Abiotic stresses like heavy metals, drought, salt, low temperature, etc. are the major factors that limit crop productivity and yield. These stresses are associated with production of certain deleterious chemical entities called reactive oxygen species (ROS), which include hydrogen peroxide (H₂O₂), superoxide radical (O₂(-)), hydroxyl radical (OH(-)), etc. ROS are capable of inducing cellular damage by degradation of proteins, inactivation of enzymes, alterations in the gene and interfere in various pathways of metabolic importance. Our understanding on ROS in response to abiotic stress is revolutionized with the advancements in plant molecular biology, where the basic understanding on chemical behavior of ROS is better understood. Understanding the molecular mechanisms involved in ROS generation and its potential role during abiotic stress is important to identify means by which plant growth and metabolism can be regulated under acute stress conditions. ROS mediated oxidative stress, which is the key to understand stress related toxicity have been widely studied in many plants and the results in those studies clearly revealed that oxidative stress is the main symptom of toxicity. Plants have their own antioxidant defense mechanisms to encounter ROS that is of enzymic and non-enzymic nature . Coordinated activities of these antioxidants regulate ROS detoxification and reduces oxidative load in plants. Though ROS are always regarded to impart negative impact on plants, some reports consider them to be important in regulating key cellular functions; however, such reports in plant are limited. Molecular approaches to understand ROS metabolism and signaling have opened new avenues to comprehend its critical role in abiotic stress. ROS also acts as secondary messenger that signals key cellular functions like cell proliferation, apoptosis and necrosis. In higher eukaryotes, ROS signaling is not fully understood. In this review we summarize our understanding on ROS and its signaling behavior in plants under abiotic stress.

The comparative alterations of short term NaCl stress and recovery on growth, water relations, ionic composition, lipid peroxidation and antioxidants in roots of two rice cultivars differing in salt tolerance were studied. Exposed for 24 h to increasing (50, 100 and 150 mmol l À1 ) concentrations of NaCl, roots of 12D Oryza sativa L. cv. Lunishree and cv. Begunbitchi decreased in fresh weight, dry weight and relative water content. Increased Na + and decreased K + ion were determined at increasing NaCl concentrations. Both peroxide content and lipid peroxidation measured in terms of MDA level increased and the ratio was higher in Begunbitchi compared to Lunishree. Recovered roots showed lower peroxide and MDA content. Ascorbate and glutathione contents increased in the stressed and recovered roots of Lunishree, but decreased in Begunbitchi with increasing NaCl concentrations. Although SOD, CAT and GR activities decreased in the stressed roots, CAT activity also increased in recovered roots of both the cultivars. The POX activity increased in stressed and recovered roots of both Lunishree and Begunbitchi. Higher free radicals scavenging capacity and more efficient protection mechanism of Lunishree against salt stress, as revealed by the lower level of lipid peroxidation and improved plant water status as well as activities of some of the antioxidants, suggest that significant cultivar differences in response to salt stress in rice are closely related to differences in the activities of antioxidants and ion content. Another possible conclusion is that improved tolerance to salt stress may be accomplished by increased capacity of antioxidative system.

The article presents an overview of the mechanism of chromium stress in plants. Chromium is known to be a toxic metal that can cause severe damage to plants and animals. Chromium-induced oxidative stress involves induction of lipid peroxidation in plants that causes severe damage to cell membranes. Oxidative stress induced by chromium initiates the degradation of photosynthetic pigments causing decline in growth. High chromium concentration can disturb the chloroplast ultrastructure thereby disturbing the photosynthetic process. Like copper and iron, chromium is also a redox metal and its redox behaviour exceeds that of other metals like Co, Fe, Zn, Ni, etc. The redox behaviour can thus be attributed to the direct involvement of chromium in inducing oxidative stress in plants. Chromium can affect antioxidant metabolism in plants. Antioxidant enzymes like SOD, CAT, POX and GR are found to be susceptible to chromium resulting in a decline in their catalytic activities. This decline in antioxidant efficiency is an important factor in generating oxidative stress in plants under chromium stress. However, both metallothioneins and organic acids are important in plants as components of tolerance mechanisms and are also involved in detoxification of this toxic metal.

Water homeostasis is crucial to the growth and survival of plants under water-related stress. Plasma membrane intrinsic proteins (PIPs) have been shown to be primary channels mediating water uptake in plant cells. Here we report the water transport activity and mechanisms for the regulation of barley (Hordeum vulgare) PIP aquaporins. HvPIP2 but not HvPIP1 channels were found to show robust water transport activity when expressed alone in Xenopus laevis oocytes. However, the co-expression of HvPIP1 with HvPIP2 in oocytes resulted in significant increases in activity compared with the expression of HvPIP2 alone, suggesting the participation of HvPIP1 in water transport together with HvPIP2 presumably through heteromerization. Severe salinity stress (200 mM NaCl) significantly reduced root hydraulic conductivity (Lp(r)) and the accumulation of six of 10 HvPIP mRNAs. However, under relatively mild stress (100 mM NaCl), only a moderate reduction in Lp(r) with no significant difference in HvPIP mRNA levels was observed. Sorbitol-mediated osmotic stress equivalent to 100 and 200 mM NaCl induced nearly identical Lp(r) reductions in barley roots. Furthermore, the water transport activity in intact barley roots was suggested to require phosphorylation that is sensitive to a kinase inhibitor, staurosporine. HvPIP2s also showed water efflux activity in Xenopus oocytes, suggesting a potential ability to mediate water loss from cells under hypertonic conditions. Water transport via HvPIP aquaporins and the significance of reductions of Lp(r) in barley plants during salinity stress are discussed.

