Cd Toxicity in Plants

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Cd Toxicity in Plants
BIO 676

• Involvement of Oxidative Response Mechanisms

by Burcu Kaplan
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OUTLINE

• Oxidative Stress Responses
• Heavy Metal Stress
• Cadmium Uptake Cd Toxicity
• Cd Oxidative Stress
• Selected Papers

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Oxidative Stress

• Various stress conditions lead to formation
• Superoxide radicals (O2-)
• Singlet oxygen (1O2)
• Hydrogen peroxide (H2O2)
• Hydroxyl radical (OH.)

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Enzymatic Antioxidant System

• Superoxide dismutases (SOD) are located in
  various cell compartments and catalyze the
  disproportionation of two O2- radicals to H2O2
  and O2.

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Enzymatic Antioxidant System

• H2O2 is eliminated by various antioxidant enzymes
  
• Catalases (CAT), located in peroxisomes/glyoxysome
  s and mitochondria thereby scavenging mostly
  photorespiratory/ respiratory H2O2.
• Peroxidases (POD) participate in lignin
  biosynthesis, IAA degradation, and convert H2O2
  to water by utilizing it in the oxidation of
  various inorganic and organic substrates
• Ascorbate peroxidase (APX) is primarily located
  in chloroplasts and cytosol and as the key enzyme
  of the ascorbate cycle, it eliminates peroxides
  by converting ascorbic acid to dehydroascorbate.
• One of the characteristic differences in the
  reaction mechanisms of these enzymes is that CAT
  without co-substrates can disproportionate H2O2,
  while POD and APX require co-substrates to
  detoxify H2O2.

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Pathways for reactive oxygen intermediate (ROI)
scavenging in plants. (a) The waterwater
cycle. (b) The ascorbateglutathione cycle. (c)
The glutathione peroxidase (GPX) cycle. (d)
Catalase (CAT). SOD acts as the first line of
defense converting O2- into H2O2. APX, GPX and
CAT then detoxify H2O2. In contrast to CAT (d),
APX and GPX require an ascorbate (AsA) and/or a
glutathione (GSH) regenerating cycle (ac). This
cycle uses electrons directly from the
photosynthetic apparatus (a) or NAD(P)H (b,c) as
reducing power.
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Biosynthetic Pathway of Mugineic Acid Family
Phytosiderophores
Mugineic Acid Family
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Heavy Metal Stress

• Various heavy metals produce different oxidative
  responses in different plants.
• The amount activity of SOD, CAT,POD,APX varies
  under metal stress conditions.

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Cd Uptake

• Cd is believed to penetrate the root through the
  cortical tissue.
• As soon as Cd enters the roots, it can reach the
  xylem through an apoplastic and/or a symplastic
  pathway complexed by several ligands, such as
  organic acids and/or phytochelatins.
• Normally Cd ions are mainly retained in the
  roots, and only small amounts are transported to
  the shoots.
• Cd accumulation in developing wheat fruits can
  occur via phloem-mediated transport (Hart et al.,
  1998).

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Cd Responses

• In a very general way, Cd in plants causes leaf
  roll and chlorosis, and reduces growth, both in
  roots and in stems.

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Effects of Cd

• Iteracts with the water balance.
• Damages the photosynthetic apparatus, in
  particular the light harvesting complex II
  (Krupa, 1988), and the photosystems II and I .
• Lowers total chlorophyll content inhibits the
  oxidative mitochondrial phosphorylation in
  Brassica napus plants.
• Indirectly inhibits the stomatal opening probably
  due to the strong interference of Cd with
  movements of K, Ca2 and abscisic acid in the
  guard cells.
• Cd significantly reduces the normal H/K exchange
  and the activity of plasma membrane ATPases.
• Inhibits the activity of several enzymes eg.
  glucose-6-phosphate dehydrogenase, glutamate
  dehydrogenase, malic enzyme, isocitrate
  dehydrogenase, Rubisco and carbonic anhydrase
• Cd may replace Zn ions in the zinc fingers, and
  that consequently Cd may interfere with the
  transcription mechanism.

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Plant Defense Mechanisms

• Immobilization
• Exclusion
• Synthesis of phytochelatins
• Compartmentalization
• Synthesis of metallothioneins
• Synthesis of stress proteins
• Production of stress ethylene.

