Accumulation and detoxification of sulfonated aromatic compounds from dye and textile industries by different plant species

Sulfonated anthraquinones are precursors of a large family of synthetic dyes and pigments, recalcitrant to biodegradation and thus not eliminated by classical wastewater treatments. In the development of a phytotreatment to remove sulfonated aromatic compounds from dye and textile industrial effluents, it has been shown that rhubarb (Rheum rabarbarum) and common sorrel (Rumex acetosa) are the most efficient plants. Both species, producing natural anthraquinones, not only accumulate, but also transform these xenobiotic chemicals. Even if the precise biochemical mechanisms involved in the detoxification of sulfonated anthraquinones are not yet understood, they probably have cross talks with secondary metabolism, redox processes and plant energy metabolism. The aim of the present work was to investigate the possible role of cytochrome P450s and peroxidases in the detoxification of sulfonated anthraquinones. Several plant species were thus cultivated in a greenhouse under hydroponic conditions, with or without sulfonated anthraquinones. Plants were harvested at different times and either microsomal or cytosolic fractions were prepared. The monooxygenase activity of cytochrome P450 toward several sulfonated anthraquinones was tested using a new method based on the fluorimetric detection of oxygen consumed during cytochromes P450 catalyzed reactions. Peroxidase was measured by spectrophotometry, with guaiacol. Results indicate that the activity of cytochromes P450 and peroxidases increased in rhubarb leaves, but not in leaves from common sorrel, when plants were cultivated in the presence of sulfonated anthraquinones. On the other hand, cytochromes P450 were able to accept as substrates anthraquinones containing sulfonated group in different positions, indicating that this enzyme was probably the first step in the metabolism of these xenobiotic compounds. These results support the idea that natural biodiversity should be better studied to use the most appropriate plant species for the phytotreatment of a specific organic pollutant.

The role of cytochromes P450 and peroxidases in the detoxification of sulphonated anthraquinones by rhubarb and common sorrel plants cultivated under hydroponic conditions

Jean-Paul Schwitzguebel

Background, aim and scope Sulphonated anthraquinones are precursors of many synthetic dyes and pigments, recalcitrant to biodegradation and thus not eliminated by classical wastewater treatments. In the development of a phytotreatment to remove sulphonated aromatic compounds from dye and textile industrial effluents, it has been shown that rhubarb (Rheum rabarbarum) and common sorrel (Rumex acetosa) are the most efficient plants. Both species, producing natural anthraquinones, not only accumulate, but also transform these xenobiotic chemicals. Even if the precise biochemical mechanisms involved in the detoxification of sulphonated anthraquinones are not yet understood, they probably have cross talks with secondary metabolism, redox processes and plant energy metabolism. The aim of the present study was to investigate the possible roles of cytochrome P450 monooxygenases and peroxidases in the detoxification of several sulphonated anthraquinones. Materials and methods Both plant species were cultivated in a greenhouse under hydroponic conditions, with or without sulphonated anthraquinones. Plants were harvested at different times and either microsomal or cytosolic fractions were prepared. The monooxygenase activity of cytochromes P450 toward several sulphonated anthraquinones was tested using a new method based on the fluorimetric detection of oxygen consumed during cytochromes P450-catalysed reactions. The activity of cytosolic peroxidases was measured by spectrophotometry, using guaiacol as a substrate. Results A significant activity of cytochromes P450 was detected in rhubarb leaves, while no (rhizome) or low (petioles and roots) activity was found in other parts of the plants. An induction of this enzyme was observed at the beginning of the exposition to sulphonated anthraquinones. The results also indicated that cytochromes P450 were able to accept as substrate the five sulphonated anthraquinones, with a higher activity toward AQ-2,6-SS (0.706 nkat/mg protein) and AQ-2-S (0.720 nkat/mg protein). An activity of the cytochromes P450 was also found in the leaves of common sorrel (1.212 nkat/mg protein (AQ-2,6-SS)), but no induction of the activity occurred after the exposition to the pollutant. The activity of peroxidases increased when rhubarb was cultivated in the presence of the five sulphonated anthraquinones (0.857 nkat/mg protein). Peroxidase activity was also detected in the leaves of the common sorrel (0.055 nkat/mg protein), but in this plant, no significant difference was found between plants cultivated with and without sulphonated anthraquinones. Discussion Results indicated that the activity of cytochromes P450 and peroxidases increased in rhubarb in the presence of sulphonated anthraquinones and were involved in their detoxification mechanisms. Conclusions These results suggest the existence in rhubarb and common sorrel of specific mechanisms involved in the metabolism of sulphonated anthraquinones. Further investigation should be performed to find the next steps of this detoxification pathway. Recommendations and perspectives Besides these promising results for the phytotreatment of sulphonated anthraquinones, it will be of high interest to develop and test, at small scale, an experimental wastewater treatment system to determine its efficiency. On the other hand, these results reinforce the idea that natural biodiversity should be better studied to use the most appropriate species for the phytotreatment of a specific pollutant.

