Arun Kumar Shanker ( AK Shanker) was born in the City of Madras now Chennai in the Southern Indian State of Tamil Nadu and was educated in the Govt school known as Kendriya Vidhyalaya in Tambaram Air Force Base. He earned his under graduate and post graduate degree in Agriculture from the Agriculture College and Research Institute, Tamil Nadu Agricultural University in Coimbatore in 1987 and 1990 respectively. Later he joined the Indian Agricultural Research Institute in New Delhi for his advanced degree in Plant physiology only to drop and take up full time researcher position with the Indian Council of Agricultural Research. Later he completed his PhD in Coimbatore in 2003 His main area of research is on Cr and plant interactions and various other abiotic stresses in crops and plants. Presently he is working as a Senior Scientist (Plant Physiology), Central Research Institute for Dryland Agriculture in Hyderabad Deccan in India.
Abstract: Expression of the Arabidopsis HARDY (hrd) DNA binding transcription factor (555 bp present on chromosome 2) has been shown to increase WUE in rice by Karaba et al 2007 (PNAS, 104:15270–15275). We conducted a detail analysis of the complete sorghum genome for the similarity/presence of either DNA, mRNA or protein product of the Arabidopsis HARDY (hrd) DNA binding transcription factor (555 bp present on chromosome 2). Chromosome 6 showed a sequence match of 61.5 percent positive between 61 and 255 mRNA residues of the query region. Further confirmation was obtained by TBLASTN which showed that chromosome 6 of the sorghum genome has a region between 54948120 and 54948668 which has 80 amino acid similarities out of the 185 residues. A homology model was constructed and verified using Anolea, Gromos and Verify3D. Scanning the motif for possible activation sites revealed that there was a protein kinase C phosphorylation site between 15th and 20th residue. The study indicates the possibility of the presence of a DNA binding transcription factor in chromosome 6 of Sorghum bicolor with 60 percent similarity to that of Arabidopsis hrd DNA binding transcription factor.
Abstract: Chromium has received relatively little attention from plant scientists compared to other heavy metals in recent times in spite of it being a very a hazardous environmental pollutant. One of the reasons for this is the complexity of the metals interactions with biological systems and the difficulty in studying them. Although the possible mode of entry into the plants, resultant toxicity mechanisms and tolerance potential has been worked out in plants there is still a need to get a complete picture of the Cr–plant interactome. With the advent of hyphenated technologies and global gene/protein and metabolite expression/quantification techniques, studies to elucidate the complete metallome are possible albeit resource intensive. This minireview focuses on the recent developments in the field of Cr–plant interactions and proposes a model using a systems biology and integrated -omics approach to decipher the intricacies of Cr–plant interaction.
Abstract: Changes in climate can be expected to have significant impacts on crop yields through changes in
temperature and water availability. The purpose of mitigation and adaptation measures is therefore to attempt a
gradual reversal of the effects caused by climate change and sustain development. There are several mitigation
and adaptation practices that can be effectively put to use to overcome the effects of climate change with
desirable results. These methods fall into the broad categories of under crop/cropping system-based
technologies, resource conservation-based technologies and socio-economic and policy interventions. These
measures are discussed to suggest effective strategies among them to combat climate change with specific
reference to India.
Abstract: A thorough and extensive wild germplasm exploration survey was undertaken and 32 high yielding candidate plus trees (CPTs) of Jatropha curcas from different locations from a latitudinal and longitudinal spread between 12 degrees 41' and 22 degrees'E longitude and 77 degrees and 84 degrees 40'N latitude covering 11 locations in an area spread of 150,000 km(2) was collected for evaluating genetic association, and variability in seed and growth characters. Significant trait differences were observed in all the seed characters viz., seed morphology and oil content as were observed in growth characters viz., plant height, and female to male flower ratio and seed yield in the progeny trial. Broad sense heritability was high in general and exceeded 80% for all the seed traits studied. Female to male flower ratio showed near to 100% heritability followed by yield (83.61) and plant height (87.73). The path analysis revealed that female to male flower ratio had highest positive direct relationship with seed yield (0.789), followed by number of branches (0.612) and number of days from fruiting to maturity (0.431). Negative indirect effects were seen in number of days from flowering to fruiting which indirectly and negatively influenced yield through plant height and number of branches. Hierarchical clustering by Ward's minimum variance cluster analysis showed phylogeographic patterns of genetic diversity. K-means clustering revealed that trees from different geographic regions were grouped together in a cluster and as were trees from the same geographical area placed in different clusters suggesting that geographical diversity did not go hand in hand with genetic diversity. In addition clustering identified promising accession with favourable traits for future establishment of elite seedling seed orchard and clonal seed orchard for varietal and hybridization programmes.
