In this article we will discuss about the stress tolerance by antioxidants production.
Abiotic stress including salt, chill and drought in plants induces production of reactive oxygen species (ROS), including superoxide radicals (O2–), hydrogen peroxide (H2O2) and hydroxyl radicals (OH). ROS production in cell is due to altered metabolic functions of cell organelles like chloroplast and mitochondria (Fig. 15.8). Their increased production is destructive and damages organellar membranes and other vital biomolecules. Production of ROS acts as signal for the activation of stress-response and defence mechanism.
Reactive oxygen species or reactive oxygen intermediate (ROI) are partially a reduced form of atmospheric oxygen (O2). Their production is due to excitation of oxygen (O2) to form singlet oxygen (O2’) or transfer of one or two or three electrons to O2 to form superoxide radical (O2–) or hydroxyl radical (HO–). Oxygen is vital for the cell, however, under stress conditions, it undergoes a series of reactions to form reactive oxygen species and jeopardize survival of cells.
Photosynthesis and respiration are the potential source of ROS production. Glycolate oxidation in peroxisomes contributes to enhanced production of ROS. In recent years, new source of ROI has been discovered. They are NADPH-oxides, amine oxides and cell wall bound peroxides. Generally, production of ROI in cells is low i.e., 240 µm SO2– and 0.5 µm H2O2 in chloroplast. Their concentration in stressed plants reaches upto 240-720 µm of O2 and 5-15 µm H2O2.
Scavenging of ROS:
Several plants under stressed condition synthesize and accumulate antioxidants as defensive mechanism. Antioxidant system in plants consists of a battery of enzymes that can scavenge oxygen radicals such as superoxide dismutase (SOD), peroxidases, catalases and glutathione reductases.
Antioxidants in excess bind to ROI molecule and detoxify or scavenge it. The SOD is the main antioxidant defensive system in most of the plants, catalyzing dismutation if two superoxide radicals converted into oxygen and hydrogen peroxide. According to their metal co-factor, different types like copper/zinc (Cu-Zn), manganese (Mn) and iron (Fe), plant organelles such as chloroplast and mitochondria contains Fe-SOD and Mn-SOD respectively.
Antioxidants such as ascorbic acid and glutathionine, which are found in excess in plastids and other cellular components (5-20 mM ascorbic acid and 1-5 mM glutathione), play a positive role for plant defense against oxidative stress. Transgenic plants, for example, with reduced ascorbic acid and glutathione levels are highly hypersensitive to stress.
Certain plants under stressed conditions naturally avoid production of ROI due to anatomical modification, physiological adaptation such as Crassulasian Acid Metabolism (CAM) and molecular mechanism that rearrange photosynthetic apparatus.
Participation of several components involved in signal transduction pathway of plants that senses ROI/ROS have been demonstrated. These components are mitogen activated protein (MAP) and AtNNPi. Involvement of calmodulin has also been examined in ROS signalling.
During signalling mechanism, MAP-kinase and calmodulin cascades are activated resulting in the activation or suppression of several transcription factors involved in the regulation of plant responses to oxidation stress.
Manipulation of ROI scavenging pathway in different cellular compartments found that contribution of mitochondria and chloroplast produce in excess of ROI leads to the recruitment of scavenging mechanisms. Information about ROI-scavenging properties of the nucleus is meagre. Attack on nucleus by ROI is comparatively less due to its redox-sensitive transcription factor.
Transgenic strategies have been utilized to study relationship between abiotic stress tolerance and anti-oxidant defence system. One of the classic examples is Mn-SOD cDNA from Nicotiana plumbaginifolia has been introduced into alfa-alfa. The Mn-SOD was expressed with the help of two different transit peptides in mitochondria and chloroplast.
Similarly, transgenic tobacco was found to express Mn-SOD in their two cell organelles. Transgenic plants survived oxidative shock and were able to grow. In another transgenic study, alfa alfa (Medicago sativa) was transformed with Mn-superoxide dismutase (Mn-SOD), specially targeted to mitochondria or chloroplast, by their peptide signals.
More than 100% survival rate was observed with transgenic alfa alfa. The ability of transgene to combat oxidative stress was assessed by employing ozone. The chloroplast specific expression of Mn-SOD was able to reduce leaf injury. In the same line of work Fe-SOD gene from Arabidopsis has been able to express in transgenic tobacco and protected photosystem II (Table 15.3).
Another approach for increasing oxidative stress tolerance relies on the expression of E. coli derived gene for glutathione reductase (GR) and Cu, Zn-SOD together in transgenic tobacco. These results clearly evidenced that both Cu Zn-SOD and GR together play a positive role in enhancing tolerance to oxidative stress.
