If plants are exposed to increased levels of abiotic stress, such as metals, all kinds of cellular processes are influenced are/or disturbed. Our research has shown that reactive oxygen forms play an important role in this: not only concerning the signalling in the plant, but also in causing damage. Therefore, this will be a central theme in further studies.
In the model plant Arabidopsis thaliana” (thale cress), the cellular responses, from gene to protein, are examined for metal contamination. In this way, we get more insight in the succession of the cellular mechanisms that occur after exposure to toxic metals: from ‘sensing’ the available metals and passing on this information to the cells upto the final reaction.
This information is used for the development of biomarkers and for the selection of plants that can be used for soil remediation.
Increased metal levels in the environment
Increased cadmium concentrations limit
Metal tolerance in mycorrhizal fungi
Organisms that are exposed to toxic concentrations of heavy metals in their environment suffer from increased stress. They can only survive in such hostile environments if they can sufficiently adjust their metabolism.
If stress takes extreme forms, populations shrink and a sharp selection pressure for genetic adaptations will arise and eventually, adapted ‘tolerant’ populations may develop through the process of natural selection. Evolutionary adaptation to metal-contaminated ecosystems is a fine example of micro-evolution.
We study a fascinating example of zinc, cadmium and copper tolerance within a group of ectomycorrhizal fungi (Suilloids) that live in symbiosis with trees thriving on metal-contaminated soils in Northern Limburg. Mycorrhizal fungi are the most important group of soil organisms that assist trees with the absorption of sufficient minerals. In exchange for this service, the fungi obtain sugars from their host plant.
A team of ‘mycorrhiza’ researchers is busy to unravel the mechanisms that are responsible for the metal tolerances in Suilloids. Molecular research, from gene to protein, is supported by physiological and biochemical experiments. The ecological implications of the metal tolerance of the fungi for host plants and for ecosystems on contaminated soils are investigated as well.
Sporocarps of ‘Suillus luteus’ (‘Slippery Jack mushroom’), an ectomycorrhiza
The interest for the use of plants for remediation of soils and shallow groundwater contaminated with both heavy metals and organic contaminants has increased enormously.
Based on many years of fundamental research and knowledge of the natural capacities of plants to take up metals and other toxic contaminants from polluted soil or water, phytoremediation has been put forward as an alternative remediation technology.
Phytoremediation can be defined as the use of plants to remove, degrade or stabilize hazardous substances from soils or water. In several cases of phytoremediation, the plant is actually not the ‘exclusive leading actor’. Very often, the plant-associated bacteria and mycorrhiza play a very important role. In this case, the main function of plants is ‘limited’ to the improvement of the growth conditions for micro-organisms.
Different forms of phytoremediations are distinguished according to the ‘function’ of the plants in the remediation process.
Phytostabilization is the use of plants, eventually in combination with suited soil additives, to stabilize the contamination. Preventing a further spread of the contamination is the main objective. Firstly, the plants often have, via exudates or via root-associated micro-organisms, an immobilizing effect on the contaminants that can be found in the soil. A well-closed vegetation cover will also prevent further spread via wind erosion and water erosion and will drastically decrease the percolation of contaminants to the groundwater.
Phytoextraction is the use of plants to remove contaminants from the soil by uptake through the roots of the plants; in many cases, the contaminants can be further transported and concentrated to the aboveground parts of the plants. The application of phytoextraction in case of contamination with metals can take place by means of hyperaccumulating plants that that can be found in nature, or artificially improved (manipulated) or selected valorizable species. Subsequently, the plant material, enriched with contaminants, can be valorized (for example for energy production) and the metals can be recycled from the remainders.
Phytotransformation is the uptake of contaminants by plants and the transformation or degradation of these contaminants into non-hazardous compounds. Phytotransformation is mainly associated with organic contaminations. Molecules are absorbed via the roots and ‘transformed’ by plant cells and/or endophytic micro-organisms, eventually even made suitable as ‘building stones’ for molecules that are naturally produced by the plant.
Strengthened biodegradation in the rhizosphere: plants excrete all kinds of organic molecules that are used as substrate by the micro-organisms. Because of this, 100 to 1000 times more bacteria can be found in the ‘rhizosphere’ (the zone close to the roots) than in the bulk soil or a soil without vegetation. If these micro-organisms can degrade the contaminants that are present in the soil, this can lead to an acceleration of the degradation in comparison with a soil without vegetation. Plants also produce humus that will increase the microbial life in the upper soil layer.
Figure: Trees can ‘pump up’ large volumes of groundwater. Next, the contaminants in the groundwater can be degraded by rhizospherebacteria or taken up by plants and ‘processed’. Willows and especially poplars are good species for this application.
Trees can ‘pump up’ large volumes of groundwater.
Role of plant-associated bacteria in growth and development of plants and in phytoremediation
Remediation options currently available for contaminated soils and groundwater are frequently expensive, environmentally invasive and do not make efficient use of existing biological resources. By consequence, they can not be employed for the treatment of large contaminated areas with diffuse pollution problems. As a complement to these conventional methods, currently emerging bioremediation technologies might provide a more cost-effective remediation approach. However, large scale application of bioremediation presently faces a number of obstacles including the levels of pollutants (being toxic for the organisms involved in remediation), the bioavailable fraction of the contaminants (being too low) and, in some cases, evapotranspiration of volatile organic pollutants from soil or groundwater to the atmosphere. Possible solutions for these problems can be found by further exploiting of plant-microbe interactions.
In case of phytoremediation of metal contaminated soils and (ground) water, plant-associated bacteria possessing a metal-sequestration system can reduce metal phytotoxcicity and enhance translocation to the upperground plant parts. Besides, rhizosphere bacteria producing siderophores and/or organic acids can increase plant availability of metals. During phytoremediation of organic contaminants, plants rely on their associated microorganisms possessing the appropriate degradation pathway to obtain an efficient degradation of organic contaminants resulting in decreases of both phytotoxicity and evapotranspiration of volatile contaminants and/or degradation intermediates to the atmosphere.
Bactera colonizing the root xylem of poplar
Health effects of air pollution
Although air pollution consists of a heterogeneous mixture of gases and particles, most recent research has concentrated on the adverse effects of particulate matter (PM). PM consists of primary particles, such as diesel soot, toxic metals but also natural components as sea salt.
Airborne PM is generally defined on the basis of the size distribution of the particles. Thus, PM10 and PM2.5 stand for PM with median aerodynamic diameters of less than 10 µm and 2.5 µm, respectively. Particles less than 2.5 µm are more deeply inhalable. In Europe the mean yearly PM concentration is het higest in Belgium, The Netherlands, North France and in the German Rurh area (see figure 1).
Persons with respiratory and cardiovascular disease are more susceptible for the acute health effects of fine dust. In children long-term exposure to fine dust is associated with lower lung function. The research within the Centre of Environmental sciences focuses on the molecular changes due to exposure to air pollution. We study the inter-individual susceptibility. We also work together with the Unit of Lung Toxicology of the KULeuven to study specific segments of the population including persons with diabetes.
Annual mean fine dust concentration