Abstract: The physiological ability to adapt for various environmental changes is known as acclimation. When exposed to sublethal level of stress, plants develop the ability to withstand severe stress, as acquired tolerance. The present study was conducted to explicate the physiological basis of acquired tolerance in rice. Rice seedlings (variety IR 20) were grown in half strength Hoagland solution, and after 22nd day, they were kept in half strength Hoagland solution containing 50 mM NaCl (sublethal dose) for 7 days followed by half strength Hoagland solution containing 100 mM NaCl (lethal dose) for another 7 days. The non-pretreated 29 days old rice seedlings maintained in half strength Hoagland solution were directly transferred to half strength Hoagland solution containing 100 mM NaCl (lethal dose) solution for 7 days. The control plants were maintained in half strength Hoagland solution without NaCl. Various morphological and physiological parameters were recorded on 29th and 36th days old seedlings from control, pretreated and non-pretreated plants. The results revealed significant reduction in growth parameters (shoot length, root length, leaf area and total dry matter production) of non-pretreated plants below that of pretreated plants. The pretreated plants showed increased values to the extreme of 19.8 per cent in leaf water potential (psi(w)), 9 per cent in relative water content (RWC), 26 per cent in photosynthetic rate (P (N)), 28 per cent in leaf stomatal conductance, and 47 per cent in chlorophyll a over non-pretreated plants. The same trend was also observed in chlorophyll a/b ratio (6.6%) and F (v)/F (m) ratio (19.3%). However, a reverse trend was seen in F (o) value. The pretreated plants showed improved ionic regulation as evident from low Na+, Cl- and high K+ contents, which is attributed to enhanced plant water status and photosynthesis. Both pretreated and non-pretreated plants had higher contents of osmolytes viz., sucrose, leaf soluble sugars and proline contents than control plants. However, starch content revealed an inverse trend. Therefore, the present study reveals that rice can acclimate to lethal dose of salinity stress by pretreatment with sublethal dose of NaCl.
Abstract: UV-B radiation on plants is now of major concern to plant biologists due to the threat to productivity in global agriculture because of stratospheric ozone depletion. Quantitative and qualitative predictions attempts of expected effects and the search for a suitable ameliorant or a stress alleviant are being met with mixed outcomes. One of the reasons for this is the complications involved in designing UV stress response experements. The study of Heijari et al. in this issue has partially resolved this issue by well designed controls. Although the proportion of UV -B stress countering effects of Se in this study was not enough to meet the criteria of Se as an ameliorant. One of the future challenges to understand Se -plant interaction will be to study its potential essentiality to the plants and to unravel the complete picture of interconversion of the Se species within the plant system.
Abstract: A pot culture experiment was conducted in green house to study the potential of chromium (Cr) phytoaccumulatory capabilities of four promising agroforestry tree species viz., Albizia amara, Casuarina equisetifolia, Tectona grandis, and Leucaena luecocephala. Possibility of enhancement of Cr uptake by chemical (citric acid) and biological vesicular arbuscular mycorrhizal fungi (VAM) amendments were also tried. Biologically stable speciation of Cr trivalent (Cr(III) and hexavalent Cr(VI) were used. Cr(VI) was more toxic to the tree growth in terms of collar diameter (CD) increment in all the tree species than Cr(III). In general, roots accumulated more Cr than shoots in all the tree species. There was more than 10 fold increase in root Cr content in comparison with shoot Cr content in all the trees at all the concentration of Cr and all sources of Cr. Citric acid significantly increased the Cr content in the tissues of roots in all the species under both speciation of Cr. The highest increase in Cr content brought by 20 mM citric acid addition was in A. amara. Unlike citric acid, VAM treatment did not bring about a significant increase in the Cr content of all the tree species studied. Results suggest that Albizia amara is a potential Cr accumulator with citric acid as soil amendment. The potential of this tree as a Cr phytoaccumulator may be investigated in long-term studies.