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Dosage Duration Dependency

• Low Cd levels (i.e. about lt 1 µM in the soil
  solution or in the culture medium), Long exposure
  time (years or, in some cell cultures, at least
  several months), it can be reasonably
  hypothesized that plant management of chronic
  Cd stress is a whole made up of general cellular
  homeostatic processes, which may be common also
  to the management of other metals and other
  stress factors.
• High Cd levels(i.e. gt 1 µM ), Short exposure time
  (hours, days or weeks), plants can manage this
  acute Cd stress by a rapidly induced full
  fan-shaped response, in order to detoxify Cd
  ions and efficiently repair Cd damage.

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Fan-shaped response to Cd stress in higher
plants. The proposed multi-component model could
allow the plantmodulating to various extents the
expression of each ray of the fanto cope
effectively with Cd stress, by means of
mechanisms of avoidance, detoxification and
repair.
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Dosage Duration Dependency
Tolerance is supported by detoxification
mechanisms, which in turn rely on homeostatic
processes. The shift between homeostatic and
fan-shaped responses can be rapid and involve
quick changes in gene expression. Differently,
the slow shift from fan-shaped response to
real tolerance is caused and affected by the
long-term selection pressure, which may increase
the frequency (and promote the expression) of one
or a few tolerance gene(s).
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Cd Oxidative Response

• Cadmium induces profound changes in the
  physiology of plants.
• Despite its very low relative concentration in
  chloroplasts, it seriously blocks the activity of
  photosynthetic processes at different routes,
  including chlorophyll degragation.

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Cd Oxidative Stress

• Cd produces oxidative stress but, in contrast
  with other heavy metals such as Cu, it does not
  seem to act directly on the production of oxygen
  reactive species (via Fenton andor Haber Weiss
  reactions).
• On the other hand, Cd ions can inhibit (and
  sometimes stimulate) the activity of several
  antioxidative enzymes.
• There are varying responses to Cd-induced
  oxidative stress in different plants this is
  probably related both to levels of Cd supplied
  and to concentration of thiolic groups already
  present or induced by Cd treatment. Thiols
  possess strong antioxidative properties, and they
  are consequently able to counteract oxidative
  stress (Pichorner et al., 1993).

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Cd Oxidative Stress

• In sunflower leaves, Cd enhanced lipid
  peroxidation, increased lipoxygenase activity and
  decreased the activity of the SOD, CAT, APX,
  glutathione reductase and dehydroascorbate
  reductase (Gallego et al., 1996).
• In mungbean Cd produced lipid peroxidation,
  decrease of catalase activity and increase of
  guaiacol peroxidase and ascorbate peroxidase
  activity (Shaw, 1995).
• In bean roots and leaves, 5 mM Cd enhanced
  activities of guaiacol and ascorbate peroxidases,
  and raised lipid peroxidation (Chaoui et al.,
  1997).
• Cd treatment notably increased lipid peroxidation
  in pea plants (Lozano-Rodriguez et al., 1997),
  whereas no peroxidation was noticed in Cd-exposed
  plants and hairy roots of carrot (Sanita di
  Toppi et al., 1998).

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Comparative studies of H2O2 detoxifying enzymes
in green andgreening barley seedlings under
cadmium stress
Attila Hegedus, Sara Erdei,, Gabor Horvath
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EXPERIMENTAL

• Hordeum ulgare L. cv. Triangle
• Hydroponics of a half strength Hoagland solution
• Either etiolated or grown in growth chamber 12
  h. light 12 h. dark, 23 C
• After 7 days transferred to different
  concentrations of CdCl2 grown in above growth
  chamber conditions.

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EXPERIMENTAL

• Cd content
• Chlorophyll content MDA (malondialdehyde)
  content (lipid peroxidation)
• Enzyme activities
• POD
• CAT
• APX

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Results

• Cd mainly accumulates in roots
• Chlorophyll content decreases
• MDA content is higher in Cd treated plants
• PODhigher in roots in all plants but upon Cd
  enhanced activity only in leaves
• CAT, no significant change
• APX, activity increases with increased Cd
  concentration and time

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MDA

• MDA shows the existence of the non-redox heavy
  metal induced oxidative stress.
• At high Cd concentration increased level of MDA
  could be detected in both leaves and roots

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POD

• POD destroys hydrogen peroxide, but there is no
  activity increase in roots
• POD increases in soybean roots upon Al.
• ( Cakmak et al. 1991)
• POD activity is not effected in carrot roots upon
  Cd (Sanita di Toppi et al. 1998)
• Leaf POD activity increases with increasing Cd
  amount time in both green greening plants
• Probably acts in cytosol as a scavenger.

