In early seedling development, it was found that its presence interferes with the root growth more than with shoots. In both parts, cadmium interfered significantly from the concentration of 0. However, the 0. In environments contaminated by cadmium, establishing the germination and development of this species may not be possible.
Molecular Mechanisms and Genetic Basis of Heavy Metal Tolerance/Hyperaccumulation in Plants
Keywords: Schinus terebinthifolius Raddi, contamination, heavy metal. Risks of ecosystems contamination by these pollutants have worried due to their major ecological consequences, being cadmium one of the heavy metals with greater phytotoxic potential He et al. Schinus terebinthifolius belongs to the Anacardiaceae family, is a tree widely found on Brazilian coast Carvalho et al. Besides being widely used as an herbal medicine Carlini et al.
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Thus, due to its wide distribution, the species can be exposed to environments with soil contaminated by heavy metals such as cadmium. In the initial stages, this heavy metal may cause reduction in the percentage of seed germination Ahmad et al. This study aimed to assess the ecotoxicological effects of lead in the germinative development of pepper mastic S.
The seeds of S. The viability assessment of the batch of seeds was analyzed through germination test, where seeds were previously disinfected with sodium hypochlorite NaClO 2.
To assess the effects of cadmium on seed germination and initial development of S. The seeds were disinfected 2. Cadmium solutions were prepared from cadmium nitrate Cd NO 3 2 in pH 6. Seeds were sown in Petri dishes mm diameter with double layer paper germitest soaked in cadmium solution in different concentrations and water. The experiment was carried out in quadruplicates with 30 seeds per replicate. The response variables to assess effects of cadmium were: the percentage of germination; the germination speed index GSI , both quantified according to Maguire , and the daily growth of shoot and root of seedlings using a digital caliper 0.
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The experimental data were collected during 20 days from the day of sowing. Statistical analyzes of the results of measurements were performed using the statistical software SPSS Additionally, the regression analysis was used to assess the relation between the variation in growth of the seedling parts and the used cadmium concentration.
The germination and seedling growth, generally was negatively and significantly influenced by the different concentrations of cadmium. The concentrations of 0. Similar trend of decreasing values was also observed for the germination speed index Figure 2. Concentrations from 0. However, higher concentrations of cadmium up to 6. Cadmium showed very high potential for inhibition of root growth of pepper mastic, where the seedlings that have been developed exposed to greater concentrations, 4.
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This high inhibitory effect was also observed in the shoot. However, the shoot was more tolerant to the presence of cadmium than the root Table 1. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account. If the address matches an existing account you will receive an email with instructions to retrieve your username. Search for more papers by this author. Tools Request permission Export citation Add to favorites Track citation.
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Managing editor: Ping HE. Citing Literature. Again, considerable variation was observed between the ability of five ectomycorrizal fungi to grow in culture with a range of nine different heavy metals Tam, The mechanisms employed by the fungi at the cellular level to tolerate heavy metals are probably similar to some of the strategies employed by higher plants, namely binding to extracellular materials or sequestration in the vacuolar compartment. Thus in the fungus Pisolithus tinctorius , tolerance to Cu and Zn was achieved by binding to extrahyphal slime Tam, , whereas detoxification of Cd in Paxillus involutus involved binding of Cd to the cell walls and accumulation of Cd in the vacuole Blaudez et al.
In relation to the role of ectomycorrhizas in metal tolerance by the host plant, most mechanisms that have been proposed involve various exclusion processes that restrict metal movement to the host roots. These have been extensively reviewed and assessed Jentschke and Godbold, and include absorption of metals by the hyphal sheath, reduced access to the apoplast due to the hydrophobicity of the fungal sheath, chelation by fungal exudates, and adsorption onto the external mycelium.
There are fewer reports on the role played by arbuscular mycorrhizas in metal tolerance. Weissenhorn et al. However, a Glomus isolate Br1 obtained from zinc violets Viola calaminaria growing on a heavy metal soil was shown to support the growth of maize and alfalfa on heavy metal soils more effectively than a commonly used Glomus isolate Hildebrandt et al. In a related study, it was shown that the maize grown in the presence of the heavy metal Glomus isolate Br1 contained considerably lower heavy metal concentrations than plants grown without mycorrhiza or with the common Glomus strain Kaldorf et al.