Abstract: Due to its wide industrial use, chromium is considered a serious environmental pollutant. Contamination of soil and water by chromium (Cr) is of recent concern. Toxicity of Cr to plants depends on its valence state: Cr(VI) is highly toxic and mobile whereas Cr(III) is less toxic. Since plants lack a specific transport system for Cr, it is taken up by carriers of essential ions such as sulfate or iron. Toxic effects of Cr on plant growth and development include alterations in the germination process as well as in the growth of roots, stems and leaves, which may affect total dry matter production and yield. Cr also causes deleterious effects on plant physiological processes such as photosynthesis, water relations and mineral nutrition. Metabolic alterations by Cr exposure have also been described in plants either by a direct effect on enzymes or other metabolites or by its ability to generate reactive oxygen species which may cause oxidative stress. The potential of plants with the capacity to accumulate or to stabilize Cr compounds for bioremediation of Cr contamination has gained interest in recent years. (c) 2005 Elsevier Ltd. All rights reserved.
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Abstract: Selenium (Se) is regarded as an antioxidant in animals and plants, even though considered as nonessential element in plants. To test its ability to counteract senescence related oxidative stress in soybean a pot culture experiment was conducted. The soybean plant was sprayed with sodium selenate (50 ppm) at 78 days after sowing (DAS). Soybean leaves were harvested at 80 and 90 DAS for analysis of oxidant production and antioxidative enzymes activity. Se positively promoted growth and acted as antioxidant by inhibiting lipid peroxidation and per cent injury of cell membrane. The antioxidative effect was associated with an increase in superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) enzymes activity. Significant increase in antioxidant enzyme activity was positively related to Se content. The decrease in antioxidative enzymes at 90 DAS was much faster in control plants than Se-sprayed plants. The reduction in SOD and GSH-Px may be associated with senescence-induced oxidative burst.
Abstract: An experiment was conducted to study the effect of selenium on senescence phenomenon in soybean. Selenium was applied both as seed treatment at 5 ppm and as foliar spray at 50 and 100 ppm on 45, 60 and 75 DAS. The impact of selenium on delaying senescence was well established by increased activity of antioxidant enzymes such as, catalase and peroxidase and more numbers of leaves. Improved source strength by retaining more leaf number and area with better partitioning efficiency was considered as the contributing factor for significant yield improvement in selenium-treated plants.
Abstract: The response of the antioxidant enzymes and metabolites of the ascorbate glutathione pathway to oxidative stress caused by equal concentration (50 muM) of Cr(III) and Cr(VI) was studied in 15-day-old seedlings of green gram (Vigna radiata (L.) R.Wilczek. cv CO 4) for 5 days after imposition of stress. Significant increase in lipid peroxidation and H2O2 generation was seen 5 h after stress in Cr(VI) as against 12 h in Cr(III)-treated plants. High rate of uptake and translocation of Cr was seen in the first 12h of treatment with roots retaining 80 times more Cr than the shoots in both the speciation. No significant increase in catalase (CAT) activity and monodehydroascorbate reductase (MDHAR) was observed under Cr(III). Superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities increased under Cr(VI) after 12 and 4 h, respectively. Under Cr(VI) stress there was a steep increase of 194.6 nM g(-1) FW in the ascorbic acid (AA) content between 5 and 24 h of treatment. In contrast to reduced glutathione (GSH) content, which reduced after 24 It after treatment, oxidized glutathione (GSSG) increased steadily through the course of the experiment under both speciation. The rate of decline in the GSH/GSSG ratio was much faster in Cr(III) than Cr(VI). Results suggest differential response to AA and H2O2 signaling by Cr(III) and Cr(VI) and that AA in combination with APX was more effective in mitigating oxidative stress as against the role of GSH as an antioxidant. (C) 2004 Elsevier Ireland Ltd. All rights reserved.
Abstract: Flower and fruit abscission was studied in PKM 1 cultivar of tomato. The plants were sprayed with 2 mM nitrophenol on 60 days after transplanting [(DAT), flowering] and repeated once on 70 DAT (fruit set stage). Observations were record-ed in the flower and developing fruit from 60 DAT with an interval of 5 days upto 80 DAT. Hydrogen peroxide, superoxide radical and lipid peroxidation increased from 60 DAT to 80 DAT. Activities of superoxide dismutase (SOD), ascorbate peroxidase (APOX), peroxidase (POX), catalase (CAT), proline and polyol in-creased upto 70 DAT and thereafter decreased in both nitrophenol and control treat-ments. Nitrophenol-treated plants maintained lower hydrogen peroxide content, superoxide radical and lipid peroxidation and higher activities of SOD, APOX, POX and CAT at 70 DAT than control. The results suggest that flower and fruit abscission is associated with higher oxidative stress and antioxidant activity declines at flowering and fruit set stages. Nitrophenol can alter the oxidant and antioxidant levels, which favour increased fruit set coupled with high yield.
