Among all
the environmental stresses, the effect of metal accumulation has been
considered one of the most disturbing factors arising in the late 19th and
early 20th centuries. Some metals, such as zinc (Zn), mercury (Hg), copper (Cu),
arsenic (As), lead (Pb), and cadmium (Cd), can be

persistent
and bioaccumulative elements, thus being potentially toxic to living organisms,
from microorganisms to animals. These elements may be introduced into the
environment by many anthropogenic activities, such as mining, fertilizer use,
metal-based pesticides, and a wide range of industrial activities, which
release metals into the environment. In the case of plants, metals in the soil
can enter the roots through symplastic or apoplastic pathways before entering
the xylem and being translocated to the shoot, although transport through the
phloem may also play a key role in delivering metals. Plants have a range of
structural and biochemical barriers that can control the loading and unloading
of elements, and these include the exodermis and endodermis, as well as the
production of metal chelators. Metals can also trigger a series of changes that
can lead to phytotoxicity. Cd is a toxic metal because of its relatively high
mobility in the soil–plant. Cd can affect cell biochemical mechanisms and structural
aspects, for example, by lowering the control of the cell redox state, so
inducing oxidative stress and disruption of membrane composition and function .Cd
can induce severe disturbances in the physiological processes of a plant, such as
photosynthesis, water relations, and mineral uptake. Hence, a complex
biochemical pathway within the cell can be triggered concomitantly with
transcription regulation of Cd-responsive genes, such as induction of
antioxidant systems and increase in expression of transcription factors.
Moreover, a cross talk between many molecules involved in the modulation of
Cd-induced signaling pathway has been explored, such as the interaction of
reactive oxygen species (ROS) or antioxidants with hormones. In fact, strong
evidence has been presented to show that hormones are a major player in the signaling
pathways of Cd-induced stress

Introduction

 The presence of cadmium (Cd) in soils,
sediments, and water is a major environmental concern. Its release into the
environment in large amounts as an industrial waste has led to its current
ranking as a major anthropogenic pollutant. Cd, with its low affinity for soil,
usually remains in the mobile bioavailable form. It is transported into
microorganisms by the energy-dependent manganese or magnesium transport
systems.It competes with and replaces other functional metals inside cells . It
also brings about the denaturation of proteins, inhibits bacterial respiration
and proton–solute cotransport, and causes single-stranded breaks in cellular DNA.Because
of the bioaccumulation of Cd and its long-term toxicity in humans, the study of
Cd sequestration by microorganisms to clean Cd-contaminated environments
becomes important. Many microorganisms have evolved mechanisms to tolerate and
overcome Cd toxicity. A major mechanism
for heavy-metal resistance involves alterations in the membrane transport
system of an organism, resulting in the reduction or denial of entry of Cd into
the organism  Alternatively, the
intracellular or extracellular sequestration of heavy metals by adsorption to
cell walls or by binding to a specific biopolymer results in tolerance to
heavy-metal toxicity. Heavy metal
tolerance can also be achieved by microbially mediated extracellular
precipitation of the metal ion into an insoluble form, using microbially
produced sulfides, carbonates, hydroxides, phosphates, or oxalates, thus
reducing the bioavailable soluble concentration of the toxic form.Another mechanism involves the utilization
of energy-dependent ion efflux pumps,
which capture and remove the cytoplasmic metals
through the cell membrane. Another
involves
the enzymatic oxidation or
reduction of the metal ion, resulting
in its detoxification or
volatilization.

In its
reduced form, Cd is nonvolatile, therefore its removal by enzymatic reduction
would be unnecessary and thus energetically unfavorable. Cd resistance
sometimes involves the secretion of a polymer, protein, or other component that
sequesters Cd from extracellular media. This sequestration occurs by means of (i)
cell surface interactions involving specific functional groups on the cell wall
or (ii) intracellular sequestration.

 

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