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The effect of short-term Pb and Cr (0, 100 and 1000µM) stress in moss Taxithelium nepalense (Schwaegr.) Broth., the possible generation of oxidative stress, antioxidant metabolism and changes in the chloroplast and cell membrane ultrastructure were investigated. In moss cells, treatment of Pb and Cr for 12 and 24 h decreased the dry mass and total chlorophyll content with marked inhibition under Pb. Both Pb and Cr accumulated after 24 h of their treatment where highest accumulation of Pb was visible than that of Cr. The ultrastructural studies at 1000 µM of Pb and Cr showed distortion of the thylakoid, distortion of chloroplast membrane and changes in the chloroplast structure. Chloroplast distortion was highly visible under Pb than that of Cr. The distortion in the cell membrane was evident at high concentration of Pb, while under Cr, minor changes were visible as compared to controls. Both Pb and Cr significantly increased the production of ROS like H 2 O 2 and O − 2 radical with marked production after 24 h under Pb than that of Cr. The alteration in metabolism of activated oxygen in moss cells was evidenced by the increase in the lipid peroxidation in moss cells, with pronounced effect after 24 h than that of 12 h after Pb and Cr treatment. The SOD activity showed an increasing trend followed by decrease in CAT, POX and GR activity after 12 and 24 h. Both ascorbate and glutathione showed higher accumulation under Pb followed by Cr. The results showed that at high concentration of metals, oxidative stress could be induced in moss cells characterized by the generation of ROS and initiation lipid peroxidation that inhibited the major antioxidant metabolism. Both physiological and ultrastructural studies suggested the possible induction of oxidative stress in Taxithelium nepalense (Schwaegr.) Broth. under Pb and Cr toxicity.

Aluminum (Al) toxicity is a major constraint for crop production in acidic soil worldwide. When the soil pH is lower than 5, Al(3+) is released to the soil and enters into root tip cell ceases root development of plant. In acid soil with high mineral content, Al is the major cause of phytotoxicity. The target of Al toxicity is the root tip, in which Al exposure causes inhibition of cell elongation and cell division, leading to root stunting accompanied by reduced water and nutrient uptake. A variety of genes have been identified that are induced or repressed upon Al exposure. At tissue level, the distal part of the transition zone is the most sensitive to Al. At cellular and molecular level, many cell components are implicated in the Al toxicity including DNA in nucleus, numerous cytoplastic compounds, mitochondria, the plasma membrane and the cell wall. Although it is difficult to distinguish the primary targets from the secondary effects so far, understanding of the target sites of the Al toxicity is helpful for elucidating the mechanisms by which Al exerts its deleterious effects on root growth. To develop high tolerance against Al stress is the major goal of plant sciences. This review examines our current understanding of the Al signaling with the physiological, genetic and molecular approaches to improve the crop performance under the Al toxicity. New discoveries will open up new avenues of molecular/physiological inquiry that should greatly advance our understanding of Al tolerance mechanisms. Additionally, these breakthroughs will provide new molecular resources for improving the crop Al tolerance via molecular-assisted breeding and biotechnology.

Alternative oxidase (AOX) is one of the terminal oxidases of the plant mitochondrial electron transport chain. AOX acts as a means to relax the highly coupled and tensed electron transport process in mitochondria thus providing and maintaining the much needed metabolic homeostasis by directly reducing oxygen to water. In the process AOX also act as facilitator for signaling molecules conveying the metabolic status of mitochondria to the nucleus and thus able to influence nuclear gene expression. Since AOX indirectly, is able to control the synthesis of important signaling molecules like hydrogen peroxide, superoxide, nitric oxide, thus it is also helping in stress signaling. AOX mediated signaling and metabolic activities are very much important for plant stress response. This include both biotic (fungal, bacterial, viral, etc.) and abiotic (drought, salinity, cold, heavy metal, etc.) stresses. The review provides a gist of regulation and functioning of AOX. Plants being sessile, are exposed to various environmental stressors, viz, drought, salinity, metal toxicity, low or high temperature, pathogen attack, nutrient deficiency, hypoxia etc; which aims to hamper their lifestyle and lifespan to a great extent. So, it has developed certain inbuilt mechanism for perception of minute changes in the environment and responders which facilitates, either tolerance or avoidance responses to alleviate the stress. This review rotates round one of these molecules which helps in stress perception and mediates a retrograde signaling pathway to architect a tolerance/avoidance response. What is Alternative Oxidase?The discovery of alternative oxidase (AOX) was at the beginning of the 20 th century from thermogenic plants during anthesis. AOXs are interfacial membrane bound, cyanide insensitive, metallo-protein involved in mitochondrial redox reactions. AOX branches off from the cytochrome pathway of mitochondria at the level of Ubiquinone (UQ) and is responsible for coupling, oxidation of ubiquinol, to 4 electron reduction of oxygen to water.1 As AOX bypasses Complex III and IV of the cytochrome pathway, it dramatically reduces ATP generation and the energy thus released is dissipated as heat.2 It helps in maintaining metabolic homeostasis and signaling dynamics in mitochondria. Stressors effect plant growth resulting in misbalance of energy demands and production. The ability of plants to maintain the delicate balance of energy production and utilization is fundamentally important for their survival. Presence of AOX forms the striking functional difference between mitochondria of higher plants (as well as some fungi and protists) and animals, i.e., presence of two terminal oxidases, AOX and cytochrome oxidase. AOX is encoded by two nuclear gene subfamily, AOX1 and AOX2, where dicotyledons possesses both gene ubfamilies, while monocots have only AOX1 genes.3 AOX is responsible for thermogenesis (attract insects for pollination) and stress tolerance. In response to stress AOX mediates a retrograde signaling pathway which...