Abstract: A study on seasonal variation in leaf physiological parameters and dynamics of nitrogen metabolism was conducted in Albizia amara, a fodder-yielding, drought-resistant and fire-hardy tree of the arid and semi-arid regions of India, in National Research Centre for Agroforestry, Jhansi. Rate of transpiration and resistance to diffusion by stomata of the leaves of Albizia amara were inversely related and showed significant seasonal variation. Higher leaf temperatures were observed in summer season (March to June). Relative water content during winter months (November to February) and monsoon (July to October) were significantly higher than that in the summer. High leaf transpiration rates reduced relative water content significantly. Total nitrogen did not exhibit a distinct linear relationship with nitrate reductase activity, although they were directly related. Significant variations in nitrate reductase activity due to season were observed. Nitrate reductase activity showed distinct relationships with diffusive resistance, transpiration rate and relative water content. Summer season showed significant difference in all the parameters recorded in comparison with that in winter and monsoon.
Abstract: A study on seasonal variation in nitrate reductase activity and total nitrogen was conducted at National Research Centre for Agroforestry, Jhansi, India, on Albizia amara, a fodder yielding, drought resistant and fire hardy tree of the arid and semi-arid regions of the country. Total nitrogen did not exhibit a distinct linear relationship with nitrate reductase activity although they were directly related. Significant variation in nitrate reductase activity due to season was observed. Nitrate reductase activity was lowest in May and June. Nitrate reductase activity and total nitrogen showed distinctly different patterns of seasonal variation. Summer months showed significant differences in both the parameters in comparison with winter and monsoon months.
Abstract: Summary
This chapter contains sections titled:
* Introduction
* Mode of Action and Toxicity of Trace Elements in General
* Specific Mode of Action of Major Trace Elements
* Specific Mode of Action of Other Metals
* Mode of Action: What is the Future?
* References
Abstract: Chromium (VI) has long been recognized as a toxin in plant systems and as a carcinogen in human and mammalian systems. The actual mutagenic or toxic species of Cr is one or more of the reactive intermediates produced in the reduction of Cr(VI) to Cr(III). Glutathione (GSH) is suspected to be a prime reductant here due its ability to produce long-lived Cr(V/IV) intermediates during the reduction of Cr(VI). GSH-Cr interactions in plants have been fairly well elucidated (Shanker et al 2004). Dichromate reacts with GSH at the sulfhydryl group forming an unstable glutathione– CrO3- complex. The Halliwell-Asada pathway is the key pathway whereby Cr toxicity or tolerance is mediated. The high contents of dihydro ascorbate (DHA) in combination with an absence of active scavenging of free radicals and blockage of normal cell cycle progression by DHA is one of the main mechanism of Cr induced toxicity in plants (Shanker et al 2004). Cr(VI) can function as a hill reagent and can inhibit electron transport both in the photosynthetic and mitochondrial apparatus thus accounting for reduced NADPH pool. The critical balance between the available NADPH pool and ROS production by Cr would decide the redox status of the cell in both plants and animals. Chromium-DNA interactions are one of the well explained mechanisms of action of Cr in causing apoptosis and carcinogenesis. Cr associates with both- DNA bases and the phosphodiester backbone and the binding occurs through both coordinate covalent binding or electrostatic/ionic interactions. The base-specific binding of Cr has revealed a general, but not absolute, preference towards the formation of Cr(III)–guanine DNA adducts (A DNA adduct is an abnormal piece of DNA covalently-bonded to a carcinogenic chemical) and polyriboguanylic acid (poly(G)) in the case of RNA (O’Brien et al 2003), Cr- DNA protein crosslinks (Cr–DPCs) have been reported to extensively develop respectively between DNA and non-histone proteins and RNA and cytoplasmic proteins in many animal systems (Reem et al 2007). Cr(VI)-containing compounds are well-documented clastogens. Clastogens are chemical agents that increase the rate of genetic mutation by interfering with the function of nucleic acids. Evidence also indicates that chromosomal abnormalities (micronuclei) and genomic instability (microsatellite instability) are possibly involved in the induction of cancer by Cr(VI) (Wise et al 2008). DNA interstrand crosslinks (ICLs) are caused by Cr interacting with reaction centers on the complementary strand of DNA. A notion that has received much attention is that intracellular Cr(VI) mediates Fenton-like reaction and produces ROS which are responsible for nearly all of the toxicity and genotoxicity caused by Cr(VI) (Shanker et al 2005).
