Perspectives of plant-associated microbes in heavy metal phytoremediation

"Phytoremediation" know-how to do-how is rapidly expanding and is being commercialized by harnessing the phyto-microbial diversity. This technology employs biodiversity to remove/contain pollutants from the air, soil and water. In recent years, there has been a considerable knowledge explosion in understanding plant-microbes-heavy metals interactions. Novel applications of plant-associated microbes have opened up promising areas of research in the field of phytoremediation technology. Various metabolites (e.g., 1-aminocyclopropane-1-carboxylic acid deaminase, indole-3-acetic acid, siderophores, organic acids, etc.) produced by plant-associated microbes (e.g., plant growth promoting bacteria, mycorrhizae) have been proposed to be involved in many biogeochemical processes operating in the rhizosphere. The salient functions include nutrient acquisition, cell elongation, metal detoxification and alleviation of biotic/abiotic stress in plants. Rhizosphere microbes accelerate metal mobility, or immobilization. Plants and associated microbes release inorganic and organic compounds possessing acidifying, chelating and/or reductive power. These functions are implicated to play an essential role in plant metal uptake. Overall the plant-associated beneficial microbes enhance the efficiency of phytoremediation process directly by altering the metal accumulation in plant tissues and indirectly by promoting the shoot and root biomass production. The present work aims to provide a comprehensive review of some of the promising processes mediated by plant-associated microbes and to illustrate how such processes influence heavy metal uptake through various biogeochemical processes including translocation, transformation, chelation, immobilization, solubilization, precipitation, volatilization and complexation of heavy metals ultimately facilitating phytoremediation.     

Parallel reduction in expression of the eye development gene hedgehog in separately derived cave populations of the amphipod Gammarus minus

Caves provide excellent settings to examine evolutionary questions. Subterranean environments are characterized by similar and consistent conditions. Cave-adapted species often share characteristics such as diminished pigmentation, elongated limbs and reduced or absent eyes. Relatively little is known about the evolution and development of troglomorphic traits in invertebrates. In this study, we compare expression of the eye development genes hedgehog, pax6, sine oculis and dachshund in individuals from multiple independently derived cave populations of the amphipod Gammarus minus. hedgehog expression was significantly reduced in cave populations, compared to genetically related surface populations. Interestingly, no differences were found in pax6, sine oculis or dachshund expression. Because hedgehog-related genes are also involved in eye reduced in Astyanax mexicanus, these genes may be consistent targets of evolution during cave adaptation. These results provide support for the hypothesis of genomic 'hotspots' of evolution and allow comparison of adaptive mechanisms among diverse animals in subterranean environments.     

Agonist-stimulated beta-adrenergic receptor internalization requires dynamic cytoskeletal actin turnover

Stimulation of beta-adrenergic receptors (betaARs) leads to sequential recruitment of beta-arrestin, AP-2 adaptor protein, clathrin, and dynamin to the receptor complex, resulting in endocytosis. Whether a dynamic actin cytoskeleton is required for betaAR endocytosis is not known. In this study, we have used beta(1)- and beta(2) ARs, two ubiquitously expressed members of the betaAR family, to comprehensively evaluate the requirement of the actin cytoskeleton in receptor internalization. The integrity of the actin cytoskeleton was manipulated with the agent latrunculin B (LB) and mutants of cofilin to depolymerize actin filaments. Treatment of cells with LB resulted in dose-dependent depolymerization of the cortical actin cytoskeleton that was associated with significant attenuation in internalization of beta(2)ARs, beta(1)ARs, and mutants of beta(1)ARs that internalize via either clathrin- or caveolin-dependent pathways. Importantly, LB treatment did not inhibit beta-arrestin translocation or dynamin recruitment to the agonist-stimulated receptor. To unequivocally demonstrate the requirement of the actin cytoskeleton for beta(2)AR endocytosis, we used an actin-binding protein cofilin that biochemically depolymerizes and severs actin filaments. Isoproterenol-mediated internalization of beta(2)AR was completely blocked in the presence of wild type cofilin, which could be rescued by a mutant of cofilin that mimics a constitutive phosphorylated state and leads to normal agonist-stimulated beta(2)AR endocytosis. Finally, treatment with jasplakinolide, an inhibitor of actin turnover, resulted in dose-dependent inhibition of beta(2)AR internalization, suggesting that turnover of actin filaments at the receptor complex is required for endocytosis. Taken together, these data demonstrate that intact and functional dynamic actin cytoskeleton is required for normal betaAR internalization.     

