Contamination of soils by heavy metals is of widespread occurrence as a result of human, agricultural and industrial activities. Among heavy metals, lead is a potential pollutant that readily accumulates in soils and sediments. Although lead is not an essential element for plants, it gets easily absorbed and accumulated in different plant parts. Uptake of Pb in plants is regulated by pH, particle size and cation exchange capacity of the soils as well as by root exudation and other physico-chemical parameters. Excess Pb causes a number of toxicity symptoms in plants e.g. stunted growth, chlorosis and blackening of root system. Pb inhibits photosynthesis, upsets mineral nutrition and water balance, changes hormonal status and affects membrane structure and permeability. This review addresses various morphological, physiological and biochemical effects of Pb toxicity and also strategies adopted by plants for Pb-detoxification and developing tolerance to Pb. Mechanisms of Pb-detoxification include sequestration of Pb in the vacuole, phytochelatin synthesis and binding to glutathione and aminoacids etc. Pb tolerance is associated with the capacity of plants to restrict Pb to the cell walls, synthesis of osmolytes and activation of antioxidant defense system. Remediation of soils contaminated with Pb using phytoremediation and rhizofiltration technologies appear to have great potential for cleaning of Pb-contaminated soils.

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Lead toxicity in plants

Plants are the target of a wide range of pollutants that vary in concentration, speciation, and toxicity. Such pollutants mainly enter the plant system through the soil (Arshad et al. 2008) or via the atmosphere (Uzu et al. 2010). Among common pollutants that affect plants, lead is among the most toxic and frequently encountered (Cecchi et al. 2008; Grover et al. 2010; Shahid et al. 2011). Lead continues to be used widely in many industrial processes and occurs as a contaminant in all environmental compartments (soils, water, the atmosphere, and living organisms). The prominence of environmental lead contamination results both from its persistence (Islam et al. 2008; Andra et al. 2009; Punamiya et al. 2010) and from its present and past numerous sources. These sources have included smelting, combustion of leaded gasoline, or applications of lead-contaminated media (sewage sludge and fertilizers) to land (Piotrowska et al. 2009; Gupta et al. 2009; Sammut et al. 2010; Grover et al. 2010). In 2009, production of recoverable lead from mining operations was 1690, 516, and 400 thousand metric tons by China, Australia, and the USA, respectively (USGS 2009).

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Lead Uptake, Toxicity, and Detoxification in Plants

Effect of salinity on growth, photosynthesis, water relations and solute composition of the potential cash crop halophyte Plantago coronopus (L.)

A critical review on speciation, mobilization and toxicity of lead in soil-microbe-plant system and bioremediation strategies.

Effect of Pb toxicity on root morphology, physiology and ultrastructure in the two ecotypes of Elsholtzia argyi.

Effects of an increased lead (Pb) content in soil on growth, photosynthesis (P
N) and anatomical parameters of Plantago major L. plants grown under controlled conditions were studied. The total dry weights of plants at 500 and 2000 mg kg−1 Pb in soil were correspondingly 70% and 54% of those of control plants. A reduced leaf area and changed leaf structure caused a decrease in P
N in the whole plant. The specific leaf weight (SLW) increased as compared with that of control plants. An increasing Pb content in soil caused a larger number of chloroplasts and larger sizes of protoplasts, a decrease in the chlorophyll a+b contents and a larger number of stomata per unit leaf area based both in adaxial and abaxial epidermis, as compared with control plants. However their conductance was from 40% to 50% lower than that in control plants. It was noted that the dimensions of conducting bundles decreased mainly because of the reduced xylem area. The lower functional activity and the changes at different structural levels of the photosynthetic apparatus caused a decrease in the growth rate of plants at a high Pb content in soil.

Plantago major plants responses to increase content of lead in soil: Growth and photosynthesis

