Тяжелые металлы в окружающей среде
HEAVY METAL TOLERANCE IN PLANTS - NEW AND EMERGING
жүктеу/скачать 6.49 Mb. Pdf көрінісі
|
- Бұл бет үшін навигация:
- Heavy metals – effects on plant functions and development
| HEAVY METAL TOLERANCE IN PLANTS - NEW AND EMERGING
BIOTECHNOLOGICAL SOLUTIONS Bojin Bojinov Head, Department of Genetics and Plant breeding, Agricultural University of Plovdiv Keywords: heavy metals, phytotoxicity, biotechnology, hyperaccumulation. The term ―heavy metals‖ in plant science refers to a series of metals and metalloids that can be toxic even at very low concentrations. Most of them (i.e. As, Cd, Hg, Pb or Se) do not perform any known physiological function in plants. Others (such as Co, Cu, Fe, Mn, Mo, Ni and Zn) are included in different plant metabolic pathways and thus are required for normal growth and devel- opment. While their presence may be needed to the plants, an excess of these elements can easily lead to poisoning as their concentration rises above optimal. These high concentrations negatively impact the organisms and such an impact is termed ―heavy metal phytotoxicity‖. It results in major changes in the functioning of the roots and leaves that affect other developmental processes such as flowering, embryogenesis and seed formation. Heavy metals – effects on plant functions and development Reduced growth is one of the most common physiological consequences of heavy metal ex- posure in plants. Metal ion-induced changes in the structural and physiological integrity of leaves reduce the rates of photosynthesis and respiration, which has deleterious effects on energy provision and the efficiency of other metabolic processes. Transpiration and transport processes between var- ious organs are also affected. Plant roots are directly exposed to the heavy metal content of contaminated soils and are the first to be affected by that exposure. Furthermore, they have rather simple and predictable structural organization and developmental zonation, which makes them ideal model systems for studying plant adaptations to abiotic stresses. Studies show that higher concentrations of heavy metals may result in alterations of numerous physiological processes due to: (1) inactivating enzymes, (2) blocking functional groups of metabolically important molecules, (3) displacing or substituting for other essential elements and (4) disrupting membrane integrity. A rather common consequence of heavy metals‘ presence is the interference with electron transport inside cells, especially that of chloroplast membranes. It results in an enhanced production of reactive oxygen species (ROS). While ROS are important signaling molecules on the one hand, they can have adverse effects on cell membranes when present in large quantities. This is likely to trigger specific defense responses in certain places – i.e., along the root axis, they can trigger cell wall cross-linking and lignification. ROS overproduction following exposure to high Cd concentra- tions leads to lipid peroxidation and subsequent cell death. Other effects may include biological macromolecule deterioration, membrane dismantling, ion leakage, DNA-strand cleavage, etc. Alto- gether these result in a plant state generally described as ―oxidative stress‖. It is interesting to notice that there are plants that survive, grow and reproduce on natural metalliferous soils as well as on sites polluted with heavy metals as a result of anthropogenic activi- ties. The majority of species that tolerate heavy metal concentrations that are highly toxic to the other plants behave as ―excluders‖, relying on tolerance and even hypertolerance strategies helpful for restricting metal entrance. They retain and detoxify most of the heavy metals in the root tissues, with a minimized translocation to the leaves whose cells remain sensitive to the phytotoxic effects. Nevertheless, a number of hypertolerant species, defined as ―hyperaccumulators‖, exhibit an opposite behavior as far as heavy metal uptake and distribution in the plant is concerned. When exposed to heavy metals non-tolerant plants display symptoms of injury such as yel- lowing and shedding leaves, necrosis and decreased growth. At this stage the metabolism is disor- dered including defects in mineral metabolism, inhibited gas exchange and damage to the DNA. Non-nutritional heavy metals are particularly dangerous because of their mutagenic potential. Hg, Pb and As injure mitosis by spindle disturbance. Mercury induced c-mitosis, i.e. doubling of the chromosome without proper sorting to the daughter cell, thereby, leading to polyploid and aneu- ploid cells, and c-tumors in plants. Lead salts cause numerous c-mitoses, low mitotic activity and inhibited root growth. Often, non-nutritional heavy metals influence the uptake and concentrations of nutritional heavy metals. The toxicity of non-nutritional toxic heavy metals