Agriculture Reference
In-Depth Information
regulated in control processes like stress responses, programmed cell death and
pathogen defense pathways. The estimated constant rate of 240 µMO 2 and steady
level of 0.5 µH 2 O 2 for ROS production in cells are optimal growth conditions. On
the other hand, stresses that destroy the cellular homeostasis of cells result in the
enhancement production of ROS (i.e. up to 720 µMO 2 and steady state level of
5-15 µMH 2 O 2 ). These stresses consist of salt, drought, chilling, heavy metal, heat
shock, UV radiations, desiccation and air pollutants such as ozone and SO 2 , nutrient
deprivation, mechanical stress, pathogen attack and high light. Stress enhanced the
production of ROS that creates threat to cells and these conditions also enhance the
expression of ROS scavenging enzymes.
Under stress conditions, ROS are produced by cells (e.g. by NADPH oxidase)
and the signals for the defence pathways are also produced. Therefore, ROS may
be considered as the cellular byproduct of stress metabolism as well as secondary
messenger for signal transduction pathway in stress response.
Plant cells require different mechanisms to regulate their intracellular ROS con-
centrations by scavenging ROS because ROS are toxic and participate in key signal
events as well. These include superoxide dismutase (SOD), ascorbate peroxidase
(APX) and catalase (CAT). The equilibrium between SOD and APX activity in cells
is considered to be crucial for estimating the steady state level of O 2 -1 and H 2 O 2 .
And the balance with metal ions like Fe and Cu by ferritin and copper binding pro-
teins is also important to prevent formation of highly toxic OH by metal-dependent
Haber-Wiess or Fenton reaction. Other antioxidants important for the defence of
plants against oxidative stress are ascorbic acid and glutathione that are found at
high concentrations in chloroplasts and other cellular compartments. Though the
ROS scavenging enzymes expression increases the tolerance of plants under abiotic
stress. However, in chloroplasts and mitochondria a group of enzymes called alter-
native oxides also decreases the ROS production in cells by alternative channelling
of electrons in electron transport chain.
Conclusion and Future Perspectives
In this review article, arsenic occurrence, distribution, sources, chemistry, physi-
ological response mechanism and oxidative stress were discussed. We found that
arsenic from both natural and anthropogenic sources have been considered as one of
the most toxic element affecting millions of people in the world. And several prob-
lems like vegetation loss, contamination of ground water and toxicity in animals,
plants has been due to arsenic contaminations. From many published reports, it is
now clear that arsenic induces cellular toxicity by damaging the oxidative defense
mechanism that can be prevented by the phytochelation method. But we need more
sound information related to arsenic as treatments of residues from smelting or
mining, preventing the use of agrochemicals containing As or simple methods for
soil/water testing in field or laboratory which will allow us in making decision for
remediation and an adequate disposal plan.
Search WWH ::




Custom Search