Springer-Verlag2009

Do crosstalks with primary and secondary metabolism limit or stimulate the detoxification of xenobiotics?

Jean-Paul Schwitzguebel

Because plants are static and live in a competitive and sometimes hostile environment, they have evolved efficient mechanisms that protect them from abiotic and biotic stresses. These mechanisms include detoxification and sequestration of xenobiotic compounds and of toxic trace elements, exploited in any phytoremediation process. However there must be a limit on the amount of pollutants that can be accumulated and detoxified without disrupting the normal plant biochemistry and physiology. This limit seems to depend not only on plant species, but also on the ecotype or cultivar. The presentation aims to highlight some biochemical mechanisms, suggested or supposed to of importance for the successful phytoremediation of organic contaminants. Enzymes involved in xenobiotics detoxification are often linked to the redox chemistry of the cell. The activities of cytochrome P450 monooxygenase, peroxidase and glutathione transferase have implications on the regulation of cellular redox status, closely related to mitochondrial respiratory chains, also involved in maintaining the cellular and plant energy balance and carbon flow. For example, overloading a plant with high concentrations of xenobiotics requiring oxidation by P450 may compete with the normal functions of these enzymes. An increase in their activity may also impose a major demand on both intracellular O2 and NAD(P)H pools, disturbing plant redox or energy status, and thus affecting both primary and energy metabolism. Plant mitochondrial respiratory chains differ from the mammalian one. In addition to Complex I, they contain at least four other dehydrogenases that enable the controlled oxidation of matrix and cytoplasmic NAD(P)H. Furthermore, plant mitochondria are characterized by the presence of an alternative respiratory pathway, through which reducing equivalents can be transferred to oxygen. This pathway branches at the level of the ubiquinone pool and comprises a single enzyme, the alternative oxidase, not coupled to the synthesis of ATP and insensitive to cyanide. Increase in the expression and/or activity of the alternative oxidase has been observed during temporal events such as seed conditioning, leaf development, elevation of salicylic acid levels, thermogenesis, fruit ripening, oxidative stress, physical wounding and plant pathogenic attack. The alternative pathway is inhibited by benzhydroxamate compounds, antioxidants such as propylgallate and copper chelators such a disulfiram. The alternative oxidase is able to oxidise several added polyphenolic substrates and could also oxidise intracellular polyphenolic compounds. It is not yet known if it could directly participate in the metabolism of some xenobiotics. Depending on the stucture of the organic pollutant to be detoxified, the secondary metabolic processes in the plant could also be affected. Many plant specific metabolites, often involved in plant interactions with its environment, have a structure similar to xenobiotics, and detoxification of the latter does probably use, at least partially, the metabolic pathways of the former. For example, most of the natural anthraquinones are glycosylated, whereas glycosyl-transferases are known to be involved in the conjugation of many xenobiotics, probably including sulphonated anthraquinones. It is not yet known if the detoxification mechanisms of sulphonated aromatic compounds are unique and specific to anthraquinone producing plants. Molecules involved in the conjugation of xenobiotics, like glutathione, also play a major role in normal plant metabolism. The presence of xenobiotic compounds can induce the biosynthesis of glutathione transferases and thus an increased use of glutathione. Plant glutathione level and redox status are thus affected under such conditions. On the other hand, phytochelatins are derived from glutathione and involved in the detoxification of trace elements with their –SH groups. In either case, implications on sulphur supply and metabolism are expected. Experimental conditions do also affect plant physiology and biochemistry, and it is not obvious if results obtained with young and small plantlets under well-defined laboratory conditions can be extended to mature plants in the field. However, the large-scale implementation of phytoremediation will be successful only if the “right plant is used at the right place”. Basic physiological and biochemical knowledge is thus required to select the most appropriate plant species, ecotype or cultivar, tolerant to the contaminants to be treated and able to accumulate and detoxify them without impacting its growth and survival.