to plants, thus, can result from (i) the displacement of essential metals such as Mg 2+ , Ca 2+ , Fe 2+ and Zn 2+ in biomolecules causing mo- lecular dysfunctions, (ii) binding of metals to thiol groups which leads to inhibited functions of the target biomolecules and (iii) as the consequence of blocked thiol groups overproduction of reactive oxygen species (ROS). To cope with the above negative effects plants resort to a series of defense mechanisms that control uptake, accumulation and translocation of these harmful elements. Different plant species utilize different strategies to cope with heavy metal toxicity, ranging from sensitivity to tolerance and heavy metal accumulation. One commonly employed evasion strategy lies in hindering the entrance of heavy metals in- to root cells through their entrapment in the apoplastic environment by binding them to exuded or- ganic acids or to anionic groups of cell walls. As the nutritional heavy metals (i.e. Zn, Cu and Mn) are beneficial to ectomycorrhizal fungi and host plants in the desirable concentrations range but are toxic at higher concentrations, ectomy- corrhizal plants can alter the cellular availability to minimize toxicity of these metals. Emerging ev- idence suggests that in this context the cellular mechanisms involved in detoxification of excess heavy metals mainly include (i) the biofilter function of ectomycorrhizal fungi and (ii) subcellular sequestration of these metals in ectomycorrhizal plants. For example, in P. sylvestris inoculated with the metal-tolerant fungus S. luteus, a significant decrease in the concentrations of Zn, Cu and Mn from the rhizomorph of the fungus to vascular tissues of host plants demonstrated filter effects of the ectomycorrhizal fungus and prevented excess metal accumulation in plant roots. Ectomycor- rhizal plants may also enhance the uptake of nutritional heavy metals and sequester them in vacu- oles and vesicles, thus favoring intracellular detoxification. These examples show that understand- ing of the benefits of the ectomycorrhizal fungi-plant interaction requires knowledge of the cellular utilization and localization of nutritional heavy metals. The toxicity of non-nutritional heavy metals makes their detoxification important to avoid toxicity to the two interacting organisms, the ectomycorrhizal fungi (EMF) and the plant. EMFs are likely to employ protective mechanisms similar to those of their host plants including binding to cell walls and sequestration in the vacuole. Further protective measures are: (i) binding of heavy metals to extracellular exudates, (ii) decreased uptake and/or pumping metal ions out of cytosol via transporters located at the plasma membrane, (iii) chelation of metal ions in cytosol by compounds, such as glutathione, phytochelatins and metallothioneins, (iv) compartmentation of metals in other subcellular structures to avoid accumulation of metal ions in cytosol, and (v) repair of metal dam- aged biomolecules. In ectomycorrhizal plants, the extramatrical hyphae and mantle constitute the first biological barrier against toxic metal ions and can decrease plant exposure by extracellular binding or sequestration in the vacuoles of the fungal cells. Higher heavy metal concentrations were detected in cell walls and vacuoles of hyphal cells of EMFs than cell walls of root cells. Ectomycor- rhizal Scots pine seedlings showed increased exudation of organic acids, amino acids, dissolved 60 61 monosaccharides and dissolved organic carbon in comparison with those of non-mycorrhizal plants. These exudates can chelate Cd 2+ and Pb 2+ and therefore, contribute to detoxification of the heavy metals and improve the performance of the seedlings. If this first line of defense fails, most of the heavy metals that do enter the plant are then kept in root cells, where they are detoxified by complexation with amino acids, organic acids or metal- binding peptides and/or sequestered into vacuoles. This greatly restricts translocation to the above- ground organs thus protecting the leaf tissues, and particularly the metabolically active photosyn- thetic cells from heavy metal damage. A further defense mechanism generally adopted by heavy metal-exposed plants is enhance- ment of cell antioxidant systems, which counteracts oxidative stress. Different pathways could lead to upregulating genes in the antioxidant defense systems, some of which include the utilization of plant hormones. In addition to the five classical plant hormones, i.e., gibberellins (GAs), cytokinins (CKs), auxins, abscisic acid (ABA), and ethylene, jasmonate (JA), brassinosteroids (BR), and sali- cylic acid (SA) are also well known for regulating many physiological processes and heavy metal stress tolerance. Furthermore, it is also expected that some more growth hormones are yet to be dis- covered in future. жүктеу/скачать 6.49 Mb. Достарыңызбен бөлісу:
|