The Role of Heavy Metals in Plant Response to Biotic Stress
Furthermore, elemental microbeam analysis indicated that the growth of maize in heavy metal soils was, at least in part, due to the selective immobilization of metals within the root tissues that contain the fungal cells. The binding properties of the cell wall and its role as a mechanism of metal tolerance has been a controversial one.
Earlier reports have been reviewed Ernst et al. Although the root cell wall is directly in contact with metals in the soil solution, adsorption onto the cell wall must be of limited capacity and thus have a limited effect on metal activity at the surface of the plasma membrane. However, Bringezu et al. One related process concerns the role of root exudates in metal tolerance.
Root exudates have a variety of roles Marschner, including that of metal chelators that may enhance the uptake of certain metals. Since the range of compounds exuded is wide, other exudates could play a role in tolerance to other metals. The clearest example of a role for root secretions in tolerance is in relation to organic acids and the detoxification of the light metal Al Ma et al. Plasma membrane function may be rapidly affected by heavy metals as seen by an increased leakage from cells in the presence of high concentrations of metals, particularly of Cu.
Heavy Metal Stress in Plants: From Molecules to Ecosystems - Google книги
Similarly, others concluded that damage to the cell membrane, monitored by ion leakage, was the primary cause of Cu toxicity in roots of Silene vulgaris , Mimulus guttatus , and wheat, respectively De Vos et al. Certainly direct effects of Cu and Cd treatments on the lipid composition of membranes have been reported Ros et al. In addition, Cd treatments have been shown to reduce the ATPase activity of the plasma membrane fraction of wheat and sunflower roots Fodor et al. Thus tolerance may involve the protection of plasma membrane integrity against heavy metal damage that would produce increased leakage of solutes from cells De Vos et al.
However, there is little evidence to show how this might be achieved. Another factor that may be involved in the maintenance of plasma membrane integrity in the presence of heavy metals could be enhanced membrane repair after damage Salt et al. This could involve heat shock proteins or metallothioneins, and evidence for this is discussed in following sections. Apart from tolerance involving a more resistant plasma membrane or improved repair mechanisms, the cell membrane may play an important role in metal homeostasis, either by preventing or reducing entry into the cell or through efflux mechanisms.
Many of these cations, of course, are essential and so complete exclusion is not possible; selective efflux may be more realistic. It appears that the metabolic penalty for having more specific uptake mechanisms, and thus restricting the entry of toxic ions, is greater than that of having inducible efflux systems Silver, The number of examples of exclusion or reduced uptake mechanisms in higher plants is quite limited. The clearest example of reduced uptake as an adapted tolerance mechanism is in relation to arsenic toxicity Meharg and Macnair, , In Holcus lanatus roots, phosphate and arsenate appear to be taken up by the same systems.
The altered phosphate and arsenate uptake system was genetically correlated to arsenate tolerance Meharg and Macnair, Further work has suggested that arsenate tolerance in H.
PCs are discussed in detail in a later section. More recently, a plasma membrane transporter in tobacco that confers Ni tolerance and Pb hypersensitivity has been described Arazi et al. Although the normal physiological function of this transporter remains to be established, it could provide a possible mechanism for Ni tolerance.
An alternative strategy for controlling intracellular metal levels at the plasma membrane involves the active efflux of metal ions, although there is very little direct evidence for such a process in plants. Efflux transporters may also play a role in metal ion homeostasis in animal cells. It was thought that Zn efflux involves some form of secondary active transport. Defects in these ATPases have been linked to two human disorders, Menkes disease and Wilson disease, that result from defective Cu export and thus the accumulation of Cu in some tissues Solioz and Vulpe, Although there is no direct evidence for a role for plasma membrane efflux transporters in heavy metal tolerance in plants, recent research has revealed that plants possess several classes of metal transporters that must be involved in metal uptake and homeostasis in general, and thus could play a key role in tolerance.
Recently, a role for the Nramps in Fe and Cd uptake has been reported Thomine et al. Increased expression, resulting from changes in the plant Zn status, led to increased Zn influx in the roots. However, the transport function, specificity and cellular location of most of these proteins in plants is as yet unknown. Heat shock proteins HSPs characteristically show increased expression in response to the growth of a variety of organisms at temperatures above their optimal growth temperature.
They are found in all groups of living organisms, can be classified according to molecular size and are now known to be expressed in response to a variety of stress conditions including heavy metals Vierling, ; Lewis et al. There have been several reports of an increase in HSP expression in plants in response to heavy metal stress. Tseng et al. Small heat shock proteins e.