2014

From green to clean: a promising and sustainable approach towards environmental remediation

Jean-Paul Schwitzguebel

The use of green plants specifically chosen or tailored for the remediation of polluted soils and brownfields, the treatment of industrial wastewater or the removal of volatile organic compounds from indoor or outdoor air is becoming essential for sustainable development. Phytoremediation options are less expensive compared to physico-chemical approaches, even if the time scale required reaching the target end-points can be a limiting factor. Phytotechnologies are now offering efficient tools and environmentally friendly solutions for the cleanup of soils and water contaminated by organic pollutants. However this goal requires a sound understanding of how plants can specifically accumulate, detoxify and metabolise xenobiotic compounds. Significant progress has been made recently to better understand the soil, rhizosphere and plant mechanisms involved in the phytotreatment of organic pollutants; to implement phytoextraction, phytodegradation, rhizofiltration, and rhizodegradation as instruments for the removal and/or management of organic contaminants present in soil and water; and to identify critical points in overcoming bottlenecks in plant removal and detoxification of xenobiotics. Depending on the physico-chemical properties of the organic pollutant to be removed or detoxified, as well as on the specific plant physiology and biochemistry, different phytotreatments are available to decontaminate water and soils. For example, aquatic macrophytes or even terrestrial plants can be grown under hydroponic conditions or in constructed wetlands to remove many xenobiotic compounds, e.g. sulphonated anthraquinones and azo dyes present in wastewater from the dye and textile industries. Advances have also been made to remediate soils contaminated with hydrophobic compounds like PCBs, petroleum hydrocarbons or pesticides (e.g. lindane or DDT), highlighting the respective roles of plants, endophytic and rhizospheric bacteria, and the importance of their interactions. These examples illustrate the potential of plants for the phytotreatment of xenobiotics, as well as their ability to cooperate with microorganisms (phytostimulation). Exploring and exploiting the biological diversity of plant metabolism is necessary. Consideration of plant taxonomy and phytochemistry should be the first steps in the appropriate use of the huge potential of plant species, since plants often produce natural chemicals whose structure is close to foreign compounds. Finally, the success of phytoremediation also largely depends on the ability of plants to tolerate the pollutant(s) to be removed. The possible effects of these contaminants on plant physiology and biochemistry must be addressed, since they will determine the ability of a specific plant to deal with specific pollutants or a mixture of them, and thus affect the efficiency of any phytotreatment. It is thus of utmost importance to determine the maximal possible amount of the xenobiotic compounds that can be accumulated and detoxified without injury, critical stress and disruption of metabolism or redox processes in the plant species under consideration. Such information will help to maintain the plant wellness, a key factor to correctly design and size the constructed wetland to remove the foreign compounds present in a particular wastewater or to estimate the time required to clean up a contaminated soil.

2010

From green to clean: a promising and sustainable approach towards environmental remediation

Jean-Paul Schwitzguebel

2010

The role of cytochromes P450 and peroxidases in the detoxification of sulphonated anthraquinones by rhubarb and common sorrel plants cultivated under hydroponic conditions

Jean-Paul Schwitzguebel

Springer-Verlag2009

Do crosstalks with primary and secondary metabolism limit or stimulate the detoxification of xenobiotics?

Jean-Paul Schwitzguebel

2014