate high amount of Cr without being damaged. Such Cr-tolerant, hyperaccumulator plants are exploited for their bioremediation property. The present review discusses Cr availability in the environment, Cr transfer to biota, toxicity issues, effect on germination and plant growth, morphological and ultrastructural aberrations, biochemical and physiological alterations, effect on metabolic processes, Cr-induced alterations at the molecular level, Cr hyperaccumulation and Cr detoxification mechanism, and the role of arbuscular mycorrhizae in Cr toxicity, in plants.

Chromium toxicity and tolerance in plants

Potential risks of the residue from Samarco's mine dam burst (Bento Rodrigues, Brazil).

The Allium anaphase-telophase test was evaluated to find out if it could be recommended in the screening of wastewater for genotoxicity. Five mutagenic or carcinogenic chemicals usually found in wastewater were tested in the Allium anaphase-telophase test. Sodium dichromate (25 μM), benzene (100 μM), dichloromethane (175 μM) and 1,1,1-trichloromethane (175 μM) increased the frequency of chromosome aberrations in the root cells, whereas formaldehyde (1 mM) was found to be non-mutagenic in this test system. Other studies where chemicals were tested in the Allium test were reviewed. For 15 chemicals the results were compared with results from the Ames test, the Microscreen assay, and carcinogenicity tests in rodents. The sensitivity of the Allium test was calculated to be 82%. In conclusion the Allium test is recommended for the screening of wastewater because it has a high sensitivity, is cheap, rapid, easy to handle, and because it can be used on wastewater without pretreatment of the sample.

Evaluation of the Allium anaphase-telophase test in relation to genotoxicity screening of industrial wastewater

Determination of genotoxic effects of copper sulphate and cobalt chloride in Allium cepa root cells by chromosome aberration and comet assays.

The effects of trivalent and hexavalent chromium on root growth, cell division and chromosome morphology of Allium cepa were studied. The concentrations used of chromium nitrate and potassium dichromate were 2 × 10-1, 2 × 10-2, 2 × 10-3, 2 × 10-4, 2 × 10-5, 2 × 10-6, and 2 × 10-7M. The results showed that both trivalent and hexavalent chromium inhibit root growth and cause mitotic irregularities comprising c-mitosis, anaphase bridges, chromosome stickiness, and chromosome fragmentation and lagging. When compared with chromium nitrate on the equimolar basis, potassium dichromate showed a stronger inhibitory and toxic effect on root growth and cell division, respectively. The possible mechanism of the Cr poisoning of root meristem cells of Allium cepa is also briefly discussed.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1601-5223.1992.tb00003.x

Maoxie Li

Papers

Effects of trivalent and hexavalent chromium on root growth and cell division of Allium cepa