The procedure of isolating the thylakoids and the thylakoid membrane fragments enriched with either photosystem I or photosystem II (PSI- and PSII-membranes) from Arabidopsis thaliana leaves was developed. It differed from the one used with pea and spinach in durations of detergent treatment and centrifugation, and in concentrations of detergent and Mg2+ in the media. Both the thylakoid and the fragments preserved carbonic anhydrase (CA) activities. Using nondenaturing electrophoresis followed by detection of CA activity in the gel stained with bromo thymol blue, one low molecular mass carrier of CA activity was found in the PSI-membranes, and two carriers, a low molecular mass one and a high molecular mass one, were found in the PSII-membranes. The proteins in the PSII-membranes differed in their sensitivity to acetazolamide (AA), a specific CA inhibitor. AA at 5 × 10−7 M inhibited the CA activity of the high molecular mass protein but stimulated the activity of the low molecular mass carrier in the PSII-membranes. At the same concentration, AA moderately inhibited, by 30%, the CA activity of PSI-membranes. CA activity of the PSII-membranes was almost completely suppressed by the lipophilic CA inhibitor, ethoxyzolamide at 10−9 M, whereas CA activity of the PSI-membranes was inhibited by this inhibitor even at 5 × 10−7 M just the same as for AA. The observed distribution of CA activity in the thylakoid membranes from A. thaliana was close to the one found in the membranes of pea, evidencing the general pattern of CA activity in the thylakoid membranes of C3-plants.

Carbonic anhydrase activity in Arabidopsis thaliana thylakoid membrane and fragments enriched with PSI or PSII

Changes in net photosynthetic rate (PN), stomatal conductance (gs), intercellular CO2 concentrations (Ci), transpiration rate (E) and water use efficiency (WUE) were measured in Plantago major L. plants grown under sufficient soil water supply or under soil water stress conditions. The plants had high PN in a wide range of soil water potential and temperature regimes. Soil water had little effect on PN under ambient CO2 concentrations, which was explained by a high carboxylation rate, but increased the dark respiration rate. Carboxylation activity at low Ci depended on RuBP regeneration, whereas at high Ci it depended on the phosphate regeneration rate. The gs and E values were low in plants under stress as compared to the controls that resulted in an increase of WUE. The results obtained show that Plantago major plants have different ways of adaptation to soil water deficit conditions.

Changes in the photosynthetic characteristics of Plantago major plants caused by soil drought stress

The expression of genes of two carbonic anhydrases (CA) belonging to the a-family, α-CA2 and α-CA4 (according to the nomenclature in N. Fabre et al. (2007) Plant Cell Environ., 30, 617-629), was studied in arabidopsis (Arabidopsis thaliana, var. Columbia) leaves. The expression of the At2g28210 gene coding α-CA2 decreased under increase in plant illumination, while the expression of the At4g20990 gene coding α-CA4 increased. Under conditions close to optimal for photosynthesis, in plants with gene At2g28210 knockout, the effective quantum yield of photosystem 2 and the light-induced accumulation of hydrogen peroxide in leaves were lower than in wild type plants, while the coefficient of non-photochemical quenching of leaf chlorophyll a fluorescence and the rate of CO2 assimilation in leaves were higher. In plants with At4g20990 gene knockout, the same characteristics changed in opposite ways relative to wild type. Possible mechanisms of the participation of αa-CA2 and α-CA4 in photosynthetic reactions are discussed, taking into account that protons can be either consumed or released in the reactions they catalyze.

Participation of Two Carbonic Anhydrases of the Alpha Family in Photosynthetic Reactions in Arabidopsis thaliana.

Effects of knockout of the gene encoding α-carbonic anhydrase 4 (α-CA4) on growth and photosynthesis of Thale cress (Arabidopsis thaliana (L.) Heynh., var. Columbia) were investigated. The shoot weight of mutant plants was found to be higher than in wild-type plants, and the leaves of mutants were enriched in starch content. The electron microscopy study revealed a considerable increase in the number and size of starch grains in chloroplasts of mutant plants. Comparison of wild-type and mutant plant leaves in terms of chlorophyll a fluorescence, coefficient of photochemical fluorescence quenching, effective quantum yield of photosystem II reaction, and nonphotochemical fluorescence quenching under steady-state illumination and saturating CO2 content in the air led to the proposal that α-CA4 participates in the development of nonphotochemical energy dissipation by accelerating the supply of protons for activation of violaxanthin deepoxidase and for structural changes in the light-harvesting complexes.

V. A. Mudrik

Papers

Plantago major plants responses to increase content of lead in soil: Growth and photosynthesis

Carbonic anhydrase activity in Arabidopsis thaliana thylakoid membrane and fragments enriched with PSI or PSII

Changes in the photosynthetic characteristics of Plantago major plants caused by soil drought stress

Participation of Two Carbonic Anhydrases of the Alpha Family in Photosynthetic Reactions in Arabidopsis thaliana.

Effect of knockout of α-carbonic anhydrase 4 gene on photosynthetic characteristics and starch accumulation in leaves of Arabidopsis thaliana