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CAT Cd

• Increase in activity is reported for soybean.
• Decrease in activity is reported for mungbean,
  bean, sunflower.
• Clijsters et al. 1999 states that most heavy
  metals do not effect the peroximal CAT activity,
  supporting results in this study.
• CAT activity is primarily regulated by the amount
  of H2O2 produced by photorespiration due to its
  peroxisomal location.

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APX

• At all Cd concentrations, the APX activity starts
  to dramatically decrease, exclusively in roots of
  green plants after the third day of treatment.
• In green leaves the decrease in APX activity is
  seen only at the highest, 1 mM Cd concentration.
  
• In etiolated leaves, after light exposure, the
  APX activity decreases in control leaves and
  after the second day of Cd treatment the elevated
  APX activity again decreases especially at higher
  Cd concentrations.
• Different response in green greening plants
• APX plays a central role in H2O2 detoxification
  at the chloroplast level

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Four barley genotypes respond differently to
cadmium lipidperoxidation and activities of
antioxidant capacity
Feibo Wua,, Guoping Zhang a, Peter Dominy
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EXPERIMENTAL

• Cd-efficient genotypes
• Zhenong 1, ZAU 3 and Mimai 114
• Cd-inefficient genotype
• Wumaoliuling
• The composition of the hydroponic nutrient
  solution (mg/L)
• (NH4)2SO4 48.2, MgSO4 65.9, K2SO4 15.9, KNO3
  18.5, Ca( NO3)2 59.9, KH2PO4 24.8, Fecitrate 5,
  MnCl24H2O 0.9, ZnSO4 7H2O 0.11, CuSO4 5H2O 0.04,
  HBO3 2.9, H2MoO4 0.01.
• pH 6.59/0.1

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EXPERIMENTAL

• On the sixth day after transplanting, cadmium (as
  CdCl2) was added to each pot to form three
  concentrations
• 0 (control), 0.1, and 1 µM.
• From the 40th day after transplanting and
  thereafter, half of the 1 µM Cd treatments was
  changed with 5 µM Cd.

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RESULTS
Wumaoliuling was the most affected among the four
genotypes
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73 76 86 65
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RESULTS

• The Cd-sensitive genotype Wumaoliuling shows much
  higher levels of MDA and a higher stimulation in
  SOD and POD activities in 1 mM Cd treatment,
  while in 5 mM Cd it had the lowest SOD and POD
  activities when compared with the other three
  resistant genotypes.
• Cd-stress elevates the levels of SOD, POD and
  CAT the effect varies with the intensity and
  duration of Cd exposure, and with genotype.
• The higher activity of SOD and POD at 5 mM Cd in
  the three resistant genotypes could explain their
  higher resistance to Cd concentration.
• The decreased SOD and POD activities observed
  when Wumaoliuling was exposed to 5 mM Cd against
  1 mM Cd was very probably due to the harmful
  effect of overproduction of H2O2 or its poisonous
  AOS derivatives, as observed by the higher MDA
  concentration. Also it can be attributed to
  Cd-induced inhibition of protein synthesis.
• CAT is the least effected among others.

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REFERENCES

• I. Cakmak, W.J. Horst, Effect of aluminium on
  lipid peroxidation, superoxide dismutase,
  catalase and peroxidase activities in root tips
  of soybean (Glycine max),Physiol. Plant. 83
  (1991) 463468.
• L. Sanita di Toppi, M. Lambardi, L. Pazzagli, G.
  Cappugi, M. Durante, R. Gabbrielli, Response to
  cadmium in carrot in vitro plants and cell
  suspension cultures,Plant Sci. 137 (1998)
  119129.
• L. Sanita di Toppi, R. Gabbrielli, Response to
  cadmium in higher plants, Environ. Exp. Bot. 41
  (1999) 105130.
• H. Clijsters, A. Cuypers, J. Vangronsveld,
  Physiological responses to heavy metals in higher
  plants defence against oxidative stress, Z.
  Naturforsch. 54c (1999) 730734.
• Gallego, S.M., Benavides, M.P., Tomaro, M.L.,
  1996. Effect of heavy metal ion excess on
  sunflower leaves evidence for involvement of
  oxidative stress. Plant Sci. 121, 151159.
• Hart, J.J., Welch, R.M., Norvell, W.A., Sullivan,
  L.A.,Kochian, L.V., 1998. Characterization of
  cadmium binding, uptake, and translocation in
  intact seedlings of bread and durum wheat
  cultivars. Plant Physiol. 116, 14131420.

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