ABSTRACT
Post Graduate Department of Botany, Hooghly Mohsin College, Chinsurah-712101, West Bengal, India, Fax: 03326810544, E-mail: soumen1995@yahoo.com
Introduction
In a normal plant cell the generation of prooxidants in the form of ROS is delicately balanced by antioxidative defense systems. Exposure of plant cells to prooxidants results in oxidative stresses that shifts the balance in
favor of prooxidants (Asada and Takahashi 1987). The reactive oxygen species capable of causing oxidative damage include superoxide (O2
.), hydrogen peroxide (H2O2 ), hydroxy radical (OH.), alkoxy radical (RO.), peroxy radical (ROO.), organic hydroperoxide (ROOH), singlet oxygen (⏐O2), excited carbonyl (RO
(Fig. 1). Superoxide radical which is reactive in hydrophobic environment such as interior of membrane, is generated in plant cell at the onset of oxidative burst of cell. Protonated form of O2
than superoxide itself, but in plant cells at physiological pH a very small proportion of O2
–. exist in this form (Eltsner 1987). However, superoxide can dismutate to form H2O2.. A more reactive OH
and H2O2 through Fe catalyzed Haber-Weiss reaction. Singlet oxygen, an electronically excited species of O2, is also very toxic and its signifi cance has been realized only recently, due to the development of methods for its
Fig. 1 Generation and inter-conversion of ROS derived from O2. Ground state molecular oxygen can be activated by excess energy (photoexcitation), reverting the spin of one of the unpaired electrons to form singlet oxygen (⏐O2). One electron reduction leads to the formation of superoxide (O2
.–) radical. Superoxide exists in equilibrium with conjugate acid, hydroperoxy radical (HO2
.–). Subsequent one electron mediated reduction then produces hydrogen peroxide (H2O2), hydroxyl radical (OH
.) and fi nally water (H2O). Metal ions that are mainly present in cells in oxidized form (Fe3+) are reduced in presence of O2
.– and consequently may catalyze the conversion of H2O2 to OH
. by Fenton or Haber-Weiss reaction. Enzymes superoxide dismutases (SOD), catalases (CAT) and peroxidases (POD) reduce ROS. POD requires a reducing substrate SH2 for the reduction.