Interrelations between oxidative stress and calcineurin in the attenuation of cardiac apoptosis by eugenol

In view of the known involvement of oxidative stress and calcineurin (Ca²⁺-calmodulin dependent protein phosphatase) in β-Adrenergic stimulated events, we examined the influence of eugenol (an antioxidant generally regarded as safe by the Food and Agricultural Organization of the United Nations) on isoproterenol-induced apoptosis in neonatal cardiomyocytes. In comparison to unstimulated controls, cardiomyocytes stimulated with 50 μM isoproterenol for 48 h demonstrated (a) increased intracellular Ca²⁺ levels (b) oxidative stress involving enhanced reactive oxygen species, decreased GSH/GSSG ratio, enhanced lipid peroxidation, increased activities of superoxide dismutase and glutathione peroxidase (c) apoptosis, evidenced by increased number of annexin V/TUNEL positive cells, enhanced membrane fluidity, decreased mitochondrial membrane potential, increased activities of caspase 3 and 9 along with (d) increased calcineurin activity. Pre-incubation of cardiomyocytes with 100 μM eugenol for 1 h, followed by isoproterenol treatment for 48 h, led to reversal of enhanced intracellular Ca²⁺ levels, oxidative stress, calcineurin activation and apoptosis caused by isoproterenol. In addition, similar treatment of cardiomyocytes with 10 nM FK506, a calcineurin inhibitor, could also attenuate isoproterenol-induced apoptosis. These results indicate the beneficial effects of eugenol in preventing cardiomyocyte apoptosis.     

Leaf chlorophyll concentration relates to transpiration efficiency in peanut

Two pot experiments were conducted in two different seasons at the University of Agricultural Science, Bangalore, India, to study (a) the relationship between chlorophyll concentration (by measuring the leaf light‐transmittance characteristics using a SPAD metre) and transpiration efficiency (TE) and (b) the effect of leaf N on chlorophyll and TE relationship in peanut. In Experiment (Expt) I, six peanut genotypes with wide genetic variation for the specific leaf area (SLA) were used. In Expt II, three non‐nodulating isogenic lines were used to study the effect of N levels on leaf chlorophyll concentration–TE relationship without potential confounding effects in biological nitrogen fixation. Leaf N was manipulated by applying N fertiliser in Expt II. Chlorophyll concentration, TE (g dry matter kg^?1 of H₂O transpired, measured using gravimetric method), specific leaf nitrogen (g N m^?2, SLN), SLA (cm² g^?1), carbon isotope composition (Δ¹³C) were determined in the leaves sampled during the treatment period (35–55 days after sowing) in the two experiments. Results showed that the leaf chlorophyll concentration expressed as soil plant analytical development (SPAD) chlorophyll metre reading (SCMR) varied significantly among genotypes in Expt I and as a result of N application in Expt II. Changes in leaf N levels were strongly associated with changes in SCMR, TE and Δ¹³C. In both the experiments, a significant positive relationship between SCMR and TE with similar slopes but differing intercepts was noticed. However, correction of TE for seasonal differences in vapour pressure deficit (VPD) between the two experiments resulted in a single and stronger relationship between SCMR and TE. There was a significant inverse relationship between SCMR and Δ¹³C, suggesting a close linkage between chlorophyll concentration and Δ¹³C in peanut. This study provides the first evidence for a significant positive relationship between TE and leaf chlorophyll concentration in peanut. The study also describes the effect of growing environment on the relationships among SLA, SLN and SCMR.